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Outdoor Free Flight Forum => Old Time Rubber => Topic started by: calgoddard on July 12, 2018, 04:45:58 PM



Title: Korda Class C Tractor
Post by: calgoddard on July 12, 2018, 04:45:58 PM
This model has been highly recommended to me by Stan Buddenbohm and Herb Kothe, both legendary free flight modelers.  My five year old Gollywock has been gaining weight from various repairs.  Therefore, I decided I needed to build a new OTR small stick model. The wing span of the Korda C is 38 inches (flat). Its wing area is 143.3-inches (projected).

I could not find a build thread on this model on HPA so I decided I would start one.

I purchased the plan and short kit from Bob Holman Plans some time ago.  They have been sitting on a shelf in my garage. Bob provides excellent quality plans and laser cut parts for a very reasonable price.

The fuselage of the Korda C is built using 3/32 x 3/32 balsa sticks.  I am a little surprised that the relatively large fuselage of this OTR outdoor duration model is not built with 1/8 x 1/8 balsa sticks. But I stuck to the plan.  It said to use heavy balsa for the longerons so I used 12# balsa for those parts. It said to use medium balsa for the uprights so I used 7# balsa for those parts. The weight of the fuselage bones is 10.88 grams. The wing seems relatively small (in both span and chord) compared to the size of the fuselage, but Dick Korda was a legendary OTR builder and flier in the 1930's so I will trust his design.

I purchased a 17-inch balsa wood prop blank from Volare Products. It will need a lot of carving and sanding. I might cap the LE and TE of the prop with strips of bass wood. Alternatively, I may cover this prop with fiberglass and epoxy. I have had good results covering balsa wood props with lightweight fiberglass cloth and epoxy resin from SIG.  It is, however, a very messy and smelly process.  Surprisingly, covering a balsa wood prop in this fashion does not add very much weight. I believe that it adds a lot of strength and durability.

I plan to cover the fuselage of my Korda C with Polyspan "synthetic tissue."  It is much more resistant to punctures than domestic tissue or Esaki tissue. The wing and tail feathers of my Korda C will probably be covered with Esaki tissue. A Korda C with plastic film covering just wouldn't look right as it is vintage model.

A published list from a seasoned old time flier named Gene Wallock gives the preferred rubber motor sizes for many OTR models. I guess it is pretty well accepted. He recommends 16 strands of 1/8 inch rubber and a length of 36 inches for the Korda C.  That sounds like a good starting point to me.  The hook-to-peg distance will be around 26 inches, so I will probably have to braid the rubber motor.

I am going to build the wing in two halves that mate in order that the wing fit in to one of my standard clear plastic Christmas wrapping paper boxes that I use to transport my models.  I did this with my KIWI KOOP F1G and the system worked out very well. I followed Hepcat's advice on how to make a relaible detachable wing connection that was easy to build and use and maintains the required stiffness.

My plan is to make the fin plug into the stab.  The fin is extremely tall, and permanently assembled tail feathers would be cumbersome to transport.  A couple of .030-inch CF rods extending from the root of the fin and a couple of .062-inch Aluminum tube sockets in the stab should do the trick. I might instead use a plug-in tongue on the lower end of the fin.

I can glue one or more balsa wood wedges onto the TE of the fin to adjust the right turn in the power phase, if needed. Stan taught me that trick.  I'll mostly rely on thrust adjustments to achieve the optimum flight pattern in the power phase.  Stab tilt will be used to effectuate a right turn in the glide. With a big free wheeling prop, I don't think much stab tilt will be needed to make the model turn right in the glide.  

My Korda C will have a small balsa box in the fuselage under the wing for carrying one of my RF trackers.  I can just see this model flying OOS.  The Korda C is so large and apparently flies so well that I am willing to accept the 3 gram weight penalty imposed by carrying the RF tracker.

If anyone has built this model and has some advice, please post the same.  Thank you.


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 12, 2018, 06:08:46 PM
Here is the best picture of a Korda Class C tractor that I could find.



Title: Re: Korda Class C Tractor
Post by: calgoddard on July 13, 2018, 09:45:27 AM
The plan for the Korda C does not indicate any wash-out in the wing tips.

The 38-inch span wing has a horizontal center section and two outer wing sections with 3 1/2-inches of dihedral.  See the photo of the Korda C model in Reply #1 to get a feel for the overall configuration of the wing. The wing has under camber which I understand provides increased lift at lower speeds but increased drag at higher speeds, compared to a wing with a flat bottom.

I normally build washout into the wing tips of my outdoor rubber powered models.  I have heard many explanations for the advantages of wash-out in the wing tips. The explanation the makes the most sense to me is that it improves lateral stability by lessening the chance that the model will spiral into the ground during the glide portion of the flight.

When I built my "New Gollywock" the plan also did not show any wash-out. I asked John Hutchison (FAC Hall of Fame member) whether I should build wash-out into the wing tips of my New Gollywock. He said it was not necessary. So I followed his advice and that model flies great.  The only time it spiraled into the ground was when I launched it without noticing that one of the tiplets on the stab had previously broken away. When it transitioned into the glide, it gradually began circling tighter and tighter until it spiraled into the ground.  Apparently the drag of the lone tiplet on the stab caused this.  When I replaced the missing tiplet, I got the same great flights as always.

For those not familiar with the New Gollywock (1941), it is basically the same as the original Gollywock (1939) except that the former has no sub-fin and instead has tiplets on the ends of the horizontal stabilizer. But I have digressed.

Dick Korda was reportedly a very accomplished flier.  It would seem that however he built his Korda C would be the way to go.  But perhaps the advantages of wash-out in the wing tips had not yet been discovered in the 1930's.

So you experts out there, should I build wash-out into the wing tips of my Korda C?


Title: Re: Korda Class C Tractor
Post by: Bigbandito on July 13, 2018, 11:26:10 AM
Thanks for posting Cal. I'll be following to learn new tricks and cheer you on.


Title: Re: Korda Class C Tractor
Post by: strat-o on July 13, 2018, 02:09:23 PM
Can this model compete in OTR with a folding prop?

Marlin


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 13, 2018, 04:03:02 PM
strat-o -

You asked an interesting question.

The Korda Class C Tractor was designed to fly with a fixed 17-inch prop.  

I am not familiar with the AMA rules or the SAM rules that would apply to Old Time Rubber ("OTR") models.  It appears that the Korda Class C Tractor would be considered a "small rubber stick" under the SAM rules. AMA does not have an "OTR" event per se. It looks like the Korda Class C Tractor could be flown in the Moffett event.

I don't know if you can change the prop from that shown on the plan under the SAM rules.

The rules of the Flying Aces Club ("FAC") prohibit folding props on all models.

The model I am building is the Korda Class C Tractor.  Don't get it mixed up with the Korda Class C World Record which is a different model.  Plans for the latter model are available at www.outerzone.co.uk


Title: Re: Korda Class C Tractor
Post by: Bargle on July 14, 2018, 12:22:08 PM
Here is the best picture of a Korda Class C tractor that I could find.

Plane looks good, but how does that guy see with that big white thing over his face? ;)


Title: Re: Korda Class C Tractor
Post by: tgwhitley on July 14, 2018, 11:19:57 PM
FFQ issue # 52 has article with useful information about Korda C Stick. Tips from several great fliers Karl Gies, Carl Redlin, Herb Kothe, Ed Hardin, and Stan Buddenbolm.  Search for ways to buy back issues in PDF format at Free Flight Quarterly.
Tim


Title: Re: Korda Class C Tractor
Post by: Hepcat on July 15, 2018, 09:38:38 AM
Cal,
I am probably about to lose all my friends so I will try to be brief so that I can run away before they get me.
Scale models may occasionaly need a little wash out on the tips if a bad subject has been selected to model.  Duration FF models should not need any.  Worries about tip stalling and dropping into a spin are usually nonsense.  FF models don't have a man inside pulling back on a stick to make the aeroplane fly slower and slower for a landing.  If a FF model is approaching a stall it is better to get it over early and settle in a few undulations. If the stall is delayed, perhaps by excess thrust or rank bad trimming then the stall may continue on and on and on until it reaches the ground. It is important to remember that a FF model almost invariably flies in circles.  If it stalls you hope it will drop the wing that is on the inside of the turn and do a turn or two, which is the quickest way out of a stall.

I think you will find that most expert follow this principle by having more incidence on the inside wing.  They will almost certainly vary on which panel has what incidence and that, of course, is a major reason of why polyhedral wings are so popular.
John.
   


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 15, 2018, 09:43:05 AM
tgwhitley -

Thanks for the tip on FFQ issue #52.  I will get a copy of the same.

Hepcat - thanks for you explanation.  Per your advice, I will not build wash-out into the wing tips of my Korda C. That makes the building process easier as well.


Title: Re: Korda Class C Tractor
Post by: danberry on July 15, 2018, 11:56:33 PM
Put some washout in it. You won't regret it.


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 16, 2018, 03:13:40 PM
I built the nose block for my Korda C Tractor.

There is nothing novel in my approach but I will summarize the same for those readers of this thread who may be new to building OTR models. Also, some of you experts may tell me how to improve my technique.

When I got started in this hobby John Hutchsion emphasized that a tight fitting nose block is extremely important. It is necessary in order to maintain the thrust lines that you arrive at during the trimming process that are critical to the model flying correctly during the power phase. Of course, he was right about this.

I laminated four rectangles of medium density 1/8-inch sheet balsa wood, cross-grain, with Titebond carpenter’s glue. The sheet rectangles were slightly larger than the width and height of the front end of the fuselage. I drew an outline of the nose of the fuselage onto one side of the cured laminated block. I used that outline as a guide when I sawed the four sides with a balsa saw so that the block was close in dimensions to the outer dimensions of the front end of the fuselage.  I glued a backing sheet of 1/64-inch plywood to the rear side of the laminated block using Titebond carpenter’s glue.

I then built a snug fitting rectangular frame of ¼-inch x 1/16-inch hard balsa wood strips inside the nose, tacking the corners of the frame together lightly with medium viscosity CA so they would not be glued to the nose. I then glued the rear side of the nose block (the plywood side) to the rectangular frame with the same CA, being careful to align the nose block with the exterior of the fuselage.  After a few minutes, I pulled the nose block out of the fuselage and liberally applied CA to the long joints between the rectangular frame and the plywood backing sheet.  After some drying time, careful sanding of the outer sides of the rectangular frame was needed so that I could fully insert the nose block into the fuselage.

The front of the nose block was sanded into a rounded shape using a Dremel tool and a sanding block.

I drilled a 3/16-inch hole through the center of the nose block to receive an Aluminum thrust bearing that I purchased from either Volare Products or Retro RC. It is designed to receive a .062-inch music wire prop shaft.  The relatively large 17-inch prop of the Korda C will be driven with a lot of torque so I am not comfortable with using anything smaller than a .062-inch prop shaft.  I suppose you could get away with a .055-inch prop shaft, but then you might have difficulty finding a suitable thrust bearing. I used 30-minute epoxy to glue the Aluminum thrust bearing into the nose block.

I coated the entire front surface of the rounded nose block with medium viscosity CA. An easy way to do this is to put a polyethylene sandwich bag over one hand and use it to spread around CA liberally applied to the nose block. The glue won’t stick to the sandwich bag.  The cured CA gives the nose block a hard outer shell to resist any possible deformation under load. The shell of CA also resists motor lube.

The upper outer edge of the rectangular frame on the rear side of the nose block was sanded round.  This allows the nose block to pivot out of the front end of the fuselage should a blade of the propeller strike the ground on landing. This lessens the chance of having a broken balsa wood prop.  The rounded edge has been colored with a red Sharpie pen to remind me that the nose block is inserted with this edge UP.

Using CA I glued two segments of wooden toothpick into corresponding 3/32-inch holes drilled into the upper and lower sides of the nose block. They hold the rubber band that secures the nose block to the front end of the fuselage. You cannot rely on the snug fit to avoid losing the nose block and prop in flight or upon landing.  The rubber band that holds on the nose block will stretch to allow the nose block to pivot if a prop blade strikes the ground upon landing.

The plan for the Korda C Tractor indicates 2 degrees of down thrust and 1 degree of right thrust. I sanded the front end of the fuselage with a sanding block and got something close to this.  I may have sanded in more like 2 degrees of right thrust. However, I will sand in further adjustments to the thrust line, as necessary, during the trimming process.  Once any final sanding of the front end of the fuselage has taken place I may coat it with CA or glue on some 1/64-inch plywood strips to make it more durable and to lock in the optimum thrust line. The finished nose block of my Korda C came is much heavier than I would like at 5.01 grams but it is extremely strong.  Besides, as several experts have told me “nose weight is your friend.”

Over the years I have learned that building a larger outdoor model strong is very important, if you want it to last.  Light is good, to a point, but not to the point where the model is prone to warping or breaking during normal handling.

Stan Buddenbohm told me he launched his Korda C at 40 inch-ounces of torque.  Wow! If I want to get near that launch torque, I better have a strong model.

Any input about my build would be appreciated. Thank you for reading this post.


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 17, 2018, 09:00:04 PM
I carved the 17-inch balsa prop from a Superior Props blank that I purchased from Volare Products.  This prop blank is shown in a photo that is attached to the first post in this thread.

I used a sharp knife and followed the usual technique.  The carving took a little over an hour.  There is a nice video on YouTube that shows the technique.

As I recall the P/D of this prop should turn out to be 1.3, which I think will be good for this model.  The plan does not indicate the P/D of the prop. It just shows how to cut the prop blank for carving.

As you can see in the photos, after some sanding, the weight is currently a little over 10 grams.  The prop blank was marked to indicate 7.5# per cubic foot density.  I still need to do some more sanding, and in particular, sanding of the rear sides of the blades to yield some under camber.

I am leaning toward covering the prop with light weight fiber glass cloth and epoxy resin from SIG.  I have done this in the past and such covering adds considerable strength and durability with surprisingly little weight gain.


Title: Re: Korda Class C Tractor
Post by: Craig G on July 18, 2018, 12:25:24 PM
Looking great Cal. I have a new-to-me Tractor that I finished setting up the DT last night, hoping to get it going in time for some official flights at the NATS next week.


Title: Re: Korda Class C Tractor
Post by: Red Buzzard on July 18, 2018, 01:33:09 PM
Hi Cal,

I am sure that George at Volare took the original prop profile into consideration. It's worth remembering freewheeling props require somewhat higher pitches to reduce drag during the glide. It will also give you a longer prop run and enhance the LONG motor this model can accommodate. Take advantage of it. If you want to make a tweak use a higher p/d ratio. The only other worthwhile(?) note I can offer is to add a bit of LEFT rudder for initial testing. The lifting rudder can give a fair amount of RIGHT turn in early testing at low torque. Both of my Lanzo Duplexes took an alarming right swoop on low turns that left me breathless. In the end I trimmed them both with 1/32" LEFT skew of the entire tail group. I could have just as easily used a tab. I flew them both right/right with the original props.

Bill


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 18, 2018, 08:19:47 PM
Craig G - thanks for the complement.

Red Buzzard - Thanks for the trimming advice. I don't want this model to crash in a low torque trim flight. I measured my carved and sanded balsa wood prop and by my calculations its P/D is approximately 1.3.

After sanding the top surfaces of the blades a bit more, and after sanding in some under camber, the weight of the 17-inch balsa wood prop is now 9.66 grams.  I am definitely going to cover both sides of the prop blades with light weight fiber glass cloth and epoxy resin.

The late Karl Gies lined the LE and TE of the blades of the prop of his Korda C Tractor with 25 pound test monofliament fishing line before covering them with fiberglass.  I am not sure this is necessary.  

I toyed with the idea of inserting a thin plywood spar into the hub of the propeller as suggested by George Bredehoft but decided against this.  I think the fiberglass covering should make the light weight balsa wood prop sufficiently strong.

The prop will be painted some bright color so I have better chance of finding it should it somehow come free from the model during a flight.  I could tell you a sad story about a dumb mistake I made that has driven me to paint this prop.



Title: Re: Korda Class C Tractor
Post by: calgoddard on July 19, 2018, 06:30:15 PM
I glued 1/64 x 1/16 Bass wood strips to LE and TE of the prop blades.

I used Titebond glue because it makes a very strong wood-to-wood joint. It sands reasonably well, unlike CA. I will sand the edges of these strips flush with the front and year surfaces of the blades

This reinforcement should lessen the likelihood of nicks being taken out of the LE and TE.  I only covered about 2/3's of the length of the perimeter of each blade.  That's what single strip would cover and I figure the innermost portions of the LE and TE nearer the hub have less need for such protection.

These Bass wood strips were sold in a bag indicating that they are HO model train lumber. The strips easily curved around the ends of each blade. No pre-soaking of the strips was necessary.

I am still planning on covering the relatively large balsa wood prop with fiberglass and epoxy.  Maybe the addition of the Bass wood strips was not necessary, but they added very little weight.  


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 20, 2018, 05:41:38 PM
As l previously indicated, I am building the wing of my Korda C so that it can be taken apart in two sections. This makes transport easier.  Several years ago, I decided to transport all of my models to our flying field safely and efficiently in IRIS brand clear Christmas wrapping paper boxes.  They have removable lids and nest nicely in stacks of three or four boxes. These boxes measure 33.9-inches long x 12.1 inches wide x 6.3 inches tall.  Why spend a ton of time building a nice model and trimming the same, only to have it damaged in transport?

I still have my first FAC Rubber Scale model - a 30-inch wing span Fairchild 24. It takes a monstrous carboard box to be safely transported as the wing and tail feathers are not removable.

When I go to a contest, space in the back of my car is at a premium. I can fit three P-30 models in one IRIS box, three Coupe models in another IRIS box, several OTR models in another IRIS box, and so forth. Because these boxes are transparent, I can identify their contents just by looking at them.  Where I fly we quit at noon because the wind comes up by then.  Depending on the time of year, I often need to lug all my models into a hotel room.  It can easily reach 120 to 130 degrees F. inside my parked car and my models would warp in such conditions.  Therefore, I often I load all my airplane boxes onto a luggage dolly and roll them into my air-conditioned hotel room. I take a lot of kidding from other fliers when they see me doing this.
 
The fuselage of the Korda C will fit diagonally in one of the IRIS boxes.  However, a fully built Korda C wing will not as the wing has a span of 38-inches (flat).

The horizontal center section of the Korda C wing is 12 inches long so I built it in two 6-inch sections that plug together. I faced the two mating center ribs with 1/64-inch plywood and added some gussets to the inner bays. One 6-inch section has a pair of .057-inch carbon fiber (CF) composite tubes that plug into segments of 3/32-inch Aluminum tubing in the other 6-inch section. These components are supported on medium density balsa wood spars that extend between adjacent ribs.  Thread was wrapped around these parts and some CA applied for added strength. The CF composite tubes are very rigid and very strong.
 
I am building the outboard dihedral wing sections separately and will join them to the 6-inch center sections later on.

The center section of the Korda C wing will be covered on top with 1/32-inch sheet balsa wood where the hold down rubber bands do their squeezing.  Unsupported tissue covering in this area would take too much of a beating. These rubber bands will also prevent the wing halves from creeping apart, although that is unlikely. There should not be any forces acting to pull the wing halves outwardly.  I used a similar set up in my first Coupe and it has served me very well.  I have not had any problems with this wing connection scheme.

Onward with my Korda C build.  



Title: Re: Korda Class C Tractor
Post by: OZPAF on July 20, 2018, 08:30:28 PM
Quote
There should not be any forces acting to pull the wing halves outwardly.  I used a similar set up in my first Coupe and it has served me very well.  I have not had any problems with this wing connection scheme.

That is supported by my experience with early RC gliders. The dihedral of the wings provides sufficient inwards holding force together with small amount of friction, to hold the wings on. As the wings became heavier, with less dihedral and ailerons - it was then necessary to have more holding power and this was actually tape on F3B class models of around 2.5kg weight.

John


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 21, 2018, 03:17:06 PM
I took a break from building in order to make up a rubber motor for my Korda C.

It is 16 x 1/8-inch, 38 grams.  I braided the rubber motor so that its length is about equal to the estimated hook-to-peg distance of 26 inches.  It will stretch some after winding and I don't want motor bunching and CG shifting. I would rather not have to install a spring tension mechanism in the nose block.  

I used a Crocket hook of course.  Some prefer T-hooks.  There is a wobbly motor peg at the rear end of the rubber motor that will fit loosely over the 3/16-inch Aluminum motor peg.  I not sure it will provide any benefit in terms of even unwinding of the braided rubber motor.  It will still make it easier to install the rubber motor in the fuselage.

I used May 2015 TSS rubber because a friend gave me several pounds of it.  I have never tested this batch of rubber.  However, I have heard that any TSS rubber batch from 2009 on is pretty good, with June 2016 being exceptional as I recall.

By my rough calculation the estimated breaking turns for this 38 gram rubber motor will be around 950.  I will do some test winding on the bench to see when the torque leaps up.  Once I get the fuselage covered with Polyspan it will have enough torsional strength to allow me to do some test motor runs on my stooge.   I am hoping I can get a motor run of longer than 60 seconds.  Of course the motor run will be slightly different for the same number of turns when the model is flying on the power portion of its fight pattern.


Title: Re: Korda Class C Tractor
Post by: Red Buzzard on July 21, 2018, 09:52:09 PM
Cal,

Nice braid! I think your basic set-up will work fine with a Garami free-wheeler. Wobbly peg works great. It's a good idea to make the wobbly peg a little wider than your blast tube so you can unload the blast tube with a broken motor in it instead of trying to fiddle with it in the fuselage. There is a chance your broken motor can kink the wobbly peg and jam it against your tube that goes through the fuselage, but stuff happens. With a wide wobbly peg you at least have a chance.

You can make a test rig to test motors outside your fuselage with a simple 2x4 and some plates to support your rear peg and your nose block. Make it the same length as your motor base - or adjustable for other models - then you just hook it up to a bench vice, wind up your motor, and watch stuff fly around your garage when the motor breaks. It's good practice, just make sure you have some room!

Bill


Title: Re: Korda Class C Tractor
Post by: LASTWOODSMAN on July 21, 2018, 11:25:24 PM
     Hi calgoddard - does that sleeve that is held by the rear rubber motor's final  loop, does that sleeve slide over the rear motor peg?   Also, could you please show how the blast tube fits onto the rear motor peg?   How do you make your blast tubes ?    How do you load a blast tube into the fuselage?   With the rubber all knotted and twisted up inside the blast tube - how does that blast tube slide off of the wound motor - don't they rub against each other?
     That is a very symmetrical braided motor.    :o  8)   One more dumb question    :-\ - is the motor 16 strands of 1/8"  rubber, each braided strand is appx  26" long, correct?  
     I am following your "Old Time Rubber"  OTR  38"  KORDA  CLASS "C" TRACTOR   Outdoor Duration   build with great interest ...    :)
     Thanks

LASTWOODSMAN
Richard


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 22, 2018, 10:33:25 AM
Red Buzzard -  All good points.  Thanks.  Does a wobbly motor peg do any good with a braided rubber motor or is it only beneficial for a non-braided rubber motor?

LASTWOODSMAN - I am probably only possess an intermediate level of skill as an outdoor builder and flier.   I only got started in this hobby about 5-6 years ago.  Hopefully the real experts on HPA will weigh in to correct me and/or supplement my explanations.

A wobbly motor peg is essentially a segment of Aluminum tubing (or a bobbin) that has an ID that is larger than the OD of the motor peg.  Preferably the difference between the ID and the OD are such that the wobbly motor peg can wobble on the actual motor peg. The wobbly motor peg should be short enough so that its ends do not rub up against the sides of the fuselage. A good example of a wobbly motor peg is a segment of 1/4-inch Aluminum tubing that slides over a motor peg made of 3/16-inch Aluminum tubing.   See the attached picture.  Note that the flared ends of the wobbly motor peg help retain the rear end of the rubber motor on the wobbly motor peg. You can flare the ends of the 1/4-inch Aluminum tubing by inserting large Phillips screw drivers in opposite ends and then pushing and twisting.

Here is some theory about wobbly motor pegs from an article I wrote a few years ago that was published in our local flying club's newsletter.

George White summed up the problem pretty well in his article entitled Taming the Rear Motor Peg in which he stated:  “As the motor unwinds it wants to squirm about and flex up and down at the rear peg. The use of a solid peg interferes with that and the motor has a gathering at the tail.” A wobbly motor peg is essentially an outer motor peg that rotates around a fixed inner motor peg.  Tom Arnold explained how the wobbly motor peg works in his article entitled An Article About Very Long Rubber Motors in which he stated:  “The explanation of how it works . . . is that by allowing the motor to shift, roll, and twist at the peg as it unwinds it prevents that big knot from forming around the peg. For some reason it works itself out of a jam and lets the full winds come out. Now you still have a long sloppy motor to slide back and forth but you have gotten every turn out of it you could before then.”    

While not required, preferably a blast tube has a bayonet lock that permits it to lock onto the motor peg with a twisting motion.  See the attached photos. Without this feature, the blast tube will creep forward during winding unless you have a bystander hold it in place.

You will need a tool called an extractor in order to use a blast tube.  This is a long piece of music wire with a handle at one end and a hook at the other end.  The extractor must be long enough so that it can fit all the way through the blast tube with the hook protruding out the rear end of the blast tube. Once the rubber motor has been installed in your model and your model is mounted on a stooge, do the following: 1) insert the extractor all the way through the blast tube; 2) connect the hook on the extractor to the Crocket hook or T-hook at the front end of the rubber motor; 3) slide the blast tube off the extractor into the fuselage of the model as far as you can; 4) lock the rear end of the blast tube onto the motor peg; 5) disconnect the extractor hook from the Crocket hook; and 6) connect your winder to the Crocket hook. Reverse the process when you are done winding and connect the prop shaft hook of your prop assembly to the Crocket hook.

You can make a blast tube out of PVC pipe.  Preferably a blast tube is made out of lightweight Aluminum tubing or clear plastic tubing from an aquarium store.

The ID of the blast tube must be large enough so that the knotted rubber motor does not bind against the inside surface of the blast tube. The OD of the blast tube must be small enough so that it can be inserted through the nose of the model.

Braiding rubber motors is much easier to do than to describe.  There is a nice YouTube video showing the technique.

The rubber motor I made for my Korda C is 16 strands of 1/18 inch rubber.  I made one very large loop of rubber (approximately 38 grams) and folded it in half three times to get 8 loops (16 strands). I then wound about thirty turns (counter-clockwise) into one group of 4 loops.  Holding that group aside, I did the same to the other group of 4 loops. I then put the two groups together on my winder and they wound themselves into the braided configuration shown in the photo that is attached to Reply #19. The braided motor is about 26 inches long. The 26 inches includes the length of the Crocket hook. Before braiding the rubber motor is a little longer.  The amount that braiding shortens a rubber motor depends on the number of strands, the width of the strands, and the amount you wind each group.  Expert F1G (Wakefield) fliers often make their rubber motors with 1/16-inch rubber for reasons that I will not get into here. When braiding, I wind each group just enough so that the groups are almost ready to start forming knots.  

Note - most experts will tell you to that when braiding a rubber motor, you should wind each group of rubber loops clockwise.   I don't like this approach, because the braiding seems to largely disappear when you wind the rubber motor for flying.  

I hope I adequately answered your questions LASTWOODSMAN. Thanks for following my Korda C build thread.  I am a slow, methodical builder, but I enjoy the process.


Title: Re: Korda Class C Tractor
Post by: Red Buzzard on July 22, 2018, 12:46:12 PM
Hi Cal,

As I stated earlier, I braid everything. Simply put, in addition to the advantage of avoiding most bunching, braiding just makes the whole motor easier to handle and deal with in general use. The wobbly peg concept, I believe, promotes even distribution of knots and consistent unwinding. It just seems to make things more-predictable. I cannot quantify any of this. BUT I do know that without the wobbly peg concept, particularly a peg that spans the blast tube's diameter, a broken motor is a real pain. The locking slots that you illustrate at the end of the blast tube are essential to keep the blast tube from flying out the front of the fuselage and spearing the winder in some broken motor events.

Do you know how long your skeins were before you put in your braiding turns? That would be the number I would use to calculate maximum turns.

Very nice explanations to Woodsman.

Bill


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 22, 2018, 12:57:36 PM
Red Buzzard -

Thanks for the follow-up on the benefits of using a wobbly motor peg.

Thanks also for the complement regarding my explanations.

I didn't measure the length of the skein of 1/8-inch rubber that I used to make up the rubber motor for my Korda C.  It was super long.

I need to correct a typo in Reply #22, in the third paragraph from the end.  I meant "1/8 inch" rubber. NOT - - 1/18 - - .  The modify icon is gone for that reply.


Title: Re: Korda Class C Tractor
Post by: Red Buzzard on July 22, 2018, 02:33:33 PM
Hi Cal,

From what you have said about your 1/8" motor, it sounds like my go-to motor that I use in my Double Feature and my Becker Unlimited (small Nostalgia). It is a 16x36" that I skein as an 8x72" motor that I fold in half, then braid with 40 turns per half. The weights would be close, mine at about 43 grams. It will support about 1100 turns at 35 in./ozs. Generally, if you try a third wind there will be one or two broken strands lurking in the skeins somewhere or they will break with low winds. It's a good motor with a calculated max of 1335 turns in my table.

Bill


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 22, 2018, 03:36:14 PM
Bill -

Thanks for the input on the size of my rubber motor.

I am thinking (hoping) my completed Korda C will come in around 80 grams (or less).

I am reluctant to use a rubber motor bigger than about 50% of the weight of the model.

With my Gollywock, I found that I got longer flights when I reduced the rubber motor size from 30 grams to 25 grams.  Less weight to carry upwards offset a shorter motor run during the climb.  The glide of my Gollywock improved noticeably when the overall weight was reduced by 5 grams.


Title: Re: Korda Class C Tractor
Post by: Red Buzzard on July 22, 2018, 04:11:02 PM
Cal,

I just mentioned my motor as a reference for your winds estimate. Your own thinking is right on. The Korda C gains over a Golly in prop diameter, motor base, and wing area so it's a great choice. At 80 grams it will be very competitive.

B.


Title: Re: Korda Class C Tractor
Post by: LASTWOODSMAN on July 22, 2018, 05:30:30 PM
    Hi Cal - thanks for your very informative reply to my questions.   I also only started back in the hobby 6 yrs ago, when I was 55, all after a 40 year absence.  I too am a slow methodical builder, which I also really enjoy doing.  (Plus I use white glue ... I am a Luddite  :) ).    
     I tried braided motors before - they have a LOT of power and torque twisting pressure on that Crocket Hook, that you are trying with all of your strength, to hold between your thumb and forefinger, and then trying to get the prop hook of the noseblock onto that Crocket Hook, and then get that noseblock positioned into the nose, which brings me to my next question.  
     Once the blast tube is hooked onto the rear peg,  you then pull the Extractor wire handle out, to take off the Crocket hook, and then put on your Winder hook to the Crocket hook, and then wind the motor up inside the tube.  You now have to slide the Blast tube out of the fuse, and over past the Crocket Hook,  and eventually hook the Crocket hook to the noseblock prop wire.  How do you do all that ?    Do you have to use the extractor again ?
       I like your noseblock setup with one elastic and four  3/32" toothpic pegs to hold it in position.   QUOTE :  "You cannot rely on the snug fit to avoid losing the nose block and prop in flight or upon landing.  The rubber band that holds on the nose block will stretch to allow the nose block to pivot if a prop blade strikes the ground upon landing."
Toothpic pegs and  3/32"  holes - definately worth a try - I have been always using masking, or clear, scotch tape, to hold the noseblock on  -  those noseblocks really want to keep coming off no matter what I do ... and tape doesn't really hold either, as I have to cut new tape strips for almost each flight it seems, as the tape seems to lose its stickiness.

LASTWOODSMAN
Richard


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 22, 2018, 06:50:38 PM
Red Buzzard - Thanks for the words of confidence in my approach. The swept area of a 13-inch Gollywock prop is approximately 133 inches.  The swept area of a 17-inch Korda C prop is approximately 227 inches!

LASTWOODSMAN -

After the rubber motor is wound, you disconnect the winder and re-connect the hook of the extractor. You rotate the blast tube to unlock the bayonet lock, and slide the blast tube forward over the extractor, all the way to its handle. You then disconnect the hook of the extractor and put the extractor (and blast tube that is still surrounding it) to the side. You then hook the prop shaft hook of the nose block assembly to the Crocket hook.

Installation and removal of a blast tube using an extractor, as I have described in Reply # 22 and here, respectively, is standard practice.  You can see it in YouTube videos of fliers winding their rubber powered outdoor model airplanes.

Note that the larger Crocket hooks contain two holes. After the rubber motor has been wound to the desired launch torque, I disconnect the winder and insert the shaft of a small Phillips screw driver through the rear hole in the Crocket hook.  This makes it easy to resist the torque of a fully wound 16 strand rubber motor. The hook of the prop shaft goes through the forward hole in the Crocket hook.

I only use two toothpick segments to hold on the nose block, one upper and one lower.  I use a single size 16 office rubber band to hold on the nose block. Position the little Bass wood wedges on the sides of the fuselage to hold the opposite ends of the rubber band with the desired about of tension - not too tight, not to loose.  Good luck to you!



Title: Re: Korda Class C Tractor
Post by: OZPAF on July 22, 2018, 08:58:40 PM
I'm impressed with your detailed explanations to Richard Cal and your follow up Red B. I've always followed your builds Cal and enjoy your painstaking description of each step. It's a great way to learn about an interesting area that I haven't tried.
Looking forward to some videos of this bird flying.
Happy building.

John



Title: Re: Korda Class C Tractor
Post by: LASTWOODSMAN on July 22, 2018, 09:02:44 PM
Hi Cal - yes thanks from me too for those detailed explanations.   I was always wondering  ??? :-\  why the Crocket Hook (I use them too), has two holes in it. ??   Now I know.   :)

LASTWOODSMAN
Richard


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 23, 2018, 10:22:31 AM
OZPAF - Thanks. I have enjoyed your input and encouragement over the years.  My goals in posting on HPA are to: 1) help others; 2) gain input from experts on better ways to do things; and 3) avoid mistakes that might lead to a damaged or under-performing model.  When I was 15 years old I built Guillows models in isolation with predictably poor results.

I covered the 17-inch balsa wood prop I made for my Korda C with lightweight fiberglass cloth and epoxy resin.  I bought the cloth, resin and hardener from SIG a couple of years ago. I have had excellent results with these materials in the past when I used them to cover several balsa wood props I have made. These include props for my King Harry (2 Bit +1) and Gollywock (OTR Stick) models built for FAC competition.

Covering a balsa wood prop with fiberglass cloth and resin is a tedious and messy process.  It is made all the more difficult by the fact that once the resin is mixed up with the hardener, you only have about 15 minutes of working time before it starts to become tacky.  However, the additional strength and durability added to the prop with this covering are worth the effort, especially since the weight gain is much less than one would expect.

The directions from SIG said to add 15 drops of hardener to one ounce of resin.  I had some liquid medicine cups in my garage cupboard marked in teaspoons.  Hmmmm – how many teaspoons are in an -ounce?  This required a trip into the house to ask Alexa.  Answer - 6 teaspoons (US) are in one ounce (US).  I poured 3 teaspoons (US) of resin into a medicine cup and added 8 drops of hardener.  I was worried this might not be enough resin to complete the job. I thoroughly mixed the two ingredients. It turns out that I had twice as much resin and hardener mixture than I needed.  
 
Note there are only 4.8 UK teaspoons in one UK ounce.  Go figure! I am not sure if the SIG directions meant US drops or UK drops :)

I wore a respirator and had both the side entry door and the main folding door of my garage open where I was doing this work.  I didn’t want to breathe in any of the super fine fiberglass strands that come loose when cutting the four separate rectangles of fiber glass cloth for the opposite sides of the blades and the hub. I also wore Nitrile gloves.

I used to cover a balsa wood prop with only two rectangles of fiberglass cloth, but it was difficult to avoid wrinkles around the hub area.  This time I had the ends of two rectangles slightly overlap at the hub on opposite sides of the prop.
 
I applied the resin and hardener mixture to the prop with a small paint brush.  I covered one-quarter of the surface area of the prop with resin and then applied one rectangle of fiberglass cloth over the same, smoothing out wrinkles as best I could. I repeated this process a total of four times. I left the brush in the cup after I was through with the covering job in order to monitor the hardening process. The paint brush is yellow and is tilted at an angle in the second picture so you can barely see it.

After 24 hours of curing, I can cut the excess fiberglass cloth off the LEs, TEs and hub and sand them smooth. I previously lined the LEs and TEs of the prop blades with 1/64-inch Bass wood strips so it won’t matter if the fiberglass cloth wraps around these edges.
 
I have never tried the conventional method of covering a balsa wood prop with tissue or silk span and coating the same with nitrate dope.
 


Title: Re: Korda Class C Tractor
Post by: applehoney on July 23, 2018, 01:23:14 PM
I admire all the work you do on a prop, the time spent on such ... and not least the safety precautions taken.

The outcome is a strong stiff propeller .... but I've only ever used tissue covering and nitrate dope - sometimes no tissue on smaller diameters.  Obviously the time input is much less and I find the product invariably of very adequate strength.   I haven't broken a freewheeler in very many years other than a sole occasion when I hit the stab at launch and put the model into the  ground at full power. Otherwise, landing or D/T impacts have been of no consequence.  I do round off the top of the noseblock plug so that the whole unit rolls out of the fuselage nose when the prop touches ground.

Cal, please understand I'm not decrying or criticising your procedure, merely comparing notes on methods.  I do appreciate the extent of your inputs on the progress of the Korda C Tractor, to point I've been idly thinking of checking a Zaic book for it when next considering a new project.

Jim M



Title: Re: Korda Class C Tractor
Post by: calgoddard on July 23, 2018, 01:44:02 PM
applehoney -  Your feedback is welcome.  In fact, I think you are right on the mark.  My finished Korda C prop came out at 17 grams!  The prop diameter is 45% of the wing span. So it is a relatively big prop and I should not expect it to weigh, for example, only 7 grams.  

I think I am guilty of overkill in my construction and finishing of my Korda C prop.  Tissue and dope would have been lighter, and, as you point out, the strength of the prop reinforced in this manner would probably have been sufficient.
Due to the large diameter of the prop and the large chord of its blades, the fiberglass and epoxy covering added an unexpected 5-6 grams. The Korda C prop has a lot of surface area.

I was a little worried about the strength of the prop without reinforcing the same.  It was cut, carved and sanded from 7.5# balsa wood and felt pretty light. I hesitated to sand the blades too thin. I compared the blade thickness of my Korda C prop blades to that of a 17-inch Coupe prop with blades apparently made by an expert flier (Harry Steinmetz) and they were similar so I stopped sanding.

Some of the added weight in my Korda C prop came from bushing the prop with a segment of 1/8-inch Brass tubing. It's pretty heavy - pushing nearly 2 grams as I remember.  I thought about using 1/8 inch Aluminum tubing for this bushing, but was worried about wear.  

Here is a picture of my completed Korda C prop, such as it is.

I will be using the typical tube-in-tube bearing with a Garami free wheeler clutch.  The inner sleeve will be a segment of 3/32 Brass tubing about 1/16 of an inch longer than the outer Brass bushing to prevent the dog on the 1/16 inch prop shaft from pressing on the hub during the free wheel stage. This is of course a pretty standard prop set up in OTR models.

I spent many hours making a durable, but overweight balsa wood prop. I have no doubt that my Korda C will still fly with this prop but I don't want a tank. This is a set back in my Korda C build. At this stage, I lack the energy (and prop blank) to make a new prop for my Korda C  :(  The one I built will probably not break or get lost in the weeds where I fly.  So I won't need to replace it except to improve performance.  Oh well, live and learn.  As I said previously, I am not an expert builder and flier.


Title: Re: Korda Class C Tractor
Post by: applehoney on July 23, 2018, 03:15:50 PM
Cal, I feel that your final sentence is much contradicted by the substance of your postings.

I share your reservations about an aluminum bushing being prone to wearing - brass is the norm.   I've never used the tube-in-tube system as I fit a spring stop to any model regardless of size and a washer soldered to the shaft behind the prop takes all loads and allows it to freewheel without restriction. No motor braiding required either.   

I have in recent years taken to cutting short lengths (3/32 to 1/8) of brass tubing of the appropriate diameter to accept the shaft and cyano'ing such into the ends of the next larger diameter tube which constitutes the noseblock bush. *    In effect the shaft is then running in two journals rather than a longer bush in which the slightest wire deformity would create friction.   Perhaps overkill but I don't trust any piece of shaft wire to be absolutely straight ... but then again is the small space between shaft and noseblock bush likely to gradually accumulate dust etc. ?  I apply a trace of sewing machine or gun oil to prop and bearing bushes before any flying session.

Very nice looking prop, by the way !

JM

*  On reflection, that tube could be aluminum as it's not taking any rotational wear but the weight saving of the length involved is hardly worth consideration.



Title: Re: Korda Class C Tractor
Post by: flydean1 on July 23, 2018, 04:13:05 PM
Cal, first off, you have made a very handsome propeller!  It should perform well, and probably last forever.

I used to use polyester resin, but kept getting behind the curve on working time.  Got introduced to epoxy when making parts for my homebuilt.

Now I use West Systems 105 which, depending on catalyst, gives as little as 9 minutes working time, up to as much as 4 hours!  The slowest cure is still solid in 24 hours.  It's a bit pricey, but of the very highest quality.  I also use their plunger pump system which correctly apportions each ingredient.

First, I spray the uncut sheet of cloth with either thin nitrate, or rattle-can spray lacquer.  The cloth should be a little stiff as if it had been starched.  That way, it doesn't fray when cut.  You can even draw complex shapes on it with a soft lead pencil or marking pen  .Also, I cut the cloth pieces such that the weave is on a 45 degree bias.  It really makes it easy when going around compound curves.

You can't believe how nice it is to not be under the gun when working with epoxy.  Instead of using the resin to adhere the cloth, I coat the item with a couple coats of nitrate dope, sanded smooth with 600 paper, then adhere the cloth by brushing thinner thru the cloth.  The resin is used to fill the weave and reinforce the fibers.  With the slow cure, I have plenty of time to blot with paper towels absorbing all excess resin which keeps the weight gain down.

This process was recommended by Don DeLoach.  Try it next time.  As I did, you will never fool with polyester again.


Title: Re: Korda Class C Tractor
Post by: OZPAF on July 23, 2018, 08:38:24 PM
That is a fine looking prop Cal. I have a another couple of suggestions for the application of light weight glass cloth if you decide to use it for props or anywhere else.

I use a cheap rattle can hair spray on the cloth to stiffen it and minimise fraying when cutting the cloth on the bias. FD's idea is similar.

Also if you lightly use a contact spray on the prop or form you can mould the cloth dry around the shape and then screed the resin through the cloth. Thus you can have everything ready before you mix any resin.

I would also only use epoxy and for something like a prop - warm it to thin it and then brush it on and then screed it with an old credit card or similar(or business card).

Finally roll a toilet roll over it to ensure the cloth is well attached and to remover excess resin.

John


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 24, 2018, 10:03:37 AM
applehoney - thanks for the comments on journals. I know what you mean about prop shafts not being perfectly straight.  I like the inclusion of a tensioner in each of your models. Unfortunately, I have never been any good at soldering, try as I might with de-greasing, flux, the correct solder, the proper soldering iron, the optimum temperature, etc.

flydean1 - thanks for all the advice on fiberglass and resin covering. It would sure be nice to have at least 30 minutes of curing time.  I forgot to orient the fiberglass cloth on the blades of my Korda C prop so that the weave was at a 45-degree angle. I remember now that this is optimum.

OZPAF - Those are great instructions for covering a prop with fiberglass cloth and resin.  I will follow your advice the next time I try this.  I am still wondering if the added weight is worth it on a rubber powered model.  

Moving on, I finished the frame of the wing of my Korda C.  

The wing span is 38 inches (flat).  The ribs have under camber. Fortunately they were all laser-cut. The main ribs are a close match to the NACA 4410 air foil. The wing uses a 10% thick airfoil with 4% camber at 40% of the chord.

The Korda C wing has a relatively high aspect ratio in comparison to a Gollywock wing as visible in the last photo attached to this post.  The Gollywock wing frame is an older one, whose covering has been stripped.  This partially explains why its balsa wood is darker.

The 1937 plan for the Korda C specifies bamboo wing tips.  I am not a fan of curving bamboo. Therefore, I made the curved wing tips of my Korda C wing by laminating four 1/64-inch x 1/16 Bass wood strips around plywood forms supplied in the Bob Holman short kit using Titebond glue.  These wing tips are very strong, and very light.

The wing of the Korda C employs the “Cleveland multi spar style” which consists of five 1/16-inch square sticks - three on top and two on the bottom.  The Bob Holman plan does not specify the density of the balsa wood for these sticks. I made the center ones with 12 - 14# balsa wood after I had a few breaks with 8# spars.

The consensus was that I should incorporate wash-out into the wing of my Korda C. So I built 1/8-inch of wash-out into each wing tip.

I was happy with the 16.08-gram weight of the frame of my Korda C wing. This is especially so considering the additional weight of the hardware I included in order to allow the wing to be disassembled into two pieces for ease of transport. Note my inclusion of 1/32-inch sheet balsa covering in the center of the wing where the hold down rubber bands will span. By way of comparison, the Gollywock wing frame in the last photo weighs 18.04 grams. It does not incorporate the CF composite and Aluminum tubing hardware that would allow it to separate into two halves.

I placed the wing frame of my Korda C on top of the edge of ruler along its center line to determine its heavier side. I glued two small pieces of Bass wood to the side of the outermost right rib to bring the wing frame into balance. I don't want a heavier wing tip to cause my model to turn left or right.

The wing of my Korda C will be covered with Esaki tissue.  I plan to shrink the tissue covering with Eze Dope, a water based “dope” product from the UK that I have been happy with after going through a learning curve.  My wife can better tolerate my model airplane building hobby if I do not use nitrate dope :)


Title: Re: Korda Class C Tractor
Post by: OZPAF on July 24, 2018, 09:02:48 PM
It's interesting to see the 2 wings together. Different approaches to bending strength and torsional stiffness. The Korda appears to relay more on the tissue covering for both using the multi spars to working together with the tissue(and braced by the tissue) while the Gollywock uses a more conventional spar for bending and diagonal bracing t help with torsion.
Of course the multi spars also have a turbulator effect as well.

They both look nice BTW.

John


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 25, 2018, 11:53:00 AM
The span of the Korda C stab is 18 3/8-inches. See the attached picture of the stab that I built for my Korda C. As you can see in the photo attached to Reply #1, the Korda C has an ugly over-size fin.  This is consistent with the trend that was apparently popular in 1937 of having the height of the fin equal to half the span of the stab.  Other well-known models of that era had the same feature, such as the Lanzo Duplex.  I will not change the proportions of the fin of my Korda C as this would violate FAC rules. Such a change might also negatively affect the flight performance of this model.  Moreover, it would be unreasonable for me to presume that I know more about this model than its legendary designer and flier, Richard Korda.
 
The stab of the Korda C is a lifting stab as it has an airfoil cross-section. The fin of the Korda C also has an airfoil cross-section with the convex surface on the left side. The latter feature will help induce a desired right turn during the climb.
 
Both the fin and stab of the Korda C have a minimal construction that includes two 1/16-inch square spars on each side.  The maximum thickness of the fin and stab of the Korda C are relatively small. This will make it challenging to include hardware that allows them to be disconnected for transport.  The assembled tail feathers of the Korda C are way too large to fit in one of my standard IRIS plastic transport boxes.

The weight of the uncovered stab of my Korda C shown in the attached photo is only 3.75 grams.  I formed the curved tips using four 1/64-inch x 1/16-inch Bass wood strips laminated around a form and bonded together with Titebond glue.  A plywood form for making the stab tips is supplied in the short kit that I purchased from Bob Holman Plans.


Title: Re: Korda Class C Tractor
Post by: vintagemike on July 25, 2018, 01:25:02 PM
Hi there, lovely model, there is a school of thought that the cambered surface should be on the right side of the fin. Having the camber on the left (as plan) introduces too tight a turn. Any one else want to comment?


Title: Re: Korda Class C Tractor
Post by: applehoney on July 25, 2018, 01:36:15 PM
18.5 inches actually but who's gonna quibble over 1/8" ??

However ....   " would violate FAC rules "

I said previously I was thinking of checking the Zaic book with the Tractor being maybe a future project -you inspired me !  I had FAC in mind but their rules dictate that maximum projected span is 36".  However. the Tractor is 38" 'flat' and a quick dihedral sketch seems to indicate that the projected span would be 37"   ....  too large to be FAC legal.   Grrrr ....  :'(



Title: Re: Korda Class C Tractor
Post by: Red Buzzard on July 25, 2018, 02:48:14 PM
Hi Cal,

Relative to Vintagemike, If you put the lifting side of the rudder to induce left turn you will then favor torque in the climb and may have trouble at high winds/torque. And it will develop lots of torque. To say nothing of likely violation of OT and FAC rules. I would leave the camber as original and be ready to apply a little left tab. With my Duplexes, I left the skew in the tail surfaces and flew it with that, always. If you need more glide turn to the right, stab tilt works just fine. I lost both Duplexes, one to a fabulous thermal at Taft and perhaps a model thief, and the second to UPS shipping. The boot mark was pretty plain and UPS paid-up.

Bill


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 25, 2018, 11:08:40 PM
applehoney - I researched the legality of the Korda C Tractor for FAC competition before I decided to build this model.

The 2018-2019 FAC rule book says, under the OTR Stick and Fuselage event heading:

- - Maximum wing span is 36” projected OR maximum wing area is 150 sq. in. - -

The wing area of the Korda C Tractor is 143.3 square inches, and thus, it meets this rule.

In regard to the orientation of the airfoil of the fin, I need to stick to the plan in order for my Korda C to be FAC rule compliant.

I have worked out a system for adjusting the angle of the fin, as needed during trimming.  I will post more on that later.

You guys are great for following my build and posting comments. Thank you.

I am still thinking about carving a lighter prop.  Actually I am thinking about carving another 10 gram prop and then not covering it with fiberglass cloth and resin in order to save 5 grams.  If this lighter prop breaks, I can use my heavy red prop.  


Title: Re: Korda Class C Tractor
Post by: Hepcat on July 26, 2018, 08:28:20 AM
Cal,
As usual I have been following your build (of the Korda C this time) with interest. Near the start you mentioned a 'Volare' prop blank. They always seem to have good products so naturally I had a look at the photograph. Very nice and they had even marked the wood weight, 7.5 lb/cu.ft. I did think that it was a pity to use such light wood but no reason to comment at the time. However I see you are now thinking of carving a new propeller so I will pass a comment.  I think the best propellers probably come from the strongest balsa you have (or even woods that are heavier than most balsa).  The blades can then be carved thinner, which will almost always make a more efficient propeller and possibly one as light as a fatter balsa one.  Propellers from a harder wood will usually give a better finish without using much glass fibre and certainly avoiding messing with epoxy resins.
John
 


Title: Re: Korda Class C Tractor
Post by: applehoney on July 26, 2018, 10:01:06 AM
Have to agree with John (not unusual ... ) in that the better of my prop selection are carved from pine and merely finished with nitrate dope to a smooth finish.   Of two 'Senator' props - one pine, one balsa - the former is a tad lighter.

Cal, thank you for the update on FAC elegibility -  I  zeroed in on the maximum span and quite overlooked the wing area ruling.  Indebted to you!


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 26, 2018, 11:17:52 AM
Hepcat and applehoney -

Thanks for the recommendation on prop carving.  I have heard others say that it is preferable to carve a prop from heavy wood so your advice is consistent with their comments. It makes sense that you would want the propeller blades as thin as possible. Also, the harder wood should eliminate the need for fiberglass and epoxy covering, as John indicated.

I might be doing a lot of whittling with a sharp knife. I'll see if Volare Products can saw me a blank from the hardest balsa wood they have in stock.  I don't have a band saw.

applehoney - don't be too hard on yourself.  It is my understanding that the Korda C was not eligible for the OTR Stick and Fuselage event in FAC competition until the 2018-2019 rules came out.  If it had been eligible, Herb Kothe would probably have been flying the Korda C in that event.  I have had the pleasure of competing against Herb on a number of occasions.  He is a real gentlemen and is always willing to share his knowledge.  Herb is a master builder and flier, and almost unbeatable in FAC competition.  Here is a picture of Herb launching his Taylorcraft in Eloy, Arizona, USA. Note that this Taylorcraft disassembles for transport.  The wing is held onto the fuselage by a single plastic machine screw.


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 26, 2018, 04:15:12 PM
The tail feathers of the Korda C are quite large.  The fin is unusually tall in proportion to the span of the stab. Therefore, I prefer to have the fin and stab separable for transport.

I thought about the practicality of various releasable fin-to-stab connection schemes including: 1) a pair of carbon fiber (CF) composite rods extending from the root rib of the fin that plug into Aluminum tube sockets in the stab;
2) small hard balsa wood or plywood tabs extending from the root rib of the fin that slide into slots built into the stab; or 3) plastic machine screws and nuts.

Loss of the fin in flight would be catastrophic. Such a failure would almost certainly result in a crash and total destruction of my Korda C.  I therefore decided to go with the third option.

I backed the bottom side of the root rib of the fin with 1/64-inch plywood.  I glued plastic nuts in captive fashion on the top side of this rib using CA and little pieces of hard balsa that butt up against one of the six side edges of each nut. I built a platform in the top of the center of the stab that supports the root rib of the fin. Two plastic machine screws extend through 3/32-inch holes in this platform and aligned 3/32-inch holes in the root rib, and thread into the plastic nuts.  If necessary, the aft hole in the platform can later be machined into a lateralling extending slot to allow minor angular adjustments of the fin during the trimming process.

I had to remove the center rib visible in the photo attached to Reply #40 and install two ribs close by each other. The platform is located between these two ribs.  I can now install .020-inch music wire hooks into these ribs. These hooks will serve to retain the ends of corresponding rubber bands on either side of the fin that will pivot the stab when the DT triggers.  

The uncovered tail feathers of my Korda C weigh 6.91 grams. This is close to the weight recited in example described in the article on the Korda C that appeared in FFQ #52. The span of the Korda C stab is over 18-inches and the height of the fin is roughly half that span. So the weight of the tail feathers I built for my Korda C is not bad considering their size.



Title: Re: Korda Class C Tractor
Post by: calgoddard on July 29, 2018, 12:07:34 PM
It’s time to talk about the de-thermalizer (DT) for my Korda C.  Much of what follows is written for the beginner.
  
If your outdoor free flight model catches a column of rising air (thermal) it may fly ought-of sight (OOS) and be lost forever.

A DT is a system that triggers movement of one of the flying surfaces to impair the overall lift of the model and cause it to come down to the ground.  The most common form of DT, allegedly discovered by model airplane legend Carl Goldberg, pivots the trailing edge (TE) of the stab upwards. The rapid pivoting motion of the stab ends when the plane of the stab extends at an angle of between about 45-60 degrees relative the centerline of the fuselage.  The model comes down gently and slowly in a horizontal orientation.  Rarely, the thermal is too powerful and the tilted stab does not bring the model back to earth. Therefore some fliers resort to a pop-up or pop-off wing DT.
 
Some type of timer is needed in a DT.  The various timers used in outdoor free flight all have advantages and disadvantages.  Let me repeat that I am trying to keep the weight of my Korda C (excluding the rubber motor) under 80 grams.  Adding an extra 3-5 grams here and there is really undesirable. With that in mind, I will review the various timers and explain my selection of a particular timer for the DT of my Korda C.

Fuses that burn have been used for many decades in our hobby. They are inexpensive, fairly accurate and generally reliable. However, they are not legal for models flown in California.  See the news coverage of the many brush fires and forest fires in California.

Electronic timers are relatively expensive ($40), weigh 3 - 4 grams with their LiPo battery, but are highly accurate.  I don’t think an electronic timer looks right on an Old Time Rubber (OTR) model designed and published in the late 1930’s.
 
Scroll timers use a clock-work mechanism to rotate a threaded cylinder about its axis until an arm engaged with the threads is driven off the cylinder and activates a flying surface to move to a new position.  These timers can be fairly expensive ($50) and the smallest one I could find weighs about 4 - 5 grams as I recall.

TOMY timers ($2) use the small wind up coil spring and gear mechanism that drives those little motion toy figures from Japan.  I have a nice TOMY timer that has been re-configured for use in a DT, which I pulled off my first P-30 model, but it weighs 4 grams.

I chose to use a viscous damper ($20) from Volare Products.  It only weighs about 1 gram.  The official name of this product is the Munson BADGE Timer - Classic. A small lever arm is connected to rotating mechanisms inside a sealed chamber that includes a viscous fluid.  When a small amount of spring tension is applied to the arm it rotates very slowly.  A loop of Spider line or other line is placed over the arm and the line is pulled by one end of a small coil spring until the loop slips off the arm.  Another line connected between the other end of the coil spring and the TE of the stab is released, allowing the stab to pivot due to the force of a pair of small rubber bands. Viscous timers are the lightest timer that can be used in a DT.  Their main drawback is that they are notoriously inaccurate.  The rate at which the arm rotates in a DT varies significantly with temperature.  The DT can trigger too soon, destroying what would have been a max flight, e.g. two minutes, in a contest.  It can also trigger too late, giving you a very long chase.  I have considerable experience with DTs that use a viscous timer and I can usually get them to trigger sometime between 120 and 150 seconds.

I will explain the DT on my Korda C in more detail, after it has been installed, using pictures. When I started in this hobby the manner of setting up a pop-up stab DT was a mystery to me.  It seemed at the time as if a lot of fliers assumed that everyone knew how to do it.  I don't recall finding a concise, clear explanation on the Internet at the time.


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 29, 2018, 05:17:59 PM
I built a small balsa wood box for holding one of my Walston RF transmitters.  It weighs around 3 grams with two hearing aid batteries installed.  I fly all my larger model airplanes with one of these RF transmitters on board.  I would have permanently lost several good models over the past 5-6 years if I hadn’t taken this precaution.  It is worth the 3-gram weight penalty.  

As some of you may know, if a model carrying an RF transmitter flies OOS you walk the last line of sight carrying a Yagi antenna connected to a receiver. You point the antenna in various directions and the beep gets louder as you determine the location of the model and walk towards it.  Successfully using an RF locator system is an acquired skill.

The box is glued inside the fuselage beneath the wing.  Access to the box will be gained through an uncovered part of the fuselage beneath the center section of the wing. The RF transmitter slides into the balsa wood box and a friction fit holds it in place.  Nevertheless, when preparing for a flight, I will thread the antenna through a hole in a small 1/64-inch plywood end plate that will be secured over the open end of the box with a small rubber band.  I don’t want the RF transmitter to somehow fall into the 16 x 1/8-inch rubber motor during winding or during a flight. The RF transmitter’s foot-long antenna will thread into small segments of wire insulation glued to the fuselage behind the wing.
  
I added grip panels made of 3/32-inch balsa wood sheet on either side of the fuselage.  I don’t want to break a longeron or puncture the covering when I grip the fuselage and toss the model into the air. I added a DT panel made of 1/16-inch sheet balsa wood behind one of the grip panels. It has a large hole in it for receiving the viscous timer button. I thought it safer to have the DT timer located aft of the grip panels to lessen the likelihood of my hand upsetting the DT on launch. If the stab were to pop up under full power, the result would not be pretty.  The crash would cause severe damage to my Korda C. The 17-inch prop that I made is heavy so mounting the viscous timer button aft should not adversely affect the optimal CG range indicated on the plan.

I don't like the large holes in the strips of balsa wood that are apparently for receiving dowels that carry the wing hold down rubber bands. They are too large.  These dowels would be heavy and are overkill in terms of structure. I should have installed my own strips of wood with smaller holes. I will check this detail on the 1937 plan.


Title: Re: Korda Class C Tractor
Post by: OZPAF on July 29, 2018, 09:07:48 PM
Cal, it is probably tool late now but I was wondering why you didn't have the Walston bug coming out of the top of the box. This would avoid any chance of it falling into the rubber motor. The box would need to be located where you have your access panel under the wing. a small section of tape across the top of the box would stop it rising and perhaps damaging the underside of the wing.

This will be a sight to see fly.

John


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 30, 2018, 09:46:33 AM
John -  That is an excellent suggestion about the orientation of the box for the RF transmitter.  Why didn't I think of that?

There is no detail on the 1937 plan for the Korda C regarding the wing hold down, so I am free to not use the relatively heavy 1/8-inch dowels indicated on the Bob Holman plan.

This is a long and detailed build thread. Thank you for following along. I use standard techniques except for a few areas, such as tissue covering, as described below.

I covered the tail feathers of my Korda C with red Esaki tissue. I am very happy with the results.

Everyone seems to have their own preferred method of adhering and shrinking tissue on balsa wood frames. Never use domestic tissue. It can be heavy and has little strength.  Only use Esaki tissue.  I understand it is made in Japan from rice.  It has enormous strength for its weight.  Easki tissue has a pronounced grain. You can readily determine the orientation of the grain by tearing the tissue. It will tear easily along the grain but it is very difficult to tear Esaki tissue across the grain.  

Here is an excerpt from an HPA post by modelace on September 5, 2014:

"Esaki has a defined grain and has greater shrinkage in the direction perpendicular to that grain."

I follow the practice of orienting the tissue so that its grain runs spanwise on the wing and stab and runs lengthwise on the fuselage.  This seems to be an area of disagreement among free flight enthusiasts.

I used to pre-shrink the tissue for the stab three times to avoid subsequent warping. That was a pain in the neck. Plus, the finished stab sometimes ended up with saggy and/or wrinkled tissue after a fourth spritzing and drying.
I can’t use nitrate dope.  Clear Krylon spray smells even worse.

Here is my current preferred method for covering the wings, stab and fins of my models with Esaki tissue.

I will describe covering the stab. I adhere un-shrunk tissue, slick side down, to the frame.  I attach the tissue with a 50/50 mixture of Elmer’s white glue and water.  I first cover the bottom side of the frame. I only glue the tissue around the perimeter of the frame  Then I cover the top side of the frame with a nice overlap at the edges. I let the structure dry overnight.

Next, I apply a 2:1 mixture of water and Eze Dope to the tissue.  Eze Dope is a great water-based “dope” product from the UK. It is odorless. I use a nice wide, soft brush to apply this mixture to the tissue, being careful to spread away any areas that appear white in color.  I coat both sides of the stab at once and then clamp the stab in a raised fashion on my building board to allow for air circulation and to maintain flatness, i.e. prevent warping.  I use magnets on my MagnaBoard set-up to do this. I let the wet tissue dry naturally with no sunlight and no heat gun.  In a few hours the tissue is drum tight and has a nice finish.  According to information I have read about Eze Dope, or heard on the company's YouTube videos, tissue-covered structures finished with Eze Dope resist warping.  I have found this to be true.

Using white glue and water to adhere the tissue to balsa wood frames allows me to soak the fuselage, wing, stab and fin in water later on and remove the tissue after it has become tattered. They can be re-covered and will look as good as new.  I glue the balsa wood frames together with CA so that when I soak them in water, they stay together.
 
Warps are your enemy in free flight, especially in the stab.  Warps in the wing and fin can also cause no end of problems.  I try to inspect a model at home for warps but sometimes they magically appear at the flying field.  I have seen fliers rubber band their stab to a flat board or a piece of Styrofoam in the hopes of preventing warps.  I have never tried this.  Some very experienced fliers just laugh when they see a stab strapped to a board.  In their view it is a complete waste of time.

I plan to work on the fuselage covering some more today.


Title: Re: Korda Class C Tractor
Post by: applehoney on July 30, 2018, 03:30:37 PM
>    a defined grain and has greater shrinkage in the direction perpendicular to that grain

Absolutely so.   Covering with the grain chordwise does give less tissue sag between ribs and thus a better general airfoil but .....  the tissue is then very prone to stress splits along the grain between ribs and/or spars.   Tried it ... discarded it.

Looking at your plan illustrated I'd wonder if to move the rear peg forward a bay .. or  maybe two ..  to get more rubber clearance.  Looks kind of narrow as drawn but perhaps due to the angle depicted.

Nice looking stab!  I do keep some lightly built stab  structures strapped to foam blanks when in storage or inbox. I do not consider anything that deters warps to be a waste of time.


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 31, 2018, 10:32:29 AM
applehoney - I think you are seeing an optical illusion due to the angle of the photo, just as you said.

I inserted my largest blast tube into the fuselage of my Korda C and was able to couple its bayonet lock to the 3/16-inch Aluminum motor peg with no difficulty.

I finished the Garami free wheeler clutch on my prop assembly.  I used epoxy and Spiderwire fishing line to fix the U-latch in position on the hub. It is not pretty, but it is functional.

There is nothing unique about the construction and operation of my Garami clutch.  For those unfamiliar with this design, the important detail that is not visible in the attached picture is that the hub is bushed with a segment of 1/8-inch Brass tube.  A segment of 3/32-inch Brass tube rotates freely and can slide back and forth within the segment of 1/18-inch Brass tube.  The inner segment of 3/32-inch Brass tube is about 1/16-inch longer than the outer segment of 1/8-inch Brass tube.  When the motor run is over, the U-latch swings away from the right angle drive dog on the forward end of the 1/16-inch music wire prop shaft.  The prop slides rearward a tiny amount. The forward end of the segment of 3/32-inch Brass tube prevents the innermost curved portion of the drive dog from squeezing down on the prop hub and impairing free wheeling. 



Title: Re: Korda Class C Tractor
Post by: calgoddard on July 31, 2018, 01:26:52 PM
 Instead of "1/18-inch Brass tube" I should have said - - 1/8-inch Brass tube - - in the last paragraph of Reply #54.


Title: Re: Korda Class C Tractor
Post by: calgoddard on July 31, 2018, 07:22:41 PM
I loaded up my covered Korda C fuselage today with a lubed 16 x 1/8-inch rubber motor - 38 grams.  I did some test runs with the motor wound and the prop installed.  My Korda C fuselage was mounted on my stooge which was clamped to the back end of my Ford Expedition. I used an Aluminum blast tube.

I first wound to 150 turns, hooked up the prop, and let it unwind.  No problems.  There was a slight oncoming breeze and the prop, with a Garami clutch, did a nice free wheel at the end of the motor run.

I then wound to 250 turns. There was no problem with the subsequent motor run.

Thereafter, I wound to 400 turns and experienced no problems with the motor run.  It was a 50 second motor run!

I decided that was enough for now. Eventually I hope to get near to 1000-1100 winds, and a launch torque of 30+ inch-ounces.

I am going to increase the motor weight to 40 grams to make the braided motor a little longer. I want less tension when I am installing it.  I will stick with 16 x 1/8-inch for the time being. I may have to go to 18 x 1/8-inch if the climb is not aggressive enough.



Title: Re: Korda Class C Tractor
Post by: OZPAF on July 31, 2018, 07:56:39 PM
Looking forward to your trim flight reports Cal. I hope you get a chance soon.

John



Title: Re: Korda Class C Tractor
Post by: Hepcat on July 31, 2018, 08:47:04 PM
Cal,
I don't understant it. You are usually so methodical and yet you seen to have missed out a build step.  You appear to have covered the fuselage of the Korda
with tissue before you have the 'Mylar' in place!!!
John.






Title: Re: Korda Class C Tractor
Post by: calgoddard on August 01, 2018, 10:45:31 AM
Hepcat -

As usual, you are very observant.

The fuselage of my Korda C is covered with Polyspan "tissue."  It is light in weight, puncture resistant, and gives very good torsional strength.  I understand that is it legal for Flying Aces Club competition and AMA competition. I am not sure about SAM rules.

The label for the Polyspan product even says "tissue." It is slightly heavier than Esaki tissue, but probably significantly lighter than a combination of Easki tissue over Mylar plastic film.

Like most techniques in our hobby, covering with Polyspan tissue is an acquired skill.  It does not like to go around curves.  Shrinking Polyspan tissue with a heat gun is a delicate process. Like a good steak, you don't want to overcook it.  

I spray painted the Polyspan tissue covering on the fuselage of my Korda C with black Design Master floral spray paint, from a "rattle can." This brand of spray paint is far lighter than conventional spray paint dispensed from a can.  I ran out of red floral spray paint (used it on the prop).  I also had some orange, blue and grey floral spray paint cans on hand but those colors did not seem suitable.

The tail feathers of my Korda C are covered with with Esaki tissue (only) and I will cover the wing with only Esaki tissue in order to save a little weight and because these structures take less of a beating than the fuselage.

Where I fly outdoors (California and Arizona) it is not common to have issues with moisture and/or high humidity, so covering with Mylar is not really necessary, and would add unwanted weight to this model.    

OZPAF - Well our flying field was 112 degrees F. the other day.  Our club does not fly in July and August due to high heat.  My first real trim flights of my Korda C will have to wait until September.  I may dare a few very low power trim flights (150 turns max) in my local six acre park.  You can see in this video of an early, very low turns trim flight of my Air Shark P-30 that I have to severely limit the turns and torque and use an early DT to avoid my models being carried over adjacent houses by the sea breeze that starts coming up early in the morning:

https://www.youtube.com/watch?v=eLWX1WOUzb8

Flying in my local park is risky for another reason.  There are often dogs off leash even though there are signs posted that warn that all dogs must be on a leash.  Some dogs are "interested" in my models when they land in this park.  


Title: Re: Korda Class C Tractor
Post by: billdennis747 on August 01, 2018, 01:03:26 PM
The label for the Polyspan product even says "tissue." It is slightly heavier than Esaki tissue, but probably significantly lighter than a combination of Easki tissue over Mylar plastic film.
'They' say tissue and mylar is no heavier than tissue alone because it needs less dope.
The most important thing with UK polyester tissue is 'shiny side out', or you get a finish like a badly-shaved pig.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 01, 2018, 01:33:40 PM
Yes the instructions I reviewed said to apply the Polyspan tissue shiny side out.  I did this. It's a bit challenging to discern shiny versus not shiny.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 01, 2018, 05:00:03 PM
I took a break from building.

I wanted to see how many turns my 16 x 1/8-inch, 38-gram Tan Super Sport (TSS) rubber motor would take. The rubber I used to make this rubber motor is May 2015 TSS. That’s the date on the shipping label.  Maybe there wasn’t a TSS batch with that date, but unfortunately the box does not have a date stamped on the same like most TSS rubber boxes.  I have not done any scientific stretch tests on this rubber, but it passed my manual pull test. A friend gave me a big box of this rubber and I plan to use it up on my outdoor models that take big rubber motors.
 
I have a winding stooge permanently set up in my garage. I hooked the eight loops at the rear end of this rubber motor over its hook. About one or two years ago I retrofitted my heavy-duty Merrill winder with a wonderful coil spring torque meter that I purchased from Volare Products. However, the latter torque meter is not calibrated in inch-ounces of torque and I wanted to read actual torque in those units as the winding progressed.  Therefore, I ganged my Wilder’s outdoor torque meter to my Merrill winder.  I hooked the S-hook on the rear side of the Wilder’s winder through the forward hole of the Crocket hook at the front end of the rubber motor.  The Merrill winder has a nice spring-loaded safety latch that I hooked through the ring on the front side of the Wilder’s torque meter.
 
I stretched the well-lubed rubber motor at least six times its unstretched length before commencing winding. I like to stretch eight times before starting winding but I did not do a precise measurement of the step out point before commencing winding. I put in 500 turns and then began walking slowly in as I continued to wind. I was monitoring the torque as the winds increased.
  
The ratio of the Merrill winder is slightly more than 3.5 to 1. After several minutes of cranking and reading the counter on the Merrill winder and the Wilder’s torque meter, I got to 950 turns and 30 inch-ounces of torque.  I saw the torque jump dramatically as I neared 950 turns so I knew the rubber motor was about to break.  I could have wound to breakage but did not want all the drama. I estimate that a slightly heavier motor (40 grams) with the same configuration made from the same batch of rubber should be able to take 1,000 turns without breaking.  While the 38-gram rubber motor produced a motor run of 50 seconds for 400 turns I can’t just extrapolate that time in linear fashion.  My fuselage was stationary and the prop will turn much faster at much higher torque if and when I might launch my Korda C with 1,000 turns.
 
As I was winding I was wearing safety googles and wearing leather gloves. After I unwound the rubber motor I inspected the same. It had no broken strands.  I was a bit surprised at this.
 
Of course, I will do many low power flights of my Korda C and gradually increase the launch torque during the trimming process. I am not sure I will ever be able to launch my Korda C at 30 inch-ounces of torque.
 
Stan Buddenbohm told me he launched his Korda C at 40 inch-ounces of torque.  But Stan is a world class rubber powered flier, not just a multiple world record holder in free flight gliders. He said his Korda C could do 5 minutes in calm neutral air, i.e. no thermals. Stan said his Korda C would climb “. . . nearly straight up for a time to tremendous altitude.”  Herb Kothe, Stan Buddenbohm, and Karl Gies, said it’s the big 17-inch prop that makes this model such a good flier.

I need to start covering the wing.  I am thinking of using the technique of first wrapping narrow strips of Esaki tissue in zig-zag fashion about the wing as described in Reply # 55 of the Casano Stick thread. Someone please tell me if this is not a good idea.


Title: Re: Korda Class C Tractor
Post by: flydean1 on August 01, 2018, 10:36:39 PM
The diagonal tissue wrap is a good idea to stiffen torsionally without adding much weight.  An earlier post mentioned that tissue shrunk more across the grain than parallel and that post suggested running the grain in that manner so it would shrink the most.  Please, we are doing this to provide torsional stiffness and tissue is MUCH stronger parallel to the grain than across it.

Dohrman Crawford originally showed me his technique.  Haven't done it on a rubber model, but it really stiffened up my 1/2A Satellite.  Added around 1/4 oz.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 03, 2018, 02:10:47 PM
I covered the different types of de-thermalizers (DTs) and timers used in DTs in Reply # 49.

I chose to use a viscous damper ($20) from Volare Products as the timer in the DT that I installed on my Korda C.  A small lever arm is connected to rotating mechanisms inside a sealed chamber that includes a viscous fluid.  When a small amount of orce is applied to the arm of the viscous damper it rotates very slowly.  A loop of Spiderwire or other line is placed over the arm and the line is pulled by one end of a small coil spring until the loop slips off the arm. Another line connected between the other end of the coil spring and the trailing edge (TE) of the stab (or the fin) is released, allowing the stab to pivot due to the force of a pair of small rubber bands.
 
Here is a link to a video showing the configuration of the DT on my Korda C and demonstrating its operation:

https://www.youtube.com/watch?v=1y0X6Hvgkb0

The leading edge (LE) of the stab butts up against a shoulder in the form of a segment of approximately 1/8-inch square balsa wood glued to the top of the rear end of the fuselage.  The stab has a pair of hooks bent from .025-inch music wire and anchored on each side of the fin with CA about 1/5 of the chord aft of the LE.  Small rubber bands are stretched between these hooks and a pair of wooden “hooks” (wedges) glued to the insides of uprights in the tail end of the fuselage.  Normally I would have a small 3/32 dowel extend through the tail end of the fuselage so that the lower ends of these rubber bands slip over the ends of the same.  However, I mounted the hooks too close together on the stab for this arrangement to work in optimal fashion.  I did not want to re-build the stab.  It is my fault for not thinking more about proper hook location in advance. The rubber bands stretch over the 1/8-inch square shoulder and their force pivots the TE of the stab upwardly.
 
A short length of monofilament fishing line passes through a segment of black wire insulation glued to the rear end of the fuselage.  A tiny O-ring tied to the end of the monofilament fishing line slips over a small hook extending from the lower rear corner of the fin.  I like to use monofilament fishing line to minimize friction as the line slides through the curved wire insulation. In many DT systems the hook extends from the center of the TE of the stab but the stab of my Korda C has a V-shaped cut-out region in this area.
 
The forward end of the plastic fishing line is secured to the rear end of a first segment of Spiderwire fishing line.  This fishing line is made of some sort of polyester and is easier to handle.  The front end of this first segment of Spiderwire fishing line is connected to the rear end of a coil spring made of .009 music wire. You can make these or buy them from Stan Buddenbohm.  Some fliers use a rubber band or a length of elastic thread instead of a coil spring. The rear end of a second segment of Spiderwire is connected to the forward end of the coil spring.  The forward end of the second segment of Spiderwire has a loop that slips over the arm of the viscous damper.

A short segment of 1/16-inch Aluminum tube is crimped over the monofilament line at a location where the stab will be at an angle of about forty-five degrees when the crimped tube segment is stopped by the forward end of the wire insulation.

I wrap the first segment of Spiderwire fishing line around the motor peg which functions as a capstan.  This prevents stab creep, i.e. gradual lifting of the TE of the stab as the spring relaxes near the end of the predetermined DT trigger period, which would induce a stall.

The length of the various parts of the DT line are critical to the proper functioning of the DT.  When I first tried to install a DT, I had a great deal of difficulty tying knots in fishing line at the appropriate locations.  I finally stumbled on the technique of feeding the various lines through short segments of 1/16-inch or 3/32-inch Aluminum tubing, adjusting a particular length as desired and then crimping down on the tubing with pliers. The attached picture shows the formation of a loop in this fashion. Note that you would feed the fishing line through a metal wire loop at one end of the coil spring before crimping if making a connection to the spring.  In other words, the loop of the spring would be captured in the loop of fishing line shown in the attached picture.

I still need to install some keys on the bottom of the stab to ensure that the fin is always in the same angular position when my Korda C is launched.  Even small variations in the angle of the fin from flight-to-flight can have a significant impact during the power phase of the flight.  




Title: Re: Korda Class C Tractor
Post by: FF Bruce on August 03, 2018, 04:07:20 PM
Thank you for sharing this build with us, just thought I'd help a little, in the second picture showing your stab stop. You might want to make groves in the stop ,where the rubber bands goes, so that the rubber pulls down on the front edge of the stab. As it is now if the stab rocks the stab has nothing to pull it back down to the stab rest. This can and has on many an airplane caused a crash. I pointed this out on a friends model he said "no I check it wouldn't happen" the next flight it happened.


Title: Re: Korda Class C Tractor
Post by: Hepcat on August 03, 2018, 05:03:14 PM
Cal,
My post #58 was intended to be humerous; from your reply in #59 I am not sure you realized that. I know we limeys have a weird 'sensayuma' that is not always understood by others. I am sorry to be so long in sending an apology
John
 


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 04, 2018, 10:30:57 AM
Hepcat -

No problem.

Your input is always valued.

I had heard of the technique of first covering the balsa wood frames with Mylar film and then covering the Mylar film with tissue.

It could well be that the results of such dual covering are superior to covering solely with Polyspan tissue.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 04, 2018, 07:09:02 PM
FF Bruce -

Thanks for catching this very important detail.  I will make grooves in the stop as you suggested for the reasons you stated.

Wow, I better check my Gollywock and Jabberwock for this same defect.

As I have said previously, one of the benefits of posting a build like this is to receive exactly the kind of important input you gave me.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 05, 2018, 09:05:30 AM
As suggested by FF Bruce in Reply #65, I cut a pair of spaced apart grooves into the upper side of the 1/8-inch square balsa wood stick that serves as a stop for the LE of the stab.  I did this with a Dremel tool equipped with a cutting disc.  I put a little CA in each groove to reinforce the stop where it was weakened by removing some of its wood.
 
The rubber bands that tilt the stab pass through these grooves.  This modification ensures that they engage and pull down on the LE of the stab to prevent it from lifting up during a flight and undesirably altering the decalage, or worse, causing a crash.

I decided to double the number of small blue rubber bands that tilt the stab.  Now there are two on each side of the fin.  I was not happy with the relatively slow speed at which the stab was tilting upwardly under the force of only two stretched rubber bands.  When the model is gliding, insufficient titling force on the stab when the DT has triggered can result in inadequate tilting, or worse, no titling.  This can lead to loss of the model because effectively the DT did not work as intended in terms of impairing the lift of the model.  

Many experts have told me to err on the side of plenty of force pulling the stab upwardly.  Oncoming air during the glide, especially in breezy conditions, can keep the stab down or cause it to flap up and down in slow fashion if the rubber band lifting force is inadequate.  


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 06, 2018, 12:56:09 PM
I applied strips of red Esaki tissue to the frames of the wing of my Korda C to impart additional torsional stiffness.  The wing is separable into two halves for ease of transport.

The strips of Esaki tissue are approximately ½-inch wide.  For reference purposes, a segment 8 inches long only weighs .02 grams.  The grain of the tissue runs along the length of the strips as recommended on HPA.

I used UHU glue stick to attach the strips to the LE and TE.  I don’t want the 50/50 mixture of Elmer’s white glue and water that I will subsequently use to adhere the finish covering also made of Esaki tissue to loosen the attachment of the strips.

I ran the strips at a 45-degree angle relative to the LE and TE per a recommendation I read on HPA. Even if the strips don’t add much stiffness, they will hardly add any weight.

I am going to add another series of strips so that the two sets cross over each other, in other words, each half of the wing will have four "X's" made of strips of red Esaki tissue.



Title: Re: Korda Class C Tractor
Post by: calgoddard on August 06, 2018, 04:17:34 PM
I added the second strips of red Esaki tissue as described in Reply #70 immediately above.  The strips crisscross on opposite sides of the wing.

I think I should shrink the strips before I put on the outer layer of Esaki tissue.

My current plan is to cover the wing with yellow Esaki tissue so that the red tissue strips will be visible through the outer covering.


Title: Re: Korda Class C Tractor
Post by: TRuss on August 08, 2018, 01:31:02 PM
Thank you for sharing this build with us, just thought I'd help a little, in the second picture showing your stab stop. You might want to make groves in the stop ,where the rubber bands goes, so that the rubber pulls down on the front edge of the stab. As it is now if the stab rocks the stab has nothing to pull it back down to the stab rest. This can and has on many an airplane caused a crash. I pointed this out on a friends model he said "no I check it wouldn't happen" the next flight it happened.

Ha ha!  It definitely happened.  You will see grooves next weekend Bruce.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 08, 2018, 07:20:50 PM
I covered one half of the wing with yellow Esaki tissue, with the grain of the tissue running parallel to the span of the wing.  I did not pre-shrink the yellow tissue.  The tissue was adhered to the balsa wood frame with a 1:1 mixture of water and Elmer's white glue using a small paint brush.  I adhered the tissue shiny side down.  I like how the non-shiny side looks after doping. I let the wing completely dry for a few hours.

Next, I applied a 2:1 mixture of water and Eze Dope to the yellow tissue on both sides of the wing section using a wide soft brush.  Both sides were covered with the water and Eze Dope mixture during the same 5-minute brushing operation.

We are having hot weather in SoCal and standing outside in the strong sunlight and a 5 mph breeze caused the tissue to shrink drum tight in about 15 minutes. During the drying of the tissue I monitored the wash-out and did a little twisting. It came out perfectly.

I can't say enough good things about the Eze Dope product.  No smell, no muss, no fuss.  The brush cleans up fast with water.

The finished wing section feels very stiff, but I can't say what part the diagonal red Esaki tissue played in the result. I do like how the red diagonal stripes look on the finished wing section.

The yellow tissue shrank tight over the three 1/16-inch square spars on the top of the wing section, forming slight ridges. These ridges will function as turbulators and should increase the lift of the wing compared to its lift without the turbulators.  I am not sure Richard Korda intended this.  


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 09, 2018, 09:26:10 AM
I have decided to make a new prop and nose block, aka “front end assembly” for my Korda C. The front end assembly I already made weighs a staggering 25 grams!
  
The first nose block and thrust bearing I made weighs around 5 grams as I recall. I made a replacement nose block already that weighs 2.49 grams.  I used lighter balsa wood for the laminations.  I also adopted applehoney’s suggestion for the thrust bearing.  It comprises a segment of 1/8-inch Aluminum tubing with very short segments of 3/32-inch Brass tubing secured in each end.  I did not back the entire nose block with 1/64-inch plywood this time, only the perimeter. I built the rectangular “frame” that inserts into the front end of the fuselage out of slightly thinner ¼-inch wide hard balsa wood.

The weight of the first 17-inch balsa wood prop I made for my Korda C was fine after I carved and sanded it - only 10 grams. However, the Bass wood LE and TE lining, the fiberglass and epoxy coating, and tube-within-a-tube Brass bushing took that up to around 17 grams. Add the weight of the prop shaft, Garami clutch and thrust washers and you get a heavy front end assembly.

I can only order Superior Props prop blanks in medium density balsa wood, not hard balsa wood or pine wood. Harder wood would allow me to make the blades much thinner.  I don’t have a band saw and do not want to attempt to cut my own block of wood by hand in order to generate a prop blank out of heavy balsa wood. I am going to reduce the axial length of the prop hub.

I will try to sand the blades thinner on my second Korda C prop. This time I will only cover the prop with a layer of CA, spread around with a sandwich bag. This coating was recommended to me by Stan Buddenbohm.  There is not much I can do to reduce the weight of the 1/16-inch diameter prop shaft.  However, I am investigating a simpler and lighter alternative to the Garami clutch that Stan suggested to me.   My goal is a new front end assembly weight of 17 grams.  However, I will be happy if the new front end assembly does not weigh more than 20 grams.
  
If the lighter prop should break at a contest, I can glue it together.  If this does not make the prop useable, I still have the “tank” front end assembly that I initially built.  I will have to move the wing forward if I use the tank because its extra weight will shift the CG forward.  Both props will have the same diameter (17-inches) and P/D (1.3) so my Korda C should fly with either front end assembly.

Well this build is taking forever. However, it is too hot in August to fly where my club flies anyway.  Thanks for following along, and thanks for your input.


Title: Re: Korda Class C Tractor
Post by: OZPAF on August 09, 2018, 06:57:02 PM
How heavy is your Korda now Cal?

John


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 10, 2018, 04:19:19 PM
OZPAF -

The total weight of my Korda C as shown in the attached photo is 81.26 grams.  This of course does not include the weight of the 40 gram rubber motor.  I still need to add two wing hold down rubber bands and some dowels for retaining them. This will add another 2-3 grams.

Here is the breakdown of the weights of the main components:

tail feathers               9.98 grams

fuselage                   22.57 grams

wing                        23.11 grams

nose block &
prop                        25.57 grams

TOTAL                     81.26 grams




Title: Re: Korda Class C Tractor
Post by: vintagemike on August 10, 2018, 06:16:38 PM
What a nice model! reminds me of its older, bigger cousin which has recently been published on Outerzone


Title: Re: Korda Class C Tractor
Post by: OZPAF on August 11, 2018, 01:28:51 AM
Thanks Cal. Checking back at your quoted wing area of 143 ins2 would indicate that even with it's heavy prop the wing loading is still good at around 0.6gms/sq inch.
 
However a lighter prop would certainly help IMHO by reducing the inertia effect of the heavy nose - making it easier to trim.

John


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 11, 2018, 10:28:49 AM
OZPAF -

Thanks for your helpful comments about wing loading.

In his book entitled Rubber Powered Model Airplanes, Don Ross says that a wing loading of 0.5 grams per square inch for a mid-size model with a wingspan of 24-30-inches is "OK".  My Korda C has a wing span that is significantly bigger than a mid-size model. It appears, however, that Ross includes the weight of the rubber motor in determining wing loading. This makes sense.  Unfortunately, if I include the weight of the 40 gram rubber motor the wing loading of my Korda C is way above 0.5 grams per square inch.

In addition to the component weights listed in Reply #76 my Korda C will carry an RF transmitter and batteries weighing a combined 3 grams.

For reference purposes I note that the Korda C built by Karl Gies weighed 71.7 grams.


Title: Re: Korda Class C Tractor
Post by: Red Buzzard on August 11, 2018, 05:07:16 PM
Hi Cal,

Let's see, at 40 grams of rubber and 81 grams of airframe, you'll still be tough to beat. Then that 17" prop and long motor base gives you a long prop run and you'll be even tougher. And a few grams will mean your Korda will take a few tumbles from the wind, lousy DTs and regrettable cross wind launches and will still be flying when it's covered with glue patches. You'll go crazy obsessing about weight when it will be more fun obsessing about reading air. As Dan Berry has said, "let's not overthink this..." Go fly your pants off!

RB


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 14, 2018, 10:15:36 AM
Red Buzzard - Thanks for the encouragement.

I just received an email notification that my new Superior Props 17-inch prop blank which I purchased from Volare Products should arrive in two days. Then the carving and sanding starts anew.

I have been playing around with a mock-up of a prop I made with a new free-wheeler clutch based on a clever design by Stan Buddenbohm.  I am sorting out the optimum dimensions. As shown in the attached photo, I used a 17-inch x 5/16-inch x 5/16-inch stick of balsa wood as a prop spar whose middle section roughly simulates the width of the hub of the second Korda C prop I will make.  The paddles glued at an angle on the ends of the spar help it free wheel in the oncoming breeze to allow me to observe the operation of the clutch.  I want to have all the details of the new clutch worked out so I don't mess up the installation on a newly carved and sanded prop. Once I do this, I will post a complete write-up of the details.  So far the mock-up performs beautifully, but I have a few tweaks I want to make here and there to ensure that the new clutch on my new prop is fail-safe.  I will mount a second mock-up on my Korda C fuselage and do a low-wind test motor run with the fuselage mounted on my stooge when it is clamped to the tail gate of my SUV that is parked in front of my house.  The on-coming breeze will simulate the air flow during a glide.

Today I plan to work on adding locators to the underside of the stab to ensure that the fin is always in the same exact angular position at launch.


Title: Re: Korda Class C Tractor
Post by: Starduster on August 14, 2018, 12:45:05 PM
Hi Cal,

Let's see, at 40 grams of rubber and 81 grams of airframe, you'll still be tough to beat. Then that 17" prop and long motor base gives you a long prop run and you'll be even tougher. And a few grams will mean your Korda will take a few tumbles from the wind, lousy DTs and regrettable cross wind launches and will still be flying when it's covered with glue patches. You'll go crazy obsessing about weight when it will be more fun obsessing about reading air. As Dan Berry has said, "let's not overthink this..." Go fly your pants off!

RB

This is, by far, probably the best bit of wisdom ever imparted on this forum!

Bravo!


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 14, 2018, 02:49:27 PM
I can't do any outdoor flying until September. It is way too hot at our flying field right now.

So I may as well mess around making a new prop!  Or, instead, I could clean out and organize my garage :)


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 18, 2018, 10:29:38 AM
As an alternative to a Garami clutch, I made a simple clutch for the new lighter 17-inch balsa wood prop that I just made for my Korda C.  The weight of my new front-end assembly for my Korda C (including prop, nose block, clutch, washers and prop shaft) is only 14.71 grams (see first picture). This is substantially less than the 25.57-gram weight of the original front-end assembly that I made for this model.  I am pleased that I have reduced the weight of my Korda C by more than 12% (not counting the weight of the 40-gram rubber motor).  See Reply #76 for a listing of the main component weights.

The clutch incorporated into my new front-end assembly embodies a Stan Buddenbohm design which he created and used successfully for many years with no problems.  Stan always comes up with clever designs for his free flight models. Like all free wheeler clutches, this new clutch allows the prop to free-wheel after the motor run is complete, i.e. nearly all of the turns on the rubber motor have been used and the model is entering its glide phase. Free-wheeling of a fixed, i.e. non-folding, prop is essential to a good glide.  A fixed prop that won’t free-wheel acts as a dethermalizer (DT) and kills the glide, or worse, can cause the model to crash.

As is the case with many relatively simple mechanical devices, it takes a lot of words to accurately describe the construction and operation of the Buddenbohm clutch.  It’s really not that complicated. Hopefully the pictures that are part of this post will help you to better understand the following detailed written description.

The hub of the new Korda C prop has an axial length of ¾-inch.  It is bushed with three segments of tubing (see second picture). An intermediate segment is 1/2-inch long, 1/8-inch OD Aluminum tubing while the segments on each end are 1/8-inch long, 1/8-inch OD Brass tubing. The three segments of tubing are held in place with thin CA that weeps all around the outer surface of the tubing segments and bonds them securely to the balsa wood hub. A ¾-inch long segment of 1/8-inch OD Brass tubing is too heavy.  A ¾-inch long segment of 1/8-inch OD Aluminum might experience excessive wear and produce subsequent prop wobble.
  
An inner segment of 3/32-inch OD Brass tubing slides freely back and forth axially inside the three outer segments of 1/8-inch OD tubing.  The OD of the 3/32-inch tubing is slightly less than the ID of the 1/8-inch OD tubing segments so the inner segment can rotate freely within the outer segments, without any wobble. The 1/8-inch long segments of 1/8-inch OD Brass tubing form journals or bearings. The inner segment of 3/32-inch tubing is approximately 1/16-inch longer than the combined length of the outer segments of 1/8-inch tubing.   The intermediate segment of 1/8-inch OD Aluminum tubing is needed to ensure that the forward end of the 3/32-inch Brass tubing does not hang up on the rear end of the forward 1/8-inch long segment of 1/8-inch OD Brass tubing.  A pair of Brass washers and a Teflon washer sandwiched between the same are positioned on the 1/16-inch prop shaft between the hub of the prop and the thrust bearing in the nose block.  The ID of the 3/32-inch Brass tubing is slightly larger than the .062-inch OD of the 1/16-inch prop shaft. Again, there is no wobble when the prop shaft rotates inside the segment of 3/32-inch Brass tubing.

A small, generally triangular-shaped catch made of .027-inch galvanized steel sheet metal is glued with thin CA in a groove sawed into the hub that is spaced 3/8-inch from the prop shaft. You can see the catch in the third picture.  I may have trimmed it down a bit after taking that picture. The plane of the catch extends perpendicular to the axis of one of the blades. The forward end of the prop shaft is bent at ninety degrees to form a drive dog. The outer end of the drive dog engages the rear straight edge of the catch when the clutch is in its drive mode (see fourth picture). A small flat is ground into the drive dog and the rear straight edge of the catch is shaped with a file to the optimum angle in order to increase their area of contact.  Small strips of 1/64-inch plywood are glued to opposite sides of the hub and straddle the leading and trailing edges of the catch. This strips are wrapped with thread and glued with CA to securely anchor the catch in the groove in the hub. A great deal of force will be exerted on the catch.

After the rubber motor is connected to the prop hook, the nose block is installed into the fuselage while pulling out on the prop to ensure full engagement between the drive dog and the rear edge of the catch.  The torque exerted by the wound rubber motor presses the drive dog tightly against the rear edge of the catch and prevents relative axial movement between the drive dog and the catch.  The rearward thrust produced by the prop during the motor run presses the prop forward on the prop shaft. Along with friction, the thrust helps maintain engagement between the drive dog and the catch during the motor run. Inadvertent decoupling of the drive dog and the catch would result in a very undesirable super high-speed unwind of the rubber motor.
  
Here is how the clutch works during a flight. At the end of the motor run, the torque of the rubber motor is insufficient to drive the prop.  The catch disengages from the drive dog as the prop begins to free-wheel, in part due to its rotational momentum. At the same time the prop slides rearward on the 3/32 Brass tubing segment due to the oncoming airflow.  On the first rotation of the prop after disengaging from the drive dog the sloped upper edge of the catch may engage the drive dog and facilitate rearward axial movement of the prop.  Thereafter the catch no longer engages the drive dog and the prop spins freely during the glide portion of the flight (see fifth picture).  The forward end of the inner segment of 3/32-inch Brass tubing engages the curved inner portion of the drive dog and prevents it from pressing down on the hub and inhibiting free-wheeling of the prop.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 19, 2018, 05:44:23 PM
As shown in the attached picture my Korda C now weighs 71.51 grams.  This includes the weight of the new front-end and a wing saddle.

I did another test motor run on my car-mounted stooge.  I wound a 16 x 1/8-inch - 40-gram rubber motor (mounted in the fuselage) to 400 turns, connected the new prop and let the motor run.  Everything appeared to be working just fine. I then wound to 700 turns. The motor run was nice and smooth. The Buddenbohm clutch worked both times.  I was pretty sure that it would.

The motor run with 700 turns was about 95 seconds.  

I think I can get 900 - 1,000 turns into this rubber motor.  I will have to creep up to that amount of turns during the trimming process.  I hope I can launch at 30 inch-ounces of torque or more.

It's beginning to cool off a bit in SoCal. I can't do any 85% of breaking turns trim flights at our club's flying field until mid-September. I won't try to wind and fly with that many turns until I first fly my Korda C with 50 turns, 100 turns, 200 turns, 250 turns, etc. while making incidence, rudder and/or thrust line adjustments along the way, as needed, one at a time of course. I will initially set the CG in the middle of the optimum range recommended on the plan from Bob Holman Plans. I don't anticipate having to make any CG adjustments during the trimming process, but I won't glue the wing saddle in place, and eliminate the rubber bands that wrap around the fuselage and temporarily secure the wing saddle to the fuselage, until I am satisfied with the location of the CG.

I may do a few test glides and 100 turn trim flights in my local 6-acre park.  


Title: Re: Korda Class C Tractor
Post by: LASTWOODSMAN on August 19, 2018, 06:30:32 PM
Good info Cal  -  your Korda C really looks like it means business   8)  - looking forward to your short test flights.

LASTWOODSMAN
Richard


Title: Re: Korda Class C Tractor
Post by: Hepcat on August 19, 2018, 09:16:37 PM
Cal,
Your Korda is up to your usual meticulous standard and I can't say anything more flattering than that.  I can't help being a bit mischievous though and asking what you are going to do with the eleven grams you have saved on the propeller assembly; move the wing back a bit to get the CG correct? <smile>.
John


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 20, 2018, 09:50:18 AM
Hepcat -

Thanks for your complement.  Did you notice that the wing hold-down rubber bands on my Korda C do not criss-cross each other?

I decided to make a dummy nose block for test gliding my Korda C in my local park.  Personally, I feel that gliding the model with the prop free wheeling gives a better indication of performance in a real flight.  But the new 17-inch balsa wood prop is very light and I wouldn't want to break it before I have the model trimmed for a decent glide.  The dummy nose block will weigh around 14.71 grams - the same as the light weight front-end.


Title: Re: Korda Class C Tractor
Post by: applehoney on August 20, 2018, 12:27:41 PM
" I feel that gliding the model with the prop free wheeling gives a better indication of performance in a real flight. "

Have to agree with that thought.   Regarding the chance of breaking the prop .... that exists whether landing from a hand glide or a flight;  but unlikely in any circumstance if you've rounded off the top of the noseblock plug.


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 20, 2018, 04:13:50 PM
applehoney -

I already rounded off the top of the nose block plug on both the front-end assemblies I built for my Korda C, per your sound advice given to me on HPA several years ago.  It has been my standard practice since then.



Title: Re: Korda Class C Tractor
Post by: LASTWOODSMAN on August 20, 2018, 05:12:59 PM
Hi Cal and Applehoney.   I have a newbie question.   Regarding  "the chance of breaking the prop .... that exists whether landing from a hand glide or a flight;  but unlikely in any circumstance if you've rounded off the top of the noseblock plug."    What part is rounded off, and is that to make the prop block "pop  out" on landing ?     I have my Speedster nose block Scotch - taped on so snug, that the nose block does NOT pop out on landing.   In fact, I ran out the rubber on the last short trimming test flight this am, and the plane ran out the rubber sitting up high, into the wind, and came down hard at a steep angle, and bent the prop shaft.  The prop block did not come off.   Should I be making it so that it pops off upon landing ?   The grass was not too short, and I have a slightly tight motor in the fuse.

LASTWOODSMAN
Richard


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 20, 2018, 07:10:47 PM
Richard -

The nose block needs to fit snug in the fuselage.  It should not move around.  You need to maintain accurate thrust lines from flight to flight.

You also don't want the nose block to fall out during a flight. If the rubber motor stays connected to the prop hook the dangling prop will act like a crude DT and bring down your model. If the rubber motor does not stay connected to the prop hook you will most likely lose your prop and nose block and not be able to find them.

If you have a balsa wood prop, the nose block should pop out on landing to reduce the likelihood of breaking the prop or otherwise damaging the prop shaft and/or fuselage.  Injection molded plastic props are pretty durable and so having the nose block pop out on landing is not as important when the model has a plastic prop.  However, if the model has a plastic prop, allowing the nose block to pop out on landing can save your model from getting a bent prop shaft.

The top edge of the nose plug should be rounded, with a sanding block, as shown in the attached photo. I mark the top edge with red ink to remind me to insert this edge up into the fuselage.  The rounded edge allows the nose block to roll forward, out of the model, when the prop blade strikes the ground, more or less in the six o'clock position.  The fit between the nose plug and the forward end of the fuselage should be firm, but not so firm that the nose block cannot roll out.

On non-scale models, the nose block is held on with rubber bands, and these yield to allow the nose block to roll out.

On scale models, tiny magnets help keep the nose block in place, along with a friction fit.

I hope I answered your questions.  Applehoney has a lot more expertise than me and hopefully he will correct and/or supplement my reply where he deems appropriate.



Title: Re: Korda Class C Tractor
Post by: applehoney on August 20, 2018, 07:34:16 PM
No correction needed.   However I do not use rubber bands to retain a noseblock - a spring tensioned shaft does that for me on any model, retaining a few residual turns.

However  ...  " having the nose block pop out on landing is not as important when the model has a plastic prop."       Agreed that a plastic may be more  durable but a 'pop-out' noseblock protects the shaft.

Cal -do you have any finish on the prop  ?  


Title: Re: Korda Class C Tractor
Post by: calgoddard on August 20, 2018, 07:45:15 PM
Applehoney -

You are right.  A spring tensioner will keep the nose block in and avoid the hassle of using rubber bands to hold it in place.  A spring tensioner will also help avoid rubber motor bunching and CG shifting.  For some reason, I have not perfected the art of building spring tensioners.  It comes down to the spring, and it can be a tricky business getting the right spring force.  I hate making those "safety pin" type springs, and my coil springs seem to produce either too much tension, or not enough tension.

Switching gears, the new 17-inch balsa wood prop that I made for my Korda C was finished by coating the LEs and TEs with medium viscosity CA.  I probably coated a band about 3/8-inches wide with CA.  After the CA dried, I sprayed on two coats of clear Krylon spray, over the entire surface of the prop.  I allowed the first coat to dry before applying the second coat.  I hate the smell of clear Krylon spray. It consists of some sort of polymer with lots of volatile chemicals. I had to spray the prop outside, wearing goggles and a respirator.  The prop is waterproof and will resist absorbing any rubber motor lube that might be on my fingers.  It is not as durable as my "red tank" fiber glass and epoxy covered prop, but the new prop weighs substantially less than the old one.


Title: Re: Korda Class C Tractor
Post by: flydean1 on August 20, 2018, 10:42:32 PM
Applehoney -

I have not perfected the art of building spring tensioners.  It comes down to the spring, and it can be a tricky business getting the right spring force.  I hate making those "safety pin" type springs, and my coil springs seem to produce either too much tension, or not enough tension.

Volare Products has a new version of the K Fags spring winder.  I have an original one, and both tension and compression springs are fairly easy to make.  Also, ACE, True Value, and other hardware stores in the USA have an assortment of springs that I have managed to make work.  The "safety" springs are even more fiddly than the coil springs.


Title: Re: Korda Class C Tractor
Post by: applehoney on August 21, 2018, 09:52:51 AM
A locksmith is a good source for small coil springs


Title: Re: Korda Class C Tractor
Post by: tgwhitley on September 10, 2018, 06:45:13 AM
Update on you C stick?

Tim


Title: Re: Korda Class C Tractor
Post by: calgoddard on September 15, 2018, 07:16:02 PM
I drove 90 miles today to our club's flying field in Perris, Califoria.

My goal was to trim my brand new Bob White coupe for our club's F1G contest tomorrow. It is a locked down coupe. I have not previously flown this model.  After two hours I gave up.  There is something wrong with the fit of the prop assembly in the front end of the fuselage. I can't adjust the thrust line as I need to.

So I switched to my Candy G coupe.  I have flown it for about five years.  It usually flies well but not at the club's last F1G contest. I made one or two thrust line adjustments, and got a nice climb. I got two maxes in a row and put it away.  In Perris, we have to stop flying as noon approaches because the wind builds.

I got out my Korda C and installed a 16 x 1/8-inch 40 gram motor.  

I started at 100 turns, then 150, then 200.  The plane seemed to be flying OK.  When I got to 350 turns (motor will take 900-1,000 turns) the plane spiraled down to the right from about 50 feet in altitude, hit the hard dirt and broke one of the blades off the light weight prop.  I glued it together with CA, added a 1/16 shim to remove some right thrust.  I wound to 300 turns and got a nice flight. I put the model away.  It was getting too hot and there were dust devils.      


Title: Re: Korda Class C Tractor
Post by: calgoddard on September 18, 2018, 07:11:31 PM
Looking back, I should have followed the advice of Red Buzzard in Reply #14. He recommended that I add some left rudder.  The huge fin of the Korda C has an airfoil cross section (convex on its left side). I think this airfoil cross section may have induced the right spiral.  Perhaps there was also too much right thrust before I shimmed the nose block with a piece of 1/16-inch balsa wood.  I will move the stab stop so that there is about 1/32-inch left skew of the entire tail feathers as recommended by Red Buzzard.  Live and learn as they say. It's certainly true in free flight! This mistake resulted in a broken light weight prop.


Title: Re: Korda Class C Tractor
Post by: calgoddard on November 21, 2018, 12:01:00 PM
The attached picture shows the repairs that I made to the broken lightweight prop of my Korda C Tractor.  Stan Buddenbohm recommended that I splice in some strong stuff across the break. I followed George Bredehoft’s method that he uses to reinforce the hub of a balsa wood prop. I cut two spaced apart radially extending slits across the break with an Xacto knife. The slits go all the way through the blade. I inserted two ¼ x 1 ¼ x 1/64 plywood strips snugly into the slits. They weighed a total of only .14 grams.  I pushed one of the side edges of each strip down flush with the surface of the blade.  I sanded off a slight amount of excess of the other side edges of the plywood strips that were protruding from the surface of the other side of the blade. I then wept thin Volare CA into the slits. It is not visible in the picture but when the blade broke it produced a scarf joint.  So there is no butt joint between the end of the broken blade and the other part of the prop that includes the hub and the other blade.  The plywood strips extend across this scarf joint and past its ends.  I put a few small strips of heavy clear adhesive tape on the backside of the tip of the non-broken blade to re-balance the prop.

Last Saturday I finally got a chance to fly my Korda C Tractor in a contest put on my local flying club.  I did some more trimming in our large flying field on Friday the day before the start of the contest.  Previously in my local park I had tried a low power flight adding 1/16-inch left rudder.  This corrected the previous nearly disastrous problem of the plane spiraling into the ground in a right turn about 15 - 20 seconds after launch at our large flying field.  The airplane now climbs right and glides right in about a 150 foot diameter circle. Apparently when the unusually tall vertical fin is positioned with its flat bottom surface in line with the center line of the fuselage, due to its airfoil shape the fin yields too much effective right rudder. I had been warned about this in a post in this topic above and should have heeded the warning.  Not doing so caused me a broken prop, which took about an hour to repair with the 1/64-inch plywood splines.

With a 16 x 1/8-inch 40-gram rubber motor I gradually increased launch torque at the large flying field in a series of practice flights. The repaired prop held up just fine. The Buddenbhom clutch described in Reply #84 works great. I ended up adding some more down thrust.  Eventually I was very satisfied with practice flight of nearly five minutes.  It was a good thing my Korda C was carrying an RF tracker on that nearly five minute flight.  It has a viscous timer for the DT and I know I set it.  The stab was popped up when I found my Korda C after its long flight.  I need more calibration information on the viscous timer as set up in the DT.  The climb of my Korda C is at about a 45-degree angle and the model gets really, really high.  I found that about 650 - 700 turns and 20 inch-ounces of torque easily produces a max. The 17-inch wide blade balsa wood prop appears to be rotating quite slowly but apparently develops a lot of thrust. It was difficult to judge the duration of the motor run on a flight launched with 600+ turns on the rubber motor because by the time the motor run ends my Korda C is so high that it is hard to discern the transition to freewheeling.  I estimate that the motor run is about 90 seconds long.

In the OTR event at the contest last Saturday my first two official flights were easy maxes.  On my third official flight my Korda C got really high, but unfortunately flew into a downer and was literally sucked down to the ground while continuing to glide forward in a nice attitude.  The time of my third official flight was only 97 seconds.  I lost the OTR event to a friend who flew three maxes with his Gollywock. That was a nice consolation.

I am going to fly my Korda C in the OTR event in another contest this Sunday.  I will first put up a practice flight very early in the morning to see if my Korda C can easily max.  If so, I will put in all three official flights early, in dead air, before any thermals (and downers) can develop.  I learned this strategy from Stan Buddenbohm.  He was the one that told me that the Korda C is a great flier when I was discussing a future build of a small OTR model. Once again, Stan was correct.  He used to launch his Korda C years ago at 40 inch-ounces of torque!  I may work my way up to 30 inch-ounces of launch torque.  Right now it does not seem necessary for me to increase launch torque unless I fly in a contest with 3 minute maxes instead of 2 minute maxes.  Of course, there is always the possibility of getting into a fly-off in a contest that has only 2 minute maxes, meaning that I will need be able to fly 2 ½, 3, 3 1/2 minutes, etc.  

During the contest several fliers commented that my Korda C is ugly.  I don't care how it looks.  I only care how long it flies :)


Title: Re: Korda Class C Tractor
Post by: OZPAF on November 22, 2018, 12:01:47 AM
How did they work that out Cal? :) Anyway as a friend once said to me - beware the flyer whose model has patches/repairs. He knows his model and it is no doubt competitive :)

Anyway - as long as you're having fun - anything else is irrelevant.

Thanks again for your detailed reports - I always find them of interest.

John


Title: Re: Korda Class C Tractor
Post by: Red Buzzard on November 25, 2018, 08:09:24 PM
Yay, Cal,

Way to go! Don't sell yourself short with that motor. There's quite a bit more available. As you work with the trim you can sometimes "trade" down thrust for right thrust as your torque goes up. That can improve your cruise while keeping the climb safe. More torque at launch balances the bit of right thrust at launch. Most of your prop run is in "cruise" and anything you can do to get the nose up even a little in cruise is good.

Best those Gollywocks!

Bill


Title: Re: Korda Class C Tractor
Post by: calgoddard on November 26, 2018, 11:12:32 AM
I flew my Korda C in my club's combined OTR-NOS Rubber contest yesterday.  Nine fliers competed in the event.

After the fog lifted we started putting in official flights.  There was a two minute max.  The winner would be the highest total of three flights - unless there was a fly-off.

I maxed with my first three flights.  So did another veteran flier with his Gollywock that had a folding prop. Another flier with a Gollywock that had a folding prop just missed his third max.

The CD decided that the fly-off would start with a first flight of 180 seconds.  I decided to go for a new motor for my attempt in the fly-off.  It was identical (16 x 1/8 - 40 grams, TSS May 2015) to the rubber motor I had used for the first three official flights.  With Stan's blessing, I wound a little higher this time - 825 turns and 25 inch-ounces of torque. I easily made my three minute flight with plenty of extra time beyond 180 seconds.  But only 180 seconds counted for me in the first round of the fly-off. The Gollywock had a technical problem and only flew 29 seconds in the fly-off.  

Based on what I have observed, the Korda C is more than a match for the Gollywock, despite its seeming handicap of a fixed prop.  I plan to gradually work up to 30-inch ounces of launch torque.  Red Buzzard - thank you for your comments about adjusting the thrust line as I increase the launch torque.

I am very happy with how my Korda C flies.