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Author Topic: Korda Class C Tractor  (Read 416 times)
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calgoddard
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« 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.
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Korda Class C Tractor
Korda Class C Tractor
« Last Edit: July 12, 2018, 04:57:03 PM by calgoddard » Logged
calgoddard
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« Reply #1 on: July 12, 2018, 06:08:46 PM »

Here is the best picture of a Korda Class C tractor that I could find.

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Re: Korda Class C Tractor
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calgoddard
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« Reply #2 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?
« Last Edit: July 13, 2018, 10:29:50 AM by calgoddard » Logged
Bigbandito
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« Reply #3 on: July 13, 2018, 11:26:10 AM »

Thanks for posting Cal. I'll be following to learn new tricks and cheer you on.
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« Reply #4 on: July 13, 2018, 02:09:23 PM »

Can this model compete in OTR with a folding prop?

Marlin
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calgoddard
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« Reply #5 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
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Bargle
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« Reply #6 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? Wink
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tgwhitley
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« Reply #7 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
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Hepcat
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« Reply #8 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.
   
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calgoddard
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« Reply #9 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.
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danberry
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« Reply #10 on: July 15, 2018, 11:56:33 PM »

Put some washout in it. You won't regret it.
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calgoddard
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« Reply #11 on: Today at 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.
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Re: Korda Class C Tractor
Re: Korda Class C Tractor
« Last Edit: Today at 03:31:06 PM by calgoddard » Logged
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