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Author Topic: Korda Class C Tractor  (Read 5194 times)
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Red Buzzard
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« Reply #25 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
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calgoddard
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« Reply #26 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.
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Red Buzzard
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« Reply #27 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.
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LASTWOODSMAN
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« Reply #28 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  Smiley ).    
     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
« Last Edit: July 22, 2018, 05:54:47 PM by LASTWOODSMAN » Logged

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« Reply #29 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!

« Last Edit: July 22, 2018, 07:04:22 PM by calgoddard » Logged
OZPAF
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« Reply #30 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

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LASTWOODSMAN
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« Reply #31 on: July 22, 2018, 09:02:44 PM »

Hi Cal - yes thanks from me too for those detailed explanations.   I was always wondering  Huh Undecided  why the Crocket Hook (I use them too), has two holes in it. ??   Now I know.   Smiley

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Richard
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« Reply #32 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 Smiley

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.
 
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Re: Korda Class C Tractor
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applehoney
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« Reply #33 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

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calgoddard
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« Reply #34 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  Sad  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.
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applehoney
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« Reply #35 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.

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flydean1
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« Reply #36 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.
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« Reply #37 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
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« Reply #38 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 Smiley
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« Reply #39 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
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« Reply #40 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.
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« Reply #41 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?
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« Reply #42 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 ....  Cry

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« Reply #43 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
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« Reply #44 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.  
« Last Edit: July 25, 2018, 11:23:04 PM by calgoddard » Logged
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« Reply #45 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
 
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« Reply #46 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!
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« Reply #47 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.
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« Reply #48 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.

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« Last Edit: July 26, 2018, 06:02:06 PM by calgoddard » Logged
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« Reply #49 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.
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