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Author Topic: Korda Class C Tractor  (Read 5333 times)
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calgoddard
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« Reply #50 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.
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« Last Edit: July 29, 2018, 05:40:25 PM by calgoddard » Logged
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« Reply #51 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
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« Reply #52 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.
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« Last Edit: July 30, 2018, 10:36:44 AM by calgoddard » Logged
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« Reply #53 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.
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« Reply #54 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. 

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calgoddard
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« Reply #55 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.
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« Reply #56 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.

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« Reply #57 on: July 31, 2018, 07:56:39 PM »

Looking forward to your trim flight reports Cal. I hope you get a chance soon.

John

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« Reply #58 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.




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calgoddard
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« Reply #59 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.  
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« Reply #60 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.
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« Reply #61 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.
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« Reply #62 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.
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« Reply #63 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.
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« Reply #64 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.  


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« Reply #65 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.
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« Reply #66 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
 
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« Reply #67 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.
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« Reply #68 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.
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« Reply #69 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.  
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Re: Korda Class C Tractor
« Last Edit: August 05, 2018, 09:19:57 AM by calgoddard » Logged
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« Reply #70 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.

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Re: Korda Class C Tractor
« Last Edit: August 06, 2018, 01:08:09 PM by calgoddard » Logged
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« Reply #71 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.
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Re: Korda Class C Tractor
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« Reply #72 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.
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« Reply #73 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.  
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Re: Korda Class C Tractor
Re: Korda Class C Tractor
« Last Edit: August 08, 2018, 07:50:55 PM by calgoddard » Logged
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« Reply #74 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.
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Re: Korda Class C Tractor
« Last Edit: August 09, 2018, 09:50:33 AM by calgoddard » Logged
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