What is the weight of aileron cove/false spar material?

[Marty57]

In the quest for an all wood wing for my 2+2, I made the curved false spar/aileron cove from birch ply. I laminated two pieces of 1/16" Finnish Birch ply to form the cove to shape; worked great and easy to lay up each 4' section. I will be able epoxy them directly to my wood ribs with out any reinforcement pieces. I suspect they are heavier than similar aluminum cove material. Does any one have a weight of a section (and length) of the aluminum cove for comparison? Mine weigh 9.6 oz for a 4' section. I am using the J3 aileron style cove the entire length of the wings, aileron to flap. Total weight for the cove for the entire wing is 2lb 3 oz. Thanks

Marty57

 

[kevin]

Looks GREAT Marty. I can't wait to see it fly!!!!!!!!!!

 

[mike mcs repair]

I don't have one to weigh, but that sounds reasonable, but are you adding the top and bottom lips to them? I would think the fabric will deform what you got there...or then again it is 1/8" thick I guess..

 

[Marty57]

Mike,
I'm not sure I need to add the lip. The cove is very strong; I think I could stand on it and it would not break. The Aluminum cove needs the lip for strength; the wood lam is strong by itself. I went with two 1/16" vs two 1/32" because I was concerned about the cover deforming the shape. Other than strength for the aluminum cove is the lip necessary as far as the covering is concerned? The Super Cub parts list shows the brace assembly; how many are used in a wing? I won't need these so there is some trade off/savings in weight.


Marty57

 

[skywagon8a]

Marty57
I admire your workmanship. It will be too beautiful to cover.

Taught fabric has an extremely powerful force. It has the ability to deform the strongest of structures. The cove has two purposes. One is to give a specific aerodynamic shape to the trailing edge of the wing. The other is to hold the fabric. When the fabric is shrunk, it has the ability to bend the center edge of the cove forward. My educated guess is that you should add a stiffener to the edges in order to prevent or minimize the distortion. I have found that no matter how much stiffening is done, there will be some distortion. It is almost impossible to prevent. Remember the WW1 Spad? the trailing edges were scalloped from the shrinking of the fabric. It was not done intentionally.

 

[Bugs66]

Marty, I have some cove in my hangar and can weigh this evening if you are still entertaining aluminum.

 

[Marty57]

Hi Bugs,
I did get the weight of the cove material so no need to weigh but thanks. Turns out that the weight is the same. I am going to add the top and bottom lip to resist the fabric pulling things out of whack and than install with screws like aluminum. I should have them ready to install in a few days; lots to do while the glue dries.
Thanks,
Marty

 

[Marty57]

Skywagon,
I sent this earlier from my phone but I think it went to Steve instead (Sorry Steve). I knew about the Spad and am very concerned about that issue. My original intent was the lips on top and bottom and I think I will add those to the cove with some blocking between the lip and the cove. I found the weight of the aileron cove in aluminum is 26 oz just for aileron length; my cove is 35oz including flap. It should be the same weight as full length aluminum even with the lip. New question is mounting; screws or glue? Could glue make the wing too ridged? Another builder saw some rib cracking on a wood Cubby wing at recover that had wood cove but I don't have any details. If I glue the clove I will have washout blocks in place, same for screws. Any thoughts on attachment? Thanks
Marty57

 

[skywagon8a]

Marty57
I am far from being an expert on wood. However, I am under the impression that wood is much more resilient than aluminum. I am inclined to think that the Cubby wood that cracked, likely was cracked during construction or was a poor piece of wood in the first place. Unless it was not a crack but a glue separation. It is not uncommon to find cracked aluminum in the wings in the area of the prop wash. The beat from the prop sets up vibrations which causes cracks unless the aluminum is properly supported. To answer your question about the fasteners. Screws only hold where they are screwed in. Cracks could form directly from the fastener hole. Glue would have more area to accomplish it's mission and will prevent localized cracking. My guess is that, with modern glues, the wood cove would be more durable.

 

[Marty57]

skywagon8a,
I keep going back and forth on this. I think you are right; not enough info regarding the cracks to make any decision based on one report. For instance, if the washout was not built into the wing when the leading edge and false spars are glued on than a very strong twist is set up in the wing when the plane is rigged. That could have a lot to do with cracking. If the plane was in for a re-cover than it must have been flying for a while and what wing looks new after twenty years? I just finished re-installing all my hangers and flap hardware and cut the access for the flap hanger and push rod in the first cove. The first couple of bays are sheeted from the spar to the cove to strengthen that area for the fuel tank so it will be pretty solid there top and bottom anyway. So, it looks like epoxy for everything. I like the idea of using as much wood as possible for a lot of reasons; one being expansion and contraction of the material. Wood and aluminum expand and contract differently. Not a big problem but something to think about when selecting materials. So I'll keep moving forward with the wood coves; that's why they call it Experimental!
Marty57

 

[skywagon8a]

Marty57
Why don't you make up a square test panel about the same dimensions as your rib spacing. Place a plain piece of cove on one side and a well reinforced piece of cove on the opposite. Place a diagonal across the square to stabilize it. Cover it, shrink it to the maximum for the fabric, and see for yourself how much distortion you get. The maximum distortion will be at the edge of the cove where the fabric will try to uncove the cove. Don't forget to reinforce the two rib sides as they will also buckle.

 

[Marty57]

Skywagon8a
Good idea on the test panel but I don't have any covering materials yet. I will be using Stewart system for covering. I am going to glue the coves in place and than add the upper and bottom lips with some corner blocks where they come together for strength. I don't expect much if any distortion. I could do the test piece before covering the wing and add bracing than if necessary. Good suggestion.

 

[Bugs66]

Marty,
I can send you a small kit of Stewarts stuff. Cement and extra experimental dacron I have. Let me know.

 

[Marty57]

Bugs,
That's not a bad idea, all I need is enough to cover a test panel that would simulate rear spar to bottom of cove and one rib to another rib with over hang. I just measured the area; about 24" from rib to rib (includes overhang) and about 22" from spar to cove (covering cove and spar). I'll just cover the top of the box, right? Won't need much glue for that; maybe a small jar or something unbreakable like shampoo bottle??. I'll glue up a piece of cove to span distance between two ribs and make a box with everything else standard to stock measurements. This will tell me if I really need the lip or not. All I need to do is glue the fabric on following the Stewart directions and shrink it down. The eco fill doesn't add any tension to the fabric, does it? You still have my address? I'll make the test box and when covered I'll post the results here. Thanks Bugs!
Marty57

 

[skywagon8a]

Marty57
I would cover both the top and bottom. Covering both sides will double the pull in a more realistic manner. You will be amazed at the amount of distortion that can show up.

 

[Marty57]

Bugs
Do you have enough scrap for two sides? I'll build the test box to fit what ever scrap you have.
Thanks,
Marty

 

[skywagon8a]

Marty57
On second thought it may be a good idea to place two diagonals in order to lock all four corners in place on the test panel. You are testing the sides not the corners.

 

[Marty57]

Skywagon8,
I'm going to build the box with diagionals so the test "ribs" won't bend in. On the cove I am going to do the top with no lip and the bottom with a 1/16" x 1" lip and cover top and bottom. That way I can test and compair both to see how they perform. I'm very curious now to see how it works out.
Marty57

 

[Bugs66]

Marty,
Sending out the stuff today. My aluminum cove was .23 oz/in. For 102" aileron run = 23.5 oz. I put a sample in the box.

 

[Marty57]

Thanks Bugs. I'm going to build the test box tomorrow. I'm looking forward to my first attempt at covering. I watched the Stewart videos last night, feel ready to try it. How do you calibrate a standard iron for the different temps needed? Something like an inferred temp sensor?
Thanks,
Marty57

 

[Bugs66]

For iron, go to Hobby shop and buy the little spring loaded temperature gauge. Go ahead and buy a model airplane iron as well, you will use it regularly. I only used the household iron for the large shrinks.

 

[Marty57]

Thanks, I'll head over to the hobby shop tomorrow and see what they have.
Marty57

 

[Bugs66]

If they have the little IR temp sensors, those are handy instead of the spring tool.

 

[Marty57]

Christian,
I got the materials today, thanks. Tomorrow I should be able to build the box and cover it on Sunday. A couple of discoveries that have surprised me. First, the weight. The piece of aluminum cove you sent me weighs .23oz per inch; the wood cove I made comes in at .20 per inch. What really surprised me is that the aluminum cove doesn't fit the end of my wood ribs, built exactly to the drawings. I attached the pictures of the two different coves so you can see. It looks like my ribs are thicker at the end of the rib where the cove fits. So, fitting an aluminum cove would prove to be difficult for sure. Another difference is the thickness. The wood cove I made is 1/8" thick compared to the aluminum at .028. I'm really curious to see if the wood cove needs any lip with that thickness. If this weight holds (I still need to trim about 1/4" of the wood cove) than the weight savings over the two wings is 12 oz. Most of the wood ply parts i need to use are 1/16" so I bought a bunch from ACS. They have a 1/32" that I could use two pieces to make the cove half the thickness and half the weight if needed; I will likely use the two 1/16" pieces if no lip is necessary from the test cover. So although I did not set out to make my light weight many of the parts are lighter than metal. I'll post again with the results of the test cover. Thanks again for the materials you sent, bit help again. For those thinking about scratch building these are the kind of things that add to the build time; and they addd up for sure.
Marty57


Metal Cove:


Wood Cove:

 

[Bugs66]

Interesting Marty. Your tail ribs are built to Wag drawings or PA-18? I assume you cannot push the cove into place? Seems real close. If you get a chance you might want to see if the aluminum cove I sent you matches the dimensions of the PA-18 aileron bay cove. Maybe D&E are slightly different.

 

[Longwinglover]

Marty57

Taught fabric has an extremely powerful force. It has the ability to deform the strongest of structures. Remember the WW1 Spad? the trailing edges were scalloped from the shrinking of the fabric. It was not done intentionally.

There are two differences here that need to be considered.

There is a huge difference between iron taughtened fabric (Stewart's) and dope tightened fabric. The dope will be tighter and continue to shrink the Stewart's will not.

The WWI SPAD (and the 1930's Curtiss P-6E Hawk both are noted for having scalloped trailing edges. They both had wire trailing edges.

John Scott

 

[skywagon8a]

There are two differences here that need to be considered.

There is a huge difference between iron taughtened fabric (Stewart's) and dope tightened fabric. The dope will be tighter and continue to shrink the Stewart's will not.

The WWI SPAD (and the 1930's Curtiss P-6E Hawk both are noted for having scalloped trailing edges. They both had wire trailing edges.

John Scott

John, you are absolutely correct. My point in using the Spad as an example was for emphasis. I covered my Cub with Stewarts and it did distort some areas. Not anywhere near that which aging dope can accomplish though. My main point is that fabric, when drawn up tight, is powerful and CAN distort structure if not properly reinforced. Once the fabric is installed, it will be too late to accomplish the reinforcements. The fabric can not be tight without pulling hard on something.

 

[Marty57]

Bugs,
The cove you sent does conform to the PA 18 drawings. The end of my rib seems to be about 1/8" taller at the corners so it won't conform to the cove. I made the test box; top had no lip bottom has a lip. Tomorrow I will cover the box to see how it holds up to the shrink of the fabric. Interesting process figuring all this out. I'll take pictures of the box before and after and post.
Marty57

 

[Marty57]

Well, here's what I learned. First the box to test the cove with fabric. Top edge no lip; bottom edge 1" x 1/16" lip:



Thanks to Bugs, I have material to cover the test box. BTW; this is my first attempt at covering. Watched the Stewart videos on UTube; ready to go:



And after shrinking first at 200, than 250, finish with 350:



Well, that didn't work! The top edge with out the lip bent a lot with the fabric shrank at 350 degrees. Bottom edge with the lip was better but still distorted more than acceptable in my book; about 1/8" in the middle of the rib span.



I guess I could try a thicker lip; maybe 1/8" top and bottom but the added weight and time is not worth the effort. I do have a nice covered box to continue learning the covering process; tapes, inspection rings, rips, etc. So, I will be using aluminum cove. I will need to modify the end of my ribs to match the cove but that's not a big deal. So, who sells the best cove at a good price?

Marty57

 

[Bugs66]

Very interesting Marty! Glad you figured this out now. I would check with D&E or Wag for cove.