Ercoupe-like cantilever wing in wood

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cluttonfred

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I have mentioned before my appreciation for Fred Weick's Ercoupe design, especially the wing construction, but I also prefer working with wood. A few wooden Ercoupes were actually designed and built during WWII in an attempt to use non-strategic material but nothing came of it. They were reportedly a little heavier but much quieter than the metal and fabric originals. The pen and ink drawing below is actually the wooden version with the distinctive reinforcing ribs on either side of the cockpit.
ercoupe wing cutaway.jpgwooden ercoupe details.jpgercoup wing upside down.jpg
I am most interested in adapting the Ercoupe-style Warren-truss two-spar wing to a simple, fabric-covered cantilever monoplane, my stab at a 21st century VP-2. The idea would be to go with a square-cut, constant-chord wing and full-span ailerons to reduce the parts count to just four ribs (end ribs, diagonals, nose ribs, aileron ribs). I could also see doing the wing in three sections (straight center section without ailerons, outer sections with ailerons and dihedral) to reduce workshop size requirements. Overall it seems like a very straightforward way to go, eliminating compression struts or ribs and drag/anti-drag bracing, but I'd love some feedback on the pros and cons.

Cheers,

Matthew
 

Riggerrob

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Sound concept.
I would go for a full plywood wrap around the leading edge, gluing it to both the top and bottom spar caps. A D-spar stiffens the wing in torsion.
 

cluttonfred

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True, but the point of the Ercoupe wing is that it forms a 3D truss to resist torsion so the D-tube is redundant. Note the wing attachment points in the upside down pic. Early Ercoupes were fabric-covered and the leading edge does not go all the way around.
 

fly2kads

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I don't see any reason this wouldn't work. There may be a very small drag increase due to the airflow crossing the ribs at an angle, but in the grand scheme of a VP-2 (ish) plane, it probably wouldn't be noticeable. An airfoil tailored for this application could minimize this effect, if desired.

I would still be inclined to use a full D-tube. With the wood L.E., the effort between a D-tube and the partial wrap on the Ercoupe is minimal. I suspect the Ercoupe scheme made riveting the skins much easier, but that's not a consideration on a wood wing, obviously.

Would a three-panel wing be necessary? I haven't check into the VP-21 thread in a while to see how that's shaping up, if that's what's guiding your thinking.
 

Victor Bravo

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Regardless of wanting or needing a structural torsion tube, there would need to be some kind of sheeting around the leading edge for aerodynamic purposes. So if you are already putting a piece of something there to provide a smooth leading edge, how much further is it to glue that piece to the spars and ribs to yield a stronger wing?
 

cluttonfred

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So if you are already putting a piece of something there to provide a smooth leading edge, how much further is it to glue that piece to the spars and ribs to yield a stronger wing?
VB, not trying to play devil's advocate here, but honestly wondering, why would you add that weight if it wasn't necessary? Ercoupe span is 30', wing chord about 5', so if that gap between the spar and the leading edge covering is 12" then we are talking about 27 sq ft of metal + rivets or plywood + glue that you don't need. A full 4' x 8' sheet of 1/16 aircraft plywood only weighs 6-7 lb but still, weight is weight.

I am not saying I wouldn't do it, just wondering how much difference it would really make in practice. I was actually thinking that plywood over foam ribs would work well there and would probably want the plywood to go from the forward spar around and back again, but more to protect and brace the foam and secure the edges of the plywood than for torsion.
 

Victor Bravo

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I was actually thinking that plywood over foam ribs would work well there and would probably want the plywood to go from the forward spar around and back again, but more to protect and brace the foam and secure the edges of the plywood than for torsion.
OK, so in the idea you mention above, you'd have the plywood going from the top of the main spar around the leading edge, and back to the bottom of the main spar. You would probably have to glue it on, because you really don't want the plywood to spring back and push the fabric away from the airfoil shape of the ribs.

If you flew the airplane before the glue was dry, you would have a non-structural plywood wrap around the leading edge, which would smooth the airfoil shape for better performance.

If you flew the airplane after the glue was dry, you'd have a "D-tube" leading edge that further stiffened the wing torsionally.

Waiting for the glue to dry will even save a little bit of weight :)
 

cluttonfred

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Understood, VB, but if it were not needed to brace the ribs, how much would the full ply D-tube really change if the wing is torsionally stiff enough without it? That's what's not clear to me.
 

Aerowerx

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.... The pen and ink drawing below is actually the wooden version with the distinctive reinforcing ribs on either side of the cockpit.
View attachment 97233
Is this drawing meant to show that the fuselage skin is spiral wound plywood? I have often thought that was a way to build a good and stiff structure. (and, yes, it might be heavier than aluminum or fabric)
 

TFF

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Think you are putting too much mystique in the design. The rear ribs will all need to be compression ribs. You would also have to determine if the corner blocks will hold the ribs on the main and drag spar and not pop free. You not completing the D with wood is like bad grammar. Just wrong, when you have the opportunity to do it right. The most important thing is the spar will be a monster, especially at the root, compared to the aluminum. Look at some pictures of the Spacewalker wing. You can make it work, but you are going to get less for your money changing materials to one that it was not designed for.
 

cluttonfred

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Would a three-panel wing be necessary? I haven't check into the VP-21 thread in a while to see how that's shaping up, if that's what's guiding your thinking.
One-piece wing would be simplest and lightest but not everyone has that size shop. Original idea was a two piece bending beam arrangement like a glider so each half is under 16’ long.

Is this drawing meant to show that the fuselage skin is spiral wound plywood? I have often thought that was a way to build a good and stiff structure. (and, yes, it might be heavier than aluminum or fabric)
I don’t have any more info on the wooden Ercoupe, but that sounds probable.

Hey, Matthew. Could you get me that center picture? I'll swap a fuselage pen-and-ink for it!
Sorry, you lost me.

You can make it work, but you are going to get less for your money changing materials to one that it was not designed for.
I guess I wasn’t clear, I am interested in the general design approach here, not trying to replicate an Ercoupe wing or otherwise in wood. In that sense I don’t think the arrangement is uniquely suited to any one material. Here is an example, the wood-foam-fabric Livesey D.L.5, an unbuilt British answer to the Volksplane.

774994A1-5419-48AD-AA78-FCD24A337740.jpeg
 

pwood66889

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Got that scan of a front view of the wooden Ercoupe fuselage, Matthew. Now if I can figure how to upload it... My scan is a .pdf.
I could not copy that picture (center one) you provided in post #1.
 

fly2kads

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Is this drawing meant to show that the fuselage skin is spiral wound plywood? I have often thought that was a way to build a good and stiff structure. (and, yes, it might be heavier than aluminum or fabric)
I believe it was, or more specifically, spiral wound veneers which made a plywood shell in-situ. This would have been in keeping with other aircraft that were developed in the same time frame with "non-strategic materials," by Fairchild, Timm, et. al. I think it would be a lot of fun to build a plane this way, but cold-molded with modern epoxy. I'd skip the concrete molds and steam pressure chambers. ;)
 
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