Cantilever Internal Braced Fabric Covered Wing

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wsimpso1

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I suspect one of the most efficient and amateur-accessible ways to make a cantilever, internally-braced, fabric-covered wing is the geodetic approach used in the Vickers Wellesley and Wellington bombers and also the simpler wooded Thalman/Yates/Greenwood etc. series of homebuilts.

View attachment 118360 View attachment 118361

That sure looks like a structural skin to me...
 

Skypilot053

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Use the compression struts and two sets of drag anti drag wires one near upper skin surface and other adjacent to the lower surface this should provide the torsional stiffness at lower speeds
 

Tiger Tim

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Do we have examples of aircraft with Cantilever Internal Braced Fabric Covered wings......I see lots of internal braced fabric wings with struts and twin spars like the Piet. I am looking for examples of double drag wire system for torsional stiffness.....I think it is the a set of drag and anti-drag wires attached at the middle of the spars at the compression ribs.....I can see how that would stop the assembly for "racking" but am sketchy about torsion unless there are 4 wires attaching the tops of the spars together and the bottoms together, but I have not seen that.....so I must be missing something.....This would be a wing with no D-nose just spars ribs small about of aluminum at the LE to form the fabric and internal bracing wires. Am thinking the outer portion past the strut attach of maybe the Kolb or some such........throw out some names,,,,, thanks in advance.
The wings of an Aeronca C-3 are built the way you describe and they feel as floppy in torsion as any other externally braced wings of the period. I suspect at least part of it has to do with spar depth or lack thereof. Perhaps it would be stiffer if the wings weren’t razor thin on a C-3 to begin with.

Another data point could be the geodetic lattice in a whole bunch of Fisher wings. I hear those are extremely stiff in torsion and functionally the diagonal strips do much the same as you’re proposing for wire/cable, though they probably spread out the load a bit more along the spar instead of high point loads right at the compression members as wires would do.
 

wsimpso1

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How so, if it's covered in fabric and mostly holes?
The premise on this thread is how to internally brace a fabric covered wing without a structural skin as the weight of any structural skin is too high. The geodetic wing takes the thin evenly distributed structural skin and replaces it with a structural skin that is mostly open and the rest thicker strips than a usual structural skin with the intersections doubled that then is fabric covered. So, it is still structurally skinned, violating the stated goal of not being structurally skinned and violating the stated goal of it being internally braced, as this is skin bracing for torsion.

If we are to dispense with initial premise and accept a search for the lightest torsional bracing of the existing metal structure, that changes the approach. Say so, and then go about designing the best we can do in each of the offered options until satisfied you have the lightest scheme for the bird in question.

Billski
 

cluttonfred

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I understood from proppastie's opening post that "internally braced, fabric covered" meant no external struts or wires and no rigid skin so to my mind a geodetic approach is sound. I also thought of one you've already mentioned, the Ercoupe, as a great way to accomplish that with very few parts.

 

proppastie

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the Ercoupe
question about the rear spar attach....is that attachment a single pivot bolt.....it is hard to see....I am finding in my investigations that the single bolt rear spar pivot ...the aft spar is moving up under load and twisting the spar allowing more twist no matter how stiff the wing is from the front spar to the rear spar.
 

cluttonfred

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No, it appears to be two bolts in a plate fitting (4a in exploded view below) attached to the wing rear spar that ties in to two holes in the center section rear spar. You can just make out the two holes in the photo below showing the center section.

1638000816108.png
1638000940442.png

question about the rear spar attach....is that attachment a single pivot bolt.....it is hard to see....I am finding in my investigations that the single bolt rear spar pivot ...the aft spar is moving up under load and twisting the spar allowing more twist no matter how stiff the wing is from the front spar to the rear spar.
 

proppastie

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I just did a warren truss layout and the twist is way less... or even reasonable (weighs more) ....thanks......I feel like I am sneaking up on it.
 
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cluttonfred

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I often come back to an Ercoupe-like, Warrent-truss wing design (but squared-off) when sketching minimalist cantilever two-seaters for a 21st-century VP-2 application. Keep the airfoil thick, say, 18% with a 5' total chord (including full-span ailerons) and you have a maximum wing thickness of 10.8". That lets you use ordinary spun aluminum cylindrical fuel tanks, say 8" diameter x 36" long for about 7.5 gallons each, just forward or just aft of the main spar.
 

proppastie

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It is somewhat difficult for my project because of the goals I have set....and the limitations of my knowledge and experience.....44' wing span, less than 254 lb. empty, 5.3 G limit load .....and metal construction except for fabric covering......the farther along I go...... the closer I am getting to some serious compromises of these goals......
 

proppastie

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I was really surprised at how the wing twisted with wire brace....vs the twist with the warren truss there was no comparison.....another surprise was the twist even when the flutter conditions were met as to the posts 55-58
 

WonderousMountain

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Compared to CAD my drawings aren't much of a favor.
From a material use standpoint, I think you'd be better
Off connecting the tips, 6(42") spans, with intermediate
Rib not shown, 21ft per semispan. False spar is weight.
IMG_20211201_113636.jpg
 
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proppastie

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Compared to CAD my drawings aren't much of a favor.
From a material use standpoint, I think you'd be better
Off connecting the tips, 6(42") spans, with intermediate
Rib not shown, 23ft per semispan. False spar is weight.
View attachment 118614
Work in progress. ...one shown has changed from steel to aluminium, more Warren truss ribs ....probably will be light weight ribs as you show to carry the fabric. Weight 1/2 one shown but deflection is higher.
 
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