Ultralight struts/cantilever/additional weight

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TFF

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With a high wing cantilevered, your head is usually fighting for the same spot as the spar carry through. Low wing you are normally sitting over it. Just easier to package.
 

wsimpso1

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Cantilevered wings carry all lift to the root, with all of the lift, drag, bending moment, and pitching moment is carried by the wings to the fuselage.

Strut braced wings instead react off much of the lift to the strut. The spot along the wing that minimizes bending moment in the wing is about 70% out the semi-span. This matters the most if you use a constant section spar. Bending moment decreases to zero at the root fittings with struts. In exchange for reduced bending moments inboard of the strut attach points, the spar is under big compression, for which the spar must also be sized and will matter to weight as well.

When the wing is stiff torsionally, usually by using a structural skin, only one strut is needed. Go instead with spars, ribs, and fabric covering, and you need two struts to carry it all.

Well, structural skins and spars that can carry all of the shear, bending, and torsion tend to weigh more than light spars, ribs, fabric, and struts. Which is why most UL are double strut braced with fabric covered wings.

So why would we ever use one strut, or even cantilever the wing. Well, for lower drag, so the ships can go faster. Save enough drag and it pays for the extra weight. But with a 252 lb weight target and really low max level flight speed, the draggy but light strut braced fabric covered wing makes sense.

Billski
 

lr27

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How much does an extra strut weigh? I think plywood for a 1/32 d-tube would only weigh 7 to 10 pounds on an ultralight. If it wasn't stiff enough, you could install it with diagonal grain on the inner panels. It might be a little trickier with other materials.

I think,the computer ate my comment that you could make a truss between two spars to get torsional stiffness. A thicker airfoil would help, of course.
 

rotax618

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It is difficult to engineer sufficient torsional stiffness into a very light cantilevered wing to resist aeroelasticity, there have been cases where there has been control reversal and in worst cases catastrophic failure from aileron flutter.
I you can afford the extra weight and/or use exotic material, by all means cantilever the wing.
 

lr27

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We're talking about ultralights here. In those examples, did any failures happen at less than , say, 80 mph? If not, they don't really apply. Any info on material for the d-tube, design, aspect ratio, etc?

It is difficult to engineer sufficient torsional stiffness into a very light cantilevered wing to resist aeroelasticity, there have been cases where there has been control reversal and in worst cases catastrophic failure from aileron flutter.
I you can afford the extra weight and/or use exotic material, by all means cantilever the wing.
 

rotax618

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The examples I was referring to were amateur design/built what we call ultralights here in Australia. One notable failure had an AR of about 6, glassfibre ribs and leading edge cover, but only a single strut. The ribs did not have sufficient strength to resist deflection of the trailing edge tube (aileron attachment). Similar designs using 2 struts had been flying for years without a problem.
All structural failures come from design or material failures. It is possible to design safe strut less wings but it is hardly worth it for a low speed sportplane of less than say 900-100lb TO weight.
It is easy to design a safe light wing if part of the bending, shear and torsion loads are taken by struts, you can use sailcloth covering and tubular battens for rib, the wing can be single surface, you can have simple 2 tube spars like the Kitfoxes and Avids. Even the Archeon jet look-alike has struts.
To have dihedral in a cantilever wing requires a complex centre splice or attachment, strutted wing require inly a shear pin.
 

Dana

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It is difficult to engineer sufficient torsional stiffness into a very light cantilevered wing to resist aeroelasticity, there have been cases where there has been control reversal and in worst cases catastrophic failure from aileron flutter.
I you can afford the extra weight and/or use exotic material, by all means cantilever the wing.
To get decent torsional strength without two struts (or wires), you need essentially a largish closed shape... in simple forms, a tube is good in torsion, a C-channel or I-beam is not. The Kolbs use a single large (5 or 6") round tube as the main spar and one strut (the Firefly uses two struts, not for structural strength, but to add drag to meet the Part 103 max speed calculation). Internally there's a single drag strut, and it's covered with Stits fabric. The Lazair, one of the lightest ultralights that's still reasonably robust, uses a singe spar with an aluminum D-tube, and styrofoam ribs and plastic film covering. I don't know what it uses for drag bracing or if it's just the D-tube.

On the early Kolb designs like my Ultrastar there is definitely some aeroelasticity, you can actually see the wing twist when the ailerons are applied, which significantly reduces the roll rate. As far as I know, there was never a failure because of it, and later designs improved the structure at the wing root to anchor the tube spar better, reducing the twistiness (engineering term meaning opposite of torsional stiffness :D).
 

b7gwap

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upload_2020-1-18_8-26-30.jpeg
from a google search.

note the clear Tedlar covering aft of the D tube, attached with tenacious double sided tape. (Although the tired restoration project pictured clearly shows some of that has let go)
 

cluttonfred

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Very cool, thanks. A few questions...
  1. In an Ercoupe-style wing, am I right in thinking that the main spar, diagonal ribs, and rear false spar together form a sort of triangular truss and that's what resists torsion, drag, and anti-drag forces?

  2. Do the false nose ribs also serve as the vertical stiffeners to the web of the Wagner beam main spar?

  3. Other than streamlining the airfoil, would there be any benefit to a full D-section leading edge in an Ercoupe-style wing?
I have been looking at the HB-204 Tornado as inspiration for my "pseudo-jet" concept and wondering if a constant-chord Ercoupe-style wing with tip tanks might be the way to go.

Cheers,

Matthew
 

BBerson

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1) yes
2) yes, I don't know if the spar has other additional thicker verticals. The nose ribs are not false, the nose loads are very high.
3) not really.
 

cluttonfred

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Thanks, BB. It occurs to me that the Ercoupe approach might also work with a three-tube system: two tubes (with reinforcements near the root) form the top and bottom main spar caps joined by truss bracing, the third tube forms the trailing edge. It ought to be possible to reduce the parts count for a constant-chord wing to the spar tubes and reinforcements, four types of rib including the nose ribs, and the diagonals for the main spar truss. It could even work with a single large tube for the main spar with the Ercoupe arrangement reducing the torsional stress on the big tube.
 

rotax618

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Chris Heinz CH701 has a fairly substantial D box leading edge and stressed metal skin yet he chose to use 2 struts for safety. I note that BBersons wing is not loaded to test torsion. Aeroelasticity in torsion is a killer, in normal flight regimes it lowers control effectiveness, but an airframe vibration caused by an out of balance prop or engine missfire can cause catastrophic flutter.
 

jedi

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Thanks, BB. It occurs to me that the Ercoupe approach might also work with a three-tube system: two tubes (with reinforcements near the root) form the top and bottom main spar caps joined by truss bracing, the third tube forms the trailing edge. It ought to be possible to reduce the parts count for a constant-chord wing to the spar tubes and reinforcements, four types of rib including the nose ribs, and the diagonals for the main spar truss. ........

With two tubes forming the top and bottom main spar caps joined by truss bracing it is relatively easy to taper the wing planform and spar height by reducing the truss bracing height towards the wing tip.
 
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