Why do "ladder" wings have no compression strut towards their wing tip?

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David Teahay

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https://scontent.flos2-1.fna.fbcdn.net/v/t1.0-9/fr/cp0/e15/q65/47010854_744236279285251_7750259899992375296_n.jpg?_nc_cat=104&efg=eyJpIjoidCJ9&_nc_eui2=AeGgDEMLKl1cgXWa68LDMFswYXRxZkiXDdeZ7BVTYxsMS_QHApCX6Gi56m3ijNgwyypHCYHCZlYexY73ndLqB3ISz6ho6elw-uDawl-EcIcOsEFNzfNxL9PfiGR3jhEB2pI&_nc_ht=scontent.flos2-1.fna&oh=c50a6d0ce4cfa3f2d9dd510dcaa10e4b&oe=5CA1BD37
That is a picture of a goat 3 ultralight and a weedhopper wing,you ll notice that the two wings dont have çompression strut and drag/anti drag wire along the outer end of the wing. My question is why is this so,does the outer end of this wings experience no anti drag and drag forces?
 

BoKu

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...the two wings dont have çompression strut and drag/anti drag wire along the outer end of the wing. My question is why is this so,does the outer end of this wings experience no anti drag and drag forces?
That's a pretty astute observation, good catch!

I have wondered about this myself; I have seen the same feature in other aircraft; I think that the Fly Baby also lacks drag/thrust bracing outboard of about .7 semispan.

I don't know the answer, but I believe that at the outboard end of the wing the drag and thrust loads are so low that the bending capacity of the spars and other structure is adequate to react them without deflecting too much.

Contrary to what you might imagine, forward thrust loads produced by pull-ups are potentially much greater than the drag loads, especially the during transitory state at the beginning of the pull-up. I would guess that the amount of forward thrust at each spanwise portion of wing is proportional to the vertical lift produced by the portion. Since spanwise lift distributions tend to be elliptical, the thrust distribution is also probably elliptical as well. If that's the case, the accumulated thrust load, and resulting bending moment, will be greatest at the side of body, and diminish very quickly as you go outboard. Out at the tip it tapers off to nothing.

--Bob K.
 

Wanttaja

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That's a pretty astute observation, good catch!

I have wondered about this myself; I have seen the same feature in other aircraft; I think that the Fly Baby also lacks drag/thrust bracing outboard of about .7 semispan.

I don't know the answer, but I believe that at the outboard end of the wing the drag and thrust loads are so low that the bending capacity of the spars and other structure is adequate to react them without deflecting too much.
Yes, Fly Baby's wing doesn't have any cross-bracing outboard of the ~80% span point:



(Page 21 of http://www.bowersflybaby.com/PB100/Guide_1.pdf)

The arrows show the location of the internal cross-bracing wires.

I believe the load distribution of the wing is low enough outboard where the stiffness of the inner portion of the wing is sufficient. In addition, the Fly Baby wing thickness is tapering down in this area, and there's a tremendously strong laminated wingtip bow tieing the spars together at the tip.

Ron Wanttaja
 

lr27

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I would guess you could cut off the last two or three feet of most small aircraft of moderate speed and replace it with Styrofoam*, and it would be strong enough. Loads drop off fast. as you get away from the centerline.


*i.e. extruded polystyrene foam, not expanded bead foam, although maybe even that would do.
 

TFF

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What you see is not nessisarily what you get. How much of a compression strut do you need for xyz design has to do with how good the engineer can run numbers. A regular wing rib might be enough as a compression rib on something flying no faster than 40 mph. The one wing had drag anti drag struts that were doing the same thing as wires, and on the tubular spar/ leading edge, that is taking a lot of that in itself. The forces are there, how and how much is up to the construction type to size and location of your parts.
 

dino

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It should be fairly easy to test the ladder of choice loaded edgewise.
 

Armilite

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Yes, Fly Baby's wing doesn't have any cross-bracing outboard of the ~80% span point:



(Page 21 of http://www.bowersflybaby.com/PB100/Guide_1.pdf)

The arrows show the location of the internal cross-bracing wires.

I believe the load distribution of the wing is low enough outboard where the stiffness of the inner portion of the wing is sufficient. In addition, the Fly Baby wing thickness is tapering down in this area, and there's a tremendously strong laminated wingtip bow tieing the spars together at the tip.

Ron Wanttaja
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Is there a reason why the Distance between C4 and C3 vs C3 & C2 & C2 & C1 Vary in Distance apart? I would think you would want them spaced the same as C4 & C3 and basically add a C0.
 

Pops

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On a geodetic type of wing construction the geo-strips take the drag/anti-drag loads. On the JMR wings, I have 4130 tubing compression struts and 1/8"x 3/4" geo-strips on 6" centers . Very overbuilt at the last bay toward the tips, could of went to wider centers. Also the reason the inter bay at the root is plywood covered.
 

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Wanttaja

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Wanttaja said:
Is there a reason why the Distance between C4 and C3 vs C3 & C2 & C2 & C1 Vary in Distance apart? I would think you would want them spaced the same as C4 & C3 and basically add a C0.
Probably related to the spanwise loading of the wing; more loading towards the roots so the innermost compression ribs are closer together. Rib C2 is the major outboard structural point; it's where the bracing wires attach.

The 45 degree angle connection at the root bay would make the bracing wires more effective.

In any case, the guy that knew has been dead for nearly 15 years....

Ron Wanttaja
 

BoKu

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Probably related to the spanwise loading of the wing...
That's my bet. Under pull-up loads, the forward bending moment applied to the wing will be roughly proportional to its upward bending moment; or rather to what that moment would be if the wing were cantilever. So you need more shear resistance at the inboard end than at the outboard end. One convenient way to get that increased shear resistance is to use a steeper angle between the wires and the spars.
 

addicted2climbing

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On a geodetic type of wing construction the geo-strips take the drag/anti-drag loads. On the JMR wings, I have 4130 tubing compression struts and 1/8"x 3/4" geo-strips on 6" centers . Very overbuilt at the last bay toward the tips, could of went to wider centers. Also the reason the inter bay at the root is plywood covered.
I have often wondered if it were possible to make a simple cantilevered fabric covered UL type wing with the tubular 6061 spars being front and rear but with a D box wrap as well as having geodetic skinning strips in aluminum linking and stabilizing the ribs? If needed the ribs could be angular like on an Ercoupe. Ie similar construction the ercoupe but instead of a central spar two tubular spars like how the Rans and most UL's are built.. Low weight aircraft in the 400lb empty weight.... The Cygnet has a similar Geodetic skinned wing but in wood and a central spar...

Thoughts?
 

mcrae0104

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Is there a reason why the Distance between C4 and C3 vs C3 & C2 & C2 & C1 Vary in Distance apart? I would think you would want them spaced the same as C4 & C3 and basically add a C0.
Yes, two reasons. But first, consider the loading. From the perspective of forces (or components of forces) acting fore-aft on the wing, the wing is a simple, cantilevered truss. Even if the spanwise loading is evenly distributed, the forces acting on the members accumulate as we get closer to the fuselage. (Look at a shear diagram for a cantilevered beam for an example of this.) The actual spanwise loading of a wing magnifies this effect (as others have pointed out).

Reason 1 for closer spacing: Either the front or rear spar acts in compression (depending if we are talking about a drag or anti-drag load). Since members closer to the root carry a greater axial load, the closer rib spacing there helps prevent column buckling in the spars.

Reason 2 for closer spacing: The wings aren't a pure truss because they are loaded all along the length of the spar (not just at the joints). Therefore, each segment of the spar is functioning in bending as well as carrying the axial compression load of the "truss". Since these fore-aft bending loads are greater toward the root (this time due to the lift distribution), the closer spacing reduces the max moment carried by the spar about its weak axis.

No "C0" member is needed because the loads there fall off to zero and both the front and rear spars are cantilevered with a long, nicely braced back span. (i.e. they're stiff enough as a cantilevered beam to carry the moment at point C1.)
 
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