Warren truss update

Just a few shots of the new wing. So far saving 3 oz from original wing. Also found other weight savings- 20 grams on new spinner, 1 oz on wheels and 1 oz on tank.

LAST EDITED ON Jul-14-04 AT 00:12 AM (CDT)
 
Looks good Sparky!! How are you going to do the wing tips in the interest of lightness etc.?? Do you have a plan yet?

I will probably make a couple of sets different ways to see which ones are lighter.

Hello,

Great pictures of a fine structure.

Two questions:
1: Why are you joining the TE planking at the middle of the wing, the high stress point? How is the joint established??
2: How is the planking at the leading edge joined? This joint has to be in the middle of the wing in any case.. One of my headaches when building a wing...

Regards,
Paul

Why does it matter where the TE is joined? I would think that spars are the only thing that cannot be joined at the center of the wing.

The way it was explained to me is that when the wing flexes up or down, its rotation point is right in the middle of the wing. This generates compression force on the spar on one side and stretching force on the other side. The leading and trailing edge are not involved at all in keeping the wing from flexing.

LAST EDITED ON Jul-14-04 AT 08:48 AM (CDT)
 
Its done with 2 hr epoxy. I built this wing to spec as the Bob Hunt vidio shows.

It is worthwhile to NOT have the TE spar joints in the center of the wing. Further, they should always be spliced on an angular cut, rather than a butt joint.

I normally leave one spar piece (nearly) full length (say 36 inches) and splice a piece on its end, which gets trimmed to length. And I weigh each spar, and arrange to cut off the heavy ends.

Same for the trailing edge planking. And place its joints on opposite wing panels.

If you can use 48" stock, this works out even better.

For the laminated sheet spars with 1/64" ply center that I use now, the joints on alternate layers are never on top of each other..

This is just "free" additional strength.

(Nice work on your wing!)

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"There comes a time in every man's life and I've had many of them." -Casey Stengel

>I normally leave one spar piece (nearly) full length (say 36
>inches) and splice a piece on its end, which gets trimmed to
>length. And I weigh each spar, and arrange to cut off the
>heavy ends.
>
>Same for the trailing edge planking. And place its joints on
>opposite wing panels.
>

Larry,
I absolutely agree on importance of keeping the spar solid at the center of the wing. I do all my spar splicing as far on the outboard wing panel as possible. I also make sure that top and bottom spar splicing is NOT done in the same wing bay. I try to stagger top and bottom spar slicing by ~2 inches.

As for the leading and trailing edges: I don't think it matters as much if at all. I still splice them, not butt join though.

LAST EDITED ON Jul-14-04 AT 01:37 PM (CDT)
 
I am just following Bob Hunt's instructions. I usally do not but joints but there is a bass wood spar not shown as of yet in the center. This is a lost foam wing.

>Why does it matter where the TE is joined? I would think
>that spars are the only thing that cannot be joined at the
>center of the wing.

As a matter of principle I never make a splice joint at the center section of the wing, with the exception of a Leading Edge square, say, on a wing where the LE tapers back and you have no choice.

This goes for spars, trailing edges, sheeting, anything.

And I always make a splice joint (angled significantly to perpendicular) rather than a butt joint where ever possible.

And never two splices in the same area. For instance, the top trailing edge sheeting might be spliced on the inboard panel and the bottom trailing edge sheeting spliced on the outboard panel. I'll cut a couple of inches off of leading edge sheeting to make sure that joint is not in the same horizontal location as another joint on the trailing edge.

Butt joints are the least resistant to bending loads and bending loads from lift are concentrated on the center of the wing (in and of itself) and at the joint of the wing and fuselage. So I move'm and splice'm, scattering any potential weakness around the wing stucture and away from the center section.

My two sense worth...

Cy

I agree with you, These wing half's are joined in the center with a 1/8 rib butt joint. When finished gluing you cut the back half away. A 1/8 x 1/2 inch bass wood spar joines the two halfs together.

Thanks for Pics. Learning how to build light and strong is sooo difficult, but it is absolutely essential if your going to take care of business. Like a lot of our forum members, I watch the Discovery Wings Channel, and it is interesting to see how major airframe manufacturers have to wrestle with the same issue of strength versus weight just like we do.

"I don't know about this tuned pipe business. I practiced on that thing until my lips turned blue, and I still can't play a stinkin note."
"Homer J. Simpson"

This really is a good looking wing, and you are doing a great job on it.

However, as per the discussion about trusses and triangular structures, I want you to notice that the angle between the ribs is very different from the ideal equilateral angle of 60 degrees. This is not to say it is not a fine structure.

But I do hear a lot of touting of these wings as being extra rigid and twist resistent. I submit that they really aren't that much better in this regard than a conventional parallel rib structure. Consider that quite a bit of whatever additional strength it has is located where it is not needed.

Try this experiment sometime. BEFORE any planking is applied to your wing structure, stand it on a table on the trailing edge. Now, if you apply downward force on the leading edge, you would see some benefit to the angled ribs (particularly if they intersect at 60 degrees). Like a bridge truss. Repeat the experiment with a parallel ribbed framed structure, and you can see that it is NOT as strong, and that it can more easily "parallelogram" when the force is applied. In fact, the triangulated structure could even have "pin" joints (free to rotate) and still be quite strong. By contrast the parallel ribbed structure would virtually collapse with pin joints - it depends on the joints themselves to maintain alignment. (Actually, the triangulated structure is much simpler to analyze, because the members are all in pure tension and compression..)

OK, great - extra strength! But when the wing is in use, how much force is applied in the direction you were testing?

So, now do a similar comparison test for "twist resistance". There you will find that the triangulated structure has a bit more strength, but it might surprise you how little more! Measure it. Here's your problem(s) - first, the triangles are far from equilateral. Second, your material is SOFT 1/16" balsa - very light, but with a poorer strength to weight ratio than medium balsa. AND, third, much of the rib sheet material is cut away. Just a small deformation in length of each angled rib, spread over many ribs, translates into quite a twist angle at the end of the wing.

Torque strength is one thing, but the wing structure bears its loads on a VERTICAL direction, in use. So, one thing generally done to strengthen the wing greatly is to add sheet "webbing" vertically between the top and bottom spars. It really helps a lot, but this structural benefit is not peculiar to wings with diagonal ribs.

If you have both framed structures and the ability to measure strength handy, continue testing as you build more of the wing. You'll find that both wings get amazingly stronger when the LE planking is added. Less so with the TE planking, it has less surface area, and almost no CURVATURE. The LE planking adds tremendous strength by virtue of a monocoque effect - stretching and compression forces distributed smoothly over a curved surface (in 3 dimensions), with a large surface area.

Before you cover the wing, you typically add cap strips. These help strength a great deal (making little T-beams). Now you've added considerable tensile/compression strength to the ribs! This will arguably benefit the diagonal rib wing's twist resistance. But it is NOT the major structure piece in this regard - the LE planking is.

Finally, when you cover the wing, shrink the silkspan, convert the silkspan into a plastic-like material by applying lacquer (dope) to it. Now the wing REALLY gets strong (unfortunately, this is the step where it is easy to build in a twist..). Once covered, a large component of twist resistance is added by distributing the forces over a very large, smooth, curved, 3D surface.

There is a good reason why the diagonal ribs are typically NOT set at 60 degrees in a stunter wing - this arrangement results in large expanses of open bay area (at both LE and TE, in particular), which is a consideration for covering.

Years ago I had a discussion with Bob Hunt, and I told him that I had looked at the diagonal ribbed wings, and concluded that they weren't that much more resistant to twist than a conventional wing. I presented the same discussion that I posted here. Bob basically agreed with me, but then he pointed out another (possible) advantage that I had not considered - the diagonal ribs show up in the wing surface contours, and may "turbulate" airflow!

Besides, they look neat!

Anyway, I think I have figured a way to make the ultimately strong wing, and thank God I have a computer to draw all the wing rib templates.. The ribs are arranged in overlapping X-configurations, sort of an "egg crate" or honeycomb structure. But here's my "twist": ribs are CANTED alternately at a "geodesic" angle!

I'm joking of course. The only reason I would go to such trouble on a wing structure would be to "show off" a little, that is, for appearance sake. If such a structure is worth all the complexity and difficulty involved, that would be because you were able to make it out of smaller, lighter structure and maintain the same (adequate) strength.

But what you need to consider about any structure are the "diminishing returns" effects. How strong does it REALLY need to be? There is some optimum point for such things.

The classical D tube structure we use for wings is amazingly good, for the simplicity involved. And the diagonal ribs illustrated here, while perhaps imperfect, are a very good compromise.

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"It's linkage I'm talking about,
and harmonies and structures
And all the various things that lock
our wrists to the past."
-Charles Wright

Larry,
Your analysis is correct.
When someone says they want, or have, a stronger wing, you have to ask whether it is in flexure or torsion. It's simple to make an open structure wing stiffer in the flexure mode: you can make the section thicker, web the spars, use higher modulus material for the spars, etc.
Torsional strength is a different ballgame- I think it is actually more critical than flexural stiffness. When you pull a high-g turn, the wing flexes- no big deal except for a loss of energy- but it also twists. This twisting can cause unpredictable responses.
A D-tube wing can be stiffened torsionally by adding diagonal struts between the ribs, from the top spar of rib 1 to the bottom TE of rib 2, from the bottom spar of rib 2 to the top TE of rib 1, etc. Here again, the thicker the section, the more efficient the diagonals are. Also, a structure like this is very resistant to warps.
Even better would be a fully sheeted wing. This construction dissipates load over a greater, 3-dimensional, area. You can expect far less deflection for a given load than a similar D-tube, open TE structure.
From here you have to consider sheeted foam. Fully monocoque, quick to build, easier to finish, aerodynamically clean.
The bottom line is the question: how strong does it need to be and how much can it weigh?
Derek Moran

I think the reason this wing was designed this way was to loose lumber. It looses every other rib. It had no leading edge. Just molded sheeting. I could be wrong but it does weigh less. It has more strength with the boxed in trailing edge. To give it more rigidity it will have two layers of polyspan in the center .