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I am interested in designing a scale model semi-monocoque wing for a glider. I've done some research on the framework and internal structure. I have found out about the spars, ribs and stringers and I am trying to produce a design with the highest strength to weight ratio. I am planning on making the wing 0.5m long with a chord length of 0.1m along the cross section (so no tapering).

Here is my design so far: design plans of glider wing

If anything is unclear please let me know. In my first diagram, I have placed 2 spars through the wing about 10% of the chord length from the leading edge and 15% from the trailing edge, leaving 75% of the chord length between the two spars.

In the second diagram is a rough plan of how many ribs would be used and where they would be placed: the solid outline is the wing outline, the thick black lines are the spars and the dashed lines are the ribs.

I have used a triangular configuration, as the triangle is the strongest shape and resists shear, torsion, compression and tension better than any other shape. Then I have placed a rib going down from the top corner of each of the 3 triangles to the bottom of each. Then I have placed ribs (or stringers) between each of the bottom corners of the triangles.

My question is, how can I improve or optimise that design? Is there a certain number of triangles I should use and how big should they be relative to the wing? Should they be equilateral triangles or is isosceles stronger? Also, how many ribs should I be considering using?

I'm not looking for anyone to figure out exact quantities. If there is some kind of calculation I need to carry out to find the optimal number or triangles and ribs, I would be happy to do that. Unless it is way beyond A-level maths/physics standard then I probably will not be able to do it. But I would like advice on how I can figure it out. If my question is vague or unclear, please inform me, and I will gladly edit it.

Thank you

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  • $\begingroup$ The idea of using spars and ribs to make a light structure to resist bending is good, but I think the details are wrong. Your drawing will resist bending of the wingtips in the longitudinal direction, but the large bending loads are actually vertical. If you need two spars joined by ribs, they should be vertically above each other to resist those bending forces. (The ratio of vertical to longitudinal loads is roughly the same as the glide ratio, which should be at least 10 for a reasonable aerodynamic wing design, and as high as 50 for a "high-tech" full-sized glider wing!) $\endgroup$ – alephzero Jul 11 '17 at 8:34
  • $\begingroup$ Good topic. Do you intentionally prefer ribs to spread out the loads to the spars? You can as well use foam for internal structure and two beams (C-sections). $\endgroup$ – Gürkan Çetin Jul 15 '17 at 14:07
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If you want to design something, the first step is to find out what the design needs to do - i.e. what are the forces which will act on the spar.

You can find the wing loading from the weight of the plane plus the maximum "g" force you want it to survive in flight (and in crash landings, while you are learning to fly it!)

You also need to think about the torsion forces on the wings - read about the difference in the chord position of the "center of pressure" and the "neutral axis" of the beam created by your wing spars.

Then, you can find the bending and torsional stresses in the spars, and calculate how big they need to be to carry those loads.

The "nice" thing about beams is that the stresses are statically determinate - i.e. changing the shape of the beam doesn't change the loading pattern (except for the change in weight of the wing, of course).

That should get you started answering your question. The lightest design will probably have spars that are tapered along their length - that may be more important than the exact layout of the "web" connecting the spars, for weight saving.

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  • $\begingroup$ Thank you for that advice, I will definitely read up on the centre of pressure and the neutral axis. In your fifth paragraph, I don't understand what statically determinate means, could you explain that in a bit more detail please? $\endgroup$ – AkThao Jul 11 '17 at 5:31
  • $\begingroup$ When you say 'shape of the beam doesn't change the loading pattern', does that mean the cross-sectional shape of the spars doesn't affect how much weight it can carry? $\endgroup$ – AkThao Jul 11 '17 at 5:33
  • $\begingroup$ Not quite. The shape of the beam doesn't change the shear force and bending moment at any position along the length of the beam, but it does change the way the stress varies over the beam cross section. It is the (maximum) stress that breaks things, not the total force. So the design process can be done in two separate steps: first find the forces and bending moments, and then find the smallest (or lightest) shape at each section that will withstand those forces without breaking. $\endgroup$ – alephzero Jul 11 '17 at 8:22
  • $\begingroup$ "Statically determinate" means the load distribution in the structure does not depend on the relative stiffness of different parts of the structure - i.e. you can calculate the load distribution, and then calculate the stresses and deflections, as two separate steps. That makes finding the effect of changes to the design very much simpler to do than in the general situation where "everything depends on everything else." $\endgroup$ – alephzero Jul 11 '17 at 8:38

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