#### Arfang

##### Well-Known Member

I have some difficulties understanding the way torsion is created by the wings and how to design the wing structure accordingly and I wondered if someone could shed some light on the subject.

- I found this formula posted by wsimpso1 on another thread to determine wing torque:
*dynamic pressure * moment coefficient * chord * projected area*. Probably a silly question but by ''projected area'' are we talking about the entire wing area? - I've read explanations describing a center of ''rigidity'' or center of torsion and the lift force resultant times the distance between the two being what creates torsion. Is that a viable method and what exactly is this center of ''rigidity''?
- One book I found says that skin along with the shear web is designed to carry torsion loads and is sized using Bredt's formula. Another source states that when using an airfoil for which the center of pressure doesn't move significantly with AOA, the spar alone could be enough, when properly designed, to resist torsion loads. Are there certain conditions that would make one choose one option over the other?
- If the spar is resisting all bending and torque loads, what is left to the rest of the structure? By that I mean if the skin isn't carrying torsion loads then what are you basing your calculations on?
- Also, if the wing creates torsion, should the wing attachment points at the fuselage be designed to resist that torsion?
- Let's now assume that I have a solid D-tube made in one piece with no separate skin, ribs or core, like an extrusion if you want, and that D-tube is designed to resist torsion loads. Would treating it as a beam in bending and torsion be the correct way to proceed?
- Following that, if I wanted to build and load-test a sample wing using different building methods, what method should I use to ''simulate'' both torsion and bending at the same time (can it be done with sandbags?) and what indicators aside from plastic deformation and cracks should I look for?

I realize now that's a lot of questions but I'm happy with whatever bit of information you want to share, I'm here to learn. Thank you in advance.