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# Rotor Blade Torsional Stiffness - Xfoil

## Rotor Blade Torsional Stiffness - Xfoil

(OP)
Greetings all,

The rotors I have been working on for the past little while seem to be suffering from torsional twist due to large Cm vs. very thin section.  The question I need to answer, though, is just -how much- are they twisting.

I've writen a Matlab program that calculates blade conditions at incremental radius stations and iteratively determines twist across the length of the blades.  Currently, I am simply using a flat-plate approximation for the polar moment of inertia of the airfoil cross-section, J, but I don't feel that this is accurate enough.

Can anyone suggest a good method for determining the torsional stiffness of a solid airfoil?

Additionally, some of you may be familiar with Mark Drela's excellent xFoil program for analysing 2d airfoils.  One feature of this program is a module that provides a structural analysis of the loaded airfoil.  This gives me a nice number for J but it doesn't seem to scale with airfoil dimensions - does anyone know how to correctly interpret the BEND function outputs of xfoil?

Thoughts?

-Paul

### RE: Rotor Blade Torsional Stiffness - Xfoil

The torsional section modulus of a closed ellipse is 0.196*length*thickness^2

That might be a better approximation for your airfoil. If the ellipse fits inside the airfoil I think it will be a conservative solution.

Otherwise you'll need to do some calculus, see Timoshenko (at a rough guess) for the method.

Cheers

Greg Locock

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