# How to calculate torque from wind pressure?

1. Aug 1, 2008

### burnit

Hi All,

I am trying to calculate the rotational torque created on a shaft from a rotating blade from wind pressure but have little idea how to go about this?

Eg:
If i have a flat piece of solid material of say 1" square or 2.54cm square facing into the wind blowing at say 30mph or 60kmh.
How can i calculate the wind pressure applied to the test piece?

Also the calculation to get torque created at the shaft from the wind pressure.

The pitch of the blade would seem to have to factor into this but as mentioned not sure how to go about this.
Could someone help me with some calculations for Shaft Torque from Wind Pressure.

Thank You

Last edited: Aug 1, 2008
2. Aug 2, 2008

### chaoseverlasting

Well, say the density of air is $$\rho$$, and its given velocity is $$v$$. Say the wind hits the fan at an angle $$\theta$$. Now I dont know how these things are usually done, but if we assume each molecule to be something akin to a ball and we assume that this is a simple case of collision between molecules and the fan, and the coefficient of restitution is e, we can calculate the effective energy transferred and the air pressure and all the rest of it. Im kinda running late right now but this shouldnt be too hard. Ill get back to you in a couple of hours.

3. Aug 4, 2008

### ice109

yes if you want to derive the pressure of a gas from zero that's how you do it, with a coefficient of restitution someone has already done all the hard work for you

http://www.knmi.nl/samenw/hydra/faq/press.html

says $P = \frac{1}{2}\rho \gamma v^2$

where $\rho$ is air density and $\gamma$ is shape factor.

then torque about is just $\int F \vdot r dr = \int P \dot A \vdot r dr = PA \int r dr$ and you can finish the rest