Expert: Ray Wilkinson - 4/26/2007

Question
hi there!

im writing a technical report on the design of some wind sails which I hope will catch the wind only from one direction to then expand like a sail and create a sort of shield or deflection.

At the moment the general design is a set of lightweight wooden panels, connected inbetween by a textile cloth and which are then threaded through a metal wire cable to slide away from eachother.

i need it to be sensistive to strong prevailing winds and not just random gusts so I need to find out how much weight can be lifted by different wind speeds/ or the amount of force which windspeeds have. (The wind speeds I want them to react to could eithet be over 6 knots or also 30 knots , whichever proves more practical)

This way I hope to calculate the proper materials and sizes for a realistic weight for the sail.
Ive only just started learning about wind to design my project, but physics is not my best subject, so I would greatly appreciate if you had any suggestions such as wind aerodynamic shapes or other things to bear in mind, or if you think its likely to work. thanks very much!

Answer
Hi

I'm not too sure I understand exactly the geometry of your device, but I can answer your general query so you have something to work with.

The lift force generated by any aerodynamic surface or object (such as a wing) is given by L=1/2 rho V^2 S CL, where

rho is the air density (1018 kg/m^3 at sea level)
S is the relevant surface area (wing area for a wing)
V is the air speed
CL is the lift coefficient at the relevant angle of attack.

The tricky bit is to determine the value of CL, which depends on the shape and the angle it presents to the air flow. Typically, a proper wing without flaps will have a CL that varies from 0.2 at zero angle of incidence to about 1.0 close to the stalling angle (about 17 degrees for most wings). As your device is effectively a thin curved plate, the best you would get might be around 0.5 at the best angle.

As you can see, it's not straightforward, but then you didn't really expect me to give a numerical answer did you? ;-) You might be able to pull some estimates from this, though. Really, it would need some wind-tunnel testing or heavy computation to simulate the real values.

Incidentally, drag is almost the same equation (as the force is similar in nature), but the CL becomes CD, and the values are different of course. In this case, the area would be the area presented by the cross-section. Your sails are effectively going to generate a mixture of lift and drag, the combination of which will provide the force you are going to use.

I hope this helps.

Regards

Ray