Expert: Mark Janus - 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
It's  difficult to visualize the orientation of your design, but wind air dynamic pressure is equal to 1/2 air density times the velocity (in ft/second) squared.  So if you multiply this by the amount of area the wind acts over (i.e. the size of your sail or capture area) you can get an estimate of the force the wind will produce on the forward face of your sail (assuming the air is also "stopped" on the back of the sail).
Some sails are designed to simply "catch" the wind, those are for downwind "sailing" others are like airfoils (or wings) which are actually designed to move at an angle to the wind to produce the force that propels the vessel forward.

Hope this helps somewhat,
mj