Expert: Paul Soderman - 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
Sheila - It is not clear from your description if your sails act more like parachutes or sailboat sails. If they act like parachutes, it is a simple matter to calculate the drag:  
D =Cd * rho * v^2 * S / 2

where D is drag in pounds
rho = air density = 0.002377 lb sec^2/ft^4 at sea level
v = wind speed, ft/sec
S = cross sectional area of the chute (perpendicular to wind), ft^2
Cd = drag coefficient ( 1 to 2), I would use 1.2

At the following website you can calculate parachute performance for a falling body.  Compare the terminal velocity with the parachute drag (in Newtons). The terminal velocity is analogous to your wind speed: http://mvhs.mbhs.edu/~brgo/drag/index.php

The aerodynamics of a sailboat sail is more complex because you have lift and drag on a sail moving at angle of attack.  The forces can be greater than in the case of a simple drag chute.  I would need more information on the geometry and flight conditions to calculate the forces.