[bobdbilder]

If you used the stepper motor you would probably want to calculate the wind force against the vent at its closed position. You would then need to calculate the torque required to hold that vent in the closed position. The bigger the area, the higher the force against the vent and the bigger the holding torque. You would then need to use a stepper motor with a suitable holding torque. A cheaper alternative to Stepper Motors are RC Servos. Biq torky omes have metal gears.

Here is some formulas you could use. Sorry, too lazy to retype this from my blog
The force on a wing is calculated as

F = 1/2 x ρ × v ² × Cd × A

Where

F = Wind Force in N

ρ = Density of Air (typically 1.146 kg/m² at 35ºC)

v = speed in m/s

Cd = Coefficient of drag

A = Surface area in m^2

One thing that we need to understand in this equation is that the Force required to overcome air resistance is heavily influenced by speed. Now we have to look at the surface area as one that is directly being influenced by the moving mass of air. Therefore, if the surface is at an angle to the air, one has to calculate this based on the degree. Based on an freely available educational information, we know that the drag coefficient Cd;

Cd = 1.28 × sin (a)

Where a is the angle of attack or inclination of the surface. Plus as in trigonometry, we know that the maximum surface area is one where the angle is at 45 degrees. Any larger, then the Cd would need a different number than 1.28. We also know that the surface area is a plank with the size of 715mm wide by 90mm long. Having an area of 0.06435 m²

Now we have the calculated force

F = 1/2 x ρ × v ² × Cd × A

= 1/2 (1.146) (200 x 10 / 36)² (1.28 sin (a) ) × A

= 2263.7 × sin (45) × A

= 1600.68 × 0.06435

= 103N

Or approximately 10.51kg

This shall be the design constraint for the servo motor. We need one that can at the very least, generate this much amount of force to be able to move the spoiler. The HITEC HS425BB has a stall torque of only 4.1kg/cm at the maximum power of 6V. The servo arm that fits our requirement based on location of the servo and distance to the control arm is 30mm (3cm). Therefore, the available force from this servo at that distance is 1.36kg or 13N. If we choose to use the same area of spoiler the capability of the spoiler is limited by

a) Speed i which it operates and

b) Angle of attack of the spoiler

We would need a servo that generates at least 31kg/cm or 431 oz in or multiple servos just to move against that amount of wind. Therefore, we have come to another fork in the road. Single or Multiple? Looks like this is going to be costly. This then brings me back to where I wanted to do this project. A variable angle of attack spoiler meant for reducing operational cost. It was for the purpose to make life a little bit easier for normal people. We would not expect them to be running at 200km/h.