# Calcutaing force needed for a rudder

1. Jan 31, 2014

### grnbaja72

Calcutaing force needed for a "rudder"

Hello everyone, I joined today in an attempt to get some help with a "rudder" im trying to build for remote control purposes.

So my problem is that I am trying to build what can best be described as a control surface that will apply force to a vertical pole that at times can be very heavy (35-60lbs) and long (7'-40'). What I am trying to figure out is if it is even feasible in the sense that I am going to be limited by the amout of force I am able to apply by the equipment available for radio controlled airplanes.

Here is the best description I can give you of what I am trying to achieve.

If you picture a varying length vertical pole moving through the air at apprx 100knots. The bottom end of this pole is weighted to about 20lbs in order to maintain vertical presentation. Attached to this weighted end is a piece of plastic approximently 10" wide by 12" tall. This piece of plastic articulates around the pole with the use of servos. What i am trying to figure out is a formula or ability calculate the strength that these servos need to have in order to deflect air to the side of the vertical pole which then raises the weighted end of the pole as it flys through the air.

I have attached a pretty cruse picture that may help explain what im looking for. I know that there are going to be other measurements that are needed in order to figure it out but if someone is willing to help please let me know and i will get any info needed.

Last edited: Jan 31, 2014
2. Feb 2, 2014

### strive

Presuming the rudder is just a flat plate (not an airfoil) you can presume that the force exerted on it is:

F = (0.5 * air_density * air_velocity˄2) * surface_area * sind(deflection)

Moment around the hinge is:
M = F * r
(r is the distance from the hinge to the center of the surface area)

If the servo is attached directly to the hinge it must produce torque higher than M (because this calculation is only an approximation).
Otherwise servo torque is:
Mt = M * rs/rr
(rr is the length of the lever attached to the hinge, and rs is the length of the lever attached to the servo)
Again the servo must produce torque higher than M.