# Drag coefficient of an airplane control surface

1. Nov 8, 2013

### phys_student1

Hello,

I was reading somewhere about how to control Rudders in RC planes. The basic equation here is the drag equation: F=0.5*ρ*v^2*C*A (take ρ=1Kg/m3)

where v is the velocity, C is the Drag coefficient of the control surface (Rudder in this case) and A is the area.

I am wondering if we have a flat plate, what would its Drag coeff. be if its area is A and it is deflected θ° to either directions? According to Wikipedia, if it is parallel, C=0.005, if it is perpendicular, C~2. I am not sure how these numbers were calculated.

Last edited: Nov 8, 2013
2. Nov 11, 2013

### Ryoko

You could get a good estimate by looking at the specs of a thin symmetrical airfoil. For example, the NACA0010 airfoil has a fairly constant drag coefficient of around 0.006 for lower angles of attack ( < 8 degrees ). But there is one other drag source you may want to consider, namely induced drag. Induced drag could be equal or possibly greater than the drag of the base airfoil at 8 degrees since rudders tend to be low-aspect wings.

Two other points to think about. The first is that airfoil specs are generally given for high Reynold's numbers. The specs are often inaccurate for slow speed craft like models since the air is not moving fast enough to keep a good flow. The second is that your rudder drag is probably insignificant compared to the wing drag and the overall form drag of the plane. Also, you probably shouldn't be flying with 8 degrees of rudder (assuming this is not a multi-engine with an engine failure).