# Rudder& elevator torque calculation

• ank_gl

#### ank_gl

hi, i m currently working on a radio controlled miniature aircraft for a local competition,,,,,,but i don't know how to calculate the torque required for the rudder & elevator motors.fuselage is about 200cms.airflow is abt 8-10m/s

please tell me what else is required for the calculations & also what type of motors to use

What school level are you at? Are you familiar with continuity and momentum in a fluid stream? Do you understand the concept of moments about an axis?

It's policy to not just spoon feed you answers to questions. You have to do some work. So let's start with...have you drawn a free body diagram?

i have attached the free body diagram...
airflow produces lift on the elevator...we get anticlockwise moment about the aerodynamic center and nose pitces down...all i want to know is how do i calculate the torque which is experienced at the hinge point coz i don't know which area to take in formula
FL=1/2*ρ*V2*CL*Area
and at what point does this force acts?

also doesn't the aerodynamic centre change when the plane pitches up or down... as far as i know, it should & then how should i take it into account?

sorry for asking such silly questions

also pleasezzzzzzzz don't yell at me...i don't have anyone to help me

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I would spend my time on what would be the worst case scenario. Making some simplifying assumptions, that would be the elevator is a flat plate almost perpendicular to the flow. In this case the force developed by the form drag would be simply

$$F=\frac{1}{2}\rho A V^2$$

Since the plate is perpendicular to the flow, the center of pressure should be at the geometric center of the elevator. That will allow you to calculate the maximum torque that the aero forces are applying to the elevator and thus the max torque your servo/linkage system must provide to move it.

thanks man...
i almost can't believe that i m so stupid

Hey. Don't think like that. It took me a little bit to sit down and think about it. The important thing is that it makes sense to you as to why I suggested that method. There are other, more elaborate ways, to do it, but why not try to break it down to the easiest scenario if you can? Usually, this will lead you to a very conservative number. You can then decide if it's worth your time to make more detailed calculations. That, in many cases, is a tough skill to develop for an engineer. When can you make simplifying assumptions that will still remain valid for your situation? When should I deviate from those simplifications? It takes practice.

Good luck.