Rudder& elevator torque calculation

[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
pleasezzzzzzzzz helpppppppppppppp

 

[FredGarvin]

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?

 

[ank_gl]

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

 

[FredGarvin]

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.

 

[ank_gl]

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

 

[FredGarvin]

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.

 

[ank_gl]

thanx for the advice sir

 

[Aero Stud]

Just to add a bit, you're probably not going to turn your rudder and elevetor beyond like maybe 20-25 degrees for a standard configuration. So you multiply that force by the sine of the angle and for a standard airframe you can assume your aerodynamic center is at about the middle of the wing and about the height of the middle of the body and doesn't move that much. So yeah this should give you a pretty good estimate. And really needless to blame yourself, because believe me that calculating these things exactly - with the speed altered because of the engine and the wing altering the flow and many many other factors it gets VERY complex. Also a good idea is get some help from someone who builds these things already or search for info on similar designs on the web - there's a lot of it.