How do i calculate the pressure that the airflow produces on the control surfaces when in flight?

If i assume that:

The rudder angle is 30degrees
The airspeed around 150km/h ( ~90mph)
The area of the control surface is ~ 75sqare inches.

I think it is possible to calculate with simple physics, but i just dont know how.
Any program for this?

Here's a program that computes the servo load...
http://members.cox.net/evdesign/page...ge_design.html
And the way the FAA does it in Part 23..
http://www.airweb.faa.gov/Regulatory...C?OpenDocument
And one for UAVs..
http://www.ncsa.uiuc.edu/EP/CSM/tmp/...cs/UAVCERT.pdf
.
Search on "control surface load"

The program is just an exel document. Anyone got it?

I thought we had some of those calculators in the sticky thread Aerodynamic Tools and Calculators. I did a google for "servo load calculator" and turned up 4 sites right away with online calculators for immediate use. The links have been added in a post in the sticky thread.

Just be sure you provide a worst case speed of what the model will be able to do in a screaming death dive. And to prevent loading the servo you should use the number given by the calculator for servo torque required and get ones that provide 1.5 to 2 times that torque calc so you know the servo won't be operating near it's stall zone.

Enjoy!

Thanks a lot! How fast can a 72" Yak 18 go, with a saito 100 in the nose? The airplane weights around 3-4kg (~8lb)

You can get a fair estimate by multiplying the pitch of the prop, by maximum rpm of the motor by about 110%.

For example, 15 cm pitch times 10000 rpm is 1500m/min or 90km/h times 1.1 = 99 km/h.

There are charts on the web for calculating this too.

I a dive the airplane will actually go over the pitch speed of the prop and wind up the engine even faster than it runs in level flight. At that point the prop is acting like an airbrake. Whether or not it's 10% over is up to the model design, weight, prop diameter and probably what you have for lunch but 10% doesn't sound too far off. Just to err on the side of being pessimistic call it 120 mph and calculate the servo load based on that then mulitply by 1.5 and go shopping for servos with at least that much torque.

As I suggested you do NOT want to be too close to the stallling point. THe power demand on the airborne battery is far higher than a stronger servo that is well within it's limits. And the stronger servo also means you'll retain the transit speed instead of a struggling longer time so the controls will react more consistently.