Servo torque requirements

[Aerobat]

This website was mentoned in the current issue of Model Aviation. It has a formula for calculating servo torque requirements, based on the size of the control surface, deflection, and aircraft speed.

The results are interesting - for IMAC type control surface deflections (i.e. 11 degrees on the elevator) not much torque is required. The numbers only get big for 3D deflections and high speeds (i.e. doing a wall maneuver).

This explains why my 40% Edge seems to fly lower-level IMAC sequences just fine with 111 oz. servos. Comments??

http://www.csd.net/~cgadd/eflight/calcs_servo.htm

[hpapilot]

Yeah... there's no way I'm getting real numbers out of that calculator. It says that for my QQ Yak, going 80 mph, I can get 35 degrees of aileron throw on less than 270 oz of torque. I actually have a total of over 500 oz of torque, and get blowback at far less than 80 mph, with less than 35 degrees of throw.

But, straight off the site:

Note the following assumptions:

1. The angle of incidence of the wing, stab, or fuse is zero (relative to the airflow).
2. Angular velocity and acceleration of the aircraft is zero.
3. Air flow may be modelled using Bernoulli's equation for dynamic pressure.
4. Conditions are: sea level, zero humidity, moderate (~55 F) temperature.
5. Control linkages have zero offset at hingeline and are perpendicular to horns at neutral.
6. Control mechanisms are frictionless and surfaces are mass-balanced.
7. The wing, stab, fuse, and control surfaces are thin, flat slabs.
8. No aerodynamic counterbalances are used. (Account for these manually, if desired.)
9. The pushrods are significantly longer than the servo and control horns.

Please note:

• The calculations are completely theoretical. No empirical "tweaking" has been done.
• The assumptions (except #6) should generally yield conservative (high) predicted torques.
• Extreme control throws are probably not practical at high speeds.
• This model is best used for comparisons. No guarantees are made of its validity.
• Maximum required servo torque may occur at LESS than maximum throw.

However, my wings are not flat slabs, and the ailerons are obviously not mass-counterbalanced. There are some real world factors not taken into account here.

[hpapilot]

It also says that my FF Gernot Yak 55 only needs 5.334 oz of torque TOTAL for the ailerons to get 55 degrees of aileron throw going 15 MPH. That just doesn't happen in the real world. 17 isn't enough, 22 seems to work ok.