Control Surface Loads

In designing modifications to planes, I have found that the servos specified have significantly higher torque capacity than that needed for my flying. My Goldberg EX-TREME 330 3D recommended standard servos or about 40 oz.in. torque. My calculation using a servo calculator program indicates I need only 3 oz.in. torque for the elevator and my flying conditions. I am very comfortable flying with the 17 oz.in. torque servo currently installed. This way I save weight which is important in my Epowered models. I welcome any experience that would confirm or correct my findings.

Dick Dietz, AMA87885

It's true that most sport flying can get by with standard or lower powered servos.. the two factors to consider are flying speed, and surface area.
The smaller servos tend to have less sturdy internals.. gears tend to be made from congealed margarine, and can be stripped in flight with high loadings on the surfaces even at slower speeds.
Basically it's the users choice based on his experience as to how he flies his planes.
I have a lot of standard servos getting dusty now that I'm doing a lot of e-flight. Just don't need the extra size of the servo case.
And I've seen standard servos used in very large scale planes satisfactorily.
3D does require some consideration in servo choice. Small, ball-bearing and metal geared is probably optimum for the ailerons and elevators.
I see some very mauverable e-planes with very large surfaces fly very well with micro-servos.

Thanks Paul for the reply. Have you used any servo load calculators in your modeling?

dick

No, I haven't used a load calculator. I'm very much belt-and-suspenders when it comes to surface deflections. I will use a larger servo than the absolute minimum.
The only time I can recall seeing the effect of a servo overloaded by airloads was an HS-81 on a rudder on a 2M electric, which stripped in-flight right at the top of a 1000' cllimb, leaving me with only elevator to get the plane back. With good use of stalls and loops, I did that.
I had a small electric fun-fly which used dental floss for the pull-pulls. I found out dental floss stretches under load, at speed....
Nice loops at slow speed, flew into the ground wings level at high speed.

It's probably wise to use servos with at least 3 times the required torque. That not only prevents gear train failures like Paul's but also ensures the servo is not working at near it's stall current so the motor and driver circuitry isn't being overly strained and over heated by being asked to operate too close to it's limits.

Also did you do the calculations based on that 70 or 80 mph death dive?

Those calculators are great for getting a feel on what's needed but don't forget the limits under special circumstances.

Thanks Paul and Bruce.

I think I am conservative, but don't want to forget in panic mode I probably would call for full deflection of contro surfaces at high speeds. 2 to 3 times calculated load will still save me size and weight in servos.

Great feedback!

Dick