OK.. I think possibly there is a misunderstanding of how servo ratings work and what they mean.

People see a servo rated at 60 ounce inches and think that's it.. 60 ounces.. Unfortunately that's not the complete picture. A torque rating is actually a combination of 2 things.. a weight, and a distance. That rating means the servo using a 1" arm, can lift 60 ounces without stalling. If you increase this servo arm to 2" long, you reduce the lifting ability of the servo by 1/2 to 30 ounces. This is why your complete pushrod geometry is so important.

Let's take the Funtana 90 aileron as an example and I'll show you some things that might startle you;

Base Values:
Aileron: 5.75" root, 3.5" tip, 28.5" span = 263 square inches of area.
Speed: 40 mph. Now reality is once you start to roll, the drag will rise and the speed will drop, but let's just say for example the speed stays the same.
Servo arm and control horn in the same plane of rotation.

With a perfect 1:1 pushrod geometry, where 35 degrees of servo movement results in 35 degrees of surface movement, the force required to move the aileron to full deflection at this speed is roughly 60 ounce inches.

If I now shorted my servo arm to .75 inches, increase my ATV's so I get 50 degrees of travel either direction from neutral, the servo moves a full 50 degrees, the surface moves the same 35 degrees, but the servo is only required to generate 37 ounce inches of torque.

Now let's go the other direction. If I lengthen the servo arm to 1.25", the servo will only move 27 degrees for the surface to move 35 degrees, but the force required jumps to just over 87 ounce inches of torque.

OK.. now the Funtana is setup slightly different since the servo rotates perpendicular to the axis of rotation of the aileron control horn. So the calculations change a bit. If we use the same references as above, same values, and so forth.. here's the numbers that come up assuming your pushrods are pefectly oriented in the neutral position.


Arm:Horn - Power Required
1":1.125" setup - 50 ounce inches required.
.75:1.125" setup - 35 ounce inches required.
1.25:1.125" setup - 75 ounce inches required.

So this again illustrates how important mechanical advantage is for simply moving the surfaces let alone combating flutter.

Did this make sense or did I totally confuse you guys?

EDIT: Updated 2nd set of numbers to reflect actual Funtana 90 setups using the bottom control horn position of 1 1/8" distance from hinge-line.