Aileron differential with one servo?

[Janusz Jawien]

I bet that this question was answered many times on the forum but somehow I am not able to find an info.
Please explain how to create aileron diferencial using one servo that will
work with arms on the bottom of the wing.
A diagram will be very helpful...
Janusz

[BMatthews]

Here's a sketch I did a while back for someone that asked the same sort of thing. I think from the notes it was intended to show how to AVOID differential by using the proper control horn and servo arm orientation but if you read the bad parts as being good you'll see how to do it to get the differential.

As you can see it's all in how the arcs of servo or control horn or other intermediary bellcranks are used to convert rotary to linear or back again.

[MarkusN]

I...that will
work with arms on the bottom of the wing...
More specifically concerning your setup: You want the connectiong points of the pushrod in front of the servo axle and behind the aileron hinge.

Make a sketch of the extreme positions of your setup first. It's very easy to inadvertedly create a knuckle lever mechanism when overdoing mechanical differentiation. (i.e having the servo run against a fully stretched linkage.)

[Janusz Jawien]

Thank you so much! I finally understand the concept! Now back to the building board....
Janusz

[BMatthews]

Mark brings up a good point. In the second control horn illustration notice that the arc of the hole travel does not cross the right most vertical line when the pushrod is pushing. That means that the control horn will be pushed back until the hinge and control horn hole with the pushrod are in line and can't go back any further and the servo will still not be at its limit which will result in the servo stalling out or the pushrod bending out of line. A very bad situation.