The importance of control horn placement

When building a RC airplane, attention to detail pays off with a more stable and predictable airplane.  Placement of control horns while seemingly not a critical assembly, I've learned that it is important to line up the control horn hole with the stabilizer - elevator hinge line.

 

To prove the importance of placing the hole in the control horn directly above the hinge line I created some construction drawings and ran several tests.

I mocked up two stabilizers and elevators with control horns, push rods and a servo driven by a servo tester.  This  photo shows the setup.  

 

 

The first stabilizer and elevator has the control horn hold directly above the hinge line of the elevator.  In the second setup I moved the hole of the control horn back 10mm.  The center of the control horn hole  was 7.6 mm above the surface of the elevator.  The resulting triangle is 12.5, 7.6 and 10 mm.  The 12.5mm is important as that’s the new radius of the arc that will describe the path of the elevator.

I took two measurements with the elevator moved up and two with it moved down.  As you can see from the 5 horizontally evenly spaced servo movements (1 through 5), the resulting elevator deflections are not proportional with the even horiztontal servo movements.  This is because the arc  that describes the elevator deflection is off-set by the 10 mm the control horn hole is offset.

Does that make sense?

DaveM