[NSRCA-discussion] NSRCA-discussion Digest, Vol 49, Issue 28

[Norman Ellis]

Phenolic horns are easy to make with a bandsaw or jigsaw and drill press. Make them to the length your throws require, so with full ATV/EPA you get the highest deflection you will want, given the length of your servo arm. I typically run wing and elevator servos off a wheel rather than an arm. A short session on the drafting board (I'm old fashioned) and you can work out the lengths, assuming you know what throws you want.
 
Phenolic cuts easily with a bandsaw, drills easily with drill press. I cut four blanks slightly oversized, drill them ganged as one block of four, block them up tight together with 4-40 machine screws (just two is all it takes) then cut to final size and drill the hole for the ball link while they're still blocked together. That makes four identical pieces. Two pieces epoxied together with end-grain balsa for a spacer make one double truss horn.
 
I shape the base of the horns identical to the cross section of the control surface. I use the hinge-point as one reference to align the horn with the surface when I epoxy the horn in. That makes a pair of ailerons or elevators with horns identically positioned relative to the hinge line.
 
Phenolic horns save considerable weight compared to steel screws or threaded rods. The finished product is light, strong, rigid, precise, durable, efficient and inexpensive (discounting labor). Aircraft Spruce has phenolic sheet cheap. I use the 1/16" thickness. Choosing your own distance from connection point to hinge-line gives you maximum leverage out of the servo torque, plus maximum resolution.
 
If you find you have not as much throw as you need (underestimated required throw) you can correct easily with a longer servo arm.



 


      
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