Direct Shaft Coupling vs Stock Horn and Arms for Micro Servos
Almost every RC plane we’ve seen so far had servo linkages like this:
However, I connected the servo shafts directly to the control surfaces on my design. Let’s find out why, how, and what the difference is.
Stock Horn and Arms
Pros
Stock horns are easy to install on every model and common parts are available everywhere.
Cons
External parts generate turbulence and it means Extra Drag and Less Lift. Servos will not fit inside the wing or fins in tiny models. You may face backlash and jiggly connections. Especially when you connect them using hard wire rods. Vulnerability of the mechanism components outside of the wing and fuselage is an issue too. During a hard landing or a crash, your servos and connections can get damaged, or even the area of the internal structure on which the servo is installed.
Direct Shaft Coupling
Pros
Easier installation and uninstall and repairability. Best choice for modular designs. There are no exposed parts outside the plane. No effect on the aerodynamics. Precise connection and control.
Cons
Additional custom couplers are needed. If we ignore the tiny amount of friction in the horn and arms mechanism, there is no difference in the amount of torque transfer unless you change the ratio on the horn from 1:1 to something else.
DIY Coupler Design
I designed this part in SolidWorks and printed it using regular PLA. Then I cut the central shaft connection out from the stock horn and glued it inside the 3D printed coupler. So, you can put it on the servo and fix it there using the screw. For the other side, I used an M3 threaded insert and a bolt, which allows us to fix the control surface axis on the other side of the coupler. This modular 3D printed approach is similar to the DIY motorized camera slider where custom 3D printed parts replace stock components.
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