FC Mount Geometry & Options

My traditional Heli has a rotor shaft with a 5° forward cant as part of the design. Unfortunately the flight controller mounting plate does NOT have a matching 5° forward cant.

Should I mount my Pixracer flight controller with shims to match the rotor mast cant or can the 5° difference be adjusted out or compensated for via software? (I also plan to mount the FC upside facing downward due to fit and access considerations).

Per Chris Olsen’s video 2 on set up, I know i must shim the Heli skids so that the main bearing block is level during QGC setup and programming. What about the FC?

  • I will make sure the FC is level (albeit upside down) during initial calibration but after that, can I initialize for take off with the FC not level or will I have to shim the Heli for each initialization before every take off?

Thanks to all.

You don’t have to shim the controller. Use the AHRS trim to level it so it doesn’t drift in the self-leveling modes like Stabilize and Alt Hold. I’m pretty sure the AHRS trims can do 5 deg. It goes to .1745 radian, which is 10 deg.

For Acro it won’t make any difference. If you want to know what actual frame attitude is in acro (for maybe like FPV flying) level the AHRS with the frame and it hovers 5 deg nose-up. With this setup it also won’t make any difference in the GPS position holding modes or auto. The self-leveling modes are the only ones it affects and since I don’t use those this is the method I use. I also don’t use any hover roll trim due to my setup method. Mine are tilted ahead by 2.5-3 degrees for better aerodynamics in cruise, the AI shows it hovers about 3 deg nose up and 2-3 degrees skid low like any full-size heli does.

@1piece it makes a difference if you have an articulated head vs rigid too. My helicopters have articulated head so the frame attitude is only loosely tied to the TPP of the rotor. Self-leveling modes don’t work all that great with articulated heads. It chases it like a student pilot learning to hover. That’s why I use acro for manual flying.

Loiter don’t work all that great either with articulated heads. But auto works fine.

Chris, thank you for the speedy and very informative answer. Your knowledge, helpfulness and dedication cannot be overstated. Thanks for refreshing me with the artificial horizon settings for my situation.

The heli I am using… I guess, has a “semi-rigid“ rotorhead. It’s a typical mid-grade flybarred sport 450 with traditional dampers inside the blade grips. No specialized articulating hinges whatsoever.

All the best,

On that one you can level the horizon to the rotor with the AHRS and it will work fine. There is not enough frame mass and cyclic authority to cause the two to diverge significantly on that size heli.

On larger ones where the rotor’s TPP can change in a range of 9-10 degrees from the frame it’s a little more critical. The usual recommendation is to put stiff dampers in the head, but rigid-head rotors don’t run smooth for UAV.