D100 + AP74 on pusher aircraft

[matt.hocker]

Hi all. I have a D100/D120/AP74 setup on my SeaMax M22 which is a "pusher" aircraft. I have a question about how to configure or possibly modify the autopilot system.

Since the propellor is facing rearwards, the elevator and rudder surfaces on the tailplane are directly in the prop wash and therefore have significantly more authority than the ailerons. This means that when you turn this aircraft, you typically lead with the rudder then make a small adjustment with the ailerons to coordinate.

My AP servos are installed on the ailerons and the elevator. However when I attempt a turn using the autopilot, the plane sort of slips because the rudder is not being used. If you try to hold a heading, the plane eventually just ends up in an uncoordinated slip as well, making the AP basically useless other than to hold an altitude, which still requires my hand on the stick, so it's not really useful and I never turn it on.

I'm the second owner of the plane and I suspect that the original owner never got this system working properly either, but after 5 years of ownership I'd like to see whether it's possible to get this working or not.

Does anyone have any ideas on how to configure it or possibly modify how the servos are attached?

Thanks.

 

[Dynon]

This doesn't help you, but for those reading along, SkyView has yaw-damping capability which can deal with this inherently.

Unfortunately, for D100 series based autopilots, there is only one possible solution that some have explored, and that is to use the roll servo to drive the rudder instead of the ailerons. I wouldn't call it a Dynon-endorsed solution, but we've heard from customers that have successfully done this on SeaRays. I'm not sure about the SeaMax. With the Searay, applying rudder both yaws in the correct direction, and also causes enough corresponding roll through its inherent aerodynamic qualities, to yield acceptable performance for some. Your milage may vary.

 

[led]

Do you have TURN RATE set to zero in the roll menu?  That may help. 

Also, as a matter of curiosity... what happens if you keep the rudder centered with your feet during a turn?  (Don't have to keep the ball centered; just keep the rudder from moving.)

All airplanes lose some stability when the controls are free to wiggle in the slipstream, and your description suggests that the AP is struggling with this.  It banks and expects the plane to 'weathervane' into the turn.  If the rudder is drifting the other way, the turn rate may be slow or even negative (!)

Lawrence Doan
Senior Aeronautical/Autopilot Engineer