Hi,

I'm trying to stabilize an airplane in pitch with CG deliberately placed well aft of the aircraft neutral point, but I want control over the angle of attack with the elevator control like a stable airplane. I don't want autopilot functions leveling the airplane, or controlling the airspeed.

I have been through the past discussions in the DIY forums, and on RCGroups, Charles River, and Steve Morris's electronically stabilized plank. The Steve Morris plank approach is basically what I would like to do, but using as much off the shelf hardware and code as I can:

http://www.b2streamlines.com/Morris.html

I would like to use Mark Drela's suggested approach to measuring the angle of attack, rather than a vane system:

http://www.rcgroups.com/forums/showpost.php?p=4139374&postcount=2

This will require two differential pressure sensors, the code to perform the math, and code to allow control over the AoA with the elevator stick. Perhaps the existing code that ArduPlane can use to maintain an airspeed can be modified to maintain an AoA? The selected AoA must respond to the elevator commands. I'm also not entirely clear what happens at stall...

I have flown an airplane with an unstable CG position with just a rate gyro on pitch to damp out the divergence. It is easily flyable as long as the speed is far above stall. At lower speeds, the pitch control deteriorates, and the stall gets aggravated as the gyro attempts to stop the nose from falling.

I haven't written any program code for 25 years (Fortran!), so this is going to take awhile to come up to speed on. I suppose what I would like is an indication from the ArduPilot experts on whether this is possible with the existing hardware, and how big of a challenge are the modifications needed?

Thanks,

Kevin Caldwell

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Replies

  • Why is there a need for external sensors? Won't the IMU be sufficient for stabilzation?

    If it can stabilize a stable aircraft why shouldn't it be able to stabilize a (moderately) unstable aircraft.

    I think you should try the stock APM first.

  • I don't follow what you're saying.

    If the actual CG is behind the design CG you have an unstable plane.  This is exactly when you would want autopilot stabilization, otherwise you're not going to be able to fly it.

    If you want to move the CG back then use a lifting tail.  This makes it more unstable, but that's the reason to use an autopilot stabilization system.

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