> I would suggest to use airspeed as input to a PID controller, your
> desired airspeed as reference and the elevator control surface as output.
> I haven't tried this, so good luck!
Works like a charm :-) I got it stable with: Kp=0.0125, Ti=30.0, Td=0.0 with the Piper. You can start from there, and I'm sure that fine tuning will get better performance.
Thanks Roy, it works better now, but still not enough to prevent crashing. It looks like I'll still need an external monitor to clamp the pitch angle. For instance, if I'm doing an idle descent at 250 kts and reset the speed to 300, I need to keep the airplane from going straight down.
50 kts is quite a large change in reference, if you could change it gradually from 250 to 300, I'm sure that would be safer. It is not uncommon, in the process control world, to only let the reference change as a ramp. A gradual change at some appropriate rate, instead of a sudden change.
If you feel that you absolutely have to control the pitch angle, I would suggest that you go for a cascade configuration. One controller to control pitch angle using the elevator and one to control airspeed using the (controlled) pitch angle. Now you can easily limit the pitch angle.
-- Roy Vegard Ovesen
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