Is my plane nose heavy? Help with log analysis

Had a first successful flight with ArduPlane a few weeks ago, and attempting to figure out if my plane is a bit nose heavy.

It seems to me that Pitch is lagging behind DesPitch according to the graph from the logs, but not sure if the amount of lag is normal.

Looking at PIDP.I, it seems below 0, which is strange because the plane felt nose-heavy to me.

Full log: planes/2021-12-11 07-42-13.bin at main · yurigorokhov/planes · GitHub

Desired vs achieved pitch will not tell much of a story regarding CG, but elevator command may be an indicator.

You can see here that the elevator command centers around a positive pre-mix value, which I think indicates that the elevator is almost constantly commanded slightly up.

That alone does not necessarily mean that you have a CG problem, but it would account for the “nose heavy” feel you described. It seems to me that either you have an elevator trim mismatch or a CG issue. Perhaps a more astute log reviewer could determine which is truly the problem.

Additionally, your achieved pitch value seems to “hang out” below the desired value, indicating that an increase in the pitch I term is likely warranted. However, you probably ought to address the “nose heavy” root cause before further tuning.

I’ll leave any specific value recommendations to someone more well versed in ArduPlane.


Here’s another view

I agree with @Yuri_Rage that the elevator is spending all it’s time off center. This could mean that the mechanical trim is not correct, or that the plane’s C of G is not correct and the FC is trying to compensate.

Use the servo tab in Mission Planner to see where the elevator is positioned when the output value is about 1410-1420. Verify that is indeed holding the nose up (Forward c of G). If so, you could probably afford to move the weight back a bit. How much depends on the plane, but the plane is flying so it’s not going to be much. I usually work in 5mm increments when I’m tweaking the battery position.

If when you set the elevator to 1410-1420 and it looks like it’s in-line with the airfoil then it’s more likely the mechanical trim needs to be fixed.

Thanks a lot for the feedback. I have resolved the issue.

I had a disconnected airspeed sensor but had ARSPD_USE set to 1. Which resulted in the perceived airspeed to be very low, which in turn disabled SERVO_AUTO_TRIM and any autotune from happening. I was lucky to land the plane in 1 piece!

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