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Pitch increases when switched to Alt Hold on bench

My Align 450 FBL is running APM 2.5 with Copter fw v3.2 which is the last version supporting APM2.5.

On bench, when I switch to Alt Hold, the CP increase to over 12 deg. Is this NOT acceptable ?
Is the CP supposed to remain the same as when mode is switched to Alt Hold ?

I spent a week to try various setting on H_COL_Max (and Mid, Min) without success. The setting of 2, 3 and 4 for H_SV_MAN do not work except 0(Disable) and 1(passthrough) for setting the collective pitches.

I have no problem setting the CP in another ALign 450 (a flybar) running Copter 4.0.5 and managed to have the CP REMAIN THE SAME when switched to Alt Hold.

Can anyone please help to walk me through the proper procedure for the outdated FW Copter 3.2.

Thank you.

I never used Arducopter on a trad heli, but no flightmode will work as expected on the bench. The PIDs will try to correct small errors while the vehicle is not moving and therefor not able to follow the commands. Especially the I-term might do what you are describing.
And do you really want to risk your heli with hard- and firmware that old and unsupported?

This can be a normal behavior. It depends on your collective position on the TX. There is a dead zone around the middle position, but if your stick is higher, then the helicopter will try to achieve the commanded climb rate – which is not possible, because it is sitting on the bench. So the autopilot will increase collective pitch as far as possible, which is the value you entered for H_COL_MAX. If you lower the collective, the swash plate should stop and then start going down.

But I also agree with Sebastian @count74 : You shouldn’t use old hardware like the APM 2.5. Most of the newer controllers have sophisticated vibration isolation methods. Getting the vibrations to a low level is crucial for traditional helicopters. And they can run the latest firmware. There have been so many improvements since v3.2. You will get a way better flight behavior and avoid many problems. Not only in flight, but also with the setup. So I’d really recommend using newer hardware.


Hi Felix,

Thank you for your advice. I agree with you that newer software can make life easier during set up.

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