Copter: twitch when entering RTL below RTL_ALT

[rmackay9]

Reported by Mihai on this thread:
http://diydrones.com/forum/topics/arducopter-3-1-released?commentId=705844%3AComment%3A1621561&xg_source=msg_com_forum

This issue is reported for AC3.1 so it may have been fixed for AC3.2 by introduction of "the onion".

[rmackay9]

I've tried to reproduce this in the simulator using the latest code but wasn't able to. That doesn't mean it's fixed, it may be just that I can't reproduce it.

In these simulations the vehicle was flying very slowly in a windless environment. The RTL_ALT was set to 50m. When RTL was engaged the vehicle's roll and pitch only changed by a couple of degrees. In the graph below the Desired Roll, Roll, Desired Pitch and Pitch are in the top graph, The 2nd graph just shows the Altitude in meters. There's no roll or pitch twitch in either case.

I think we need to retry this test with master or get more information on how to recreate it.

[rmackay9]

Leonard and I spent a fair bit of time looking for twitches when switching between modes. We found a bad twitch when RTL is engaged while the vehicle is moving but couldn't find any issues which depended upon whether the vehicle was above or below the RTL_ALT.

In any case, as we reviewed the RTL behaviour looking specifically for twitches and resolved them wherever we found them, I think this is fixed in AC3.2.

It'll be added to the to-be-tested list for the AC3.2-rc1.