"FLY TO HERE" = OVAL yet LOITER_UNLIM = PERFECT CIRCLE

I am using APM 1 (firmware 2.33) in a HIL setup with X-Plane 10. Everything works well so far but one thing is strange...

When I set up a waypoint with the LOITER_UNLIM command the plane does as it should and settles into pretty much a perfect circle around the waypoint. However, when I use the FLY TO HERE feature (Flight Data tab, right-click on the map -> Fly To Here) the plane follows more of an oval (sharp turns at either end farther away from the center and almost straight sections closer to the center). Interestingly, when I switch to RTL mode the plane flies home and sometimes settles into a nice circle but other times does the oval. It does not randomly do a circle for one turn and an oval the next, if it is a circle it will do the circle forever and if it is an oval it will do the oval forever. Wether in RTL mode it does a circle or not may be connected with using the FLY TO HERE feature at some point before within the same session/mission.

Any ideas what the problem could be?

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Hello Daniel,

I have been running HIL with X-Plane 9 and I have seen the very same effect.  I have noticed that the oval tracks seem to coincide with altitude gains.  If I send a "Fly to Here" command, with a significantly higher altitude, I see the oval pattern.  Unfortunately, it doesn't happen consistently.  I'm still trying to identify exactly why this is happening.

This is a critical effect for me, as I have installed a side looking camera on the right side of my (yet to be flown)Skywalker.  My intent is to add code that will send an appropriate PWM to a video switch, chaning the video feed from front to side mounted camera when the aircraft has reached it's commanded "Fly to" point, to observe the area of interest.  With an oval shaped pattern, this technique wont work without pan/tilt, and I really dont want to add that kind of complexity/weight/power draw. 

I hope someone more knowledgeable chime sin here...

Hi Mike,

I don't see the altitude influence. When I took the screenshot (attached to the first post) I had the plane circle around waypoint 1 at altitude 100 for a while and then I used "Fly to here" to command the plane to the position marked "Guided Mode" at altitude 100 and yet I still got the oval flight path. It is almost like it does not use the same technique for circling in both cases.

I dont see it consistently. 

What bothers me most is that I never saw it prior to a week ago.  Every loiter/fly to execution prior was always circular.

I guess thats the nature of the beast here, the SW is constantly changing with very little indication of what is changing or why, uless you dig for it...

That is interesting that you never had the problem until recently. Now it makes sense that I couldn't find anything on it. I only set up the HIL last week and haven't been using anything other than Mission Planner 1.1.68 and 1.1.69 and Arduplane 2.33 with the HIL setup. I think I will try and reinstall an older version and see what I get.

I just tested it using the 2.28 firmware version but I get the same results. Which firmware were you running when you did not experience the oval flight pattern?

I get a similar thing happening with different modes, Xplane 10, APM Planner 1.1.75. When flying to a waypoint, the track is very windy. However when activating RTL (Set at same altitude as Waypoint). the plane flies in a dead straight line! I can only think that the PIDs are different depending on the type of mode the plane is in. Weird

try disableing crosstrack, in rtl, crosstrack isnt used,

That must be what it is, cheers Michael :-)

Thanks for the reply but I don't think that is what it is. I set the xtrack gain to zero and then tried again to LOITER_UNLIM around a waypoint which produced a perfect circle but RTL and FLY TO HERE did the same oval pattern as before. Also I do not quite understand why xtrack should make a difference. If the autopilot uses a perfectly fine working algorithm to LOITER_UNLIM around a waypoint (with or without xtrack) then why would it not use the same algorithm to circle around home when in RTL and to circle around the position that was clicked when FLY TO HERE was selected? Why would one scenario use xtrack and the other not, after all each scenario is just circling around a point?

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