hex strange behaviour

[tonygoole]

Hi I have just been flying my hex (pixhawk) this was 4th test flight(12mph winds in loiter). Flying great for 5 minutes. Hex started to gain a little height so I cut the throttle to loose some height and it started yawing quite violently and pitching forward gave it more throttle and it kind of went back to normal. Dropped the throttle and it started yawing again really struggling to loose height tried a few times to land it but too uncontrollable.  I put it in to stabilise mode and landed it best I could.

Checked the hex over only thing I can say when I tried spin the motors they felt quite tight to spin and you could not get a full turn. 20minutes later they feel fine and spin properly very freely. Anyone any ideas?

[Digit]

Is that 20 mins with the battery plugged in or unplugged?
Just a thought, it could be the motors expanding when heating and then freely turning when given time to cool down

[DarkButterfly]

Motors feeling "tight" after flying is normal, there is still residual current in the windings, give them a few minutes and you';ll be able to turn them by hand freely.

As for the pitch and yaw issues, check your tuning also make sure the compass is calibrated and set correctly, it could also be electrical noise affecting it, check your connections, re locate the compass if necessary.

[DVM]

It';s normal. A motor also behaves as a generator, when there is no load on a generator (nothing connected) they spin freely. When a load is applied (wires shorted together for example) the generator is harder to turn.

After a flight the FETs in your ESC can stay turned on just a little because there is some charge left on the FET';s gate. This presents a load to the motor (generator) and makes them stiffer to turn.

After a while, in your case 20 minutes, the charge on the FET';s gate has dissipated, the motor doesn';t see  a load anymore and the motor spins freely

You can get the same effect by connecting all three wires of a motor together - it gets much harder to spin. Bruce (RCModelReviews) has a good video explaining this effect

Edit: Here it is

https://www.youtube.com/watch?v=RaCjJbEB078

[jar]

if it is stable when not descending then you may be getting whats called (i think) "vortex ring state". When you descend through your own prop wash some craft get quite unstable, and wobble around or lose even more lift than expected.

I have a hex that does this. Either limit the descent rate or put some forward motion on it while descending, which should help.

John

[Elmattbo]

Stuff on vortex ring state here. With these two videos this is probably one if the most informative threads out there!
http://youtu.be/LCret4rv0HE

[tonygoole]

Thanks for the replies guys. In further up north the th UK on holiday and Internet signal is a problem. I';ve watched the videos which is something to take on board

[tonygoole]

Tell you one other thing I did notice the motor mount that fastens on the front arm via a screw thru a hole (tarot 680 pro) had become a little loose and I had to turn it a little to make the prop sit parallel to the table. I wonder if that could cause the problem . I';m guessing so as it would not be running true.?????

[atomiclama]

How easy is it to get into this vortex thingie?

Just that I have never seen this before. Part of my testing and PID tuning is to drop the quad under minimum throttle and see how stable it is then to see how much wobble I get under full power to recover.

One thing though ESCs with COMP_PWM / braking are a lot more stable. Do your ESCs support braking?

[Elmattbo]

Don';t know about the ESC';s but the wobble you see is vortex ring sting(!). I think 250';s get out of it by having such great thrust to weight available that even with stalled blades just enough lift is created to arrest the descent and break out of the vortex. Try it with a big battery on and you might find it just keeps descending!