# 25kg delta UAV starts oscillating during descent

Hello

I made around 30 flights with a delta wing uav with a weight about 25kg. APM 2.5 fm 2.68

So the plane was tuned at about 30m/s in horizontal cruise flight.

So when i start descent from waypoint one to waypoint to

- distance between waypoint one around 1500m and a change of alt about 300m

the airspeed overshoots and the plane starts oscillating. Even when i turn the airspeed pitch PID to zero.

What i figured out is that the plane gets to fast during descent even when the engine is almost off.(Idle)

then the PID parameter don't fit for that speed.

also tried do change speed before descent to a higher speed during descent.

but no chance....

Why this question: Our uav is measuring weather datas on a pre programmed path.

for me the flight path has a very high priority, speed is actually not really important.

Does somebody has an idea to get away from this oscillation?

Maybe there is now airspeed to gain adjustment for the PIDs?

(in higher airspeeds less gain, slower airspeeds more gain)

Best regards

Manuel

Views: 883

### Replies to This Discussion

What do you mean by oscillating with respect to your aircraft?

If you mean roll or pitch oscillations then try reducing the corresponding Proportional gains a bit to make it less sensitive at higher air speeds.

Hey Tim

Thank you for your repley.

I did that already.

I saw in the code there is a airspeed scaler. but the gains should be corrected with differential airpressure ant not air speed. differential air pressure or even airspeedˆ2.

What are you thinking about?

Hi,

That's pretty interesting. So the effect of a control is (about) proportional to the square of the airspeed?

Try change it! I suppose you could find critical P values at 3 different airspeeds and that should give you all you need to calculate a scaling function. Some other ppl .have had to widen the scaling limits to something more than the [0.5 .. 2] they are now.

PS that is one big bird! Would love to see it.

Regards

Soren

We don't have much information about you UAV except that it must be big and heavy. The oscillation can just as well be a mechanical problem with wing flutter caused by excessive airspeed. If so, no amount of PID tuning is going to prevent this, and you where very lucky the plane stayed in one piece.

http://www.airspacemag.com/video/Wing-Flutter-on-a-747.html

i attached a picture here, DA-50 powered, parachute landing

Hi Manuel,

Your aircraft and launcher are superb! If you have a link to it being launched or flying, it would be greatly appreciated. Is it constructed of foam and, are those slick/slender wings causing your oscillations at high speed?

What's needed is gain scheduling. Attopilot had this years ago.

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