Wing severely oscillating in stabilize mode

Posted by Benjamin Harvey on August 10, 2014 at 6:53am

Hi, im wondering if anyone can help. 

When flying my wing in fairly windy conditions my wing will occasionally start oscillating in the pitch axis, it starts gentle but then becomes progressively more violent. To escape the problem i return to manual wait a few seconds then i can switch back to stabilize. 

From what ive worked out it primarily occurs when heading into the wind.

Im using the standard PID's as they seem to work fine for rtl, if i lower them the plane navigates poorly.

Im using an apm 2.5. non 3dr. 

Apart from this issue ive been very pleased with the APM and it has already saved my bacon on more than one occasion.

Thanks in advance, Ben.example, of what happens

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Replies

  • Glenn Spacht August 24, 2014 at 3:30pm
    What you are seeing is called aeroservoelastic instability. It occurs when the aerodynamic forces from an oscillating control system exceed the damping forces inherent in the structure. In other words, the controller and servos are shaking the airplane and there isn't sufficient structural stiffness to dampen the shake. The stiffness is essentially constant with airspeed but the aerodynamic forces increase as the square of the the airspeed. Therefore, it doesn't oscillate at low airspeed but oscillates at higher airspeeds. Turn down the p gains for pitch attitude, pitch rate and roll,rate by 10 percent. If that doesn't solve the problem, dial the gains down another 10 percent.
  • Moderator
    Graham Dyer August 11, 2014 at 1:01am

    Sounds like the "zagi-flap", on flying wings with insufficient stiffness the wing tips remain stationary longitudinally while the center portion oscillates up and down usually at speed or when over powered. Will destroy the airframe on occasion as I found out a few years ago. Possibly acerbated by your KFM airfoil and depron construction.

    Stiffen the wing with a spar or multiple spars, eg carbon tube or even better some sort of I-beam arrangement (http://en.wikipedia.org/wiki/I-beam).

  • John Gladin August 10, 2014 at 10:47am

    I'm not up to speed on the current versions but I'll give a couple of thoughts on it based purely on my experience as an aero engineer.

    I noticed you were in a dive at the time.  How does your speed correlate to the problem?  If it does happen only at high airspeed it could suggest two things: either aileron flutter or a structural design issue.  Does your model have a swept forward 1/4-chord line or is it flexible in twist about the y-axis (span)?

    Secondly, if the previous idea doesn't seem to be it perhaps your gain(s) in the roll axis are a bit too high.  You could try backing them off a little.

    • Benjamin Harvey > John Gladin August 10, 2014 at 3:04pm
      The one in the video was a dive, could well be structural but will never know as it died shortly after. It's still happening though on a new wing, with less flex, during level flight at a fairly modest speed.

      I've a theory that I might have too sensitive throws, I fly with around 20-30% expo! perhaps I should dull down the throws! this should probably add a damping effect and give the apm more modulation.

      What do you think
    • John Gladin > Benjamin Harvey August 10, 2014 at 3:21pm

      Dial down those gains.  I bet that will do it. 

    • 100KM
      Hein du Plessis > John Gladin August 10, 2014 at 11:07pm

      Yep, also double check your C.G.

    • Moderator
      Gary Mortimer > Hein du Plessis August 10, 2014 at 11:31pm

      Looks like flutter at speed to me. Be very careful flying there in the week you will have aircraft looking up at you.

This reply was deleted.