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Quadplane altitude loss on transition


(Sriram Sami) #1

Hi there,

My quadplane keeps losing altitude during its transition to forward flight. It dips about 10 - 15 metres every time it transitions, and I can’t seem to figure out why.

The quadplane frame is physically able to maintain altitude during a QHOVER --> FBWA transition if I hold some up elevator and keep the throttle at 100%. However, the aircraft gets a commanded -5 degrees of pitch and also reduces forward throttle during the transition which causes the aircraft to dive towards the ground before being able to pull up.

Things I’ve tried

  • TECS_SPDWGT from 1 to 0.7 to 0.5 (favour altitude control with pitch)
  • ARSPD_FBW_MAX from 23 -> 30 to make sure throttle doesn’t cut off before reaching max airspeed
  • TRIM_THROTTLE from 17 -> 20 to hold a higher airspeed during transition - worse altitude drop.
  • Increase Q_TRANSITION_MS from 5000 ms to 15000 ms, and from 15000 ms to 2000 ms. Both almost resulted in a crash as the aircraft dived harder.

I’ve attached a log with two flights demonstrating the issue:

Any idea what causes this?

Thanks :slight_smile:

(Liang Tang) #2

you can decrease Q_TILT_RATE_DN,

(Sriram Sami) #3

I believe Q_TILT_RATE_DN is for tiltrotor aircraft? I’m using a quadplane.

(Greg Covey) #4


The drop in altitude may be the orientation between the quad motors and the plane’s frame (angle of attack). Can you post some photos of your setup?

(cala2) #5

What angle you used in your x79 Greg? Thank’s

(Greg Covey) #6

I haven’t flown the FX-79 yet but we learned from Sam on the mini Talon project that it is good to have a forward pitch on the quad motors of about a 5 degrees.

(Sriram Sami) #7

Hi Greg, the quad motors are more or less in line with the plane’s frame. Did you get the same altitude drops in the mini Talon?

New photo by Sriram Sami

(Greg Covey) #8


No, the mini Talon transitions without altitude drop. It looks like you are using a Durafly Tundra. The extra weight of the quad assembly may be loading it down until it gets sufficient speed. If your Q_ASSIST_SPEED is still set to 0, try changing it to 10 or 12. This is the speed (in m/s) below which the quad motors will provide stability and lift assistance in fixed wing modes. Zero means no assistance except during transition.

If the plane transitions without altitude loss but the quad motors never go off then decrease Q_ASSIST_SPEED by 1 or 2. If the quad motors do go off but the transition still has altitude loss then increase Q_ASSIST_SPEED by one or two.

(Sriram Sami) #9

Hi Greg,

Yes, that’s a Durafly Tundra! Thanks, I’ll try changing Q_ASSIST_SPEED the next time I fly, but I’m mainly concerned about the current numbers not making sense: the altitude drops I’m seeing occur beyond the aircraft’s stall speed (13 m/s in flight with the quad assembly installed) during the transition.

It seems to be a combination of a negative commanded pitch and the forward throttle being reduced at the same time. I’ve increased ARSPD_FBW_MIN to bump up the speed at which the transition is considered complete, but I still saw similar altitude loss. I’m not sure whether the aircraft is just not responding to altitude errors fast enough.

(Rolf) #10

I would try to reduce Q_TRANSITION_MS perhaps to 0.
In tiltrotor VTOLs, Q_TRANISITION_MS > 0 only causes the set time to pass once the transition speed has already been reached, until you have full authority over the throttle output. But be aware, just my suspicions from experiences with tiltrotors.

Regards Rolf

(Sriram Sami) #11

Hi Rolf,

In quadplanes, Q_TRANSITION_MS is the time over which the quad motors will reduce their output once the transition airspeed is reached. I believe with a Q_TRANSITION_MS of 0, once the transition airspeed is reached, the quad motors will immediately cut off. I’m not sure whether that will work out, but I can also try this during my next test.


(cala2) #12

Your batts y/o power system is ok for high consumption transition?

(Sriram Sami) #13

Yes, probably just one takeover and landing though, especially if Q_ASSIST is on, and/or Q_TRANSITION_MS Is long.

(cala2) #14

I disable q assist for first flights, be attend to the switch.