Quadplane altitude loss on transition

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: https://drive.google.com/open?id=1_AfBhg7mkQOlIf3N8l1t0K1C66eEG63L

Any idea what causes this?

Thanks :slight_smile:

you can decrease Q_TILT_RATE_DN,

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


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?

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

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.

1 Like

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


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.

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.

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

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.


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

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

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

Hi all thanks for the help so far; as a member of Sriram’s avionics team, I have been trying to help him debug this problem for the past few days. So far I have been trying to run some simulation flights on SITL in an attempt to find out what causes the altitude drop, and have made the following observations:

  1. With default parameters (and with quadplane mode enabled), the quadplane on SITL has no observable altitude drop during transition, but experiences a negative commanded pitch of -3 to -5 degrees similar to what we observed for our Durafly Tundra.

  2. There is a throttle drop observed during transition, after transition airspeed has been reached.

This suggests that the negative pitch and throttle drop are normal occurences in plane 3.8.4 firmware, but it has somehow become magnified in our case as a significant altitude drop. I have opened a new thread to discuss this issue:
Why does quadplane experience a negative pitch during transition?

Any help would be greatly appreciated!

Full disclosure: I’m not a coder/programmer and I use PX4 Pro stack not ArduPilot but I’m always interested in anything VTOL development. I hope it sparks some thought to look where to implement the fix. Your issues were quite similar to what I went through during my setup and configuration of a Ranger EX 757-3 conversion. Actually I almost lost it during the maiden due to the aggressive altitude loss. I’m not familiar at all with ArduPilot’s terminology so it’s possible it may be a repeat of what someone had already said here but I’ll leave it up to you to decipher.

The following were the issues and diagnosis:

  • The pitch down command was a reaction to what was perceived as a stall condition.
  • The throttle drop was the difference between the transition and cruise speed throttle settings.

The corrective actions:

  • Flight controller level horizon was recalibrated to quad motor booms. This gave the wing a positive angle of attack during the transition and to presumably add lift as the airspeed built up to maintain level flight.
  • The CG ended up being moved 15mm farther back than the OEM specs.
  • The transition airspeed was originally set at calculated stall speed plus 5 mph and was changed to be equivalent with the cruise speed. That worked out to be about plus 10 mph.
  • The minimum transition time was changed from two to four seconds.
  • The transition throttle setting was changed to 100% instead of cruise setting. This made it build up airspeed quicker to achieve target speeds.

From this last flight, it still experienced a little bit of a dip but I think that’s caused by the wings twisting during the transition as the quad motors were compensating for the torque induced by the prop. I’ve added guy wires to it since similar to what @GregCovey did with his. I’ve been lazy to go flying so I haven’t had a chance to test the mod. Anyway, hopefully there’s something here that can help you resolve your issues.

Good luck.

@rollys many thanks for the insightful info! We’ll take into account your suggestions for our next flight (:

Edit: Do you also have the logs from the flights which you have mentioned above? Would appreciate if I could see them!

The maiden flight is in PX4 Logmuncher.

The last flight is in PX4 Flight Review.

Alright thanks! Also, the flight videos look great!

Hi all, checking in again with updates to the debug process:

Based on @rollys’s suggestions, we have narrowed down the following parameters that may help in reducing altitude drop:

  • TRIM_ARSPD_CM: Airspeed which the autopilot aims for in auto flight. This should be set to the desired cruise speed
    • Preliminary runs in SITL (bin file attached) also show that increasing TRIM_ARSPD_CM results in a higher throttle output during transition. This is desired, as it will help build up airspeed quickly to reach the required cruise speed.
  • ARSPD_FBW_MIN: Minimum airspeed in FBWA mode, which also determines the transition speed after which the autopilot drops throttle in auto mode. This parameter was recommended to be set to equal or slightly lower than the cruise speed.
  • Q_TRAN_PIT_MAX: Maximum pitch during transition to fixed-wing flight. Will reducing the value of this parameter help to reduce the amount of negative pitch experienced during transition?
    • A test run in SITL (see bin. file over here) shows that when this parameter is set to 0, the amount of negative pitch decreases from -5 to -2. This is a desired outcome, but why is there still -2 pitch when the limit is set to zero? What else is over-riding the parameter to generate negative pitch?

We will look at how these parameters affect the quadplane’s performance for our next test flight. In the meantime, if anyone else out there has experienced the same altitude drop problem and has been able to solve the issue (thanks again to rollys for sharing!), we would really appreciate if you could share with us your debug process!