Assisted Fixed-Wing Flight

The QuadPlane code can also be configured to provide assistance to the fixed wing code in any flight mode except MANUAL or ACRO. VTOL motor assistance is enabled if Q_ASSIST_SPEED, Q_ASSIST_ALT , or Q_ASSIST_ANGLE are non-zero.

When Q_ASSIST_SPEED is non-zero then the quad motors will assist with both stability and lift whenever the airspeed drops below that threshold. This can be used to allow flying at very low speeds in FBWA mode for example, or for assisted automatic fixed wing takeoffs.


If you are not using an airspeed sensor, airspeed will be determined by the synthetic airspeed generated internally as a backup in case of airspeed sensor failure. This estimate can be very inaccurate at times. You may want to consider not enabling Assisted Fixed Wing Flight if not using an airspeed sensor to prevent false activations when airspeed really is above the threshold, but is being misrepresented by the internal airspeed.

It is suggested that you do initial flights with Q_ASSIST_SPEED set to zero just to test the basic functionality and tune the airframe. Then try with Q_ASSIST_SPEED above plane stall speed if you want that functionality.

From the 3.7.0 release an additional assistance type is available based on attitude error. If Q_ASSIST_ANGLE is non-zero then this parameter gives an attitude error in degrees above which assistance will be enabled even if the airspeed is above Q_ASSIST_SPEED.

And as of Plane-4.0 and later, a third trigger to provide assistance is Q_ASSIST_ALT . This is the altitude below which QuadPlane assistance will be triggered. This acts the same way as Q_ASSIST_ANGLE and Q_ASSIST_SPEED, but triggers if the aircraft drops below the given altitude while the VTOL motors are not running. A value of zero disables this feature. The altutude is calculated as being above ground level. The height above ground is given from a Lidar used if available and RNGFND_LANDING =1 or from terrain data if TERRAIN_FOLLOW =1, or comes from height above home otherwise.

Assistance will be activated Q_ASSIST_DELAY after any of the above enabling thresholds are reached.

Assistance can also be enabled, disabled, or forced by setting an RC switch to RCx_OPTION = 82. If that channel is below 1200us (LOW), then assistance is unconditionally disabled, if above 1800us, (HIGH) then assistance is always enabled. For other RC values, assistance will be enabled as explained above.

Assistance can also be forced active all the time by setting Q_OPTIONS bit 7 to “1”. For Tailsitters, assistance for tailsitters can be limited only to VTOL motors by by setting Q_OPTIONS bit 8 to “1”. This can increase stabiity during assistance by not using the copter style pid gains on the flying surfaces as well as the VTOL motors, or for use with copter tailsitters without servo controlled flying surfaces.


Assistance is available for all QuadPlane frame types except the single motor and non-tilt dual motor tailsitter frames.

What assistance the quad motors provides depends on the fixed wing flight mode. If you are flying in an autonomous or semi-autonomous mode then the quad motors will try to assist with whatever climb rate and turn rate the autonomous flight mode wants when assistance is enabled (ie. airspeed is below Q_ASSIST_SPEED or attitude error is above Q_ASSIST_ANGLE , or altitude is below Q_ASSIST_ALT ). In a manually navigated mode the quad will try to provide assistance that fits with the pilot inputs.

The specific handling is:

  • In AUTO mode the quad will provide lift to get to the altitude of the next waypoint, and will help turn the aircraft at the rate the navigation controller is demanding.
  • In fixed wing LOITER, RTL or GUIDED modes the quad motors will try to assist with whatever climb rate and turn rate the navigation controller is asking for.
  • In CRUISE or FBWB mode the quad will provide lift according to the pilots demanded climb rate (controlled with pitch stick). The quad motors will try to turn at the pilot demanded turn rate (combining aileron and rudder input).
  • In FBWA mode the quad will assume that pitch stick input is proportional to the climb rate the user wants. So if the user pulls back on the pitch stick the quad motors will try to climb, and if the user pushes forward on the pitch stick the quad motors will try to provide a stable descent.
  • In AUTOTUNE mode the quad will provide the same assistance as in FBWA, but it is not a good idea to use AUTOTUNE mode with a high value of Q_ASSIST_SPEED as the quad assistance will interfere with the learning of the fixed wing gains.
  • In MANUAL, ACRO and TRAINING modes the quad motors will completely turn off. In those modes the aircraft will fly purely as a fixed wing.
  • In STABILIZE mode the quad motors will try to provide lift if assistance is turned on.