Flying a QuadPlane

While flying a QuadPlane can actually be easier than flying a conventional fixed wing aircraft there are some things you need to understand. Please read the following sections carefully.

VTOL vs Fixed-Wing Level Trim

Often fixed wing “level” trim, which is the pitch attitude stabilization modes attempt to maintain, is set to be several degrees positive with respect to the wing chord line in order to provide lift while cruising. See Accelerometer Calibration and Tuning Cruise Configuration for more details.

However, when in VTOL modes, this can result in the vehicle leaning “backward” a few degrees, building in a tendency to drift backwards. This can be eliminated by setting the Q_TRIM_PITCH parameter to correct this. This can also be used to correct minor CG imbalances caused by VTOL motor placement not being exactly balanced around the CG.

Manual Forward Throttle in VTOL Modes

By setting an RC channel option (RCx_OPTION) to “209”, that channel can provide a separate throttle input to the forward motor(s) in QSTABILIZE, QACRO, and QHOVER VTOL modes. This allows forward movement without having to tilt the QuadPlane forward requiring throttle stick repositioning in QSTABILIZE and QACRO to maintain altitude, and present more forward flat plate resistance to forward movement in all modes. The maximum percentage throttle that will be applied by this channel is set by Q_FWD_MANTHR_MAX.

Radio or Throttle Failsafe

If flying in a plane mode or AUTO, behaviour is determined by the FS_SHORT_ACTN and FS_LONG_ACTN parameter settings (see Plane Failsafe Function). QuadPlanes can be set such that instead of normal plane behaviour on Failsafe induced RTLs, to transition to QRTL and land once at the rally point or home, if Q_RTL_MODE =1. If Q_RTL_MODE =2, then a fixed wing approach followed by a loiter to alt and QRTL will be executed, similar to that described in the “AUTO VTOL Landing” section of QuadPlane AUTO Missions.

If lying in any VTOL mode (QHOVER,QSTAB,etc.) and not flying a mission, failsafe will evoke QLAND , QRTL or RTL, depending on how Q_OPTIONS, bits 5 and 20, are set.

VTOL Landing Repositioning

During the final descent phase of QRTL/QLAND or a mission VTOL LAND command, repositioning the vehicle or pausing the descent is usually not possible unless switching to a pilot controlled mode like QLOITER. However, there are two options that can be enabled to give the pilot control:

Throttle Descent Control

If Q_OPTIONS bit 15(+32768 to param value), is set, it will allow pilot to control descent during VTOL AUTO-LAND phases, similar to throttle stick action during QHOVER or QLOITER. However, this will not become active until the throttle stick is raised above 70% momentarily during the descent at least once.

Horizontal Repositioning

If Q_OPTIONS bit 17(+131072 to param value), is set, it will enable pilot horizontal re-positioning during VTOL auto LAND phases using the pitch and roll sticks like QLOITER, momentarily pausing the descent while doing so.

What Will Happen?

Understanding hybrid aircraft can be difficult at first, so below are some scenarios and how the ArduPilot code will handle them.

I am hovering in QHOVER/QLOITER and switch to FBWA mode

The aircraft will continue to hover, setting forward thrust/throttle at whatever the throttle stick position dictates and gaining speed. If you zero throttle during the transition, the aircraft will continue to hold the current height and hold itself level, slowing to a halt. It will drift with the wind as it is not doing position hold.

If you advance the throttle stick then the forward motor will throttle-up and the aircraft will start to move forward. The quad motors will continue to provide both lift and stability while the aircraft is moving slowly. You can control the attitude of the aircraft with roll and pitch stick input. When you use the pitch stick (elevator) that will affect the climb rate of the quad motors. If you pull back on the elevator the quad motors will assist with the aircraft climb. If you push forward on the pitch stick the power to the quad motors will decrease and the aircraft will descend.

The roll and pitch input also controls the attitude of the aircraft, so a right roll at low speed will cause the aircraft to move to the right. It will also cause the aircraft to yaw to the right (as the QuadPlane code interprets right aileron in fixed wing mode as a commanded turn).

Once the aircraft reaches an airspeed of AIRSPEED_MIN (or Q_ASSIST_SPEED if that is set and is greater than AIRSPEED_MIN) the amount of assistance the quad motors provide will decrease over 5 seconds. After that time the aircraft will be flying purely as a fixed wing.

I am flying fast in FBWA mode and switch to QHOVER mode

The quad motors will immediately engage and will start by holding the aircraft at the current height. The climb/descent rate is now set by the throttle stick, with a higher throttle stick meaning climb and a lower throttle stick meaning descend. At mid-stick the aircraft will hold altitude.

The forward motor will stop, but the aircraft will continue to move forward due to its momentum. The drag of the air will slowly bring it to a stop. The attitude of the aircraft can be controlled with roll and pitch sticks (aileron and elevator). You can yaw the aircraft with rudder.

I switch to RTL mode while hovering

The aircraft will generally transition to fixed wing flight. The quad motors will provide assistance with lift and attitude while the forward motor starts to pull the aircraft forward. Depending on the Q_RTL_MODE, different behaviors can be selected as it returns to the return point (rally or home). See Return to Launch for details.

If you have RTL_AUTOLAND setup then the aircraft will follow the mission configuration.

I switch into QRTL close to HOME

If closer than 1.5X the larger of either RTL_RADIUS or WP_LOITER_RAD, then the vehicle will proceed toward home in VTOL mode and land. If greater, it will transition to fixed wing, climbing toward RTL_ALTITUDE and executing a normal QRTL. Depending on how far from home, the vehicle may only briefly climb and then switch back to approach or airbrake phases. The further away, the higher the climb as it flies back toward home. If the approach behavior has ben disabled with Q_OPTIONS bit 16, then it will just switch to VTOL (if not already in that mode, navigate to home and land).

I have an EKF Failsafe

ArduPilot provides a failsafe mechanism to protect VTOL operation in case the EKF becomes unhealthy. In normal fixed wing operation, ArduPilot “falls back” to another inertial guidance filter, DCM, if the EKF becomes unhealthy. However, if operating in AUTO mode in a VTOL mode when an EKF failure occurs, QuadPlanes will switch modes to QLAND, and to QHOVER in all other position control VTOL modes (QLOITER, QRTL, QLAND, QAUTOTUNE).

Typical Flight

A typical test flight would be:

  • VTOL takeoff in QLOITER or QHOVER

  • switch to FBWA mode and advance throttle over 50% and start flying fixed wing

  • switch to QHOVER mode to go back to quad mode and reduce throttle back to 50% for hover.