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.

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 behviour on Failsafe induced RTLs, to transistion 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 not flying a mission, and are flying in any copter mode (QHOVER,QSTAB,etc.), failsafe will evoke QLAND or QRTL, depending on how Q_OPTIONS, bit 5, is set.

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 ARSPD_FBW_MIN (or Q_ASSIST_SPEED if that is set and is greater than ARSPD_FBW_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 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.

The normal Plane RTL flight plan will then be run, which defaults to circling at the RTL altitude above the arming position or nearest rally point. If you have RTL_AUTOLAND setup then the aircraft will do a fixed wing landing.

If you set Q_RTL_MODE to 1 then the aircraft will switch to a VTOL landing when it gets close to return point.

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 ALT_HOLD_RTL 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).

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.