Tailsitter Planes

In ArduPilot tail-sitters are any VTOL aircraft type that rotates the fuselage (and autopilot) when moving between forward flight and hover.

Despite the name, not all tails-sitters land on their tails. Some are “belly landers”, where they lie down flat for landing to improve takeoff and landing stability in wind. Some may have an undercarriage for wheeled takeoff and others may have a stand or other landing aid.

All tails-sitters are considered types of QuadPlanes in ArduPilot. You should start off by reading the QuadPlane documentation before moving onto this tailsitter specific documentation.

Vectored and non-Vectored

ArduPilot sub-divides tail-sitters into two broad categories:

  • vectored tailsitters can tilt their rotors independently of the movement of the fuselage, giving them vectored thrust
  • non-vectored tailsitters have fixed rotor orientation relative to the fuselage, and rely on large control surfaces for hover authority

Tailsitter Config

The key parameter to make a plane a tailsitter is to set Q_FRAME_CLASS=10. That tells the QuadPlane code to use the tailsitter VTOL backend.

The tailsitter backend is a bit unusual, as it is the only Q_FRAME_CLASS setting that doesn’t have any motors associated with it. The way the backend works is that it provides roll, pitch, yaw and thrust values to the fixed wing control code. These values then control your ailerons, elevons, elevators, rudder and motors.

This has a nice benefit when setting up the tailsitter that you can follow the normal fixed wing setup guide in MANUAL and FBWA modes, and then when you switch to hover all of your control directions will be correct.

It also means that you can fly any fixed wing aircraft that is capable of 3D flight as a tailsitter, and fly it in modes like QSTABILIZE, QHOVER and QLOITER.

The key differences between fixed wing flight and hovering for a tailsitter are:

  • when hovering the copter PID gains will be used (the ones starting with Q_A_RAT_*)
  • when in fixed wing flight the fixed wing PID gains will be used (the PTCH2SRV_* and RLL2SRV_* gains)
  • when hovering the nose of the aircraft will try to point up for “level” flight
  • when in fixed wing flight the nose of the aircraft will try to point forward for “level” flight

Q_TAILSIT_THSCMX defines the maximum throttle scaling that will be applied to the control surfaces, this should be reduced if oscillations are seen at throttles bellow hover throttle.

Q_TAILSIT_RLL_MX allows the roll limit angle limit to be set differently from Q_ANGLE_MAX. If left at zero both pitch and roll are limited by Q_ANGLE_MAX. If Q_TAILSIT_RLL_MX is nonzero roll angle will be limited and pitch max angle will still be Q_ANGLE_MAX. This should be set if your tailsitter can achieve much larger pitch angle than would be safe for roll (some airframes can’t recover from high-speed knife-edge flight).

Q_TAILSIT_ANGLE specifies how far the nose must pitch up or down before a transition is complete: down for transition from VTOL mode to FW mode, and up for transition from FW to VTOL. So a value of e.g. 60 degrees results in switching from copter to plane controller (forward transition) when the nose reaches 30 degrees above the horizon (60 degrees down from vertical). For the back transition, the plane controller would be used until the nose reaches 60 degrees above the horizon. So the larger the value of Q_TAILSIT_ANGLE, the later the switch from one controller to the other.

Q_TRANSITION_MS specifies a timeout for transition from VTOL to FW flight. Even if the angle specified by Q_TAILSIT_ANGLE has not been reached before this interval has elapsed, the transition will be considered complete. The timeout for back transitions (from FW to VTOL flight) is hardcoded to 2 seconds.

Orientation

The AHRS_ORIENTATION, the accelerometer calibration and AHRS trim should all be done for fixed wing flight. Fixed wing flight is considered “normal” orientation for a tailsitter.

Vectored Thrust

If your tailsitter has vectored thrust then you should set the SERVOn_FUNCTION values for your two tilt servos for the left and right motors.

For example, if your left tilt servo is channel 5 and your right tilt servo is channel 6, then set:

  • SERVO5_FUNCTION=75
  • SERVO6_FUNCTION=76

you also need to set the right SERVOn_REVERSED values, and the right SERVOn_TRIM, SERVOn_MIN and SERVOn_MAX values.

Vectored Gains

There are two vectoring gains available. One controls the amount of vectored thrust movement in hover, and the other controls the amount of vectored thrust movement in forward flight.

The Q_TAILSIT_VHGAIN parameter controls vectored thrust in hover. A typical value is around 0.8, which gives a lot of control to vectored thrust in hover. This control is combined with control from your elevon mixing gain (controlled by MIXING_GAIN).

The Q_TAILSIT_VFGAIN parameter controls vectored thrust in forward flight. A typical value is around 0.2, which gives a small amount of control to vectored thrust in forward flight. This control is combined with control from your elevon mixing gain (controlled by MIXING_GAIN).

By adjusting the relative values of Q_TAILSIT_VHGAIN, Q_TAILSIT_VFGAIN and MIXING_GAIN you can adjust how much control you have from elevons and thrust vectoring in each flight mode.

Tailsitter Input

You can change how control inputs while hovering a tailsitter will be interpreted using the Q_TAILSIT_INPUT parameter. The choices are:

  • Q_TAILSIT_INPUT=0 means that in hover the aircraft responds like a multi-rotor, with the ****yaw stick controlling earth-frame yaw, and roll stick controlling earth-frame roll. This is a good choice for pilots who are used to flying multi-rotor aircraft.
  • Q_TAILSIT_INPUT=1 means that in hover the aircraft responds like a 3D aircaft, with the yaw stick controlling earth-frame roll, and roll stick controlling earth-frame yaw. This is a good choice for pilots who are used to flying 3D aircraft in prop-hang, but is not very useful when flying around, due to the earth-frame multicopter control inputs.
  • Q_TAILSIT_INPUT=2 and 3 mean that the aircraft responds like a 3D aircraft with the yaw stick controlling earth-frame yaw and the roll stick controlling body-frame roll when flying level. When hovering, these options behave the same as types 0 and 1, respectively. This is accomplished by splitting the roll and yaw command inputs into bodyframe roll and yaw components as a function of Euler pitch.

Note: Due to the rotation of the tailsitter body frame with respect to the multicopter body frame, the roll limits are set by parameter Q_YAW_RATE_MAX (in degrees), and the yaw rate limits are set by parameter Q_TAILSIT_RLL_MX (in deg/sec). The pitch limit is set by parameter Q_ANGLE_MAX (in centidegrees), and this also serves as the yaw rate limit if Q_TAILSIT_RLL_MX is zero. If any rate limit is too high for the airframe, you may experience glitches in attitude control at high rates.

Tailsitter Input Mask

To support people flying 3D aircraft and wanting to learn how to prop-hang manually, you can set the Q_TAILSIT_MASK to a mask of channels that will have full manual input control while hovering.

The mask of manual channels is enabled using a transmitter input channel, specified with the Q_TAILSIT_MASKCH parameter.

For example, if you are learning how to fly 3D aircraft, and you want some assistance learning how to best control the rudder, then you can set:

  • Q_TAILSIT_MASK=8 (for rudder)
  • Q_TAILSIT_MASKCH=7

then when channel 7 goes above 1700 the pilot will be given full manual control of rudder when hovering. This provides good 3D piloting practice on one or more axes at a time.

Center of Gravity

The center of gravity for a tailsitter is important in an extra dimension. When hovering it is important that there is not too much weight in the belly of the plane or on its back, so that it leans forward or back. This is particularly important for non-vectored tail-sitters.