Return to Launch (RTL)

When flying a QuadPlane you have a choice of several methods of handling return to launch. The choices are:

  • circle about the return point as a fixed wing
  • fly as a VTOL aircraft to the return point then land vertically
  • fly as a fixed wing aircraft until close to the return point then switch to VTOL and land vertically

In each case a key concept is the return point. This is defined as the closest rally point, or if a rally point is not defined then the home location. See the Rally Points page for more information on rally points.

Return to launch behavior is determined by the Q_RTL_MODE parameter.

Fixed Wing RTL

The default behaviour (Q_RTL_MODE = 0)of the RTL mode is the same as for fixed wing. It will fly to the nearest rally point (or home if no rally point is defined) and circle as a fixed wing aircraft about that point. The VTOL motors will not be used unless the aircraft drops below the airspeed defined in Q_ASSIST_SPEED. The altitude the aircraft will circle at will be the altitude in the rally point, or the ALT_HOLD_RTL altitude if a rally point is not being used.

Hybrid RTL

The another option for RTL in a QuadPlane is to fly as a fixed wing aircraft until it is close to the return point at which time it switches to a VTOL RTL as described above. To enable this type of hybrid RTL mode you need to set the Q_RTL_MODE parameter to 1, 2, or 3.

Q_RTL_MODE = 1

The initial altitude that will be aimed for in the fixed wing portion of the hybrid RTL is the same as for a fixed wing RTL. You should set your rally point altitude and ALT_HOLD_RTL parameters appropriately to ensure that the aircraft arrives at a reasonable altitude for a vertical landing. A landing approach altitude of about 15 meters is good for many QuadPlanes. This should be greater than or equal to the Q_RTL_ALT values.

The distance from the return point at which the aircraft switches from fixed wing to VTOL flight is set using the RTL_RADIUS parameter, or if that is not set then the WP_LOITER_RAD parameter is used. The aircraft will then slow down as it approaches the return point, aiming for an altitude set by Q_RTL_ALT.

Once the return point is reached the aircraft begins to descend and land, exactly as described in the VTOL RTL mode above.

Q_RTL_MODE = 2

Setting Q_RTL_MODE to 2 results in behavior similar to above, but with the vehicle returning like normal fixed wing RTL until it reaches Q_FW_LND_APR_RAD, then loitering in fixed wing mode down to Q_RTL_ALT altitude, and then exiting facing the wind and executing a QRTL to the home position. Be sure the loiter portion is set up to clear any obstacles.

Q_RTL_MODE = 3

Setting Q_RTL_MODE to 3 results in behavior similar to a normal QRTL. The vehicle will enter an “APPROACH” phase, and will return at ALT_HOLD_RTL and at a calculated distance, start a descent towards Q_RTL_ALT. As it approaches the landing position, ArduPilot starts an “airbraking” phase in non-tailistters to slow the vehicle and once slowed enters full VTOL mode and proceeds to execute a VTOL landing. This behavior is also used by default for the QRTL mode unless Q_OPTIONS bit 16 is set to prevent the Hybrid operation above.

In effect, this enables the QRTL mode for any RTL actuation: failsafe actions, mode change to QRTL, or completion of a mission (unless the last mission item prevents RTL).

Note

This mode is also used by default in all mission VTOL_LANDINGs unless the Q_OPTIONS bit 16 is set to disable it.

This fixed wing “approach” allows VTOL landings to be used without needing to setting up approach waypoints to reduce altitude and get close enough to proceed in VTOL mode toward the landing point. If disabled by bit 16, the vehicle will instantly transition to VTOL mode upon that mission items execution, or upons mode changes to QRTL, and navigate to its landing point in VTOL before doing a QLAND. This means that you should be very close to the landing site if the FW approach mode is disabled in a mission since it will proceed in VTOL flight to the land point.

The image below details the phases of the approach and landing with the default setting of bit 16 (ie, not enabled):

../_images/approach.jpg

The phases of the approach are:

  • further than 2 times the greater of either RTL_RADIUS or WP_LOITER_RADS (MAXRAD) plus a calculated distance needed to descend from ALT_HOLD_RTL to Q_RTL_ALT, the plane will attempt to climb or descend to ALT_HOLD_RTL. If within that range, it will attempt to climb/descend to a linear descent slope, meet it, and continue to descend, as shown above.
  • if started further than 2X “MAXRAD” but closer than above, at 2x MAXRAD it will continue in fixed wing mode at Q_RTL_ALT.
  • when it reaches a point that is within the VTOL stopping distance of the landing point (at the VTOL deceleration parameter limits and current speed), it will transition to VTOL mode and send a message that it is in “VTOL Position1” and continue moving to the land point. If the vehicle is NOT a tailsitter, an “AIRBRAKING” phase may occur before the VTOL transition, spinning up the VTOL motors to create additional braking.
  • once the QuadPlane is within 5 m of the land point and moving less than 2 m/s, it will semnd a GCS message declaring that it is in “VTOL Position2), and final position over the land point and begin its landing descent, which will also be indicated by GCS messages
  • if the approach is entered less than 1.5X MAXRAD, it will immediately move to VTOL Position1 state, and move toward the landing site attempting to obtain Q_RTL_ALT as it does so.
  • if in VTOL mode at greater than 1.5X MAXRAD, the plane will transition to fixed wing, and attempt to navigate to home, executing the approach. The climb and turn toward the landing point will occur at even low altitudes, just like normal non-Quadplane RTLs, so the FLIGHT_OPTIONS bit 4 for “Climb before turn in RTL” and/or Q_OPTIONS bit 0 for “Level Transitions” might be worth considering.

VTOL RTL (QRTL)

If you prefer to do return to launch as a pure VTOL aircraft (like a multirotor would do) then you can use the QRTL flight mode, but with Q_OPTIONS bit 16 set in order to disable its default behavior, which is like the hybrid RTL described above with Q_RTL_MODE = 3. The vehicle will transition to VTOL flight and then fly at the Q_WP_SPEED speed towards the return point, at an altitude of Q_RTL_ALT.

Once the return point is reached the aircraft will start a vertical descent towards the ground for landing. The initial descent rate is set by Q_WP_SPEED_DN. Once the aircraft reaches an altitude of Q_LAND_FINAL_ALT the descent rate will change to Q_LAND_SPEED for the final landing phase.

In the final landing phase the aircraft will detect landing by looking for when the VTOL motor throttle drops below a minimum threshold for 5 seconds. When that happens the aircraft will disarm and the VTOL motors will stop.