The Fun Cub Quad VTOL is a standard tailplane aircraft that has been retrofitted with a quadcopter system. The Fun Cub is a relatively affordable plane that is, in its native state, relatively easy to fly. After the conversion the plane is significantly heavier and significantly less aerodynamic but it still flies quite well, even if it does need around 75% throttle in forward flight.

This HowTo provides some background on how to upgrade the aircraft to a VTOL aircraft, how to configure the autopilot and how to get through your first flights. While it is dealing with the Fun Cub Quad VTOL much of what is outline applies to any tailplane aircraft with either a pusher or puller (aka tractor) motor.

Here is a video of the maiden flight:

• Multiplex FunCub
• Iris propulsion set (motors and ESC)
• Aluminum frame for mounting the quad motors (10x10mm square tube, 1mm wall thickness)
• 10×4.7 props for quad motors
• 10×7 prop for fixed-wing motor
• 4S battery
• TOW is ~2.3kg with a 4200mAh 4S battery

1. Run QGroundControl
2. Flash the master firmware
3. In the Setup tab select the Fun Cub Quad VTOL airframe
4. Alternatively, go to the Advanced Parameters section and set:
   • SYS_AUTOSTART to 13005
   • VT_TYPE to 2 (standard airframe)
   • SYS_AUTOCONFIG to 1
   • VT_MOT_COUNT to 4

In QGroundControl assign a switch of your remote to channel AUX1 during the RC calibration step. This allows you to switch between the multicopter- and fixed wing mode. The switch in the off-position means that you are flying in multicopter mode.

Before you attempt your first transition to fixed wing flight you need to make absolutely sure that your VTOL is well tuned in multirotor mode. One reason is this is the mode you will return to if something goes wrong with a transition and it could be it will be moving fairly quickly already. If it isn't well tuned bad things might happen.

If you have a runway available and the total weight isn't too high you will also want to tune fixed wing flight as well. If not then you will be attempting this when it switches to fixed wing mode. If something goes wrong you need to be ready (and able) to switch back to multirotor mode.

Follow the the respective tuning guides on how to tune multirotors and fixed wings.

Once you have a well tuned multirotor mode going you are ready to try your first transition.

While it might seem that you are dealing with a vehicle that can fly in two modes (multirotor for vertical takeoffs and landings and fixed wing for forwards flight) there is an additional state you also need to tune: transition. Getting your transition tuning right is important for obtaining a safe entry into fixed wing mode, for example, if your airspeed is too slow when it transitions it might stall.

Parameter: VT_TRANS_THR

Transition throttle defines the maximum throttle to use during the transition. Don't set this too low otherwise you will never reach the transition airspeed. If you set it too high it will just use more power than you may want. For your first transition you are better off higher than lower here.

Parameter: VT_F_TRANS_DUR

A forward transition refers to the transition from multirotor to fixed wing mode. This is the amount of time in seconds that should be spent ramping up the throttle to the target value (defined by VT_TRANS_THR). A value of 0 will result in commanding the transition throttle value being set immediately. If you wish to smooth the throttling up you can increase this to a larger value, such as 3.

Note that once the ramp up period ends throttle will be at its target setting and will remain there until (hopefully) the transition speed is reached.

Parameter: VT_ARSP_BLEND

By default as the airspeed get close to the transition speed multirotor attitude control will be reduced and fixed wing control will start increasing continuously until the transition occurs.

Disable blending by setting this parameter to 0 which will keep full multirotor control and zero fixed wing control until the transition occurs.

Parameter: VT_ARSP_TRANS

This is the airspeed which, when reached, will trigger the transition out of multirotor mode into fixed wing mode. It is critical that you have properly calibrated your airspeed sensor. It is also important that you pick an airspeed that is comfortably above your airframes stall speed (check FW_AIRSPD_MIN) as this is currently not checked.

Parameter: VT_FW_PERM_STAB

Activating permanent stabilisation will result in fixed wing flight being stabilised by the autopilot at all times. As soon as a transition to fixed wing occurs it will be stabilised.

Note that if you have not yet tuned your fixed wing mode you should leave this off until you are sure it behaves well in this mode.

As already mentioned make sure you have a well tuned multirotor mode. If during a transition something goes wrong you will switch back to this mode and it should be quite smooth.

Before you fly have a plan for what you will do in each of the three phases (multirotor, transition, fixed wing) when you are in any of them and something goes wrong.

Battery levels: leave enough margin for a multirotor transition for landing at the end of your flight. Don't run your batteries too low as you will need more power in multirotor mode to land. Be conservative.

Make sure you are at least 20 meters above ground and have enough room to complete a transition. It could be that your VTOL will lose height when it switches to fixed wing mode, especially if the airspeed isn't high enough.

Transition into the wind, whenever possible otherwise it will travel further from you before it transitions.

Make sure the VTOL is in a stable hover before you start the transition.

Start your transition. It should transition within 50 - 100 meters. If it doesn't or it isn't flying in a stable fashion abort the transition (see below) and land or hover back to the start position and land. Try increasing the transition throttle (VT_TRANS_THR) value. Also consider reducing the transition duration (VT_F_TRANS_DUR).

As soon as you notice the transition happen be ready to handle height loss which may include throttling up quickly.

When you transition back to multirotor mode bring your aircraft in on a straight level approach and reduce its speed, flip the transition switch and it will start the multirotor motors and stop the pusher/puller prop immediately and should result in a fairly smooth gliding transition.

Consider that the throttle value you have when you transition will command the amount of thrust your multirotor has at the moment of the switch. Because the wing will still be flying you'll find you have plenty of time to adjust your throttle to achieve/hold a hover.

It's important to know what to expect when you revert a transition command during a transition.

When transitioning from multirotor to fixed wing (transition switch is on/fixed wing) then reverting the switch back (off/multirotor position) before the transition happens it will immediately return to multirotor mode.

When transitioning from fixed wing to multirotor for this type of VTOL the switch is immediate so there isn't really a backing out option here, unlike for tilt rotor VTOLs. If you want it to go back into fixed wing you will need to go through the full transition. If it's still travelling fast this should happen quickly.