This Drone Takes a Unique Approach to VTOL

VTOL (vertical take-off and landing) solves a big problem for aircraft design and flight dynamics. A traditional fixed-wing airplane requires a long runway, because it has to reach a sufficient speed before it starts generating lift. A helicopter doesn't need a runway at all, but its flight is slow and inefficient compared to the airplane. A VTOL aircraft, like the famous V-22 Osprey or Harrier jump jet, provides the best of both worlds. But while those designs make a lot of sense for manned aircraft, Nicholas Rehm's drone takes a unique approach to VTOL that is more suitable for unmanned aircraft.

Tilt-rotor aircraft, including the Osprey, work by taking off with their rotors facing upwards. Then when they're in the air, those rotors transition to face forwards for normal flight. That is ideal, because it keep the body (and passengers) level. But it requires actuation to tilt the rotors between positions and that added complexity isn't necessary for an unmanned aircraft like a drone. Instead, as Rehm demonstrates, the entire aircraft can change orientation.

This is a "tailsitter" design and the name comes from the way the aircraft sits on its tail before it takes off. There have been many prototype designs for manned tailsitter aircraft over the last century, but they're always abandoned because piloting them during VTOL is difficult. That isn't an issue for a drone and so Rehm built this prototype tailsitter to work out the practicalities of the transition between VTOL and normal flight.

Rehm's tailsitter uses the same parts you'd see on any hobbyist drone or airplane. To maintain stable flight, the flight controller constantly monitors the drone's orientation using an IMU (Inertial Measurement Unit) and gyroscope module. But because that orientation is supposed to change with a tailsitter design, Rehm had to develop some custom code.

Rehm's code does a few important things. First, it swaps the incoming roll and yaw commands when the pilot flips a switch. That's necessary because the axes orientation changes between flight modes, which would be difficult for the pilot to account for manually. Second, it tells the flight controller to change the "vertical" axis between modes so the stability algorithms keep working to maintain level flight.

The cool thing about this approach is that it lets the drone automatically switch between VTOL and normal flight. When Rehm flips the toggle, the flight controller automatically adjusts the aircraft to achieve the new level. But that happens instantly, which is a little jarring. To ease the transition, Rehm implemented some "fade" code. That slowly adjusts the vertical axis value over a couple of seconds, so the flight controller smoothly reorients the drone.

This works very well, but this drone is just a prototype. Rehm plans to use the lessons he learned to build a more polished aircraft soon.