One of the basic and most important skills that any human helicopter pilot learns and must practice on a regular basis is the ability to autorotate safely. This is so that in the event of engine or tail rotor failure the aircraft can be safely descended and landed under control. Although there are obviously no human aircrew, for helicopter UAVs there is also a requirement to minimize (or indeed eliminate) any danger to people on the ground and also any damage to the UAV which might be caused by an engine or tail rotor failure.
Autorotation is defined as a state of flight in which the main rotor system of a helicopter or similar aircraft turns by the action of air moving up through the rotor disc, as with an autogyro, rather than engine power driving the rotor. Autorotation is therefore analogous to the gliding flight of a fixed wing aircraft.
In normal powered helicopter flight, air is drawn into the main rotor system from above and exhausted downward, but during autorotation, air moves up into the rotor system from below as the helicopter descends. Autorotation is permitted mechanically because of both a freewheeling unit, which allows the main rotor to continue turning even if the engine is not running, as well as aerodynamic forces of relative wind maintaining rotor speed.
The use of an autopilot to make a controlled autorotation descent can be split into two phases:
- Phase 1 – Autorotation. During this phase, the autopilot must detect the engine or transmission failure and the subsequent lack of power to the main rotor. It then must manage collective pitch, ensuring a balance between rotational speed and lift. To achieve this the UAV must be equipped with a sensor which feeds main rotor RPM information to the autopilot.
- Flare and Landing. As the helicopter approaches the ground it is necessary for the autopilot to reduce the rate of descent in order to produce a safe touchdown speed (the ‘flare’ manoeuvre). In order to measure precisely the remaining distance between the aircraft and the ground a radar or laser altimeter or similar device must be fitted to the UAV. The ability of the aircraft to perform the flare will also depend on the amount of energy stored in the rotor disc, hence the requirement for the autopilot to manage the descent correctly.
UAV Navigation’s VECTOR autopilot is one of very few autopilots to have a fully functional autorotation capability. This capability was developed as part of the requirement for a Tier1 customer using an Enstrom 480B helicopter which was converted from a standard manned aircraft into a UAV in 2017. The functionality has been further tested and demonstrated on other smaller UAVs, including a conventional configuration 2m rotor disc, piston-engined helicopter.