The Main Rotor

The Main Rotor

The main rotor is a coaxial one. The upper rotor spins in a clockwise direction whilst the lower one spins in the opposite direction. Why this complexity?  There are three primary reasons,

  • It’s a simple and stable rotor to manage because it is aerodynamically symmetrical in all flight modes – hover, forwards, backwards and sideways. Unlike a single rotor, used by the vast majority of helicopters flying today, which is essentially asymmetric in forwards flight as it has a blade travelling through the air at (aircraft forward speed plus rotor speed) on one side and (rotor speed minus aircraft speed) on the other. This causes all sorts of control and aerodynamic problems, especially at higher speeds, that the coaxial rotor just doesn’t suffer from.
  • It does not require the aircraft to have a noisy tail rotor because the torques absorbed by the upper and lower rotors generally cancel out, so obviating the need for one. Yaw control is through the use of differential torque.
  • With a separate forwards propulsion system, using a ducted fan, in the rear the aircraft, it is able to fly ‘flat’ at all speeds, so reducing the aircraft’s fuselage drag. Also, it can be given a wing to offload the main rotor at high speed, so reducing the main rotor’s drag and increasing the range and / or cruise speed of the machine.

All in all, the Autocopter’s unique main rotor configuration will confer on it great flexibility.

It is under extreme failure conditions that the advantages of the Autocopter’s coaxial rotor and sophisticated flight controller become fully apparent.

Following a total engine failure, the pilot can fly the aircraft almost as he would a fixed-wing. That is by using the main rotor as a wing with ailerons and the V-tail as an elevator to supplement main rotor pitch control. The ‘safety pilot’ within the flight controller then first implements the process of putting the main rotor into autorotation by dumping its pitch to keep it spinning and allowing the forward speed to bleed off until the rotor starts to autorotate. All this time the pilot is looking for a place to put the aircraft down and uses the joystick to point the Autocopter in the direction he decides to go. The ‘safety pilot’ interprets the joystick movements and uses the main rotor and V-tail to implement them. Once near to the ground, the ‘safety pilot’, using the height signal from the aircraft’s downward pointing LIDAR, turns rotational energy in the rotor into lift to slow the aircraft’s descent and enable a soft landing. Power to drive this process comes from a battery buffer system between the engine and drive system.

Following a total flight control failure, the process is simpler but this time the flight controller is in what is called ‘primary mode’ which means there is a direct connection between the joystick, the main rotor and V-Tail actuators and engine fuel controller. He is then flying the aircraft manually and easily because of the benign characteristics of the coaxial rotor. The one problem that has to be overcome through the use of differential torque is the way in which the main rotor can make the aircraft yaw and that again is taken care of by the ‘safety pilot’.