Autorotation
Autorotation is a rotation of the main rotor of a gyroplane or helicopter driven by the airstream . The air flows diagonally from below against the main rotor, which is driven like a wind turbine. The rapid movement of the rotor blades through the air creates a force through dynamic lift that counteracts gravity .
If the autorotation is strong enough, this buoyancy will be enough to keep the device in the air. When gyroplane this is the normal flight condition. They are driven by a motor-driven propeller to ensure that there is sufficient airflow over the long term . In helicopters, autorotation is an emergency measure that prevents it from falling to the ground without braking if the engine fails. The main rotor must be brought by the pilot at a suitable angle to the airstream. During the autorotation, the helicopter descends quickly. However, it remains controllable so that an emergency landing is possible.
Autorotation in rotary wing aircraft
Gyroplane
In a gyroplane , a non-powered rotor is used to generate lift instead of fixed wings. Propulsion is carried out by a motor and propeller or by towing. This means that the gyroplane - in contrast to the helicopter - is permanently in autorotation.
helicopter
The autorotation makes it possible to land a helicopter without the main rotor driven by the motor shaft . It is the basis of the emergency maneuver after failure of the motor drive or necessary throttling of this drive after failure of the tail rotor . It is comparable to gliding an airplane without a (functioning) engine and is divided into two phases:
In the first phase, the amount and speed of the aircraft in a controlled, but relatively steep glide converted into speed of the rotor, the air flow from below ( " wind ") causes the drive of the rotor. For this purpose, the angle of attack of the rotor is set very low with the collective lever ( pitch ). This means that the rotor has little drag, but only generates little lift. The principle of lift generation in this flight condition is similar to that of the gyroplane. In this phase, as much rotational energy as possible should be absorbed in the rotor with a controllable speed and sink rate . A speed of 110 to 130 km / h with a descent rate of 5 to 10 m / s is considered optimal, depending on the type of aircraft. The rotor speed is kept in the range around 100% - the control takes place with the collective. In order to "stretch" the flight path, the rotor speed can be reduced, depending on the pattern, to up to 85 percent of the rotation speed while increasing the speed at the same time. This allows a somewhat more distant emergency landing site to be reached. The aim of this phase is to overcome obstacles and reach a suitable emergency landing site.
In the second phase, the final approach to the ground, the rotor blades (pitch) is currently more lift generated to brake the sinking and horizontal velocity (above ground) and a short through increased pitching flare ( flare ) to carry out. The kinetic energy stored in the rotating rotor (the momentum) is quickly dissipated, so that only a short time frame is available for the maneuver. The aim is to achieve "target braking" with as little impact as possible (vulgo soft) on the ground.
The autorotation landing places high demands on the pilot and requires regular training, as the right amount and timing of the pitch change must be precisely taken. This is mainly due to the fact that the kinetic energy of the rotor is only available once to increase lift (necessary to brake the rapid descent during the descent phase). If the angle of attack is increased too early, the helicopter is already braked at too great a height and will then "sag" when the rotational speed of the main rotor decreases. If the angle of attack is increased too late, the remaining flight altitude is no longer sufficient to slow down the helicopter sufficiently.
For the feasibility of an autorotation landing, the flight altitude is decisive: a rotor that rotates too slowly (due to an engine failure, for example) can only regain sufficient speed in the first phase of the autorotation landing if it is high enough above the ground .
Tandem rotor
Tandem rotor helicopters like the Boeing-Vertol CH-47 cannot land by autorotation if one of the rotary wings fails.
literature
- Chapter 18 Autorotation. In: Willem J. Wagtendonk: Principles of Helicopter Flight. 2nd edition, reprinted. Aviation Supplies & Academics, Newcastle WA 2010, ISBN 978-1-56027-649-4 , pp. 141-156.
- Ernst Götsch: Aircraft technology. Introduction, basics, aircraft science . 3. Edition. Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8 .