Autorotation

This article is about autorotation of rotary wing aircraft. Not to be confused with Autorotation (fixed-wing aircraft) or Autorotation (airborne wind energy).
Airflow through a helicopter rotor. Above, the rotor is powered and pushing air downward, generating lift and thrust. Below, the helicopter rotor has lost power, and the craft is making an emergency landing.

Autorotation is a state of flight in which the main rotor system of a helicopter or other rotary-wing aircraft turns by the action of air moving up through the rotor, as with an autogyro, rather than engine power driving the rotor.[1][2][3] The term autorotation dates to a period of early helicopter development between 1915 and 1920, and refers to the rotors turning without the engine.[4] It is analogous to the gliding flight of a fixed-wing aircraft. Some trees (for example maple trees) have seeds that have wing-like structures that enable the seed to spin to the ground in autorotation, which helps the seeds to disseminate over a wider area.

The most common use of autorotation in helicopters is to safely land the aircraft in the event of an engine failure or tail-rotor failure. It is a common emergency procedure taught to helicopter pilots as part of their training.

In normal powered helicopter flight, air is drawn into the main rotor system from above and forced downward, but during autorotation, air moves 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. It is the means by which a helicopter can land safely in the event of complete engine failure. Consequently, all single-engine helicopters must demonstrate this capability to obtain a type certificate.[5]

The longest helicopter autorotation in history was performed by Jean Boulet in 1972 when he reached a record altitude of 12,440 m (40,814 ft) in an Aérospatiale SA 315B Lama. Because of a −63 °C (−81.4 °F) temperature at that altitude, as soon as he reduced power, the engine flamed out and could not be restarted. By using autorotation he was able to land the aircraft safely.[6] Autorotation is the normal operating mode of autogyros; the distance record is 1653 km.[citation needed]

  1. ^ Rotorcraft Flying Handbook (PDF). U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2001. pp. 16–1. ISBN 1-56027-404-2. FAA-8083-21. Archived (PDF) from the original on 2013-04-20. a gyroplane rotor system operates in autorotation
  2. ^ Bensen, Igor. "How they fly – Bensen explains all Archived 2014-06-26 at the Wayback Machine" Gyrocopters UK. Accessed: 10 April 2014. Quote: "air.. (is) deflected downward"
  3. ^ Charnov, Bruce H. Cierva, Pitcairn and the Legacy of Rotary-Wing Flight Archived 2016-03-03 at the Wayback Machine Hofstra University. Accessed: 22 November 2011.
  4. ^ "Autorotation", Dictionary.com Unabridged (v 1.1). Random House, Inc. 17 April 2007 Archived 17 March 2012 at the Wayback Machine
  5. ^ USA Federal Aviation Regulations, §27.71 Autorotation performance Archived 2016-12-08 at the Wayback Machine
  6. ^ R. Randall Padfield; R. Padfield (1992). Learning to Fly Helicopters. McGraw Hill Professional. p. 151. ISBN 978-0-07-157724-3.