Autorotation is a state of

flight Flight or flying is the motion (physics), motion of an Physical object, object through an atmosphere, or through the vacuum of Outer space, space, without contacting any planetary surface. This can be achieved by generating aerodynamic lift ass ...

in which the main rotor system of a

helicopter A helicopter is a type of rotorcraft in which Lift (force), lift and thrust are supplied by horizontally spinning Helicopter rotor, rotors. This allows the helicopter to VTOL, take off and land vertically, to hover (helicopter), hover, and ...

or other rotary-wing aircraft turns by the action of air moving up through the rotor, as with an

autogyro An autogyro (from Greek and , "self-turning"), gyroscope, gyrocopter or gyroplane, is a class of rotorcraft that uses an unpowered rotor in free autorotation to develop lift. A gyroplane "means a rotorcraft whose rotors are not engine-d ...

, rather than engine power driving the rotor. Bensen, Igor.
How they fly – Bensen explains all
" ''Gyrocopters UK''. Accessed: 10 April 2014. Quote: "air.. (is) deflected downward"Charnov, Bruce H
Cierva, Pitcairn and the Legacy of Rotary-Wing Flight
''

Hofstra University Hofstra University is a Private university, private research university in Hempstead, New York, United States. It originated in 1935 as an extension of New York University and became an independent college in 1939. Comprising ten schools, includ ...

''. Accessed: 22 November 2011. The term ''autorotation'' dates to a period of early helicopter development between 1915 and 1920, and refers to the rotors turning without the engine."Autorotation", ''Dictionary.com Unabridged (v 1.1)''. Random House, Inc. 17 April 2007
It is analogous to the

gliding flight Gliding flight is heavier-than-air flight without the use of thrust; the term volplaning also refers to this mode of flight in animals. It is employed by flying and gliding animals, gliding animals and by aircraft such as glider (aircraft), gl ...

of a fixed-wing aircraft. Some trees (for example

maple ''Acer'' is a genus of trees and shrubs commonly known as maples. The genus is placed in the soapberry family Sapindaceae.Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 nd more or less continuously updated si ...

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 or other mechanical issue which disconnects the engine from the rotor system. Consequently, all single-engine helicopters must demonstrate this capability to obtain a

type certificate A type certificate signifies the airworthiness of a particular category of aircraft, according to its manufacturing design (''type design''). Certification confirms that the aircraft of a new type intended for serial production is in compliance w ...

. 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. Autorotation is the normal operating mode of

s; the distance record is 1653 km.

Descent and landing

For a helicopter, "autorotation" refers to the descending maneuver in which the engine is disengaged from the main rotor system and the rotor blades are driven solely by the upward flow of air through the rotor. The ''freewheeling unit'' is a special clutch mechanism that disengages any time the engine rotational speed is less than the rotor rotational speed. If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor, allowing the main rotor to rotate freely. The most common reason for autorotation is an engine malfunction or failure, but autorotation can also be performed in the event of a complete

tail rotor The tail rotor is a smaller rotor mounted vertically or near-vertically at the tail of a traditional single-rotor helicopter, where it rotates to generate a propeller-like horizontal thrust in the same direction as the main rotor's rotation. ...

failure, or following loss of tail-rotor effectiveness, since there is virtually no

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

produced in an autorotation. If altitude permits, autorotations may also be used to recover from a vortex ring state, also known as settling with power. In all cases, a successful landing depends on the helicopter's height and velocity at the commencement of autorotation (see height-velocity diagram). At the instant of engine failure, the main rotor blades are producing

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...

and

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

from their

angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a Airfoil#Airfoil terminology, reference line on a body (often the chord (aircraft), chord line of an airfoil) and the vector (geometry), vector representing the relat ...

and

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

. By immediately lowering collective pitch, which must be done in case of an engine failure, the pilot reduces lift and drag and the helicopter begins an immediate descent, producing an upward flow of air through the rotor system. This upward flow of air through the rotor provides sufficient thrust to maintain rotor rotational speed throughout the descent. Since the tail rotor is driven by the main rotor transmission during autorotation, heading control is maintained as in normal flight. The collective must be immediately lowered to enter autorotation. Any up-collective will cause the blades to generate lift with associated drag. This drag will quickly cause the rotor RPM to decay to an unsafe speed potentially causing the blades to stall. If the blades stall, the subsequent very-high increase in drag will stop the rotor from turning. This is non-recoverable. Once an autorotation is established, the collective is used to control rotor RPM to keep it below redline and above rotor stall RPM. Several factors affect the rate of descent in autorotation:

density altitude The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as ...

gross weight In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition. Some st ...

, rotor rotational speed, and forward

airspeed In aviation, airspeed is the speed of an aircraft relative to the air it is flying through (which itself is usually moving relative to the ground due to wind). In contrast, the ground speed is the speed of an aircraft with respect to the sur ...

. The pilot's primary control of the rate of descent is airspeed. Higher or lower airspeeds are obtained with the cyclic pitch control just as in normal flight. Rate of descent is high at zero airspeed and decreases to a minimum at approximately 50 to 90 knots, depending upon the particular helicopter and the factors previously mentioned. As the airspeed increases beyond the speed that gives minimum rate of descent, the rate of descent increases again. Even at zero airspeed, the rotor is quite effective, as it has nearly the drag coefficient of a parachuteJohnson, Wayne
Helicopter theory
p109, ''Courier Dover Publications'', 1980. Accessed: 25 February 2012. despite consisting of blades. When landing from an autorotation, the kinetic energy stored in the rotating blades and the forward movement of the aircraft are used to decrease the rate of descent and make a soft landing. A greater amount of rotor energy is required to stop a helicopter with a high rate of descent than is required to stop a helicopter that is descending more slowly. Therefore, autorotative descents at very low or very high airspeeds are more critical than those performed at the minimum rate of descent airspeed. An autorotation landing consists of multiple stages: glide, flare, landing. A stable glide at an appropriate speed for the helicopter is maintained during descent. At some altitude above the ground, typically 50-100 feet, the pilot raises the nose and applies up collective. Raising the nose leads to more airflow through the rotor and raising the collective converts this airflow into more rotor lift which will slow the helicopter's forward speed. Raising the collective also keeps the rotor RPM from exceeding redline. Depending on the helicopter, before airspeed has completely decayed, the helicopter is leveled. Most helicopters at this point will still have forward velocity and will perform a run-on landing using the remaining rotor momentum. Some helicopters have enough momentum in the rotor system that the helicopter can be brought to a standstill and lowered to the ground with no forward velocity. Each type of helicopter has a specific airspeed at which a power-off glide is most efficient. The best airspeed is the one that combines the greatest glide range with the slowest rate of descent. The specific airspeed is different for each type of helicopter, yet certain factors (density altitude, wind) affect all configurations in the same manner. The specific airspeed for autorotations is established for each type of helicopter on the basis of average weather and wind conditions and normal loading. A helicopter in autorotation can be flown backwards, sideways or vertically with no lateral velocity. A slip can be used to decrease the glide angle. A helicopter operated with heavy loads in high density altitude or gusty wind conditions can achieve best performance from a slightly increased airspeed in the descent. At low density altitude and light loading, best performance is achieved from a slight decrease in normal airspeed. Following this general procedure of fitting airspeed to existing conditions, the pilot can achieve approximately the same glide angle in any set of circumstances and estimate the touchdown point. This optimum glide angle is usually 17–20 degrees.

Autorotational regions

During vertical autorotation, the rotor disc is divided into three regions—the driven region, the driving region, and the stall region. The sizes of these regions vary with the blade pitch, rate of descent, and rotor rotational speed. When changing autorotative rotational speed, blade pitch, or rate of descent, the sizes of the regions change in relation to each other. The driven region, also called the propeller region, is the region at the end of the blades. Normally, it consists of about 30 percent of the radius. It is the driven region that produces the most drag. The overall result is a deceleration in the rotation of the blade. The driving region, or autorotative region, normally lies between 25 and 70 percent of the blade radius, which produces the forces needed to turn the blades during autorotation. Total aerodynamic force in the driving region is inclined slightly forward of the axis of rotation, producing a continual acceleration force. This inclination supplies thrust, which tends to accelerate the rotation of the blade. Driving region size varies with blade pitch setting, rate of descent, and rotor rotational speed. The inner 25 percent of the rotor blade is referred to as the stall region and operates above its maximum angle of attack (stall angle) causing drag, which slows rotation of the blade. A constant rotor rotational speed is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the deceleration forces from the driven and stall regions. By controlling the size of the driving region, the pilot can adjust autorotative rotational speed. For example, if the collective pitch is raised, the pitch angle increases in all regions. This causes the point of equilibrium to move inboard along the blade's span, thereby increasing the size of the driven region. The stall region also becomes larger while the driving region becomes smaller. Reducing the size of the driving region causes the acceleration force of the driving region and rotational speed to decrease.

Broken Wing Award

The Broken Wing Award is a

United States Army The United States Army (USA) is the primary Land warfare, land service branch of the United States Department of Defense. It is designated as the Army of the United States in the United States Constitution.Article II, section 2, clause 1 of th ...

award for successful execution of an autorotation under emergency conditions. The requirements for the award, as stated in Army Regulation 672-74, are, "An aircrew member must, through outstanding airmanship, minimize or prevent aircraft damage or injury to personnel during an emergency situation. The aircrew member must have shown extraordinary skill while recovering an aircraft from an in-flight emergency situation."

References

External links

* Popular explanation o
autorotation
written by Paul Cantrell.
Pilot's 'exceptional flying' saves $540,000 helicopter
' – ''

The New Zealand Herald ''The New Zealand Herald'' is a daily newspaper published in Auckland, New Zealand, owned by New Zealand Media and Entertainment, and considered a newspaper of record for New Zealand. It has the largest newspaper circulation in New Zealand, ...

'', Monday 18 February 2008 {{aircontent Helicopter aerodynamics Types of landing

Descent and landing

Autorotational regions

Broken Wing Award

See also

References

External links