Spin (flight)

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In aviation, a spin is an aggravated stall resulting in autorotation wherein the aircraft follows a downward corkscrew path. Spins are characterized by high angle of attack, low airspeed, and high rate of descent. Spins are not spiral dives, which are characterized by low angle of attack and high airspeed, and are not a type of stall.

1 How a spin occurs
2 Phases
3 Types
4 History
5 Entry and recovery
6 Aircraft design
7 References
8 External links

[edit] How a spin occurs

Image:Aerodynamic spin diagram.png

Many aircraft have to be forced to spin, but generally speaking, an aircraft will spin if it is stalled while in a slip or skid. Under these circumstances, one wing stalls before the other. The wing that stalls first will drop, increasing its angle of attack and deepening the stall. The other wing will rise, decreasing its angle of attack, and the aircraft will yaw towards the more deeply-stalled wing. The difference in lift between the two wings causes the aircraft to roll, and the difference in drag causes the aircraft to yaw.

One common scenario that can lead to a spin is an uncoordinated turn towards the runway. A poorly-trained pilot who is overshooting the turn to final approach will be tempted to apply rudder to increase the rate of turn while applying opposite aileron to decrease bank angle. Taken to its extreme, this maneuver will result in a fully uncoordinated turn with sufficient angle of attack to cause the aircraft to stall.

Spins can also be entered intentionally for training, flight testing, or aerobatics.

[edit] Phases

A spin has four phases in aircraft that are capable of recovering from a spin:

Entry - The pilot provides the necessary elements for the spin, either accidentally or intentionally.
Incipient - The aircraft stalls and rotation starts.
Developed - The aircraft's rotation rate, airspeed, and vertical speed are stabilized.
Recovery - The angle of attack of the wings decreases below the critical angle of attack and autorotation slows. The nose steepens, after which autorotation stops.

In aircraft that cannot recover from a spin, there are only three phases—the developed phase continues until the aircraft hits the ground.

[edit] Types

Normally, a spinning aircraft has a high angle of attack and steep downward pitch angle. However, there are other types of spins that do not share these characteristics.

Inverted spin - In an inverted spin, the aircraft has a negative angle of attack.
Flat spin - A flat spin is characterized by flat pitch (the aircraft is horizontal) and is usually caused by the aircraft's center of gravity being too far aft. It is very difficult to recover from a flat spin because there is little or no smooth airflow over the control surfaces.

[edit] History

In aviation's early days, spins were poorly understood and often fatal. Proper recovery procedures were unknown, and a pilot's instinct to pull back on the stick served only to make a spin worse. Because of this, the spin earned a reputation as an unpredictable danger that might snatch an aviator's life at any time, and against which there was no defense.

The spin was initially explored by individual pilots performing ad-hoc experiments (often accidentally) and by aerodynamicists. In August 1912, Lieutenant Wilfred Parke RN became the first aviator to recover from an accidental spin when his Avro biplane entered a spin at 700 feet AGL in the traffic pattern at Larkhill. Parke attempted to recover from the spin by increasing engine speed, pulling back on the stick, and turning into the spin, with no effect. The aircraft descended 450 feet, and horrified observers braced themselves for a fatal crash.

Parke was disabled by centrifugal forces but was still considering a means of escape. In an effort to neutralize the forces pinning him against the right side of the cockpit, he applied full right rudder, and the aircraft leveled out five feet above the ground. With the aircraft now under control, Parke climbed, made another approach, and landed safely.

In spite of the discovery of "Parke's technique," pilots were not taught spin recovery procedures until the beginning of World War I.

The first documented case of an intentional spin and recovery is that of Squadron Leader J.C. Brooke. In the summer of 1915, Brooke recovered from an accidental spin, and was urged by his students to repeat the exploit. He agreed, and found that he could easily recover from a spin using Parke's technique.

In 1917, Frederick Lindemann conducted a series of experiments that led to the first understanding of the aerodynamics of the spin.

[edit] Entry and recovery

Note: Some aircraft cannot recover from a spin, and must not be allowed to spin under any circumstances. If an aircraft has not been certified for spin recovery, it should be assumed that spins are not recoverable and are unsafe in that aircraft.

When spin recovery is possible, entry and recovery procedures vary with the type and model of aircraft being flown, but there are general procedures applicable to most aircraft:

Verify that the aircraft has been certified as recoverable from the spin anticipated.
Verify that local aviation regulations do not prohibit spins in the area.
Perform a thorough preflight inspection. Special attention should be paid to loose items in the cockpit, the aircraft's weight and balance, and the aircraft's controllability.
Climb to an altitude no lower than 2500 feet AGL.
Ensure that the area is clear of traffic.
Slowly reduce power to idle, while simultaneously raising the nose to a pitch attitude that will ensure a stall.
As the aircraft approaches a stall, apply full rudder in the direction of the desired spin rotation while applying full up elevator pressure.

If the aircraft manufacturer provides a specific procedure for spin recovery, that procedure must be used. Otherwise, to recover from a spin, the following generic procedure may be used:

Reduce power to idle.
Position the ailerons to neutral.
Apply full opposite rudder against the rotation.
Apply a positive, brisk, straight forward movement of the elevator control to break the stall.
After rotation stops, neutralize the rudder.
Taking care not to cause a secondary stall or exceed the aircraft's load limits, apply up elevator pressure to raise the nose to level flight.

[edit] Aircraft design

Some powered aircraft and gliders are designed to be difficult or impossible to spin, even in an uncoordinated stall. These designs have a greater angle of attack towards the center of the wing than the tips, so that the centre stalls first and the ailerons remain effective. This wing design is known as 'washout', a good example of which can be found on the Grob 103 glider.

Many training aircraft are resistant to entering a spin though some are intentionally designed and certified for spins. A well known example of this is the Piper Tomahawk, which is certified for spins though the Piper Tomahawk's spin characteristics remain controversial. Aircraft that are not certified for spins may be difficult or impossible to recover and the pilot who intentionally enters a spin in such an aircraft has just become a test pilot.

Although it has been removed from most flight test syllabuses, there are some countries that still require you to have been taught how to recover from a spin. Of those that do not, the rationale is usually that since spin recovery training requires an actual spin, an improper recovery could result in a crash. In the US spin training is required only for flight instructor candidates. A spin occurs only after a stall so the FAA decided to concentrate on training pilots in stall recognition, prevention, and recovery as a means to reduce the accident rate due to unintentional stalls and/or spins.

A spin may sound intimidating at first, however most flight instructors and some other pilots will testify that safely spinning is an interesting experience. In a spin the occupants of the aircraft will feel a falling turning sensation not unlike a descending rolling turn on a roller coaster.