Spin (aerodynamics)


In flight dynamics, a spin is a special category of stall resulting in autorotation about the aircraft's longitudinal axis and a shallow, rotating, downward path approximately centred on a vertical axis. Spins can be entered intentionally or unintentionally, from any flight attitude if the aircraft has sufficient yaw while at the stall point.
In a normal spin, the wing on the inside of the turn stalls while the outside wing remains flying. It is possible for both wings to stall, but the angle of attack of each wing, and consequently its lift and drag, are different.
Either situation causes the aircraft to autorotate toward the stalled wing due to its higher drag and loss of lift. Spins are characterized by high angle of attack, an airspeed below the stall on at least one wing and a shallow descent. Recovery and avoiding a crash may require a specific and counter-intuitive set of actions.
A spin differs from a spiral dive, in which neither wing is stalled and which is characterized by a low angle of attack and high airspeed. A spiral dive is not a type of spin because neither wing is stalled. In a spiral dive, the aircraft responds conventionally to the pilot's inputs to the flight controls, and recovery from a spiral dive requires a different set of actions from those required to recover from a spin.
In the early years of flight, a spin was frequently referred to as a "tailspin".

How a spin occurs

Many types of airplanes spin only if the pilot simultaneously yaws and stalls the airplane. Under these circumstances, one wing stalls, or stalls more deeply than the other. The wing that stalls first drops, increasing its angle of attack and deepening the stall. At least one wing must be stalled for a spin to occur. The other wing rises, decreasing its angle of attack, and the aircraft yaws 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 continue yawing.
The spin characteristics diagram shown in this section is typical of an aircraft with moderate or high aspect ratio and little or no sweepback which leads to spin motion which is primarily rolling with moderate yaw. For a low aspect ratio swept wing with relatively large yaw and pitch inertia the diagram will be different and illustrates a predominance of yaw.
One common scenario that can lead to an unintentional spin is a skidding uncoordinated turn toward the runway during the landing sequence. A pilot who is overshooting the turn to final approach may be tempted to apply more rudder to increase the rate of turn. The result is twofold: the nose of the airplane drops below the horizon, and the bank angle increases due to rudder roll. Reacting to these unintended changes, the pilot then begins to pull the elevator control aft while applying opposite aileron to decrease bank angle.
Taken to its extreme, this can result in an uncoordinated turn with sufficient angle of attack to cause the aircraft to stall. This is called a cross-control stall, and is very dangerous if it happens at low altitude where the pilot has little time to recover. To avoid this scenario, pilots learn the importance of always making coordinated turns. They may simply choose to make the final turn earlier and shallower to prevent an overshoot of the runway center line and provide a larger margin of safety. Certificated, light, single-engine airplanes must meet specific criteria regarding stall and spin behavior. Spins are often entered intentionally for training, flight testing, or aerobatics.

Phases

In aircraft that are capable of recovering from a spin, the spin has four phases. At low altitude, spin recovery may also be impossible before impacting terrain, making low and slow aircraft especially vulnerable to spin-related accidents.
  • Entry – The airplane is stalled by exceeding the wing's critical angle of attack, while allowing the aircraft to yaw, or by inducing yaw with rudder initiated skidding uncoordinated flight.
  • Buffeting – At the critical angle of attack the boundary layer of airflow begins to separate from the wing airfoil, causing a loss of lift and resulting in oscillations of the control surfaces from turbulent airflow.
  • Departure – The aircraft can no longer maintain steady flight in a stalled condition and deviates from its original flight-path.
  • Post-stall gyration – The aircraft begins rotating about all three axes, the nose pitch attitude may fall, or in some cases rise, the aircraft begins yawing, and one wing drops.
Spins can be classified using the following descriptors:
  • Incipient – With the inside wing stalled more deeply than the advancing wing, both the roll and yaw motions dominate.
  • Developed – The aircraft's rotation rate, airspeed, and vertical speed are stabilized. One wing is stalled more deeply than the other as the aircraft spins downward along a corkscrew path.
  • Recovery – With appropriate control inputs, the yaw rotation is slowed or stopped and the aircraft nose attitude is lowered, thus decreasing the wing's angle of attack and breaking the stall. Airspeed increases quickly in a nose low attitude and the aircraft is no longer in a spin. The controls respond conventionally and the airplane can be returned to normal flight.

    Modes

The U.S. National Aeronautics and Space Administration has defined four different modes of spinning, defined by the angle of attack of the airflow on the wing.
Spin modeAngle-of-attack range, in degrees
Flat65 to 90
Moderately flat45 to 65
Moderately steep30 to 45
Steep20 to 30

During the 1970s, NASA used its spin tunnel at the Langley Research Center to investigate the spinning characteristics of single-engine general-aviation airplane designs. A 1/11-scale model was used with nine different tail designs.
Some tail designs that caused inappropriate spin characteristics had two stable spin modes—one steep or moderately steep, and another that was either moderately flat or flat. Recovery from the flatter of the two modes was usually less reliable or impossible. When the center of gravity was further aft, the spin was flatter, and the recovery was less reliable. For all tests, the center of gravity of the model was at either 14.5% of mean aerodynamic chord or 25.5% of MAC.
Single-engine airplane types certified in the normal category must be demonstrated to recover from a spin of at least one turn, while single-engine aircraft certified in the utility category must demonstrate a six-turn spin that cannot be unrecoverable at any time during the spin due to pilot action or aerodynamic characteristic. NASA recommends various tail configurations and other strategies to eliminate the flatter of the two spin modes and make recovery from the steeper mode more reliable.

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. In early aviation, individual pilots explored spins by performing ad-hoc experiments, and aerodynamicists examined the phenomenon. Lincoln Beachey was able to exit spins at will, according to Harry Bruno in Wings over America.
In August 1912, Lieutenant Wilfred Parke RN became the first aviator to recover from an accidental spin when his Avro Type G biplane entered a spin at 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, and horrified observers expected a fatal crash. Though disabled by centrifugal forces, Parke still sought an 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 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", spin-recovery procedures were not a routine part of pilot training until well into World War I. The first documented case of an intentional spin and recovery is that of Harry Hawker. In the summer of 1914, Hawker recovered from an intentional spin over Brooklands, England, by centralizing the controls. Russian aviator Konstantin Artseulov, having independently discovered a recovery technique, somewhat different from Parke's and Hawker's, on the frontlines, demonstrated it in a dramatic display over the Kacha flight school's airfield on September 24, 1916, intentionally flying his Nieuport 21 into a spin and recovering from it twice. Later, Artseulov, at the time an instructor at the school, went on to teach this technique to all of his students, quickly disseminating it among the Russian aviators and beyond.
In 1917, the English physicist Frederick Lindemann conducted a series of experiments in a B.E.2E that led to the first understanding of the aerodynamics of the spin. In Britain, starting in 1917, spin recovery procedures were routinely taught by flight instructors at the Gosport School of Special Flying, while in France, at the School of Acrobacy and Combat, Americans who had volunteered to serve in the famous Lafayette Escadrille were by July 1917 learning how to do what the French called a vrille.
During the 1920s and 1930s, before night-flying instruments were commonly available on small aircraft, pilots were often instructed to enter a spin deliberately to avoid the much more dangerous graveyard spiral when they suddenly found themselves enveloped in clouds, hence losing visual reference to the ground. In almost every circumstance, the cloud deck ends above ground level, giving the pilot a reasonable chance to recover from the spin before crashing.
Today, spin training is not required for a private pilot licence in the United States; added to this, most training-type aircraft are placarded "intentional spins prohibited". Some models of Cessna 172 are certified for spinning although they can be difficult to actually get into a spin. Generally, though, spin training is undertaken in an "Unusual attitude recovery course" or as a part of an aerobatics endorsement. However, understanding and being able to recover from spins is certainly a skill that a fixed-wing pilot could learn for safety. It is routinely given as part of the training in sailplanes, since gliders often operate slowly enough to be in near-stall conditions while turning. Because of this, in the U.S. demonstration of spin entry and recovery is still expected of glider instructor certification. Also, before their initial certifications both airplane and glider instructors need a logbook endorsement of proficiency in spin training which, under Federal Aviation Regulations 61.183, may be given by another instructor. In Canada, spins are a mandatory exercise to get the private and commercial pilot licenses; Canadian recreational pilot permit candidates must do a stall and wing drop and must recover from a stall and wing drop as part of training.