Spin (aerodynamics)

A spin is a special category of stall resulting in autorotation about the vertical axis and a shallow, rotating, downward path. 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 is a special category of stall resulting in autorotation about the vertical axis and a shallow, rotating, downward path. 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'. A method used to control a spin before it fully develops is a maneuver called the falling leaf. Many types of airplanes spin only if the pilot simultaneously yaws and stalls the airplane (intentionally or unintentionally). 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 (thus increasing the angle of attack and load factor) 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. In aircraft that are capable of recovering from a spin, the spin has four phases. Some aircraft are difficult or impossible to recover from a spin, especially a flat spin. At low altitude, spin recovery may also be impossible before impacting terrain, making low and slow aircraft especially vulnerable to spin-related accidents. The U.S. National Aeronautics and Space Administration (NASA) has defined four different modes of spinning. These four modes are defined by the angle of attack of the airflow on the wing. 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. The further aft that the center of gravity was located the flatter the spin and the less reliable the recovery. For all tests, the center of gravity of the model was at either 14.5% of mean aerodynamic chord (MAC) or 25.5% of MAC.