A spin is a yaw aggravated stall which results in rotation about the spin axis. The aircraft follows a steep, “corkscrew” like, downward path. Spins can be entered, either intentionally or unintentionally, from any flight attitude and at practically any airspeed.
The Incipient Spin (Autorotation)
If the wing drops at the stall, the resulting rolling action alters the direction of the relative airflow on to the wing. It increases the angle of attack of the down-going wing and reduces the angle of attack of the up-going wing.
This alters each wing’s coefficient of lift and drag. The down-going wing becomes more stalled, leading to a reduction in the coefficient of lift and an increase in the coefficient of drag.
Conversely, the up-going wing becomes less stalled, leading to an increase in the coefficient of lift and a reduction in the coefficient of drag.
The difference in lift between the wings produces a rolling moment and the aircraft rolls in the direction of the down-going wing.
These moments lead to autorotation; the aircraft continues to roll, a side-slip develops, and the nose drops. Without corrective action, the rate of rotation steadily increases, resulting in a fully developed spin.
RECOVERY
》To recover from the incipient spin: Ease the control column forward to unstall the wings.
》Apply rudder to prevent further yaw.
》As the airspeed increases, level the wings using co-ordinated rudder and aileron inputs. Apply power and recover any lost altitude.
•The Fully Developed Spin
The Fully Developed Spin
The development and characteristics of a spin depend on the aircraft’s design, the distribution of its masses, and the operation of its control surfaces. It is usual for the aircraft to rotate several times around a spin axis before settling into a steady state spin, with the final pitch attitude depending mainly on the position of the centre of gravity. The spin can be either flat or steep.
C.G.
The position of the centre of gravity, even if it remains within its permitted safety limits, affects the spin as follows: Forward Centre of Gravity This results in a steeper spin and a faster rate of sink. This makes the recovery easier because the spin is less stable. If the centre of gravity is forward of its permitted limits, it significantly reduces the likelihood of a spin occurring, and instead results in an unusually steep spiral descent, during which the indicated airspeed increases.
Aft Centre of Gravity This results in a flatter spin and a lower rate of sink. This makes the recovery more difficult, because the spin is more stable. If the centre of gravity is aft of its permitted limits, it significantly reduces the likelihood of recovery from a settled spin condition.
When an aircraft is in a steep spin rotation it is primarily in roll, whereas in a flat spin it is primarily in yaw
RECOVERY
Spin recovery, like recovery from a simple stall, requires re-attachment of the separated airflow over the wings. For a successful recovery from a spin, it is necessary to first stop the yawing moment and reduce the rolling moment. The usual recovery technique is to:
》Centralise the control surfaces and reduce the power setting.
》Verify the direction of spin on the turn and balance indicator and apply full opposite rudder.
》Allow the rudder to become effective, then ease the control column forward to reduce the angle of attack and unstall the wings.
》As the rotation stops, centralise the rudder, level the wings, and gently pull out of the ensuing dive.
》Apply power and climb the aircraft to regain any lost altitude.
Note: When pulling out of the ensuing dive, be careful to prevent an accelerated or ‘g’-stall, and subsequent entry into another spin.
