Spinning Part 2, The Aerodynamics of a Spin

Adrian Willis
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When an aircraft wing “stalls”, the airflow detaches from the wing and reduces lift and increases drag. The stall is the name given to this condition which is caused by the angle of attack (alpha) being excessive or going beyond critical. The speed this occurs at varies according to various factors including G, weight, power setting, air density etc. and is largely irrelevant from the pilots point of view as it is the critical alpha that is relevant, not the speed. It is convenient to know the approximate speed of reaching critical alpha at a certain configuration. In terms of aerobatics this will be a power off, 1 G condition at normal weight and normal height, clean stall. We know also that with power on, critical alpha will be at a lower speed, all other factors being equal. In the Extra 200, the aircraft we teach aerobatics and spinning in at the British Aerobatic Academy and what most of our tutorial will be based on, this is approximately 60kts.

If full left rudder is applied when the aircraft is stalled the yaw will cause the left wing to move backwards and the right wing forwards, the local airspeed difference will cause the right wing to rise and the left wing to drop and as the rudder is held in, this will continue. If the rudder were to be neutralised, at this very early stage, the spin would stop. This early stage where auto rotation has not become self-sustaining is known as the incipient spin.

With the rudder held in, the rolling and yawing movement will cause the left wing to have a greater angle of attack than the right, i.e. “be more stalled” and this will cause more drag and less lift which will cause the autorotation to be self-sustaining. This is known as a fully developed spin and occurs after a different number of rotations in different aircraft. It also occurs after a different number of rotations depending on how the spin is entered. In the Extra for example, if you enter the spin from significantly below the 1 G stalling speed, say 40 knots, it will take a long time to become fully developed and will be very slow to start spinning at all. This is because both wings will be heavily stalled and when rudder is applied, unless it is applied very vigorously, the lift difference between wings is not great so the aircraft just sinks with very little wing rise and therefore the alpha on each wing is not very different so autorotation is slow to develop.

From this description it is important to understand that the myth that if you let go of all the controls during a spin, it will recover itself, is wrong. This is only true if the spin is insipient and cannot be true, by definition, in a fully developed spin. What is certainly true in many aircraft is that the insipient stage can last a long time and during this period if you remove all control inputs, the aircraft will recover.

It is important to understand the effect of aileron. If you are spinning to the left and you apply right aileron, it will slow down recovery and increase the rate of rotation a little. This is because right aileron increases the angle of attack of the left wing and so reduces lift and increases drag which creates a stronger auto-rotative force. The effects of aileron vary according to aircraft type. In a Sukhoi 29 for example, full out spin aileron will prevent recovery. In an Extra it will significantly slow it down. The bottom line is to resist the natural instinct to use out spin aileron and consciously check the stick is neutral during recovery. The engine and propeller also have a big effect on the characteristics of a spin. If you are flying a lycoming powered aircraft, with a propeller that rotates to the right as seen from the pilots seat, then the gyroscopic force of propeller will lift the nose in a left hand spin. The amount of this force will vary according to the power setting. Therefore in all cases you must ensure the throttle is fully aft when entering a spin.

Later on I will write about flat spins, this is when we will push the throttle forward and thus flattening the spin, and inverted spin, this is where the aircraft enters inverted. It is the same as an erect spin except you are upside down and consequently the roll is opposite to yaw.  – subscribe to our newsletter to make sure you don’ t miss this!

Spinning is a complicated topic and is misunderstood by many, if you have any questions about spinning, please write a comment below and I will answer it as quickly as I can. If you want to learn how to spin properly, why not book a spinning and upset recovery course – or go the full hog and sign up for the Basic Aeros course?

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