Rufus Heald discusses spinning

Spin recovery. Once it was part of the PPL but now it's not even necessary to experience a spin during training (although some instructors will make the effort). The very mention of a spin worries some pilots but the first step to overcoming that fear is to understand the principles behind a spin - and the recovery.

To make a positive recovery from a spin, it is necessary to assist the anti-spin forces so that the aircraft stops yawing and rolling. Normally roll control of the aircraft is produced by deflecting the ailerons so that one goes down, thereby increasing the angle of attack on that wing only, and the other goes up, reducing the angle of attack on the other wing. This causes an imbalance of forces and rolls the aircraft.

However, in a spin the inside wing, which is also the down-going wing, is already stalled; applying normal aileron to raise that wing by increasing the angle of attack will be counter-productive. Such action will increase the angle of attack of the already stalled wing and still further reduce its lift and increase its drag.

It is therefore essential that corrective 'out-spin' aileron is not applied. If, however, aileron is applied in the same direction as the spin then the aileron on the inside/downgoing wing is raised and the other is lowered. This will reduce the angle of attack of the inside wingtip and so tend to unstall it and thereby reduce its aerodynamic drag.

As I explained earlier, the rolling motion in the case of a spin is produced by the yawing of the aircraft. If this yaw is eliminated, the roll will cease and both wings will be at the same angle of attack and generating the same amount of lift. The aircraft will then be able to accelerate downwards until normal flying speed is reached and it can then be manoeuvred in the normal manner back into its desired flight path. The normal way to control the angle of attack of the wings is by elevator movement. Since the stall has resulted from an excessive angle of attack, forward movement of the control column will tend to reduce the angle of attack enabling the aircraft to recover from its stall. Indeed this is the standard recovery from a stall which is taught to every pilot in the world.

 

Yaw and balance

You can see from the foregoing that to recover from a spin it is necessary to use rudder to counteract the yaw and resultant roll, and to use elevator to reduce the angle of attack until the aircraft is unstalled. The aircraft can then be flown in the normal conventional manner.

There are several factors which affect recovery from a developed spin. First, I will deal with the position of the centre of gravity. If this is too far forward, the aircraft will be over-stable and may have insufficient elevator control to enable it to enter a spin from gliding flight. If it is too far aft then the control movements required in the pitching mode will be very light and may become unstable at large angles of attack. This is why the manufacturer has placed limitations on the position of the centre of gravity: to ensure that it is neither so far aft that it inhibits recovery nor so far forward that it inhibits manoeuvrability such that the aircraft will not spin from a glide. The next factor which affects the spinning characteristics of an aeroplane, and hence its handling in a deliberate spin, is the engine. This has two elements which affect the spin. Firstly, at high power the airflow generated by the propeller slipstream blowing over the rudder and tailplane will significantly increase the effectiveness of these controls. The effectiveness of an aerodynamic surface is proportional to the square of the speed of the airflow over it, so at high power the rudder and elevator will probably be capable of inducing a spin at almost any speed and centre of gravity combination, and for this reason the flight manual states that a deliberate spin should be entered from a glide.

The second engine aspect is the question of centrifugal force and gyroscopic precession. The engine and propeller are a reasonably large rotating mass, rotating, in the case of, say, a Cherokee PA28-140, in a clockwise direction as seen from the cockpit. The effect of yaw in the spin will cause the engine to exert a pitch-up force in a spin to the left (left spins are more common than right spins because the slipstream tends to make the aircraft yaw to the left), and a nose down force in a spin to the right, as a result of gyroscopic precessional forces. The size of this force will be proportional to the rpm the engine is producing. The effect of slipstream during a spin will also tend to yaw the aircraft to the left in opposition to any corrective rudder which might be applied to recover from a spin to the left. For these reasons the throttle should be closed for recovery from an unintended spin, and should be closed throughout in the case of a deliberate spin.

In practical terms, when an aircraft is reluctant to enter a deliberate spin a pilot will normally use a 'fast tickover' rather than a fully closed throttle so as to enhance the effectiveness of the tail controls. Where idle rpm in the air would normally be around 1000rpm, pilots will use around 1200rpm to help initiate the spin, closing the throttle once the spin has been established.

 

Weighing it up

The third factor which will affect the spinning characteristics of an aircraft are its total weight and the disposition of that weight. At a large angle of attack, the nose is well above the longitudinal axis and during either a rapid roll or a rapid yaw, inertia of the mass of the engine will tend to cause it to perform a circular movement.

Centrifugal force around a spin axis will tend to cause the nose to pitch up into an even larger angle of attack. If the flight path is in a downward direction, as it must be in a spin, then the centrifugal effect of the engine mass will be to tend to flatten the aircraft's attitude in relation to the earth and to the axis of the spin.

Finally, variations in engine rpm at the moment of spin initiation will modify the yawing and pitching forces acting on the aircraft. There are an almost infinite number of combinations of possible weight distribution and engine power settings.

When attempting to make an aircraft spin, the 'normal' entry is from a glide with the engine throttled back. If the aircraft is reluctant to enter a spin, it is usual to slightly increase the airspeed of spin initiation and also the engine rpm, to provide more effective controls. These increases are of the magnitude of 5-8 knots of airspeed and idle rpm being increased from about 1000 to 1200. As soon as the spin is initiated and established, the throttle should be closed.

Finally, if you're keen to trying spinning (and spin recovery!), do it with an instructor.

 

This article was originally published in the May 1999 issue of FLYER