Part 1 – The incipient spin and autorotation
The subject of aircraft spins is vast, almost limitless, in fact. This is the first of a four-part series focused on the phenomenon In later articles, I’ll cover the fully developed spin and recovery, as well as flat, inverted and accelerated spins. Since many of the principles of autorotation are contained in the execution of “flick” or “snap” rolls, these will also be explained.
There are three phases associated with the build-up to a spin. The first is the incipient spin and the second is autorotation, when newly created aerodynamic factors begin to drive the spin. The third is when the spin is fully developed. This article I’m going to focus on the first two phases.
Demonstrating an incipient spin
In order to best demonstrate an incipient spin, we put the aircraft into a situation which, for one reason or another, could play out in any pilot’s career.
Firstly, power is reduced slightly and the aircraft is put into a nose-high attitude with the airspeed decaying steadily. Then, enough of a rudder input is made to cause the aircraft to be flown with crossed controls and out of balance.
As the speed decays, further back pressure on the stick will need to be applied to keep the nose up. This causes the wings to steadily approach the stalling angle of attack.
Rudder in either direction may be applied, but in order to avoid an unnecessary duplication of the explanations later on in this series, we will choose right rudder and remain with it throughout. Up to this very moment, the relative wind would still have been coming entirely from the front of the aircraft.
The wing drops…
When the aircraft eventually stalls, the right wing is going to drop because application of right rudder causes the aircraft to yaw clockwise around its vertical axis. By so doing, this wing slows down and generates less lift. The left wing, being on the outside of the yawing motion, travels faster and generates more lift. The aircraft therefore tends to roll to the right.
In attempting to counteract the roll, the pilot moves the stick to the left, “crossing the controls”. The right aileron moves downwards and has the effect of increasing the angle of attack of that wing, bringing it even closer to the stalling angle of attack.
So, in the instant that the aircraft stalls, the right wing will drop. This will be the point at which the incipient spin will be considered to have occurred.
However, within a heart-beat the incipient spin stage will be over and done with.
Either an instantaneous recovery from the situation will have been made or the aircraft will enter the next phase of the spin, which is known as the autorotation.
If a recovery from the situation was initiated, this would be accomplished by promptly pushing the stick forward so as to reduce the angle of attack of the wings whilst simultaneously making an appropriate rudder input that would stop any yaw that was present. At the same time, full power from the engine would be restored and the aircraft would fly away safely.
If a recovery from the incipient spin was not made, then, within a second heart-beat, the process of autorotation would begin.
As the stall occurred the rudder input to the right would have been present. The stick would still have been commanding a nose-high attitude, so the angle of attack would have remained above and beyond the critical angle.
But since we actually wish to see a full-blown manifestation of what autorotation is all about, we go a step further and pull the stick fully backwards, thereby keeping the aircraft fully stalled. The amount of right rudder input is also increased to full deflection to achieve a maximum yawing effect.
There are different and dangerous effects that power from the engine and propeller could have at this stage. For the time being, I’ll keep it as simple as possible. Therefore, as the autorotation phase commences, the action of throttling back completely will be adopted.
As the right wing drops, it experiences a new and additional component of the relative wind from underneath the wing. The resultant relative wind meets the wing at an angle of attack that is far above the wing’s stalling angle of attack. This wing now experiences two things: a greater loss in lift and a greater increase in drag.
Conversely, the upward going wing, experiences a new and additional component of the relative wind from above. Its resultant relative wind meets the wing at an angle of attack that’s below the wing’s stalling angle of attack. It therefore experiences both an increase in the lift compared to what it was generating at the moment of the stall and a reduction in drag.
It is this differential in lift and drag between the two wings that provides the automotive force to “drive” or “propel” the auto-rotation of the aircraft. The aircraft now rotates around its vertical axis losing height very rapidly.
Autorotation will continue for as many as two or three turns until all the aerodynamic and inertial moments associated with the movement of the aircraft have worked themselves into a state of equilibrium. That point marks the beginning of the progression into a fully developed spin. This will be covered in Part 2.
Many pilots have failed to contain an incipient spin that took their aircraft into the auto-rotation stage.
If this ever occurs at altitudes of around 800ft or less above ground level, depending on the type of aircraft, the chances of survival will be minimal. The aircraft would make contact with the ground long before it had settled into a fully developed spin.