Smoke On Go

The incipient spin and auto-ration – part 2

From Auto-ration to the fully developed, steady state spin

Part one dealt with how the phenomenon of autorotation provides the “motor” to drive a spin. If your memory is hazy as to how and why this happens, please read part one again.

During the early stages of the autorotation, there are only four significant aerodynamic moments present, those being:

  • The low lift and high drag of the down-going wing.
  • The higher lift and less drag of the up-going wing.
  • The effect of up-elevator input in keeping a high value of angle of attack.
  • The rudder input that caused the yaw and induced a wing to drop.

However, other forces start manifesting as the aircraft auto-rotates.

Remember that, in order to avoid continued repetition, we have made a deliberate choice to explain what happens in a spin to the right. Since it is the right wing that has dropped and the aircraft is yawing very positively to the right, the nose will drop down below the horizon. This will cause the aircraft to side-slip in the direction of the autorotation. The whole right hand side of the fuselage and empennage will experience a relative wind. Because of the greater area and moment arm of the fuselage behind the centre of gravity, the aircraft “weathercocks” around its vertical axis towards the dropped wing in a clockwise direction. The auto-rotative “motor” then becomes more powerful. This is therefore a fifth aerodynamic moment, and it can be considered to be “pro-spin”.

A sixth aerodynamic moment now comes into play.

As the aircraft is yawing to the right in a clockwise direction around its vertical axis, a relative wind will also be generated on the left side of the fuselage towards the area behind the centre of gravity. Because of the greater moment arm of the fuselage between the centre of gravity to as far backwards as the fin, the aircraft will tend to weathercock around its vertical axis in an anti-clockwise direction, but AWAY from the dropped wing. This resistance to the clockwise yaw slows the auto-rotation down and the auto-rotative “motor” becomes weaker. This is a sixth   aerodynamic moment and it can be considered to be “anti-spin”.

Inertial moments

Throughout the auto-rotation stage, inertial moments are beginning to take effect. These are caused by the distribution of matter within and throughout the aircraft. Specifically, we talk about the matter concentrated in the fuselage of the aircraft and the matter concentrated in the wings of the aircraft. The moments of inertia of the fuselage will have an effect on the body angle that the aircraft spins at, and the moments of inertia of the wings will have an effect on the angle of bank that is achieved during the spin.

So, basically speaking, a heavy engine up front would be flung forwards and outwards from the perimeter of the aircraft’s path of rotation. This inertial moment would be strengthened if there was a really heavy person sitting in the back seat. The rear part of the aircraft would be flung backwards and outwards. Over all, the aircraft could be expected to take up a flatter attitude in a spin compared to if the back seat was empty. By the same reasoning, if, hypothetically, you were spinning an aircraft with wing-tip tanks that were full of fuel, both wings would tend to be flung out and away from the aircraft and it would have a flatter angle of bank in the spin compared to if the wing-tip tanks were empty.

Within two or three full turns since the onset of auto-rotation, all six aerodynamic moments will reach a state of balance and the aircraft will have settled into a state of equilibrium. A fully developed, steady state spin will then be said to exist.

During the spin there can be a lot of buffeting from the disturbed airflow over the wings and also over the tail surfaces. The whole experience can in fact be very disconcerting and for someone experiencing a spin for the first time, either in training or having entered one inadvertently, the experience could in fact be very frightening!

On the other hand, and as a positive, the spin may now in fact be held indefinitely up until a height is reached where recovery action must be commenced. The speed does not build up in the spin.  If one was to steal a glance at the airspeed indicator, one would see that the speed was very low.

A perfect state of equilibrium might not always be maintained in the spin. From time to time certain of either the inertial or the aerodynamic moments might predominate temporarily. The aircraft will then take up an oscillatory motion with the nose pitching up and down and the spin rate accelerating and decelerating throughout the spin.  This will happen until equilibrium is once again restored.

Enough for now! Try and save parts 1 and 2 of this subject so that you can refer to these notes. Part 3 will deal with the effects that power and the misplacement of controls can have on spoiling your day and then thereafter, the recovery from steady state spins.




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