Smoke On Go

Incipient spins, autorotation and spinning – part 3

Keeping it safe during a fully developed spin

Part two dealt with how an aircraft transitions from the autorotation phase to the fully developed spin.

If your memory is hazy as to how and why this happens, please read part two again here.

An engine stoppage is always a possibility when doing a spin. In an American aircraft the propeller turns clockwise as viewed from the cockpit. Therefore, in a spin to the right, the aircraft will be moving in the same direction as the rotation of the propeller so that there is often a very noticeable “slowing down” of the r.p.m. In cases where the engine’s idling speed has been set at a lower-than-normal r.p.m., some have been known to stop during a spin.

There are many aircraft that have propellers that turn anti-clockwise as viewed from the cockpit. The Tiger Moth and Chipmunks are examples. The slowing down of the r.p.m. then occurs when the aircraft is spinning to the left

It is dangerous to make inappropriate control inputs and power changes during a spin.

During a spin, there are certain aerodynamic and gyroscopic factors that could cause a complete departure of the aircraft from its state of equilibrium. The aircraft would then have cause to gyrate differently, possibly also in a manner that is somewhat frightening. During such “departures” from the norm, a pilot could lose all situational awareness and then find it impossible to know how to put a stop to such seemingly “out of control” spin behavior.

The ailerons, elevators and rudder are often mishandled whilst an aircraft is spinning.

In an intentional spin the stick should be held fully back and should not budge from that position until the point arises for it to be moved firmly forwards so as to un-stall the wings for the recovery. A “rocking” movement of the stick, born of fright, disorientation, uncertainty, confusion, or panic will continuously change the aerodynamic pitching moment and therefore the state of equilibrium that the aircraft should otherwise be in. Depending on how actively the stick is moved forwards and backwards, the rate of rotation of the spin will vary considerably. A forward movement results in an acceleration of the spin rate. The reason for this is because of the “conservation of momentum” phenomenon. This, for example, is observed when you see a figure skater on ice doing a pirouette with arms out wide.  The rotational rate accelerates noticeably as the arms are brought into a position close to the body axis. The same happens to a spinning aircraft.  As forward elevator is applied, the aircraft’s body attitude steepens, the spin radius decreases, and the spin rate increases. When the stick is moved backwards, the body attitude becomes flatter, the spin radius increases, and the spin rate reduces.

The ailerons should be kept neutral, and the desired rudder input should be kept at full deflection.

Movement of the ailerons in either direction from the neutral position influences the amount of lift and drag that each of the wings is producing. This affects the overall lift/drag differential of the two wings and therefore the rate of rotation of the aircraft. With so many different aircraft types in existence, it is very difficult, without judicious experimentation on each particular type, to determine exactly what effect the application of aileron would have when applied in either the opposite or the same direction of the spin. Such applications could, quite definitely, precipitate either a “pro-spin” or an “anti-spin” reaction.

The amount of rudder applied would affect the rate at which the aircraft was yawing at. It should be kept at full deflection, thereby commanding a fixed rate of spin, right up until the time comes for the recovery.

Moving the throttle backwards and forwards in a spin will produce strong gyroscopic forces that will cause immediate and significant changes in pitch, yaw, and roll rates.

The propeller, being a rotating mass, exhibits the phenomenon of gyroscopic precession.  If a force is applied to any spinning or rotating mass, that force will not act at the point where it is being applied. Instead, it will precess and act at a point 90 degrees removed in the direction of rotation of the mass. Knowledge of this rule enables us to understand why certain situations associated with engine and propeller r.p.m. arise in a spin,

Let’s assume that we are in a fully developed, steady state spin to the right, with the stick being held fully back, the ailerons neutral and the throttle closed. The propeller is turning clockwise as viewed from the cockpit. There would, in all probability, be very little out of the ordinary spin behavior being exhibited by the aircraft. The pitch attitude of the aircraft would be remaining more or less constant, and the aircraft would be yawing at a constant rate towards the right. There is therefore a force that is being continually applied, from front to back, at the three o’clock position on the propeller’s rotating disc. This force precesses to the six o’clock position, where, if it was strong enough, it would tend to “push” the nose downwards around the aircraft’s lateral axis. However, at this stage, this force would be very weak because the engine is throttled back, and the propeller is turning slowly.

If the throttle is advanced, it will cause the propeller to turn faster and the strength of the effect of the force to intensify. The front to back force acting at the six o’clock position now actively pushes the nose steeply downwards. As discussed previously, a shorter spin radius occurs, and the aircraft’s spin rate accelerates dramatically, perhaps becoming so blindingly fast that the view of the ground is just a blur. Associated with this event is plenty of noise and vibration and the fact that the pilots are being flung over towards the left-hand side of the cockpit. This is about as frightening as a spin can be!

If, under exactly the same circumstances, we were to be doing a spin to the left, a force would exist that was being applied, from front to back, at the nine o’clock position on the propeller’s rotating disc. This force precesses to the twelve o’clock position from where it tends to “push” the nose upwards around the aircraft’s lateral axis. The aircraft would nevertheless remain in a fairly docile spin until the throttle was advanced and the propeller started to turn faster. At that point the force is significantly magnified, the nose moves upwards, and the spin “flattens”. With an increase in the spin radius, the rate of rotation reduces slightly. For the pilot, the aircraft would now be in a FLAT SPIN, a situation that is both exciting and frightening, the recovery from the maneuver will also be somewhat more complicated.

The bottom line is that any experimental flying associated with an aircraft’s spinning characteristics should be left to the certification test pilots at the factory. Try and save parts 1, 2 and 3 of this subject so that you can refer back to these notes. In the next issue, part 4 will deal with recoveries from steady state spins…




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