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Spinning

Current flight training no longer requires spinning to be taught as part of the Private Pilot License syllabus. Pilots are taught to avoid the conditions, which may lead to the incipient stage of a spin and through regular stall training a safety margin can be maintained (in theory). Due to this, there is a lack of understanding as to what a spin is, how it develops and how it is recovered from. Due to the lack of exposure, there is a lot of ‘unease’ around the subject.


Defining the Spin


The spin has been defined as a motion in which an airplane in flight at some angle of attack between the stall and 90 degrees, descends rapidly towards the earth while rotating about a vertical axis. The spinning motion is very complicated and involves simultaneous rolling, yawing, and pitching while the airplane is at high angles of attack and sideslip.

The spin manoeuvre has 3 phases (Not including Entry):

1.) Incipient - Occurs from the time the aeroplane stalls and rotation starts until the spin axis becomes vertical or nearly vertical. During this time the airplane flight path is changing from horizontal to vertical, and the spin rotation is increasing from zero to the fully developed spin rate. The incipient spin usually occurs rapidly for light airplanes (4 to 6 seconds, approximately) and consists of approximately the first two turns.

2.) Developed - In the developed spin the attitude, angles, and motions of the aeroplane are somewhat repeatable from turn to turn, and the flight path is approximately vertical. The spin is maintained by a balance between the aerodynamic and inertia forces and moments. The spinning motion is made up of rotation about the aeroplane center of gravity.

3.) Recovery - The third phase is caused by a change in the moments so as to upset the balance between the aerodynamic and inertia moments. Such a change in the moments is obtained by deflecting the controls of the aeroplane. The specific control movements required in any particular aeroplane depend on certain mass and aerodynamic characteristics.



Aerodynamics in the Spin

A spin is an aggravated stall that typically occurs from a full stall occurring with the aeroplane in a yawed state and results in the aircraft following a downward corkscrew path. As the aeroplane rotates around a vertical axis, the outboard wing is less stalled than the inboard wing, which creates a rolling, yawing, and pitching motion. The airplane is basically descending due to gravity, rolling, yawing, and pitching in a spiral path. The rotation results from an unequal AOA on the airplane’s wings. The less-stalled rising wing has a decreasing AOA, where the relative lift increases and the drag decreases. Meanwhile, the descending wing has an increasing AOA, which results in decreasing relative lift and increasing drag.


A spin occurs when the aircraft’s wings exceed their critical AOA (stall) with a sideslip or yaw acting on the aeroplane at, or beyond, the actual stall. An aeroplane will yaw not only because of incorrect rudder application, but because of adverse yaw created by aileron deflection, engine/prop effects (p-factor, torque, spiraling slipstream), gyroscopic precession, wind shear, and wake turbulence. If the yaw had been created by the pilot because of incorrect rudder use, the pilot may not be aware that a critical AOA has been exceeded until the aeroplane yaws out of control toward the lowering wing. A stall that occurs while the aircraft is in a slipping or skidding turn can result in a spin entry and rotation in the direction of rudder application, regardless of which wingtip is raised. If the pilot does not immediately initiate stall recovery, the airplane may enter a spin.



The diagram above shows the difference in drag and lift acting on each wing in the spin.


Factors Affecting the Spin

Without going into too much detail, there are 3 major factors affecting spin characteristics:

1.) Mass Distribution - The way in which the mass of an airplane is distributed between the wing and fuselage is the most important single factor in spinning because it determines the way in which the aeroplane, while spinning, responds to control movements, especially to elevators and ailerons. An aircraft rotating in a spin can be considered to be a large gyroscope. Since there are mass and angular rotation about all three axes, inertia moments

are produced about all three axes.

2.) Relative Density - Aircraft relative to the density of air which it is flying in. This only varies slightly in light aircraft with changes in altitude and load (fuel).

3.) Tail Configuration - Rudder and Elevator effectiveness, use of ventral and dorsal fins.

Spin Procedures


When flying the manoeuvre in the aircraft, 2 key things must be considered. The aircraft must be approved to conduct intentional spinning (as per the AFM) and attention to the pre-flight checks becomes even more critical. Emphasis on excess loose items, which may affect weight, centre of gravity and controllability of the aeroplane. The C of G envelope can be assured by correct loading and plotting the mass and balance before flying.

Entry

In the entry phase, the pilot intentionally or accidentally provides the necessary elements for the spin. The entry procedure for demonstrating a spin is similar to a power-off stall. During the entry, the pilot should slowly reduce power to idle, while simultaneously raising the nose to a pitch attitude that ensures a stall. As the aeroplane approaches a stall, smoothly apply full rudder in the direction of the desired spin rotation while applying full back (up) elevator to the limit of travel. Always maintain the ailerons in the neutral position during the spin procedure unless AFM/POH specifies otherwise.

Recovery (Within the Incipient stage, no more than 2 rotations)

The recovery phase occurs when rotation ceases and the AOA of the wings is decreased below the critical AOA. This phase may last for as little as a quarter turn or up to several turns depending upon the aeroplane and the type of spin.

To recover, the pilot applies control inputs to disrupt the spin equilibrium by stopping the rotation and unstalling the wing. To accomplish spin recovery, always follow the manufacturer’s recommended procedures. If there isn’t any in the AFM, follow this generic procedure:

Spin Recovery Template

1.) Power - Idle

2.) Aileron - Neutral

3.) Full opposite rudder to rotation direction

4.) Forward elevator (forward of neutral)

5.) Neutralise rudder once rotation stops

6.) Pitch up to return to level flight

1.) Power aggravates the spin. It can lead to a flatter spin attitude and usually increases the rate of rotation.

2.) Dependent on type, into spin aileron may help with recovery. Out of spin aileron can flatten the spin attitude and delay recovery. Best procedure is to remain aileron neutral.

3.) This is the most important control for recovery. Apply it confidently to full-deflection and hold until the rotation stops.

4.) Do not wait for the rotation to stop before doing so. This will lead to a decrease of AOA and directs the aircraft towards installed flight. Hold the controls in this position until rotation stops.

5.) Failure to do so as the aircraft accelerates leads to yawing/sideslip.

6.) Progressively apply to ease out of the dive. Avoid excessive G-loading and potential secondary stall. This has to be balanced with avoidance of accelerating through Airspeed limitations.

Again, it is important to emphasise that the spin recovery procedures and techniques described above are recommended for use only in the absence of the manufacturer’s procedures.

Common Errors

Common errors in the performance of intentional spins are:

  • Failure to apply full rudder pressure (to the stops) in the desired spin direction during spin entry

  • Failure to apply and maintain full up-elevator pressure during spin entry, resulting in a spiral

  • Failure to achieve a fully-stalled condition prior to spin entry

  • Failure to apply full rudder (to the stops) briskly against the spin during recovery

  • Failure to apply sufficient forward-elevator during recovery

  • Waiting for rotation to stop before applying forward elevator

  • Failure to neutralise the rudder after rotation stops, possibly resulting in a secondary spin

  • Slow and overly cautious control movements during recovery

  • Excessive back elevator pressure after rotation stops, possibly resulting in secondary stall

  • Insufficient back elevator pressure during recovery resulting in excessive airspeed

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