|Last Updated: Mon Jan 27 11:18:09 UTC 2014|
Pitts S-2A (©
1993, Carlo Kopp).
The spin is one of the basic aerobatic manoeuvres, and one which every pilot should be comfortable with. Spinning is a mode of flight in which the aircraft is stalled and in autorotation about its vertical axis, either upright or inverted, and is a condition during which substantial altitude may be lost and a high rate of descent maintained.
What Happens in a Spin
Spinning characteristics are inherently type specific, as the aerodynamic shape and mass distribution of aircraft are type specific. How an aircraft will spin, and its ability to recover from a spin, either through natural stability or pilot control input, are highly sensitive to the aircraft's design. This discussion will concentrate on the typical behaviour of aerobatic trainers which are the class which a student of spinning is most likely to encounter.
All spins can be divided into three phases, the transient entry phase, termed the incipient spin, the fully developed spin and the recovery, if the latter is possible. A typical condition for entry into a spin is a stall, coupled with some yaw/roll input. Under these conditions the upgoing wing is less stalled than the downgoing wing, and the roll damping effect which maintains the aircraft wings level under unstalled conditions now generates a moment which rolls the aircraft toward the downgoing wing, resulting in a self-sustaining yawing-rolling rotation. This condition is initially unstable and will settle, over a period of several rotations into a stable spinning condition, the fully developed spin. The motion of an aircraft during a spin entry involves often a very complex combination of moments, and a period of initial oscillation is not uncommon for some types.
Once in a fully developed spin the aircraft will usually be in a state of stable autorotation, although some types may also stably oscillate in addition to autorotating. This continues until recovery or impact. Some types will remain in a state of sustained incipient spin, never reaching the autorotating state characteristic of the fully developed spin.
Recovery requires the unstalling of the wings and the arresting of the yaw, the latter depending critically on the ability of the aircraft's controls to counter the rotational energy built up in the autorotation. If for reasons of aerodynamic design or mass distribution of the aircraft this cannot be achieved, the spin may not be recoverable.
The amount of rotational energy accumulated is critically dependent upon the mass distribution of the aircraft, more mass further from the CoG results in more rotational energy, an aft CoG condition is a typical instance, full wingtip tanks, or a long nose and engine well forward of the CoG are others. Another factor which may exacerbate this is the effect of a large and heavy prop, which under power on conditions may add a pro-spin gyroscopic force to the aerodynamic spinning moments. Some WW2 fighter aircraft were typical instances and unrecoverable once a spin fully developed.
The effectiveness of the rudder is of critical importance in spin recovery, as it is the principal means of arresting the yawing motion. Should the effectiveness of the rudder be reduced, the spin may become unrecoverable. An aft CoG condition is a common cause of reduced rudder effectiveness, as it reduces the nose down attitude of the aircraft in the spin thus shielding the rudder. Another common cause of reduced rudder effectiveness is the position and input to the elevator, in some types stick forward will contribute to rudder shielding.
Entry and Recovery of Spins
Spin entries can be intentional or inadvertent. Inadvertent entries usually result from stalling an aircraft under conditions where rolling/yawing moments are present, the initial wing drop in the stall developing into a full spin if not handled properly.
Intentional spin entry should have a clean stall, which in most types is achieved by reducing airspeed and maintaining a nose up attitude of at least 14 degrees. At lower nose attitudes the stall will be less pronounced and the aircraft may need to be forced into the spin, which will cost points in competition.
As the aircraft enters the stall the nose will drop, at which point full back stick and full rudder should be applied in the intended direction of the spin. Typically the aircraft will assume a nose low attitude, as the incipient phase is passed and the spin fully develops. Throughout the spin full back stick with aileron neutral must be held (some specific types may also need aileron), to maintain the wings in a stalled condition.
The horizon will be moving in the direction opposite to the direction of the spin, and it is important to be aware of this, particularly where the entry is inadvertent and hence not planned. The direction of the spin is of critical importance in determining the required input to recover the spin.
Recovery is initiated by applying rudder opposite to the direction of the spin, to slow and eventually arrest the rotation. When the yaw rate has dropped or stopped, backpressure should be eased off to unstall the wing, while neutralising the rudder. As the wing unstalls the aircraft will typically assume a near vertical nose down attitude, and speed will rapidly build up if this attitude is maintained. Therefore it is necessary to pull out of the dive, and it is important to apply the right amount of backpressure to prevent the wing from stalling again, but also to ensure that Vne is not reached during the recovery. Once the elevator is applied to the recovery, power can be applied to enhance the effectiveness of the tail controls and effect lesser height loss.
Much the same applies for inverted spins, with attention being paid to the direction of the elevator input which has been reversed. Because the direction of rudder action is maintained in inverted flight, the application of rudder opposite to the direction of rotation will achieve the desired effect. With most types the rudder will be more effective in the inverted spin, as less of its area is shielded by the elevator. A common problem in inverted spinning is disorientation, where the pilot confuses the direction of the yawing motion with the rolling motion, as the horizon is typically obscured by the nose.
The spinning behaviour of the aircraft will be dependent upon the direction of the propeller's rotation. A clockwise turning prop (from the cockpit) will add a gyroscopic force which will flatten a left upright or right inverted spin. The magnitude of the gyroscopic effect will be proportional to the size of the prop and power setting.
What Can Go Wrong
The sensitivity of the spin to the aerodynamic and mass distribution characteristics of each and every type of aircraft, and the idiosyncrasies of type specific recoveries make the spin a potentially lethal manoeuvre in the hands of the unfamiliar. There are two common ways in which spins and recoveries can go badly wrong.
The first instance is where the recovery cannot be effected and the aircraft continues spinning until it impacts the ground. Failure to recover a spin may result both from improper technique or pilot disorientation as to the direction of the spin, or from inappropriate choice of aircraft or configuration for the manoeuvre.
Some aircraft will be reluctant to exit a spin as the effectiveness of their controls will be inadequate to arrest the rotation, as discussed above. This condition may also result from inappropriate loading or filling of tanks, both of which can change the mass distribution to a configuration where more rotational energy (ie having increased the moment of inertia) can be stored in the aircraft, than what the controls can overcome. Some types may not be recoverable, others may take a lot of altitude to recover. This means that several rotations may pass before the rate of rotation is lowered to the point where recovery can be effected, and this must be accounted for in the height budget for the manoeuvre.
Therefore it is important to ensure that you practice spinning in aircraft types with loadings which are known to be within the safe envelope for this manoeuvre, and always ensure that the instructor has experience in multiturn spins and understands the limitations of the aircraft.
Another area where spins can go badly wrong is in improper recovery technique, resulting in the overstressing of the airframe. One technique which is popular in spite of having caused fatal accidents is that of letting go of the controls and allowing natural stability to take its course, the aircraft recovering on its own. While this may work for some types under some conditions, it may not work at all and the spin may continue, or it can lead to a dangerous situation during the recovery.
A typical situation would involve the use of this technique to exit the incipient spin, where the aircraft is in a condition more akin to a spiral dive, with the elevator acting more like a speedbrake. Relaxing backpressure will allow the aircraft to rapidly accelerate its spiral dive, possibly passing through Vne in the process, with potentially fatal effects. Even where an aircraft will naturally recover from a spin upon releasing the controls, this situation may develop, and common sense therefore suggests that a technique using positive control inputs is always more appropriate. Hands off recovery technique is not recommended for spins of more than two turns under any circumstances.
The cardinal rule in spinning is to recognise that particular entry and recovery techniques will always be specific to the type at hand and its loading, and no one technique is suitable for all aircraft types. It is therefore of great importance that the aerodynamic features and mass distribution of any type are studied before you spin it and understand what impact they will have on how the type spins. Above all, don't be shy about asking somebody who knows. It could save your life one day.
Acknowledgements: Chris Burns,
Victorian Unlimited Aerobatic Champion (the author's aerobatic coach at
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