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03 Aug : 16:27

Flight Dynamics
SPINNING
By Keith Ashman CFI

Part 1 (Part 2 follows bellow)

Statistics show worldwide that the spin is responsible for more fatalities in our sport than any other type of accident.

It is probably the least understood and practiced manoeuvre in gliding and as a result requires special attention to prevent spin accidents. Please read the following article carefully in order to fully grasp this complex dynamic behaviour.

The investigation into a long span two-seater accident in the USA showed that it is very important to understand the dynamics of the spin and the mode of spin entry. To summarise, the glider entered what appears to be a cross-controlled entry whilst thermalling.
This results typically in a steep nose down entry with rapid wing roll-off into the spin. The glider recovered (as it will do due to rapid speed build-up) with the nose ± 45 degrees below the horizon. The nose was then lowered to almost vertical and the glider gained speed quickly with airbrakes being used together with a high “G” pullout. The structure failed due to over-speed and over-stress. The failure mode is expected as the airbrake is mounted in the inboard panel of the wing and the outboard panels failed close to their roots.

The concern therefore exists that often a spin recovery is simply performed according to a well-documented process. In the case of this glider it appears that the recovery had been affected before lowering of the nose to build speed. The implication being that for it to recover there was sufficient speed and no need to lower the nose further. In modern gliders the speed builds quickly resulting in recoveries approaching or past VNE.

The problem with the well-documented stick forward opposite rudder or opposite rudder stick forward process is that it does not prompt the pilot to consider “flying the spin”, it is simply a mechanical control input and therefore prone to misuse.

There does exist a need to understand the mode of spin entry in order to fly the spin recovery in a manner that will effect a quick safe recovery.

For this understanding to be possible, knowledge of the aerodynamic layout of the glider and of how this affects the spin behaviour, is essential.

Unfortunately not all gliders behave similarly and many factors affect the spin entry and recovery mode. Some factors are listed to help understand their influence on spin behaviour.

AILERONS AND FLAPS:

Flaps increase the angle of attack of the wing as does downward aileron input. The angle of attack increase also causes an increase in DRAG. This increase in DRAG slows the wing and together with the increased ANGLE OF ATTACK eventually stalls the wing.

The portion of the wing affected by the downward moving aileron or flap will then stall more readily than if the aileron or flap were in a neutral position. This complicates the behaviour as the wing that was originally designed to stall from the root towards the tip may stall at the tip first or in the case of flapperon-equipped gliders the wing may stall all at once.

This causes a rapid “roll-off” and wing drop during a cross-controlled entry or an entry resulting from aileron input as an attempt to raise the dropping wing.

The size of the aileron as a percentage of the tip chord of the wing at its tip will affect how rapidly the positive aileron input stalls the wingtip. Aileron differential reduces the downward movement of an aileron to reduce tip stalling.

THE PLANFORM OF THE WING

Many modern gliders have elliptical sheared back planforms to maximise the lift distribution over the wing. This is done to allow the whole wing to remain unstalled at maximum lift to a speed just above stall. This means that the wing is designed to stall all at once. The result of this planform is very little stall warning buffet (the wing roots are still flying close to the stall whereas some less efficient planform shapes have started to stall at the root resulting in a pre-stall buffet) and the potential for a large portion of the wing to stall with any positive aileron input or massive yaw input.

THE SIZE OF THE RUDDER

The glider will always spin in the direction of excess yaw not
always in the direction of the turn.
A small rudder means that some gliders do not enter a spin using the traditional stall and rudder input techniques. These machines can be spun from a gentle turn by simply using excess rudder first and then stalling the glider, some opposite aileron will often help.

Thinking about the above, this is the mode of spin entry that kills!

Consider a low slow turn with too much pro-turn rudder and a tip that is being held away from the ground with opposite aileron and the speed is low. This ultimately results in the cross-controlled entry mode with the glider rolling off with a RAPID WING DROP ANDA RAPID NOSE DOWN ATTITUDE if not past vertical . This mode is the most common, most violent and least understood entry mode not to be confused with an incipient spin as there is not a real incipient stage.

Here the spin must be flown in order to recover efficiently and safely.

This involves using sufficient opposite rudder input to stop the rotation and sufficient (if any) stick forward input to unstall the glider. Usually with a cross-controlled entry the use of a neutral stick position with sufficient control input to recover from the dive to prevent excess speed build-up (theVentus 2CT recovers from a normal cross-controlled entry at VNE) and stay below VNE is required. This means that a cross-controlled entry near the ground is normally fatal if not understood properly in order to recover quickly.

A large rudder could result in a spin out of the turn if too much rudder is used very early in the incipient stage of a spin entry recovery.

In the next article the dynamics of the cross-controlled entry will be explained and some practical tests discussed in terms of efficient recovery.

- End of Part 1 -

SPINNING
By Keith Ashman CFI

Part 2

In the previous section the basics of flying the spin were discussed. Practically this needs to be applied with specific aerodynamic layout knowledge to ensure a quick recovery.
Many flapped gliders will recover rapidly from a crossed control entry by simply moving to a less positive flap setting. This has the effect of unstalling the stalling/stalled wing to speed up recovery.

In the case where the aileron drag is high pro-turn aileron ie; aileron towards the spin turn also effects a recovery which with opposite rudder will speed up recovery.

Although not an action many pilots would naturally use the increased drag on the flying wing slows it down and the aileron portion of the stalled/stalling wing tends to unstall.

This can be demonstrated on some types where the aileron drag effects are so great that a recovery with pro-turn aileron is possible.

What is being highlighted here is the need to fully comprehend how the aerodynamic layout of the glider is likely to affect the spin entry and recovery.
This is particularly relevant with the crossed control entry.

Remember that the crossed control entry leaves the glider with a nose down entry and a rapid roll and yaw into that position, this of course means that stopping rotation is critical and recovery from the dive very important.
The recovery in terms of the build up of speed is helped by the nose down attitude which will allow speed to build quickly. Rotation can be stopped with sufficient opposite rudder and the recovery from the dive must be prompt without causing a high speed stall or allowing too much speed to build.

It is often claimed that some modern trainers are reluctant to spin off a crossed entry. Twin Astirs are an example where the spin is helped by; first initiating excess yaw, then stalling the glider, then applying opposite aileron.

This allows a gentle turning type of crossed control entry so typical of many spin-in accidents. The entry is so violent that it often surprises pilots given the apparent docile nature of the type.
Generally those that enter easily will recover quickly as well and those that require a lot of input to spin take a little longer to recover.


There is no fixed rule of thumb for the most efficient recovery on various types other than to unstall the glider by reducing angle off attack and stop the rotation.

Blindly using wrote learned recovery techniques without considering their suitability can be fatal. 

One particular open class glider accident report stated that the glider was seen to enter a spin whilst thermalling steeply. The rotation stopped with the glider pointing nose steeply down and thereafter the nose was lowered further and the glider broke up due to an application of airbrake at a speed and g-loading too high for that glider during the dive recovery.
Without full details it is possible to speculate that this fits the crossed entry scenario with standard recovery implemented, resulting in an overspeed in the dive.

The rotation had stopped so in fact dive recovery could have been implemented to keep the recovery speed low. The entry was most probably cross controlled, as this is how big gliders are configured whilst turning steeply.


Food for thought, but more importantly food for understanding how and why we spin.

- End of Part 2 -