The first aircraft I ever spun was the AT6. This was way back in 1964 when I was a 17-year-old Pupil Pilot in the South African Air Force. At the time that I started my flight training, the AT6 Harvard had already been in service as a trainer with our Air Force since the closing stages of World War II and would continue to be the backbone of Training Command’s inventory for another 30 years, before being replaced by the Pilatus PC 7 Mk II. Over a time span of more than fifty years, thousands of students learned the basics on this wonderful machine, and spinning was covered in great detail within the syllabus of instruction.
My experience in learning to spin the aircraft was no different from that of the thousands of young Pupil
Pilots that had gone before me and of those that would follow. We all received excellent theoretical training in the aerodynamics of spinning, and each and every lesson in the aircraft was preceded by an exhaustive briefing on the subject.
Spinning was accomplished as part of the syllabus of instruction before going solo, and total proficiency in the execution of the exercise had to have been achieved. Shortly after having completed the solo circuit and landing consolidation phase, a “solo” spin had to be demonstrated just off the airfield boundary to the student’s Instructor, the Flight Commander and the Chief Flying Instructor, all of whom who took up a position on the balcony outside of the control tower, armed with binoculars and a two-way portable VHF radio, in case verbal intervention or advise was required to be given to the student.
Spinning continued to be done throughout the general handling phase of the course and also as part of the instrument flying syllabus. Here the student, who sat in the back cockpit under a canvas hood which blocked out the entire canopy was taught to enter, maintain and recover from a spin, all on limited panel, the artificial horizon and directional indicator having been caged before the exercise.
Now, some fifty-five years after I was trained on the AT6, I still teach spinning on the aircraft from time to time.
Regrettably, I will not be able to avoid getting somewhat technical as we move along, but I will try and keep it as basic as possible. So here goes…..
The first stage of any spin in any aircraft is the auto-rotative stage. This is brought about by the fact that one wing might drop at the stall.
Ideally, a wing should never drop at the moment of the stall of an aircraft. This would only occur if the aircraft was perfectly rigged, was being flown perfectly balanced, and was in smooth air and if it incorporated design features to alleviate wing-tip stalling. However, the AT6 is prone to dropping a wing at the stall. It’s stalling characteristics, by modern-day standards, are less than ideal, and this, in my opinion, is one of the factors that made the aircraft such a good trainer.
There are a variety of reasons as to why a wing could drop at the stall. Many AT6s are “bent” to a certain degree, having suffered the abuse of thousands of bounced landings during training. Many of the aircraft have been rebuilt after accidents, and many have had their “g” limitations exceeded at the hands of some young and inexperienced, but nevertheless fearless and enthusiastic youngsters. Inevitably there are differences in the angles of incidence between the two wings, some being slightly washed out, and some slightly washed in.
The rotational effect of the slipstream from the propeller also has an effect, but we will ignore this in this article, as the subject we are leading into is all about power off intentional spins. A highly localized gust could also cause one wing to stall and not the other, thus causing it to drop.
Then too, the presence of a pitot assembly out on the right wingtip leads to a disturbance of the airflow in that region, and this, in a perfectly rigged AT6 that is being perfectly flown, could lead to the right wingtip stalling before the left one.
Finally, a wing could drop at the stall because of an unintentional rudder input by the pilot. Any yawing moment that was present at the moment of the stall would then cause the wing that was in the direction of the yaw to drop.
Now, let’s get ourselves up to at least 5000 feet a.g.l. and have a look at spinning the aircraft…..
We have prepared the machine for the exercise, the usual checks before spinning having been accomplished. This is followed by a steep turn in which an inspection of the area below us and a lookout for other aircraft is made.
In the early 60’s we had an AT6 spin right through the centre of a 3 ship formation, taking out the lead aircraft in the process. There was only one survivor, that being the instructor in the back seat of the
aircraft that came spinning down. He was thrown clear and his first conscious moment thereafter, was when he realized that he was floating down in his parachute.
We thus keep a very good lookout in a 360-degree steep turn, and as we roll the wings level, we reduce power and pull the pitch lever to the fully coarse setting. We do this as the aircraft will be spinning fairly rapidly, and we wish to reduce the gyroscopic loads on the propeller and crankshaft.
The nose is raised as the speed reduces in order to maintain height and gradually the wings approach the stalling angle of attack. At the moment of the stall, a wing could drop, this being either the left or the right one. However, to take the guesswork out of the equation, we take matters into our own hands and induce the wing of our choice to drop, by introducing yaw through rudder input in the desired direction of the spin.
Immediately prior to the wing dropping, the aircraft is moving forward in either level flight or a slight climb. The relative wind comes entirely from the front of the aircraft.
Now, let us assume that it is the right-wing that drops at the stall. This is most often the case in the AT6 because of the location of its pitot assembly. However, we have now made certain of the fact that the right-wing will drop, because we have made a right rudder input at the moment of the stall.
As the right-wing drops, it experiences a new and additional component of the relative wind, this being from underneath the wing, which is the direction in which the wing is moving. The resultant relative wind meets the wing at an angle of attack that is far above the stalling angle of attack of the wing. This wing now experiences two things:
A far greater loss in the lift, and….
A far greater increase in drag.
Conversely, the upward going wing, now experiences its new and additional component of the relative wind from above. Its resultant relative wind meets the wing at an angle of attack that is below the stalling angle of attack of the wing. This wing thus experiences an increase in the lift compared to what it was generating at the moment of the stall and also a reduction in drag.
The autorotation of the aircraft now begins. The right-wing is developing very little lift and a huge amount of drag and the left-wing is developing greater lift and less drag. It is this differential in lift and drag that will provide the automotive force to “drive” or “propel” the auto-rotation of the aircraft.
And so, accompanied by a fair amount of buffeting due to separation of the airflow over the wing and also due to the turbulent flow of the airflow over the tail surfaces, the aircraft starts auto-rotating.
The fact that the right-wing has dropped and that the aircraft is also yawing very positively to the right brings the nose well down below the horizon. Up until this point, there have only been four significant
aerodynamic moments present, those being:
The lift and drag of the downgoing wing
The lift and drag of the upgoing 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, as the aircraft yaws and its nose falls below the horizon, the aircraft side-slips in the direction of the autorotation and the whole right-hand side of the fuselage and empennage experiences a relative wind and therefore a fifth aerodynamic moment. Because of the greater moment arm of the fuselage behind the centre of gravity, the aircraft “weathercocks” towards the dropped wing and the auto-rotative “motor” increases.
We are now only through about a half a turn of spin and have five significant aerodynamic moments affecting the aircraft. A sixth one now comes into play, this one being the resistance to the yaw that is being offered by the left-hand side of the fuselage as the aircraft rotates in a clockwise direction.
Six significant aerodynamic moments are having an effect on the autorotation of the aircraft at this stage. The strength of these moments is very variable indeed and can vary from strong to weak. For example, the application of aileron either in the direction of the dropped wing or opposite to it, would have an effect on the lift and the drag that that wing was producing, thereby affecting the overall lift/drag differential of the two wings. By way of additional examples, the amount of elevator applied would affect the aerodynamic pitching moment, and the amount of rudder applied would affect the rate at which the aircraft was yawed and the extent to which a wing would be made to drop.
The placement of controls thus has an effect on the auto-rotative power that will ultimately be generated.
And so, for absolute uniformity, we were taught a standard entry for our spins in the AT6. We would use full aft stick, full rudder in the desired direction of the spin, and we would always keep the ailerons neutral.
No experimentation with the aileron was allowed, save to explain that application of aileron would affect both the yawing and rolling aerodynamic moments of the aircraft.
We still haven’t got past the auto-rotative stage yet, but at least we should have a good understanding of what is happening up to now.
Specifically, as we rolled out of our inspection turn, we reduced power to about 17 inches manifold pressure and pulled the propeller pitch back to full coarse. The 17 inches leaves us with a fair amount of slipstream over the tail surfaces so that we have reasonable rudder and elevator effectivity.
We have decided that we will spin the aircraft to the right. At about 70 knots we are close to the stall and the angle of attack is nearing the CL-Max. At this stage, we pull the stick back right into our stomach and hold it there. Simultaneously, we apply full right rudder, since this is our chosen direction of spin. We hold the rudder fully in. We also take care not to make any aileron inputs, as we don’t want any surprises.
The aircraft enters the auto-rotative stage, and as it starts, we remember to bring the throttle right back. The landing gear warning horn sounds as we do so, and it will be with us throughout the duration of the spin. The slipstream provided by the engine running at 17 inches m.p. has served its purpose. It has given us a clean, crisp, break at the stall and also good rudder effectively. However, to leave any power on during the spin could really complicate matters…..but, this is a subject for another article.
We have got our auto-rotative stage going well and truly now and all the aforementioned aerodynamic moments are coming into play. Within a very short time span, we are through one and a half to two turns,
and by now other moments are developing.
These other moments are what we call inertial moments, and these are caused by the distribution of matter within and around 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 the bank that is achieved during the spin.
So, basically speaking, with a heavy engine up front and a heavy person in the back seat, you could expect the aircraft to take up a flatter attitude in a spin compared to if the back seat were empty. If, hypothetically, the AT6 had its bomb and rocket racks plus machine guns fitted and it was carrying its weapons and ammunition, then in a spin the wings would take up a flatter angle of bank in the spin compared to if the wings were clean and not carrying arms and ammunition.
By the time the aircraft has completed about two to three full turns of autorotation, the inertial moments have become fully developed. When the aerodynamic forces are balanced by the inertial forces, equilibrium is achieved and the aircraft is said to be in a steady state of spin.
The spin may then be held indefinitely and until a height is reached where recovery must be effected. 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. The speed indication is somewhat higher in a spin to the left than it is to the right, this being because the pitot assembly is way out on the right-wing and in a spin, to the left, it is on the outside of the spin axis and moving faster than in a spin to the right.
The perfect state of equilibrium is not usually maintained in the AT6. From time to time certain of either the inertial or the aerodynamic moments predominate temporarily until equilibrium is once again restored. The aircraft thus takes up an oscillatory motion with the nose pitching up and down and the spin rate accelerating and decelerating throughout the spin.
There are a number of aspects involved in the spinning of the AT6 that are worthy of mention here:
Firstly: The presence of a heavy person plus military parachute in the rear seat of the aircraft leads to a greater inertial pitching moment and a tendency for the aircraft to spin in a flatter attitude. In extreme cases, the inertial pitching moment could exceed the capability of the elevators to introduce an aerodynamic pitching moment in order to effect a recovery from the spin. It was for this reason that our Pilot’s Handling Notes included a statement to the effect of “Recovery from a spin might be facilitated by the occupant of the rear seat bailing out first”. South Africa is noted for its huge rugby-playing forwards and we had many such giants as instructors. Whilst Air Force legend talks about some interesting moments with such giants in the rear seat, no cases were ever recorded of an AT6 failing to recover from an intentional spin.
Secondly: In spin to the right the aircraft is moving in the same direction as the rotation of the propeller. There is a very noticeable “slowing down” of the r.p.m. and many times it looks as if the engine is actually stopping. In cases where the idling was set too low to start with, engines have been known to stop when spinning to the right.
Thirdly: We usually trained our students to spin through five to seven revolutions of spin before starting the recovery. About two to three turns alone were required to get through the auto-rotative stage and then we had another few revolutions to demonstrate the steady-state where we would show the aspects that we have already discussed.
Fourthly: There would be a lot of buffeting from the disturbed airflow over the aerofoil and also over the tail surfaces. There would also be a fair amount of rattling of the canopy window frames. The whole experience is, in fact, a little disconcerting or intimidating at first, but one gets used to it.
As a seventeen-year-old Pupil Pilot I decided to put all of what I had been taught to the test and climbed an AT6 as high as she would go, at which stage I put her into a spin. I recall initiating a recovery after twenty turns of spin and noticing no abnormal behaviour from the aircraft whatsoever.
We are about to complicate matters a little further………Whilst the aircraft is spinning there are three significant gyroscopic factors that are also present. The first is the propeller and the engine’s rotating parts. We endeavour to minimize the effects of this gyroscope by keeping the engine throttled back all the way and also by pulling the pitch back into full coarse. Gyroscopic effects from a fast turning propeller, moving clockwise as viewed from the cockpit, would affect the spinning characteristics of the aircraft, causing it to flatten in a spin to the left and to steepen in a spin to the right. Also, whether this was true or not, our standard operating procedures called for the pitch to be pulled to fully coarse so as to minimize the gyroscopic loads on the propeller and crankshaft to prevent wear and tear. Be it as it may, who were we to argue, and so it was that the pitch was coarsened for every spin.
The other two gyroscopic effects come from the fuselage and the wing. Remember that a gyroscope is any
rotating mass. This is true of all wheels or other masses that are in a state of rotational movement. Thus, as the fuselage and wings rotate in the spin, they become gyroscopes, the fuselage being commonly known as the “B” (for the body) gyro and the wings as the “A” (for aerofoil) gyro.
These gyroscopes exhibit the properties of all gyroscopes, namely rigidity, which is a reluctance to change from the plane of rotation, and precession. Precession is defined as such: – “If a force is applied to a spinning gyroscope, the force will not act at its point of application, but rather at a point 90 degrees removed, in the direction of rotation of the gyroscope”.
Grab hold of a model aircraft now and hold it in front of you so that you are looking down on it. Assume the aircraft is spinning to the right. The fuselage makes up the one gyro and the wings make up the other. The engine has been taken almost completely out of the equation because of the low r.p.m. and the propeller pitch setting.
The spin recovery process is about to begin……What we need to do first is stop the rotation of the aircraft and so opposite rudder to the direction of spin is applied. The rudder on the AT6 is very powerful and has no problem in counteracting the aerodynamic auto-rotative moments. If opposite rudder was to be held fully applied and the stick was to be held fully back, the rotation would stop and then the aircraft would start spinning to the other side.
However, at a very early stage in the recovery procedure, the stick must be moved well forward so as to unstall the aircraft. After all, we do want to recover as expeditiously as possible and with a minimum loss in height. There is no purpose in prolonging the recovery.
So, the opposite rudder is applied and two beats later the stick is moved forward. The tempo is akin to the military timing when changing direction on the parade ground where troops move to the beat of WUN, two, three, WUN. The force required to move the stick forward is fairly heavy, and many pilots actually use two hands to do so.
The elevator is used to break the inertial pitching moment and to get the nose down so that the wings meet the air at an angle below the stalling angle of attack. A number of things happen simultaneously at this stage. Firstly, as opposite rudder is applied the rotation rate starts to slow down slightly. However, as the stick is moved forward the spin actually accelerates dramatically and the aircraft might rotate for as many as three turns, all at a very fast rate. This could actually leave you in some doubt as to whether the aircraft was in fact actually recovering. The important thing here is not to lose focus and pull the stick back, because then the aircraft will never recover from the spin.
The reason for the acceleration in spin rate is mainly two-fold….. Firstly there is the “conservation of momentum” aspect, whereas the spin radius decreases the spin rate increases. Next time you see a figure skater doing a pirouette with arms out wide, notice how the rotational rate accelerates as the arms are brought in to a position close to the body axis.
Secondly, we come back to the fuselage, which is, in fact, a gyroscope since its mass is rotating clockwise in our right-hand spin. As the stick is moved forward, the same force, but being applied in two different places can be looked at. Firstly, there is a force being applied to lift the tail and unstall the aircraft. This force precesses through 90 degrees and acts upwards on the left wingtip. The very same force from the elevators can also be said to be applied to the top side of the engine, acting downwards. This force acts at 90 degrees to where it was applied and transfers to the right wing-tip, where it acts downwards.
The net result is that the very auto-rotative forces that gave rise to the onset of the spin are increased dramatically, as the downgoing wing is pushed down resulting in a greater degree of stalling
with even less lift and more drag being generated than there was at the beginning of the manoeuvre. On the left side, the wing that moves upwards experiences an increase in lift and a reduction in drag.
The lift/drag differential between the wings is increased and the auto-rotative force or “motor” is enhanced, thus causing a very high rotation rate.
The number of “accelerated” spins the aircraft will do during the recovery will depend on how soon the stick is pushed forward after rudder is applied. If the stick forward application is rushed and occurs before the rudder has started slowing the rotational rate down, the effects of the conservation of momentum will be high. Additionally, the strength of the “B” gyro will also be high and the enhanced precessional effects will give rise to aerodynamic auto-rotative effects that cause very high rotational rates and where up to three accelerated spins could occur.
On the other hand, if the stick forward application is delayed somewhat more and the rudder had already started to reduce the spin rate, the effect of the stick forward application would result in fewer accelerated spins.
Please be aware that we are talking about fractions of seconds here and that there is an extremely fine line between whether the aircraft will spin through one turn of accelerated spin or up to three turns of accelerated spin.
As the spin stops and the aircraft unstalls, the rudder must be centralized, the wings levelled and the aircraft pulled out of the dive.
Recovery is usually to the 90-knot climbing attitude. As the aircraft cuts the horizon, a little throttle is applied and the landing gear warning horn silences. The pitch lever is moved forward slowly to about the 2000 r.p.m. position and then very slowly and smoothly the throttle is advanced so as not to cause the engine to “cough” or to cause an Overspeed of the propeller.
I am aware of the fact there has been a huge amount of information that has been absorbed here, and if I have not lost you yet, let’s summaries the whole exercise:-
Unless you have vast spinning experience, please do not attempt spins on your own in the AT6. The experience is like nothing you have ever experienced in most modern-day training aircraft. If you have minimal spinning experience and/or you have never spun the AT6 before, make sure there is a pilot with you that is indeed familiar with the AT6’s spinning idiosyncrasies.
Do your pre-spinning checks and ensure that your gyro flight instruments are caged.
Complete a 360-degree lookout steep turn and make sure the airspace you will be spinning into is clear of aircraft.
On exiting the steep turn, throttle back to 17 inches m.p., raise the nose and pull the pitch lever to fully coarse.
As the speed decays to about 65 to 70 knots, pull the stick back briskly into the pit of your stomach and apply full rudder in the direction you wish to spin in.
As the “break” occurs, throttle back completely.
Concentrate on keeping the placement of controls constant. Do not allow the stick or rudder position to vary and do not apply any aileron. Any movement of the flight controls will affect the aerodynamic moments and therefore the aircraft’s spinning characteristics.
If the aircraft is “flicked” (as the British say) or “snapped” (as the Americans say), into the manoeuvre at too high an airspeed, the aircraft will go right over onto its back during the first turn of auto-rotation, before resuming the upright attitude.
Hang on patiently as the aircraft transitions from the auto-rotation stage to the spinning stage.
Hold the aircraft in the spin for as long as you wish to. Take note of the airspeed reading, the buffeting and the oscillatory instability in pitch and rotation rate.
Initiate the recovery by applying full opposite rudder and then two beats later pushing the stick well forward of the neutral position. Be prepared to use two hands if necessary.
Avoid any negative “g”. This is uncomfortable and will cause momentary engine failure.
As the aircraft recovers from the spin, centralize the rudder, roll the wings level and ease out of the dive. Restore power gently.
In conclusion, allow me to state that in putting Pupil Pilots through flight training over a 50 year period, I am not aware of any AT6 that never recovered from an intentional spin. We had many unintentional stall/spin accidents and one situation where there was a structural failure of an elevator which gave the pilots a very nasty moment.
Every single pilot that ever spun our Air Force AT6’s was prepared for the manoeuvre in an extremely thorough and responsible manner.
We were prohibited from spinning the “G” model aircraft that were on the base and I cannot recall exactly why. It was either because they had artificial horizons that were uncageable and damage would have eventually resulted to the instruments, or it was because we carried additional radio equipment behind the rear cockpit and which altered the c. of g. of the aircraft. The “G” models were readily identifiable by their different canopy fenestration and lack of a “power push” lever for the hydraulic pressure that would power the landing gear and flap extension and retraction. They also had a lesser aileron and elevator travel.
I would say that the owners of G model AT6’s should consult their handbooks carefully to see whether spins were allowed in that model before going out and doing them.
Hope you enjoyed it all……!
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