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Groundschool – Theory of Flight

Control


Revision 49a — page content was last reviewed 6 August 2012

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ln this module we examine how the 3-axis aircraft is controlled to initiate and maintain normal rotations about the three axes; i.e. to manoeuvre in three dimensions. There are some unusual control practices that provide useful flight manoeuvres and some techniques for recovery from unusual attitudes.


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8.1 Control in pitch

In section 6.3 we learned that movement of the elevators provides a pitching moment about the lateral axis, that initiates a change in the aoa. Once the new aoa is established, then — provided the elevators are held in that deflected position by pilot pressure on the control column or a trim device — the pitch moment returns to zero and the aircraft maintains that aoa (and there is a direct relationship between aoa and IAS). In the manoeuvring forces module we established that aoa and power combinations provide (a) increased speed or climb and (b) decreased speed or descent — or varying degrees of either.

Thus, control in pitch (i.e. of aoa) combined with throttle control allows an aircraft to take-off, climb, cruise at various speeds, descend and land. However, control in pitch involves more than initiating a discrete pitching moment to effect an aoa change and subsequent attitude change. In section 1.10 we found that to sustain a turn, an additional force must be continuously applied towards the centre of the curve or arc — the centripetal force. This is achieved by an increased aoa — greater than the normal for a particular straight and level airspeed — held with control column back-pressure. The increased aoa provides the centripetal force, and that force keeps the aircraft constantly pitching 'up' in the longitudinal plane, into the direction of turn.

8.2 Control in yaw

Aircraft perform much better if their longitudinal axis is accurately aligned, in plan view, with the flight path; i.e. with the relative airflow. Also, an aircraft flying with a constant angle of slip is part way to ending up in an unusual flight attitude. Thus, the primary task of the fin and rudder combination is to act as a stability and trim device so that the directional stability system will restore flight to the proper state of zero sideslip angle. If the rudder has no trim tab, as would be the case with most light aircraft, then the pilot would have to keep the aircraft in trim by applying pressure to one rudder pedal. This pressure will vary with airspeed and the sideslip angle. This trimming task also includes the use of rudder to overcome adverse yaw when initiating a turn, and to keep the turn balanced or 'coordinated'.

slip_skid ballThe very simple flight instrument provided to indicate slip — or skid in a non-coordinated turn — is the balance ball. A metal ball is enclosed in a short, transparent, slightly curved tube where movement is somewhat damped by the restriction of the tube. When the aircraft is flying with zero sideslip, the ball will be centred at the bottom of the curve; when the aircraft is slipping into (or skidding out of) the turn, the inertial forces will move the ball left or right in the direction of the slip. To trim the aircraft, the pilot applies pressure on the rudder pedal on the side to which the ball has moved; i.e. 'steps on the ball'. When the aircraft is slipping, the pilot will also feel those inertial forces apparently pushing his/her weight in the same direction as the ball, hence the expression 'flying by the seat of your pants'.

The amount of pilot-induced yaw, at a given airspeed, is dependent on the degree of rudder deflection. If the pilot holds the rudder deflection, the aircraft will continue yawing and sideslipping. But, as the aircraft rotates about the normal axis, the wing on the outside of the rotation must be moving a little faster — and the inner wing a little slower — so there will be a small lift differential. The differing lift moments will raise the outer wing and lower the inner wing and the aircraft will enter a banked turn.

A pilot would not initiate a sustained turn by using rudder alone, but there are occasions when it is appropriate and effective to alter the aircraft's heading a few degrees by using just rudder — or perhaps rudder plus a little opposite aileron to stop the bank. Such occasions are when finally aligning the aircraft with the runway centre-line or compensating for small changes in wind direction when landing. We will cover this in the 'Circuit, approach and landing' module. Having said that, there is still an occasion where a rapid 180° change in direction is achieved solely with rudder; see the following.

The ultimate use of rudder to yaw the aircraft is demonstrated when the pilot of an aerobatic aircraft executes a 'stall turn' or 'hammerhead'. The former term is perhaps a misnomer, because the wing does not reach the critical aoa during the manoeuvre. The manoeuvre involves pulling the aircraft into a full-power vertical climb then, as the airspeed decays to somewhere near the normal stall speed, applying full rudder so that the aircraft is yawed 180°, about the normal axis, into a vertical descent. The slipstream supplies the energy to the rudder for the turn. The interesting thing is that, although the aircraft is at or below normal stall speed, the wings are nowhere near the critical aoa as, towards the end of the climb, the control column is held forward of the neutral position — and because of the low aoa and low V², the wings are not producing much aerodynamic force*. The length of the vertical 'up-line' depends on the aircraft's power/weight ratio — and thrust power and momentum are reducing quickly. Of course, if the pilot delays too long before applying full rudder, the weight vector will take over and cause the aircraft to slide vertically backwards — a tailslide.

*In fact the wing could be at the 'zero lift' aoa where the aerodynamic forces on both sides of the wing are equal and opposite.



8.3 Control in roll

We discussed how ailerons produce a rolling moment in section 4.10, so what happens when the ailerons are normally deflected, by the pilot moving the control column to the left or right? Initially the aircraft will start to roll, and if the control column is then returned to the neutral position the roll will cease but the bank angle reached will tend to remain. To level the wings, the column has to be moved to the opposite side — then returned to neutral once the wings are again level. Which indicates there always tends to be a sort of 'neutral stability' in the lateral plane. However, that situation doesn't exist because, as we found in section 7.4, other forces come into play when the aircraft is banked — creating sideslip, then yaw and eventually a turn. So the prime reason for introducing a roll is as the first step in turning.

However, before going on to the turn, let's just look a little further at the effect of aileron deflection while elevator and rudder are held in the neutral position, when the aircraft's velocity is high; i.e. pure roll.
The simplest aerobatic manoeuvre is the aileron roll, which (in aircraft that have been certificated for aerobatics) is usually accomplished by first gaining some extra kinetic energy, applying full power and raising the nose so that it is pointing 20° or so above the horizon with the balance ball centred. The control column is then firmly moved left or right to the full extent of travel and held there, while the elevators and rudder are both held in the neutral position; i.e. the roll is produced solely by the aileron deflection. The aircraft will then continue to roll about its longitudinal axis — because the ailerons being positioned towards the wingtips produce a strong rolling moment about the axis — more or less at a steady rate of roll, until the accumulated extra energy is exhausted. If the column was not moved to its full extent, the aircraft will still roll, but the rate of roll will be slower yet still steady. The time to complete a full 360° roll is very much dependent on the aircraft design and that roll rate dictates how many complete rolls can be produced before the aircraft ends up with the nose pointed well below the horizon — because of the combined effects of slip, yaw and the full 360° inclination of the lift vector. The rate of roll for a competition-class aerobatic aircraft is around 360° per second; for the more mundane aerobatic aircraft, it is around 60° per second, which will probably produce only one full 360° roll. A properly executed roll will result in a continuing 1g load throughout the manoeuvre.






8.4 Control in a turn

We come then to the question 'how do you turn an aircraft?' Well, you can make the aircraft's nose turn just a few degrees without banking by just applying pressure on the rudder pedal in the direction you want the nose to move and at the same time moving the control column a little sideways in the opposite direction to stop the consequent bank. Applying pressure to the rudder in one direction with opposite aileron is cross-controlling. Normally, this is a very sloppy way to fly but also a habit that can lead to trouble — particularly in low-speed descending turns, such as that made in the approach to landing.

Although not directly related to turns, this extract from an RA-Aus incident report illustrates how easy it is to get into difficulties if you don't realise you are cross-controlled at low speeds.
"The student had completed two solo circuits and landings without incident. During the third the landing appeared normal, the aircraft touched down without bouncing but then veered left and the left wing lifted. The student applied full power but the aircraft failed to climb normally and appeared to be staggering and slowly orbiting to the left. The aircraft only gained about 40 feet height then gradually descended, striking the ground nose low and left wing low. The student was not injured. It was found that he had maintained full left rudder when he applied full power and was using aileron to counter the yaw — the aircraft basically sideslipped into the ground."

The normally recommended way to initiate a level turn (to the left) is to move the control column to the left until the required bank angle is achieved, then return the control column to neutral. At the same time as applying aileron, just sufficient bottom (left) rudder is applied to balance the turn so that there is no slip or skid and the balance ball stays centred. Also, the amount of rudder required increases as airspeed decreases. As the aircraft banks, the lift vector departs from the vertical, so the aoa must be increased sufficiently that the vertical component of lift always exactly balances weight. This means increasing back-pressure on the control column as bank is applied. As aoa increases, induced drag increases. So, to maintain V² throughout the turn, power must be increased. Thus a properly balanced, constant rate and constant-speed turn implies a smoothly coordinated application of aileron, rudder, elevator and power.

In some aircraft, particularly slower aircraft with high aspect ratio wings, it is necessary to lead the turn with quite a bit of rudder (because of aileron drag) before adding aileron. In other aircraft it is quite easy to initiate and continue a turn without using rudder at all, but the turn will be uncoordinated — i.e. the balance ball not centred — and such conditions are not desirable.

During a banked level turn, the outer wing is moving very slightly faster than the inner wing and will consequently produce more lift; the bank will tend to increase and the turn to wrap-up even though the ailerons are in the neutral position. In order to maintain the required bank angle it is necessary to apply a slight opposite pressure to the control column, which is known as 'holding off bank'. This is relative to level and climbing turns, but different physics apply to descending turns.

In a climbing turn, the outer wing has a slightly greater aoa than the inner wing, and thus additional lift. Combined with its slightly faster speed, this reinforces the tendency for the bank angle to increase and the need to hold off bank.

The reason for the higher aoa of the outer wing is because of a difference in relative airflow. Imagine an aircraft doing one complete rotation of a continuing climbing turn. Obviously all points on the airframe are going to take the same time to achieve the higher altitude; however, the upward spiral path followed by the outer wingtip must have a larger radius than that followed by the inner, and therefore the path followed by the outer wingtip is not as steep as that followed by the inner. The less steep path of the outer wing (i.e. the relative airflow) means that the aoa of the outer wing will be greater than that of the inner. You might have to think about it a bit!

The reverse occurs in a descending turn — the steeper path of the inner wing in the downward spiral means that it will have a larger aoa than the outer wing, which may compensate, or overcompensate, for the faster velocity of the outer wing. In order then to maintain the required bank angle it is necessary to apply an inward pressure to the control column; i.e. in a descending turn the bank must be 'held on'. If the pilot tends to hold off bank in such a turn, there will be an excess of 'bottom' rudder and the aircraft must be skidding. Whenever an aircraft is slipping or skidding, the wing on the side to which the rudder is deflected will stall before the other, with a consequent instantaneous roll in that direction. So the situation we've described — holding off bank in the descending turn with excess bottom rudder — means that should the aircraft inadvertently stall — a cross-controlled stall — it is going to roll further into the bank and enter an incipient spin. Hence the old adage — 'never hold off bank in a gliding turn'. A cross-controlled stall typically occurs in the turn onto final approach for landing.

If you must fly cross-controlled when banked, then it is better to fly with an excess of top rudder, as in the sideslip manoeuvre. Thus, if the aircraft should stall, the roll will be in the direction of the upper wing; i.e. towards an upright position. And never apply an excess of bottom rudder in an attempt to tighten any turn, particularly when the airspeed is low for the bank angle employed and/or height is low. This is discussed further in the 'Safety: control loss in turns' module.

A breaking turn is a defensive flying manoeuvre, which every pilot should be able to perform rapidly and automatically to avoid collision, particularly in the circuit. It involves very rapid transition, usually into a steep descending turn, but a steep climbing turn may be necessary. A level turn is an unlikely choice, but whatever turn is chosen you must be able to perform it instinctively while your head is continually swivelling to ascertain the location of other aircraft — without falling out of the sky by inadvertently applying back-pressure on the control column and thus exceeding the critical aoa.

Before we go on to the sideslip, another very simple aerodynamic demonstration — but only for aerobatic aircraft whose engine and airframe are able to take the loads, and absolutely never for any other aircraft — is the flick roll.

The flick or snap roll asymmetric aerobatic manoeuvre is an accelerated stall combining a rapid increase in aoa with a full yaw. It is brought about — when cruising straight and level at a speed less than Va — by pulling the control column firmly back as far as it will go while applying full left or right rudder. The wing in the direction of the applied rudder stalls first and the aircraft flicks into a 360°, roughly horizontal, roll. The roll will continue while the rudder and elevators are held at their limits (and cease when they are returned to the neutral position) but the aerodynamic drag produced by the manoeuvre slows the aircraft quite rapidly and the aircraft will enter a vertical spin if the roll is held too long. The faster the entry speed, the higher the torsional stress on the rear fuselage and empennage, the engine mountings and perhaps even the engine crankshaft; but the slower the speed, the greater the likelihood of immediately entering a spin, and recovery technique is dependent on several variables. The roll is usually a lot snappier in the opposite direction to propeller rotation. Flick rolls may be executed only in aerobatic aircraft designed to withstand the extreme stresses.

Such a simple control action, whether or not while turning, demonstrates how easily misuse of rudder can end up in an unusual and dangerous attitude, and where the possibilities increase as speed decreases. And be aware that you don't have to actually push on the rudder pedal, you can easily achieve misuse by inadvertently slipping one foot off the rudder bar at a critical time — the turn onto 'final approach' for example.

[The next section in the airmanship and safety sequence deals with 'Awareness of angle of attack increase in a turn']






8.5 Sideslip as a manoeuvre

Types of sideslip vary in degree — from inadvertently flying cross-controlled in the cruise (i.e. one wing slightly low and compensating with opposite rudder) to a fully-fledged cross-controlled turn where the aircraft is steeply banked in a descending turn with full opposite rudder applied. All sideslips reflect uncoordinated flight and result in increased drag.

Note: in aerodynamic terms, any time it is evident that the aircraft's longitudinal axis is at an angle to its flight path (in plan view) then the aircraft is sideslipping (i.e. its motion has a lateral component), and the angle between the flight path and the axis is the sideslip angle. Aerodynamicists don't generally distinguish between sideslip and 'slip' or 'skid', but many pilots use 'slip' as the general term, 'skid' to describe slipping away from the centre of a turn and 'sideslip' to describe a particular type of height-loss manoeuvre.

The sideslipping manoeuvre is only for the pilot who has a very good feel for their aircraft because, among other things, the ASI will most likely be providing a false airspeed indication. High sideslip angles combined with high aoa must be avoided. There seem to be as many definitions of the types of slip as there are exponents of sideslip techniques, but the safe execution of all sideslips requires adequate instruction and continuing practice. Here are some types:
The straight or steady-state sideslip approach to landing
The helmet and goggles crowd who, very sensibly, like to fly biplanes and other open-cockpit aircraft not equipped with flaps, need a manoeuvre for use on the landing approach to a short strip that enables them to lose height quickly without increasing airspeed and which provides a good view of the landing area. The answer has long been the cross-controlled steady state sideslip; a manoeuvre designed to lose height over a short distance, dumping the potential energy of height by converting it to drag turbulence rather than kinetic energy. Such sideslips may also be a requirement when executing a forced landing, and the same type of slipping approach may also be necessary for those aircraft where, in a normal approach, the pilot's view of the runway is obstructed by the nose.

Once established on the approach descent path at the correct airspeed, the aircraft is banked with sufficient opposite (top) rudder applied to stop the directional stability yawing the nose into the relative airflow and thus turning. Slight additional backward pressure on the control column may be needed to keep the nose from dropping too far. The aircraft sideslips in a moderate to steep bank with the fuselage angled across the flight path, giving the pilot a very good view of the landing area. The greatly increased drag, from the exposure of the fuselage side or 'keel' surfaces to the oncoming airflow, enables an increased angle of descent without an increase in the approach airspeed. The execution of a sideslip to a landing varies from aircraft to aircraft and it may not work particularly well where there is a lack of keel surface — an open-frame aircraft like the Breezy, for example.

The sink rate is controlled by aileron and power is held constant, usually at idle/low power, and the sideslip must be eased off before the flare and touchdown. When recovering, care must be taken to coordinate relaxation of the back-pressure, leveling of the wings and straightening of the rudder — otherwise the aircraft may do its own thing or stall, particularly in turbulent conditions.

The straight sideslip is limited by the maximum rudder authority available; there will be a bank angle beyond which full opposite rudder will not stop the aircraft from turning.

Although this manoeuvre usually comes under the proprietorship of the 'stick and rudder' people, the use of the sideslip, by the captain of a Boeing 767, undoubtedly saved the lives of many people in an extraordinary incident that occurred in 1983 when, due to a train of errors — as are most accidents/incidents — an Air Canada 767 ran out of fuel at 41 000 feet. The captain subsequently glided the aircraft to a safe landing on an out-of-service runway, which was being used for a drag racing event at the time. The aircraft was sideslipped through several thousand feet to lose excess height on the approach. For more information about this magnificent demonstration of airmanship (following an execrable demonstration of preflight procedure by many people; keep the old adage in mind — "proper pre-flight procedure precludes poor performance"!) google the phrase 'Gimli glider'.

The sideslipping turn
Slipping whilst turning is a manoeuvre often used in non-flap equipped aerobatic aircraft where it is desirable to perform a curving landing approach. This is also a useful emergency manoeuvre if it is necessary to increase the sink rate during a turn — such as the turn onto final approach in a forced landing when an overshoot of the landing site is apparent. It is just a sideslip where insufficient top rudder is applied to stop the aircraft turning while slipping. The rate of turn and the rate of sink are controlled by the amount of bank and the amount of rudder but it is an uncoordinated descending turn. Dangerously high descent rates are achieved if the bank angle applied exceeds the full rudder authority.
Fishtailing
Fishtailing is a series of sideslips where the wings are held level in the approach attitude with (alternating) aileron while the aircraft is repeatedly yawed from side to side by applying alternate rudder; the increased drag increases the sink rate and is possibly used as an emergency measure if overshooting a forced landing. The manoeuvre is generally not recommended, because uncoordinated control use at low levels may lead to dangerous loss of control. Also, excessive alternating rudder reversals may overstress the fin/rear fuselage.
Sideslip to a crosswind landing
In a sideslip to a crosswind landing, the aircraft is always banked with the into-wind wing down so that the sideslip can be smoothly decreased to a forward slip (below) before the roundout. Most aircraft tend to be slower in the slip, so the nose will need to be a bit lower than that needed to maintain the normal approach speed. A smoothly executed sideslip approach requires much practice, but displays considerable finesse to a ground observer.

The forward slip crosswind approach
A 'forward' slip is a moderate sideslip application designed only to compensate for crosswind during approach and landing. The slip can be applied throughout the final approach or just in the last stages, and it usually follows a full sideslip approach in crosswind conditions. The into-wind wing is lowered with sufficient bank so that the slip is exactly negating the crosswind drift, while opposite (top) rudder is applied to stop a turn developing and to align the aircraft's longitudinal axis with the flight path — and the runway centreline. If drifting off the path, just add or remove some aileron pressure and, at the same time, add or remove some rudder pressure to maintain direction. An approach speed 2–3 knots above normal is set up, the sink rate (which will be greater than usual because of the inclined lift vector) is controlled by the power setting, the into-wind main landing gear will touch down first and the aircraft is held straight with rudder by pivoting on that one wheel until ground speed has reduced to a safe level.

The forward slip is the particularly recommended technique for crosswind landings in high-wing taildragger aircraft. Incidently, a useful technique for a high-wing taildragger in a significant crosswind is to also perform the take-off run on one main wheel.

If there is any real difference between the straight sideslip and the forward slip it is just the amount of pressure applied to the controls. In a sideslip, the aileron pressure dictates the angle of descent and the rudder pressure dictates the amount the fuselage is deflected across the flight path. In a forward slip, the aileron pressure is just enough to compensate for the crosswind drift and thus maintain position on the extended runway line, and the rudder pressure just enough to keep the fuselage aligned with both the landing path and the flight path.

There is one manoeuvre for certified aerobatic aircraft that demonstrates what might be considered a reversal of all we have stated in this module. This is the 'four-point slow roll' or 'hesitation roll' where the aircraft is rolled through 360° in level flight around a point on the horizon, but the roll is paused for a second or two at each 90° point; i.e. when the wings are first vertical, when the aircraft is upside down, when the wings are again vertical and when the aircraft returns to normal attitude.

The roll is started (to the left) with normal aileron and a bit of left rudder, then as the roll progresses through the first 90° top (right) rudder is increasingly applied to negate the yaw, and also to hold the nose up. During the slight pause at the 90° position the aircraft is being held in a nose-up attitude by the rudder whilst the elevators are used to stop the nose wandering to the left or right across the horizon, and the ailerons are neutral. Some lift will be generated by the fuselage having an aoa because the nose is being held up. Then the roll is restarted until, at the 180° position, the aircraft is inverted and the nose is held up by a large forward movement of the control column and the aoa is negative; i.e. the lift is being generated by a reversed aerofoil. And so the roll continues. Of course, all the control movements involve gradual increase/decrease in pressures throughout the sequence.




8.6 Spins


The 'stall/spin' phenomenon
When an aircraft is held in a turn, and near the critical aoa with excess bottom rudder applied (i.e. a cross-controlled skidding turn, which often happens when the pilot tries to 'hurry' the turn with bottom/inside rudder instead of increasing bank) the lower, partly blanketed, wing will be producing less lift than the upper wing. Any tightening of back-pressure on the control column (or any inadvertent back-pressure applied when, for instance, looking over your shoulder; or even any encountered turbulence or wind shear) may take the aoa past the critical angle. The lower wing will drop sharply in an 'uncommanded' roll, and thus become more deeply stalled than the upgoing wing — which may not be stalled or just partly stalled. The high aoa of the lower wing causes greatly increased induced drag, yawing forces in the same direction as the lower wing come into action, the nose swings down and the aircraft enters an incipient (i.e. partly developed) spin condition, where it is about to start autorotation (below). All of this happens quickly, and in some aircraft very quickly indeed.

If the aircraft is properly weight-balanced (i.e. weight less than MTOW and cg within the defined fore and aft limits), this is readily countered by quickly easing stick back-pressure to reduce aoa below critical aoa — which immediately restores full control — applying sufficient rudder to stop further yaw, adding full power and rolling the wings level with aileron while keeping the balance ball centred. However, if the slew, exacerbated by the yaw, is allowed to develop past perhaps a 45° movement in azimuth, then there is a stall/spin situation. You sometimes hear about this when an unaware pilot allows such to develop close to the ground — often when performing a climbing turn after departure from an airfield; or in a turn-back to the runway following engine failure after take-off; or when in the descending turn from base leg on to the final approach to landing where, because of illusory ground reference cues, there may be a tendency to increase the rate of turn by applying additional bottom rudder whilst maintaining the bank angle with opposite aileron — 'holding off bank' as mentioned in section 8.4.

Ultralights, with their very low wing loading, normally display quite benign stall characteristics when slowly decelerated to stall speed in straight and level flight. But they may exhibit quite nasty behaviour in an accelerated stall or when a stall is initiated during a turn; and such are the usual unintentional stall/spin modes. Under these circumstances the height lost in the incipient spin — the initial entry to autorotation — may be 100 to 400 feet. Thus, an incipient spin condition is highly dangerous when occurring in the circuit pattern or in any other low-level flight situation. See the flick roll box above and read the sections dealing with 'limiting climbing turns during take-off' and 'accelerated stalls'.

There may be uncertified light aircraft where characteristics, such as rolling inverted in a relatively mild stall/incipient spin situation, are evident. However, a commercially manufactured aircraft with such characteristics should not receive a CASA Certificate of Type Approval.

[ The next section in the airmanship and safety sequence is the section below Spin recovery confidence building ]


Autorotation — the fully developed spin
With adequate training, an incipient spin is readily anticipated and easy to correct — provided the aircraft weight and balance are within the stated limits. But, if the correction is not done before the nose has swung maybe 90° or so, it may develop into autorotation where the aircraft is descending in a stabilised, usually nose-down, rotation — rolling and yawing in the same direction at a constant airspeed at or slightly above Vs1 — a full-blown spin with each 360° rotation taking only 2–4 seconds in a very light aircraft. The height loss during each rotation — 200 to 400 feet or more, depending on the stall speed and the steepness of the spin — plus the considerable height loss during the pull-out from the recovery dive, is insignificant at a reasonable height but will be critical at lower levels.

In a normal turn the aircraft's longitudinal axis is more or less aligned along the flight path — which is the periphery of the turn — the cg moves along the flight path and the inner wing is pointing towards the centre of the turn. But in fully developed autorotation the vertical axis of the spin is located somewhere in the 90° sector between the lateral and longitudinal axes and not so far from the aircraft's cg — perhaps less than two fuselage lengths. Thus, the aircraft is not turning in the normally accepted meaning of the word; it is 'spinning' around that vertical axis, while it's also rolling and yawing about the aircraft's cg; and also pitching somewhat.

The lower wing is more deeply stalled than the higher producing less lift but, being on the back slope of the CL curve, more induced drag so providing the asymmetrical yawing and rolling moments. Those aerodynamic forces produced by the wings drive the spin while the resistance of the rear fuselage and empennage reaches a point where it prevents the yaw from developing further; the aircraft's inertia resists change in angular momentum so producing the stable autorotation condition. Usually the structural loads are only a little above normal during autorotation.

In a steep spin, the nose is pitched down perhaps 50–60°, the aoa of the lower wing is 20–30°, there is a fair bit of bank and the roll motion dominates. The spin axis will be perhaps somewhere near one or two fuselage lengths forward (more or less) of the aircraft's cg — further away for a steeper spin. The cg will be following a helical flight path.

In a flat spin, the nose is pitched down perhaps 10–20°, with an aoa around 60–70° due to the high vertical component of the relative airflow. With very high induced drag and little bank, the angular rotation winds up and yaw motion dominates. The spin axis will be much closer to the aircraft's cg, perhaps even within the airframe, and particularly so if the cg is in an aft position. The closer the spin axis is to the cg, the harder it is to break out of the spin. If the axis coincided with the cg, break-out would be impossible — unless the aircraft was equipped with a ballistic parachute recovery system.

Most very light tractor-engined aircraft spin steeply to moderately steeply, so spin recovery early in autorotation is usually — but not always — straightforward: close the throttle, ailerons to neutral, stop the yaw (by applying full rudder opposite to the rotation direction apparent through the windscreen or shown by the turn indicator (not the balance ball/needle), then unstall the wings to stop the spinning (generally by applying full forward stick rather than just moving it to or past the neutral position until the spin stops). Control movements must be carefully sequenced and positive. The aircraft will be in a steep descent when the spin has ceased; the aerodynamic loads during the subsequent pull-out from the descent may lead to an accelerated stall if the aircraft is nearing the surface and the pilot applies extreme back-pressure. The height loss just during the pull-out stage may exceed 400 feet, so that the total loss of height during spin entry and recovery could easily exceed 1000 feet.

The problem for a pilot who is conscious of the need to avoid stall conditions when in the circuit by always maintaining a safe speed near the ground, and has had ample training in stall and incipient spin recognition and recovery, occurs when a spin is inadvertently induced at altitude. If that pilot has never previously encountered full autorotation then the disorientation associated with the first experience can be frightening. The pilot may also experience a ground rush illusion where the surface features rapidly spread out to fill the entire field of view and the ground appears to rapidly rise; the reaction is to freeze or to pull back on the control column, which just ensures that the aircraft is held in the stalled condition even though there may be ample height available to recover.

Fallen Tiger The photo at left was taken about 1949 and shows what happened when a student pilot got himself into a spin, evidently retained back-pressure on the control column and allowed the Tiger Moth to spin all the way to the ground from above 2000 feet. The spin developed into a flat spin, with relatively low vertical and horizontal speed, enabling the pilot to walk away with minor injuries. Also the Tiger Moth had a tough steel tubing fuselage frame, which absorbed much of the impact energy. You can see that the fuselage aft of the engine compartment firewall seems practically undamaged.

The pilot of any aircraft will not be exposed to the risk of an unintentional stall/spin if they always remain situationally aware, maintain an appropriate energy balance, does not indulge in very low-level manoeuvring and, above all, flies the aircraft. Don't practice stalling below 3000 feet agl; and remember spins result from a loss of lateral and directional stability at the critical aoa, and the only way to get into a spin is to first exceed the critical aoa. Also, sufficient forward stick movement will immediately decrease aoa below the stall angle and restore full control in any stall or near-stall condition; but not in autorotation where opposite rudder and full forward control column movement is necessary because (1) the aoa developed will be well past the critical aoa and (2) the control surfaces will not be as effective as usual — the fin and rudder could be screened by the tailplane and thus in a low energy, turbulent airstream.
Intentional spinning
Stall/spin events occurring at a safe altitude are insignificant; they may become nasty accidents when they occur at lower levels. But intentional spins can be fun in an aerobatic aircraft that has been certificated for intentional spinning and, given sufficient height, quite easy to recover from — provided the cg position is within the forward and aft limits. The challenge is in aiming for a precise number of turns, half-turns or quarter-turns and the exact direction the aircraft will be heading at recovery.

Spin characteristics are very complex and vary greatly between aircraft. Generally the intentional spin is induced from level flight by closing the throttle, bringing the aircraft to the point of stall in a nose-up attitude, holding ailerons in the neutral position then applying full rudder in the direction you want the aircraft to rotate and, at the same time, pulling the stick right back. Hold the neutral aileron, full rudder and back stick. The reason for the excessive control movements is to ensure a swift and definite entry into autorotation. The higher the nose is held above the horizon at the point of stall the more violent will be the spin entry.

Aircraft that tend to spin with the nose pitched well down will recover more quickly than aircraft where the spin attitude is relatively flat. However, if allowed to continue past two or three full turns, then centrifugal forces become well established — which tend to make all parts of the aircraft rotate in the same horizontal plane. Then, a nose-down spin may turn into a flat spin, which will then speed up rotationally, the rate of descent decreases, spin radius decreases and break-out will take longer, or may not be possible because it may be impossible to lower the nose. The spin axis may be very close to the pilot which would be very disconcerting. Recovery control forces required usually increase as the spin winds up; also, after initiating recovery action, the spin may increase a little before the action takes effect.

Engine power — and its associated effects — also tends to flatten the spin. The flatter the spin, the closer the spin axis is to the cg and the greater the aoa, maybe 75° or more! Also, at such angles, the rudder may be completely blanketed by the fuselage/tailplane, making that control quite ineffective. Structural stresses increase as the spin progresses. A flat spin might be induced if, at the point of stall, full opposite aileron is applied with full rudder.

If an aircraft stalls when inverted, it may enter an inverted spin if the control column position was held well ahead of neutral at the stall. It only happens during aerobatic routines — such as a poorly executed entry into a half-roll off the top of a loop, or messing up a stall turn. The recovery from an inverted spin involves correcting the yaw and increasing stick back-pressure until rotation ceases, then rolling level when speed has increased sufficiently; but the great danger in an inverted spin is pilot disorientation.

One thing is certain — NEVER, NEVER intentionally spin an aircraft that has not been through the complete spin certification process; they may be incapable of recovery from fully developed autorotation, or the recovery attempt may result in a violent manoeuvre that overloads the airframe.

Spin restrictions are not confined to non-aerobatic aircraft; for example, intentional spins were prohibited in the Seafire 47 and Sea Fury, very fast naval fighters of the late 1940s early 1950s, because of the time to recover (if recovery was possible) and the consequent extreme height loss.

Spin recovery confidence building
Developed spin recovery training is not included in the RA-Aus Pilot Certificate syllabus or the General Aviation Private Pilot Licence syllabus, but stall and incipient spin awareness and recovery are normal parts of the syllabi. A spin is usually classified as an aerobatic manoeuvre and, as all RA-Aus registered aircraft are prohibited from such manoeuvres, they shall not be allowed to enter an intentional developed spin. More to the point, no ultralight (and rather few light non-aerobatic aircraft) has ever been through the complete flight test schedule for spin recovery.

However, gaining some experience and confidence in recovery from full autorotation can be readily and cheaply obtained by practicing a half-dozen spin recoveries with an instructor in a two-seat glider or powered aerobatic GA aircraft. It is probably better experience in the glider, as you are also exposed to the fact that every glider landing is achieved easily without using any chemical energy; on the other hand, a GA aircraft provides more opportunity to also explore the basic aerobatic manoeuvres — rolls, loops and stall turns. Incidently, 30 minutes thermalling in a glider will also demonstrate the absolute need to coordinate rudder and aileron in every turn — the ailerons on the long slender wings provide a large adverse yaw moment.

These intentional spins should not be made from level flight, as described above, but should be made from those flight situations where unintentional spins are most likely to occur; i.e. in climbing and descending turns. These defensive flying lessons will also expose the student to the fact that it is very easy to invoke an accelerated stall during the pull-out after breaking out of the spin, if excessive control column back-pressure is applied — which is an automatic reaction if the ground is rising up to smite you!

Spin recovery training in a spin-certified aircraft does invoke a recognition of the behaviour of that aircraft type immediately prior to an incipient spin event, but these warning signs will vary between aircraft types — or even aircraft of the same type. However, knowing how to recover from a stall/spin situation is of no help if it develops at a height that does not provide sufficient height for recovery — circuit height, for example. At such heights the aircraft must always be operated at a safe airspeed (1.5 × Vs1) and restricted to gentle manoeuvres.

[ The next section in the airmanship and safety sequence is contained within section 2.9 Take-off, climb, descent and landing speeds ]

The 'falling leaf' manoeuvre
During World War I, a very simple manoeuvre called the 'falling leaf' was developed as an incipient spin training exercise consisting of repeated entries into incipient spins while the ailerons are held in the neutral position. Although simple, the exercise requires precise timing — so that speed remains at and just above stall — and a good 'feel' on the controls plus very good 'hands and feet' coordination. It involves the initiation of an incipient spin by bringing the aircraft to the point of stall in level flight, then pulling back on the control column whilst applying full rudder. As the wing drops, the control column is moved forward to the neutral position to unstall the wings, then opposite rudder is applied and held, which actions stop the yaw and the incipient spin. Then the control column is pulled back again to stall the wings whilst the rudder is held in the same position; thus an opposite-direction incipient spin is started. Those sequences are repeated so that as the aircraft mushes down, in and out of the stalled condition, it slips from side to side in a series of small arcs, supposedly as a falling leaf may descend. Nowadays the falling leaf is classified as an aerobatic manoeuvre, thus performing the exercise in an ultralight is prohibited. Obviously before any falling leaf exercise is attempted the pilot must receive appropriate spin recovery training.

The falling leaf term is also used to describe the technique of 'walking or pedalling down' a stalled aircraft by picking up a dropping wing with opposite rudder and then leaving the rudder applied a little longer than necessary so that the other wing starts to drop. In the latter technique, which is also a good developmental exercise in smooth air, the aircraft shouldn't be allowed to display much lateral movement during the descent. One of Bob Hoover's popular airshow demonstrations, the 'Tennessee Waltz', is a graceful falling leaf manoeuvre.

Again, this exercise should not be attempted unless the pilot has appropriate spin recovery training — and ample height because of the substantial height loss in all falling leaf manoeuvres — but all these types of control exercises do provide an excellent means of familiarising yourself with the feel of your aircraft at low speed and its particular stability foibles.
Picking up a dropping wing with rudder
There is occasionally some debate about the merits of using the secondary effect of rudder to pick up a dropping wing when flying at or near the stall. The dropping wing has not been arrested by roll stability because it is partly or fully stalled. (The reason for proposing use of rudder rather than aileron is because if the dropping wing is near the critical angle of attack the use of aileron will increase the camber of that section of the wing taking it into, or further into, the reducing lift zone of the wings CL curve.) As demonstrated in the falling leaf, using only the rudder to 'pick up' the wing does nothing to remove the stall condition, and excessive input will lead to the opposite wing dropping and the aircraft entering an opposite-direction incipient spin. This technique of picking up a dropping wing with opposite rudder should not be applied during normal stall recovery, unless there is ample height for recovery from an induced spin. The wing must be unstalled by moving the control column forward so that normal aileron control actions can be taken and rudder used to check any yaw.

The aircraft manufacturer's recommendations for stall recovery should be followed. But in their absence, the recommended technique in normal stall recovery is always to unstall the wings by easing forward on the control column — which is immediately effective — use sufficient rudder to check any further yaw, at the same time apply full power and then level the wings with aileron. For further information see Standard recovery procedure for all stall types

However, when in the final stages of landing, and just above the surface in ground effect (should you want the aircraft to touch down in a stalled condition), gentle application of rudder using opposite yaw to pick up a dropping wing coupled with a slight easing of control column back-pressure may be an alternative to applying power for a go-around. But it depends very much on the particular wing — form, washout, flap setting, slats and slots — on how the stall develops along the wing and on the pilot's knowledge of the particular aircraft. It also depends on how crosswind is being countered. In some aircraft, the use of aileron to pick up the dropped wing will increase induced drag on the lower wing, and the consequent adverse yaw may swing the aircraft towards the ground.

[ The next section in the airmanship and safety sequence is in section 2.9 "Take-off and landing speeds" ]

The spiral dive
As explained above, a spin and a turn are completely different beasts. Also, in a spin, the airspeed is relatively low and constant, the vertical speed is relatively low (2000–4000 fpm) and the angular rotation is fast. In a diving turn or 'spiral dive', the rotation is slower because of the wide (but tightening) turn radius, and the airspeed and height loss both increase rapidly.

In the lateral stability section, the possibility of entering a spiral dive condition was mentioned. In a well-developed steep spiral dive — the 'graveyard spiral' — the lift being generated by the wings (and thus the load factor) to provide the centripetal force for the high-speed diving turn, is very high and the turn continues to tighten. The pilot must be very careful in the recovery from a fully established spiral dive, or excessive structural loads will occur. See recovery from a spiral dive.

8.7 The stick force gradient

An aircraft's control systems must provide the pilot with handling qualities appropriate to the task in hand plus adequate feedback of the aerodynamic forces being generated by the control surfaces — particularly the elevators.

To avoid inadvertent airframe overstress it is required that the pilot must always apply an increasing pressure to the control column if increasing the elevator's aerodynamic force and thus the load on the airframe. Increasing back-pressure if the manoeuvre is a turn or a pull-up, increasing forward-pressure if a push-down.

That control column pressure requirement is known as the 'stick force gradient' and the pressure applied is specified as the 'stick force per g'.

The stick force that can be applied to achieve a particular elevator deflection depends on the length of the control column and the degree of travel available in the fore-and-aft arc; i.e. the stick's mechanical advantage. If the stick travel is short then the force required to deflect the elevators will be greater and the control system will probably feel too sensitive. Also the cg position affects the stick force required to increase the aerodynamic load; an aft cg reduces the stick force, a forward cg position increases the stick force required.

To reduce the possibility of inadvertent application of airframe loads exceeding the design positive load limit, the control system must be set up so that the stick force required to reach that limit must be at least a specified minimum value. FAR 23.155 specifies that value as the aircraft's mtow/140 or 15 pounds (6.8 kg), whichever is greater. Take for an example a 600 kg mtow aircraft: 600/140 = 4.3 kg stick force, but as the 6.8 kg minimum is greater, then FAR 23 would require 6.8 kg force as the minimum — for 'control column' systems. However it would not be difficult for the average male to apply a 7 kg one-handed pull on the control column. FAR 23.155 states that the stick force need not be greater than 35 pounds (16 kg). There is a different FAR 23.155 standard for 'control wheel' systems.

FAR 23.155 also requires that 'There must be no excessive decrease in the gradient of the curve of stick force versus maneuvering load factor with increasing load factor.'

Even design by professionals may not provide a guarantee that the aircraft is safe. Read this United States Federal Aviation Administration special review team report [pdf format] which identified issues with a LSA category aircraft's wing structure, flutter characteristics, stick force gradients, airspeed calibration, and operating limitations.

The control system must be set up so that the stick force required to increase load by 1g (the stick force per g) is always greater than a minimum value. Perhaps 1–2 kg force would cover the recreational aircraft range from the lightweight minimum aircraft to the 600 kg light sport aircraft. The lower the value the more sensitive the aircraft is to elevator inputs.

If the stick force per g was 2 kg then the pilot would apply a back pressure of 2 kg to increase the load from 1g to 2g for a 60° banked level turn. Once applied that force must be held-on by the pilot; i.e. if the pilot has to ease-off pressure to hold the aircraft in a constant rate 2g level turn then the aircraft is exhibiting signs of instability.


The next module in this Flight Theory Guide discusses aircraft weight and balance.



Things that are handy to know

  • Top rudder refers to the relative position of the rudder pedals — 'top' being the rudder pedal opposite the lower wing. Thus, if the aircraft is banked and turning to the left, then pressure on the right rudder pedal will apply top (or outside) rudder; pressure on the left rudder pedal will apply bottom (or inside) rudder.


Stuff you don't need to know
  • The Wright brothers were the first to realise that control in each of the three axes was necessary for sustained stable flight. They added vertical tail rudders to their canard configuration 1903 Flyer and arranged simultaneous rudder deflection with the wing warping control (the trailing edge of one outer mainplane was pulled down by cords to increase camber while the other was pulled up) thus providing a lateral stability system and countering adverse yaw when initiating a turn. Glenn Curtiss was the first to patent the use of ailerons [but not the first to use] in place of wing warping in the hope of beating the Wright patent for three-axis control/stability systems. A long and bitter patent battle among the United States aircraft manufacturers (that had inhibited U.S. aircraft development and manufacture for 10 years) ended in early 1917 when the U.S. Congress forced all manufacturers into pooling their patents by threatening to seize all their patents.

    When the USA entered World War 1 in 1917 the nation was forced to purchase 10 940 combat aircraft from Europe while only 5064 mainly training aircraft were supplied to the U.S. army and navy by U.S. manufacturers.

    View the Wright Brothers 1906 patent. Note that the patent uses the term 'aeroplane' in lieu of 'mainplane' or 'wing'.



Groundschool – Flight Theory Guide modules

| Flight theory contents | 1. Basic forces | 1b. Manoeuvring forces | 2. Airspeed & air properties |

| 3. Altitude & altimeters | 4. Aerofoils & wings | 5. Engine & propeller performance | 6. Tailplane surfaces |

| 7. Stability | [8. Control] | 9. Weight & balance | 10. Weight shift control | 11. Take-off considerations |

| 12. Circuit & landing | 13. Flight at excessive speed | 14. Safety: control loss in turns |


Supplementary documents

| Operations at non-controlled airfields | Safety during take-off & landing |



Copyright © 2000–2012 John Brandon     [contact information]


Page edited 2008 by RA-Aus member Dave Gardiner www.redlettuce.com.au