Pilot Elementary Training: RAF Flying Cadets' Handbook - Part III
MANOEUVRES - Part II
13 - S P I N N I N G
How the Aircraft Moves in a Spin. The spin, like the stall, of which it can be regarded as a secondary result, is a state of affairs into which you should not get yourself unintentionally. It arises from mishandling of the controls, from bad flying, and from failure to understand why and when the aircraft stalls.
It is necessary to practise deliberate spins, so that the spin can be instantly recognized, its full implications understood, and the appropriate correcting action be at once taken should a spin ever start.
A spin occurs if, when the aircraft is stalled, it is already moving in the yawing plane, or conditions are such that the stall itself will lead to a movement in the yawing plane. During a spin an aircraft is, in fact, moving in all three planes at once, in the rolling, yawing and pitching planes, and these combined movements result in a rotation about the axis of the spin, while at the same time the aircraft descends vertically. The speed of descent depends on the steepness of the spin, or on what we may call the 'pitch' of the spiral track it is following. (You know how the threads of a screwed rod may be close together, as in a small bolt, or quite wide apart as in a corkscrew ; the distance between the thread is called 'pitch.') If the spin is one of coarse pitch, it is steep, and if it is of fine pitch, it is flat. The diagram on the opposite page shows the attitude of the aircraft in a steep and in a flat spin.
You can see from this diagram that the aircraft is rotating about the axis of the spin and, if you think it out carefully, you will be able to see the movements in the aircraft's three planes of movement. The movements in the rolling, yawing and pitching planes are not unlike those involved in a gliding turn, except for the important difference that in a turn you are providing the movements by means of the controls, while in a spin, the forces acting on the aircraft provide the movements themselves. Now what are those forces ? To understand them, we must go back to the question of the stall, and to the principles governing the lift and drag of an aerofoil.
The Forces Acting in a Spin. Just consider the wings of an aircraft at the moment of the stall. If there is no movement in the yawing plane, and if one wing does not drop more than the other wing, we know that the result of the stall is that the nose goes down, the angle of attack of the wings to the airflow is thus reduced, and this reduction is heightened by the effect of the increase in speed (which we get when the nose goes down) of the relative airflow. The reduction of the angle of attack unstalls the wings.
But now suppose that the aircraft were moving in the yawing plane when the stall occurred. This means that the outer wing was moving faster than the inner wing and therefore had rnore lift. Consequently, when the stall occurs, the two wings will not be equally stalled ; the inner wing will be more stalled than the outer wing. Therefore it will drop faster than the outer wing, that is, the aircraft will move in the rolling plane towards the inner wing. This aggravates the situation, since the down-going inner wing continues to meet the airflow at an increasing angle of attack (see page 15) ; since the inner wing is stalled, it means an increasing degree of stall. At the same time, the outer wing, which is not dropping so rapidly as the inner wing, meets the airflow at a smaller angle of attack, and therefore becomes less stalled, it may even become unstalled.
If there is nothing to stop this state of affairs, the aircraft will continue to move in the rolling plane, turning over and over. This is called 'autorotation.' So long as an aircraft is not stalled, we control its movements in the rolling plane by use of the ailerons. When the aircraft is stalled, however, the ailerons are ineffective for this purpose in fact, their normal use merely makes matters worse.
This is easy to understand if you think of the fundamental principles involved. If you are banked to the left, and wish to bring the left wing back to the level position again, you naturally move the stick to the right ; this depresses the left-wing aileron and raises the right-wing aileron. The depressed aileron meets the airflow at a greater angle of attack and the raised aileron at a less angle of attack. When the aircraft is not stalled, this means that the left wing gets more lift and right wing less lift, so that the aircraft levels itself. But if the aircraft is stalled, the reverse effect follows. Then the left wing with its aileron at a greater angle of attack gets more stalled because it is already past the angle of maximum lift (see graph, page 13), while the right wing with its aileron at a less angle of attack becomes less stalled. Furthermore, the drag of the left wing is increased, while that of the right wing is decreased ; this of itself produces movement in the yawing plane in a direction in which we least want it.
If, at the moment of the stall, one wing drops a little, the result is a movement in the yawing plane due to the weather cocking-effect produced from the tendency of the aircraft to slip in towards the lower wing (see page 33). If you attempt to 'pick up' the dropped wing by the normal use of the ailerons, not only do you make the dropped wing fall lower, but the increased drag on its aileron adds to the movement in the yawing plane. As a result, what might have been no more than an incipient spin, which might not have developed into a full spin, promptly becomes the real thing. Therefore, remember this as another golden rule. If a wing drops at the moment of the stall, do NOT try to pick it up by normal use of ailerons.
The only way to lift up a dropped, stalled wing is to apply opposite rudder, so that the speed of movement of the dropped wing can be increased, and more lift achieved.
The Forces Required to Stop a Spin.. In order to stop an aircraft spinning, once it has gone into the spin, we must first stop its rotation about the spinning axis. This means that some force must be applied to oppose the forces which are providing the rotation. It does not require much thought to realise that whether the spin is steep or flat, we have only one means at our disposal of stopping the rotation, and that is the rudder.
Consequently, to recover from the spin, the first thing to be done is to apply full opposite rudder; that is, if you are spinning to the left push your right foot forward on the rudder bar, and push it as hard as you can. The force which you are trying to oppose is much greater than anything you will try to counteract with the rudder in normal flight. Don't be misled by the feel of the rudder bar into thinking that you have full rudder on when, in fact, you have not. Always thrust out your foot with your full strength.
If you are spinning to the left. it means that the tail is revolving round the spinning axis in an anti-clockwise direction if looked at from above (see the previous diagram). Applying full right rudder, therefore, means that you are pulling the rudder round to the right, so that it offers greater resistance to the airflow. (Think of it, if you like, as meeting the airflow at a greater angle of attack and therefore having more lift.) This resistance is acting in the direction opposite to that of the rotation, and therefore is a brake on the aircraft's rotation which finally slows it down and stops it. If you were to leave the opposite rudder on after the rotation had stopped, you would start to rotate, i.e. spin, in the opposite direction, since as yet you have done nothing to unstall the aircraft.
Therefore when you see that the rotation has practically stopped, press the stick forward (the amount of forward pressure on the stick varies for different types of aircraft-for most training types. it will not be great), and finally ease off the rudder. It is no use pressing the stick forward while the aircraft is in full rotation ; the elevators will then not have the effect you are seeking, although they may steepen the spin, which is sometimes desirable (see next section). They will in fact delay, or even prevent, the rudder from stopping the rotation. T
This is because they will shield the rudder to some extent from the airflow, and thus reduce its effectiveness.
The effect of the elevators, when the rotation has stopped, is to convert the still vertical, stalled descent of the aircraft into a dive, and the principle of this part of the recovery from the spin is that of the recovery from an ordinary stall. You reduce the angle of attack of the wings to the airflow and, by gaining more speed, increase the forward component of the relative airflow, so that the angle of attack is still further reduced. The wings thus become unstalled and you recover from the dive in the ordinary way, that is by gently easing back the stick until the aircraft is flying level.
How to Practise the Deliberate Spin, and the Recovery from it. You must never attempt to spin your aircraft deliberately, unless the altitude is at least 5,000 feet, and you must complete a spin at a minimum height of 3,000 feet. Before going into a spin, take great care to see that there are no other aircraft nearby, and particularly beneath you. This means that before beginning a spin you should do a 360° turn so that you can be sure the air below is clear. This complete 360° turn is important, as otherwise you might miss seeing an aircraft hidden beneath one of your own wings. To go into the spin, fly level, close the throttle and hold the nose up by backward pressure on the stick ; in other words, stall the aircraft. Just before the stall occurs, put on rudder in the direction in which you want to spin, and the spin results.
The first time you experience a spin, which, of course, will be with your instructor, you will probably find it rather disconcerting. A lot of things seem to be happening at once, and though your sense-organs register the sudden change, they may not on the first occasion give you the full idea of what the aircraft is doing. What it is doing, as was explained in a previous section, is moving in all three planes, rotating about an axis which is probably just in front and to one side of the nose (it varies with different aircraft), and descending vertically.
The easiest way to watch what is happening is to look along the nose, on the side towards which you are spinning ; that is, in the direction in which you applied the rudder. You will then see that a patch of the earth down below is revolving round the nose, and you will be able to appreciate the attitude of the aircraft, and at least the main fact of rotation round the spinning axis.
To recover from the spin, (and in the early days at any rate, you should not let the spin continue beyond two or three complete turns), apply rudder in the opposite direction to the spin, as hard as you can. As the rotation slows down, move the stick forward. Not right forward, but only a little beyond the central position is required.
When the rotation stops, centralize the rudder and press the stick gently backward to recover from the resulting dive.
Note. If you continue to apply rudder after the rotation has stopped, it will begin again, in the opposite direction. Remember that up to this stage the aircraft is still stalled. If you try too suddenly to pull the aircraft out of the dive which forms the second part of the recovery from the spin, you will probably stall again (see page 26).
With the types of aircraft in which you will fly during elementary training, full opposite rudder, followed by a little forward pressure on the stick, will stop a spin in all normal circumstances. This type of aircraft usually spins in a fairly steep attitude from which recovery is easier than from a flat spin. Since recovery depends in the first case on the rudder, you can understand how this arises ; in a flat spin the tail plane and elevators shield the rudder from the airflow, thus rendering it comparatively ineffective.
As a spin is continued, so it tends to get flatter ; the greater the proportion of the total weight of the aircraft which is situated behind the centre of gravity, the flatter the spin. Consequently, it may happen even with an elementary training type in which the load is so disposed that more of it than usual lies behind the centre of gravity, that a spin which has been allowed to continue for a number of revolutions may become much flatter than that to which you are accustomed.
As was said above, the reason why the flat spin is more difficult to recover from than the steep spin is that the rudder is shielded by the tail plane and elevators. If, therefore, the aircraft does not cease rotating within 1,000 feet of your applying full opposite rudder, the following methods should be tried. They should not be resorted to otherwise.
1. Throughout, always keep full opposite rudder applied.
2. Press the stick right back and keep it there to remove or reduce the shielding effect of the elevators.
or 3. Rock the stick sharply, full forward and full backward, pausing for about a second at each extreme position. The aim is to achieve a rocking motion in the aircraft so that it may swing into a position favourable to recovery. This rocking may be intensified by opening the throttle as the stick is pressed forward and closing it as the stick is pressed backward.
If a persistent spin of this nature shows no signs of lessening when you have come down to 3,000 feet, you must prepare to abandon the aircraft. Don't forget that you want to be sure, in such circumstances, that you can be out of the aircraft by the time it reaches an altitude of 1,000 feet. (For a summary of procedure in abandoning an aircraft, see page 97.)
It is possible for an aircraft to get into an inverted spin, either intentionally or if it is carelessly handled while flying inverted, or in some acrobatic manoeuvres. Should this occur, simply centralize the controls, which is normally sufficient to stop the spin.
Note. Before practising spins, make sure that the slat-locking lever, if one is fitted to your aircraft, is in the locked position.
14 - S I D E S L I P P I N G
General Principles. The manoeuvre known as sideslipping may be roughly described as making the aircraft descend through the air broadside on, while gliding. The direction in which the aircraft travels is at an angle to the direction in which the nose is pointing. We can sideslip on a turn as well as when flying straight. The advantage of the sideslip is that it permits us to increase our rate of descent, that is, to lose height more quickly, without increasing our forward speed. By varying the amount of slip, we can vary the rate of descent. The force which provides the sideway movement comes from the lift and gravity, just as a component of lift provides the centripetal force required in a turn. This is shown in the diagram overleaf :
The aircraft moves forwards, downwards and sideways ; the downward and sideway movement being along the line of the resultant of lift and weight. Drag limits the speed of the sideslip.
This sketch explains that in order to sideslip, the first requirement is that the aircraft should be banked, and also that the steeper the bank, the greater the degree of sideslip. If, however, we merely banked the aircraft, the sideslip would only begin, and would be promptly checked and converted into a turn, albeit probably a faulty one, by the movement in the yawing plane which would be produced by the pressure of the air bearing on the fin and keel surfaces. So, besides applying bank, we have to apply opposite rudder, in order to sideslip. The degree to which any aircraft can be sideslipped depends on the extent to which the rudder is capable of overcoming the I weathercocking I of the aircraft, provided by the air pressure on the fin and keel surfaces. This imposes a, limit to the amount of bank which can usefully be applied and thus to the amount of sideslip.
How to Sideslip on the Glide. You can sideslip the aircraft so that it follows any flight-path you want, but we will first assume that you wish to continue to glide in the same direction, but wish to sideslip at the same time. First select a mark on the ground towards which you wish to fly : then bank the aircraft either to the left or right and apply opposite rudder, swinging the nose in the opposite directionuntil the resulting sideway path of the aircraft points towards your mark on the ground. Keep your glide-path steady by the appropriate use of the rudder and elevators, and hold the bank constant with the ailerons. You have also to keep the nose in its correct position relative to the horizon, which is rather above that of the normal glide. If you let the nose drop, the speed will increase, which you wish to avoid. On the other hand do not allow the speed to fall off by holding the nose too high:
When you decide that you have lost sufficient height, recover from the sideslip by taking off rudder and allowing the nose to swing back until it is in the correct position in relation to the horizon for gliding, and pointing towards the mark towards which you were sideslipping.
At the same time apply opposite bank, to bring the aircraft back to a level keel in the rolling plane.
If you wish to sideslip so that your flight-path is changed, that is, that you glide at an angle to your original path, go into the sideslip in the same way. Once more, use the rudder and elevators to control your direction and speed down the desired path, and come out of the sideslip as before. Remember that you must come out with the nose pointing in the direction in which the aircraft is actually flying.
If you were originally gliding into wind, coming in to land, and you decide to sideslip across wind to avoid some obstacle, or to change your landing path, turn the aircraft sufficiently, before coming out of the sideslip, to ensure that your flight path is then into wind:
Side-Slipping on a Tttrn. A slipping turn is very useful if you wish to land on a piece of ground which is bounded on its leeward side by high trees or other obstacles. Instead of gliding in over the obstacles, you glide parallel to the boundary on its windward side and sideslip during I he final turn in, thus losing the necessary height. To do this, increase the bank, with the ailerons, beyond that required for the ordinary turn, at the same time keeping the nose up with top rudder, and decreasing the rate of turn by easing the stick slightly forward. Recover (do this with a good margin of height, because of the rapid rate of descent) by reducing the bank and top rudder together until the aircraft is level in the rolling plane, with its nose pointing in the direction in which the aircraft is moving, which should, of course, be into wind.
15 - PRECAUTIONARY LANDINGS
A precautionary landing becomes necessary if, while on a cross-country flight, you run into bad weather, lose your way, or find yourself running dangerously short of fuel. It is not the same thing as a forced landing ; with a precautionary landing you have rather more time to select your landing ground, and you have the engine to assist in your approach.
We will suppose that circumstances have arisen which make a pre-cautionary landing prudent. If it is possible, of course, you should find an airfield, but this may not be practicable, and you will have to find a field on which to land. Check the direction of the wind, and begin the search for a suitable piece of ground. In bad weather, your search and final approach will probably have to be made at low height. Special points about low flying are discussed in the next chapter ; remember them in connection with precautionary landings in bad weather.
In seeking your landing place, remember that it should be clear of obstacles, not only on the leeward but on the windward side also. (After all, later you will want to take-off again) It should give you sufficient space for landing without running into the far boundary, and its surface should be reasonably smooth.
This last point is one on which it is not easy to be certain when one is looking down from 2,000 feet. Consequently, before making your actual landing, you do a dummy run-up, using the normal approach, but instead of landing, fly over the field some 50 feet above it, taking a good look at the surface and, if it is reasonably smooth, selecting the actual path down which you will land. If the surface proves on examination to be dangerously rough, or if you find that there are obstacles which will make landing, or subsequently take-off, unduly difficult,
Note. Practice in precautionary landings must only be carried out in the area indicated by your instructor. If you think for one moment of the farmer's point of view, you will quite understand why to abandon the idea of landing on that particular field and go on to select one more suitable.
When you have decided from your dummy run that the chosen field is satisfactory, climb again and make an engine-assisted approach, but at a flatter angle and at a slower speed than are normal. This is because you want to limit the time between passing over the near boundary and the touch-down to as short a period as possible. The field is not likely to be so big that you will have ample space to play with. You achieve your object by holding the nose up, so that the speed is just above the stalling speed and open the throttle a little more to prevent the air:craft sinking at too high a rate. The usual methods in the engineassisted approach for correcting overshooting and undershooting are employed (see page 73). Since descent is made at a flatter angle than normal, the amount of flattening out required will be less than normal. The speed should not be reduced below the normal approach-speed for an engine-assisted approach until you have completed the turn into wind and are making the final approach to land. Normally you should not attempt to take-off again yourself if you have to make a precautionary landing (other than at an airfield) while flying solo during your elementary training. Once you have landed you should follow the instructions given in chapter 19 on Forced Landings.
16 -LOW FLYING
During elementary training, low-flying practice must be carried out only when an instructor is in the aircraft with you, never when you are solo.
Low flying is always avoided as far as possible because
I . if the engine fails you are on the ground a few seconds later, and cannot pick and choose a place to land
2. map reading is difficult ;
3. it is easy to think you are going to miss a tree and hit it instead
4. it annoys civilians and frightens livestock ;
5. to manoeuvre near the ground calls for concentration and smooth execution. People showing off frequently neglect one or the other and crash.
However, it is sometimes imposed by tactical needs or by bad weather conditions, such as thick banks of low-hanging clouds. You must, therefore, be thoroughly familiar with low-flying conditions and methods. Low-flying practice is usually carried out at a height between 250-400 feet and over an area specially set apart for this purpose. Low-flying practice must be limited to this special area.
Low flying on account of bad weather generally involves a reduction in speed, so practise at speeds rather below the normal cruising speed. If the aircraft is fitted with flaps, lower them partially. This has two advantages: it gives more lift, despite the slower speed, and it puts the aircraft into a rather nose-down attitude, which helps your visibility. First, therefore, partially lower the flaps, throttle back somewhat, and fly across wind. You will notice that the drift is more apparent than when you are flying at a normal altitude. Next turn up-wind. When turning near the ground, always open the throttle a little, since you are flying below your normal cruising speed, and, of course, you need additional thrust and lift on the turn. Fly up-wind and you will notice that the ground speed is reduced, though the airspeed remains constant. Then turn and fly down-wind; you will notice that now the ground speed is increased, although the airspeed remains constant.
You may experience the temptation to take so much note of the apparent change in speed, particularly the increase when flying downwind, that you want to change the throttle opening. Such a temptation must be sternly resisted; it is airspeed, not ground speed, which you have to consider, so if you ever feel tempted to alter the throttle opening when flying low (except for a turn), look at the airspeed indicator first, and you will probably find that no change in throttle is required.
On turns, when flying low, there is an optical illusion, due to drift, which may cause some confusion at first. If you fly into wind and then turn across wind the aircraft appears to slip inwards, though in fact you are doing a correct turn. If you think for a moment of drift effect, you will understand why.
Similarly, if you fly downwind and then turn across wind, the aircraft appears to skid outward, which again is an optical illusion due to drift. Consequently, in turns, take care to avoid being misled by these illusions, and check that you are turning correctly by the turn and sideslip indicator.
The important things in low flying are
1. Maintain your airspeed steady, rather less than normal cruising speed.
2 Make sure that your turns are accurate.
3. Avoid turning steeply ; limit yourself to turns of not more than rate 2.
17 - STEEP TURNS AND C L I M B I N G T U R N S
First re-read pages 56-62, which deal with the forces coming into play, and the use of the controls, in turns.
Steep Turns with Engine Going in. First look all round for other aircraft; then, if the coast is clear, open the throttle fully and go into the turn. This you do in the same way as for a gentle turn, that is by applying bank and bottom rudder and by pressing the stick backwards at the appropriate time. But go on increasing the bank until it is at the desired angle, at the same time taking off bottom rudder, and pressing the stick further backwards to increase the rate of turn to that appropriate to the degree of bank. The tendency for the aircraft to overbank (page 62) is less the steeper the bank, so that holding off of bank, though still necessary, is not so marked as in a gentle turn.
Staying in. Keep the angle of bank constant, and hold the nose steadily on the horizon, and moving round it. Remember that when you are steeply banked, - the rudder moving the aircraft in the yawing plane makes the nose move up or down the horizon, and that the elevators moving the aircraft in the pitching plane move the nose round the horizon. Slipping or skidding are corrected as for a gentle turn (page 65).
Coming out. Apply opposite bank and top rudder together, and also press the stick well forward to prevent the aircraft adopting a climbing attitude when it has emerged from the turn. When the aircraft is flying straight and level, centralize the controls, and close the throttle to the normal cruising point.
Steep Gilding Turns
Before going into a steep turn from the glide, you must increase your speed markedly by pressing the stick forward and depressing the nose. To attempt a steep gliding turn at too low an airspeed is to invite a stall.
Going in. First look round as usual before beginning a turn, and then press the stick well forward to gain the extra speed. which is essential ; go in as for a gentle turn, that is with bank and rudder together, followed by a backward movement of the stick. Increase the bank until it is sufficiently steep, taking off bottom rudder gradually, and increasing the backward pressure on the stick to provide the appropriate rate of turn. There is no need to 'hold off' bank ; this applies to all gliding turns, and the explanation which was deferred on page 62 is as follows.
In a gliding turn the aircraft is descending in a spiral path, but the spirals followed by the two wings are not identical, that of the inner wing being steeper. The inner wing therefore has more downward movement relative to its forward movement than has the outer wing. Both wings are, of course, moving downward by the same amount, but the outer wing has farther to go, in the same time, in its circular path than the inner wing. Therefore, although the angle of attack of both wings is increased by the downward component of the relative airflow, the angle of attack of the inner wing is larger than that of the outer wing. The increase in lift which the inner wing gains for this reason more than counterbalances the increase in lift which the outer wing obtains from its greater speed. This is why there is no need to 'hold off ' bank in a gliding turn, and why you have in fact to 'hold on' bank in gentle gliding turns.
Staying in. Maintain the bank constant and keep the nose in the correct position below the horizon, and moving round it at the appropriate rate by use of the rudder and elevators. The rudder will now be primarily concerned with keeping the nose at the right level compared with the horizon, while the elevators will move it round the horizon.
Coming out. Come out in the same way as from a gentle turn, by applying opposite bank and top rudder, and at the same time pressing the stick forward. You must take care to maintain the correct gliding speed ; accordingly the stick must be pressed well forward.
Climbing Turns
In a climbing turn, the rate of turn cannot be anything more than moderate. The reason is easy to understand. In order to climb, you must increase the thrust, and the power of the engine is largely absorbed to gain additional height. In a turn, you must have more lift to provide the necessary centripetal force, which means an increase in the angle of attack, with a consequent increase in drag; more thrust is therefore necessary to keep the speed constant at the larger angle of attack.
In a climbing turn, therefore, you need more thrust for two purposes, both to climb and to turn, and the limit on the total thrust available, which depends on the power of the engine, determines the steepness of the turn you can make while climbing.
A climbing turn should be made when you are climbing at the best climbing speed without full throttle. In this case, open the throttle fully before going into the turn. (If you were already climbing on full throttle, or at less than the best climbing speed, you must depress the nose a little before going into the turn.) Do not start a climbing turn below 500 feet unless such a turn is really essential to avoid some obstacle. This is how a climbing turn is carried out.
Going in. First, as always before beginning a turn, look round and make sure that the way is clear. See that you are climbing at the best climbing speed and then open the throttle fully. (If you have the throttle fully open already, depress the nose a little.)
Go into the turn as for a level turn, that is with bank and rudder applied together, followed by backward pressure on the stick. The rate of turn must be kept very moderate ; on training aircraft it should not exceed rate 2, and in your early attempts you should keep it at rate 1 or less.
Staying in. Hold the bank steady; you will find that the 'holding off' pressure on the stick must be greater than in a level turn. This is because the outer wing, besides moving at a greater speed than the inner wing, is on the outside of the spiral, and therefore meets the airflow at a greater angle of attack than does the inner wing. This is because its relative movement is more forward and less upward than is the case with the inner wing. It is just the reverse of the conditions in a gliding turn.
As you will realise on thinking about the effect of the controls, you will require very little bottom rudder to keep turning correctly in a climbing turn, since the nose has to be kept turning round the horizon and above it. Bottom rudder, when the aircraft is banked, brings the nose down below the horizon.
Some movement in yaw in the direction of the turn is, of course, necessary; but this may be sufficiently supplied by that coming from the pressure of air on the keel surfaces, i.e. the 'weathercocking' of the aircraft.
Coming out. You come out of a climbing turn in the same way as for a level turn, by applying opposite rudder and opposite bank, and pressing the stick slightly forward. Take care, however, to keep the aircraft in its proper climbing attitude and not to depress the nose too much.
Special Notes on Turns
Load Factor. The load factor in steep turns can increase very considerably. For example, when we turn at approximately 40° of bank, the load factor will be about 1 -3 ; in a turn at approximately 70° of bank, it will be 2 -9. In a turn with 80°of bank, it will be nearly 6.
Incidentally, if you execute a bad turn, with a lesser angle of bank than is necessary for the rate of turn, the load factor is the same as if the turn were a correct one.
The following graph and diagram show the increase in the load factor in turns requiring varying degrees of bank for their correct execution; the figures apply to all aircraft:
Stalling on Steep Turns. The high load factor corresponds to conditions demanding greatly increased lift to provide the necessary centripetal force (page 57). Consequently, a steep turn which is executed without providing either the extra thrust required by opening the throttle or the extra speed by lowering the nose, may easily result in a stall. And since, when the stall occurs, the aircraft is moving in the yawing plane, the aircraft will probably spin. And it will not necessarily spin in the direction in which you are turning: this depends on the nature of the mistake you have been making in your turn. Suppose that you are attempting to turn with inadequate thrust, and with rather too much bottom rudder: the aircraft will skid outwards and the nose will drop. This may by itself be sufficient to cause the stall ; but if you press the stick backwards, wrongly thinking thereby to raise the nose, you will increase the rate of turn, and the angle of attack, and you will stall and spin inwards.
If you are doing a turn with less than adequate thrust or speed and rather too much bank for the rate of turn, the aircraft will slip in ; the pressure of the air on the keel surfaces will turn the aircraft in the yawing plane, and the nose will drop. If you try to hold the nose up to its correct position in relation to the horizon with top rudder, the forward speed of the aircraft will be reduced still more ; the lift will decrease, with this the sink must increase, and so the angle of attack will be increased. Consequently the aircraft will stall and, since top rudder has overcome the yawing tendency due to sideslip it will flick over to the side opposite from the direction of the turn and go into a spin on that side. It is well to ask your instructor deliberately to demonstrate this type of spin, as it can be distinctly disconcerting the first time you experience it. You ought not to find yourself in this position by accident, since it can only arise through rank bad flying; but it is well to be prepared.
The danger of imagining that stalling only occurs at one particular speed is demonstrated by stalls on a turn. The relationship between the speed at which the stall occurs and the angle of bank required for the execution of turns is shown in the following graph and diagram:
This graph and these figures relate only to one particular type of aircraft; and you should remember the shape of the curve, rather than the actual figures. Note particularly how steeply the curve rises when the angle of bank increases above 50°.
Don't forget the fact that an aircraft which stalls in level flight at 45 m.p.h., does so because the angle of attack passes the stalling point when you attempt to keep the aircraft flying level at that speed without the engine. Similarly, the fact that an aircraft turning with an angle of bank of 70° stalls if the speed drops to 80 m.p.h. means that at that speed the attitude of the aircraft to the airflow is such that the angle of attack passes the stalling point.
As you remember from our elementary discussion of lift and drag, we can only increase lift at the expense of increasing drag. Drag acts as a brake, and consequently during a turn when we have to increase lift, the increased drag reduces our forward speed. Therefore, the steeper the turn, the shorter the interval for which we can remain in it without danger of a stall. Incidentally, a turn at an angle of bank of 90° can only be executed on an aircraft with a very powerful engine, and maintained for a very short time. The fact that it is possible at all is due to the momentum of the aircraft as it goes into the turn at high speed; this keeps it moving forward for a time, although the lift is now all directed horizontally and there is no vertical component to sustain the weight of the aircraft. An aircraft in a turn with 90°' of bank begins to lose height immediately the effect of the momentum is lost, and if the attempt were made to maintain the turn, it would mean that the aircraft slipped in, or rather downwards, rapidly.
More on Load Factor. One more point about the load factor; on page 57 it was explained that, for any aircraft, the centripetal force required depends on the rate at which its direction is changed, and on the airspeed at the time. Consequently, if you are flying at maximum speed, there is a limit to the rate of turn you can safely execute. If you attempt to do a very 'tight' turn, that is to follow a circle of small radius, at a high rate of turn, the centripetal force will have to be very large. Such conditions impose a very severe strain on the structure of the aircraft, and there is naturally a limit to the strain an aircraft can bear. (It is possible to break up an aircraft by sudden application of the large centripetal force which is necessary for a very tight turn at high speed.) The conditions will also impose a big strain on you. Just as the aircraft resists the change of direction involved in a turn, because its momentum tends to carry it on in its old path, so your momentum causes you to resist the change. (This applies to any loose objects in the aircraft.) The necessary centripetal force to overcome this resistance is applied to you through the seat. Consequently, as the load factor increases, you feel as though you were being pressed down on your seat ; your feet are pressed to the floor or rudder bar, and a big effort is necessary to raise them.
The blood in your veins is not subject to the centripetal force in the same way as your body, but it is forced away from the head - as a result, first temporary dimming and then loss of vision occurs ('blackout' as it is popularly known), and this is rapidly followed by unconsciousness if the load factor goes on increasing.
After some experience of conditions of high load-factors, you will begin to be able to estimate the position from the pressure forcing you down on your seat. Judgment of this pressure will help you to recognize the point at which blackout is approaching, and permit you to prevent the further increase of load factor by holding or decreasing the rate of turn. This note is really in anticipation of conditions which you will encounter on service types. You should not normally execute turns at so high a rate in elementary training aircraft. Incidentally, the steep turns which you will carry out at the E.F.T.S. will appear to you much steeper than they really are 'when you think you are doing a turn banked to nearly 90° you will probably. in fact be banked to about 70°.
18 - S P E C I AL E M E R G E N C I E S I N T H E A I R
Action in the Event of Fire. This practice is carried out in your elementary training, only when you have an instructor with you in the aircraft. You must not repeat it when you are flying solo.
Except when caused by enemy action, fire in an aircraft in the air is a happening of very rare occurrence ; it almost invariably begins at the engine. Consequently, your first action must be to cut off the fuel supply to the engine and stop it running.
Therefore, should fire break out:
1. Turn off the petrol.
2. Open the throttle fully (to clear the carburettor and fuel pipes).
3. Switch off after the engine has ceased firing.
4. Operate the fire extinguisher if one is fitted. This is usually controlled by a handle beside your seat.
You can also sideslip the aircraft in the direction opposite to the side of the engine on which the carburettor is fitted, so that the flames and smoke are kept away from the cockpit. But this may be done only with single-engined aircraft. With a multi-engined aircraft it would involve the risk of the fuel tanks being set on fire.
If the fire continues and seems to have a good hold, abandon the aircraft and take to your parachute.
Abandoning an Aircraft Apart from enemy action, circiumstances may arise when you will have to abandon your aircraft in the air and take to your parachute. Happily, with modem aircraft, these circumstances are very rare, but the possibility is there you must be prepared for such a situation by knowledge of what to do.
There are two considerations to be borne in mind, for your own safety is only one. The second is that the abandoned aircraft shall cause the minimum damage to life and property when it finally comes to earth.
Therefore, if you are forced to take the decision to abandon your aircraft, first of all, if it is at all practicable
1 Turn off all petrol taps.
2. Turn off all engine switches.
3. Close the throttle.
4. Head the aircraft towards open country.
5. If a rudder-trimmer is provided, trim the aircraft for straight flight at normal gliding speed.
How you leave the aircraft depends on the circumstances. If the aircraft is under control and in the level attitude you open the hood, release the Sutton harness and the speaking tube to your helmet (if you should forget the latter it is not important, as it will-come away quite easily), climb out over the side of the cockpit, and keeping your right hand on the release ring of the parachute, dive away from the aircraft. Provided you abandoned the aircraft at not less than 1,000 feet, count three slowly, as you fall through the air, and then pull on the release ring. Pull it right out of the harness, so that it comes away in your hand. The reason you count three is to get well away from the aircraft before the parachute opens, so that there is no risk of it fouling any part of the aircraft.
If the aircraft is under control, and for some reason you have difficulty in climbing out, you can let the aircraft drop you out. This you do by opening the hood and releasing the Sutton harness and speaking tube, and rolling the aircraft on to its back with the ailerons.
If, however, the aircraft is not under control, as may be the case if there has been a structural defect, or if it has been damaged in combat, .your action must depend on circumstances. For example, if the aircraft is diving vertically, you are being held in by the straps of the harness, and if you release them without due precautions you may damage yourself by falling forward on to the dashboard. In such a case, open the hood, place one foot on the dashboard, release the harness, and push yourself out sideways and forwards, by a thrust with your leg.
If the aircraft is under control, but circumstances are such (for example, flames blowing into. the cockpit) that speed of exit is of first importance, act as follows-the method is drastic but very effective.
Open the hood and release the harness and speaking tube ; then put one foot behind the stick and push abruptly with all your strength. You will be ejected from the aircraft, and when you have had time to realize that you are out and falling through the air, pull out the parachute release-ring.
If you have to abandon the aircraft when it is approximately in the normal attitude, but at a very low height, say below 500 feet, the principal object is to ensure that the parachute is open well before you reach the ground. Therefore act as follows : open the hood, release the Sutton harness and speaking tube, and stand up in the cockpit. Pull the release ring of the parachute, so that the slipstream may catch the parachute and blow it, and you, well clear of the aircraft.
Re-starting the Engine in Flight. It is possible for the engine to stop while the aircraft is in the air without any defect having developed. You might accidentally close the petrol tap or run a tank dry, or knock the switches off, for example. Again, in some aerobatics the engine may stop if the manoeuvres have not been accurately performed. Consequently, you have to learn how to re-start, in the air, an engine that has stopped. But do not practice this when you are flying solo, only when your instructor is with you.
If the engine stops, first make sure that the petrol tap is 'ON,' that there is fuel in the tank and that the switches are 'ON.' It is no use trying to start the engine otherwise.
In order to get the engine started, you have to force the propeller to revolve, and this you do by putting the aircraft into a steep dive with the throttle half open. When a: certain minimum speed has been obtained, the pressure of the air on the blades of the propeller overcomes the compression of the engine, the propeller revolves, and the engine begins to fire once more.
Then pull out of the dive in the normal way. If, by any chance, you have to start to pull out of the dive before the engine has re-started, the act of recovery may itself start the engine ; but don't rely on this as a normal method of re-starting. Always continue the dive if possible until the engine starts.
Before starting the dive, take a look at the ground; and if one region seems more likely to provide better conditions for a forced landing than others, make your dive in that direction. After all, your engine may have stopped through some defect which will prevent it starting again.
Similarly, if the engine fails to re-start after you have been diving and you have come down to 2,000 feet, pull out of the dive and prepare to carry out a forced landing. It is no use diving on and on in the hope that the engine will start, and finally find you have not enough height left to let you select a suitable spot for a landing and to select it in comfort.
19 - F O R C E D L AN D I N G S
General Principles. A forced landing is one enforced on the pilot by circumstances ; it differs from a precautionary landing in that the pilot has less time to select a suitable field, and since engine failure is the comnonest reason for a forced landing, it calls for greater judgment and skill. He cannot'go round again if he misjudges his approach.
Although forced landings are happily not often called for, when the need does arise, it is urgent, and therefore every pilot should always be prepared for the possibility.
This means that he should always know the direction of the wind, and check it frequently ; that he should be constantly noting the features of the landscape beneath him, and selecting fields suitable for forced landings.
As you can imagine, the time at your disposal to effect a forced landing depends on the circumstances, particularly your height. If the engine should fail soon after taking off, or when you were engaged in a low-flying manoeuvre, you would have little choice of where to land; you must fly straight on, turning only slightly to avoid major obstacles, and make the best possible landing ahead.
If the engine fails when you are flying at 5,000 feet, you have quite a lot of time before you will actually touch down ; you have therefore more choice of where you will land, and although the details of the earth's surface are not recognizable at 5,000 feet, you can select one direction in preference to another, if it promises pasture, as opposed to forest, for example.
Another most important use to which you can put the time given you by extra height is to search for the cause of engine failure. Make sure the petrol tap is on, and also the switches. If there is more than one fuel tank, see if the one from which you were drawing fuel is empty, and if so change to another tank.
If you can't find the cause of failure, close the throttle. A sudden spluttering burst from the engine just as you are concentrating on the actual touch-down can be very disconcerting. Incidentally, if the engine should start again after you have begun your approach for the forced landing, that is, after you have come below 1,000 feet, don't attempt to carry out your original trip, but continue with your landing. The engine may fail again later when you are in a much less satisfactory position than before for landing.
Selecting the Landing Ground. It is not easy to judge the earth's surface from even 2,000 feet above it, and the ability comes from experience and observation. Always have this matter in mind when flying and, in particular, never miss an opportunity of observing carefully areas which are known to you on the ground. The following hints will help you to recognize different types of surfaces :
1. Grass appears a dull uniform green or, in autumn, a brownishgreen colour. It may often be recognized by the presence of horses, cattle or sheep grazing on it.
2 Growing crops. The colour of these vary according to the seasons, but are much brighter in spring and lighter in colour than either grass or stubble. They can be recognized by their regular appearance and by the spaces generally found between rows. Crops should be avoided, especially in the summer. In a strong wind, high crops and long grass show ripples or waves as the wind passes through them.
3. Stubble appears buff-coloured in the autumn, according to the time which has elapsed since the crop has been cut. In stubble regular rows can generally be seen. Stubble is one of the best surfaces for landing on.
4 Root crops appear dark green from the air with regular rows. Root crops make a bad landing ground, but may be used as such in an emergency if an aircraft is landed slowly and parallel to the rows or furrows of the crop, provided that the speed and direction of the wind allow.
5. Ploughed land is more general in late autumn and winter, and has a rich brown or red colour which varies according to the soil of the district. Ploughed land makes bad landing ground except in very dry or frosty weather.
6. Sand appears very light yellow or almost white when dry, and varies greatly in its suitability as a landing ground. Dry, shifting sand is generally too soft for landing, but sand that is moist and firm is very suitable. On a beach the sand a few yards from the water's edge is generally suitable.
7. Snow is very uncertain and possesses the disadvantage of concealing the surface and of rendering obstacles almost invisible. Scan the surface for shadows which may indicate an obstacle or depression in the ground, and endeavour to execute a very slow landing.
Obstacles also are often difficult to see from above; wire fences, for example, can be detected only by recognition of the posts which support them. The following points should be noted :
I . Inequalities in the ground surface may often be discovered by the shadows they cast.
2. The lowest part of a field can sometimes be recognized by the presence of a stream or pond.
3. Roads and railways are often lined by telegraph wires, and, as these are invisible at an altitude, sufficient height should be allowed to ensure the aircraft passing over them. The presence of telegraph wires or high-tension cables may generally be recognized by the poles that carry them.
4. Obstacles on the leeward side of a landing ground over which a pilot has to pass before landing necessitate a much longer ground, whilst obstacles to windward possess the disadvantage that the aircraft will have to pass over them while taking off. A skilful pilot can often avoid obstacles such as high trees by sideslipping or by landing and taking off across wind.
The Manoeuvres Involved in a Forced Landing. Immediately the circumstance arises which necessitates a forced landing, let us say engine failure, assume the correct gliding speed, and trim the aircraft for gliding. Decide upon what looks the best place to land within comfortable reach, and, bearing in mind the direction of the wind, glide down to leeward of it; in the meantime, look round the cockpit to see if you can find the reason for the engine failure. If you cannot find what is wrong, close the throttle.
When you have reached a point on the leeside of your selected landing place, from which you can make a comfortable final approach, lose any surplus height, by gliding to and fro across wind. Before beginning this part of the manoeuvre, partly lower the flaps if they are fitted.
As you glide to and fro, always turn in towards your landing place, so that you make up for the distance lost through drift. (The wind is naturally taking you away from your field as you glide across wind.) Watch drift and estimate from it the strength of the wind. Look out at the area you have chosen, and examine the surface as carefully as you can ; look for any obstacles or obstructions, and decide upon the landing path you will take. Go on gliding to and fro across wind, until you are at about 1,000 feet and in a position from which a normal gliding approach can be made. Two leisurely, sweeping turns, as shown in the diagram opposite, make judgment easier than half a dozen. tight turns, also illustrated.
When you come opposite your selected landing path, turn into the wind and glide down to land. But instead of aiming to land near to the near boundary as usual, aim to land well in the field.
Do this as an insurance against errors of judgment. It is much better to overshoot somewhat and run into the far boundary with the aircraft on the ground, travelling slowly, than to hit the near boundary while you are still airborne, at flying speed. Also if you overshoot considerably, you can lose some surplus height, by putting on full flap and by sideslipping, when it is quite certain that you will clear, the near boundary. If you undershoot, you cannot remedy your mistake, since you have no engine to help you: trying to creep over the boundary by holding the nose up will only make matters worse, besides involving you in the risk of a stall.
A forced landing calls for very accurate judgment of all the aspects of approach and landing. It is a very sound rule to keep some surplus height in hand until you are quite sure that you won't need it ; then sideslip it off.
After You Have Landed. When you have successfully landed, your job is to notify the nearest R.A.F. station of your landing, so that the aircraft may be protected and repaired or removed.
Do not, however, just leave your aircraft to its own devices, and go off and look for a telephone. Find some responsible person to stand guard over it. Unless you have landed in a very desolate countryside, someone is pretty sure to have been attracted by your landing. Ask them to take a message to the nearest military unit or police station, asking for a guard to be sent. If there is no possibility of finding such a guard, entrust the guard duty to a responsible civilian.
When your aircraft is safely guarded, find out where the nearest telephone is and from this give a message to the appropriate R.A.F. station, telling them of your landing and whereabouts, and if any damage has been done to the aircraft.
While waiting for an R.A.F. guard to arrive, you will have an opportunity of examining the aircraft to try to find out the reason for the engine failure, if that was what caused your forced landing. If you can correct the trouble yourself, well and good ; but don't take-off again. Always give the fullest information to the R.A.F. station, when you finally get in touch with it. It will permit them to decide the type of assistance to send you. Don't underestimate your requirements : it only causes more trouble in the end, and wastes a lot of time.
Imagine you are the Flight Commander about to receive the information you are going to send. Get inside his mind. What does he want to know ?
1. Rank and name of pilot.
2. Aircraft type, number and unit.
3 Time landed. Damage to (a) yourself. (b) aircraft.
4. How much fuel left.
5. EXACT pinpoint. Give copious references. They can be used as cross-checks. A small aircraft is often very hard to see in a field,
6. Can instructor land in and fly out of your field ? If in any doubt, say NO. It may annoy him, but not nearly as much as getting there by air and then having to return again without landing. Can hedges be pulled down to give a 'double' run across next field ? Is there a bigger field next door ?
Other points to remember are:-
7. If ringing up an airfield other than your own, ask them to tell your Flight Commander at once. This will save him starting a search.
8. Give a telephone number where they can get hold of you. If necessary, hire a small boy to run to and fro with messages from aircraft to telephone-house and vice versa. Nothing annoys a Flight Commander more than to want to send you a message and to find your number is an empty public call-box.
9. If an instructor flies out for you, spread a white tablecloth, light a bonfire with thick white smoke, fire Verey lights, etc., to attract his attention (see 5 above).
10. Keep crowds off the aircraft.
11. Pay for hospitality. Country folk are often poor as well as generous.
12. Get names and addresses of all who help you. The C.O. will want to write letters of thanks.
13. Be a sensible and responsible person, not just a careless pilot much relieved in mind.
You must also take steps to see that the aircraft does not receive further damage while on the ground. It should be pegged down, and the engine, propeller and cockpit covered with any tarpaulins or other covers which can be obtained.
Before you leave the scene of your landing, obtain the name and address of the owner or occupier of the ground where you have landed, and make a summary account of any damage to property, crops, etc., which you may have caused. Admit no financial liability.
When you get back to your unit, write a detailed account of the occurrence and fill up a forced-landing report.
20 - T A K I N G O F F A N D L A N D I N G O U T O F W I N D
General Principles. We do not take-off or land across wind, unless circumstances compel us to do so. Sometimes this is necessary ; this is the case when the landing ground (it may even be a runway) is long and narrow, and the wind is not blowing down the long side. Also if there are obstructions on the boundary, or on the landing ground itself, we may have to take-off or land across wind in order to avoid them.
The first point to remember is never to take-off or land further out of wind than is absolutely necessary. The second is the effect of the wind while the aircraft is running along the ground out of wind ; the main effects are:
1. Its pressure on the fin and keel surface tends to swing the nose of the aircraft into wind.
2. Its pressure on the fin and keel surface tends to make the aircraft drift ; this imposes a strain on the undercarriage which it is not designed to withstand and tends to tilt the aircraft on to the downwind wing.
3. It tends to get under the windward wing and blow the aircraft over on to. the opposite wing.
The best way to counter these difficulties is, during the take-off to ensure that the wheels are pressed hard down on the ground until the aircraft leaves it, and for the landing to make certain that at the moment of the touch-down the wheels are pointing in the direction in which the aircraft is moving.
Taking Off. During a cross wind take-off you want to hold the wheels hard down on the ground, and the most difficult part of the take-off is when the aircraft is just about to become airborne. It is then that you must avoid bouncing, since you will probably bounce on one wheel, which throws a heavy strain on the undercarriage, and gives the wind the opportunity to get under the wing and tilt the air.craft over.
The most convenient method of taking off across wind varies a little; according to whether you are using a. runway or the grass surface of an airfield. In the first case, the smooth surface does not give rise to the unwanted bounce, and you can keep the wheels hard down on the ground by getting the tail well up until you have attained a speed rather above flying speed, when a slight backward pressure on the stick will take you clear of the ground. On a grass surface, however, there is more chance of a bounce as the aircraft approaches flying speed, and it is best to take-off by following a track which curves slightly down-wind. This flat turn down-wind puts the same strain on the aircraft as the cross-wind is applying, but in the opposite direction.
The usual preliminaries for any take-off, set out on page 66, must be carried out. Then find some point well ahead, to enable you to keep the aircraft on the course desired. Open the throttle fully, and press the stick well forward, to raise the tail well above the level flying position. This forces the wheels down on to the ground. If on a runway, keep the aircraft straight, and when the speed has risen slightly above flying speed, ease the stick back and take the aircraft off the ground.
If you are on grass, use the rudder to make the aircraft follow a slightly curved track towards the down-wind direction, and when the speed has reached the necessary figure, complete the take-off as before.
Landing. The aim in making a cross-wind landing is to bring the aircraft in so that its track over the ground coincides with the desired approach and the landing direction, and then, just before the touchdown, to swing the nose so that it points along the line of the track, that is, in the direction in which the aircraft is actually moving. Consequently, proceed as follows.
Carry out the first part of your approach in the normal fashion, but after you have turned on to the last leg, which will be to some degree across wind, head the aircraft slightly into wind of the apparent downward path, during its last part. Counteract the drift by controlling the nose so that the track of the aircraft over the ground corresponds to the line of your chosen landing path. Flatten out and hold off as usual, and then, just before the touch-down, keeping the aircraft level with the ailerons, swing the nose a little down-wind until it is pointing in the direction in which the aircraft is actually moving. Touch down, and keep the aircraft travelling straight along the ground, by use of the rudder, as usual.
21 - A E R O B A T I C S
Introduction. By aerobatics, we mean any manceuvre other than normal flying, turns, sideslips, spins, dives and landings. You are taught aerobatics not so much because the actual mnoeuvres may be of value in operational flying, as because they teach you how to control your aircraft in unaccustomed attitudes. Should you find yourself in any unforeseen position, your training in aerobatics will help you to
Take appropriate action. Again, you will find that your confidence in yourself and in your aircraft grows as you attain skill in aerobatics ; this confidence has a real foundation, for the successful execution of aerobatic manoeuvres calls for accurate flying of a high standard.
There are certain general points which have to be learnt. First, all aerobatics must be executed so that they are completed and normal flight resumed, at a height of not less than 3,000 feet. This means that you should climb to 5,000 feet before beginning an aerobatic manoeuvre.
Secondly, it is essential that the Sutton harness be tightly fastened ; this is particularly important in manceuvres involving inverted flight. If you ever make the mistake of embarking on a slow roll, for example, with the harness on the loose side, you will realize why. In this manoeuvre your weight is taken at one stage by the harness, and if it is loose, you leave your seat and feel as if you are dangling in the air, suspended by your shoulders. This not only feels unpleasant; it may be dangerous. If the harness is loose enough to allow the parachute to come away from the hollow seat, it may not bed back into it again when you are once more flying the right way up. In this case, it may foul the stick, and you may find yourself in a dive which you can't easily stop.
Always, therefore, before setting off to do aerobatics, do up the harness very tightly, so that there is no slackness after you have pressed yourself up against the straps. Make sure, however, that you have sufficient freedom of movement to be able to push the stick and rudder bar to their full extent in either direction.
The third point is. that you must take particular care to ensure that there are no other aircraft in the vicinity before beginning aerobatics. Since most aerobatics entail some loss of height, and since you are often in a position where you can't keep an all-round lookout, this is most important, and you must look well below and above you, as well as round about you.
Flying Upside Down Some aerobatics involve inverted flight. By inverted flight is meant the condition when the aircraft is flying upside down in relation to the earth, and when the direction of lift is not upwards, but downwards, in relation to the normal upper side of the wing.
You are not engaged in inverted flight, merely because your head is pointing down towards the earth ; it is the direction of lift which determines the question. Thus, at the top of a loop, the lift is still operating as usual to provide part of the centripetal force required for the continued change in the aircraft's direction. In a slow roll, however, you are really flying inverted, since the direction of the lift is skywards although the wing is now upside down, as shown in the diagram overleaf.
In inverted flight you are kept in the aircraft by the straps of the harness, and if you have left any objects, such as maps or gloves, loose in the cockpit, they will fall out, since they are naturally subject to gravity. On the other hand, at the top of a loop, although you are flying upside down, you remain pressed on your seat, and anything loose in the cockpit will not fall out. You have applied centripetal force to make the aircraft change its direction throughout the loop, while you yourself and any loose articles have tried to continue throughout in a straight line, which means that you are forced on to the seat and they are forced against the floor of the cockpit.
In training aircraft, inverted flight is only possible while gliding, since the carburettor and engine are not specially adapted for flying inverted.
The Loop. In the execution of a loop, the aircraft makes a complete revolution in the pitching plane, like this:
Throughout the loop, centripetal force must be supplied by the lift from the wings. Since the aircraft is in the normal position in the rolling plane, it is the ordinary vertical component of lift which provides the necessary force. You will see from the diagram that the lift at the top of the loop is directed down towards the earth.. At this point, the lift combines with gravity to provide the centripetal force.
Notice that a loop is not a perfect circle, but more egg-shaped. If the path were circular, the load and strain imposed on the aircraft at the beginning of the loop would be very high. The most important thing to remember in learning the loop is that the change in direction from level flying must be smooth, and not too sudden. The whole maeouvre is carried out as follows.
From level flight, look all round and above, then put the aircraft into a moderate dive until the speed has increased (your instructor will tell you the figure appropriate to the particular type). Then begin, very gently, to ease the stick back ; as the nose rises above the horizon, open the throttle fully, and keep on pressing the stick steadily back, and, when the opposite horizon comes in view, close the throttle smoothly ; as it dives down the last part of the loop, gradually bring the aircraft from its dive to level flight once more. Throughout the loop keep the aircraft flying straight by use of the rudder as necessary.
The Stall Turn. The stall turn provides a rapid method of turning through 180° ; it is effected by increasing the rate of climb of the aircraft smoothly but rapidly until it is at the point of the stall ; then, when the rudder is applied, the aircraft cartwheels over and goes down in a dive towards the direction from which it was formerly coming. It is performed thus. Look all round, and then, from level flight, at normal cruising speed, ease the stick back gently but steadily, until the nose of the aircraft is pointing almost vertically upwards. As the point of the stall is approached, apply full rudder in the direction. in which you wish to turn, and close the throttle. The aircraft then falls into a vertical dive: centralize the rudder, and ease gently out of the dive.
The Slow Roll. In a slow roll, the aircraft carries out a complete revolution in the rolling plane, during which a period of inverted flight is involved. You will find the manoeuvre much easier to learn if you think out in advance the forces acting on the aircraft at the various stages of the roll. In describing the control movements, we therefore add, in brackets, the purpose of each movement.
First, make sure that the Sutton harness is really tight, then look all round, and finally select a point well ahead on to which to fly. Open the throttle and dive gently in order to increase the speed (your instructor will tell you the appropriate figure). Ease the stick back gently until the nose of the aircraft is just above the horizon, and then press it right over to the side to which it is intended to roll (to provide the movement in the rolling plane). At the same time apply a little rudder in the same direction (to counter aileron drag). As the bank becomes steep apply top rudder (to hold the nose up ; as the aircraft slips in, due to steep bank, the pressure of the air on the keel surface makes the nose fall). As the aircraft passes the vertical and begins to roll on to its back, ease the stick gently forward, while keeping it pressed hard over against the side of the cockpit (to keep the nose up on the horizon). As the aircraft gets over on to its back, close the throttle (the engine will not run in inverted flight), and ease the stick still further forward (to keep the. nose on the horizon). At the same time keep th e aircraft straight with the rudder ; opposite rudder will probably be required (to counter aileron drag, which is now transferred to the opposite wing, since the aircraft is on its back).
As the aircraft rolls from off its back, ease the stick gently back (it is now not so much concerned with holding up the nose), and apply top rudder (to hold the nose up on the horizon ; since aileron drag has also to be counteracted, more top rudder will be required than was the case when the aircraft was beginning the roll). When the vertical position is passed, open the throttle gradually and ease the stick still further back (forward pressure now has no effect on holding up the nose on the horizon), and reduce the pressure on the top rudder (its effect in holding up the nose is being lessened as the aircraft approaches the normal attitude).
As the level position is reached, centralize all the controls and resume level flight.
Throughout a slow roll, keep the stick hard over to side towards which you are rolling ; use the elevators and rudder to keep the nose on the horizon, that is, to keep the fuselage parallel to the earth, throughout the manoeuvre. If you think of it in this way, the rather complicated description of the control movements set out above becomes clear, and the value of the manoeuvre in teaching control over the aircraft is plain.
If you experience difficulty in keeping the nose on the horizon, when flying inverted, it is a good plan, before you begin the roll, to ease the tail trimmer well forward, so that the aircraft is 'nose heavy.' This means that when she is upside down, the nose will tend to rise above the horizon.
The Half Roll. A half roll is begun in the same way as the slow roll, though it may be commenced with rather less speed. When the aircraft is in the inverted position, with the throttle closed, centralize the stick and the rudder. Then press the stick gently backwards, so that the aircraft completes the second half of a loop, finally resuming level flight as in the case of a loop.
This manoeuvre means that you change your direction of flight by 180° and resume level flight at a lower altitude, because of the height lost in completing the second part of a loop. See the diagram opposite.
The Half Roll off the Top of a Loop. This manoeuvre is the reverse of the half roll. The half roll you begin as a roll and finish as a loop; in this case you begin a loop, and finish as a slow roll. It means that you change your direction by 180° but maintain or gain height in the process.
Your speed at the top of a loop is bound to be low ; it is unlikely to be much above the normal 'stalling speed' and may be below it. The reason you are not stalled is that the wings have not passed the critical angle of attack ; the load factor is less than one, since the lift from the wings is combining with gravity to provide the centripetal force necessary
This means that you must avoid being involved in true inverted flight when you are rolling off. Therefore start the loop at a higher speed than normally, and begin the roll well before you reach the top of the loop.
First make sure that the Sutton harness is tight and that there are no other aircraft in the vicinity. Then begin a loop, but at a somewhat higher initial speed than normally, say 20 per cent. higher. This means either a steeper dive to begin with, or opening the throttle a little more during the dive. As the aircraft loops, look upwards, and as she approaches the top of the loop, when you catch your first view of the opposite horizon, start the roll.
Do this by pressing the stick to one side of the cockpit, keeping straight by use of the rudder (pressure will be needed in the same direction as that in which you apply bank; when vertically banked you sometimes need a lot of top rudder to keep the nose up) and elevators; as the aircraft rolls off, apply forward pressure progressively to the stick, and complete the second half of a slow roll in the normal manner.
The Barrel Roll. in a barrel roll the aircraft carries out a revolution in the rolling plane, but, unlike the case of the slow roll, it is not involved in true inverted flight. The flight path is a corkscrew one, with the aircraft flying along the edge of a screw of very coarse pitch. The lift from the wings is directed inwards towards a line through the centre of the corkscrew, and the pilot remains firmly. pressed on his seat throughout the roll.
Having made sure that the Sutton Harness is tight, and that there are no other aircraft in the neighbourhood, begin the barrel roll like this. Select a point on the horizon, and fly level towards it ; then, for a roll to the left, dive to the right of your chosen point until the approp airspeed is attained. Your instructor will tell you what this should be ; it will be more than that required for a slow roll. When you have attained the correct speed, apply backward pressure on the stick, and as soon as the nose rises well above the horizon to the right of your selected point, apply left bank and a little left rudder. Keep the stick well over to the left and maintain a steady backward pressure on it. Keep both pressures on the stick, and that on the rudder bar applied until the aircraft is once more the right way up and the wings are nearly level. Then centralize the controls and resume level flight.
Inverted f´Flying. When thinking about this, don't try mentally to , invert yourself ; imagine the chair you are sitting in is the aircraft cockpit, and the ceiling of the room is the earth.
General Principles. On training aircraft, it is possible only to glide inverted, since the engine is not specially equipped to operate in the upside down position. Furthermore, since the wings are much less efficient at providing lift in an upward direction (related to the earth), when they are upside down, the glide path has to be steeper than normal. Accordingly, gliding inverted means a rapid loss of height. Remember this fact, and also that all aerobatics must be completed and normal flight resumed by 3,000 feet.
The Effect of the Controls. You will probably find it helpful to understand this paragraph if you make a simple paper model and bend tips of wings and tailplane to represent control-surface movements. The primary effects of the controls on the aircraft are just the same when it is flying inverted as in normal conditions. But the secondary effects show themselves in the reverse direction. Thus, if while flying inverted, you press the stick to the left, the left wing will go down (down to your eyes, but up in relation to the earth), but aileron drag will tend to make the aircraft yaw to your left, instead of to the right as normally. Similarly, the aircraft will tend to slip up to your right instead of down to the left, so that the movement in yaw resulting from the pressure of the air on the keel surfaces, will be to your right instead of to the left as normally.
Again, if, while flying inverted, you apply left rudder, the aircraft will turn towards your left (which is to the right, so far as the aircraft's direction of movement to an outside observer is concerned). As usual, the outer wing gets more lift than the inner wing because of its gr