Pilot Elementary Training: RAF Flying Cadets' Manual - Part I
First Principles:
1 - THE AIRCRAFT AND ITS COMPONENT PARTS
AN AIRCRAFT consists of the following principal components:
The Fuselage. The fuselage is that part of the aircraft stretching from nose to tail, which contains the cockpit or cabin in which the pilot and passengers take their place, and in which the controls are mounted. In the case of single-engined aircraft, the engine is also mounted in the fuselage, normally in the nose. The fuselage consists of a framework of wood or metal, covered with fabric or metal sheeting. The sides of the fuselage are known as the keel surfaces.
The Wings. The wings also consist of a framework of wood or metal covered with fabric or metal sheeting. They are built horizontally into the fuselage towards its forward end, that point joining the fuselage being known as the root, and the extreme end, as the tip. In the case of a biplane, a second wing is mounted above the first and joined to it by struts.
The forward edge of the wing is called its leading edge, and the rear edge its trailing edge. Forming part of the wings are the ailerons and, in the case of most aircraft, the flaps. Both ailerons and flaps are built into the trailing edges, and are hinged at their own forward edge, so that they can be lowered or raised by the pilot. The ailerons, one in each wing, can be moved both above and below the wing surface; when one is raised the other is automatically lowered. The flaps move only below the wing surface, and both move at once in the same direction.
The Tail Unit. The tail unit consists of the tail planes, the elevators, the fin and the rudder. These are of similar framework and sheeting construction to that of the fuselage and wings.
The tail planes are set horizontally in the fuselage in the same manner as the wings; the fin is set vertically in the fuselage (or, in the case of twin fins, in the tail planes).
Attached to, and forming part of the tail planes, are the elevators. These are hinged to them in the same way as the ailerons are hinged to the wings. Attached to the fin is the rudder ; this is hinged to it, so that it can be turned to right or left,
The Controls. The principal controls are the control column or stick, the rudder bar and the throttle ; other subsidiary controls will be referred to later. The stick is so placed that it comes between the pilot's knees. Movement of the stick away from and towards the pilot, controls the elevators ; movement of the stick laterally controls the ailerons.
The rudder bar is placed so that the pilot's feet rest upon it ; pressure with the left foot pulls the rudder round to the left and pressure with the right foot pulls it to the right.
The Throttle. The throttle controls the power output of the engine, which is delivered to the propeller. If the pilot eases the throttle lever away from him, the engine gives increased revolutions per minute, thus more power ; if the pilot eases the throttle lever towards him, the engine gives less power.
The Undercarriage and the Tail Skid or Wheel. The undercarriage consists of a system of supports mounted below the forward part of the fuselage or below the wings. It carries the wheels on which the aircraft moves on the ground. In most aircraft the pilot can retract the undercarriage into the wings or fuselage, so that it presents less resistance to the air. The tail skid or wheel is mounted at the extreme rear of the fuselage; it supports the tail part of the aircraft when it is on the ground.
The Engine and Propeller. Aircraft may be fitted with one or more engines, each with its propeller. In the case of a single-engined aircraft, the engine is normally mounted in the nose. In the case of multi-engined aircraft, the engines are usually mounted in the wings. The engines are of the internal-combustion type, generally using petrol as fuel.
The Instruments. The aircraft is fitted with a number of instruments, which provide the pilot with information about his speed, height, direction, the state of the engine, etc. The elementary pupil need only concern himself with certain of these instruments, the most important being:
THE AIRSPEED INDICATOR. This is calibrated in miles per hour, or in knots ; it does not record the speed of the aircraft over the ground, but the pressure of the air which the aircraft is encountering, which is equivalent to the speed of the airflow past the aircraft.
THE ALTIMETER. This records the height of the aircraft, either above sea level, or above the airfield from which the aircraft took off, according to the way in which the pilot set the instrument. It is calibrated in 100's of feet.
THE COMPASS. This shows the direction in which the aircraft is flying.
THE ENGINE-REVOLUTION COUNTER. This records, in revolutions per minute the speed at which the engine crankshaft rotates.
THE OIL-PRESSURE GAUGE. This records the pressure, in pounds per square inch, of the oil in the circulation system of the engine.
THE TURN-AND-SIDESLIP INDICATOR. There are two needles on the face of this instrument. The lower one records the rate at which the aircraft turns. The scale is marked to show Rate-1, 2, 3 and 4 turns. A Rate-1 turn means that the aircraft is flying on the circumference of a circle at a speed of 3° per second. Thus a Rate-1 turn of 360° takes 2 minutes.
The upper needle records 'side slip' or 'skid'. These terms will be explained at a later stage in this manual.
THE ARTIFICIAL OR GYRO HORIZON. This instrument tells the pilot what the attitude of his aircraft is in relation to the horizon. Naturally its use is only necessary when the real horizon cannot be seen.
THE DIRECTION INDICATOR. The direction indicator is a device to permit the pilot to steer any given course. It is set by the pilot to the course required, from the reading given by the compass, and is easier to watch and to follow than the compass itself.
THE RATE-OF-CLIMB INDICATOR. This instrument shows the rate, in 100's of feet per minute, at which the aircraft is gaining or losing height,. when it is climbing or descending.
2 - WHAT KEEPS THE AIRCRAFT IN THE AIR: THE FORCES ACTING ON IT
Introduction. The aircraft is able to rise into the air, and to keep in the air, because of the force exerted by the air, when it is moving relative to the aircraft. The relative movement comes from the movement of the aircraft through the air, and this is derived from (a) the power provided by the engine, (b) the pull of gravity on the aircraft.
It is essential that you should understand how the force exerted by the air supports the aircraft, since in flying you are always utilizing this force, by means of the controls, to make the aircraft behave in the way you wish. We shall therefore have to discuss some very elementary aerodynamics.
Aerofoils. The wings, tail planes, elevators, fin and rudder are not flat sheets ; they are built up frameworks of wood or metal covered with a skin of fabric or metal sheeting. This method of construction has definite mechanical advantages and, furthermore, it allows the use of shaped surfaces which have valuable aerodynamic qualities. All these units, called aero foils, are made to follow the streamline principle, which ensures that their movement through the air involves the least waste of engine power in overcoming unwanted resistance.
The tail planes, of which the elevators form a part, and the fin and rudder are usually built to a symmetrical section like this, cambered on both sides:
The wings are of a different shape. In section, a typical wing is like this:
Lift and Drag. If an aerofoil is placed in a moving stream of air at an angle to the airflow, the force of the air pushes it upwards and backwards, and we can show the result in a diagram like this:
Notice that we said that the air pushes the aerofoil upwards and backwards. This is literally true ; the air has these two effects, which though they are seen combined, can be considered separately, like this:
The upward effect we call Lift and the backward effect Drag. The direction of the lift is always considered as at right angles to the airflow, and that of the drag, parallel to it. The combined effect of lift and drag is called Total Reaction.
The effect of the force exerted by the air is not resolved equally into lift and drag. The amount of lift and the amount of drag vary according to the angle at which the airflow meets the aerofoil. This angle is called the angle of attack. If the aerofoil is at a large angle to the air flow, a considerable part of the force of the air is felt as drag ; if we use the length of the lines to represent the proportion of lift and drag, we get a picture. like this:
Notice that the total reaction is pointing much more backward than in the earlier diagram. If the aerofoil is at a smaller angle to the airflow, we get a very different picture:
Lift/Drag Ratio. We have been talking of the amount of lift and drag, relative to each other: their relationship is known as the lift/drag ratio; and to be specific the ratio is highest in a training aircraft when the angle of attack is about plus 4½° This does not mean that the lift is greatest at an angle of attack of 4 ½° because, in fact, the lift goes on increasing well beyond this angle of attack as you will see from the lift curve shown below; but from the drag curve you will see that the drag increases rapidly with the angle of attack. When, therefore, we combine the lift and drag curves into one lift/drag ratio curve, we find the best lift/drag ratio at about 4½°.
Variation of Lift and Drag with Angle of Attack. If we plot lift and drag, against the angle of attack, for any aerofoil we get graphs which, though varying from one aerofoil to another, are of the general shape shown below:
Note particularly the difference in the shapes of the lift and the drag curves. In the case of lift, the curve is almost a straight line for the first part of it, up to about 10° of angle of attack. This means that an increase in angle of attack of, say, 1° has much the same effect on the lift if the increase is made from 3° to 4°, or from 7° to 8°. But the drag curve is very different in shape. An increase in angle of attack from 7° to 8° has a much larger effect on the drag than an increase from 3° to 4°. You will realize the importance of this fact in flying, at a later stage.
Similarly the graph of lift/drag ratio looks like this :
Try to memorize these curves of lift, drag and lift/drag ratio for a typical aerofoil, since they tell you much of what you want to know about lift and drag and will help you to understand many problems of flying.
You may be somewhat puzzled to see that there is some lift at a negative angle of attack, with an aerofoil. This is because, the aerofoil being curved, we have to take an arbitrary straight line through it to give us our angle with the air flow. When this line is parallel to or at a slight negative angle to the airflow, some part of the aerofoil will, in fact, be at a positive angle to it, and thus give some lift.
Angle of Maximum Lift- You will notice from the lift curve that the lift increases as the angle of attack increases up to a certain point, which, in the case of the aerofoil to which this particular curve refers, is about 15°. Afte. this point is reached the lift falls off abruptly with further increases in the angle of attack. This is a very important matter in practical flying, as you will learn later. The angle of attack at which the lift is greatest is known as the angle of maximum lift, or the critical angle.
How the Angle of Attack Alters. The angle of attack between the airflow and the aerofoil can be altered in three ways. First, if we imagine the aerofoil moving forward on a level path through the air, the angle of attack can be changed by tilting it, like this:
Secondly, if the aerofoil, while continuing to move forward, also moves downward, the angle of attack is changed, even though the original angle to the horizontal remains unaltered. Remember, it is this angle with the airflow that matters. These two diagrams show the situation:
You will see that the angle of attack has been increased. Thirdly, if the aerofoil, while continuing to move forward, also moves upwards, the angle of attack to the airflow is changed, this time reduced:
Lift and Drag Formulae. So far, we have been assuming that the airflow remained at a constant speed, and we have made no mention of the density of the air. If, however, the speed of the airflow is increased, while the angle of attack remains constant, the amount of both lift and drag is increased. If the density of the air is reduced, the amount of lift and of drag is reduced. If we wish to calculate the lift and drag for any aerofoil in given conditions, we have to take account of one more factor, its shape, particularly the amount of camber of its upper surface.
The whole position can be written in two simple formulae which, if you memorize them, will help you to keep the various factors clearly in mind. These are the formulae :
In the formulae:
CL is the lift coefficient Which takes account of the shape of the aerofoil and any given angle of attack to the airflow.
CD is the drag coefficient which takes account of the shape of the aerofoil and any given angle of attack to the airflow.
r, a Greek letter (pronounced 'roe'), is a symbol representing the density of the air.
V is the velocity of the airflow : you will notice that the lift and drag vary according to the square of the velocity.
S is the surface or area of the aerofoil.
Now lift is that which counteracts the weight of the aircraft and permits it to keep in the air. Drag is a disadvantage to us, but it is inseparable from lift; it is in effect the price we have to pay for lift. Drag acts as a brake on the progress of the aircraft through the air, and the designers seek to build aircraft so that the drag is kept as low as possible. Although in this section we have been speaking only of the drag of an aerofoil i.e. of a wing, drag is caused by the passage of the other part of the aircraft through the air, by the fuselage, tail unitt, undercarriage, etc. This type of drag is known as 'parasite drag', since it is all hindrance and is not associated with any appreciable lift. The largest part of the total drag, however, comes from the wings. Consequently, since a high speed in level flight is usually a main requirement, the wings are usually set in the fuselage so that, in level flight, they present an angle of attack to the airflow that gives as little drag as is possible, taking account of the minimum lift necessary to support the weight of an aircraft. This means that the angle of attack in level flight is rather less than that which would give the best lift/drag ratio, though the latter gives what we may call the most economical angle.
In the case of the wing, whose characteristics are illustrated in the graphs on paragraps before the angle of attack which gives the best lift/ drag ratio is + 4 ½°, while the lowest drag is found between - 2° and + 2°. With such a wing in a normal general-purpose aircraft, the wings would be set in the fuselage so that in level flight they made an angle of attack with the airflow of some 2° to 2½°. At this angle the lift at cruising speed would equal the weight of the aircraft and the drag would be nearly the lowest obtainable.
Let us now consider the position when we wish to carry out a manoeuvre, such as a turn, which calls for an increase in the lift. How are we to obtain the necessary increase ? Look at the lift formula
We cannot alter the area of our wings, obviously, nor can we change the density of the air which surrounds us. This leaves only the factors CL and V which we can deal with. CL includes both shape of wing and angle of attack ; since we cannot alter the shape, we are left with angle of attack and airspeed. We may either increase the angle of attack, but not beyond the angle of maximum lift, or we can increase the speed.
Thrust and Weight. We have already referred to the fact that the movement of the aircraft through the air is derived from the power provided by the engine and from the pull of gravity on the aircraft. The engine drives the propeller which pushes the air behind it thus propelling the aircraft forward. This forward force exerted by the propeller is called thrust. The force of the pull of gravity on the aircraft depends on the weight of the aircraft, and we use the word weight to describe this force.
The Four Forces. We have thus four forces working on the aircraft, lift, drag, thrust and weight. If, of course, we have the engine throttle closed so that the engine is only just ticking over, the thrust it develops is negligible and can be ignored for practical purposes.
These four forces do not always act in the same direction in relation to each other. But each is regarded as acting in a constant direction to some factor in the total situation. These constant directions are
Lift always acts at right angles to the airflow;
Drag always acts parallel to the airflow ;
Thrust always acts along a line which we may regard as running lengthwise through the fuselage from tail to nose ;
Weight always acts vertically downward from the aircraft to the earth.
Diagrammatic Representation of the Four Forces. When we represent these forces in diagrams, we do so by drawing lines which point in the direction in which the forces are working, and we represent the strength or value of the forces by the lengths of the lines (this is what was done in the diagrams in the previous section).
For instance, in the case of weight, such a line represents the total weight of the aircraft, and though in fact the weight is distributed over the structure and acts vertically downwards all along it, we add all the weight together and show it as one straight line with the force acting from the centre of gravity, as in the diagram above.
Centres of Gravity, Pressure, Thrust and Drag. If you are not familiar with the term 'centre of gravity', perhaps we can explain it as the point at which, if you placed a pivot, the aircraft would be perfectly balanced. If you cut a model aircraft out of cardboard and stick a pin through it at any point except the centre of gravity, the model will not remain level, but will tilt when you hold it up by the pin. If you get your pin through the centre of gravity your model will. be perfectly balanced on the pin. Similarly a line representing lift in a diagram shows the sum of all the lift coming from the aerofoil surfaces of the aircraft. Lift is obtained from all over a wing, from every square inch of it, though not equally from every square inch. We can put it diagrammatically like this:
In effect, we add together all the lift, and show it as one straight line rising from one point on the wing which we call the centre of pressure. The centre of pressure, CP, has the same relation to lift as the centre of gravity has to weight.
Similarly, we represent thrust which is shown as acting from the centre of thrust, and drag which is shown as acting from the centre of drag. Ideally, the lines of lift, drag, thrust and weight should meet at the same point so that the centres of pressure, drag, thrust and gravity are the same. In fact, in a real aircraft they do not exactly coincide, though the designer seeks to bring them as close together as possible. However, for the purposes of our diagrams we show the lines of lift, drag, thrust and weight all starting from the centre of gravity, since we can adequately show the forces in this way.
A diagram of an aircraft flying straight and level, at a steady cruising speed shows the following disposition of forces :
You can see that the lines representing lift and weight are the same length and point in exactly opposite directions. The lines representing thrust and drag are also equal in length and point in opposite directions. This is so because if the aircraft were flying level, lift must equal weight; if lift were more, the aircraft would start to climb, and if it were less, the aircraft would lose height. Similarly, if the aircraft is flying at a steady speed, the thrust must equal the drag. If the thrust were greater than the drag, the aircraft's speed would steadily increase; if the thrust were less than the drag, the speed would decrease.
Equilibrium. An aircraft flying straight and level, as shown in the diagram, is said to be in a state of equilibrium. Whenever an aircraft is flying in such a way that the four forces (or three if the throttle is closed) are operating so that they balance one another, either one against one, or two together against the other two together, or two together against the remaining one, then the aircraft is in equilibrium.
The combined effect of any two forces is called the resultant : to arrive at the resultant, which is also represented in our diagrams by a straight line, is very simple. It is done by regarding our two lines representing forces as two sides of a parallelogram, adding the other two sides, and drawing the diagonal across it. The diagonal gives the strength of the resultant force and the direction in which it acts.
Notice that in the first diagram on the next paragraph the length of OD is not the arithmetical sum of the lengths OL and OD, but is less. It represents the total force in a direction resulting from considering the combined effect of the two forces.
You will recognize that these are vector diagrams about which you have learned, or will learn, in other parts of your training course.
When an aircraft is gliding with the throttle closed, the forces are like this:
In this case, the resultant of lift and drag (or total reaction) is equal to the weight, consequently although the aircraft is losing height, it is in equilibrium and is moving in its forward direction at a constant speed.
Disturbance of Equilibrium. If an aircraft flying in a state of equilibrium has its airspeed increased or its direction of flight changed, it ceases for the moment to be in equilibrium. Now an aircraft is built so that, when disturbed from a state of equilibrium, it readjusts itself in such a way that it reaches a new state of equilibrium in which the forces, although perhaps regrouped, once more balance each other. The way it does this is by changing its attitude to the airflow, that is, by changing the angle of attack of the wings to the airflow. As you remember from previous discussion, a change in the angle of attack means a change in the amount of lift and drag, but not the same relative change in each case. A change in drag affects the airspeed, since drag acts as a brake.
Consequently, when an aircraft seeks to return to a state of. equilibrium by changing the angle of attack to the airflow, it alters the lift, the drag, and the airspeed. It cannot, of course, alter the weight.
Now it does not necessarily follow that the aircraft's own ideas on the state of equilibrium it will settle into after some disturbance, will coincide with ours. In such a case we use the controls to regulate one force, say lift, and so compel the aircraft to adopt an attitude which gives a state of equilibrium, which in turn will allow us to follow the path we want at the speed we want. For example, suppose we are flying straight and level at 80 miles per hour, and we wish to keep straight and level but to fly at 100 miles per hour. In order to gain speed, we open the throttle and increase the thrust. What is the result if we do nothing to alter the angle of attack ?
The increase in thrust gives an initial increase in airspeed, which means more lift and also more drag. But we are not in equilibrium, and the aircraft seeks to return to equilibrium. The way in which it succeeds depends on its design. In some aircraft the nose will go down, in some it will remain in the same attitude in relation to the horizon; but in the majority it will go up. We will take as an example, this last case. Work out for yourself what happens in the other two cases.
If the nose goes up, it means that the lift is increased, not only on account of the initial increase in speed, but because the angle of attack is also increased; in consequence our flight path changes, and we begin to climb. The nose rises until the angle of attack to the new direction of the airflow reaches the point when the drag is so increased that the airspeed is considerably reduced below the original 80 miles per hour. (You will remember that the drag increases very rapidly with an increase in the angle of attack.) Although the lift is also increased at this greater angle of attack, this increase is more than cancelled out by the reduction in speed caused by the increased drag. This drag is also reduced with the reduced speed, but its increase due to the greater angle of attack is not cancelled out by the reduction in speed. Even if you are not mathematically minded you should be able to see this quite clearly from the curves for lift and drag on page 13, read in conjunction with the formulae
The final result, then, is that our direction of flight is changed (we are climbing), our speed is reduced, the lift is reduced and the drag is increased, which is just the reverse of what we wanted to happen. Three stages in the situation are illustrated in the following diagram:
You will now think 'There must be something wrong here. You say we are climbing, but that our lift is less than when we were flying level. At first sight it certainly seems all wrong, but the resolution of forces shown in the right-hand diagram makes it clear. What has happened is that the thrust is now carrying us upwards, with the assistance of the lift, while drag is combining with weight to hold us downwards and backwards. But we could not have begun to climb had the lift not been actually increased by the increase in speed, following on the increase in thrust provided by opening the throttle, combined with the increase in the angle of attack due to the rise of the nose. In other words, if we want to climb, we must always first increase the lift to change our direction. In fact, too, the reduction in lift at the angle of climb employed, is very small, and can be ignored for practical purposes.
However, in the particular case we began by considering, we did not want to climb; we wanted to fly straight and level at a higher speed. What do we have to do to achieve our aim ?
It will be easier to understand if we look at the state of equilibrium we finally reach when we are flying straight and level at 100 miles per hour ; the forces are disposed as in the next diagram (N.B. Our flight path is level in relation to the earth, though our nose is pointing downwards slightly towards the earth) ;
The thrust is now acting slightly downwards, and, combined with the weight, is equalled by the combination of lift and drag. The weight remains constant, the drag is a little less than the thrust, and the lift is a little more than the weight. Once more, the increase in lift is only small.
We arrive
at this position. by reducing the angle of attack as we open the
throttle, to prevent the increase in airspeed from increasing the
lift, and by going on reducing the angle of attack as the airspeed
increases, until the airspeed ceases to rise. The speed ceases to
rise because as it increases, the drag also increases (
).
Furthermore, since when we began the manceuvre our wings were meeting the airflow at the angle of attack which gives a very low drag, say 2°-2½° (see page 16), the fact that we are decreasing the angle beyond this point means that we are on the point of increasing the drag. This is shown by the drag curve on page 13.
Furthermore, drag on all the parts of the aircraft, not only the wings, increases as the speed increases. We can sum up the position by saying that if we wish to increase the speed in level flight we must increase the thrust, and steadily reduce the angle of attack of the wings to the airflow, so that no initial increase in lift is obtained, but that as the thrust points downwards the lift is very slightly increased to counteract the downward force of the thrust.
We cannot go on raising our speed in level flight indefinitely since, with the throttle full open, the speed at which the drag and lift equal the thrust and weight - must be reached, and this is the aircraft's maximum in level flight.
Terminal Velocity. Even if an aircraft were to be dived vertically with the throttle full open, it would not go on gaining speed indefinitely. Finally the drag and lift would equal the thrust and weight combined, and no further increase would occur. This final speed is called the terminal velocity.
The Stalling of an Aerofoil. On page 14 we referred to the fact that if the angle of attack of an aerofoil to the airflow is increased, the lift is increased, up to the angle of maximum lift, and that thereafter, the lift rapidly decreases. You can see this plainly from the lift curve (page 13) where the angle of maximum lift is clearly 15°. If the angle of attack is increased beyond this point, the aerofoil is said to be 'stalled'. In order to describe the condition called stalling, we shall have to consider what happens to the airflow when it encounters an aerofoil set at an angle to it. It can be shown diagrammatically like this:
The airflow is split by the aerofoil, some of the air passing over the top surface, and becoming deflected downwards, and some passing under the aerofoil, and finally turning slightly upwards as it joins the air coming from the fop side of the aerofoil.
What happens is that the air passing under the aerofoil increases in pressure and exerts an upward thrust on it; while the pressure of the air passing over the top of the aerofoil is reduced, and this exerts an upward sucking effect on the aerofoil. Thus both the air passing under and the air passing over the aerofoil has a lifting effect. In fact the amount of lift provided by the upward sucking effect is much greater than that provided by the upward thrust effect: diagrammatically it is like this :
Now this upward sucking effect is only maintained so long as the airflow over the top of the aerofoil remains smooth, as it was shown in the first diagram on this page.
If the airflow becomes disturbed or 'turbulent', the upward sucking effect is very greatly reduced ; the greater the turbulence the less the lift. The turbulent airflow can be shown diagrammatically like this
The air is actually moving up the top.side of the aerofoil from the back to the front. It is when the air over the aerofoil is in this turbulent state that the aerofoil is said to be stalled. Stalling occurs when the angle of attack of the aerofoil to the airflow is increased beyond a certain point (which depends on the design of the particular aerofoil). The result of the stalling is, of course, that the lift is greatly reduced. The stalling angle, as you can see from the lift curve in the diagram on page 13 is that immediately following the angle of maximum lift. Stalling always occurs when the angle of attack is increased beyond the point of maximum lift, and it cannot occur unless the angle of attack is so increased. As you will also see from the lift curve, once the angle of maximum lift is passed, the lift goes on decreasing. This reflects the fact that there are degrees of stall ; the greater the angle of attack (the stalling point having been passed), the greater the degree of stall.
The Stalling
of an Aircraft. In flying we are concerned with the stalling of
the aircraft ; this depends on the stalling of the aerofoil surfaces
which form part of the aircraft, of which the wings are the most
important. Remembering that stalling is dependent on the angle of
attack, consider the circumstances when the wings are likely to
be brought past the stalling angle. Suppose that the wings of an
aircraft will stall at an angle of attack of 15°, and that the aircraft
is flying level at a cruising speed of 80 miles per hour, the wings
meeting the airflow at an angle of 2½°'. Think what this means ;
it means that the lift -provided by the wings, which depends on
the shape of the wings, the angle of attack, the density of the
air, the velocity of the airflow, and the surface area of the wings
(
)
is just sufficient to counter-balance the weight of the aircraft,
no more and no less. Now suppose we close the throttle and reduce
the thrust of the engine, which is providing the speed of the airflow.
Since we have reduced V in the formula, it follows that the lift
must be reduced. Therefore the aircraft will, unless we do something
to prevent it, begin to lose height, and from flying level, will
begin to fly down at an angle towards the earth, since it is gravity
which is now providing the main source of airflow.
But if we wish to try to continue to fly level, we must retain the lift at the amount at which it stood before the engine was cut off. The only element in the formula which we can now change is the angle of attack; if we increase that, we increase the lift. But if we try to keep the aircraft flying level we must, in the absence of the engine power, lose speed, since we are not allowing gravity to act so as to pull us forward as it does if we are content to lose height. Since we are losing forward speed, the lift is steadily falling off, and we can only maintain it by still further increasing the angle of attack. And, of course, since the drag increases with the lift this also will reduce our forward speed.
The speed steadily falls, and therefore we must steadily increase the angle of attack to keep the lift constant. Finally, the angle of attack reaches the stalling point, and the wings and the aircraft stall. When this occurs, the speed will be at a certain point, for example 45 m.p.h., which is called the stalling speed for that particular aircraft. It is perhaps an unfortunate term, since it suggests that stalling is dependent on speed, but this is not the case. The aircraft will, in fact, stall over a wide range of speeds if the stalling angle is reached at such speeds. The so-called stalling speed is therefore the speed at which the stalling angle is reached when the attempt is made to keep the aircraft flying level without the engine.
Stalling While Changing the Direction of Flight. Now let us consider the case of an aircraft which is diving; its path and therefore the direction of the airflow is like this :
If we try to pull out of the dive, that is to change the direction to fly level, or upward, too quickly, what happens ? A moving mass like an aircraft in a dive, resists the attempt to change its direction; in fact after the attitude of the aircraft to the earth has been changed, so that its direction of movement may be changed, it still continues to move in the original direction as well as in the new desired direction. Consequently the direction of the airflow which it is encountering is not from the new direction towards which it points, nor from the old direction, but from a combination of the two as shown by the arrows in this diagram :* (There is thus a third source of relative air movement, besides that provided by the thrust from the engine and from gravity; it is that derived from the momentum of the aircraft. If we seek to make an aircraft turn, or dive or climb, its momentum tends to carry it forward along its original path for a certain period of time, a fact which is very important in many manoeuvres.)
You can see from the arrows how the angle of attack is therefore increased, and the more suddenly and violently we seek to change the direction of the aircraft, the greater will be the difference between the direction in which the aircraft is pointing and the direction of the airflow, and this increases the angle of attack. Consequently we can very easily stall if we try to pull out of a dive too quickly, even though our speed is very high.
In actual flying conditions, we do not deliberately stall the aircraft except for landing and for training purposes, so that we can learn how to make the aircraft unstalled. But we can, if we fly carelessly, accidentally stall the aircraft by bad flying, on turns, or on pulling out of a dive. If the stall takes place sufficiently high up, there is no danger but, if the aircraft stalls close to the ground, it will probably crash before we can unstall it. This is because since the lift is so greatly reduced when the wings are stalled, we lose a great deal of height very quickly. Further details of the stall and how to recover from it will be found in a later section ; the importance of this discussion is that it explains why the aircraft stalls, it is always because the angle of attack TO THE AIRFLOW passes beyond the angle of maximum lift.
The Relationship Between Angle of Attack and Airspeed. In every kind of manoeuvre in the air, flying straight and level, climbing, gliding, turning, aerobatics, etc., when we are seeking to make the aircraft behave in a certain way, we are concerned with a number of factors, one of which is the angle of attack of the wings to the airflow. This has its own peculiar problem in that the aircraft is not provided with an instrument which records in degrees the angle of attack. Consequently we have to use an indirect indicator, which is provided by the Airspeed Indicator, since on every aircraft there is a relationship between airspeed and angle of attack when the aircraft is not changing direction. When there is a change of direction, the normal relationship breaks down and airspeed provides no indication of angle of attack.
The point is illustrated by considering the conditions in which the angle of attack can pass the stalling angle, which we discussed in the previous section. We saw there how, if we attempt to fly level without the power provided by the engine, seeking to maintain the lift by increasing the angle of attack, the wings will finally stall, and that, when this occurs, the aircraft will be flying at a certain speed. This so-called stalling speed, as shown on the, airspeed indicator, reflects the fact that the wings have reached the stalling angle. But in another sort of stall which we described, which may occur if we try to pull out of a dive too quickly, that is to change our direction too quickly, the wings will pass the stalling angle though our airspeed may be anything up to three or four times the so-called stalling speed.
The Relationship in Level Flight. Let us consider the case of straight and level flight. If we are flying with the throttle half open, at 80 miles per hour, the angle of attack of the wings to the airflow is at, say, 2½°. If we want to fly faster, and open the throttle, we must reduce the angle of attack, as we have seen in a previous section, to fly level at the same time. This we do so that the increase in lift gained from the greater speed is counter-balanced by the reduction in lift due to the reduced angle of attack. For practical purposes we can regard the situation as one where we have to keep the lift constant, and equal to the weight. We could draw a graph which would show the relationship of speed to angle of attack for any aircraft in level flight. It would show that as the speed increased, the angle of attack was reduced, and as the speed is reduced, the angle of attack gets greater.
Climbing and Gliding. We could draw similar graphs for climbing and gliding ; although they would not be exactly alike nor resemble precisely the graph for straight and level flight, they comply with the same principle.
There is no need at this stage to go into the question of why the three curves are not exactly the same, as it would take us much too far into aerodynamics ; in any case, with a simple elementary training aircraft, the difference is not great.
The Use of Airspeed as an Indication of Angle of Attack. We use the relationship of airspeed to angle of attack to permit us to find the attitude of the aircraft which gives the angle of attack necessary (a) to maintain level flight at cruising speed, (b) to climb at the best rate, (c) to glide at the best gliding angle. How we do this is described in later sections. But remember that when you are told to control the aircraft so that it glides, say, at 65 miles per hour, this really means that you are to control it so that the wings make a certain angle of attack to the airflow.
How the Relationship varies with the Load Carried by the Aircraft. You can easily understand that for a particular aircraft the relationship between angle of attack and airspeed is affected by the total weight of the aircraft. This can be seen, since for practical purposes we can regard the lift, when the aircraft is not changing direction nor gliding, as equal to the weight. Imagine a bomber weighing 20,000 lbs. fully laden, flying level on its way to its target, at 200 miles per hour. The lift equals 20,000 lbs., and the angle of attack is, say, 4½°. Now think of the bomber returning after having unloaded 3,000 lbs. of bombs, and having used up 1,000 lbs. of petrol. If it is still flying at 200 miles per hour, the angle of attack must be less than 4½°, since when it was 4½° and the speed was 200 miles per hour, the lift equalled 20,000 lbs. Now the lift is only 16,000 lbs., but the speed is the same ; therefore the angle of attack must be less than the former 4½°.
In your elementary training the question of considerable variations in the weight does not arise, although there will, of course, be a difference between flying dual with full petrol tank, and flying solo with a nearly empty tank.
Load Factor. Load factor is a term which you will frequently hear used, and will encounter in your other reading about flying; consequently we will give a brief note on the subject here.
An aircraft has naturally a certain weight ; it may weigh 1½ tons unladen, or with petrol, armament, pilot, etc., 2½ tons. When flying level all the weight has to be borne in the air by the lift obtained from the surface of the aircraft, principally, of course, from the wings. Consequently, we can relate the total weight of the aircraft to the wing area, and express the weight as a 'wing loading' figure of so many pounds per square foot. This is the normal wing loading when the aircraft is in steady level flight, when the lift per square foot has to be just equal to the weight per square foot to keep the aircraft in the air on a level course.
In certain circumstances the lift per square foot has to be increased, sometimes very considerably, and therefore the load on the wings increases. In other circumstances less lift is necessary and therefore the load on the wings is reduced. These changes in load are expressed as a ratio to the weight of the aircraft, and we talk of load factors of, say, ¾, 1½, 2, 3, etc., etc. By a load factor of 2, we mean that twice as much lift is necessary as when the aircraft is flying level..
Centripetal Force. You may wonder how it comes about that the lift has to be so considerably increased, but it is really quite easy to understand. Think what happens to you when you are in a car going round a corner. Imagine that you are in the right-hand rear seat, and that the car, travelling at a good speed, turns to the left. -You feel yourself pressed tight against the side of the car; in other words, the side of the car is pushing you away from the direction in which you were going and forcing you to go round the corner with the car. You can feel this force and know that it is there. The same force is, of course, acting on the car and also forcing it round the corner. This force is called Centripetal Force, and is that force which is required to keep a heavy body moving in a circle and must therefore act towards the centre of that circular path.
Now any force required to change the direction of an aircraft must be in addition to that force which is always required to hold an aircraft in the air. The only force which the pilot has at his disposal is the lift from the wings, so that this lift has to provide for the constant weight of the aircraft as well as that extra required to provide the necessary centripetal force in a turn. The magnitude of this centripetal force varies both with the rate of turn and the speed of the aircraft. If the rate of turn is doubled by turning at the same airspeed but at half the radius, the centripetal force required to hold it in the turn is doubled. If the rate of turn were kept constant, but the airspeed were doubled by flying at twice the radius, the centripetal force would again be doubled ; while if the speed and rate of turn were both doubled, the centripetal force would become four times as great.
The next diagram shows clearly how, in a turn at 45° of bank, the centripetal force required to hold the aircraft in the turn is equal to the weight, and the lift required from the wings to supply this extra force as well as to lift the weight is some 1 -4 times the weight ; while in a 60° banked turn the lift required becomes double the weight or, as we saw in the preceding paragraph, the load factor becomes 2.
This centripetal force is, of course, present in all manoeuvres entailing a change of direction. In pulling out from a dive, for instance, the rate at which the aircraft is pulled out governs the centripetal force required to force it out of this dive. Very high loads can be applied to an aircraft if it is pulled out rapidly, but the rate at which aircraft can be pulled out of a dive is governed by the maximum lift which can be obtained from the wings to supply the necessary centripetal force.
A car can be forced round a corner only up to the limit of the grip of the tyres on the road. If the road is slippery the car will face round the corner but move nearly straight on. An aircraft, similarly, can be made to change its heading from a steep nose-down dive to a level attitude ; but its path will still be mostly down and the angle of attack will be very high. A reference to the figure on page 27 will show that with the angle of attack well beyond the critical angle, as it would be, the lift coefficient becomes correspondingly low. The centripetal force necessary to force the aircraft out of a dive rapidly would not therefore be available, and the aircraft would continue to sink.
If the aircraft is just going into a dive, or is on the top of a loop, for example, part of the centripetal force required is supplied by gravity so that the lift necessary is reduced, and the load factor becomes less than 1.
3 - THE CONTROLS AND THEIR EFFECTS
Before we go on to consider the effect of the controls, be careful to note this point : we are describing the action of the controls on the aircraft without any reference to the horizon, or any point on the earth. The controls always have the same primary effect on the aircraft, no matter what its position may be in relation to the earth, except when the aircraft is stalled. When you have thoroughly mastered their effects, we can go on to consider how to use the controls to change the attitude of the aircraft in relation to the earth.
Primary Effect of Controls-The Ailerons. As we said in the first chapter, the ailerons are part of the wings, and are hinged along their own forward edge like this.
They can be moved up and down, and the movement is effected by moving the control column-or stick-from side to side. The ailerons in the left and right wings are linked together, so that when one moves down, the other moves up. There are a number of different types of ailerons, giving different degrees of downward movement of the down-going aileron for a certain degree of upward movement of the up-going aileron. There is no need for us to go into the technical reasons for the different designs at present.
Movement of the stick to the left raises the left aileron and depresses the right aileron. Now what is the result ? The left aileron, having been raised, presents a reduced angle of attack to the airflow, and therefore has less lift. The right aileron, having been lowered, presents an increased angle of attack to the airflow, and therefore has more lift. The natural result is that the left wing drops and the right wing rises, and the aircraft rolls to the left round a line drawn through the fuselage from nose to tail. This is called movement in the Rolling Plane. So long as you hold the stick to the left, so long will you continue to roll. If therefore you want to put the aircraft in a different attitude in the rolling plane, you move the stick until the attitude is reached and then centralise it once more. When the aircraft lies at an angle in the rolling plane to the normal position, it is said to be 'banked.'
The Elevators. The elevators form part of the tail plane, being hinged along their own leading edge. They can be raised or depressed by the stick by a movement to or from the pilot. When the stick. is pressed backwards, the elevators rise and they offer resistance to the airstream striking their upper surface. The pressure of the air results in the tail of the aircraft falling in relation to the nose or, as it is usually thought of, in the nose rising and trying to 'chase the tail'. Similarly, when the stick is pressed forward, the elevators are depressed, the tail rises, and the nose goes down. This is known as movement in the Pitching Plane, the aircraft pivoting about its centre of gravity.
The Rudder. The rudder is hinged to the trailing edge of the fin; it can move to either right or left, and is controlled by the rudder bar. When the rudder bar is pushed forward by the right foot, the rudder moves out to the right-hand side, and therefore offers resistance to the airflow. As a result, the tail of the aircraft is pushed round to the left, the aircraft pivoting on the centre of gravity. The nose, therefore, is pushed to the right and moves round in the direction of the right wing tip. When the rudder bar is pushed with the left foot, the reverse happens. This is known as movement in the Yawing Plane. The way to think of the action of the rudder is that it causes the nose to move round in the direction of the wing tip-to the left if the left foot is pushed forward on the rudder bar, and to the right if the right foot is pushed forward.
Remember, no matter what the attitude of the aircraft in relation to the earth, the controls always have the same primary turning effect on the aircraft (except when the aircraft is stalled). The ailerons give control in the rolling plane, the elevators in the pitching plane, and the rudder in the yawing plane.
The Further Effects of the Controls, and Aileron Drag. When the ailerons are used they have a secondary effect beside that of moving the aircraft in the rolling plane. This is due to the resistance they offer to the airflow; that is, to their drag. This drag acts in the same way as the resistance encountered by the rudder when it is moved ; it turns the aircraft in the yawing plane. If you think of it carefully, you will see why. The aileron which is depressed, and has therefore more lift, so that its wing rises, has also more drag, because we can only get increased lift (when we increase the angle of attack) at the expense of more drag. Now drag is resistance to the airflow ; this resistance tends to turn the aircraft in the yawing plane in the opposite direction to that in which the bank is applied: i.e. if the stick is moved to the right, the aircraft yaws to the left; if the stick is moved to the left the aircraft yaws to the right. This is known as Aileron Drag. This effect is much more pronounced at low than at high speeds.
Yet another effect follows from the use of the ailerons, a movement in the yawing plane which is in the opposite direction to that produced by aileron drag. With modem aircraft of clean design, this movement cancels out that produced by aileron drag, except at low airspeeds. When the aircraft is banked, it tends to slip in towards the lower wing. When this happens, the pressure of the air on the fin and on the keel surface turns the aircraft in the yawing plane, since there is more effective keel surface behind the centre of gravity of the aircraft than in front. The turning movement is in the direction of the lower wing.
The rudder also has a secondary effect ; when an aircraft is turning in the yawing plane, the outer wing is naturally moving rather faster than the inner wing. The greater speed of the airflow past it gives it more lift; so it rises, giving a movement in the rolling plane, or bank.
Note that any movement in the yawing plane will tend to bank the aircraft.
There are no further effects of the elevators.
The Stability of the Aircraft in Flight. An aircraft is designed to be stable in flight ; this means that it should tend to keep the same attitude in which it is set, and to return to it if it is displaced by local air disturbances. Since the aircraft, as already described, moves in three planes, so far as it is concerned, ignoring the earth, stability has to be provided for in each of these three planes. The great advantage of stability is that it saves the pilot much effort and tends to make the aircraft 'fly itself'.
Stability- in the Rolling Plane. This is achieved by setting the wings into the fuselage at a slight upward angle known as the Dihedral Angle. All, or part of the wing, may be set at a dihedral angle like this:
Why does the dihedral angle give stability in the rolling plane ? Should the aircraft, by reason of some air disturbance, become banked when flying straight, it will begin to slip through the air, down towards the lower wing, like this :
In consequence, as you can see, the lower wing will meet the airflow at a greater angle of attack than the upper wing, and will therefore have more lift. This greater lift restores the aircraft to the normal level position.
Stability in the Pitching Plane. This is provided by the tail plane, and is achieved on the same principle as stability in the rolling plane.
As was described on page 15, if an aerofoil while moving forward is also moving downward, its effective angle of attack in relation to the airflow is increased. Similarly, if the aerofoil is moving upward as well as forward, its effective angle of attack to the airflow is reduced. In the first case, the aerofoil will have more lift, in the second, less lift.
We can consider the tail plane to be set in the fuselage at a zero angle of attack to the airflow and, since it is symmetrical, this means that it has no lift at this angle. Now if, as the result of some air disturbance, the tail plane is displaced downward from the line of level flight, it will have positive lift, and accordingly rise again to the level .position. Similarly, if it is displaced upward, it will have negative lift, and therefore fall back to the level position. In each case, as it returns to the level position the lift also returns to normal zero, and stability results. The fact that the tail plane is well behind the centre of gravity permits it to control the stability of the whole aircraft in the pitching plane.
Stability in the Yawing Plane. Stability in the yawing plane is provided by the fin and by the sides of the fuselage (called the keel surfaces). Should the aircraft be moved in the yawing plane, the fin and the keel surfaces offer resistance to the movement, and the airflow tends to force the rear part of the aircraft, pivoting about. the centre of gravity, back to the former position. This is called Directional Stability.
The Engine and Propeller. The engine, by driving the propeller provides the power to maintain the aircraft in the air by giving the air movement necessary to lift and to move it through the air as required. The propeller drives the aircraft through the air by pushing the air immediately behind it backwards towards the tail; this air is called the slipstream. Consequently, the speed of the airflow over the tail and elevators, and past the fin and rudder is greater than that over the wings. Thus the speed of the airflow over this part of the aircraft varies with the engine speed much more than does the air speed over the wings. Consequently the degree of effect of the elevators and rudder varies with the speed of the engine ; the faster the propeller is rotating, the greater the speed of the airflow over the elevators and rudder, and consequently the greater the pressure on them when they are deflected from the normal position and the greater their effect in their respective planes.
Propeller Reaction. A slipstream from a propeller does not flow straight back past the fuselage, but twists round in corkscrew fashion. With a propeller which turns clockwise when viewed from the front, the aircraft will yaw to the right due to the slipstream striking the upper fin surface on the starboard side. This is compensated either by offsetting the fin, or by some form of rudder bias so that at cruising speed the aircraft will fly straight without any pressure on the rudder bar. This means that at full throttle the aircraft will tend to swing to the right, and when gliding with the throttle closed it will tend to swing to the left.
Subsidiary Controls-Tabs and Trimmers. An aircraft is fitted with certain subsidiary controls known as tabs or trimming devices. These take several forms, sometimes a spring loading which tends to pull the rudder or elevator in a certain direction, sometimes a separately movable part of the ordinary control, like this:
It should be noted that the tab and the control surface move in opposite directions. If the tab is moved down, it will force the elevator up, and that in turn will force the tail down. These trimming devices are usually under the pilot's control, though on some aircraft the rudder trimmer may be adjustable only when the aircraft is on the ground. Training aircraft are not usually fitted with aileron trimmers. The most important trimmer from the point of view of the elementary pupil is the tail, or elevator trimmer. Its effect is the same as that of the elevator, and the object of the trimmer is to relieve the pilot of work in keeping the attitude of the aircraft in the pitching plane to that desired. If, for example, the pilot wishes to climb, he presses the stick back to effect the necessary change of attitude of the aircraft in the pitching, plane, and then adjusts the tail trimmer until the aircraft maintains that attitude without any need to keep pressure on the stick. The trimmer is also adjusted whenever any change in engine speed is made, since this changes the air speed over the tail and consequently the effect of the elevators.
Slats. A slat is a subsidiary aerofoil fitted in some aircraft in front of the leading edge of the wing, like this :
The space between the slat and the wing is called the Slot. The slat is so set that its angle of attack is much less than that of the wing. When the wings are at normal angles of attack, the slat lies flat against the leading edge of the wing, but when the angle of attack of the wing approaches the critical angle, it comes away from the leading edge, opening the slot. The air rushes through the slot and is deflected by the slat over the top surface of the wing, and prevents the flow becoming turbulent. The effect is to postpone the stall, and to permit the angle of attack of the wing to be increased beyond that at which it would normally stall if the slat were not fitted. In most training aircraft the slat can be locked back against the wing by means of a control lever in the cockpit. The only point about the slat with which you need concern yourself in your elementary training is that when you are about to engage in spinning or aerobatics you should put the controlling lever to the 'locked' position. Don't forget to return it to 'unlocked' when you have finished your aerobatics.
Flaps are fitted to most aircraft. They are control surfaces incorporated in the trailing edges of the wings, between the ailerons and the fuselage; sometimes they also extend below the fuselage. There are many different types of flaps, but in essence the flap consists of a surface, which is hinged at its forward end to some part of the wing, so that it can be lowered below the line of the wing by means of a control in the cockpit. It can be lowered through different angles at will. The flaps in both wings go down evenly together. Here are illustrations of two types of flap:
When the flaps are lowered, the angle of attack to the airflow is increased, and consequently the lift and the drag from the wings, of which they form part, are also increased. Whether the lift is increased more than the drag, or vice versa, depends partly on the design of the flap and partly on the angle through which it is lowered. Since the angle is under the pilot's control, he can either obtain a greater increase in lift than in drag by lowering the flaps through a small angle, or a greater increase in drag than in lift, by lowering them through a large angle.
Flaps are used principally in the approach to land and landing, and sometimes at the take off. In the approach to land, the flap has two effects; it makes it possible to glide down to the ground at a steeper angle, but at a slower speed than is the case if flaps are not used. During the latter part of the landing (the 'hold-off' as it is called), when the aircraft is just skimming above the ground, the drag from the flaps helps us to lose speed quickly and reduces the length of landing field we need. In the same way, the drag assists us to pull up more quickly after we have touched down and our wheels are running along the ground.
In the take off, the additional lift is valuable but the drag is a serious disadvantage. Consequently, for taking off, if used at all, are only lowered through, a small angle, so that more lift can be obtained at the minimum cost in added drag. The effect is that takeoff speed is lowered. Acceleration of theaircraft is also reduced.
You will understand this note on flaps better when you come to learn about landing and taking off.
Wheel Brakes. Most aircraft are fitted with wheel brakes, although some elementary training aircraft have no brakes. Tail-skids fitted with shoes help to bring them to rest. The brakes are used to assist in controlling the movement of the aircraft on the ground ; to keep it stationary, to reduce its speed arid to help in turning it.
It is common for the brakes to be linked with the rudder bar, so that when you wish to turn to the left and therefore push your left foot forward on the rudder bar, the left wheel brake is applied if you wish. This holds back the left side of the aircraft while the right side is free to turn.
Always use the brakes very moderately and apply them gently. If the brakes are applied harshly, there is a tendency for the aircraft to tip forward on to its nose.
4 - BEFORE YOU LEAVE THE GROUND
The Parachute and its Harness. Remember that though it is very unlikely that you will need to take to your parachute in the air, if you do your life will depend upon it. Therefore take care of your parachute, and report at once anything that seems to be defective with the harness or quick-release knob. If you should have to abandon your aircraft in flight the procedure, so far as the parachute is concerned, is as follows. Grasp with your right hand the handle of the rip cord ; the handle lies on the broad band of the harness on your left side. Once you are out of the aircraft (you may jump out or let the aircraft throw you out) count three slowly (unless you are less than 1,000 ft. from the ground) ; this is to give time for you to get clear of the aircraft so that the parachute doesn't get caught on any part of it as it opens. Then pull the rip cord-handle right out, so that it comes right away in your hand. The parachute will then open and you will float gently down to earth. When you are about thirty feet from the ground, turn the milled knob of the central fastening of the harness upwards towards you in a clockwise direction. When your feet are just about to touch the ground, strike the knob sharply with your fist. This releases the harness, the parachute floats down, and you are not dragged along by it as you might be if the wind were strong and you did not release it.
Climbing into the Aircraft. Be careful to put your hands and feet only where indicated by the instructor. Certain parts of the wings are stiffened to take the weight of your body as you stand on them ; others are not, and you will damage the fabric if you tread in the wrong place ; similarly you may push your hand through the fabric if you don't use the proper hand-holes.
The Sutton Harness. This consists of four straps with numbers 1, 2, 3, 4 embossed on them. Nos. 1 and 4 go over your shoulders and Nos. 2 and 3 come up from the floor past your thighs. One has a thick stud tied to it. Put this stud through the appropriate hole in strap 1 (probably the third hole from the end). Then put the first hole of strap No. 2 over the stud, and then the first hole of No. 3 strap over the stud. Finally put the appropriate hole (the same as in the case of No. 1 strap) of No. 4 strap over the stud. Finish off by pushing through the slot in the stud the locking-pin device which is attached by a cord to one of the straps. When the locking pin is pulled out, the straps fall apart most easily if they have been done up in the right order.
The harness should feel distinctly tight and possibly a little uncomfortable when you do it up : you will find that it eases very quickly and becomes comfortable in a few minutes.
The locking-pin device acts as a quick release ; when it is pulled out the harness is freed so that an emergency abandonment of the aircraft is not hampered.
Do not release the harness on landing before the aircraft has come to a stop.
Sundry Adjustments. Connect the tube from your helmet to the tube leading to the. instructor's mouthpiece ; after the engine is started see that your helmet straps are fairly tightly adjusted (otherwise you won't be able to hear the instructor well) ; see that your goggles and the windscreen are clean, and that the straps of your goggles are not twisted, and that the goggles sit snugly on the helmet on your forehead, ready to be pulled down over your eyes when you want them.
See that any part of the fuselage which has been opened or let down to permit of entry to the cockpit is securely fastened in place again.
The Cockpit Check. There is a certain standard routine check which you must always carry out before the engine is started, and which must be repeated (with amplifications) before you take-off. This cockpit check is most important, and you must get in the habit of performing it from the first; even while your instructor is dealing with starting the engine, taxying and taking off, you can do all the visual checking in your own cockpit.
The routine check varies for different types of training aircraft, but you must carry out the various checks in the same order every time ; one simple method is to go round the cockpit, starting from left to right, passing from point to point in regular order. Your instructor will issue you with the correct cockpit drill for the type of aircraft on which you are being trained.
Starting the Engine and Moving off. Always give your instructions to the aircraftman in a loud clear voice, and make sure you hear him repeat them before taking any action. Similarly, always repeat the messages he gives you, to indicate that you have heard him.
Handling the Controls. In all handling of the controls, be gentle; do not move the controls about abruptly or violently. Try to think, not so much of moving a control as of applying pressure to it in the desired direction. Your handling of the controls should always be smooth ; in the early stages you will find that you may tend to apply excessive pressure too suddenly to the controls ; try to check this. Since an aircraft is heavy you cannot suddenly alter its attitude or direction of flight ; a certain amount of time must always elapse before a control movement produces the desired effect.
Remember that when you change the engine speed, you will have to change the position of the tail trimmer ; and that when you have changed the position of the aircraft to climb or to dive, you should adjust the tail trimmer to save the trouble of applying a constant pressure on the stick.
The 'feel' of the controls is not the same all the time: when taxying on the ground you will find that the controls feel 'sloppy'; a large movement is necessary to obtain the desired result on the track of the aircraft. This is because there is not much air pressure on the control surfaces, because the air speed is low and the engine revolutions moderate.
In the air you will find the controls very responsive: only light pressure is normally required.
Remembering what has been said about the effect of slipstream on the elevators and rudder, you will understand that the feel of the stick in its fore-and-aft movement, and of the rudder bar, change when the engine speed is changed.
The Effect of the Controls in Relation to the Horizon. We have described in pages 32 and 33 the effect of the controls on the aircraft in its three planes of movement. The controls have always the same primary effects, no matter what the attitude of the aircraft is in relation to the horizon, or to the surface of the earth, provided the aircraft is not stalled. But in flying, the controls have to be used to govern the movements of the aircraft in relation to the horizon and to the earth. You are seeking to make the aircraft adopt or maintain an attitude to the. earth, and the speed required for a particular manoeuvre. Your use of the controls depends, in the main, on your visual judgment of these movements. In cloud or in darkness, the instruments provide you with exactly the same indications on which to base your judgments.
Consequently, the effect of the controls has to be related to the movements of the aircraft, not only in space, but in relation to the earth below. Thus, if when the aircraft is flying level, the rudder bar is pushed forward with the left foot, the nose swings round the horizon to the left.
The effect of the rudder, then, when the aircraft is flying level in the rolling plane, can be said to be to move the nose round the horizon. But if the aircraft is banked, the effect of the rudder is then not to move the nose round the horizon, but to move it up or down across the horizon like this:
When, therefore, you think of the effect of the controls as giving movements of the aircraft in relation to the horizon, you have to bear in mind the attitude of the aircraft at the moment ; otherwise you will find yourself tending to think too much of the effect in the normal flying attitude. The result will be to think of the elevators, for example, as controlling the movement of the nose exclusively up and down in relation to the horizon, a fact which is true only when the aircraft is on a level keel. Consequently, although it may seem a somewhat roundabout method, you will find it best to think first of the effect of the controls on the aircraft, i.e. the rudder always moves the nose round towards one or other wing tip : the elevators always move the nose either down towards the undercarriage or up towards the fin ; the ailerons roll the aircraft.
You must then translate the movement of the aircraft in relation to the horizon, which you require to effect, into the necessary movements of the aircraft in its three planes : you then use the controls to provide these movements, and thus attain your end.
You will find this method particularly helpful in the early stages of your elementary training, when you are learning turns, and in the later stages when you come to aerobatics.
The Question of the Wind. Never miss an opportunity of checking the direction from which the wind is blowing. On the airfield you have the wind-sock as a guide: from the air the smoke from chimneys or bonfires (but not from moving railway trains) should always be looked for. You can also detect the direction of the wind from the drift of the aircraft. By drift is meant the sideways movement, over the ground, when you are flying across wind. Drift is much more noticeable at low altitudes than when you are-higher up ; it looks as if the earth is moving sideways beneath you towards the direction from which the wind is blowing. This diagram illustrates drift :
There are a number of other ways in which the wind direction can be learned, which you will find described in your Navigation Manual. The purpose of this note is to emphasise the importance of taking every opportunity of checking the wind direction.
