An Outline of the Principles Governing Modern Aviation Design
THE MAIN CONTROLLING FACTORS OF FLIGHT are four in number. The engine controls the height at which the aeroplane flies. The elevators force the tail down or up, into the attitude for climbing or descent. The remaining controls -ailerons and rudder - enable the aeroplane to turn and bank correctly. The aeroplane in the illustration is a Westland Lysander monoplane.
IN some parts of the world, where there are hurricanes, tornadoes and other kinds of high winds, people are often blown off their feet, and the roofs of buildings, sheds and all kinds of heavy objects are lifted by the wind and carried long distances in the air.
A really fierce hurricane seldom blows at more than between 80 and 100 miles an hour, and a wind which seems as if it would blow a man off his feet is probably not blowing more than 40-50 miles an hour at the most.
Winds, however, do bring nearer to everybody a realization of the force which air can exert when it is moving fast. It is because use has been made of this fact that aeroplanes are able to fly.
To understand how an aeroplane flies, it is necessary to realize that the force which air can exert is the same, for a given speed, whether you are still and the wind is blowing past you, or whether the air is still and you are moving, through the air. This may be easily demonstrated. On a windy day you have to hold your hat on because of the wind. But if you are travelling on an express train on a calm summer’s day, and put your head out of the window, you will feel exactly the same thing. You will have to hold your hat on to prevent it from being blown off. In one instance the air is rushing past you and in the other you are rushing through the air. The force which is exerted on your hat by, say, a fifty miles-an-hour wind, when you are standing still, is exactly the same as the force trying to blow away your hat when you lean out of a railway train which is moving at fifty miles an hour.
The modem aeroplane moves twice as fast as the worst hurricane. For the most part, the force of the wind increases as the square of the speed. A wind of 100 miles an hour, for example, exerts a force four times as great as one of 50 miles an hour or nine times as great as one of 33⅓ miles an hour. It follows, therefore, for a given aeroplane, that the faster it can move the more it will be able to lift, other things being equal. That is one of the reasons, though not the only one, why modern fast aeroplanes are able to lift more than the older slow-flying aeroplanes.
The simplest form of air flow is over a flat surface. Fig. 1 shows a section of a flat plate over which the air is flowing. The edge of the plate which first meets the air is called the leading edge (L.E.) and the edge where the air leaves the plate the trailing edge (T.E.). At the leading edge the particles of air crowd together, and the pressure is increased, for the particles are pushed up against the edge of the plate before they can get round it. At the trailing edge the opposite occurs. The streams of air, rushing along the surface of the plate, do not join up again for some little distance away from the trailing edge. There is less pressure there and a partial vacuum is formed, causing a sucking action.
FIG. 1. A FLAT SURFACE offers resistance at its edges to the flow of air. The leading edge (L.E.) is in front and the trailing edge (TE.) behind.
The figure shows what happens if the plate is held still and the air flows past it, but the same thing happens if the plate is moved rapidly through the air. The piling up of the particles at the leading edge and the suction at the trailing edge cause a resistance to the movement of the plate through the air, and this resistance has to be overcome. The faster the plate moves through the air the greater is the pressure in front and the less the pressure behind. In general, the resistance increases with the square of the speed in the same way as the lift does.
When a flat plate is held as in Fig. 1 the air is not able to lift it. To be lifted, it must be held at an angle to the air flow, as in Fig. 2. The air flowing over a surface inclined as in Fig. 2 is deflected over the top surface, roughly as shown in the figure, and the air forms little whirls or eddies. These eddies lessen the lift on the plate, and it is because of this phenomenon that it was found many years ago that a flat surface would not lift well enough to be used for the wings of an aeroplane. Anything which upsets the smooth flow of the air increases the resistance and lessens the lift.
FIG. 2. AN INCLINED FLAT SURFACE deflects the air stream, which forms eddies as shown. Eddies lessen the lift obtainable; so curved surfaces have been evolved to minimize the effect of eddies.
Many of the earlier experimenters who were trying to discover the secrets of flight noticed that the wings of birds were not flat surfaces, but were curved, and began to experiment with them. It was soon found that a properly curved surface lessened the eddies which are formed as in Fig. 2, and so lessened the resistance. Little by little the best kind of curved surface was evolved from experiments, and in the modern aeroplane the top and bottom surfaces of the wings are curved. The curve on the upper surface is considerably greater than that on the lower surface. The curves are carefully arranged for different kinds of aeroplanes, because no curves have been discovered which are suitable for an aeroplane which is required to fly fast and also for one which is required to lift a heavy weight. Fig. 3 shows some of the wing sections now used on aeroplanes.
FIG. 3. SECTIONS OF AEROFOILS, showing the curved wing surfaces which have been designed for different kinds of aeroplanes. No curves have been discovered that are suitable for an aeroplane which is required to fly fast and also for one which is required to lift a heavy weight. In general the thin sections apply to high-speed aircraft; but many standard types of wing section are now in use for different types of aircraft.
Over these curved surfaces the air flows more smoothly than over flat surfaces. By means of smoke, and in other ways, the air flow may be photographed, and Fig. 4 is a photograph of air flowing over a section of a model aeroplane wing.
There are now many hundreds of different shaped curved wings in existence and the search is still going on for the best shape. The increase in speed, for instance, introduces many difficulties which do not occur at low speeds.
Taken generally, it is the alteration in maximum camber of thickness of a wing which most affects its lifting power and its resistance. The relation between the lift and the resistance is, generally speaking, greatest when the maximum thickness is about one-twentieth of the distance of the chord from the leading edge. The lift is greatest, apart from any consideration of resistance, when the greatest thickness is about one-twelfth of the distance of the chord from the leading edge. The chord line is the straight line through the centres of curvature of leading and trailing edges.
An alteration in the top curve of the wing of an aeroplane has a greater effect than an alteration in the bottom curve. A compromise is made, and the best place for the maximum depth is a little more than one-third of the chord’s distance from the leading edge.
FIG. 4. THE AIR FLOW CAN BE PHOTOGRAPHED with the aid of smoke or other means, and its effects can be studied in detail. This photograph shows (in section) a model aeroplane wing and the air flowing over its surface.
The upper and lower surfaces of a wing lift and the amount that the wing lifts depend upon the angle it makes with the direction of the flow of air. When the angle becomes too great, however, the smooth air flow breaks up into eddies and the wing ceases to lift. When an aeroplane wing does this it is said by pilots to stall. It is then that an aeroplane may suddenly try to dive to the ground or to spin.
For a long time it was thought there was no way of preventing an aeroplane from stalling suddenly, until F. Handley Page invented his now famous slot. This, in effect, is a little movable curved surface along the leading edge of an aeroplane wing and it normally fits closely on to the wing. When the air flow is nearly breaking down and the wing is likely to stall, the small curved surface opens out away from the wing and causes the air to flow smoothly again. Fig. 5 shows diagrammatically the Handley Page slot, and Fig. 6 the slot in an aeroplane. Fig. 7 shows the flow of air over a wing with the slot closed and with it open. It can be seen that, when the slot is closed, the flow of air does not follow the wing at all - it is stalled - but immediately the slot is open the air flow becomes smooth and the wing lifts as it should.
FIG. 5. THE SLOTTED WING, invented by F. Handley Page, embodies a small movable curved surface along the leading edge of the aeroplane wing, normally fitting closely to the leading edge, as shown above by the dotted lines. When the air flow is nearly breaking down and the wing is likely to stall the small curved surface opens out and causes the air to flow smoothly again.
The discovery of the curved wing, which gives greater lift and less resistance than a flat wing, was only one step forward towards making an aeroplane fly. The next step came with the development of the petrol engine, which provided an engine powerful enough, yet light enough, for driving the aeroplane through the air. But there was still another discovery to make.
FIG. 6. WHEN THE SLOT IS OPEN it can be seen that the small curved surfaces of the slotted portion are themselves of aerofoil shape, like the wing itself. When closed they fit snugly to the wing and conform to its shape.
In the earliest aeroplanes it was found that the resistance of the undercarriage and wheels, the body or fuselage, the bracing wires and so on was so great that aeroplanes flew badly and required powerful engines to make them fly fast. It was soon learnt that various parts of the aeroplane were offering resistance to the air because they were badly shaped. By shaping them properly the air was made to flow smoothly round them. The correct shape is known as streamline. The air streams follow this shape and set up fewer eddies than with other shapes. As it is not possible to make aeroplane wheels and other parts streamlined in shape, light streamline “fairings” are fitted for this purpose over wheels and other parts of aeroplanes. In most modern aeroplanes the undercarriage is retractable; that is, it can be drawn up into the wings so that it offers no resistance at all. By smoothing the surfaces, shaping them properly, or fairing them by streamline covers, the resistance of the modern aeroplane has been made much less than that of the early types. Thus the modern aeroplane can fly faster with less power than its predecessors.
FIG. 7. STRIKING DIFFERENCES in the air flow when the slot is closed and when it is open are shown here. In the upper picture the flow of air does not follow the wing at all - it is stalled; but immediately the slot is open the air flow becomes smooth and the wing lifts as it should.
Once the aeroplane is in the air it is not difficult to fly. There are only four practical controls. The first is the engine, which controls the height at which the aeroplane flies. The second is on the tail plane of an aeroplane. Here there are surfaces, called elevators, which can be moved so that the tail is forced up or down, for coming down or climbing, respectively. The elevators and engine, between them, control the speed and climb of the aeroplane.
On the wings of an aeroplane, along the trailing edge, are the third controls, movable surfaces called ailerons. The ailerons, with the rudder, the fourth control, enable an aeroplane to turn and bank properly. One aileron moves up while the other moves down, causing a greater resistance on one wing than on the other and a greater lift on one wing than on the other. If the aeroplane were not banked properly in this way it would sideslip, just as a car does when it is driven round a corner too quickly. The controls are worked by means of a rudder bar and a wheel in the pilot’s cockpit. The only control which is possibly difficult to understand is the engine. It is not always realized why the engine controls the climbing of an aeroplane.
If an aeroplane is flying horizontally, the horse-power which the engine is exerting is exactly right to keep the weight of the aeroplane in the air on a level path.
Difficulties Increased by Speed
When the throttle is opened, so that the engine is exerting more power, the airscrew begins to revolve faster and the aeroplane begins to move faster. The faster the speed, the greater the lift, so that the aeroplane will now be lifting more than its own weight; in other words it will begin to climb. Similarly by lessening the power of the engine - closing the throttle - the power for horizontal flight is lessened and the aeroplane begins to come down as the lift is less than the weight of the aeroplane.
The lift of the wings depends not only on the speed of the aeroplane, but also on the angle at which the wings are inclined to the direction of air flow. By altering this angle the lift can be varied, so that, for a given angle, a higher or lower speed is required for horizontal flight. The elevators put the nose of the aeroplane up or down as required, and so alter its angle, and in this way govern the speed at which the aeroplane flies.
Once an aeroplane is set upon its course, if the weather is fine, the pilot need do little except keep his eyes on the instrument board and make necessary adjustments to the controls.
When an aeroplane comes in to land it is brought into the wind as far as possible, so that its speed over the ground shall be as low as possible and the speed of the air stream past the wings as high as possible. An aeroplane whose landing speed is, say, fifty miles an hour, when landing against a thirty miles-an-hour wind pouches the ground at only twenty miles an hour. If it tried to land down wind its ground speed would be eighty miles an hour, and there would be always the danger of the wind getting under the tail of the machine and tipping it over on its nose.
Even today there are many things to be discovered about the way in which air flows, especially at the high speeds which aeroplanes are now reaching— speeds which are introducing all kinds of new and difficult problems.
THE BEAUTIFUL STREAMLINED FUSELAGE of the modern aeroplane is well illustrated by the De Havilland Albatross. A low-wing monoplane of special construction, powered by four Gipsy Twelve supercharged engines, the Albatross is designed for fast and luxurious passenger flights and for long-distance mail and freight transport.