The Modern Single-seater Monoplane is Based on Racing Seaplane Design
ETHYLENE GLYCOL INSTEAD OF WATER is used for cooling on modern fighters. Ethylene glycol, and similar substances, can be worked at a higher temperature than water. They therefore require a smaller cooling area. The radiator is arranged at the end of a specially shaped air scoop. Such a scoop is seen underneath the fuselage of this British fighter, the Hawker Hurricane. Not only has this aircraft a retractable undercarriage, but the tail wheel is also retractable.
SPECIAL interest attaches to the fighting aeroplane because it is, above all things, dedicated to speed. It is the fastest military machine in existence, and its speed is its dominant characteristic.
The reason for this is that the aeroplane fights not only with guns, but also with speed, climb and powers of manoeuvre. It is sometimes suggested that too much emphasis is placed upon speed in the air; yet when the test comes there is not a pilot with experience of aerial fighting who would not prefer speed to almost any other factor, including that of weight of armament.
This chapter will deal with modern fighter design and not with the design of the fighters of the past. But it is essential to refer to some of the earlier types so that the latest types shall be seen in true perspective.
The fighter grew out of the observation machine. Originally, aircraft were regarded merely as reconnaissance machines. When reconnaissance was done by a machine carrying only one person it was natural that that machine should be described as a “scout”.
Its function as a scout, however, was soon overshadowed by its function as a fighter. For it was found that it could shoot down enemy machines with greater certainty than could larger types. This was because it was faster and could therefore close with the enemy machines and break away again at will.
So the scout turned into the fighter, though for a time it retained its title of scout. Its qualities came to be recognized as fighter qualities. They were speed, high rate of climb and great powers of manoeuvre. Such machines had little range and could not carry much load. But they were fast and quick and they could climb rapidly.
That has ever since been the accepted type of fighter. It is true that variations have occurred. Two-seater fighters - necessarily slightly slower than the single-seaters - have been used successfully and have formed part of the equipment of the Royal Air Force. Today, however, the single-seater is the only type in production for the expanded Royal Air Force.
Recent active service experience has led some observers to doubt the value of speed and to suggest that slower speed is not so serious a drawback to a fighter as is generally supposed. But even these observers agree that, if the speed is reduced, the powers of manoeuvre must be increased in compensation. Thus the qualities originally determined remain the ones which must be sought in all fighter designs.
A machine in which speed plays so important a part is closely related to the racing types. The Schneider Trophy seaplanes of 1927 and 1929 were the ancestors of the fighting aeroplanes of 1938 and 1939. The resemblance is not merely superficial. Much information about structural methods was obtained during the races, and designers of the new fighters have taken advantage of this information.
The general arrangement of a modern fighting aeroplane may now be outlined. It is a monoplane and is, for its power,
of small size. The extremely small fighters of the Bat Bantam type no longer exist, but that is partly because engine powers have risen to such an extent that exceptionally small wings are no longer practicable. Yet, for their power, the modern fighters are still small.
In common with nearly all modern types of aeroplane, they are monoplanes, generally low-wing monoplanes. This arrangement perches the pilot, as it were, fairly high above the wing surface, so that he enjoys a good outlook. In addition, as the wing is mounted on the lower part of the fuselage, its underside can be used for the recesses into which the landing wheels retract when the machine is in the air.
Another important feature common to all the latest fighters is the enclosed cockpit for the pilot. With speeds approaching 400 miles an hour in still air it has become increasingly important to give the pilot the utmost protection. Even in a well-sheltered, open cockpit he could not fly with the ease and comfort that are needed if he is to be efficient at such high speeds.
The engine will probably be liquid-cooled and have twelve cylinders in V formation. It will be supercharged and will have two other features of the utmost importance, for they exercise an influence upon the performance of the modern fighter. Ethylene glycol or some similar substance will be used as the cooling medium instead of water; and the radiators, both for this liquid and for the oil, will be arranged in a kind of tunnel formed in the underside of the fuselage of the machine and having only a small aperture to the front.
Liquid cooling has been used for fighters, although air cooling is now used for so many other types, because up to the present higher speeds have been attainable with a liquid-cooled than with an air-cooled engine. This is because the liquid-cooled engine presents, for its power, a smaller frontal area. Thus the body of the aeroplane can be made smaller, so that it causes less resistance. Engine power has steadily increased since the early days. At present it is rather more than 1,000 horse-power, and there are indications that it may go still higher in the near future.
The special radiator, which is now used in all the latest Royal Air Force fighters and also in one or two bombers, is remarkably ingenious. If an ordinary water radiator were used, its size for an engine of 1,000 horse-power would be unduly large. It would therefore cause big air resistance and prevent high speeds from being reached.
The first step towards bringing its resistance down is the use of a cooling substance, such as ethylene glycol, which, as it will work at a higher temperature, can be satisfied with a smaller cooling area. The next step is the enclosure of the radiator element in a tunnel whose interior shape is carefully devised so that air entering it is first slowed down considerably and then again speeded up as it leaves.
THE OBSERVATION MACHINE was the forerunner of the fighter. Early in the war of 1914-18 single-seater reconnaissance aircraft were termed “scouts”. Later is was discovered that their manoeuvrability made them valuable as fighters. The Bristol Scout shown in this picture provides a typical instance of this change-over. An 80 horse-power Gnome engine was fitted to the Bristol Scout.
The object of the tunnel, whose aperture can just be seen under the fore part of the fuselage of a modern fighter, is to control the air flow through the radiator proper. It does this, according to well-known laws, by its shape, and it juggles with the heat, pressure and speed of the air so that in the end the cooling produces almost no drag.
When the ducted radiator patent was taken out it was suggested that eventually it might be possible so to design these radiators that they would cause no drag at all, but, on the contrary, exert a small thrust - an apparent instance of “getting something for nothing”. In practice, however, the waste heat from the engine, instead of being thrown away down the exhaust pipe and through the radiator, would help to propel the machine.
Ducted radiators of this type are effective only at high speeds and cannot work at much below 300 miles an hour. They are among the most important of recent contributions to fighter design, and they were developed in Great Britain.
Not only do the undercarriage wheels retract into recesses in the undersides of the wings when the machine is in flight, but the tail wheel also retracts into the rear end of the fuselage. Thus, the aeroplane, when in flight, has a clean line without excrescences on wings or fuselage other than the small, narrow mouth of the ducted radiator.
The airscrew may or may not be of variable pitch. For the high-speed machine with a reasonably large wing area variable pitch may not be needed. It seems fairly certain- that in the future the variable-pitch airscrew will be used for fighter as it is for bomber and other types. The Hawker Hurricanes in service with the R.A.F. have fixed pitch airscrews at the present time. If they were changed to variable pitch airscrews certain modifications in the wing arrangement would be desirable. We have now a fairly comprehensive picture of the modern fighter - a small, 1,000 horse-power aeroplane, with retractable undercarriage and tail wheel, enclosed cockpit, ducted radiator, and wings set low on the fuselage. It is armed as heavily as possible without undue detriment to its performance. Details of the armament used in the latest Royal Air Force machines may not be given, but some general remarks are possible as to one important feature of single-seater fighter armament.
So far axial fire has been the only sort for which provision has been made. The guns are all fixed and fire directly forwards in the line of flight of the machine. There are at present in modern single-seater fighters no guns which can be swivelled, traversed or otherwise aimed in the ordinary sense.
The reason is that the technique of single-seater fighter work is based upon the view that the aeroplane itself, and not the gun, should be aimed at the adversary. The pilot turns and twists his machine until he has obtained a sight through a fixed tube or other fixed sight set in the fore-and-aft line of the machine.
At first one gun was used in single-seater fighters and it was synchronized so that its bullets did not hit the blades of the airscrew as they passed in front of the muzzle. The synchronized gun, or, rather, pair of guns, is still used in certain fighters, but it is generally supplemented by several other guns, mounted probably in the wings. They are completely sunk into the wings, and the muzzles only form small holes in the leading edges. They are fixed, as are the guns in the fuselage, to fire forwards in the line of flight.
Six or eight guns can be fitted, and in some countries there is an increasing tendency to supplement them with “cannon” mounted either between the banks of the engine cylinders or in the wings. These “cannon” are merely large machine guns firing explosive shells. In Great Britain, up to the present, cannon have not been much used, but their possibilities are not being overlooked.
The fighter mounts, then, a number of guns, most of them in the wings, but sometimes with a pair in the fuselage The fire control may be hydraulic or pneumatic. The guns can be selected at will by the pilot, who can fire them in groups or singly by means of a trigger fitted to the top of the aeroplane control stick.
THE SCHNEIDER TROPHY seaplanes of 1927 and 1929 were the ancestors of the modern fighter. A strong resemblance is noticeable between this British Schneider Trophy seaplane of 1929, and the fighter illustrated on page 439. Much of the information about structural design which was obtained from the Schneider Cup races has proved valuable in the design of the fighting aircraft of today.
Tactically, the use of the fighter is determined by this arrangement of fixed guns and high performance. The pilot has to place the machine so that he can use his guns, and his entire attention is concentrated upon the flying. Thus the powers of manoeuvre are of great importance. But here there has been a change in recent years. Speeds have mounted to such an extent that it is thought by some authorities that the day of the “dog fight” is over and that fighter tactics will be confined to approaching, striking and turning away again. They believe that no two machines, and certainly no groups of six, twelve or more machines, will ever in the future maintain contact with one another for an extended period.
In the war of 1914-18 this form of combat was common. Large formations would clash, and there would develop a “dog fight” lasting for perhaps a quarter of an hour or more, in which the machines remained, as it were, locked together, whirling round and round, diving, zooming and turning, but always keeping together.
It is thought that nowadays a fighter formation, meeting another, would approach, open fire and then turn away. If it were attacking a formation slower than itself it would remain within range for brief periods only. Whether this theoretical reasoning is sound is open to doubt.
In the Royal Air Force horse-powers and speeds have been increased to such an extent that powers of manoeuvre are necessarily somewhat restricted. A machine capable of doing 400 miles an hour on the level and 500 miles an hour in a dive cannot turn in small circles. It must take curves of big radius and make its alterations of course relatively gently.
Whatever might be the tactics of fighter pilots if there were another major war, it may be laid down that designers of the fighting aeroplane will continue to seek speed and yet more speed and climb and yet more climb, and that they will, if necessary, permit the powers of manoeuvre in a small space to be reduced. But that is not to say that these powers are to be disregarded. The latest machines, though capable of speeds in excess of 400 miles an hour, are also capable of flying at 70 miles an hour.
Slots for Slow Speeds
To obtain this speed range, most of the fighters have wing slots and wing flaps, sometimes slotted flaps. These devices give the machine lift at low speeds and also aid in maintaining its stability at low speeds. When the machine’s speed has fallen below a certain point the wing slots open automatically, the air stream itself producing the pressure effects which cause the opening. The wing flaps can be operated by the pilot by means of hydraulic gear similar to that used for raising the undercarriage wheels.
When the machine is flown slowly, the flaps are lowered by the pilot and the slots automatically open. The wing contour is changed and its curve appears to have become more marked. In other words, the wing is altered to give the necessary conditions for slow flying. When a pilot is bringing a fast fighting machine in to land he lowers the wing flaps. The slots open themselves. Thus the pilot is able to fly slowly while retaining full control, and this enables him to put the aeroplane down into aerodromes which would otherwise be too small.
Although the wing flaps might not be used, the slots would automatically operate if the machine were flown slowly. The powers of manoeuvre are to some extent improved by this change, and if, during combat, a pilot wished to make a turn of small radius, he could do so by reducing speed as a preliminary.
Structurally, the modern fighter is built entirely of metal. The Air Ministry decided some years ago to adopt the principle that metal construction should be used for all first-line aeroplanes. Although training machines and machines needed for emergency expansion purposes may be built of wood, the general principle remains.
The spars and longerons and the general structure are always of metal for fighting machines built for the Royal Air Force, but occasionally parts are fabric-covered. Wing coverings, however, are of metal when the design arranges for the wing surfacing to take the stresses. This is the “stressed skin” construction (see pages 170-174) which has proved eminently well suited to modern aircraft design.
ABILITY TO FLY SLOWLY as well as fast is required in the modern fighter to enable sharp turns to be made. If these are attempted at exceptionally high speeds, the pilot experiences a “black out” of vision. Trailing edge flaps (shown in the lowered position) enable low flying speeds to be achieved and simplify the landing of this Hawker Henley.