HEAVILY LOADED MONOPLANE IN FLIGHT. This Hendy Heck is equipped with such devices as Handley Page slots and flaps, which allow a high top speed and enable a landing to be made at a reasonable speed. The higher the wing loading the higher is the stalling speed of an aeroplane. Many devices have been evolved to overcome the drawbacks which accompany the advantages of high wing loading.
THE wing loading of an aeroplane is a figure obtained by dividing the weight of the machine by the area of its main planes. In the British Commonwealth and America wing loading is expressed in terms of pounds per square foot; in other countries metric units are used. The expression wing loading generally means the “all-up weight”, including full commercial (or military) load and full tanks. In the course of a flight petrol and oil are used up and the weight, and hence the loading, gradually become less.
Wing loadings have shown a constant tendency to increase from the earliest days of flying up to the present time, and much higher loadings than any at present used are foreshadowed by practical experiments and research which are now being carried out. Before 1914 a wing loading of 10 lb. per square foot of wing was considered high, but more recently biplanes have been loaded up to 18 or 20 lb. per square foot. With monoplanes it is possible - for reasons which will be explained - to go to much higher loadings. Some of the recent high-speed monoplanes have been given loadings as high as 30 or 40 lb. per square foot. Although we quote an aeroplane as having a wing loading of so many pounds per square foot, this must be understood to be an average over the whole area. The distribution of the load varies from the lower surface (where there is pressure) to the upper surface (where there is suction) and also from the outer tip to the centre and from the leading edge (front) to the trailing edge (rear). The distribution varies also with the attitude of the aeroplane, whether it is climbing, flying level at different speeds or landing, although the “loading” may be the same. Yet wing loading is a convenient and important measure of the performance of an aeroplane.
The advantages of a high wing loading will be apparent after consideration of the two items - weight and wing area - which make it up. The more weight which a commercial air liner can carry the more efficient it will be. This weight may be in the form of paying passengers, thus increasing the revenue, or in the form of fuel, thus increasing the range of the machine. Alternatively, the wing loading may be increased by the use of a smaller wing area for the same all-up weight.
If a smaller area were used the aeroplane would travel faster for the same output of engine power and the same consumption of petrol, because there would be less structure to drag through the air. For a given tank capacity, allowing a certain number of hours’ flying time, it would travel farther at the higher speed; or on a certain stage, say from London to Paris, the flying time and thus the consumption of fuel would be less. Whichever way we look at it, then, higher wing loadings are economical because they allow a larger revenue for a lower expenditure. There is also the further point that the smaller the wing area, the smaller the weight of the wing. This again allows an increase in payload for the same all-up weight.
Higher wing loadings are equally important in certain types of military aircraft, notably long-range bombers and flying boats used for reconnaissance work. For fighters the advantage is not so marked, and this brings us to the reverse of the picture. Why have not loadings been increased far beyond their present limit? What is the limit? There are two limits on the speed of an aeroplane in flight, a maximum and a minimum. At the upper limit, as with any other vehicle or moving body, the fastest speed of an aeroplane is reached when the greatest propulsive force is equal to the maximum “drag” or resistance to propulsion. Moreover, the aeroplane, unlike any other vehicle, whether it be a train, motor car or ship, has a lower limit of speed as well. Other vehicles may gradually accelerate from a stationary position to a maximum speed. An orthodox aeroplane cannot be stationary when it is flying.
There is a minimum speed at which it is sustained in the air by the forces exerted by the flow of air over the wings. This minimum speed depends on a number of factors, the most important of which is the wing loading. If the loading is 10 lb. per square foot the minimum or “stalling” speed will be about 50 miles an hour. At 20 lb. per square foot the stalling speed will be about 70, and at 40 lb. per square foot over 90 miles an hour, assuming the wing to be a normal one without special devices (to be described later) which allow a lower speed.
High minimum flying speed affects the aeroplane in two important ways. It raises the speed at which it can leave the ground, thus lengthening the run across the aerodrome. It makes landing more difficult and lengthens the run after landing. At the same time a high wing loading, other things being equal, reduces the rate at which the machine can climb away from the ground, and also reduces the height to which it can ultimately climb. Further, it has the effect of making the aeroplane less easy to manoeuvre.
Thus the design has to be a compromise between the requirements of a high cruising speed and of reasonable handling qualities at low speeds. Every effort must be made to remove the handicap which high loadings put on the aeroplane during the take-off, climb and landing. Manoeuvrability is not so important, except for a limited class of military aircraft.
Telescopic Advantages
The problem has been attacked in many ways and at present it may be said that the highest practicable wing loading for a landplane is about 40 lb. at the take-off and not much over 30 lb. for landing, when the fuel has been used up and the weight correspondingly reduced. A flying boat is not so limited, as most stretches of water from which it would operate are not as restricted in area as are aerodromes, nor are they generally surrounded by high buildings, trees and other dangerous obstacles. Wing loadings of over 40 lb. per square foot may therefore be used and will be found on new designs projected for transatlantic routes by British and American manufacturers.
Various methods have been used to overcome the effects of high wing loadings on the take-off, climb and landing. The first and most important subject for research has been the wing itself. Investigations are still being made mathematically, in the wind tunnels and by full-size trials on every dimension and proportion of the wing shape. It has been found possible to develop shapes capable of carrying higher loads per square foot at the same speed, or, putting it another way, to carry the same load at lower stalling speeds. Such wings are now found on all modern aircraft, and their main characteristics are evident at a glance.
The wing section is relatively thick at the root where it joins the fuselage, perhaps as much as one-fifth of the chord (the distance between the leading and the trailing edges). It tapers in thickness towards the tip and at the same time the leading and trailing edges converge from the centre towards the tip. The upper surface is boldly convex; the lower surface is almost flat over the rear three-quarters of its width. Because the wing is thick, it has been found possible to enclose within its surfaces all the structure needed to support it; thus it contrasts with the older and thin-sectioned biplanes which were externally braced. Further, because such wings will carry higher loadings, it has been possible to use smaller areas. Thus we find the modern tapered wing monoplane displacing the older wire-braced biplane with thin wings.
TAPERED MONOPLANE WING of the Hendy Heck, with the leading edge slot open and the flaps at the trailing edge down as for landing. A higher speed in the air is made possible by the use of the retractable undercarriage, which, when the aeroplane is in flights is withdrawn into the cavities in the wings.
In addition to wing research, designers have investigated the possibilities of devices such as leading edge slots and flaps on the under surface, which allow even greater loadings to be carried at the same stalling speeds. For the same stalling speed a modern monoplane requires only perhaps two-thirds of the wing area of the biplane, and thus has a top speed for the same engine power and fuel consumption as much as forty or fifty per cent greater. The range and earning capacity are correspondingly increased.
It has frequently been suggested that an advantage would be gained by a wing built telescopically so as to have an extension to push out at the tip during the take-off and landing. Patents have been taken out for a wing of this sort and at least one has been built. The Fowler wing, developed in America, which combines the advantages of variable wing area with those of the trailing edge flap, has been successfully used on recent aeroplanes, notably the Lockheed 14 transport monoplane.
The Tricycle Undercarriage
Another problem which has been attacked is that of designing an airscrew to deliver more power from the engine. An ordinary airscrew will be efficient over a relatively narrow range of speeds only. If it is designed for an aeroplane having a top speed of, say, 100 miles an hour, it will still be reasonably efficient for taking off the ground at 50 miles an hour. If, however, the top speed is 300 miles an hour, the airscrew will be ineffective at the take-off. It will tend to whirl round without developing much thrust at low speeds where thrust is most necessary. Now, however, variable pitch airscrews are being made in large quantities. They have blades which can twist in the hub and adjust themselves automatically to the forward speed of the aeroplane. They are much more efficient at the takeoff speed than were the older fixed wooden airscrews, and they allow higher loadings to be lifted from the ground.
If still higher loads per square foot of wing surface can be lifted into the air by means of variable pitch airscrews and wing flaps, the aeroplane has still to be put back on a ground at a landing speed which is not impossibly high. The airscrew will not help here, as the engine is throttled right back. We do get help from the flaps and also a little from the fact that some of the load in the form of petrol will probably have been used up. This problem has been helped by the resurrection of an old forgotten type of undercarriage not used since the earliest days of flying - the tricycle undercarriage.
For many years it has been the universal practice to put the two main landing wheels forward of the wing and to have a small skid or wheel at the tail end. This arrangement was satisfactory with low landing speeds, but if the landing speed were high and the wheel brakes were applied too soon the machine would tip up on its nose, to the great discomfort - or worse - of the occupants. In the tricycle undercarriage the two main wheels are put under the trailing edge of the wing, behind the centre of gravity, and there is a third robust wheel at the nose. However hard the pilot applies the brakes on the two main wheels, he cannot somersault such an aeroplane. It can therefore be brought in at a higher speed (that is, with a higher wing loading) without risk. With such an undercarriage it is, moreover, possible to accelerate more quickly for taking off.
THREE-BLADED VARIABLE-PITCH AIRSCREWS of a Short Empire flying boat built for Imperial Airways. The blades may be rotated in pitch through a considerable range so that they can be adjusted to the speed of the aircraft. They can thus be made more efficient at the relatively slow speed of the take-off.
Further aids to efficiency may now be considered. Some of these are still in the early stages of experiment, others are well tried.
The oldest of these is the catapult, as fitted to many of the battleships and cruisers of the Royal Navy. These ships, unlike the large aircraft carriers, have no flying deck from which an aeroplane can take off.
No wing loading is low enough to allow aircraft to fly away from the restricted space available in a ship. If, however, the aeroplane is loaded on to a catapult fired by a cordite charge, which will accelerate it to its flying speed within the twenty yards’ length of the catapult, then the take-off is easy. It is true that so rapid an acceleration is not comfortable, but comfort is not regarded as being of the first importance to a naval pilot.
The catapult retracts from its twenty yards’ length into a comparatively small space when not in use. The aeroplane cannot land back on the catapult, and hence the aircraft used for this work are always fitted with floats. They may then alight on the water alongside the warship and be hoisted on board by means of a crane.
Such catapults are of necessity short and the acceleration is therefore great, but longer catapults with more comfortable accelerations may be used either on land or on water for commercial purposes. By using a catapult of a hundred yards’ length the acceleration may be reduced so that it would not be greater than that experienced in normal flying manoeuvres. An aeroplane will remain air-borne with a much higher wing loading than it can normally take off the ground. If the take-off can be aided by a gentle acceleration on a long catapult it is possible to envisage much higher loadings than any at present used. If the aeroplane is a flying boat used on long-range work with a large proportion of the load in the form of consumable fuel, then there should be no great difficulty in making it alight on such sheltered water as is to be found at most big seaports in Great Britain and abroad.
Refuelling in the Air
An alternative scheme which is passing from experiment to the stage of practical operation is known as refuelling. Originally this was used on record-breaking flights. Normal aeroplanes were refuelled by means of long hosepipes from other machines flying over them and they were able in this way to remain in the air for days at a time.
The economic value of the idea has now been realized. If a machine is loaded with passengers, freight and a small quantity of fuel up to the take-off limit and filled up with petrol from a flying tanker immediately it has reached a convenient height, its range (or earning capacity) is greatly increased. Without refuelling it is scarcely possible to cross the Atlantic or Pacific with a revenue-paying load. With its help, such crossings become profitable. Another method of making long transoceanic crossings profitable has been developed by Major R. H. Mayo in his composite aircraft.
It is dangerous to attempt prophecy in such a rapidly developing science as aeronautics. So many research workers are engaged on so many different problems that their findings alter the position almost daily.
With the data at present available and assuming that no revolutionary discoveries are made in the next few years, it may be prophesied that the future of long-range ocean transport depends on the use of flying boats. These will be multi-engined monoplanes fitted with flaps and variable pitch airscrews, working at a constant speed. They will be loaded at the take-off, which will be assisted by some form of catapult mounted on a long barge, to over 50 lb. per square foot.
When the flying boats are in the air they will be fuelled up during the first two hours of their journey, to over 70 lb. per square foot. Cruising at over 300 miles an hour, they will fly from Southampton to New York between dawn and sunset, and they will do this on a profit-earning basis.
LONG-RANGE BIPLANE of the Handley Page type V/1500, an early type designed for carrying heavy loads over long distances. Wing research has caused this type of aircraft to be largely superseded by the tapered wing monoplane, in which all the supporting structure is carried within the wing, thus eliminating the need for external bracing.
For many years it has been the universal practice to put the two main landing wheels forward of the wing and to have a small skid or wheel at the tail end. This arrangement was satisfactory with low landing speeds, but if the landing speed were high and the wheel brakes were applied too soon the machine would tip up on its nose, to the great discomfort -