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A study of the trend of development in aeroplane construction renders it possible to predict with some degree of certainty the leading characteristics of the aeroplane of the future, and especially of that type which will be built purely for racing purposes. The following is an attempted study along these lines. The writer has no wish to assume the role of prophet, and the accompanying drawings and description are based mainly upon a survey of the work which has been done during the present year of phenomenal development by the designer, the constructor, and the airman. The keynote of this development is to be found in the fact that the speed of the aeroplane in straight-away flight has risen during the past year from fifty to seventy-five miles an hour, and that the "blue ribbon" of the air (if we may borrow a nautical phrase) has passed from the biplane to the monoplane, the type which, at the present writing, is so far in the lead, as to speed, lightness, and stability, that it stands in a class by itself.
The racer of the future, then, will be a monoplane; and in narrowing down to this type, man is but treading in the footsteps of Nature, the master builder, who, working through a process of evolution that has stretched out over millions of years, has produced, .in that wonderful bird, the albatross, the perfect flying machine. The future high-speed flyer, then, will possess the same tapering, rounded body and the widespread wings of narrow width which characterize the swiftest of the birds. Langley, in his classic researches, showed that it was the leading portion of a plane which was the most efficient, and this for the reason that it was continually moving on to fresh bodies of undisturbed air. He showed that as the after portion of the plane had to do its work upon air which had already received from the forward portion a downward velocity, this air was unable to provide the effective reaction which was exerted by air absolutely inert. Hence it follows that a plane 5 feet in width by 10 feet in length would be rendered more efficient if it were divided longitudinally, and the same area were presented in a plane 2-1/2 feet wide and 20 feet in length. The wings of the racer will be long and narrow; and when they come to be built of metal instead of the present wood and fabric, it will be possible to give them those sweeping, rounded forms which prevent eddy-making, and also, from a constructional point of view, add not a little to the strength. The body will be of a generally circular or oval section; and to allow of a long and gradual taper, for ease in traversing the air, the body will have considerable length. The greater length also will add greatly to the fore-and-aft stability in flight.
The present wood-canvas-and-wire construction will have to go. It is makeshift work at the best, and was adopted because, in the early days of experiment, it offered a cheap and light combination of material, and one which, in the event of the inevitable breakages, incidental to experimental work, could be cheaply and quickly repaired, Its place will be taken by some one of the many remarkable alloys of steel which are now available-metals of enormous strength and toughness in proportion to their weight. The use of these, coupled with careful designing by the skilled engineer, will make it possible to produce an aeroplane of much greater strength, that will weigh no more than the present machine, and will present far less resistance.
The principal resistances encountered by an aeroplane when in flight are those due to the lift and the head surface. The resistance due to the lift is fairly constant; for as the speed increases the angle of incidence decreases, and there is always an adjustment between the two which provides sufficient vertical reaction at all times to lift the weight of 500 to 1,000 pounds, as the case may be. The head resistance, however, increases approximately as the square of the speed; and if it is 100 pounds, say, at 40 miles per hour, it will rise to 400 pounds at 80 miles per hour. Hence the great importance, in a racing machine, of reducing the head surface to the least possible limit consistent with structural requirements.
It is this consideration of head resistance which has doomed the biplane as a purely racing type. When Octave Chanute built the first biplane glider, with its light, but very rigid Pratt trussing of vertical wooden struts and diagonal wire ties, he produced an excellent piece of engineering construction, which has proved to be ideally adapted to the early experimental stage which is now drawing to its close; but for high-speed results, because of the large amount of head surface presented, the Pratt truss was doomed to ultimate extinction. Unquestionably, the higher speeds which have been attained by the monoplane are due largely to the fact that its trussing is simpler, and the head surface, particularly of the wire stays, is relatively much less.
Now a word as to the resistance offered by a mass of tightly-strung wires, Prof. Langley showed in his whirling-table experiments that the resistance of a wire is much greater than that which would be due to its projected area. As the speed increased, the resistance would rise rather quickly until, at a certain critical point, at which the wire would sing with a peculiar note, there was a sudden and very large jump in the resistance. This is explained by the fact that the rate of vibration of the wire under the rush of air is so great that it practically presents a solid surface, whose width is equal to the amplitude of vibration. Hence a tightly-strung wire presents a resistance to the air which is seemingly out of all proportion' to its actual surface.
It follows, then, that even the simple king-pin trussing of the Blériot and Antoinette types must go if we are to achieve the high speeds which are predicted for the future racing machine. Now this will be possible only if some high-grade sheet metal is substituted for the canvas of the wing surface, and the necessary transverse bending strength. is secured by means of plate-steel members inclosed within the wing surfaces and strongly riveted to the structure of the main body of the machine.
The form of wing shown in our drawings will afford a sufficiently strong construction in metal. The wings should widen considerably as they approach the body; for this would provide increasing space between the upper and under surfaces, and allow the depth of the channels to be increased proportionately to the bending stresses. These channels. would be carried into the main body and riveted to transverse diaphragms, which should be so cut that the metal of the diaphragm would extend unbroken for some distance into the wings. We are convinced that by careful designing, the selection of the highest grades of steel, and by first-class workmanship, it will be possible to provide wings of ample strength without exceeding the limit of weight imposed by aeroplane requirements. Buckling in the fore and aft direction will be provided against by rolling the metal of the wing surfaces with shallow corrugations, as shown in the drawings. The main body of the aeroplane will be built also of thin sheet metal, and will be generally elliptical in cross section; two very light trusses, one horizontal; the other vertical, extending from the operator to near the tail. The chords of these trusses will be formed of light T-iron.
To provide for the heavy loads and stresses that are concentrated at the wings and motor, the T-irons of the trusses will be increased in depth and run entirely around the forward end of the body, forming, at their intersection in the nose, a strong construction for carrying the motor. Additional strength will be provided by transverse diaphragms. It is understood, of course, that all of this metal work will be specially rolled in extremely light sections, and that the material will be some alloy, such as vanadium steel, which in experimental specimens, as noted on our editorial page, has shown elastic limits running up to over 200,000 pounds to the square inch. The motor, of from 75 to 150 horse-power, according to the size of the aeroplane, will probably be of the revolving type; the Gnome motor having shown itself to be the ideal aeroplane drive.
Turning to nature for guidance again, we find that the fast-flying birds fold their legs snugly beneath them when in flight. The racing aeroplane must do the same. We show a suggested arrangement for a folding chassis, hinged just below the body, and provided with a yoke which leads from the axle up to the crosshead of a piston rod, which with the guides and cylinder is carried by the T-iron that forms the bottom member of the vertical truss. The cylinder is provided with a two-way valve and connections, by which compressed air can be introduced to the forward or after end of the cylinder. When the chassis is down and in operation, the compressed air acts as a cushion to provide a certain amount of fore and aft movement to the wheels. As soon as the machine rises, a throw of the valve introduces compressed air at the forward end of the cylinder, and the chassis is drawn up snugly against the body. A small tank of compressed air, which supplies the folding mechanism, also supplies a small cylinder of similar construction placed transversely to the car, which operates the movable wing tips. The two-way valve of this cylinder is controlled by a small gyroscope, which may be thrown out of gear when the airman wishes to make a turn, or perform other evolutions.
In answer to the question as to what speed may be expected from a machine of this general design, we think it will be agreed that, in view of its sweetness of form, the complete absence of wires, struts and other energy-consuming surfaces, and the fact that because of the smoothness of the steel surface, skin friction will be reduced to a minimum-it is conservative to expect from such a machine, after it has been developed by experimental work, speeds of from 100 to 125 miles an hour.