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Aviation History
1945
1945 - 0067.PDF
JANUARY IITH, 1945 FLIGHT 3.1 (7) As the density of a heavily loaded high-aspect-ratio wing will be high, only a small proportion of the petrol load will be accommodated in the wings. If we start by reviewing the physical bulk of the main items that have to be carried, we can discuss the best basic outline of a machine to contain them. Assuming for a moment that our aircraft will be able to cruise at 250 m.p.h. on 1,300 h.p., it will need an endurance of 16 hours at that speed, plus a fighting allow- ance of, say, one hour, at nearly full"throttle. That means about 1,500 gallons of petrol. If 600 gallons for the out- ward journey can be carried Ur some form of jettison tank, and 100 gallons can be accommodated in the wings, that leaves 800 gallons to be carried in the fuselage or nacelles, requiring about 130 cubic feet. Possible Layouts Two engines of, say, 2,500 h.p. for take-off, will require about 60 cubic feet each. A crew of two will need at least another 60 cubic feet. These items can be arranged in several possible forms: — (1) The conventional twin nacelle and fuselage com- bination. (2) A central nacelle and twin booms, as in the Lightning. (3) With the engines buried in the fuselage and shaft drive to the nose, as in the Airacobra, or to the tail. (4) With the engines in the fuselage and right-angle shaft drive to the airscrews in front of/t>r behind either wing, as in Major Seversky's, and several other projected long-range fighters. (5) As an asymmetrical combination of a nacelle and pusher airscrew, and tractor airscrew and fuselage, as in the accompanying illustrations. Layout (1), though accepted as the orthodox form, is particularly unsuitable in this case because, as already pointed out, to obtain the necessary economy in fuel con- sumption, provision must be made for cruising on one engine ,0nly. The large yawing couple produced when a conyieritional twin is running on one engine would con- siderably reduce the cruising efficiency. In any case, this arrangement has many inherent disadvantages, of which the turbulent flow from the necessarily short nacelles and the destruction of laminar flow over a great part of the wing are important factors in the present design. No. (2) eliminates turbulent flow from the nacelles to the tail, but is otherwise no better than (1). No. (3) has attractions, but in the present instance the power would appear to be too great to be absorbed by one airscrew unit. If a five-bladed airscrew is necessary to absorb 2,000 h.p. at great heights, then for 4,000 h.p. one would have to visualise a ten-bladed contra-prop. After the air had passed through that, it would probably be resolved into its constituents of hydrogen, oxygen and nitrogen, and though this might have surprjamg effects on the boundary layer, it probably would not give very much traction. No. (4) is the best of those considered so far, and has generally been accepted as the form for any long-range fighters or fast bombers of the future. For cruising, one engine can turn both airscrews. To keep the centre of gravity right it will normally be necessary to make the airscrews tractors rather than pushers, so that although the nacelle bogey is removed, the slipstream will still destroy laminar flow over much of the wing. In the case of a very-long-range fighter such as we are discussing, however, this arrangement does not fit in so well because of the sheer bulk of fuel and engines to be accommodated. As already laid down in our basic assumptions, weights must be concentrated as near the centre of gravity as pos- sible. Now, to get ali our weights together near the e.g. in one fuselage would mean piling engines and petrol tanks on top of one another, which is obviously impossible. Stringing them out along the fuselage, as would have to be done (see diagram), would be bad for manoeuvrability, and, carried to excess, bad for stability. Is there a better solution? I venture to suggest that the asymmetrical arrangement provides one possible answer. Weight dispersal : This diagram shows, in somewhat ex-aggerated form, the heavy items distributed along the fuselage, an arrangement which gives rise to large momentsof inertia and therefor.' reduces manoeuvrability. A combination of a nacelle and "pusher," with a tractor airscrew and fuselage, offers the following advantages: — (1) Best possible concentration and balance of weight. (2) Yawing couple when crilising on one.engine materi- ally reduced, as compared with the conventional twin. (3) No shaft drive required, except a short extension on the pusher. (4) When cruising on the pusher engine only, there is no interruption of the air flow over the wings. (5) Wetted area and structure weight at the minimum. (6) Turbulent flow from short nacelles eliminated. The main features of the suggested layout are obvious from the diagram. Although there is probably no par- ticular aerodynamic advantage in wing radiators, fitting them to the leading edge of the centre section would further the cause of concentration of weight. The fuel load is split between the nacelle and the fuselage, the greater part being in the fuselage. The nose of the nacelle provides a convenient receptacle for the armament, which could be made up from snch combinations of cannon and machine-guns as tactical con- siderations require at the time. The pilots can be accom- modated in a side-by-side pressure cabin behind the guns. The Specification By careful design, perfect balance between thrust and drag could be obtained without needing any correcting force from the rudder. When running on one engine, with the other airscrew feathered, correction should be well within the limits of the trimming tab. Assuming internal capacity for 900 gallons of fuel, the remaining 600 gallons to be carried by methods discussed later, I think that the following rough specification could be filled by an aircraft of this type: — Take-off power .. . . 2,500 h.p. per engine. Span .. .. .. 74ft. Length 37ft. ' Aspect ratio .. ..11. Wing area. .. .. . . 50a sq. ft. Tare weight .. .. 20,000 lb. Gross weight (take off) 29,000 lb. Wing loading (take-off) .. 58 lb./sq. ft. Power loading (take-off) . . 5.8 lb./h.p. Max. speed .. .. 450 m.p.h. at 20,000ft. Absolute range . . .. 5,000 miles. Consideration of how best to carry the fuel for the out- ward journey suggests the employment of a glider con- sisting virtually of a streamlined fuel tank with wings and tail surfaces. It would be towed by the fighter, the tow rope forming a pipe-line. This glider would weigh about 5,000 lb. loaded, with a high-aspect-ratio wing of lighter loading than that of the tighter. At take-off it would become airborne first, and slightly assist the take-off of the larger machine. With the layout suggested, it would be necessary to cruise on the tractor engine whilst towing the glider. The offset drag of the glider would then balance the drag of the asymmetrical nacelle. The pusher airscrew would be used on the homeward journey. [The notion that, because its wing loading is lower than that of the fighter, the glider would help to take-off is intriguing, but one rather doubts its validity. Even on the assumption that the tow-rope, in this case the pipeline, is pointing up at some 45 deg., the engine of the fighter, via its airscrew, still has to overcome the drag of the glider, which is added to the drag of the fighter.—ED.]
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