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Aviation History
1941
1941 - 0686.PDF
226 MARCH 2OTH, 1941. THE SLIP-WING BOMBER (Continued) specification laid down by Capt. Macmillan for 5,000 miles, five tons, 400 m.p.h., a range which would not be required for the job of bombing Germany, which is the only job that matters in this particular war. For'the latter purpose I am in favour of a much smaller 1ype of bomber, and the slip-wing scheme is, I feel, most suitable for the development of a small high-speed bomber with about 250 sq. ft. of wing, and thus requiring a slip- wing with about 300 sq. ft. of wing, and weighing about 1,400 lb., which "is, of course, an easy proposition to hoist into position. 'The post-war' development of commercial flying will equally require high wing loadings to make possible economical large aeroplanes. In such cases, when the pro- portions of both components become very great, it will be necessary to provide take-off bases with a gallows or ramp so that the slip-wing can be wheeled into position. But in the case of the small slip-wings I recommend for war purposes, they could be man-handled if necessary. The P.B. 49 In view of the fact that it was little more than a " sketch design," detail criticisms of the particular lower compo- nent illustrated are worth considering only in so far as they may apply to the small high-speed bomber in general. An early contributor raised the question of the propor- tion of the undercarriage weight to the gross weight at take off, but this has already been answered by a later letter from a more thoughtful reader, pointing out that, as tin loads encountered in take-off should not be as great as those incurred in landing, it is permissible to use reduced factors for take-off where a large " droppable " or "burn- able " load is carried. That this is officially recognised and&pproved is illustrated by the use of overload take-offs with existing bombers, in which case the undercarriage weight automatically becomes a smaller proportion of the new gross weight. In the particular case of the P.B.49, the undercarriage weight is 4.2 per cent, of the combined weight at take-off, and more than 12 per cent, of the landing weight, compared with 6.5 per cent, of the landing weight for the D.C.4, which, being a very large machine, is not in any case a fair comparison. The Need for Assisted Take-off To those who doubt the value of the slip-wing scheme, 1 do not think any argument could be more convincing than to compare the figures for P.B.49 with those sent in by Mr. John W. Morrison in Flight of December 5th, 1940, for a bomber to achieve a similar performance withbut assisted take-off. The slip-wing bomber has 400 sq. ft. of wing, and a gross weight of 41,460 lb. It requires about 1,900 gallons of fuel for 5,000 miles, and has altogether 4,000 h.p. The "Bomber A '' suggested by Mr. Morrison has 3.900 sq. ft. of wing, and weighs 190 400 lb., or 85 tons, and requires 20,000 h.p. and 52,500 lb. of fuel. Even these figures are attained only by assuming for "Bomber A" a fuel consumption far b£low the best at present achieved, and an overall L/D of 24, which pre- supposes a miraculously streamlined design. On a basis of first cost and production time, the slip-wing is ^200,000, or eight times cheaper, and on operating costs it is about ^4,500 cheaper per flight, or twelve times, on fuel alone, without consideration of the oil at about 7s. 6d. per gallon, or the crew of ten as compared with three. The unassisted take-off aeroplane would charge altogether, taking 5 per cent, as the War Risk Insurance, about £20,000 to deliver its five tons of'Dombs. compared with about £1,000 for the slip-wing bomber; and, although there are no bombers so large at present in service, the relative advantage of the slip-wing is approximately the same in all classes. The Buried Engine There is insufficient space in this article to consider in any detail the numerous problems associated with the buried engine and remote drive, but this is a question which I have dealt with very fully in the book, The Aeroplane of To-morrow, mentioned above. There cannot be any argument about whether the submerged engine is ' desir- able " or not, because one just cannot attain very high speeds without it. The critical speed for a radial engine nacelle at sea level is about 400 miles per hour, compared with over 600 for an elliptically nosed fuselage, and tractor airscrews cannot be used at more than about 500 miles per hour. But even for slower aeroplanes the buried engine is an inevitable development. The disadvantages of outboard engine nacelles are legion. The principal among them can be mentioned :— (1) Effect on airscrew of large obstruction immediately behind it. (2) Slipstream drag. (3) Drag of nacelles is usually high because their bluff shape necessitated by the short wing chord causes imme- diate separation of flow and turbulence. (4) Turbulent boundary layer round nacelle causes early transition point on wing, and thus increases the drag over a considerable portion of the span. (5) Elliptical lift distribution is upset, causing an increase in induced drag. (6) For similar reasons, nacelles cause premature stall- ing of wing. (7) Large outboard weights have a bad effect on stability and control. (8) Gyroscopic loads due to both airscrews rotating in same direction. (9) In single-geared flight, thrust from one engine and airscrew drag from the other cause large yawing couple. Feathering airscrews alleviate, but do not eliminate, this trouble. (10) Nacelles upset flow over tail and cause large pitch- ing moments. (11) Considerable portion of wing area is lost. (12) Large interference drag between nacelle and wing. (13) Bad weight distribution may hinder, or even pre- vent, recovering from a spin. These effects are relative but real. Increased wing load- ing reduces the size of the wing relative to the nacelle, and thus greatly accentuates these bad features, so that the buried engine is an essential accompaniment to high- wing loading. To weigh against these disadvantages one can say in favour of outboard nacelles that they can be used to "mass balance" the wing where the flutter criterion is high, and provide "convenient receptacles " for the un carriage, which is rather like trying to get a murderer acquitted on the ground that he killed his victim in the most economical manner that he could. The Slip-wing Fighter One correspondent has ventured some criticisms of the slip-wing fighter which rather suggest that he did not read the whole of the original article In that I amplified the two points which are raised—that of the effect of the weight of the upper component on the undercarriage of the fighter, and that of the position of the centre of gravity as a composite. I showed that the weight of the slip-wing did not reduce the factors on the fighter undercarriage below those nor- mally permissible for overload take-offs, and, in view of the fact that the loads on landing, when the two compo- nents are separate, are greater than at take-off, this is not dangerous. The relatively low weight of the slip-wing makes the centre of gravity, as a composite biplane, low, and the mean thrust is also low. An engine of T50 h.p. was sug- gested for the upper component, not 500 h.p., as quoted by a later contributor. This correspondent, and, indeed, other people to whom I have mentioned it, are enthusiastic about the idea, also contained in the article, of dropping flares to light up the enemy raiders, whether or not the slip-wing is adopted.
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