FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1935
1935 - 0881.PDF
APRIL 18, 1935- FLIGHT. 425 etc., which is of considerable resistance, and it would not produce a proportionate increase of speed to leave this alone while reducing too much those parts of the aeroplane con cerned in lifting the load. It has, however, been found in practice that heavy loadings and high speeds are practicable up to a considerable degree for twin-engine machines. The gradual working up of traffic without a basis of fixed contract loads or other Government assistance is a slow process which has developed further in this country than elsewhere. High-speed transport must take a long time to develop under such conditions, and operators cannot be expected to embark on the larger capital cost necessary to get high speed without being able to foresee the traffic which would follow. Variable-pitch Propellers Variable-pitch propellers have an overwhelming influence on the possibility of making fast, economical aeroplanes. The propellers of slow machines are working under tolerably good conditions during take-off. The blades are not stalled and the r.p.m. are not unduly low. As the design speed increases the blade angles have to be made relatively coarser to deal with the cruising speed, and this leads to stalled blades and low r.p.m. during take-off. Variable pitch enables these angles to be reduced so that, full r.p.m. and efficient blade angles may be used during take-off. Were it not for this, 50 per cent, of paying load might have to be omitted in high-speed machines to secure the necessary take-off, apart altogether from the difficulty of getting engines to stand up to low r p m and full throttle. Supercharging It is necessary and legitimate to take more power from an engine during the short period of take-off than tor cruising. If the cruising power were 70 per cent, of the take-off power, full throttle could be used at 10,000 ft. for cruising and reduced throttle below that height, but up to 10,000 ft., in this case, there would be no use for a supercharger unless it was necessary to take-off from high altitude aerodromes. By suitably arranging the compression ratio, etc., eugines can be "ground boosted" up to greater powers than the same engine without a blower. This is equivalent to instilling a more powerful engine, but a lighter way of doing it. A super charger which did not enable maximum power to be developed for take-off would be of limited use commercially. "Comet" as an Illustration The "Comet's" engines are of a relatively heavy commer cial type. When fitted with -V,P. propellers, air pumps tor instruments, etc., they weigh 2.3 lb. per h.p. The other things tending towards weight are the flaps, retractable under carriage, and the fact that the wing is a full cantilever of small thickness. These are, however, overbalanced by its small dimensions and high loading, both of which permit of a low structure weight (26^ per cent.) which, together with its speed, give it the extremely high value of 9.6 ton-miles per gallon for 500 miles range, at its operating height of 10,000 ft. If the machine was scaled up to four times the power—still quite moderate—the fuselage would become a commodious •m.. ORAQON l-45O0lbs. 2-4200 IM(»40) BH.P.Mw 10 5CYU.& 2-PERSCUS l-DRA&ONRW0E(t«0') 2-OH 86 (f*5') I-ENVOY 2-VICEROY ? ' Screen DOUGLAS D.C.2A BOEING 241 0 , COMET //EOKKER F36 '/ .YULTEE VI A ^-- F0K.KERf.20 MEINKEL Hi. TO _. _ , rIG.I. 1« 20 30 40 , |i WHERE S • EQUIVALENT MONOPLANE SPAN MAX 5BEE0 AT SEA LEVEL -HPH IS) 270~ & • • MONOPLANES (Mostly Canhltner) • <_ • BIPLANES JT & HAWK MOTH \ HERCULES ^GOUfiE J00OO01& aOATfESTIMATE) DH.6I DRAGOfc DH 86 TIGER MOTH FIG.4 KAPIDE ~ 10 "t SPAN-* AREA \'-? - \FACTORED WEIGHT/'. ' Notes on the Above Diagrams FIG. 1.—The curve Marked F.P. shows a relation between span and power loadings which determines the weight at which an aeroplane fitted with fixed pitch airscrews will com ply with the British and I.C.A.N. normal take-off require ments (66 ft. height attained after 656 vards from rest). The curve marked V.P. shows the same thing when fitted with variable pitch airscrew. Aeroplanes whose plotted points lie above the appropriate reference line would experience more difficulty in complying than those below, conversely, those below might be expected to clear the screen by some margin; in such cases the excess height above the screen, where known, is indicated in brac kets after the name of the aircraft. The curves are purely general, and only take account of me most important factors influencing take-off and climb, tor instance, aerodynamic cleanness would have some influ ence on the position of the. reference lines. In general aeroplanes whose plotted points lie toward the right-hand side of the figure would be fast; and those to the left, slow. The effect of high top speed on airscrew charac teristics at take-off speed in their relation to take-off has been included in the curves. FIG. 2.—Speed depends closely on frontal area per h.p. for a given degree of cleanness. Span is here used as a measure of frontal area. This is particularly convenient because span also determines fa:rly closely the w./h.p. which can be lifted over the screen. This figure is thus linked with Fig. 1 through span and h.p. FIG. 3.—Same as Fig. 2, but illustrates the applicability of span and h.p. as a measure of speed over a wide range of speed, and for widely differing types of aeroplanes. FIG. 4.—Variation of structure weight with size and load ing. The curses are strictly speaking empirical, being based on a large fund of well attested data for widely different types of aircraft. They are also in close agreement with theoretical laws for various types of structural members. Figs. 1, 2 and 4 embrace the three most fundamental and interpendent factors in tt'e problem under discussion.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
