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Aviation History
1946
1946 - 0554.PDF
284 FLIGHT MARCH 2IST, 1946 Jet Propulsion and >lililnrv Aircraft Abundance of Thrust Can be Used Effectively for Climb By W. G. CARTER F.R.Ae.S. 20 a. 15 < ujcr z- UJ O UJ a THE association of gas turbines with aircraft designedto make effective use of the special characteristicsof jet propulsion has brought the trend of develop- ment into a state of transition and uneasy experiment. Already speeds equivalent to a low-level Mach number of 0.80 have been recorded under observed conditions. This has been done by a standard twin-jet fighter not specially designed to accomplish speeds exceeding a low- level Mach number of 0.66. The general handling quali- ties of the aircraft have been good, and although changes in the aerodynamic characteristics are just beginning to be noticeable, these are regarded as moderate in effect and generally in line with predictions which had been made. It may, therefore, be possible to make more advanced explorations into the transonic region with similar twin-jet installations. The percentage increase in drag over the low-speed value, as shown by Fig. 1, gives a clear indication that the prospects are there if successfully exploited. The curve is not necessarily an accu- rate representation, as the available thrust has yet to be calibrated in flight. Up to a Mach number of 0.825 it may, however, be regarded as reasonably accurate, and the ex- trapolation beyond this figure is the best guess that can be made at the present time. This curve provides a clear indica- tion that the twin-jet arrangement belies somewhat the first impression that, because of wing nacelles, it may not be so favourable for speed development as compared with its counterpart, the single-jet fighter. The low-speed drag value of the twin will obviously be higher, perhaps as much as 50 per cent higher, than the single-jet machine designed to satisfy the same general operational requirements. But per- centage increase in drag is the critical factor, and experi- ence is beginning to show that wing nacelles do not greatly influence the rise in drag due to compressibility effects. Like the fuselage, they are three-dimensional bodies and, if precautions are observed to harmonize the associated air- flow characteristics of wing and nacelle, there seems to lie no reason to suppose that flight velocities exceeding sonic conditions could not be accomplished with engines on the wings. Nacelle Effects The main concern with the twin-jet arrangement is to minimise the severity of buffeting effects due to turbulence. Substantial improvements in this direction have been made by lengthening the nacelles of the Gloster Meteor, a feature combining to improve their fineness ratio and separate more widely peak suctions of wing and nacelle as separate items. This, at one step, put the limiting Mach number up to 0.84, whereas unfavourable effects had been recorded with, short nacelles at a Mach number of 0.75. No doubt a further increase of Mach number could be achieved by lengthening the nacelles another stage, at the same time improving their shape so as to delay shock-wave effects. 0-65 O-7O 0-75 MACH Fig. 1. Increase of drag with Machnumber. The dotted portion is the best extrapolation that can be made at present. th It would be a great advantage if research along these lines could be explored in the wind tunnel, but at high Mach number it is impracticable to do these tests except by full-scale flight. The work done at the R.A.E., which was effective up to a Mach number of 0.80, was, how- ever, of incalculable value in providing information with- out which the process of designing the Gloster E28 and the Meteor could hardly have proceeded on a satisfactory basis. - Housing the Thrust With engines installed on the wings, it may be possible to accommodate more thrust in terms of all-up weight than could be done with the single-jet fighter; or a three- engined arrangement could be considered to get even better results in improving the thrust /weight ratio. This over- abundance of thrust, so far as fighter types are concerned, is required for climbing performance rather than for speed. As an indica- tion of the trend in this direction, the Gloster Meteor now has a com- bined thrust of 7,000 1b. At one time 4,000 lb thrust was looked upon as a reasonable provision for aircraft of this type, and a corresponding figure of 3,000 lb good enough for the •single-jet machine. Now the outlook is that, however much thrust can be crammed in, it is unlikely to be more than can be used effectively, especi- ally for climbing performance to great altitudes in the shortest pos- sible time. This has become neces- sary in order to intercept bomber or reconnaissance types which, with jet engines, will be able to cruise at twice the speed and twice the height that has hitherto been possible. As climb- ing characteristics depend on the thrust/weight value of the aircraft, present trend of development will mostly be concerned with giving this the highest possible value. The outlook is that this can best be done by having two engines instead of one, but before this premise can be firmly established it will be necessary to provide jet engines giving much more thrust per unit frontal area than is obtainable from those having centrifugal compressors. These are in- evitably bulky in terms of diameter as compared with the axial, and there is no room for corpulence in the jet fighter whether it is propelled by one or more jets. The Rolls- Royce Nene engine, rated at 4.500 lb thrust, is already outside the diameter limit for fighter-aircraft nacelles, so that the most significant comparison which can be made between various engines concerns their respective thrust value in terms of frontal area more than any other installa- tional feature. In making these comparisons, there is not much to go on at the present time. The background is scanty enough with centrifugal engines, but with those having axial-flow compressors it is almost non-existent. The first, and so far the only, axials produced in this country were those designed by Metro-Vickers and flown in one of the Meteor prototypes. As the first British experimental type it could be compared, on the basis of performance, with the Whittle • / ' • t t / / // • 0-80 O-85 NUMBER O-90
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