FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1954
1954 - 1582.PDF
FLIGHT, 28 May 1954 701 POWER FOR AIR TRANSPORT The Choice of Engines: Dr. Russell's Wright Memorial Lecture Part II LAST week we printed abstracts from the opening sec tions of the notable paper by Dr. A. E. Russell, D.Sc, * F.R.Ae.S., F.I.A.S., which, with the title The Choice of Power Units for Civil Aeroplanes, formed the 42nd Wilbur Wright Memorial Lecture, given at the Royal Insti tution before members of the Royal Aeronautical Society. In the further abstracts which follow, the lecturer compares, in various characteristics, aircraft fitted with the contrasting types of power unit which he reviewed in his introductory passages. Dr. Russell is a director, and chief designer, of the aircraft division of the Bristol Aeroplane Co., Ltd. Cruising Economy.—Much had been written by various authorities on cruise procedures suitable for high-performance turbine-engined aircraft, the most well-known factor to obtain fuel economy being the need to fly at high altitude. So the assumption that all suitable aircraft would cruise near, or above, the stratosphere needed no justification. This was a conclusion of some convenience because of the resulting simple relationships which governed the variation of engine power and fuel flow with altitude at constant true air speed and turbine r.p.m. Con sequently, simple relationships could also be established for the aircraft and the factors in these equations permitted a fair under standing of the cruise economy problem. The aerodynamic gains theoretically possible due to increasing aspect-ratio were not fully realized, because of a limitation on the maximum usable value of lift coefficient. It could be shown that for a given value of profile drag, if the same ratio of cruising speed to minimum drag speed was maintained, increased aspect-ratio implied a higher value of cruising lift coefficient. A finite limit was therefore imposed by the need to provide an adequate margin to cover manoeuvres and turbulence without fear of buffeting or loss of control. For a fixed value of CDO, benefit was derived from increase of aspect-ratio despite the fact that this resulted in cruising farther away from the minimum drag condition. In addition, the improvement in L/D resulting from decreased profile drag and increased cruise lift coefficient was well marked. For a fixed body size, CDO was reduced by increasing the wing area. If the wing area was very large with a resulting low value of CDO, high values of L/D could still be achieved when the aspect-ratio was as low as 3 and the aircraft cruised at, or near, minimum drag speed. These were the conditions appropriate to delta wing configurations. The foregoing considerations dealt solely with the aerodynamic standpoint and certain of the conclusions needed qualification when structural effects were taken into account. For example, choice of aspect-ratio was governed by balancing the gains arising from higher lift/drag ratio against greater structural weight. The most important features which determined wing weight were aspect-ratio, thickness/chord ratio, angle of sweep and wing loading. At first glance it appeared that a higher aspect-ratio could be adopted on a straight wing than on a swept wing, for in the latter case the structural span was increased by the sweep. However, the different permissible thickness/chord ratios within the limitation imposed by the drag rise tended to reduce this effect. This quality, however, could not be isolated from other important factors; notably wing loading, the use of high values Fig. 5. Variation of initial cruising alti tude with wing load ing, and with wing loading x power load ing. This product is to a first approxima tion a measure of take-off distance. always being advantageous pardy by reason of an enhanced capacity to meet aero-elastic requirements. Another significant effect from another cause, and which applied to all types of aircraft, was associated with the weight and distribution of the fuel carried. Structural strength in the flight cases was largely determined by the "tanks empty" condition. As a corollary, it was frequendy found possible to increase the gross weight of an aeroplane with relatively minor structural altera tions when such increase corresponded to a greater fuel load carried within the outer wing box. To take all these effects into account when choosing each aspect- ratio was clearly an involved procedure. Fortunately, both experi ence and detailed analysis made elsewhere showed that, for the main purpose, precision in this instance was unnecessary, because the optimum value for a prescribed wing planform was not in fact sharply critical. It was only in the final stages of refinement that a second approximation for the best value of aspect-ratio became desirable. So, in the interests of simplification, for wings with approxi mately 35 deg of sweep, an aspect-ratio of 6.5 would be used for podded engines, and 5 for buried engines. The L/D ratios were then approximately equal. A value of 8 would be used for straight wings. It was probable that these values were not unreasonable and would not unfairly penalize one type of aeroplane against another. Effect of Engine Power on Cruise Economy.—When the separate relationships which controlled engine power and aircraft cruising conditions were combined together, and if certain major features of an aeroplane were known—namely, type of engine, body size and aspect-ratio,—a connection could be estab lished between basic engine thrust, CDO and cruise CL. This could lead to an examination into a family of aeroplanes the essential characteristics of which, for example, might be as follows:— Type of engine Body size Aspect-ratio Design cruise speed Cruise lift coefficient Turbojet Mean diameter 12.5ft Length 135.0ft 5.0 0.85 Mach number 0.30 With these data in mind it was apparent that, when larger engines were fitted, in order to satisfy the conditions of constant speed and lift coefficient the operating altitude and the wing area was increased. It followed that with constant body size, CDO was reduced while cruising L/D ratio was increased. It was also true that the speed for minimum drag approached closer to the cruising speed. By reason of the constant value of lift coefficient, wing loading was proportional to relative density. Drag estimates might now be made by standard methods. The body size was fixed, and appropriate wing sweep, thickness/chord ratio and tail surface areas could be selected. Then the pro duct of wing loading and power loading might be plotted against altitude for a number of different sizes of engine (see Fig. 5). As this product was, to a first approximation, a measure of take-off distance, the curves showed that, in general, the longer the take off run, the lower would be the initial cruising altitude. Take-off Performance.—Two constant values of the product of power loading and wing loading in Fig. 5 corresponded roughly to "balanced field lengths" of 6,000ft and 8,000ft, i.e., when the distance from start to clear a 50ft screen, after an engine failure at take-off, was equal to that required to decelerate and stop. Besides power loading and wing loading, the balanced field length was also affected by the take-off safety speed of an air craft and the temperature and altitude of the airport. Fortunately there was no need to indulge in complicated arithmetic to cal culate these distances, for it could be shown from statistical evidence applying to modern transport aeroplanes, that a good enough approximation to the field length required was given by the formula:— Take-off distance where ws Wo a K KwsWp/<TCL max wing loading power loading effective relative density 38 Aircraft Weights.—Rather than use some particular set of formulae for estimating the weight of various components of the structure for a large number of aeroplanes, a generalized approach had been adopted. Weight formulae which included the 36 35 40 42 44 46 ALTITUDE (ft xljOOO) 50
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
