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Aviation History
1953
1953 - 0562.PDF
556 FLIGHT, i May 1953 CIVIL JET OPERATIONS B.O.A.C's Comet Experience By CAPT. A. M. A. MAJENDIE, M.A., A.F.R.Ae.S. A LECTURE of unusual interest, delivered on April 23rd by Captain A. M. A. Majendie, flight captain of B.O.A.C.'s Comet Fleet, before the Royal Aeronautical Society—a "main" lecture at the Glasgow branch—finds a particularly appropriate place in this issue of Flight. It is printed below—practically in full—with diagrams prepared from several of the illustrations. The paper was confined strictly to the operational experience of B.O.A.C. gained in a year's operations with the de Havilland Comet 1 with four Ghost 50 turbojets. Symbols and abbreviations used were: Vm&=minimum drag speed; W=weight; E.A.S.= equivalent air speed; T.A.S.=true air speed; 1 kg=2.205 lb. The lecturer dealt with his subject under separate headings. CRUISING.—Characteristics of a turbojet aircraft are peculiar, and bear little resemblance from an operational point of view to those of piston-engined propeller aircraft. In particular, the dictates of efficiency allow very little basic freedom in the selection of cruising procedures, although a fair latitude exists in their detailed application. The need to fly high for long-range high-speed operation with a jet aircraft follows from the engine requirement of high r.p.m. in order to achieve a good specific consumption j the only method of equating thrust output against airframe drag for aerodynamic efficiency then being to use altitude in place of a throttle control. By this means it is possible to select the cor rect lift/drag ratio for optimum range at the same time as the best engine r.p.m. for specific consumption, equating thrust output with drag by flying at the correct altitude to achieve this for any given aircraft weight. In practice, with a centrifugal compressor-type engine, best engine specific consumption (lb fuel/lb thrust) per hour is generally obtained at the maximum permitted r.p.m. consistent with a satisfactory overhaul life. For the purpose of a simple analysis the engine r.p.m. therefore can be assumed constant, variations of thrust output with altitude being dependent only on air density to a first order of accuracy. Because of the high fuel consumption of jet engines in comparison with piston ones, a relatively greater percentage of the aircraft all-up weight is burnt as fuel each hour of flight, and the total fuel load at take-off can approach 50 per cent of the total weight. It follows that there is a large progres sive reduction of all-up weight throughout the flight, and that the thrust requirements demanded of the engine for a given lift/drag ratio decrease in the same proportion. To achieve this at constant r.p.m., height has to be increased progressively as weight diminishes, and this leads to the now well-known cruising-climb technique. Under level flight conditions, if engine thrust can be varied without change of specific consumption, it can be shown that best range will be obtained by flying at the lift/drag ratio appropriate to a speed a little more than 30 per cent above that for minimum drag, as long as this speed is below the drag critical Mach number. In practice, for level operation at low altitude, considerable throttling is needed to achieve this speed. As engine specific consumption worsens with reduction in r.p.m., it is advantageous on multi-engined aircraft to derive the greater part of the total required thrust from one or two engines operating near optimum r.p.m., and to idle or shut down the remaining engines. The same argument holds good when holding at minimum consumption (i.e. maximum endurance conditions), which is achieved by flying as close to the speed for minimum drag as possible, consistent with altitude stability. When engine r.p.m. is held constant at optimum engine conditions, any required lift/drag ratio can be selected by flying at the equivalent air speed appropriate to it for the aircraft weight at that time. It is con venient to relate this air speed to that for minimum drag, as by so doing it can be expressed independently of aircraft weight for a given lift/drag ratio or lift coefficient. Under these conditions of set engine r.p.m. and selected equivalent air speed at a given percentage above that for mini mum drag, thrust required varies only with weight, and cruising height will settle down with the engine altitude-throttled to give the required figure, height increasing as fuel is consumed and weight diminishes. Operating in this way best range conditions are achieved at a rather slower air speed than for the level flight case already discussed, as true air speed does not fall as fast as equivalent air speed when the latter is reduced, owing to the increase in cruising height associated with such a reduction. Furthermore, below the tropopause, the temperature lapse rate gives an improvement in engine performance, over isothermal con ditions, for a reduction of equivalent air speed, and consequent increase in cruising height, making an even slower operation desirable in this case. [Four graphs were shown to illustrate these effects; two are repro duced here. The curves were for still air, the wind component being shown as a fraction of Kmd. Based on this simple analysis, the effect of wind and temperature on range are shown in Figs. 1 and 2.] It might be thought that a limitation on the selection of a cruising equivalent air speed would be imposed on grounds of stability, because of the need to preserve a suitable margin above Vma. It will be realized that this is not so since no attempt is being made to fly at a set altitude, and any temporary excess of drag over thrust will result in a descent to a level at which adequate engine output is again available. There is no theoretical or practical objection to a cruising-climb at, or near, Vmi on this account, as long as other limitations do not intervene. To carry out an actual operation successfully and efficiently, it is necessary to devise a practical cruise control procedure, so that the operation can be kept within practical limits of the optimum condition, without placing an undue burden upon the operating crew and within the limits of available instrumentation. The acceptable tolerances in this connection are much smaller for a jet than for a piston-engined aircraft, owing to the much greater weight of fuel burnt every hour expressed as a percentage of available payload capacity. As a result, relatively small increases in specific range (air miles/lb fuel) can improve considerably the payload capacity under limiting conditions. Apart from being workable and efficient, the cruise control procedure adopted must enable a satisfactory presentation to be made of all those operational factors needed in the air and for flight planning. These in clude true air speed, time, distance, fuel, weight, height, endurance and so on, for an operation under given conditions. A vital factor in air safety is the presentation of this information to the operating crew in a simple and unambiguous manner, particularly for jet operations when time itself is at a premium. [The lecturer, discussing measured performance, here referred to two charts of specific range against I.A.S. and T.A.S. respectively, drawn for temperature conditions of 15 deg assuming a continuous lapse rate and no tropopause. The second of these is illustrated, Fig. 3.] For practical purposes it is convenient to suppress the tropopause and so to avoid discontinuities in presentation, particularly as most current operations are conducted in the troposphere. The variation of specific range with indicated air speed for a given weight is not as critical as might be expected theoretically; nevertheless, it is as well to stress again 70 SO RELATIVE 90 IOO 110 CROUND MILES/LB 120 Fig. 1 (left) shows the effect of wind on range. A and B are "optimum-incidence" curves for I.C.A.N, and isothermal atmo spheres, respectively. C is for a constant-incidence cruise such that Vi/Vmd= 1.3. Vi=£.A.S., Vimd = E.A.S. for minimum drag. Fig. 2 (right) indicates the varia tion of range with air tempera ture. The curves are drawn for stabilized fight at a given weight and a fixed incidence. + 30 t N o S4IO OlICAN 3 < a. 5' 10 -20 -30, •9 1 \ \ 1\ ? \ vs. \ \ \ \ X, s \ / 65 90 95 IOO 105 110 RELATIVE AIR MILES PER LB OF FUEL 115
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