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Aviation History
1962
1962 - 2333.PDF
602 FLIGHT International, 11 October 1962 ANALYSING THE P.1127 Propulsion Problems It is also appropriate to report briefly upon a paper by R. M. Denning, of Bristol Siddeley Engines, which is today due to be read before the Society of Automotive Engineers in Los Angeles. After discussing the optimum engine thermodynamics for V/STOL supersonic strike aircraft, the author turns to the propulsion systems which will be needed for more advanced, highly supersonic aircraft like the Hawker P.l 154. Plenum-chamber burning will be essential for the BS.100 engine of this aircraft, but the aircraft designer is nevertheless still presented with a wide range of powerplant possi bilities, one of the principal variables being the proportion of the aircraft lifted by specialized lift engines. Assuming a brochure vertical thrust equal to 120 per cent of the aircraft weight, and aircraft with identical drag characteristics, wing loading and mission—one hour at Mach 0.9 at sea level—it is possible to examine three basic designs: A, single lift/thrust-engined aircraft; B, powered by lift/thrust engine of minimum size for supersonic operation and an appropriate amount of specialized lift engines for take-off; and C, a specialized multi-engined aircraft having separate lift and propulsion engines. Assuming a constant propulsion-engine thrust: weight ratio of 6£ :1 for all engines, the extremes of the spectrum A and B would be of equal performance if an installed thrust: weight ratio of just over 11:1 could be achieved. Some studies have indicated that the installation penalty on, say, a 16:1 bare thrust:weight ratio can be such as to reduce the true installed thrust: weight ratio to 8:1. Such installation ratio penalties have not been uncommon in the past on conventional powerplants and just to maintain the same installed ratio the aircraft designer would have to reduce the installation penalty by the same factor as the engine designer (3:1). It can be shown that, for aircraft B, the thrust/weight ratio of the pure lift engines is relatively unimportant. This aircraft generally shows an advantage of about five per cent in total powerplant •+ fuel weight. Aircraft C could have a deflector valve on the pro pulsion engine, making it a more complex version of B involving severe problems in the event of propulsion-engine failure. Alterna tively, rotating tip pods could be employed, but this would appear to be inelegant in comparison with B. In discussing possible variations in the design of the basic air craft, Mr Denning comments on the increased profile drag of the multi-engine aircraft C, which also suffers from high drag due to the reheat nozzle area, which could be as high as seven per cent and be reduced only by variable geometry at a considerable weight penalty. In contrast, the lift/thrust engine can have its nozzles suitably faired in the sides of the fuselage in such a location as to recover a considerable amount of the potential con-di nozzle thrust by favourable interference with the boat-tailed rear fuselage. In view of its smaller size, higher r.p.m. and greater duct Mach number and wetted area, the specialized propulsion engine might be expected to suffer higher duct pressure-loss during subsonic cruise. On the other hand, in the single-engined aircraft considerable ingenuity is needed to achieve low static intake loss and minimum spillage drag during subsonic cruise at low r.p.m. Take-off and transition to wing-borne flight when using minimum-time techniques occupy only some 20sec from the time of deflection of the nozzles, and of this period it is only necessary to have maximum thrust during the first 8-10sec. It has been sug gested that this might be reflected in the time of operation of the maximum take-off rating. A higher boost rating for a short time would be followed by automatic reduction in thrust to a normal maximum rating for use at all other times. Such a rating would be reflected in the engine type-test schedule and would lead to an increase in maximum thrust for a given blade creep life. In conclusion, Mr Denning makes the following general points:— The basic engine cycles which would be optimum for lift/thrust engines are generally near optimum for all types of aircraft in the low-level subsonic cruise regime. Overall design cycle pressure- ratio is not an important feature of the engine within the range of values considered. Aircraft with the ability to cruise at or near Mach 2.0 require similar powerplant + fuel weights, although some advantage is evident for an aircraft with a lift/thrust engine plus a limited amount of specialized lift-engine thrust. Development to speeds in excess of Mach 2.0 would almost certainly lead to propulsion-engine sizes capable of providing the whole of the vertical take-off thrust. The standard argument used against the single-engined aircraft— that it is vastly oversized for subsonic cruise—is shown to be without foundation at the speeds likely to be used. The Ability to cruise economically at low level at speeds in excess of Mach 1.0 requires not only the thrust of the single-engined aircraft, but also a by-pass ratio on the lower limit for thrust balance, i.e., a by-pass ratio around 1.0. A lift/thrust aircraft with a small amount of specialized lift- engine thrust, while not suffering from the excessive complication of the multi-engined concept has, in some part, its less advantageous features. In view of its apparent performance advantage it could have a place as a more sophisticated aircraft with limited supersonic speed targets. Where it is shown that competiting systems have little to chose between them on powerplant + fuel weight, it is suggested that choice should be made on the more qualitative factors of simplicity, cost, ease of pilot control, incidental performance advantages and future development. These all tend to point to the lift/thrust- engined aircraft. While the author cannot claim to be impartial, considerable care has been devoted to ensuring that the conclusions were arrived at using a set of assumptions which, if anything, are biased in favour of the multi-engined configuration. a x §= 2 2 ! §2 0 P a 2 w z O o IB "6 /: BOCV v UOC # / , 1200/ 00/ $/ 7 /z 6OO 60( / V BY-PASS BURNING ' TEMP. - °K 1 r T -*/l400 /I400 /1200 U X1200 00 ^^yiooo L^^SOO 3 • Q-4 o-6 THRUST TAKE-OFF WEIGHT l-O « 1-2 A comparison between powerplants for an aircraft of P.l 154 calibre. Left, thrust plotted against specific consumption at Mach 2.0 at the tropo- pause. Right, a similar plot for Mach 0.9 at sea level, assuming a by-pass ratio of 1.0, design overall pressure-ratio of 14 : I, and design max tur bine entry temperature of 1,400"K \ \ 1 |LEVE sj .. >IC. 1 1 1 L FLICH £5j£r . r REGIME 04 OS THRUST TAKE-OFF WEIGHT
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