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Aviation History
1951
1951 - 0209.PDF
1 February 1951 135 The prototype Aspin I, rated fortake-off ox 440 Ib thrust, has successfully completed a 1,000hour test. ASPIN 1 Unit with Variable Augmentation, by Ducted Fan WITH even the most modern airscrew, propulsiveefficiency begins to fall fairly rapidly at flying speedsin excess of about 440 m.p.h., while the propulsive efficiency of the thermal jet is much inferior to that of the airscrew until a flying speed of about 550 m-p.h. is attained. To power the larger airliners and transports of the future . (which are the aircraft most likely to be operated at speeds between these limits) a jet unit with ducted-fan augmenta- tion, Le., one intermediate to the turbojet and the turboprop, would appear to deserve serious consideration. This type has been studied and developed in Bngiand by Metropolitan- Vickers and in France by Rateau. More recently a second French unit, the Turbomeca Aspin I has concluded a successful test. It is, however, a much smaller unit than either of its precursors. In all systems of air propulsion the applied force F (thrust) is equivalent to the acceleration of an air mass: m (jp — V) where m=air mass «)=efflux velocity V=intake velocity (aircraft speed) The energy E imparted to the air is equal to the difference between the respective intake and efflux kinetic energies and is expressed: — E=iwi (ro'-V) from which: E=*F (w+V) With a given expenditure of energy E, any increase of the applied force F implies a diminution of the factor (ro + V) and this, in practice, means a reduction of efflux velocity w since intake velocity V is regarded as constant for any particular evaluation. However, w must remain higher man V and any reduction of w entails an increase of mass m if the resultant thrust F is to be maintained. The aim, therefore, is to utilize the greatest possible air mass intake, and an efflux velocity only slightly higher than the aircraft speed. On the basis of this reasoning Turbomeca have so designed the Aspin I that part of the energy is used to drive an axial fan to induce an excess flow of air which serves to dilute and lower the velocity of the propulsive jet A diffusing entry duct slows down the intake air to a velocity conducive to the good functioning of the fan. If the aircraft speed rose above 550 m.pJh. and approached sonic values the efficiency of the fan would fall away and augmentation cease. In that range of aircraft speeds, however, the propulsive efficiency of the pure jet is the best available. Assuming an identical central assembly—compressor, combustion system and turbine—the same air mass flow and the same consumption of fuel, an estimation of the fan- augmented turbine unit relative to the turbojet and die turboprop can be made. The relationship varies with air- craft speed and also with the dilution ratio A of the aug- mented jet. This is the ratio of the secondary air mass flow to the primary air mass flow and curves are drawn on the graphs showing specific thrust and propulsive efficiency for A values of 3.0, 5.0 and 7.0. As regards propulsive efficiency, the turboprop would appear to be superior to the augmented jet up to an aircraft speed of 540 m.p JL If the ratio of dilution can be varied, it becomes possible to arrange for the maximum propulsive efficiency of an augmented jet to be attained at a desired aircraft speed. It should be noted that this speed may be considerably higher than that at which the airscrew functions at its optimum efficiency. In short, the turboprop is •markedly superior only at speeds up to, say, 350 or 400 m-p.h. The rate of decrease of thrust with increase Comparative curves of specific thrust plotted against aircraft speed. Three ratios of dilution are shown for *e augmented jet. 224 448 MPTRAFT <;PFFT» 1 672 Propulsive efficiencies as a function of aircraft speed for the three types of turbine power units. 224 336 448 56O 672
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