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Aviation History
1963
1963 - 1881.PDF
688 REGENERATIVE TURBOPROP.. lower fuel consumption. In our opinion, one of the major reasons for our selection in this industry-wide competition is our highly successful experience with the T56-series turboprop engine now operational with the US military services and with the air forces of several foreign nations. With the regenerative engine we will continue a gas turbine development and production tradition that has brought Allison to a position of world leadership. This new engine is a concept further advanced than any known turboprop in the world today.' Mr Dice described the basic principle of regeneration as the reclamation of turbine exhaust heat to increase the temperature of compressor discharge air, thus reducing the amount of fuel required for the most efficient engine operating temperature. •'Designated the Model 545-B2, the regenerative engine will be designed and developed primarily for use in anti-submarine warfare (ASW) aircraft such as the four-engine Lockheed P-3A Orion powered currently by Allison T56 engines. The regenerative engine will greatly extend the long-range and 'on station' capabilities of such aircraft. The engine also will be adaptable to airborne early warning (AEW) aircraft based aboard Navy carriers, for large transports operational with other branches of the military service, and in other applications where long endurance or low fuel con sumption is of prime importance. The 545-B2 utilizes an axial- flow compressor and a new Allison development—hollow, air- cooled turbine blades capable of operating in higher inlet temper atures for a major improvement in engine performance. A unique, unitized propeller reduction gearbox designed to provide greater reliability and simpler maintenance will result in a lighter, more compact overall package." Interior details of the 545 (which is designated T78 by the Navy Bureau of Weapons) are classified confidential, but an idea of an earlier Allison heat exchanger is afforded by the accompanying sectioned sketch. This illustrates a rotary-drum heat exchanger similar in principle to that successfully employed in General Motors' GMT-305 Whirlfire automotive engine. The big drum rotates so that each portion of it is alternately being heated by the exhaust gas and cooled by the compressor air. It seems fairly evident that in the T78 the drum rotates about a longitudinal axis, like the main rotating assembly of the engine; and this seems more logical from all points of view. Clearly one can plot a simple graph showing the weights of a simple turboprop and fuel for a given flight endurance compared with that for a regenerative engine (Fig 3). At some point the FLIGHT International, 24 October 1%J curves cross, because the fuel consumption of the regenerative engine is less; and beyond this point the regenerative engine plus fuel weight is less, despite the greater installed weight of the engine alone. Allison have for several years maintained a technical liaison with Rolls-Royce which has been of assistance to both companies as reported in our issue of August 1 last. Earlier, on February 1 1962, this journal reported on the high powers achieved by experi mental engines derived from the T56 but having new compressors of greater capacity and pressure ratio and air-cooled turbine blades utilizing Rolls-Royce techniques carried across to American cast materials and allowing an entry temperature of 1,400°K (1,127T) ENDURANCE (hr) Fig 3 "Flight International" plot of relative weights of simple and regenerative engines for given mission endurance: A, installed weight of simple turboprop engines; B, installed weight of regenerative engines; C, engines-p/us-fue/ weight for given endurance; D, regenerative engines-plus-fuel weight, showing to advantage for missions longer than 2±hr As the announcement of the 545 states, cooled blades will be used in the regenerative engine. A factor likely to limit the introduction of regeneration is that the general rise in compressor pressure ratio is accompanied by a corresponding rise in delivery air temperature, so that the temper ature difference across the heat exchanger becomes less. An engine like the T78 is of value chiefly for modest pressure ratios, high turbine entry temperatures, low flight speed (below Mach 0.8) with propeller propulsion, and very long flight endurance. W. T. G. Fig 4 Allison drawing showing how a heat exchanger of Whirlfire type might be coupled to o turboprop. This geometrical layout appears most unlikely, but is included here in view of the absence of any other Allison regenerative drawings
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