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The past 15 years of concept development leading to today s aircraft regenerative turbine engine are broadly reviewed. With this as background, current regenerative turboprop engine technology is discussed. Comparisons are made of the range and endurance capabilities of aircraft equipped with simple and regenerative cycle engines; projections of further development potential are made. The application of regeneration in turbofans, particularly in the moderate to high bypass engines, is considered. Performance characteristics are shown for various flight conditions and bypass ratios to illustrate the potential gains from regeneration. The question is one of determining whether the associated weight and drag penalties offset the reduction in fuel consumption. The problem offers a major challenge to the ingenuity of the engine designer.

Recently there have been many new developments in aluminum-base diffusion coatings as well as with other coating materials for protection of metal parts in high temperature oxidizing environments. Evaluation is described of many of these coatings on three separate laboratory test devices, for determining (1) resistance to erosion in a high velocity high temperature oxidizing atmosphere, (2) resistance to cracking and oxidation in a thermal fatigue test, and (3) sensitivity to light impact damage at elevated temperatures. Experience with diffused aluminum base coatings on some turbine engine components is discussed, and limitations of the evaluated coatings are cited.

The mission capability of some gas turbine powered vehicles can be greatly enhanced by the application of regeneration to the engine thermodynamic cycle. The application of regeneration to aircraft propulsion presents many unique design problems to the designer as a result of the need for very lightweight components and high performance. The many design parameters that must be evaluated, and the many tradeoff studies that must be accomplished to arrive at the best regenerator and gas turbine design are discussed. Also, the results of a series of studies are presented to graphically portray the fuel economy which results from the use of regenerative engines.

The Allison 250 turboprop program is reviewed including engine characteristics, the development program, FAA certification, and development to higher power ratings. Market and installation programs are reviewed including general aircraft turbine engine trends, turbine engine market potential, and a discussion of the justification and merits of small turboprop aircraft. Various 250 turboprop installations are discussed.

The need for surface protection of nickel base alloys to prevent hot corrosion and/or sulfidation is discussed. Results of controlled engine test cycling and the rig testing of turbine blades are discussed to establish laboratory test correlation. The relative corrosion resistance of a number of commercial alloys is shown, and the response of these alloys to corrosion resistance with protective coating is covered in relation to their limitation in erosion/oxidation deterioration. Finally, some technology results and general methodology applied to electrophoretic processing for applying coatings of aluminum and combinations with chromium are described. The processing advantages and disadvantages of this coating process and general results are compared with present production.

This paper describes the structural design of a composite material front housing for the T56 turboprop reduction gear case. The objective of the composite gear case is demonstration of the feasibility of composites for stiff, lightweight gear reduction cases and the advancement of structural and material technology. Turboprop reduction gear assemblies have typically used magnesium or aluminum for the case structure. Magnesium has reasonable strength properties and low density but the modulus is also low; furthermore, it exhibits poor corrosion resistance. Aluminum has sufficient strength but the specific stiffness, E/ρ, is similar to magnesium so the case is heavy for many applications. In addition to these disadvantages, the mounting requirements for propellers, engine, and transmissions dictate high loads on the gear case structure.

Allison has developed and obtained FAA type certificate for the Model 250 turboprop engine rated at 317 shaft horsepower. This engine has been flight tested in several experimental installations. Development of the engine to 400 shp is continuing. Weight savings from the lighter turboprop engine result in increased aircraft useful load; the reduced drag of the smaller turbine engine coupled with ram recovery and higher continuous ratings provide higher performance at comparable reciprocating engine power levels. These factors result in increased aircraft productivity and net worth to offset the higher price associated with the turbine engine.