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Cooling System Optimization in High-duty Aerospace Transmissions

The long-term future of large aircraft engines is seen to lie in ‘ultra high bypass ratio’ (UHB) powerplant. One possibility for such engines is to use reduction gearboxes to allow the relatively high-speed turbine and the low-speed fan to run at their optimum efficiencies. An important constraint upon the application of these gearboxes is the weight and drag of the cooling system. This paper presents an analysis of cooler requirements for a transmission with its own integral oil system. Results are presented for a 4.5 MW gearbox typical of the more powerful turbo prop engines currently in civil service. The importance of transmission efficiency and scavenge temperature in defining the necessary size (thus weight) of the cooler is demonstrated. It is shown that active control of the cooler duct results is further efficiency improvements.