"Is the Size Really Indifferent?”, or How the Helicopter Turboshaft Engines Are Penalised by Their Own Small Size

Abstract

The answer for the question in the title is obvious. The size is extremely important. Examining the thermal efficiency and specific net work output of gas turbine engines, it is well proved that the above mentioned performance indicators of helicopter turboshaft engines are greatly lower than the average of other gas turbine categories, which is today at take off power over 40% and 400 kJ/kg. Meanwhile 30% thermal efficiency and 200-250 kJ/kg specific net work output are considerable at helicopter turboshafts. This negative effect mainly comes from the small size of their compressor and the accordingly low blade length, especially in the rear stages (higher relative blade tip clearance) and the low Reynolds number of the flow. In this paper the compressor size and its correlation with the above mentioned performance indicators are examined.