Starting Torque Characteristics of Small Aircraft Gas Turbines and APU’s

Starting torque characteristics of small self-sufficient gas turbine engines often present a belated problem to the development engineer, particularly when proceeding into final engine qualification testing over the required range of operational environments. A common procedure is therefore to conservatively over-design the start system with consequent penalties in weight, cost, and size. The various aspects of engine starting are discussed and it is shown that the dominant factor controlling start characteristics at sub-zero ambient temperatures is lubricant viscous shear effects. The magnitude of this effect is such that stored energy start system weights for rapid starting of small aircraft auxiliary power units may weigh as much as the power unit itself. The same auxiliary power units, if used as starters for larger main propulsion gas turbines, are burdened with the incompatibility of torque output dependent upon air density, yet main engine cranking torque is dependent basically upon lubricant viscosity. It is concluded that start system over-design philosophy will tend to persist pending the application of external starting torque directly to the high speed shaft itself with uncoupling of all but necessary viscous parastic shear sources, or until such time as detailed research and development is devoted in applied tribology to further reduce viscous shear effects in high speed turbo-machinery.