Ejector Thrust Augmentation - Fact or Fiction?

An aura of mystery has always surrounded the thrust augmentation properties of ejectors, largely through lack of agreed bases for comparison with conventional propulsion systems. Twelve definitions of thrust augmentation are discussed, and two are selected as rational criteria for ejector thrust performance, based on thrust, energy input and size. Analysis shows that thrust augmentation results from the surface stress distribution over the total ejector surface, and is strongly influenced by the flow pattern at entry as suggested by von Karman, as well as by the diffuser. Reported experimental values are far below the ideal limits, and suggest that research on entry design is needed. Ejectors are inherently inferior to plain jets, size for size, and are only justified as propulsors in special cases. The Coanda nozzle is a special form of ejector, in which the Coanda jet may encourage favourable entry flow.