Abstract:
The advent of the big high by-pass ratio engines with their inherently large inlet diameters has resulted in engine installations featuring lower ground clearances than hitherto. Under these oiroumstances the vortex formed beneath the intake during ground running can be an extremely potent means of lifting ground debris into the intake. This Report oonsiders the impulsive lifting of particles over which the vortex core passes both on a theoretical and an experimental basis. Scaling laws are derived to relate model and full-scale tests and experimental evidence is cited to demonstrate their validity. For a given inlet velocity the strength of the vortex, and thus the maximum size of particle that can be lifted, is very much a function of the ambient vorticity, and of the strength of the wind blowing on to the intake. The ability of atmospheric vorticity to vary from one instant to the next leads to some difficulty in specifying the maximum size of particle that the vortex might lift. Nevertheless it seems reasonable on the basis of the evidence submitted that a 6 ft diameter intake operating with the centreline 6 ft above the ground, as is representative of some current projects, would be capable of ingesting spherical pieces of concrete up to 12 in. in diameter. Various methods of reducing ingestion were examined experimentally, and although some undoubtedly decreased the frequency of ingestion none provided an absolute safeguard. However, an operational technique which might ease the ingestion problem involves a progressive throttle opening as the aircraft initially accelerates on take-off so as to achieve as high a value as possible for the ratio of the wind velocity blowing on to the intake to the intake throat velocity.
