LOW temperatures and pressures existing in the upper regions of the atmosphere have been the cause of numerous difficulties arising in high-altitude flight testing, some of which are discussed by these authors.
Troubles with propeller feathering, for instance, have led to various measures designed to improve feathering at altitude, including relocating the feathering pump at a point beneath the oil tank, bleeding warm oil through the feathering line, reducing the feathering pump capacity, using a propeller dome spinner, and faired-knee cams or reindexed standard cams.
Several solutions are also suggested to the oil-foaming problem, which resolves itself into supplying sufficient inlet pressure and oil density at the pump to prevent reduction of pump efficiency.