From time to time attempts have been made to produce a dynamometer capable of measuring the power output of an aero-engine during flight at various atitudes. The type that has proved the most suitable up to date in flight tests is the Bendemann hub acting on the hydraulic principle which was originally designed in Germany. The principle has recently been tested in America with moderately satisfactory results, and experiments with a modified form of the apparatus are at present being conducted at the Royal Aircraft Establishment. The Bendemann hub and its accessories are somewhat complicated and are difficult to instal in a machine and to maintain in a serviceable condition, also the apparatus will not work properly at altitudes above 20,000 feet owing to increased viscosity of the working fluid at low temperatures.

The apparatus described below is simpler in construction, lighter and more robust than the Bendemann hub, and should be capable of giving equally good results irrespective of altitude; moreover, the work of fitting it to a Service machine would not be great and would not necessitate the employment of specially skilled mechanics.