A start is made by considering why the subsonic propeller was abandoned when aircraft speeds approached the shock stall. It is true that the shock stall also occurs on the propeller blades under these conditions, causing a loss of lift and an increase of drag, but this is not the predominant effect which produces the marked decrease of propeller efficiency associated with high-speed aircraft. The main aerodynamic difficulty associated with the shock stall on the wings of an aircraft is due to the separation of flow caused by the shock wave rather than the presence of the shock wave itself. This difficulty is reduced on a propeller, as any separation of flow which might occur on a propeller blade due to the shock stall, would tend to cause the air to be stagnant relative to the blade, and would thus experience a large centrifugal force by virtue of the propeller rotation. Thus the propeller shock stall would never develop to the same degree that it does on the wings of an aircraft in steady flight.

The term Aircraft Instrumentation is used to describe the special instruments which in a variety of circumstances are needed to determine aerodynamic, structural and power plant characteristics. Such circumstances may be flight tests of prototype aircraft, tests in the air or on the ground of the aeroplane, its structure or power plant, or the acquisition of statistical data on the conditions to which aircraft are subjected in civil or military operation. These special instruments therefore are additional to, and usually different from, the instruments needed in an aeroplane to enable it to fulfil its military or civil role.