The concept of stability under constraint had been developed by Neumark and applied with considerable success to predict the speed instabilities in the flight of aircraft below minimum drag speed. This concept involves the removal (or partial removal) of one or more degrees of freedom of an aircraft by the deliberate application of some control to keep a chosen element of disturbance at a constant value. A partial constraint could thus be introduced in practice by a pilot trying to keep the wings level with the horizon by the use of aileron control, or keeping attitude constant with the use of elevator. Pinsker has recently used bank angle constraint stability of an aircraft as a means of defining a minimum acceptable value of nv.

This present note considers some aspects of longitudinal stability under constraint as applicable to the flight of a helicopter in the forward and the hover modes.