The dynamic behaviour of gas turbines is of great practical importance, but unfortunately it cannot be investigated experimentally until late in the development programme when the mechanical integrity of the engine has been established. In the case of aero engines there is a requirement for fast engine accelerations and all engine transients must be achieved with complete safety. Thrust response rates must be guaranteed by the engine manufacturer at a very early stage and clearly it would be of great value to study engine transient behaviour at the design stage; this should lead to a significant saving in development time and cost by decreasing the amount of test bed running required.

The force on a circular cylinder inclined to a uniform flow may be resolved into three components, one perpendicular to the plane defined by the axis of the cylinder and the upstream velocity vector (described in this report as the out-of-plane force) one within this plane and perpendicular to the axis (the in-plane normal force) and one within this plane and along the axis (the in-plane axial force).

It has become common practice, since Allen and Perkins, to model the flow over a circular cylinder inclined to a uniform flow by resolving the incident flow into two perpendicular component flows. One flow is taken to be parallel to the axis of the cylinder (axial flow) and the other normal to it (cross flow). The cross flow is not taken to be uniform over the whole length of the circular cylinder. Instead it is considered to be started impulsively at the nose and to develop over the length of the body in the same way that the impulsively-started flow over a cylinder develops with time.