Evaluation of helicopter agility through inverse solution of the equations of motion

Helicopter agility in nap-of-the-earth flight is widely recognised to be of great importance. Despite this. a general method of quantifying agility does not exist. All previous attempts to quantify agility have been restricted either to flight tests or to simple kinematic modelling - both with obvious disadvantages. A method of quantifying helicopter inherent agility. the agility of the configuration independent of the pilot. utilising inverse solutions of the equations of motion has been developed. A value for the inherent agility of a helicopter is given by studying its performance over a series of standard manoeuvres. The manoeuvres used represent typical tasks undertaken by the configuration under study. The combination of these tasks represent the helicopter's operational role. The helicopter's performance over these standard manoeuvres is found by using an inverse solution of the equations of motion - calculation of the control. and resulting state. time histories needed to fly a given flight path. A six degrees of freedom non-linear mathematical model is used to simulate single main and tail rotor helicopter flight dynamics. The helicopter's performance over each manoeuvre is rated by a quadratic performance function of the state and control variables. The performance function is weighted in such a manner as to penalise undesirably large displacements in the state and control variables of particular importance to that manoeuvre (e.g. large nose down attitude changes in accelerated flight are heavily penalised). An Agility Rating is awarded to a helicopter on the basis of its performance over a wide range of similar manoeuvres. a measure of total inherent agility being a function of the agility ratings for all the manoeuvres relevant to the helicopter's role. The method is illustrated by applying it to two agility studies. Firstly. it is used to show how an optimum tailplane area can be calculated for manoeuvres in the longitudinal plane. Then an "Advanced Rotor Helicopter" is compared with a contemporary battlefield helicopter