Simulation Modelling

It is beyond dispute that the observed behaviour of aircraft is so complex and puzzling that, without a well developed theory, the subject could not be treated intelligently.

wrote these words in relation to fixed-wing aircraft over 50 years ago and they still hold a profound truth today. However, while it may be ‘beyond dispute’ that well – developed theories of flight are vital, a measure of the development level at any one time can be gauged by the ability of Industry to predict behaviour correctly before first flight, and rotorcraft experience to date is not good. In the 1989 AHS Nikolsky Lecture (Ref. 1.2), Crawford promotes a ‘back to basics’ approach to im­proving rotorcraft modelling in order to avoid major redesign effort resulting from poor predictive capability. Crawford cites examples of the redesign required to im­prove, or simply ‘put right’, flight performance, vibration levels and flying quali­ties for a number of contemporary US military helicopters. A similar story could be told for European helicopters. In Ref. 1.3, the author presents data on the percent­age of development test flying devoted to handling and control, with values between 25 and 50% being quite typical. The message is that helicopters take a considerable length of time to qualify to operational standard, usually much longer than originally planned, and a principal reason lies with the deficiencies in analytical design meth­ods.

Underlying the failure to model flight behaviour adequately are three aspects. First, there is no escaping that the rotorcraft is an extremely complex dynamic system and the modelling task requires extensive skill and effort. Second, such complexity needs significant investment in analytical methods and specialist modelling skills and the recognition by programme managers that these are most effectively applied in the formative stages of design. The channelling of these investments towards the critically deficient areas is also clearly very important. Third, there is still a serious shortage of high-quality, validation test data, both at model scale and from flight test. There is an old adage in the world of flight dynamics relating to the merits of test versus theory, which goes something like – ‘everyone believes the test results, except the person who made the measurements, and nobody believes the theoretical results, ex­cept the person who calculated them’. This saying stems from the knowledge that it is much easier, for example, to tell the computer to output rotor blade incidence at 3/4 radius on the retreating side of the disc than it is to measure it. What are required, in the author’s opinion, are research and development programmes that in­tegrate the test and modelling activities so that the requirements of the one drive the other.

There are some signs that the importance of modelling and modelling skills is recognized at the right levels, but the problem will require constant attention to guard against the attitude that the ‘big’ resources should be reserved for production, when the user and manufacturer, in theory, receive their greatest rewards. Chapters 3, 4 and 5 of this book are concerned with modelling, but we shall not dwell on the defi­ciencies of the acquisition process, but rather on where the modelling deficiencies lie. The author has taken the opportunity in this Introduction to reinforce the phi­losophy promoted in Crawford’s Nikolsky Lecture with the thought that the reader may well be concerned as much with the engineering ‘values’ as with the technical detail.

No matter how good the modelling capability, without criteria as a guide, heli­copter designers cannot even start on the optimization process; with respect to flying qualities, a completely new approach has been developed and this forms a significant content of this book.