Modelling review

Assuming that the reader has made it this far, he or she may feel somewhat daunted at the scale of the modelling task described on this Tour; if so, then Chapters 3, 4 and 5 will offer little respite, as the subject becomes even deeper and broader. If, on the other hand, the reader is motivated by this facet of flight dynamics, then the later chapters should bring further delights, as well as the tools and knowledge that are essential for practising the flight dynamics discipline. The modelling activity has been conveniently characterized in terms of frequency and amplitude; we refer back to Fig. 2.14 for setting the framework and highlight again the merging with the loads and vibration disciplines. Later chapters will discuss this overlap in greater detail, emphasizing that while there is a conceptual boundary defined by the pilot-controllable frequencies, in practice the problems actually begin to overlap at the edges of the flight envelope and where high gain active control is employed.

Much of the ground covered in this part of the Tour has utilized analytic approx­imations to aircraft and rotor dynamics; this approach is always required to provide physical insight and will be employed to a great extent in the later modelling chapters. The general approach will be to search among the coupled-interacting components for combinations of motion that are, in some sense, weakly coupled; if they can be found, there lies the key to analytic approximations. However, we cannot escape the complex­ity of both the aerodynamic and structural modelling, and Chapter 3 will formulate

expressions for the loads from first principles; analytic approximations can then be val­idated against the more comprehensive theories to establish their range of application. With today’s computing performance and new functionality, the approach to modelling is developing rapidly. For example, there are now far more papers published that com­pare numerical rather than analytic results from comprehensive models with test data. Analytic approximations tend, nowadays, to be a rarity. The comprehensive models are expected to be more accurate and have higher fidelity, but the cost is sometimes the loss of physical understanding, and the author is particularly sensitive to this, having lived through the transition from a previous era, characterized by analytic modelling, to the present, more numerical one. Chapters 3 and 4 will reflect this and will be packed full with the author’s well-established prejudices.

We have touched on the vast topic of validation and the question of how good a model has to be. This topic will be revisited in Chapter 5; the answer is actually quite simple – it depends! The author likens the question, ‘How good is your model?’ to ‘what’s the weather like on Earth?’ It depends on where you are and the time of year, etc. So while the initial, somewhat defensive answer may be simple, to address the question seriously is a major task. This book will take a snapshot of the scene in 1995, but things are moving fast in this field, and new validation criteria along with test data from individual components, all matched to more comprehensive models, are likely to change the ‘weather’ considerably within the next 5 years.

In the modelling of helicopter flight dynamics, of principal concern are the flying qualities. The last 10 years has seen extensive development of quality criteria, and the accurate prediction of the associated handling and ride qualities parameters is now at the forefront of all functional validation which conveniently leads us to the next stage of the Tour.