Category HELICOPTER FLIGHT DYNAMICS

Figure 2.5 illustrates the conventional main rotor collective and cyclic controls applied through a swash plate. Collective applies the same pitch angle to all blades and is the primary mechanism for direct lift or thrust control on the rotor. Cyclic is more

complicated and can be fully appreciated only when the rotor is rotating. The cyclic operates through a swash plate or similar device (see Fig. 2.5), which has non-rotating and rotating halves, the latter attached to the blades with pitch link rods, and the former to the control actuators. Tilting the swash plate gives rise to a one-per-rev sinusoidal variation in blade pitch with the maximum/minimum axis normal to the tilt direction. The rotor responds to collective and cyclic inputs by flapping as a disc, in coning and tilting modes. In hover the responses are uncoupled with collective pitch resulting in coning and cyclic pitch resulting in rotor disc tilting. The concept of the rotor as a coning and tilting disc (defined by the rotor blade tip path plane) will be further developed in the modelling chapters. The sequence of sketches in Fig. 2.6 illustrates how the pilot would need to apply cockpit main rotor controls to transition into forward flight from an out-of-ground-effect (oge) hover. Points of interest in this sequence are:

(1) forward cyclic (p1s) tilts the rotor disc forward through the application of cyclic pitch with a maximum/minimum axis laterally – pitching the blade down on the advancing side and pitching up on the retreating side of the disc; this 90° phase shift between pitch and flap is the most fundamental facet of rotor behaviour and will be revisited later on this Tour and in the modelling chapters;

(2) forward tilt of the rotor directs the thrust vector forward and applies a pitching moment to the helicopter fuselage, hence tilting the thrust vector further forward and accelerating the aircraft into forward flight;

(3) as the helicopter accelerates, the pilot first raises his collective (pc) to maintain height, then lowers it as the rotor thrust increases through so-called ‘translational lift’ – the dynamic pressure increasing more rapidly on the advancing side of the disc than it decreases on the retreating side; cyclic needs to be moved increasingly forward and to the left (n1c) (for anticlockwise rotors) as forward speed is increased. The cyclic requirements are determined by the asymmetric fore-aft and lateral aerodynamic loadings induced in the rotor by forward flight.

The main rotor combines the primary mechanisms for propulsive force and control, aspects that are clearly demonstrated in the simple manoeuvre described above. Typical

control ranges for main rotor controls are 15° for collective, more than 20° for longi­tudinal cyclic and 15° for lateral cyclic, which requires that each individual blade has a pitch range of more than 30°. At the same time, the tail rotor provides the antitorque reaction (due to the powerplant) in hover and forward flight, while serving as a yaw control device in manoeuvres. Tail rotors, or other such controllers on single main rotor helicopters, e. g., fenestron/fantail or Notar (Refs 2.5, 2.6), are normally fitted only with collective control applied through the pilot’s pedals on the cockpit floor, often with a range of more than 40°; such a large range is required to counteract the negative pitch applied by the built-in pitch/flap coupling normally found on tail rotors to alleviate transient flapping.

The helicopter is required to perform as a dynamic system within the user-defined operational flight envelope (OFE), or that combination of airspeed, altitude, rate of climb/descent, sideslip, turn rate, load factor and other limiting parameters that bound the vehicle dynamics, required to fulfil the user’s function. Beyond this lies the manufacturer-defined safe flight envelope (SFE), which sets the limits to safe flight, normally in terms of the same parameters as the OFE, but represents the physical limits of structural, aerodynamic, powerplant, transmission or flight control capabilities. The margin between the OFE and the SFE needs to be large enough so that inadvertent transient excursions beyond the OFE are tolerable. Within the OFE, the flight mechan­ics of a helicopter can be discussed in terms of three characteristics – trim, stability and response, a classification covered in more detail in Chapters 4 and 5.

Trim is concerned with the ability to maintain flight equilibrium with controls fixed; the most general trim condition is a turning (about the vertical axis), descending or climbing (assuming constant air density and temperature), sideslipping manoeuvre at constant speed. More conventional flight conditions such as hover, cruise, autorotation or sustained turns are also trims, of course, but the general case is distinguished by the four ‘outer’ flight-path states, and this is simply a consequence of having four independent helicopter controls – three for the main rotor and one for the tail rotor. The rotorspeed is not normally controllable by the pilot, but is set to lie within the automatically governed range. For a helicopter, the so-called inner states – the fuselage attitudes and rates – are uniquely defined by the flight path states in a trim condition. For tilt rotors and other compound rotorcraft, the additional controls provide more flexibility in trim, but such vehicles will not be covered in this book.

Stability is concerned with the behaviour of the aircraft when disturbed from its trim condition; will it return or will it depart from its equilibrium point? The initial tendency has been called the static stability, while the longer term characteristics, the dynamic stability. These are useful physical concepts, though rather crude, but the keys to developing a deeper understanding and quantification of helicopter stability comes from theoretical modelling of the interacting forces and moments. From there come the concepts of small perturbation theory and linearization, of stability and control derivatives and the natural modes of motion and their stability characteristics. The insight value gained from theoretical modelling is particularly high when consider­ing the response to pilot controls and external disturbances. Typically, a helicopter responds to a single-axis control input with multi-axis behaviour; cross-coupling is almost synonymous with helicopters. In this book we shall be dealing with direct and coupled responses, sometimes described as on-axis and off-axis responses. On-axis responses will be discussed within a framework of response types – rate, attitude and translational-rate responses will feature as types that characterize the initial response following a step control input. Further discussion is deferred until the modelling sec­tion within this Tour and later in Chapters 3, 4 and 5. Some qualitative appreciation of vehicle dynamics can be gained, however, without recourse to detailed modelling.

A typical operational requirement will include a definition of the environmental con­ditions in which the helicopter needs to work in terms of temperature, density altitude, wind strength and visibility. These will then be reflected in an aircraft’s flight manual. The requirements wording may take the form: ‘this helicopter must be able to operate (i. e., conduct its intended mission, including start-up and shut-down) in the following conditions – 5000 ft altitude, 15°C, wind speeds of 40 knots gusting to 50 knots, from any direction, in day or night’. This description defines the limits to the operational capability in the form of a multidimensional envelope.

Throughout the history of aviation, the need to extend operations into poor weather and at night has been a dominant driver for both economic and military effectiveness. Fifty years ago, helicopters were fair weather machines with marginal performance; now they regularly operate in conditions from hot and dry to cold, wet and windy, and in low visibility. One of the unique operational capabilities of the helicopter is its ability to operate in the NoE or, more generally, in near-earth conditions defined in Ref. 2.1 as ‘operations sufficiently close to the ground or fixed objects on the ground, or near water and in the vicinity of ships, oil derricks, etc., that flying is primarily accomplished with reference to outside objects’. In near-earth operations, avoiding the ground and obstacles clearly dominates the pilot’s attention and, in poor visibil­ity, the pilot is forced to fly more slowly to maintain the same workload. During the formative years of ADS-33, it was recognized that the classification of the quality of the visual cues in terms of instrument or visual flight conditions was inadequate to describe the conditions in the NoE. To quote from Hoh (Ref. 2.4), ‘The most crit­ical contributor to the total pilot workload appears to be the quality of the out-of – the-window cues for detecting aircraft attitudes, and, to a lesser extent, position and velocity. Currently, these cues are categorized in a very gross way by designating the environment as either VMC (visual meteorological conditions) or IMC (instrument meteorological conditions). A more discriminating approach is to classify visibility in terms of the detailed attitude and position cues available during the experiment or proposed mission and to associate handling qualities requirements with these finer grained classifications.’ The concept of the outside visual cues (OVC) was introduced, along with an OVC pilot rating that provided a subjective measure of the visual cue quality. The stimulus for the development of this concept was the recognition that handling qualities are particularly affected by the visual cues in the NoE, yet there was no process or methodology to quantify this contribution. One problem is that the cue is a dynamic variable and can be judged only when used in its intended role. Eventually, out of the confusion surrounding this subject emerged the usable cue en­vironment (UCE), which was to become established as one of the key innovations of ADS-33. In its developed form, the UCE embraces not only the OVC, but also any artificial vision aids provided to the pilot, and is determined from an aggregate of pilot visual cue ratings (VCR) relating to the pilot’s ability to perceive changes in, and make adjustments to, aircraft attitude and velocity. Handling qualities in de­graded visual conditions, the OVC and the UCE will be discussed in more detail in Chapter 7.

The MTE and the UCE are two important building blocks in the new parlance of flying qualities; a third relates to the aircraft’s response characteristics and provides a vital link between the MTE and UCE.

Flying qualities change with the weather or, more generally, with the severity of the environment in which the helicopter operates; they also change with flight condition, mission type and phase and individual mission tasks. This variability will be empha­sized repeatedly and in many guises throughout this book to emphasize that we are not just talking about an aircraft’s stability and control characteristics, but more about the synergy between the internals and the externals referred to above. In later sections, the need for a systematic flying qualities structure that provides a framework for describ­ing criteria will be addressed, but we need to do the same with the mission and the associated flying tasks. For our purposes it is convenient to describe the flying tasks within a hierarchy as shown in Fig. 2.2. An operation is made up of many missions which, in turn, are composed of a series of contiguous mission task elements (MTE). An MTE is a collection of individual manoeuvres and will have a definite start and finish and prescribed temporal and spatial performance requirements. The manoeuvre sample is the smallest flying element, often relating to a single flying axis, e. g., change in pitch or roll attitude. Objective flying qualities criteria are normally defined for, and tested with, manoeuvre samples; subjective pilot assessments are normally conducted by flying MTEs. The flying qualities requirements in the current US Army’s handling qualities requirements, ADS-33C (Ref. 2.1), are related directly to the required MTEs. Hence, while missions, and correspondingly aircraft type, may be quite different, MTEs are often common and are a key discriminator of flying qualities. For example, both

utility transports in the 30-ton weight category and anti-armour helicopters in the 10-ton weight category may need to fly slaloms and precision hovers in their nap-of-the-earth (NoE) missions. This is one of the many areas where ADS-33C departs significantly from its predecessor, Mil Spec 8501A (Ref. 2.2), where aircraft weight and size served as the key defining parameters. The MTE basis of ADS-33C also contrasts with the fixed-wing requirements, MIL-F-8785C (Ref. 2.3), where flight phases are defined as the discriminating mission elements. Thus, the non-terminal flight phases in Category A (distinguished by rapid manoeuvring and precision tracking) include air-to-air com­bat, in-flight refuelling (receiver) and terrain following, while Category B (gradual manoeuvres) includes climb, in-flight refuelling (tanker) and emergency deceleration. Terminal flight phases (accurate flight path control, gradual manoeuvres) are classified under Category C, including take-off, approach and landing. Through the MTE and Flight Phase, current rotary and fixed-wing flying qualities requirements are described as mission oriented.

To understand better how this relates to helicopter flight dynamics, we shall now briefly discuss two typical reference missions. Figure 2.3 illustrates a civil mission, described as the offshore supply mission; Fig. 2.4 illustrates the military mission, described as the armed reconnaissance mission. On each figure a selected phase has been expanded and shown to comprise a sequence of MTEs (Figs 2.3(b), 2.4(b)). A typical MTE is extracted and defined in more detail (Figs 2.3(c), 2.4(c)). In the case of the civil mission, we have selected the landing onto the helideck; for the military mission, the ‘mask-unmask-mask’ sidestep is the selected MTE. It is difficult to break the MTEs down further; they are normally multi-axis tasks and, as such, contain a

number of concurrent manoeuvre samples. The accompanying MTE text defines the constraints and performance requirements, which are likely to be dependent on a range of factors. For the civil mission, for example, the spatial constraints will be dictated by the size of the helideck and the touchdown velocity by the strength of the undercarriage. The military MTE will be influenced by weapon performance characteristics and any spatial constraints imposed by the need to remain concealed from the radar systems of threats. Further discussion on the design of flight test manoeuvres as stylized MTEs for the evaluation of flying qualities is contained in Chapter 7.

Ultimately, the MTE performance will determine the flying qualities requirements of the helicopter. This is a fundamental point. If all that helicopters had to do was to fly from one airport to another in daylight and good weather, it is unlikely that flying qualities would ever be a design challenge; taking what comes from meeting other performance requirements would probably be quite sufficient. But if a helicopter is required to land on the back of a ship in sea state 6 or to be used to fight at night, then conferring satisfactory flying qualities that minimize the probability of mission or even flight failure is a major design challenge. Criteria that adequately address the developing missions are the cornerstones of design, and the associated MTEs are the data source for the criteria.

The reference to weather and flying at night suggests that the purely ‘kinematic’ definition of the MTE concept is insufficient for defining the full operating context; the environment, in terms of weather, temperature and visibility, are equally important and bring us to the second reference point.

We begin by introducing four useful reference points for developing an appreciation of flying qualities and flight dynamics; these are summarized in Fig. 2.1 and comprise the following:

(1) the mission and the associated piloting tasks;

(2) the operational environment;

(3) the vehicle configuration, dynamics and the flight envelope;

(4) the pilot and pilot-vehicle interface.

With this perspective, the vehicle dynamics can be regarded as internal attributes, the mission and environment as the external influences and the pilot and pilot-vehicle interface (pvi) as the connecting human factors. While these initially need to be dis­cussed separately, it is their interaction and interdependence that widen the scope of the subject of flight dynamics to reveal its considerable scale. The influences of the

mission task on the pilot’s workload, in terms of precision and urgency, and the external environment, in terms of visibility and gustiness, and hence the scope for exploiting the aircraft’s internal attributes, are profound, and in many ways are key concerns and primary drivers in helicopter technology development. Flying qualities are determined at the confluence of these references.

In aviation history the nineteenth century is characterized by man’s re­lentless search for a practical flying machine. The 1860s saw a peculiar burst of enthusiasm for helicopters in Europe and the above picture, show­ing an 1863 ‘design’ by Gabrielle de la Landelle, reflects the fascination with aerial tour-boats at that time. The present chapter takes the form of a ‘tour of flight dynamics’ on which the innovative, and more practical, European designs from the 1960s – the Lynx, Puma and Bo105 – will be introduced as the principal reference aircraft of this book. These splendid designs are significant in the evolution of the modern helicopter and an understanding of their behaviour provides important learning material on this tour and throughout the book.

2.1 Introduction

This chapter is intended to guide the reader on a Tour of the subject of this book with the aim of instilling increased motivation by sampling and linking the wide range of subtopics that make the whole. The chapter is likely to raise more questions than it will answer and it will point to later chapters of the book where these are picked up and addressed in more detail. The Tour topics will range from relatively simple concepts such as how the helicopter’s controls work, through to more complex effects such as the influence of rotor design on dynamic stability, the conflict between stability and controllability and the specialized handling qualities required for military and civil mission task elements. All these topics lie within the domain of the flight dynamics engineer and within the scope of this book. This chapter is required reading for the reader who wishes to benefit most from the book as a whole. Many concepts are

introduced and developed in fundamental form here in this chapter, and the material in later chapters will draw on the resulting understanding gained by the reader.

One feature is re-emphasized here. This book is concerned with modelling flight dynamics and developing criteria for flying qualities, rather than how to design and build helicopters to achieve defined levels of quality. We cannot, nor do we wish to, ignore design issues; requirements can be credible only if they are achievable with the available hardware. However, largely because of the author’s own background and experience, design will not be a central topic in this book and there will be no chapter dedicated to it. Design issues will be discussed in context throughout the later chapters and some of the principal considerations will be summarized on this Tour, in Section 2.5.

This book contains seven technical chapters. For an overview of the subject of helicopter flight dynamics, the reader is referred to the Introductory Tour in Chapter 2. Engineers familiar with flight dynamics, but new to rotorcraft, may find this a useful starting point for developing an understanding of how and why helicopters work. Chapters 3, 4 and 5 are a self-contained group concerned with modelling helicopter flight dynamics. To derive benefit from these chapters requires a working knowledge of the mathematical analysis tools of dynamic systems. Chapter 3 aims to provide sufficient knowledge and understanding to enable a basic flight simulation of a helicopter to be created.

Chapter 4 discusses the problems of trim and stability, providing a range of an­alytical tools necessary to work at these two facets of helicopter flight mechanics. Chapter 5 extends the analysis of stability to considerations of constrained motion and completes the ‘working with models’ theme of Chapters 4 and 5 with a discussion on helicopter response characteristics. In Chapters 4 and 5, flight test data from the DRA’s research Puma and Lynx and the DLR’s Bo105 are used extensively to provide a measure of validation to the modelling. Chapters 6 and 7 deal with helicopter flying qualities from objective and subjective standpoints respectively, although Chapter 7 also covers a number of what we have described as ‘other topics’, including agility and flight in degraded visual conditions. Chapters 6 and 7 are also self-contained and do not require the same background mathematical knowledge as that required for the modelling chapters. A unified framework for discussing the response characteristics of flying qualities is laid out in Chapter 6, where each of the four ‘control’ axes are discussed in turn. Quality criteria are described, drawing heavily on ADS-33 and the associated publications in the open literature. Chapter 8 is new in the second edition and contains a detailed treatment of the sources of degraded flying qualities, particularly flight in degraded visual conditions, the effects of failures in flight system functions and the impact of severe atmospheric disturbances. These subjects are also discussed within the framework of quantitative handling qualities engineering, linking with ADS-33, where appropriate. The idea here is that degraded flying qualities should be taken into consideration in design with appropriate mitigation technologies.

Chapters 3 and 4 are complemented and supported by appendices. Herein lie the tables of configuration data and stability and control derivative charts and Tables for the three case study aircraft.

The author has found it convenient to use both metric and British systems of units as appropriate throughout the book, although with a preference for metric where an option was available. Although the metric system is strictly the primary world system of units of measurements, many helicopters are designed to the older British system. Publications, particularly those from the United States, often contain data and charts using the British system, and it has seemed inappropriate to change units for the sake of unification. This does not apply, of course, to cases where data from different sources are compared. Helicopter engineers are used to working in mixed units; for example, it is not uncommon to find, in the same European paper, references to height in feet, distance in metres and speed in knots – such is the rich variety of the world of the helicopter engineer.

An EH101 Merlin approaching a Type 23 Frigate during
development flight trials
(Photograph courtesy of Westland Helicopters)

It seems to be a common feature of book writing that the end product turns out quite different than originally intended and Helicopter Flight Dynamics is no exception. It was planned to be much shorter and to cover a wider range of subjects! In hindsight, the initial plan was perhaps too ambitious, although the extent of the final product, cut back considerably in scope, has surprised even the author. There are three ‘major’ topic areas, originally intended as separate chapters, that have virtually disappeared – ‘Stability and control augmentation (including active control)’, ‘Design for flying qualities’ and ‘Simulation validation (including system identification tools)’. All three are referred to as required, usually briefly, throughout the book, but there have been such advances in recent years that to give these topics appropriate coverage would have extended the book considerably. They remain topics for future treatment, particularly the progress with digital flight control and the use of simulators in design, development and certification. In the context of both these topics, we appear to be in an era of rapid development, suggesting that a better time to document the state of the art may well be in a few years from now. The absence of a chapter or section on simulation model vali­dation techniques may appear to be particularly surprising, but is compensated for by the availability of the AGARD (Advisory Report on Rotorcraft System Identification), which gives a fairly detailed coverage of the state of the art in this subject up to the early 1990s (Ref. 1.7). Since the publication of the first edition, significant strides have been made in the development of simulation models for use in design and also training simulators. Reference 1.8 reviews some of these developments but we are somewhat in mid-stream with this new push to increase fidelity and the author has resisted the temptation to bring this topic into the second edition.

The book says very little about the internal hardware of flight dynamics – the pilot’s controls and the mechanical components of the control system including the hydraulic actuators. The pilot’s displays and instruments and their importance for flight in poor visibility are briefly treated in Chapter 7 and the associated perceptual issues are treated in some depth in Chapter 8, but the author is conscious of the many missing elements here. In Chapter 3, the emphasis has been on modelling the main rotor, and many other elements, such as the engine and transmission systems, are given limited coverage.

It is hoped that the book will be judged more on what it contains than on what it doesn’t.

Experience has shown that a large percentage, perhaps as much as 65%, of the life­cycle cost of an aircraft is committed during the early design and definition phases of a new development program. It is clear, furthermore, that the handling qualities of military helicopters are also largely committed in these early definition phases and, with them, much of the mission capability of the vehicle. For these reasons, sound design standards are of paramount importance both in achieving desired performance and avoiding unnecessary program cost.

This quotation, extracted from Ref. 1.4, states the underlying motivation for the de­velopment of flying qualities criteria – they give the best chance of having mission performance designed in, whether through safety and economics with civil helicopters or through military effectiveness. But flying quality is an elusive topic and it has two equally important facets that can easily get mixed up – the objective and the subjective. Only recently has enough effort been directed towards establishing a valid set of flying qualities criteria and test techniques for rotorcraft that has enabled both the subjective and objective aspects to be addressed in a complementary way. That effort has been orchestrated under the auspices of several different collaborative programmes to har­ness the use of flight and ground-based simulation facilities and key skills in North America and Europe. The result was Aeronautical Design Standard (ADS)-33, which has changed the way the helicopter community thinks, talks and acts about flying quality. Although the primary target for ADS-33 was the LHX and later the RAH-66 Comanche programme, other nations have used or developed the standard to meet their own needs for requirements capture and design. Chapters 6, 7 and 8 of this book will refer extensively to ADS-33, with the aim of giving the reader some insight into its development. The reader should note, however, that these chapters, like ADS-33 itself, address how a helicopter with good flying qualities should behave, rather than how to construct a helicopter with good flying qualities.

In search of the meaning of Flying Quality, the author has come across many different interpretations, from Pirsig’s somewhat abstract but appealing ‘at the moment of pure quality, subject and object are identical’ (Ref. 1.5), to apoint of view put forward by one flight dynamics engineer: ‘flying qualities are what you get when you’ve done all the other things’. Unfortunately, the second interpretation has a certain ring of truth because until ADS-33, there was very little coherent guidance on what constituted good flying qualities. The first breakthrough for the flying qualities discipline came with the recognition that criteria needed to be related to task. The subjective rating scale, developed by Cooper and Harper (Ref. 1.6) in the late 1960s, was already task and mission oriented. In the conduct of a handling qualities experiment, the Cooper – Harper approach forces the engineer to define a task with performance standards and to agree with the pilot on what constitutes minimal or extensive levels of workload. But the objective criteria at that time were more oriented to the stability and control characteristics of aircraft than to their ability to perform tasks well. The relationship clearly is important but the lack of task-oriented test data meant that early attempts to define criteria boundaries involved a large degree of guesswork and hypothesis. Once the two ingredients essential for success in the development of new criteria, task – orientation and test data, were recognized and resources were channelled effectively, the combined expertise of several agencies focused their efforts, and during the 1980s and 1990s, a completely new approach was developed. With the advent of digital flight control systems, which provide the capability to confer different mission flying qualities in the same aircraft, this new approach can now be exploited to the full.

One of the aspects of the new approach is the relationship between the internal attributes of the air-vehicle and the external influences. The same aircraft might have perfectly good handling qualities for nap-of-the-earth operations in the day environ­ment, but degrade severely at night; obviously, the visual cues available to the pilot play a fundamental role in the perception of flying qualities. This is a fact of operational life, but the emphasis on the relationship between the internal attributes and the external influences encourages design teams to think more synergistically, e. g., the quality of the vision aids, and what the symbology should do, becomes part of the same flying qualities problem as what goes into the control system, and, more importantly, the issues need to be integrated in the same solution. We try to emphasize the importance of this synergy first in Chapter 2, then later in Chapters 6 and 7.

The point is made on several occasions in this book, for emphasis, that good flying qualities make for safe and effective operations; all else being equal, less accidents will occur with an aircraft with good handling qualities compared with an aircraft with merely acceptable handling, and operations will be more productive. This statement may be intuitive, but there is very little supporting data to quantify this. Later, in Chapter 7, the potential benefits of handling to flight safety and effectiveness through a probabilistic analysis are examined, considering the pilot as a component with failure characteristics similar to any other critical aircraft component. The results may appear controversial and they are certainly tentative, but they point to one way in which the question, ‘How valuable are flying qualities?’, may be answered. This theme is continued in Chapter 8 where the author presents an analysis of the effects of degraded handling qualities on safety and operations, looking in detail at the impact of degraded visual conditions, flight system failures and strong atmospheric disturbances.

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.