Category Helicopter Test and Evaluation

2.1 INTRODUCTION

Unlike a conventional fixed wing aircraft, which has separate means of generating forces for lift and forward propulsion, the helicopter uses only the thrust from a rotor to meet these two essential requirements for sustained flight. This limitation does, however, afford the helicopter a unique advantage over most fixed wing aircraft: the ability to generate a lift force even when the vehicle is stationary. Understanding the basic manner in which a rotor system develops thrust is fundamental to any study of the measurement of rotorcraft performance. It is also required in any introduction to the stability and control attributes of a typical helicopter. There are two basic theoretical approaches to understanding the generation of thrust from a rotor system: momentum theory; and blade element theory. Other more complex theories tend to build on the fundamentals introduced by these two approaches. Since many standard texts on helicopter performance cover momentum theory and blade element theory [2.1 to 2.7] it is only necessary for us to review the key points here. Forward flight produces asymmetric flow across a rotor disk and thus it is desirable for us to start by restricting our discussion to purely axial flight.

2.2 AXIAL FLIGHT: MOMENTUM THEORY

Momentum theory was initially developed by Rankine and Froude from their study of ship propellers or water screws. It later found application in the design of airscrews

[2.3] and is used here to represent a climbing rotor. The theory makes certain assumptions:

• Air is an inviscid and incompressible fluid.

• The rotor acts as a uniformly loaded or ‘actuator’ disk with a infinite number of blades so that there is no periodicity in the wake.

• The flow both upstream and downstream of the disk is uniform, occurs at constant energy and is contained within a streamtube.

• No rotation is imparted on the fluid by the action of the rotor.

These assumptions necessarily limit the accuracy of the Froude theory. The momentum approach has, however, been extended to cover the more realistic case of a compressible fluid [2.7]. Other real effects such as swirl and unsteady flow can be accommodated by appropriate empirical factors [2.4, 2.6 and 2.8].

1.4.1 Trials reporting

The deliverable from most trials is a written report which addresses the trials objective and records the test data. Careful planning of the report takes place before the trial is conducted to ensure that the final product is of good quality. This planning will scope the document, produce a skeleton structure, and define responsibilities. One of the most important planning stage actions, particularly for large, multi-discipline reports, is to nominate a single person to be in overall charge of the report. He or she can then ensure that the style and content are consistent across the authoring departments and eliminate duplication, or worse, contradictions. Each report has an introductory section that contains all the information relating to how the trial was conducted. This includes who performed the trial, when it was performed, the state of the aircraft or equipment tested, the instrumentation used, and the tests that were performed. The next section documents the results of the trial, including the conclusion of the test team and any recommendations made. In many test establishments this is written in a set layout known as the seven-part format. The aim of this format is to ensure that a person reading the report understands exactly what was done, what the results were, what the data mean, what implications a deficiency would have in operations, what the conclusion is, what the recommendations are, and whether or not the article under test meets the relevant specification or not. The seven-part format is shown below:

• Establish the test conditions;

• Present the data;

• Analyze and discuss the data;

• Relate to the operational role;

• Conclude;

• Recommend;

• Specification compliance.

Most reports have a summary and a section that groups together the recommenda­tions. In all cases a successful report will make it clear what was done during the trial and will provide a convincing argument to implement the test team’s recommendations.

1.4.2 Learning from the past

Human nature being what it is, there is a tendency to believe that a trial is complete once the report has been delivered and accepted by the customer. However it is important to have a trial’s closure procedure which ensures that all the data gathered is retained. This data may be required to provide baseline information for comparison purposes when the aircraft is modified in the future. Given the long periods that rotorcraft are kept in service the data may have to be retained for a considerable number of years. As well as keeping copies of the customer reports all the other trials documentation such as trials instructions, risk assessments, etc. are retained to serve as templates for the future. A ‘lessons learned’ system is also maintained where information concerning mistakes, problems, solutions and good ideas can be retained even though the trials participants may move on.

1.3.1 Establishing the test conditions

For the results of a trial to have any meaning the conditions which pertained at the time of the test will be of critical importance. Thus, for example, if a performance trial is being conducted the test pilot will have to fly the aircraft at exactly the right speed, with the right value of sideslip, and at the correct altitude. The environmental conditions such as temperature, winds, turbulence levels, etc. will also have to be appropriate. In many trials a variety of conditions will be required and often at opposite extremes. For example, very still air conditions may be needed to obtain accurate results but high levels of turbulence may also be required to give the test pilot conditions representative of operational flying to allow him or her to make qualitative judgements of the handling qualities. In all cases the test conditions need to be recorded for inclusion in the flight report.

1.3.2 Moving into the unknown

Nearly all test flying involves going from a known condition into an unknown one and it is how this is done which is important. Although there will always be an element of risk in this situation, provided the correct methodology is applied nasty surprises can mostly be avoided. The main part of this methodology is to adopt the incremental approach, i. e. only small changes are made in a single parameter when moving from a known condition to the next test point. As an example if control response tests are being conducted by making step cyclic inputs into a control fixture the size of the inputs would be increased very gradually. After each input the change in aircraft response is compared with the previous results with the aim of determining the relationship between changes in the input parameter and the aircraft output. Once a relationship has been established this can be used to predict the output for the next test point to determine if the test should continue. Any change in the relationship can also serve as a warning that extra caution is needed. To establish this input/output relationship a ‘how goes it plot’ is often constructed which in its simplest form consists of a graph where the two parameters are cross-plotted. Also marked on the graph will be the pre-determined test limits. Each test point adds to the trend line and the line is extrapolated prior to each subsequent point to ensure that the predicted result lies inside the test limits. For this system to work it is vital that only one parameter is changed at a time. Thus an input size change should not be made at the same time as the aircraft speed is changed. In addition to establishing trend lines for the test results the team also need to have a sound idea of the expected results prior to the trial taking place. This information can come from a variety of sources, such as results of previous trials under similar circumstances or as a result of computer modelling. Again the predicted trend information is compared with the actual results and major differences serve as a warning that all factors have not been accounted for.

A set of test cards is normally produced for each test flight. These cards serve two purposes: firstly they act as an aide-memoire of the tests to be conducted and limitations that apply, and secondly they provide a means of recording manual data to back up flight test instrumentation data. It would be hard to overemphasize the importance of having carefully produced test cards. Each card defines the test being made with the conditions required and has sufficient space to record data if required. The limits for that particular test are written clearly together with any reminders. On each card the planned start and finish time for the test are included with the expected fuel state. It is often helpful to colour code the cards to indicate the priority of tests, which allows easy airborne re-planning in the event that the flight falls behind time. When producing the cards it is best to sit in the cockpit and note the most efficient order to gather information from the aircraft or test instruments. Once the cards have been made the crew again sit in the aircraft and go through a sortie rehearsal to ensure that the cards are satisfactory.

The TI is the main output from the planning phase and is the document used to authorize and control the trial. In the TI is contained all the information that a senior person in the trials organization needs to be able to understand what is being planned and allows him or her to approve the trial. Thus the document needs to define the test methods, test procedures, and test conditions that will be used. The test methods and procedures are often standard and only require a reference to other documents which define them. The test conditions, on the other hand, always require careful thought as these vary with the trial being made. It is not simply a case of defining the target conditions for the trial but rather a range of permissible values of variables is defined so that the trial can proceed even if the exact desired conditions are not present. The TI also details the trials programme including such aspects as the size of increments in control parameters between successive tests and the criteria for progressing to the next test or phase. A key element of the TI is a section which details the safety precautions that will be taken. This safety section comes mainly from the risk assessment which is also included in the document so that the authorizing officer can ensure that the process has been conducted correctly.

1.2.3.1 Sortie planning

Each sortie in a test programme requires careful planning to ensure that the flight follows a safe progression and that the maximum amount of high quality data is gathered in the flight time available. Where possible tests are conducted concurrently, for example, information on flight control positions and engine bay temperatures can be gathered during level flight performance tests. This clearly requires careful liaison between the different parts of a flight test organization if they are all involved in the same trial. The individual test points are organized such that successive points can be achieved quickly. The principle is that no flight time is wasted. It is better to have too many test points in a sortie plan than too few but the plan still needs to be reasonable and achievable. The possible effects of weather are considered as well so that tests are organized into high level and low level sorties to ensure that time is not wasted if the cloud base is low. For each sortie the go/no go criteria is decided upon at the planning stage, for example, this might be a minimum light level or a maximum wind strength. Part of the sortie planning procedure is to produce a written flight brief, which will be used during the pre-flight briefing to ensure that all trials participants are fully prepared. Included in this brief will be the flight safety points, priorities, limitations, go/no go criteria, possible alternate tests, criteria for stopping the tests, and responsibili­ties of individual participants.

Once the trials risks have been identified and the RRMs have been employed there will still be a certain amount of residual risk which is then quantified. This will then lead to the trial being categorized as high, medium, or low risk. The main reason for allocating a category of risk is to alert the responsible senior officer to those trials which require particularly careful supervision. One definition of risk categories used at the UK test establishment at Boscombe Down is as follows:

• Low risk: the residual risk is no higher than normal peacetime flying.

• Medium risk: there is an increased risk of injury to crews or damage to the aircraft over normal operations.

• High risk: there is an increased chance of loss of life or loss of an aircraft.

1.2.2 Trials documentation

Part of the planning process which is often not given sufficient attention, is the technical documentation and reporting requirement. All the documents needed to conduct the trial safely require definition. These include TIs, instrumentation schedules, limitations documents, and risk assessments. In most test establishments the manage­ment procedures call for these documents and define their content. The eventual trials report or reports also require identification early in the planning stage.

Having identified the risks the next stage is to decide how they might be mitigated as much as possible. The measures which can be taken are known as risk reduction measures (RRM) and are typically divided into five main categories:

(1) Training: Correct and comprehensive training of the test team is vital to reducing the risk on any test programme. This training will be required on the aircraft itself and in the case of initial trials on a new aircraft encompasses the ground instruction, simulator training, conversion course and continuation training. On other aircraft it may only involve currency training on type. The training may also involve the practise and perfection of the test techniques to be used or the flight skills to be employed during the programme, such as night flying, NVG instrument flying. Safety and survival training for all crew members is also conducted.

(2) Supervision: The close supervision of the test programme from the initial inception of the requirement, through the raising of the trials instruction (TI), the test plan and the individual flight authorization is possibly the single most important RRM. It is often the greater experience and the objectivity of those in a supervisory position which is able to identify possible risks that may not be apparent to those who are intimately involved in the programme itself. The individuals with supervisory responsibility may also be able to identify any inadequacies with the performance of the trials personnel and take action accordingly.

(3) Procedures: The application of logical procedures enables any potential risks to be reduced. These include the method to be used for setting up test points, gathering data and recovering to a normal flight condition. In addition proce­dures are developed to cover emergencies and other unexpected events. These procedures are laid out in detail in the test plan for the flight(s) and are reviewed during the pre-flight briefing prior to authorization. Procedures are also developed for the dissemination of results and information within the test team and within the test establishment; this is particularly important where changes may be made to the test aircraft during the test programme.

(4) Limitations: Every flight and test programme is subject to a variety of limitations some of which are imposed by the manufacturer; the others are imposed by the test establishment. The limitations may be imposed for structural considerations, to prevent damage to systems, for handling reasons, or to enhance flight safety. In every case it is essential that all participants are aware of every limitation and that they are respected during all flights. The limitations are detailed in the test plan and revised in the pre-flight brief.

(5) Monitoring: The monitoring of a test programme or flight can take a variety of forms all of which may be employed if appropriate. This monitoring usually involves the comparison of the actual results with the anticipated results to ensure that the programme is progressing in the expected manner: a lack of correlation between these results may indicate that not all factors have been addressed correctly during the planning stage and therefore the plan should be reassessed. The comparison of actual with anticipated results is also conducted for individual flights. In addition the actual results in flight are assessed for each test point prior to moving on to the next increment; this may require the use of telemetry to a ground monitor or the use of in-flight plots to predict the likely results from the next point. External monitoring of the flight may also be employed through the use of chase aircraft and ground observers.

Test flying by its very nature involves a degree of risk above that which is routinely accepted during normal operations. To ensure that tests are conducted as safely as possible a process of identifying and reducing risk is employed. The aim of this process is to determine what the risks are, decide on the best way to reduce them and then to decide if the residual risk is acceptable.

1.2.2.1 Conducting a hazard analysis

To identify risk areas the programme or flight is divided into sections and the potential dangers considered in each one: this will involve asking the question at each stage ‘What could possibly present a hazard?’ and ‘What would be the possible consequences?’ This procedure relies to a large extent on the experience of the test pilot or FTE, however, a thorough and logical assessment of each phase of flight should lead to the identification of individual risks. It is normal to involve a number of people by calling a review meeting where the group examine each aspect of the flight. Any databases available are also examined to ensure that the lessons learned on previous trials of a similar type are incorporated. On completion of this process a list of Identified Risk Elements (IRE) can be produced for each section of the programme or flight.

The Flight Test Process

1.1 INTRODUCTION

Flight test is an expensive activity, which by its very nature attracts levels of risk higher than normal operations. To ensure that all trials are conducted as efficiently and safely as possible, a flight test process has been developed over the years. This process is used, with only minor variations, in nearly all test organisations throughout the world whether they be military, civilian or based at a manufacturing facility. Many of the steps in this process have evolved as the result of painful lessons and, therefore, the authors consider them vital to the overall test activity.

It is perhaps true to say that at the beginning of their careers, flight test personnel show greater interest in the test methods that they need to apply than in the overall system used to approve, authorize and regulate test flying. Consequently it was decided to make the test process the first chapter of this book to demonstrate the importance that the authors attach to this subject. It is our belief that without an understanding of the process the knowledge contained within the rest of the book cannot be applied effectively.

Brief details of the flight test process are given below. For more details the reader is referred to the AGARD flight test techniques and instrumentation series [1.1 and 1.2]. Generally the flight test process can be broken down into three major areas:

• Flight test planning.

• Conducting the test.

• Post-test actions.

1.2 FLIGHT TEST PLANNING

Thorough planning is vital for all flight trials to ensure that they are conducted safely, efficiently and that the trials objective is met. When a trial is first proposed to a test establishment a management plan is produced which includes a work breakdown structure defining the individual elements of the trial. For each element the department or departments which are to undertake the work are determined together with costs and timescales. Part of the management plan is to define the exact project technical objectives and it is at this level that this book will examine the planning process.

1.2.1 Technical objectives

To define the project technical objectives the exact requirements of the customer calling for the trial have to be understood clearly. Once this has been done then the next step is to decide on the assessment philosophy, in other words ‘What are we going to do? – How much of it are we going to do?’ Deciding on what approach to take depends on a number of factors such as the level of expertize within the test organization, the resources available, and if the trials results need to be compared with earlier results using a particular test method. The approach to the trial and the scope of the work both depend on the requirement to collect evidence and to identify where that evidence is located.

Determining what evidence is needed to be able to make well-supported recom­mendations relies upon the professional knowledge of the test team, although previous trials may provide a guide. The evidence itself does not necessarily need to come from a flight conducted as part of the trial, indeed because flight testing is such an expensive activity other sources of evidence are invariably considered first. These sources may be the aircraft manufacturer, who may have conducted the required test, earlier test results held at the test establishment, read across from other similar aircraft, another test organization, or possibly an operator who already has experience of the test article. Modelling and simulation are also used extensively. In each case the evidence and the source are considered carefully to determine to what extent it can be relied upon. If the existing evidence is determined to be reliable then it may be used without further testing or, more likely, a limited number of tests would be planned to ‘spot check’ the evidence to increase confidence in its validity.

Once the existing evidence has been established then the difference between this and the evidence required is what the flight trial must address, this defines the scope of the test. The team then decides how much testing is required to gather the missing evidence to allow the test objective to be met. From this is defined the detailed work breakdown structure including test techniques, required environmental conditions, trials location, the order and interdependency of tests, and the allocation of tasks to parts of the organization. Having defined the tests to be made, the facilities required to conduct them have to be identified: these usually include the aircraft build standard, ranges, test instrumentation and equipment, the test crew and their training standard, and data replay and analysis equipment.

The authors of this book have been employed at the Empire Test Pilots’ School (ETPS), part of QinetiQ, for the past ten years instructing student test pilots and flight test engineers on helicopter testing. Alastair Cooke has a masters degree in flight dynamics from Cranfield University and graduated as a flight test engineer from ETPS in 1989. Eric Fitzpatrick is a former military helicopter pilot and instructor who graduated as a test pilot from ETPS in 1986.

This book has been produced using the experience of the authors in flight test and flight test training gained over a combined period of 25 years in the field. Much of the material has its origin in the training notes produced by ETPS. These have been developed over a period of over forty years since the start of specific training for helicopter test pilots. The book has been designed to appeal to professionals working in the area of rotorcraft test and evaluation but it is hoped that it will also prove useful to a wider audience. In our experience, we have found that helicopter pilots are generally not well informed about the process that has led to their aircraft entering service, nor about why it has certain limitations imposed on it. We hope this book will provide pilots with this information as well as being a useful text for practising engineers and technologists. The rotor theory presented is more extensive than is found in most aeronautical degree courses and so the book should prove useful to graduates specializing in rotorcraft technology. Perhaps uniquely, this work approaches this important subject from both the theoretical and practical viewpoints.

For each topic the theory is explained briefly and is then followed by details of the practical aspects of testing a helicopter. These details include the safety considerations related to the anticipated tests, planning the tests themselves, and, where appropriate, the most efficient way to conduct individual flights. Following a description of each type of test, typical results are examined and an explanation given as to why they would be important to the clearance process. Whenever possible examples of actual test results have been presented and used in the subsequent discussion. The book is split into four main sections:

• Introduction: covering a methodology for testing and general aspects of test programmes.

• Performance: in this section level flight, vertical and climb/descent performance is addressed. The planning of performance trials is covered together with the methods for airborne data gathering and analysis of results.

• Handling qualities: this is a major section and covers the basics of helicopter stability and control testing. Also included are frequency domain methods and the use of mission task elements.

• Systems: in this section the major systems required to enable a helicopter to fly are
covered. This includes assessment of the cockpit, air data systems, engine control systems, and automatic flight control systems. Also addressed in this section is the testing of system failures.

Although highly specialized, the topic of helicopter testing is still vast and no single text could hope to cover everything. The authors have attempted, therefore, to concentrate on the most important aspects using their own knowledge of the subject as a guide. Inevitably, a number of important areas have not been covered; for example, it has not been possible to include information on specialist areas such as underslung load trials, deck operation trials or armament testing. The amount of information on systems testing to include in the book was a difficult decision due to the plethora of systems that can be fitted to a modern rotorcraft. Consequently, it was decided to detail the general methodology used in this type of testing and then to concentrate on systems which are intrinsic to the operation of all helicopters. Thus, it has not been possible to include the testing of hydraulics, electrics and lubrication systems. Similarly the testing of a number of cockpit systems such as piloting vision aids, navigation systems, weapons, and mission displays were considered to be beyond the scope of this book. Finally, it was not possible to include some types of testing which often play a large part in the life of a test pilot such as environmental trials, notably cold weather and icing. Despite these omissions, it is believed that the book covers all the essential areas of rotorcraft testing and will prove useful to a large part of the helicopter community. As the authors’ background has been in military test and evaluation this has clearly influenced the subject matter that has been presented. However, most of the information given in the book can be applied, with only minor modification, to the testing of civil rotorcraft.

Before closing we would like to acknowledge the help of Mr Mike Cook of Cranfield University, Mr Mark Roots of QinetiQ Ltd and Miss Julia Burden of Blackwell Pub­lishing in helping us develop our manuscript for publication.

Alastair Cooke and Eric Fitzpatrick ETPS, QinetiQ Ltd MOD Boscombe Down

Copyright acknowledgements

The following organizations are gratefully acknowledged for granting permissions for the use of copyright material.

The UK MOD for the cover photograph. NASA for Figure 3.37 and the AHS for Figures 3.18, 3.20 and 3.22.

The example helicopter

On several occasions throughout this book quantitative calculations have been made to support the theoretical trends being discussed. Where possible the calculations have been made using the same baseline data, referred to as the example helicopter. The

details of this helicopter, which is loosely based on the Westland Lynx, are summarized below: