Advances In Aircraft Flight Control

MB Tischler

• Publisher: Routledge
• Year: 2018
• ISBN:
  9781351468435
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

This book provides a single comprehensive resource that reviews many of the current aircraft flight control programmes from the perspective of experienced practitioners directly involved in the projects. Each chapter discusses a specific aircraft flight programme covering the control system design considerations, control law architecture, simulation and analysis, flight test optimization and handling qualities evaluations. The programmes described have widely exploited modern interdisciplinary tools and techniques and the discussions include extensive flight test results. Many important `lessons learned' are included from the experience gained when design methods and requirements were tested and optimized in actual flight demonstration.

PART ONE

Specification and Validation Methods

1

Handling-qualities specification – a functional requirement for the flight control system

ROGER H. HOH and DAVID G. MITCHELL

1. Introduction

The objective of any flight control system is to provide the pilot with a means to control the aircraft safely and effectively throughout its flight envelope. That is, to provide good handling qualities.† Therefore, the handling-qualities specification represents a logical source of functional requirements to guide the development of the flight control system. Experience has shown that, with a few exceptions, this viewpoint is confined to the handling-qualities community. The use of the handling-qualities specification as a source of functional requirements for the flight control system is rarely, if ever, carried out in practice. In the ‘real world’, flight control design is accomplished as a portion of the avionics system, and handling qualities are studied as a subset of aerodynamics. The lack of high-quality communication between these disciplines has been formally recognized as a problem since 1982 (see Reference 2). The flight control engineer tends to focus on issues related to sensor and actuation hardware, and related fault detection and isolation logic. The control laws are usually relegated to the category of a relatively minor issue to be resolved on the simulator. It is common for the control law selection to be based on the past experience of the engineers and pilots in charge, as opposed to the mission requirements of the subject aircraft.

Handling qualities are usually first considered explicitly during early flight testing of the prototype aircraft. Historically, this approach has led to aircraft that are reasonably safe as long as the pilots are well trained and current. However, with the advent of active control technology (ACT) flight control systems, it has led to serious problems that include divergent pilot-induced oscillations (PIOs) that, in some cases, have led to loss of the aircraft. In most cases these PIOs can be traced directly to handling-qualities deficiencies that are predicted by criteria that exist in current handling-qualities specifications.

The primary goal of this chapter is to familiarize flight control system designers with the latest concepts and criteria that have been implemented into, or are planned for, the handling-qualities specifications. We intend to present these concepts and criteria in the context of functional requirements for the flight control system.

2. Status of handling-qualities specifications

The current fixed-wing handling-qualities specification in the United States is MIL-STD-1797A. This specification supercedes the MIL-F-8785 series (A through C). The differences between MIL-F-8785C and MIL-STD-1797A are summarized below.

• MIL-STD-1797A is in the approved military standard format. The basic specification is a list of criteria with blanks in place of numbers for the quantitative boundaries. Appendix A of MIL-STD-1797A consists of a collection of criteria. The user is expected to select the appropriate criterion values from Appendix A to build a custom standard for a given application.

• MIL-STD-1797A contains criteria that are specifically oriented towards ACT-type control systems.† The database that supports the criteria in MIL-STD-1797A contains substantially more data for highly augmented aircraft than that supporting previous specifications.

The development of MIL-STD-1797 was accomplished during the period between 1980 and 1982, resulting in a proposed specification that is documented in Reference 3. That report was converted to the actual MIL-Standard during the period 1982 through 1987. Since that time, work has been accomplished to develop a more mission-oriented specification. Most of this work was done to develop a flying-qualities specification for US Army helicopters, resulting in an Aeronautical Design Standard, ADS-33C. ADS-33C (Reference 4) was used to specify the handling-qualities requirements for the Boeing/Sikorsky RAH-66 ACT rotorcraft. It has since been refined and subjected to the tri-service review process, see Reference 5. Reference 5 is expected to supersede MIL-H-8501A as the military rotorcraft specification in the United States within a year. In this chapter, we shall refer to Reference 5 as the rotorcraft MIL-Standard under the assumption that final acceptance is imminent.

While there has been considerable work to upgrade MIL-STD-1797A to include the now-accepted mission-oriented concepts, that work has not been incorporated into the specification as of this writing. Plans are to accomplish a major upgrade from MIL-STD-1797A to MIL-STD-1797B in 1997. It is recommended that control system designers consult References 6 and 7 to augment MIL-STD-1797A as a source of functional requirements.

3. Handling-qualities specifications – are they necessary?

There is substantial resistance to specifications in the name of cost cutting and paperwork reduction. What is overlooked is the fact that most specification requirements are a result of lessons that have been learned at great cost. Nearly all serious handling deficiencies that have occurred can be traced to noncompliance with existing specification criteria. The YF-22 PIO, which led to a crash on the runway at Edwards AFB, is a good example. This aircraft fails the Equivalent System Time Delay and Bandwidth Phase Delay criteria in MIL-STD-1797A by a wide margin. Such all-too-common cases of noncompliance with specification criteria are an indication that these criteria are not commonly used as functional requirements for the flight control system.

One probable reason for this situation is the complexity of the handling-qualities criteria. Handling qualities are difficult to specify because they inherently involve the quantification of pilot workload. The tools available to measure pilot workload require considerable experience and background knowledge to apply. For example, the Cooper-Harper subjective rating scale (Reference 1) is the basis for the criteria in all current handling-qualities specifications. Cooper-Harper handling-qualities ratings (HQRs) have proven to be reasonably reliable under certain test conditions, but subject to serious shortcomings in others (Reference 8). Specifically, the scale has been shown to produce consistent and reliable results when used properly in a research experiment where the pilot subjects have no vested interest in the outcome. However, when there is a vested interest, experience has shown that subjective opinions and ratings can be unreliable, even with the most experienced and conscientious test pilots (e.g. see Reference 8). Those not familiar with this phenomenon (or who do not believe that it exists) tend to place excessive reliance on piloted simulation to design empirically the flight control system. This approach often has appeal to management because it makes use of a very expensive company asset. Unfortunately, the limited visual and motion cues that are available in even the most advanced simulators, in combination with the potential for basing control laws on the preconceived notions of a few pilots, have a long history of failure.

Quantitative handling-qualities criteria are based on the results of research experiments that include variable-stability flight tests using pilots with no vested interest. This is the fundamental reason why the quantitative specification criteria should be met, even if this does not seem intuitively necessary.

Consider the following scenario. During the preliminary design review of a new aircraft, the engineers note that it does not meet certain handling-qualities criteria in the handling-qualities MIL-Standard. It is noted that the work required to modify the flight control system would involve considerable additional cost, and would have an adverse impact on the schedule. Management is not sure that the problem is real, and does not want to spend money and time unless it is absolutely necessary. The company test pilots are asked their opinion based on simulator and/or flight evaluations. They indicate that the flying qualities are satisfactory. In this environment, it is not practical (or a good career decision) for the engineers to question the validity of evaluations made by the test pilots. If there is no contractual requirement to meet the flying-qualities specification, the decision to ignore the ‘problem’ is inevitable. And this in spite of the fact that such a decision is based on the opinion of pilots with a strong vested interest, and often with experience that is limited to unstructured testing using ad hoc tasks.

In this all-too-common scenario, pilot opinion is judged to have precedence over specification criteria. This implicitly ignores the fact that the quantitative criteria are based on the collective opinions of test pilots, with no vested interest, performing highly structured tasks, usually in a variable-stability flight test environment. The only way to avoid this pitfall is to make the most current handling-qualities specification a contractual requirement. Feedback from engineers on a recent ACT rotorcraft development indicated that having the ADS-33C handling-qualities specification as a contractual requirement was extremely valuable in obtaining management approval to deal with identified problems.

The need for good handling qualities is subtle because most handling-qualities deficiencies can be compensated for by a competent pilot (albeit with training and practice). Once a pilot has managed to develop the skill required to overcome a handling-qualities deficiency, he or she may even consider it an enhancement. The personality factors that result in this phenomenon are complex. They are almost certainly related to the competitive nature of an individual drawn to a profession that involves continuous testing of his or her mental and physical skills. Implicit in this ‘good handling qualities are for sissies’ attitude is a synergy with management where the primary goal is to limit costs and to stay on ‘success-oriented’ schedules. However, we are learning that even the most skilled pilots make fundamental errors when confronted with highly stressful real-life scenarios, e.g. tired crew, short runway, turbulence, windshear, night, etc. While such an unfortunate combination is rare, it is precisely when safety dictates good handling qualities.

Manufacturers of large aircraft have often taken a position that the boundaries for short-period frequency in MIL-STD-1797A are too stringent for large aircraft in the power-approach flight condition (Category C). They argue that many existing aircraft that do not meet these boundaries are routinely landed safely by average pilots. Few operational pilots will admit that under some conditions, significant recent training and skill is required to accomplish the required tasks successfully. The vested-interest implications are obvious in the context of this discussion, and very subtle in the real world of flight control design.

In summary, the need for a flying-qualities specification is subtle, but of primary importance to maintain the desired level of safety in critical flight conditions. In military operations, t...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Introduction
• Foreword
• List of Contributors
• Part One Specification and Validation Methods
• Part Two Rotorcraft and V/STOL
• Part Three Transport Aircraft
• Part Four High-performance Aircraft
• Index