Secondary power systems

We have looked at an aircraft's primary power system, its propulsion, and will now examine its secondary power systems (SPS). Secondary power is usually provided by an aircraft's main engines when they are operating but can be supplied by other sources such as an auxiliary power unit (APU) in the aircraft or an external source on the ground.

Almost all present aircraft SPS are of three types, shown in Fig. 6.1.

Power is extracted from turbo-fan powerplants in two forms. Bleed air is tapped from one or more points along the engine compressor to provide pneumatic power. Drive shafts, from the engine's high pressure shaft, drive an accessory gear box on which are mounted generators and hydraulic pumps as well as the engine's own accessories. A typical large turbo-fan is shown in Fig. 5.13.

Turbo-jet powerplants can also supply bleed air and shaft power for systems, but turbo-prop powerplants in general have difficulty in supplying sufficient bleed air for airframe requirements. In such a case extraction of shaft power to drive separate compressors may be considered.

SPSs are often linked with each other, but it is convenient to separate them into the types described below.

Air-conditioning and pressurization

On many civil transports the largest amounts of secondary power are extracted in the form of bleed air and the largest continual user of this power is the aircraft's air-conditioning and pressurization system.

Introduction

The design, manufacture and operation of aircraft are very complex and potentially risky activities. There is a natural tendency towards trusting tried and tested configurations, and adopting an evolutionary approach. There are many cases, however of revolutionary design changes, such as the advent of the jet engine, or the delta wing. Revolution involves more risk, but can be very rewarding. The history of aircraft design has had many examples of excitingly innovative, not to say weird designs. A minority of these are successful, some marginally successful, but many have been failures, some of them disastrously expensive failures! This chapter will describe a number of different projects and attempt to classify the reasons that led to their difficulties.

On a more positive note, the chapter will conclude with describing of a number of innovative designs which might have a great influence on the future of aeronautics – positively or negatively!

Aircraft that suffered from requirements that were too restrictive, too ambitious or were changed during development

Ward contains many examples of such projects from the depressing record of British aircraft developments since 1945, and several examples will be repeated here, both civil and military. Other sources were used for later aircraft.

The Hawker Siddeley Trident

This aircraft (Fig. 11.1) had many years of successful service in the UK and in China, and was a pioneer in the field of automatic landing, but sales were disappointing.

This book acts as an introduction to the full breadth of both civil and military aircraft design. It is designed for use by senior undergraduate and post-graduate aeronautical students, aerospace professionals and technically-inclined aviation enthusiasts.

The book poses and answers pertinent questions about aircraft design, and in doing so gives information and advice about the whole aircraft design environment. It asks why we should design a new aircraft and gives examples of market surveys and aircraft specifications. It then answers the question ‘why is it that shape’ and gives the rationale behind the configurations of a wide range of aircraft from micro-lights and helicopters to super-jumbos and V/STOL aircraft, with many others in-between. Having examined the shape, the book then examines and describes what is under the skin in terms of structure, propulsion, systems and weapons. Later chapters answer questions about aircraft costs and conceptual design and draw lessons from past projects and then look into the future. A major part of the book answers the question ‘what help can I get?’ It is a combination of bibliography, lists of data sheets, computer tools and 100 pages of appendices of design data vital to aircraft conceptual designers (most of it previously unpublished).

The book concentrates on fixed-wing civil and military aircraft, with some reference to light aircraft and rotorcraft, but does not address the design of sailplanes, airships, flying boats or spacecraft.

Military aircraft types

Fighters

These aircraft have had several designations over the years, including pursuit and interceptor aircraft. There are many types, and definitions become blurred between fighters and bombers and ground-attack aircraft. True fighters may be divided into two major classes. The first class is that of relatively simple short-range interceptor aircraft (class 1), whilst the second used to be called ‘all-weather’ fighters which tend to have longer range and more avionic equipment (class 2). Figure 4.1 shows a plot of combat wing loading against thrust/weight ratio, which is a good indication of the manoeuvrability of combat aircraft (see Chapter 3 for the definition of thrust/weight and wing loading). Recent requirements for high agility have moved aircraft towards the top left-hand corner where high thrust/weight and low wing loading improve climb, sustained turn and attained turn performance. Most aircraft in this region are class 1 short-range fighters and class 2 are closer to the bottom-right quadrant, where the higher wing loading leads to better long-range cruise.

The class 1 short range, high performance interceptor carries the minimum of equipment and maximum speed is always important. However for such an aircraft the rate of climb and manoeuvrability may be even more important. This arises from the fact that due to the fighter's necessarily short range it cannot take off until a target is definitely located, but must then rapidly climb to interception.

Avionic systems

Avionics is one of the most rapidly developing fields of aircraft design. Its importance and range has increased over recent years and as much as 40% of the cost of a new aircraft can be attributed to avionics. There is a bewildering range of avionic systems, each of which usually requires the use of many acronyms.

Figure 7.1 shows an avionics fit for a small executive aircraft. This aircraft is intended for flight with a crew of one or two pilots. If this aircraft had been in operation during the early post-war years, three additional crew-members would have been required, namely navigator, wireless operator and flight engineer. Modern efficient, reliable avionics have dramatically simplified these operations and have eliminated to need for these crew members.

The growth in the capabilities of military avionics has been even more dramatic and make it possible for pilots of single-seat aircraft to navigate, communicate, detect and attack targets at heights of 100 ft and speeds approaching the speed of sound. Figure 7.2 shows a schematic of such an avionic system whilst Fig. 7.3 shows the installation of components on the instrument panel.

Having looked at typical aircraft installations the next stage is to examine the functions of various types of avionic systems in the following major groups: communications, navigation systems, radar systems and others.

INTRODUCTION

Definition

In engineering, an experimental event (here called a trial) is often constrained to admit only two possible outcomes, usually labeled success s and failure f.

Suppose the engineer contemplates a sequence of n such trials. If the population, from which the sample sequence is randomly chosen, is of finite size N, then it will contain some number M of items that would each produce a trial success s. The number x of successes s that could turn up in the sample sequence may then be of interest to the engineer.

The above experimental situation is characterized by the following conditions:

1. The sampled population is of finite size N.

2. A sample of n < N trials is randomly selected.

3. Each trial admits only two outcomes: s or f.

4. There are 0 ≤ M ≤ N successes s in the population.