1l| Aircraft Sizing, Engine Matching, and Variant Derivative

11.1 Overview

Chapter 6 proposes a methodology with worked-out examples to conceive a “first – cut” (i. e., preliminary) aircraft configuration, derived primarily from statistical infor­mation except for the fuselage, which is deterministic. A designer’s past experience is vital in making the preliminary configuration. Weight estimation is conducted in Chapter 8 for the proposed first-cut aircraft configuration, revising the MTOM taken from statistics. Chapter 9 establishes the aircraft drag (i. e., drag polar), and Chapter 10 develops engine performance. From these building blocks, finally, the aircraft size can be fine-tuned to a “satisfactory” (see Section 4.1) configuration offering a family of variant designs. None may be the optimum but together they offer the best fit to satisfy many customers (i. e., operators) and to encompass a wide range of payload-range requirements, resulting in increased sales and profitability.

The two classic important sizing parameters – wing-loading (W/S) and thrust­loading (TSls/W) are instrumental in the methodology for aircraft sizing and engine matching. This chapter presents a formal methodology to obtain the sized W/S and Tsls/W for a baseline aircraft. These two loadings alone provide sufficient infor­mation to conceive of aircraft configuration in a preferred size. Empennage size is governed by wing size and location on the fuselage. This study is possibly the most important aspect in the development of an aircraft, finalizing the external geometry for management review in order to obtain a go-ahead decision for the project.

Because the preliminary configuration is based on past experience and statistics, an iterative procedure ensues to fine-tune the aircraft for the correct size of the wing reference area for a family of variant aircraft designs and matched engines selected after discussion with engine manufacturers. Reference [1] provides an excellent pre­sentation on the subject.

11.1.1 What Is to Be Learned?

This chapter covers the following topics:

Section 11.2: Introduction to the concept of aircraft sizing and engine matching

Section 11.3: Theoretical considerations

Coursework exercises for civil aircraft Coursework exercises for military aircraft Sizing analysis and variant designs of civil aircraft Sizing analysis and variant designs of military aircraft Sensitivity analysis Future growth potential

11.1.2 Coursework Content

This chapter is important for continuing the coursework linearly. Readers compute the parameters that establish the criteria for aircraft sizing and engine matching. The final size is unlikely to be identical to the preliminary configuration; the use of spreadsheets facilitates the iterations.

11.2 Introduction

In a systematic manner, the conception of a new aircraft progresses from generating market specifications followed by the preliminary candidate configurations that rely on statistical data of past designs in order to arrive at a baseline design. In this chap­ter, the baseline design of an aircraft is formally sized with a matched engine (or engines) along with the family of variants to finalize the configuration (i. e., external geometry). An example from each class of civil (i. e., Bizjet) and military (i. e., AJT) aircraft is used to substantiate the methodology.

As of the circa 2000 fuel prices, the aircraft cost contributes to the DOC three to four times the contribution made by the fuel cost. (Fuel price fluctuates consid­erably. Of late, fuel price has shot up, making its contribution to DOC substantially higher. In this book, circa 2000 price level is maintained. That level of price held for a long time and large number of literature use this approximate value.) It is not cost-effective for aircraft manufacturers to offer custom-made new designs to each operator with varying payload-range requirements. As discussed previously, aircraft manufacturers offer aircraft in a family of variant designs. This approach maintains maximum component commonality within the family to reduce development costs and is reflected in aircraft unit-cost savings. In turn, it eases the amortization of non­recurring development costs, particularly as sales increase. It is therefore important for the aircraft-sizing exercise to ensure that the variant designs are least penalized to maintain commonality of components. This is what the introductory comments in Section 4.1 referred to in producing satisfying robust designs; these are not nec­essarily the optimum designs.

Multidisciplinary optimization is not easily amenable to this type of industrial use; it is currently explored more as research work. The industry uses a more sim­plistic parametric search for satisfying robust designs.

To generate a family of variant civil aircraft designs, the tendency is to retain the wing and empennage almost unchanged while plugging and unplugging the con­stant fuselage to cope with varying payload capacities (see Figure 11.4). Typically, the baseline aircraft remains as the middle design. The smaller aircraft results in a wing that is larger than necessary, providing better field performances (i. e., takeoff and landing); however, cruise performance is slightly penalized. Conversely, larger aircraft have smaller wings that improve the cruise performance; the shortfall in

takeoff is overcome by providing a higher thrust-to-weight ratio (TSLS/W) and pos­sibly with better high-lift devices, both of which incur additional costs. The baseline – aircraft approach speed, Vapp, initially is kept low enough so that the growth of Vapp for the larger aircraft is kept within the specifications. Of late, high investment with advanced composite wing-manufacturing method is in a position to produce sepa­rate wing sizes for each variant (large aircraft), offering improved economics in the long run. However, for some time to come, metal wing construction will continue with minimum change in wing size to maximize component commonality.

Matched engines are also in a family to meet the variation of thrust (or power) requirements for the aircraft variants (see Chapter 10). The sized engines are bought-out items supplied by engine manufacturers. Aircraft designers stay in con­stant communication with engine designers in order to arrive at the type of family of engines required. A variation of up to ±30% from the baseline engine is typically sufficient for larger and smaller aircraft variants from the baseline. Engine design­ers can produce scalable variants from a proven core gas-generator module of the engine – these scalable projected engines are known loosely as rubberized engines. The thrust variation of a rubberized engine does not affect the external dimensions of an engine (typically, the bare engine length and diameter change only around ±2%). This book uses an unchanged nacelle external dimension for the family vari­ants, although there is some difference in weight for the different engine thrusts. The generic methodology presented in this chapter is the basis for the sizing and matching practice.

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