Section 11.4: Coursework exercises for civil aircraft

Section 11.5: Coursework exercises for military aircraft

Section 11.6: Sizing analysis and variant designs of civil aircraft

Section 11.7: Sizing analysis and variant designs of military aircraft

Section 11.8: Sensitivity analysis

Section 11.9: 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