of the wing box. The baseline and variant details are worked out in Chapter 6 as the preliminary configuration, followed by the undercarriage design in Chapter 7. Aircraft mass is calculated in Chapter 8. After obtaining the aircraft drag, this chapter finalizes the size of the baseline along with the two variants.

The final sized aircraft came very close to the preliminary baseline aircraft configuration suggested in Section 6.10. Therefore, iterations to fine-tune the aircraft mass, drag, and so forth have been avoided. It is unlikely for a coursework exercise to be that fortunate. It is highly recommended that any exercise should make at least one iteration in order to get a sense of the task. Setting up a spreadsheet is part of the learning process; all equations in this book are provided to set up the required spreadsheet.

11.7 Sizing Analysis: Military Aircraft

This extended section of the book on military aircraft sizing analysis can be found on the Web at www.cambridge.org/Kundu and includes the following:

Figure 11.5. Aircraft sizing – military aircraft

11.7.1 Single-Seat Variant in the Family of Aircraft Design

Figure 11.6. Variant designs in the family of military aircraft

11.8 Sensitivity Study

The sizing exercise offers an opportunity to conduct a sensitivity study of the physical geometries so that designers and users have better insight in making finer choices. An example of an AJT wing-geometry sensitivity study is in Table 11.7 showing what happens with small changes in the wing reference area, SW; aspect

Table 11.7. AJT sensitivity study

Figure 11.7. Further growth of the military AJT/CAS design

ratio, AR; aerofoil t/c ratio, t/c; and wing quarter-chord sweep, 1/ . (A Bizjet air-

4

craft sensitivity study is not provided in this book.)

A more refined analysis could be made with a detailed sensitivity study on various design parameters, such as other geometrical details, materials, and structural layout, to address cost-versus-performance issues in order to arrive at a satisfying design. This may require local optimization with full awareness that global optimization is not sacrificed. Although a broad-based MDO is the ultimate goal, dealing with a large number of parameters in a sophisticated algorithm may not be easy. It is still researched intensively within academic circles, but the industry tends to use MDO conservatively, if required in a parametric search, by addressing one variable at a time. The industry cannot afford to take risks with an unproven algorithm simply because it bears promise. The industry takes a more holistic approach to minimize costs without sacrificing safety, but it may compromise performance if it pays off.

11.9 Future Growth Potential

Previous military aircraft designs laid the foundation for future designs. Even a radically new design extracts information and, if possible, salvageable component commonalities from older designs. Figure 11.7 is a conceptual example showing how far newer designs can benefit from older designs through their systematic exploitation. The figure summarizes designs from the AJT to the light air superiority aircraft. It could be debated about how effective the last two designs could be (without the stealth consideration); however, at this stage, it only reflects a scheme.

The designs of the AJT and CAS are sized in detail. The advanced CAS (ACAS) is an AB version of the CAS with a new wing for high-subsonic flight. Missions for these aircraft are more suited to the counterinsurgency-type role, where the 1960s and 1970s designs are still creating havoc. The 6,900-lb thrust can reach

10,000 lb with AB that should enable the ACAS to carry a high weapon load (≈5,000 lb) (this design has not been properly checked).

The ultimate extension can be toward the air superiority role (two possibilities are shown in Figure 11.7). In this case, it is unlikely that the baseline engine can be further extended; therefore, re-engine work with a more powerful turbofan (i.e., an AB producing around 18,000 lb) and a totally new wing (i.e., SW ≈ 22 m2) are required. A clean aircraft weight would reach approximately 5,700 kg, pushing the supersonic speed to approximately Mach 1.8, but with a very tight turning capability at subsonic speed. However, such a design may be controversial because its viability in combat would be questioned. A new combat aircraft design should have a stealth factor, which is not discussed herein due to having few backup data. However, there may be substantial commonalities in the forward fuselage and the systems design.