13.7 Summary

of the payload-range estimation, as a customer requirement, is not possible during the sizing exercise beacuse it requires detailed engine performance data. Subsequently, with detailed engine performance data, relevant aircraft performance analyses are conducted more accurately to guarantee airworthiness and market requirements.

A more detailed aircraft performance is estimated during the Detailed Definition Phase, which is beyond the scope of this book. The full aircraft performance does not affect aircraft configuration and mass unless the design review results in new demands for changes. These are management issues that are reviewed with potential customers to decide whether to give a go-ahead. Once a go-ahead is obtained, a full-blown detailed definition study ensues as Phase 2 activities, with significant financial commitments.

Figure 11.3 (Bizjet) and Figure 11.5 (AJT) show the lines of constraints for the various sizing requirements. The sizing point to satisfy all requirements shows a different level of margins for each capability. Typically, the initial enroute climb rate is the most critical to sizing. Therefore, the takeoff and maximum speed capabilities have a considerable margin, which is desirable because the aircraft can do better than what is required.

From statistics, experience shows that aircraft mass grows with time. This occurs primarily due to modifications resulting from mostly minor design changes and changing requirements – at times, even before the first delivery is made. If new requirements demand several changes, then a civil aircraft design may appear as a new variant. However, military aircraft design holds a little longer before a new variant emerges. It is therefore prudent for designers to maintain some margin,

especially reserve thrust capability – that is, keep the thrust-loading (T/W) slightly higher. Re-engining with an updated version is costly.

It can be seen that field performance requires a larger wing planform area (SW) than at cruise. It is advisable to keep the wing area as small as possible (i.e., high wing-loading) by incorporating a superior high-lift capability, which is not only heavy but also expensive. Designers must seek a compromise to minimize operating costs (see Chapter 16). Iterations were not needed for the designs worked out in this book.

13.7.1 The Bizjet

The sizing point in Figure 11.3 shows a wing-loading, W/SW = 64 lb/ft2, and a thrustloading, T/W = 0.32; there is little margin given for the landing requirement. The maximum landing mass for this design is at 95% of the MTOM. If for any reason the aircraft OEW increases, then it is better if the sizing point for the W/SW is somewhat lower than 64 lb/ft2 – for example, 62 lb/ft2 and/or increase T/W to 0.35. A quick iteration resolves the problem; however, this choice is not exercised to keep the wing area as small as possible. Instead, an aircraft is allowed to approach landing at a slightly higher speed because the LFL is generally shorter than the TOFL. This is easily achievable because the commonality of the undercarriage for all variants starts with the design of the heaviest (i.e., for the growth variant), and then the bulky components are reduced for lighter weights. The middle variant is used as the baseline version; its undercarriage can be made to accept the MTOM growth as a result of the OEW growth instead of making the wing larger.

Civil aircraft are recommended to come in a family of variants in order to cover wider market demands to maximize sales. However, none of the three variants is optimized, although the baseline is carefully sized in the middle to accept one larger and one smaller variant. Even when development costs are front-loaded, the variant aircraft cost is low by sharing the component commonality. The low cost is then translated to a lowering of the aircraft price, which absorbs the operating costs of the slightly nonoptimized designs.

It is interesting to examine the design philosophy of the Boeing 737 and the Airbus 320 families of aircraft variants in the same market arena. Together, more than 8,000 aircraft have been sold in the world market, which is no small achievement in engineering. The cost of these aircraft is about $50 million each (in 2005). For airlines with deregulated fare structures, making a profit involves complex dynamics of design and operation. The cost and operational scenario changes from time to time (e.g., increases in fuel cost and terrorist threats).

As early as the 1960s, Boeing recognized the potential for keeping component commonality in offering new designs. The B707 was one of the earliest commercialtransport jet aircraft to carry passengers. It was followed by a shorter version, the B720. Strictly speaking, the B707 fuselage relied on the KC135 tanker design of the 1950s. From the four-engine B707 came the three-engine B727 and then the twoengine B737, both of which retained considerable fuselage commonality. This was one of the earliest attempts to utilize the benefits of maintaining component commonality. Subsequently, the B737 started to emerge in different sizes of variants, maximizing the component commonality. The original B737–100 was the baseline

design; all other variants that came later, up to the B737–900, are larger aircraft. This posed certain constraints, especially on the undercarriage length. Conversely, the A320 (serving as the baseline design) was in the middle of the family; its growth variant is the A321 and its smaller variants are the A319 and A318. Figure 4.7 illustrates how the OEW is affected by the two examples of family variants. A baseline aircraft starting in the middle of a family is better optimized; therefore, in principle, it provides a better opportunity to lower production costs of the variants.

The simultaneous failure of two engines is extremely rare. If it happens after the decision speed is reached and there is not enough clearway available, then it is a catastrophic situation. If the climb gradient is not in conflict with the terrain of operation, it is better to take off with higher flap settings. If a longer runway is available, then a lower flap setting can be used. Takeoff-speed schedules can slightly exceed FAR requirements, which stipulate the minimum values. There have been cases of all-engine failures occurring at cruise due to volcanic ash in the atmosphere, as well as in the rare event of fuel starvation. Fortunately, the engines were restarted just before the aircraft would have hit the surface. An all-engine failure due to a bird strike occurred in 2009 – miraculously, all survived after the pilot ditched the aircraft in the Hudson River in New York.

13.7.2 The AJT

Military aircraft serve only one customer, the Ministry or Department of Defense of the nation that designed the aircraft. Frontline combat aircraft incorporate the newest technologies at the cutting edge to stay ahead of potential adversaries. Development costs are high and only a few countries can afford to produce advanced designs. International political scenarios indicate a strong demand for combat aircraft, even for developing nations that must purchase them from abroad. Therefore, military aircraft design philosophy is different than civil aircraft design. Here, designers and scientists have a strong voice, unlike in civil design in which the users dictate the requirements. Selling combat aircraft to restricted foreign countries is one way to recover investment costs.

Once a combat aircraft performance is well understood over years of operation, consequent modifications follow capability improvements. Subsequently, a new design replaces an older design – there is a generation gap between the designs. Military modifications for the derivative design are substantial. Derivative designs primarily result from revised combat capabilities with newer types of armament, along with all around performance gains. There is also a need for modifications – perceived as variants rather than derivatives – to sell to foreign customers. These variants are substantially different than civil aircraft variants.

AJT designs have variants that serve as combat aircraft in CAS. AJTs are less critical in design philosophy compared with frontline combat aircraft, but they bear some similarity. Typically, an AJT has one variant in the CAS role produced simultaneously. There is less restriction to export these types of aircraft.

The military infrastructure layout influences aircraft design; here, the LCC is the primary economic consideration. For military trainer aircraft designs, it is best to have a training base located near the armament practice arena to save time. A dedicated training base may not have a runway as long as a major civil runway. This

is reflected in the user specifications necessary for beginning a conceptual study. The training mission includes aerobatics and flying with onboard instruments for navigation; therefore, the training base should be located far from the civil airline corridors.

The AJT sizing point in Figure 11.5 shows a wing-loading, W/SW = 58 lb/ft2, and a thrust-loading, T/W = 0.55, which is a significant margin, especially for the landing requirements. The AJT can achieve a maximum level speed over Mach 0.88, but this is not demanded as a requirement. Mission weight for the AJT varies substantially; the NTC is at 4,800 kg and, for armament practice, it is loaded to 6,600 kg. The margin in the sizing graph encompasses an increase in loadings (the specification used in this book is for the NTC only). There is a major demand for higher power for the CAS variant. The choice of an uprated engine or an AB depends on the engine and the mission profile.

Competition for military aircraft sales is not as critical compared to the civilaviation sector. The national demand supports the production of a tailor-made design with manageable economics. However, the trainer-aircraft market has competition – unfortunately, it is sometimes influenced by other factors that may fail to result in a national product, even if the nation has the capability. For example, the Brazilian design Tucano was re-engined and underwent massive modifications by Short Brothers of Belfast for the United Kingdom, RAF, and the BAe Hawk (UK) underwent major modifications in the United States for domestic use.