8.1 Design Motivation

The last major task in defining a conceptual aircraft configuration is sizing, determining how large the aircraft must be to carry enough fuel and payload to perform the design mission(s). This task includes, but is more than, making sure everything the aircraft must carry inside it will fit. Because the fuel required to fly the mission depends on the size of the aircraft and the size of the aircraft depends in part on how much fuel it must carry, the sizing problem must be carefully formulated and solved in order to obtain a useful result. Aircraft size has a very profound effect on cost, and cost is often one of the most important constraints on a new aircraft design. In the 1950s and 1960s, aircraft performance requirements were often the primary design drivers, especially for military aircraft, and higher-than-planned costs were often accepted by customers in order to get the performance they desired. In the present day, cost is far more important to customers, and performance requirements are frequently revised in order to allow a new aircraft design to meet its cost goal.

Consider, for instance, the U.S. Air Force F-15 and F-16 tactical fighter aircraft shown in Fig. 8.1. Both aircraft have approximately the same P _s and performance characteristics at all but high Mach numbers. (The F-15's variable engine inlets give it more thrust and higher P _s