11.3 Theory

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 baselineaircraft 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.

11.3 Theory

The parameters required for aircraft sizing and engine matching derive from market studies that reflect user requirements. In general, both civil and military aircraft use similar specification parameters, as discussed herein, as the basic input for aircraft sizing. All performance requirements in this chapter are at ISA day and all field performances are at sea level. The parameters are as follows:

1.Payload and range (fuel load): These determine the MTOW. This is not a sizing exercise but needs to be substantiated (see Chapter 13).

2.Takeoff field length (TOFL): This determines the engine-power ratings and wing size.

3.Landing field length (LFL): This determines wing size (baulked landing included).

4.Initial maximum cruise speed and altitude capabilities determine wing and engine sizes.

5.Initial rate of climb establishes wing and engine sizes.

These five requirements must be satisfied simultaneously. The governing parameters to satisfy TOFL, initial climb, initial cruise, and landing are wing-loading (W/S) and thrust-loading (TSLS/W).

Additional parameters for military aircraft sizing are as follows:

turn performance g-load

maneuver g-load

roll rate g-load