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: [24]

These three parameters are primarily dependent on control-surface sizing (as well as engine sizing, to an extent), which is not addressed in this book. It is assumed that engine size for fast initial climb rates are sufficient and that enough control surface is available to perform the g requirements. A lower aspect ratio for the wing is considered for higher roll rates to reduce the wing-root bending moments.

As mentioned previously, an aircraft must simultaneously satisfy the takeoff field length, initial climb rate, initial maximum cruise speed-altitude capabilities, and LFL. Low wing-loading (i. e., a larger wing area) is required to sustain low speed at liftoff and touchdown (for a pilot’s ease), whereas high wing-loading (i. e., a low wing area) is suitable at cruise because high speeds generate the required lift on a smaller wing area. The large wing area for takeoff and landing results in excess wing for high-speed cruise. To obtain the minimum wing area and satisfy all require­ments, a compromise for sizing of the wing area must be found; this may require suitable high-lift devices to keep the wing area smaller. The wing area is sized in conjunction with a matched engine for takeoff, climb, cruise, and landing; landing is performed at the idle-engine rating.

In general, W/S varies with time as fuel is consumed and T/W is throttle – dependent. Therefore, a reference design condition of the MTOW and TSLS at ISA + SL are used for sizing considerations. This means that the MTOW, TSLS, and SW are the only parameters considered for aircraft sizing and engine matching. In general, wing-size variations are associated with changes in all other affecting parameters (e. g., AR, X, and wing sweep). However, at this stage, they are kept invariant – that is, the variation in wing size only scales the wing span and chord, leaving the general planform unaffected (like zoom in/zoom out).

At this point in the discussion, readers require knowledge of aircraft perfor­mance, and the important derivations of the equations used are provided in Chap­ter 13. References [2] through [6] are textbook sources for the detailed derivation of the performance equations. Other proven semi-empirical relations are in [4]. Although the methodology described herein is the same, the industry practice is more detailed and involved in order to maintain a high degree of accuracy.

Worked-out examples continue with the Learjet 45 Bizjet class for civil aircraft and the BAe Hawk class for military aircraft. Throughout this chapter, wing-loading (W/S) in the SI system is in N/m2 to align with the thrust (in Newtons) in thrust loading (TSLS/W) as a nondimensional parameter.

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