Maximum Takeoff Mass versus Engine Power
The relationships between engine sizes and the MTOM are shown in Figure 4.9. Turbofan engine size is expressed as sea-level static thrust (TSLS) in the ISA day at takeoff ratings, when the engine produces maximum thrust (see Chapter 10). These graphs can be used only for preliminary sizing; formal sizing and engine matching are described in Chapter 11.
Thrust-loading (T/W), is defined as the ratio of total thrust (TSLSJot) of all engines to the weight of the aircraft. Again, a clear relationship can be established through regression analysis. Mandatory airworthiness regulations require that multiengine aircraft should be able to climb in a specified gradient (see FAA requirements in Chapter 13) with one engine inoperative. For a twin-engine aircraft,
Figure 4.10. Empennage area versus wing area
failure of an engine amounts to a 50% loss of power, whereas for a four-engine aircraft, it amounts to a 25% loss of power. Therefore, the T/W for a two-engine aircraft would be higher than for a four-engine aircraft.
The constraints for engine matching are that it should simultaneously satisfy sufficient takeoff thrust to meet the (1) field length specifications, (2) initial climb requirements, and (3) initial high-speed cruise requirements from market specifications. An increase in engine thrust with aircraft mass is obvious for meeting takeoff performance. Engine matching depends on wing size, number of engines, and type of high-lift device used. Propeller-driven aircraft are rated in power P in kw (hp or shp), which in turn provides the thrust. Turboprops are rated in power loading, P/W, instead of T/W.
Smaller aircraft operate in smaller airfields and are generally configured with two engines and simpler flap types to keep costs down. Figure 4.9a shows thrust growth with size for small aircraft. Here, thrust-loading is from 0.35 to 0.45. Figure 4.9b shows midrange statistics, mostly for two-engine aircraft. Midrange aircraft operate in better and longer airfields than smaller aircraft; hence, the thrust-loading range is at a lower value, between 0.3 and 0.37. Figure 4.9c shows long-range statistics, with some two – and four-engine aircraft – the three-engine configuration is not currently in use. Long-range aircraft with superior high-lift devices and long runways ensure that thrust-loading can be maintained between 0.22 and 0.33; the lower values are for four-engine aircraft. Trends in family variants in each of the three classes are also shown in Figure 4.9.