Effects of variable specific fuel consumption

The foregoing discussion has assumed constant SFC but a real engine will almost certainly not conform to this. Specific fuel consumption will almost certainly vary

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Fig. 3.10 Estimating VME from variations in power for level flight.

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Fig. 3.11 Variation of SFC with referred power – test data.

with power, altitude and RRPM, usually falling as power and altitude increase and as RRPM decreases. Figure 3.11 shows a typical variation of SFC with referred power (P/8V0). As power increases, the engine operates more efficiently and SFC decreases. It is, therefore, often better to set a higher RRPM and fly at a higher true airspeed than that predicted by the simple theory if maximum SAR is to be achieved. Likewise, the endurance may be improved by operating at a slightly higher rotor speed. Based on research by Langdon [3.5] it can be shown that the extra power required to fly is offset by the improved SFC.

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