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


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


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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