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Previously we made a preliminary performance summary, which did not include consideration of engine efficiency. Based on the preceding web page we can now make a final summary of range and endurance performance for different types of aircraft.
Piston AircraftThe piston engine is efficient at any rpm within its range. Therefore, the pilot should try to achieve optimum advance ratio for the propeller while operating at full throttle. At low altitude full throttle will like be too much power for best range speed (AOA) therefore, efficiency will improve with altitude until the full throttle altitude is reached.
Wind will be the most important factor determining the optimum cruising altitude.
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The graph to the left shows that the theoretical max SR speed for a C-172 is about 75 KIAS. But, this is very slow. Most pilots are unwilling to fly that slow. On the other hand the more typical cruise speed of 100 knots or more does bring with it a significant fuel penalty. |
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An intermediate speed of 85 or 90 knots would be a better compromise providing good speed with only a very small fuel penalty compared to the L/Dmax This is quite typical of most aircraft. The pilot can fly as much as 10% faster than ideal with typically only a 2% loss in fuel efficiency. Considering the value of time it is usually more economical overall to cruise 10% faster than the best range speed. |
Fuel is only one cost associated with operating an aircraft. Maintenance is performed on a schedule based on hours flown. Revenue usually is based on the miles flown. Therefore, it will usually be better, to fly faster than the ideal range speed developed in this chapter. The exact optimum speed depends on many factors.
Most modern airlines employ computerized systems designed to optimize the overall efficiency of their operation taking all the above factors into account, including the price of fuel, maintenance costs, revenue and the forecast winds.
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