Flying Frugally
Route Kansas City to Dallas with Winds Aloft at 12,000 ft There are several definitions of efficient flight. For example, a flight could be considered most efficient if (1) the least amount of fuel is used to cover a given distance; or if (2) the least amount of time is spent in the air for the given distance; or if (3) the least amount of fuel per knot of speed is used. In the landmark technical paper Fuel Efficiency of Small Aircraft (AIAA-80-1847, 1980), B. H. Carson argues that the third criterion, which minimizes expenditure per knot of airspeed, is a good compromise between the first two criteria, which minimize fuel used or time in the air. The airspeed producing the least expenditure per knot is now known as the Carson speed . The optimal speed for the first criterion is the best-glide speed . Carson's paper establishes a neat formula that links Carson speed and best-glide speed. It says that the Carson speed is the best-glide speed times sqrt ( sqrt (3)) = 1.31607...