In my earlier column on flight,1 we estimated the energy cost of flying and saw that the competition between the energy needed to fight drag and the energy to generate lift results in an optimum, minimum-energy flight speed (Fig. 1). Now let's assume that planes fly at this cruising speed. Then we can estimate how much energy is required and, from it, how far planes can fly.
Here was the estimated energy cost, with generating lift represented in the first term and fighting drag in the second term,2 (1)where m is the plane's mass, ρ is the air density (at cruising altitude), L is the plane's wingspan (tip-to-tip), is its drag coefficient, A is its cross-sectional area, v...
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