Doubling your speed can multiply the power needed to beat drag eightfold
Air and water push back against anything moving through them, and the faster you go the harder they push. At high speeds drag grows with the square of velocity, so the power needed to overcome it rises with the cube. Even a top sprinter spends about 5 percent of their energy just shoving air aside.
Drag, also called fluid resistance, is a force acting against the motion of an object relative to a surrounding fluid, whether a car in air, a hull in water or fluid flowing through a pipe. Unlike ordinary friction, it depends on speed. In slow, syrupy flow it rises in simple proportion to velocity; in fast flow it rises with velocity squared. Engineers tell the two regimes apart with a quantity called the Reynolds number. Sports physics needs drag to explain the flight of balls, javelins, arrows and frisbees.
Two forms affect everything. Form drag comes from pressure as fluid is forced around an object, so it depends on the object's cross-sectional shape and size. Skin friction comes from the fluid rubbing along a surface, inside or out. Bodies dominated by pressure drag, such as road vehicles, are called bluff; those dominated by friction are streamlined. Together these make up what aviation calls parasitic drag, which does nothing useful for an aircraft.
Flight adds more. Wings pay for lift with lift-induced drag, partly from swirling vortices trailing behind them. At low speed a plane needs a steep angle of attack, so induced drag dominates; as it speeds up, induced drag falls but parasitic drag grows, and near the speed of sound wave drag from shock waves joins in. Designers attacked that problem on Concorde: stretching the rear fuselage of the production aircraft by 3.73 metres, following a principle called the area rule, trimmed its wave drag at Mach 2 by 1.8 percent. Ships face a cousin, wave resistance, from the surface waves they make.
Progress shows up in the numbers. Aeronautical engineers compare aircraft by an equivalent parasite area, the size of a flat plate facing the airflow that would create the same drag. The Douglas DC-3 comes out at 2.20 square metres; the DC-9, designed about 30 years later, manages 1.91 while carrying five times as many passengers.
Source: Drag (physics)
