Fuel economy in aircraft

Aircraft must consume fuel to supply the energy needed to move the vehicles and their passengers. Fuel economy is a measure of how much fuel an aircraft needs to operate, and can be expressed in several ways, for example by the liters of fuel consumed per passenger per kilometer. Aerodynamic drag, which exerts a force on the aircraft in the opposite direction from the velocity, is a principal determinant of energy consumption in aircraft because they operate at such high speeds.

Each model of aircraft has a maximum range speed for a given total load (fuel plus payload), which is the speed at which it is most fuel efficient. Flying slower or faster than this optimum speed increases fuel consumption per mile flown. There is an optimum speed for efficiency because the component of drag resulting from airframe skin friction against the air increases at a square function of air speed, but the drag resulting from generating lift decreases with air speed. (These are technically called parasitic drag and induced drag, respectively.) The desirability of a low maximum range speed to reduce environmental and climate impacts is at odds in aircraft design with the benefit from the revenue generated by making the design speed higher thereby increasing the passenger miles flown per day.

Aircraft weight is also a factor in fuel economy, because more lift-generating drag (induced drag) results as weight increases. If the airframe weight is reduced, engines that are smaller and lighter can be used, and for a given range the fuel capacity can be reduced. Thus some weight savings can be compounded for an increase in fuel efficiency. A rule-of-thumb being that a 1% weight reduction corresponds to around a 0.75% reduction in fuel consumption.

Flight altitude affects engine efficiency. Jet-engine efficiency increases with altitude up to the tropopause, the temperature minimum of the atmosphere; at lower temperatures, the engine efficiency is higher.[1] Jet engine efficiency is also increased at high speeds, but above about Mach 0.85 the aerodynamic drag on the airframe overwhelms this effect.

Above that speed, shockwaves begin to form that greatly increase drag. For supersonic flight (Mach 1.0 or higher), fuel consumption is increased tremendously.

Modern jet aircraft have twice the fuel efficiency of the earliest jet airliners. Late 1950s piston airliners like the Lockheed L-1049 Super Constellation and DC-7 were 1% to 28% more energy intensive than 1990s jet airliners which cruise 40 to 80% faster. The early jet airliners were designed at a time when air crew labor costs were higher relative to fuel costs than today. Despite the high fuel consumption, because fuel was inexpensive in that era the higher speed resulted in favorable economical returns since crew costs and amortization of capital investment in the aircraft could be spread over more seat miles flown per day.

...
Wikipedia