How to build a fast and efficient airplane

Airplanes are fast and efficient because they can operate in a low friction environment. There are no wheels (at least in flight) that have to be constantly be rolled over the ground, and at higher altitude the air is thinner which reduces air drag dramatically.

Apart from creating more efficient engines that propel an aircraft, designing an efficient and fast aircraft is conceptually simple: make the aircraft as aerodynamically efficient as possible to reduce air drag. The less air drag the aircraft generates, the faster it can go, and the less fuel it burns to get from A to B.

Designers typically attempt to give the aircraft an overall shape that resembles a droplet. This so called streamlined shape reduces form drag by minimizing turbulence created as the craft moves forward through the air. How well this can be done depends on the designer but sometimes also on other parameters. Large passenger jets for example deviate from the droplet shape and are tubular because the tremendous cost of making a body as large as a passenger jet droplet shaped outweighs the benefits. Quite early in the history of aircraft did designers realize that any kind of strut, wire, landing gear, anything that sticks out from the main body of the aircraft dramatically increases air drag, reducing speed, endurance and range of an airplane. Single, externally unsupported wings and retractable landing gear soon became standard.

Minimizing the wetted surface area is aimed at reducing skin friction and has led to the design of planes that are basically one single body with wings. Splitting up a larger body into a number of smaller bodies, with the same overall volume and hence cargo and passenger space, increases the surface area and hence skin friction. This is because for example of a sphere, the volume increases with the cube of its diameter while surface area only increases with the square, meaning a larger sphere has a lower relative skin area than a smaller one. Ongoing research and development focuses partially on learning lessons from nature in creating skin surfaces that reduce friction with the oncoming air.

Long narrow wings, with the same wing area, generally allow for faster and more efficient flight than short stubby wings, because the wing tip area is reduced relative to the whole wing which in term reduces the formation of parasitic tip vortices. These vortices form on any wing tip because high pressured air from the bottom of the wing is pushed upwards towards the lower pressur at the top of the wing. These vortices are the reason that birds in flocks fly in V shaped formations as the upward moving air crated on the wing of the bird in front helps the bird behind/to the side generate lift more easily.

 Winglet/wingtip-fence

Winglet/wingtip-fence

A fairly recent development is that of the winglet, wingfence or sharklet. These are small extensions at the tip of the wing that reduce the formation of parasitic tip vortices by creating a barrier for the high pressure air below the wing. This is preventing most of this high pressured air from leaking out to the top side of the wing - which would be an energy loss - thereby making the wing more energy efficient. 

An interesting physical effect is that up to a point, airplanes become more fuel efficient the faster they go. The underlying reason is that while air friction, or drag, increases with the square of the velocity (going twice as fast, requires four times as much energy and eight times as much power) the lift that the airplane wing produces, also increases with the square of the velocity. As the speed of an airplane increases, it gets more lift from its wing, which means it can reduce the angle of attack of the wing, pull back flaps and rise to a higher altitude, all measures which reduce drag. Up to a point, this means that the faster an airplane goes, the more fuel efficient it becomes, it requires less energy to move from point A to B than if it was flying at a lower speed.

Cars on the other hand, once in highest gear, the faster they go, the less fuel efficient they become, because there is no mitigating factor that counteracts the increased air drag that the increased velocity induces on the car.