In aviation -
Aspect Ratio refers to the shape of the wings of an aircraft
, i.e. long and skinny or short and fat.
The simplest expression of AR is span
, where span is the distance from wingtip to wingtip and the chord is the distance from the leading edge
to the trailing edge
of the wing. This works for rectangular wings such as those found on Cessnas
and other small airplanes
. For tapered
wings the average chord is needed. In these cases a more useful equation for AR is span^2/area
Aspect Ratio relates to the performance
of a wing, that is to say, its efficiency
(at a given speed or speed range). Aerodynamic drag
is the chief foe of wing efficiency, and comes in two forms: parasite drag
and induced drag
. The former is the result of airflow
disruption by struts, wheels, fuselage
, etc. Induced drag is a consequence of the production of lift
. A certain amount of energy goes into moving air in ways that don't lift the aircraft; an example is the tip vortex
, where air swirls from the bottom to the top of the wing at the tip. The effects of drag increase as the square of airspeed
Induced drag varies inversely
with aspect ratio, so a wing with infinite
span would have zero induced drag. High performance sailplanes
, with their very long
and skinny wings, have ARs of 30 or higher. The Space Shuttle
has an AR of maybe 1.5. Hang gliders
have ARs in the 5.5 to 8.5 range.
Low aspect ratios generally work better for wings designed to produce a lot of lift at low speeds, high aspect ratios are better for higher speed applications where minimal lift (beyond supporting the aircraft) is needed. Lower AR wings are generally much more maneuverable
than high AR wings as well.
factors limit the attainable aspect ratio using a given material
or materials, and the neccesary light weight means that very high AR wings tend to flex
dynamically under load, which further complicates designing for efficiency and realizing
the theoretical benefits of high AR.