Aspect ratio (wing)

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For other meanings of this term, see Aspect Ratio

The low aspect ratio wing of a Piper PA-28 Cherokee

In aerodynamics, the aspect ratio is an airplane's wing's span divided by its standard mean chord (SMC). It can be calculated more easily as span squared divided by wing area:

$AR = {b^2 \over S}.$

Informally, a "high" aspect ratio indicates long, narrow wings, whereas a "low" aspect ratio indicates short, stubby wings.

Aspect ratio is a powerful indicator of the general performance of a wing. Wingtip vortices greatly deteriorate the performance of a wing, and by reducing the amount of wing tip area, making it skinny or pointed for instance, you reduce the amount of energy lost to this process, induced drag. This is why high performance gliders have very long, skinny wings; with no engine power, they must be as efficient as possible in every respect in order to stay aloft.

Why don't all aircraft have high aspect-ratio wings? There are several reasons:

Structural: the deflection along a high aspect-ratio wing tends to be much higher than for one of low aspect ratio, thus the stresses and consequent risk of fatigue failures are higher - particularly with swept-wing designs.
Maneuverability: a high aspect-ratio wing will have a lower roll rate than one of low aspect ratio, due to the higher moment arm on the drag and greater moment of inertia, thus rendering them unsuitable for fighter aircraft.
Pitch Stability - low aspect ratio wings tend to be more naturally stable than high-aspect ratios. This confers handling advantages, especially at slow speeds. You can try this at home with paper planes - make one with the paper in landscape and one in portrait - you will see how the higher AR plane will tend to pitch up or down much more than the low AR one.
Parasite Drag - While high aspect wings create less induced drag, they create more parasite drag, (drag due to shape, frontal area, and friction). This is why aircraft that fly at high speed and low altitudes (fighter-bombers and racing planes) typically have low aspect wings. Some aircraft use a swing-wing to gain the advantages of both high and low aspect wings at the cost of increased weight and complexity.
Practicality - low aspect ratios have a greater useful internal volume, which can be used to house the fuel tanks, retractable landing gear and other systems.

The high aspect ratio wing of a USAF B52 bomber

High aspect ratio wings abound in nature. Most birds which have to cover long distances e.g. on migratory routes have a high aspect ratio, and with tapered or elliptical wingtips. This is particularly noticeable on soaring birds such as albatrosses and eagles. In addition, the V-formation (echelon) often seen in flights of geese, ducks and other migratory birds can be considered to act as a single swept wing with a very high aspect ratio - the vortices shed by the lead bird are smoothly transferred to the next and so on. This confers a huge efficiency advantage to the flight as a whole - perhaps as much as a 100% improvement compared to a single bird in flight. Note that the usual common explanation of the V-formation - that following birds are "shielded" from air resistance by the bird in front - may be misleading. While birds do "take turns" at being the lead bird, it is probably to give those at the tips a rest - they are the ones that will experience the most drag when the vortices are finally shed. However, the full explanation of this behaviour is still the subject of research and debate; scientists do not claim to have fully understood the phenomenon yet.