Aspect ratio = wingspan/mean chord or area/chord^2 or wingspan^2/area
All 3 are relevant and all can be derived from the initial version by
multiplying the top and bottom by the same term.
Low aspect ratio wings tend to be short and stubby. The wing tip chord
is relatively large and allows an increased spillage of air from the lower
surface onto the upper surface. The wingtip vortices are consequently increased
and this results in increased induced drag. Wings with a high aspect ratio tend
to be long and narrow, with a small wing tip chord, so spillage of air around
the wingtip is reduced and induced drag decreases.
Consider a rectagular wing and a swept back
wing having the same surface area. The sweptback wing will have a lower aspect
ratio as the wingspan decreases and the chord increases. Furthermore,
the higher the aspect ratio the more aerodynamically efficient the wing.
Note: induced drag is proportional to 1/Aspect Ratio
If a high aspect ratio wing is acted on by an upgust
the increase in the angle of attack will result in a larger increase in CL
compared to a wing of low aspect ratio. A straight wing will therefore be more
susceptible to an upgust than a wing with sweepback.
Typical question: If a wing has a chord of 3m and an area of 30m^2, the
aspect ratio will be area/chord^2 = 30/3^2 = 3.33
If a wing has a chord of 5m and an area of 30m^2, the aspect ratio will
be area/chord^2 = 30/5^2 = 30/25 = 1.2
This shows that the larger the wing tip chord the lower the aspect ratio
and the greater the induced drag.