What is aspect ratio? How is it measured? What is it's effect on flight character?

Aspect ratio is the ratio of the wing span to the average wing chord. Span 60", chord 10" , AR = 6. 120" span, chord 8", AR = 15 etc.

Simplifying a bit higher aspect ratios are more efficient (see any sailplane), low aspect ratios have a better roll rate, i.e. more aerobatic (see any fun fly).

Steve

You will not only find high aspect ratio wings on sailplanes but on planes designed for duration and long distance flight. The 10 foot span aerosonde that flew from Nova Scotia to Scotland (about 1700 miles) nonstop had an aspect ratio of about 14. Bert Rutan's plane that flew nonstop around the world without refueling had high aspect ratio wings. The reason is that high aspect ratio reduces induced drag due to the vortices at the wing tips. At medium and low speeds, induced drag is more than half the total drag of the airplane. Anything that reduces drag reduces the thrust needed and increases fuel economy, range and duration.

Another beneficial feature of a high aspect ratio wing besides a smaller tip chord (to minimize vortex losses), is the fact that long, skinny wings are more efficient at producing a given amount of lift for a given amount of drag (higher L/D ratio).

As a wing passes through the air, it deflects the air molecules downward, resulting in an action-reaction force upward (i.e., lift). Together with the pressure differential between the top and the bottom of the wing, this downward deflection and resulting upward force is what defies the effects of gravity. A longer wing involves a larger swath of air, with less perturbation of the air molecules, and so produces the same amount of lift as a shorter wing, but at a smaller cost in drag.

For a crude analogy, think of a short wing pushing down mightily on the air molecules to produce the necessary lift, where a long, skinny wing pushes down gently on more air molecules to get the same lift.

Racing airplanes put minimal drag over everything else, and use short wings with thin, low-cambered sections to disturb as few air molecules as little as possible as they rocket along through the sky.

STOL airplanes and heavy lifters put lift above all else, at the expense of drag, and use slats and flaps and high cambered sections to produce as much lift as possible regardless of the drag penalty. They hit their top speed pretty quickly after take-off!

Sailplanes and long-distance aircraft, OTOH, need enough lift to sustain their weight while at the same time keeping the drag to a minimum. Long, skinny wings are the solution.