Why long thin wings, and what they are for

Aspect ratio, and the trade between efficiency and manoeuvrability.

Long, narrow wings waste less energy stirring up swirling air at their tips than short, wide wings of the same area do, which is why gliders and airliners favour long wings for efficiency while fighters and aerobatic aircraft favour short ones for the ability to turn and roll quickly.

The short version

A wing's proportions, how long it is from tip to tip compared with how wide it is from front to back, is called its aspect ratio, and it matters more than most people expect. A long, narrow wing of a given area produces the same lift as a short, wide wing of the same area, but does it while wasting noticeably less energy, which shows up as less drag for the same amount of lift. That difference in efficiency is the entire reason gliders have such long, slender wings while fighter jets and stunt biplanes have short, stubby ones, and the trade is never free in either direction.

The physics of a wingtip vortex

The waste happens at the tips. A wing generates lift by keeping the air pressure below it higher than the air pressure above it, but at the very tip of the wing there is nothing stopping that higher-pressure air underneath from spilling around the end of the wing to reach the lower-pressure air above it, since the wingtip is simply where the wing runs out. That spillage rolls up into a tight, spinning vortex trailing behind each tip, and spinning that air costs energy that produced no useful lift, only drag. A long, narrow wing has proportionally less tip relative to how much wing area is doing useful work in the middle, so a smaller fraction of its total lift is lost to this tip leakage. A short, wide wing of the same total area has comparatively more tip and less middle, so a bigger share of its effort goes into spinning up wasted vortices instead of holding the aircraft up.

The paddle comparison

The same spillage happens at the blade of a canoe paddle or a rowing oar pulled through water. Pushing a paddle blade through still water, the face doing the pushing carries higher pressure than the water on the back of the blade, and at the edges of the blade, especially near the tip furthest from the shaft, water spills around from the high-pressure face to the low-pressure back, visible as a swirl peeling off the tip as the paddle moves. A long, narrow paddle blade wastes proportionally less of its push to that tip swirl than a short, wide one of the same area, for exactly the reason a long wing wastes less lift to its tip vortex. Anyone who has felt a paddle catch and swirl at the end of a stroke has felt the water version of what a wingtip does to air.

The one number worth remembering

The difference in proportions between an efficiency-focused wing and a manoeuvrability-focused one is large. A sailplane built purely for efficient, slow, long-distance soaring might have a wing more than twenty times as long as it is wide. A fighter jet built for tight turns and high speed might have a wing only two or three times as long as it is wide, sacrificing a great deal of tip-loss efficiency in exchange for a wing structure that can survive hard manoeuvring and roll quickly, since a short wing has less far to twist its tips through when it rolls.

What follows from this

Because a longer wing pays for its efficiency with a longer structure that has to resist bending and twisting under load, and because a longer wing rolls more slowly since its tips have further to travel for the same roll rate, no aircraft gets to have both extremes at once. Gliders accept slow, gentle handling in exchange for staying airborne on almost nothing, and aerobatic and combat aircraft accept a real efficiency penalty in exchange for being able to change direction abruptly, and the aspect ratio chosen for any wing is really a statement of which of those two things its designer decided mattered more.