Wing Loading

Wing loading is weight divided by wing area, and it is the first number quoted about any new aircraft because so much follows from it. Stall speed goes as its square root. Approach speed, landing distance and takeoff roll follow the stall speed. Minimum turn radius follows it. Ride quality in turbulence goes the other way — a heavily loaded wing punches through gusts that a lightly loaded one rides over — and so does the gust load the structure has to survive.

The range in service is enormous. A hang glider sits near 3 to 5 kg/m² of equivalent loading, a light aircraft around 60 to 90 kg/m² (about 12 to 18 lb/ft²), a modern airliner near 600 kg/m², and a high-speed strike aircraft higher still. Each of those is a deliberate answer to a different question, and the tradeoff is always the same one: low wing loading gives short fields, slow approaches, tight turns and a bumpy ride; high wing loading gives a smooth ride, high cruise efficiency and a runway requirement.

Two errors are common and both are quiet. The first is entering a MASS where a WEIGHT belongs. Wing loading is a force per unit area — pascals, or newtons per square metre — and an aircraft of 1,200 kg mass weighs about 11,772 N. Putting 1,200 into the numerator understates the loading by a factor of 9.81 and, since stall speed follows the square root, understates the stall speed by a factor of about three. The second is treating wing loading as a fixed property of the airframe. It is not: it falls steadily through a flight as fuel burns away, which is why a long-range aircraft that took off at its maximum weight lands far more slowly than it departed.

A note about units, since this is a page a pilot will land on. Wing loading is quoted in pounds per square foot almost everywhere in aviation, and this site has no lb/ft² unit to offer, so the imperial view shows psi instead and the numbers come out inconveniently small. The conversion to keep in your head is that 1 lb/ft² = 47.88 Pa, so a typical light aircraft at 15 lb/ft² is about 718 Pa. Divide a lb/ft² figure by 144 to get psi.

Wing loading and aspect ratio together characterise a wing about as far as two numbers can. They are independent — you can have any combination — and they answer different questions. Wing loading says how fast the aeroplane must fly; aspect ratio says how much that will cost.