Ever wonder why some aeroplanes toss you around in turbulence more than others? The answer lies, to a great extent, in a specification often listed for just about every aircraft written up for a flight review. It’s the plane’s wing loading, and its numerical value can tell you a fair bit about the flying qualities of the aircraft to which it pertains. Here’s a quick refresher as to what it’s all about.
The wing loading of an aeroplane is the gross weight of the aeroplane divided by its wing area. It’s expressed in terms of pounds per square foot, and describes how much weight an aircraft’s wing must support (at its maximum gross weight) to lift the machine into the air, and keep it there.
By way of example, your standard Cessna 150 at 1,600 pounds will have a wing loading of 10.2 lbs/sq ft sustained by its 157 square feet of wing area. In other words, every square foot of a C150's wing must lift 10.2 pounds of the aircraft’s weight to generate its lift. Contrast that figure to that of a Boeing 747-400 with its 800,000 pound gross weight and 5,825 square feet of wing area. Its wing loading of 137.3 lbs/sq ft offers up the other end of the wing loading spectrum. What are these figures telling you?
Wing loading affects stall speed, rate of climb, takeoff and landing distances, and turning ability. Aircraft designers generally settle on a wing shape and size that offers the best compromise vis-a-vis performance, weight, and manufacturing cost, while keeping an acceptable stall speed a foremost consideration throughout the design process. FAR 23 certified single-engine aircraft must, in fact, stall at no more than 61 knots; therefore, as a designer, you’ll be looking to design a wing with a load carrying capacity that meets that specific stall speed criterium.
Stall speed is directly determined by wing loading. A low wing loading translates into a low stall speed – each square foot of wing is not having to “hold up” as much of its share of the aircraft’s weight to keep it aloft as would be the case with a higher wing loaded plane. What follows then, is that aircraft with low wing loadings tend to have larger wings relative to their gross weight.
Given that wings produce their lift owing to the motion of air over their surface, a larger wing will move more air and, thus, generate more lift. An aircraft with a larger wing relative to its mass (i.e. a low wing loading) will provide more lift at any given speed. Such an aircraft will then takeoff and land at lower speeds, have a superior rate of climb for its rated power, provide for greater load carrying capacity, and cruise more efficiently owing to less thrust being required to maintain its lift in straight and level flight.
Low wing loadings provide for greater sustained turning ability because they generate more lift for their given quantity of thrust. However, the greater induced drag created by the lower wing loading – larger wings produce more drag – will reduce that aircraft’s ability for quick changes in direction. A high wing loaded aircraft will react instantaneously to a control input, but it will take longer to complete its turn.
As for handling gusty conditions, a small highly-loaded wing has less surface area upon which turbulent air can act meaning you’ll enjoy the smoother ride. By contrast, with a larger wing-to-mass ratio, you’ll feel like the fruit you just added to your cuisinart.
It may be just one specification out of many, but a glance at wing loading can tell you quite a lot about what you’re about to fly.