Is It Loaded?

Nothing sizzles and nothing sells quite like performance numbers. Two of the most commonly used metrics in describing how an airplane will fly, or when making comparisons between them, are terms that may seem to the uninitiated to belong perhaps to the agricultural sector, or maybe the building of bridges. (So no, despite the provocative title, this isn’t about firearms and anti-terrorism.) Among the many parameters used to describe an airplane’s abilities and engineering endowments, and which arise from the juggling act that aircraft designers must perform to arrive at the best compromise among them, two parameters in particular provide an excellent snapshot of what a pilot can expect. They are both known as loadings, and they are: wing loading, and power loading.

Wing loading is just the ratio of an airplane’s maximum gross weight to the surface area of its wings, which in the still-commonly used English units are expressed in pounds per square foot. It quantifies the extent to which an airplane’s wing is burdened. By default and by definition, that is weight under normal gravitational acceleration. Also, it is effectively an average number, as lifting potential isn’t the same over the entire wing, and of course it can be reduced at lesser weights. So practically speaking, it’s really a range. If you are in a bank, the wing loading effectively goes up, according to the reciprocal of the cosine of your bank angle, because the airplane’s apparent weight also goes up. But to keep the playing field equal, we’ll assume one ‘g’ here. An airplane with a low wing loading might at one extreme be a hang glider, and at the other extreme, one with a very high wing loading, perhaps something like the old F-104 Starfighter, the Space Shuttle (around 95 lb/ft2 at landing weight), or at the greatest extreme of which I am aware, the B-1B bomber (at well over 225 lb/ft2). Among the performance characteristics typically predicted by wing loading (or at least the first one I usually think of) is what the ride will be like in turbulence-after that, its stall speed, landing speed, maneuvering speed, etc. Stall speed for example is proportional to the square root of wing loading, and cruise speed likewise increases by the square root of the weight change. (That’s why glider racing involves copious amounts of ballast.) The simplest way to think of it is, will it float like a dandelion seed, or drop like a brick? The job of a wing is to generate lift, and the job of the rest of the airplane (including the wing) is to succumb to the relentless pull of gravity. Wing loading doesn’t address how efficiently a particular wing will work, or the configuration of any additional refinements such as highly efficient flaps, only how much of them there are. But generally, if there is a large expanse of wing generating an upward lifting force without too much dead weight pulling in the opposite direction, you’ll be going forward more than downward. It’s no coincidence that gliders with their high aspect ratio (i.e., long and skinny) wings and very low wing loadings have such relatively spectacular glide ratios.

Power loading on the other hand is also a ratio and also involves two things, one of them again being dead weight in the numerator, but this time in the denominator is a number representing: (big surprise) power. It specifies how many pounds must be borne by each horsepower generated by the airplane’s power source. Just as wing loading quantifies the burden borne by the wings, power loading puts a number on that of its engine (or engines). To use English units again, that would be pounds per horsepower (at maximum rated power). Note that for turbofan or turbojet aircraft, power loading is calculated by dividing pounds of weight by the number of pounds of thrust. Like wing loading, power loadings are realistically a range rather than a single number. (The simple act of flying is accompanied by a weight decrease, and thus a drop in power loading.) Unlike wing loading, where high wing loading usually means high performance, when looking at a number for power loading, it’s the other way around; lower numbers mean more muscle. Near one extreme that is at least conceivably accessible for many of us might be a Pitts biplane, with ample horsepower and relatively little weight to slow it down, yielding single-digit power loadings (something like six lb/hp). The lower the power loading generally, the better will be climb performance, and also somewhat, cruise performance. (Indeed, the F-16 has relatively so much power with an effective thrust-to-weight ratio less than unity at lower weights that it can accelerate going straight up.) Cruise performance is less dramatic because cruise speed only increases according to the cube root of power; if you want to go twice as fast, you need eight times the power. Of course, the more horses you add, the greater the penalty in avoirdupois, as well as the greater amounts of fuel a thirsty engine needs to bring along, and so you can’t carry as many people, or as much of anything else.

What about some practical comparisons? A Cessna 152 might have a wing loading of 10.5 pounds per square foot. That’s pretty light and sprightly as far as production airplanes go. A Kitfox might approach a wing loading one half of that, and a Schweitzer 2-33 glider’s wing loading is actually even lower: less than half that of the C152. (Then again, the wing loading of even the ungainly Canada goose is around four lb/ft2, although…that of a parachute might be just one pound per square foot.) The wing loading for a Cessna 172 is a bit higher, more like 14 or so. At the other extreme for single-engine airplanes is something like the Aerospatiale (Socata) TBM 700 which has a wing loading of about 35 lb/square foot, although it uses nearly full-span flaps to meet the maximum 61 knot stall speed requirement for landing single-engine airplanes. Without such contrivances, the usual upper limit is about 25 lb/sq.ft. That said though, the heavier 172 may actually be able to fly more slowly than its littler brother, again because of its large and very effective flaps. The full-flap stall speed for the C172P is listed as 33 knots, indicated. For an airplane, that’s a pretty slow pace indeed. But generally, wing loadings have increased over time. The days of aircraft needing low wing loadings for their relatively small engines to allow their pilots to fly out of short unimproved fields have mostly become the exception rather than the rule. The difference in wing loadings again become apparent once we’ve taken to the sky; the ride in a Cessna 152 on a blustery winter day will be nothing like the ride in a Malibu Mirage for example, with a wing loading approaching twice that of the Cessna. (By the way, in case you wondered, the wing loading of the Wright Flyer was just under 1.5 lb/sq.ft.)

When it comes to comparing power loadings among the types of airplanes most of us might expect to fly (now or ‘maybe someday’), here are some rough numbers. The power loading of a Cessna 152 is about 15.2, and for the 172 (R-model), about 15.3. (There isn’t too much difference; the C152 weighs about 1670 pounds and the engine develops roughly 108 horsepower, while the 172 weighs about 800 pounds more, but with a 160-hp engine, things are about the same. A Cessna 152 might climb at 650 feet per minute; the climb rate for the 172 will be something only a bit higher, perhaps 700. Put a 195-hp engine in the 172 though, and the climb differential goes up to about 20%.) That TBM 700 weighs over three tons, but with those 700 shaft hp, the power loading goes down into the single digits: nine-point something. But most high-performance single-engine airplanes come out with somewhere around 14 down to about 12 lb/hp. (Twelve is about what a Mooney or an SR22 have; the power loading for the Lancair IV is about 10.) Generally, there isn’t usually much of a wide range, simply because airplanes are built to move people and things around. The more stuff you put in there (like people, baggage, and extra fuel for better range), the more wing you need for a given maximum stall speed and the more power you need for a respectable climb. (Some airplanes with the same power loading may perform better in the climb-taking the Malibu Mirage and a Socata TB21 Trinidad as examples-simply because of a cleaner design, or a higher aspect ratio wing.) Then again, as a particular airplane design evolves over time, like the pilots who fly them, they tend to put on extra weight. Over the last few decades, there has been more navigation gear and more avionics, Also manufacturers tend to increase engine power over time, and that costs in terms of added weight. The one time that power loading changes suddenly, by the way-I did italicize ‘usually’ just a minute ago-is when a twin-engine airplane loses an engine. Then, power loadings go from somewhere in the neighborhood of 10 or less, to well over that of the famously featherweight performance of even a Cessna 152…numbers like 18, 19…21, 22…which isn’t surprising given the known performance liabilities of a twin with one engine out. (Again just for fun, a note from the history books: The power loading of the Wright Flyer was extremely high, at 62.5 lb/hp.)

There are other performance predicting metrics also, such as the aspect ratio of a wing, and wetted area. We’ll look at those another time.