Wing and power loadings influence how your airplane flies
You'll see them on the specifications page and often referred to in aircraft reports. They are two numbers — power loading and wing loading — that are good predictors of an airplane's performance.
How we get to these numbers is actually quite simple. Power loading is a figure that describes how much weight each of the engine's horsepower must carry, expressed in pounds per horsepower at maximum-rated power. Wing loading is a similar measurement, describing how much weight the wing must lift at the aircraft's maximum gross weight, expressed in pounds per square foot of wing.
At the risk of making rash generalizations, power loading influences mainly climb performance, with a lesser effect on cruise performance. No surprise there — the more power you've got, the faster the airplane will go — to a point. Wing loading, meanwhile, plays a big part in how an airplane rides through turbulence, as well as its stall speeds, which in turn influences approach and maneuvering speeds.
As you might imagine, these numbers vary significantly among aircraft types and are skewed by various design choices. For instance, a Cessna 150, with some 157 square feet of wing holding up a mere 1,600 pounds, has a wing loading of just 10.9 lb/sq ft; it is at the lower end of the production-airplane scale in this regard. (Several of the so-called bushplane homebuilts have still lower wing loading.) Wing loading for single-engine airplanes tends to top out at around 25 lb/sq ft, mainly because the regulations call for a 61-knot maximum landing-configuration stall speed for singles.
To carry any more weight, the single must rely upon high-lift devices, like large flaps, to make a small wing perform well at low speeds. There are notable exceptions to the single-engine wing-loading norms, including the Aerospatiale TBM 700, which has a wing loading of 34.9 lb/sq ft; it uses nearly full-span flaps — spoilers for roll control — to meet the stall-speed requirements.
Power loading also varies throughout the fleet, although not to as great a degree. The Cessna 150's 100 hp gives it a power loading of 16 lb/hp, while at the other end, the TBM 700's namesake 700 shaft hp, working against a maximum gross weight of 6,579 pounds, gives a power loading of 9.4 lb/hp. Most high-performance piston singles post power loadings in the range of 14 lb/hp down to 12 lb/hp.
Why such a narrow range? Certification requirements, together with the desire on the part of the aircraft designers to provide the best possible useful load, creates this form of orthodoxy. Let's take four examples of singles to help illustrate how power loading and wing loading affect performance. They are the aforementioned Cessna 150, the Cessna 172, the Socata Trinidad TC, and the New Piper Malibu Mirage.
As is pretty obvious, weight plays a role in each of these calculations. The more weight you want the airplane to carry, the more wing it will need for a given stall speed and the more power it will need to have adequate climb performance. Naturally, the more an airplane weighs before you add people and fuel, the higher the maximum gross weight will have to be.
Airplane designs, as they mature, inevitably gain weight. In part, extra equipment is to blame — 25 years ago, a pair of nav/coms, an ADF, and a transponder were considered a lot of equipment for a light single. Today, we expect even the simplest of airplanes to come with all the electronic bells and whistles — and even though the weight of the individual electronic elements has fallen greatly in the intervening years, overall weight has still crept up.
Both of the Cessnas, the 150 and 172, evolved from earlier models with smaller, less-powerful engines. As they were being designed, it was desirable to have relatively low wing loading; this improves short-field operations because of the low stall speed and makes it easier for the modestly powered engine to get the airplane into the sky. So they kept their relatively large wings — and grew even more effective flaps — giving them low wing loadings. Ride around in a 150 on a blustery day and you'll see — graphically demonstrated — the effects of low wing loading. The little trainer seems to respond to even the slightest of gusts.
Although the Skyhawk is certainly not bizjet-solid in the bumps, it's significantly better than its smaller cousin. Amazingly, the 172 also has a lower landing-configuration stall speed than the 150 — even though its wing loading is 13.8 lb/sq ft — because it has large, effective flaps. According to the handbook for the Cessna 172P, the full-flap stall speed is 33 knots indicated. As an added benefit, the Skyhawk's glide ratio is good, and chances are that whatever you hit after a power loss will be with a modest amount of forward velocity — this is, no doubt, a prime contributor to the airplane's excellent safety record.
Airplane designers usually select a wing shape and size that they feel is the best compromise of performance, weight, and building cost. Based on what this wing can do, the designer then increases weight until either the stall speed reaches an unacceptable level (which may be the legal limit, or some other fixed point based on the airplane's intended mission) or until climb performance becomes inadequate.
Compare the 150 and 172 again, this time in power loading; the two-seater carries 16 pounds for every horsepower, while the Skyhawk lifts a bit less — 15 lb/hp. As a result, even the larger, somewhat draggier Skyhawk turns in better climb performance — 700 fpm at sea level versus 670 for the Cessna 150. Pilots who have installed larger engines in the Skyhawk have also noticed that the main advantage of the extra power arrives in the climb. Even with a 195-hp Continental IO-360 (in the Hawk XP), the airplane is doing well to reach 130 knots true, yet its rate of climb is some 20 percent better.
It's been fashionable for the quickest homebuilt airplanes to wear small, thin wings — the theory is to aim for less induced drag, the drag produced by the very act of creating lift, and get more speed. Such a tactic has also been tried on the production line in the Socata (nee Aerospatiale) Caribbean series. All the airframes share a 128-square-foot, constant-chord wing; the airfoil was chosen as much for ease of construction as anything else. In particular, when you get to the Turbo Trinidad, with a 3,086-pound maximum gross weight, the wing starts to influence performance greatly. Stall speed in landing configuration is right at the 61-knot limit, which undoubtedly has limited the ultimate max-gross weight of the airplane. (The Mooney TLS, with an additional 20 hp, can carry nearly 300 pounds more, largely because of its greater wing area.)
In the Trinidad, the 61-knot stall was achieved only after dedicating two-thirds of the wing's trailing edge to flaps. As a result, the ailerons were squeezed for space, further forcing the designers to select surfaces with a lot of chord. (In fact, the control skins for the flaps are merely two of the aileron skins joined at the centermost rib.) Short-span, deep-chord ailerons are difficult to make powerful without high control efforts, and the TB21's execution is no different. The airplane has a heavy, ponderous feel in roll, a condition that is not helped by the pushrod control linkages, which themselves limit available leverage.
On the upside of the Trinidad's performance is an excellent ride in turbulence. The French airplane slices through bumps that would really toss around the pilot of another brand of airplane. And its climb performance is respectable — the power loading is 12.3 lb/hp — particularly when you consider that a short, constant-chord wing is not generally considered to be optimal for good climb rates. Moreover, with a generous amount of flap area and high wing loading, the Trinidad can be coaxed into truly thrilling rates of descent — good for when you have made that downwind-to-base turn a bit early, and not so good if the engine quits.
Sometimes, aircraft designers pick an airfoil — and, in particular, a certain wingspan — to benefit a certain type of performance. In the case of the Piper Malibu Mirage, the 43-foot wingspan (comprising 175 square feet of wing) was selected to give the high flyer the best altitude performance possible. Two of the Mirage's qualities stand out in comparison to those of the Trinidad. First, even with marginally higher wing loading — 24.6 lb/sq ft, compared to the TB21's 24.1 lb/sq ft — it has a slightly lower (by 1 knot) stall speed. And even though it has the same power loading — 12.3 lb/hp — the Mirage turns in better climb performance by 100 fpm at sea level.
Why? In part, it's the Mirage's better aerodynamics. Despite being considerably larger, the six-place single is cleaner, and that long wing really pays off in climb rate. (You need only to look back at the U-2 spyplane to see the ultimate expression of high-aspect-ratio wings in action.) Additional span also allows for longer-span flaps without crowding the ailerons and forcing truckish roll response; indeed, considering that long span, the Mirage, while certainly no Pitts, is fairly sprightly in roll.
Power loading, as mentioned, portends climb performance — that's why an airplane like the Mirage, almost a ton heavier than the Skyhawk, can absolutely wax the Cessna in climb. But there are practical limits. Unless the landing-configuration stall speed is your limiting factor — which it often is for singles, but is not even a consideration for twins — adding more power does not offer a limitless payback. Eventually, the airplane ends up carrying around so much engine — and fuel to feed it — that payload suffers, even though climb performance may be spectacular. Over the last 30 or 40 years, long-lasting designs have been fitted with larger and larger engines, very often with a commensurate increase in maximum gross weight. And each one, now having to carry more fuel, ends up having scarcely more full-fuel payload than before.
Performance, though, is what sells, and it can be measured, in general terms, by just two numbers on the specs page. And while there are a lot of other factors involved in any specific airplane's performance envelope, wing loading and power loading can give you a couple of useful thumbnail sketches of what to expect.
E-mail the author at [email protected].
Related Articles
