The Travels of Mr. Fowler

The historical context is interesting. John Anderson of the Smithsonian, and other historians of aeronautics, have pointed out the curious lack of connection between the evolution of early aircraft designs and the pure science that anticipated them by many years. Theoretical aerodynamicists had scant contact with the designers and builders of actual airplanes. Flow phenomena had been studied in the laboratory decades before they were encountered anew on the flying field by scientifically illiterate builders and pilots; on the other hand, it sometimes happened that innovations were investigated in the laboratory only after they had appeared in the field. The development of lift-enhancing flaps displays that irregular intercourse between workshop and laboratory.

The evolution of flaps progressed with surprising speed, considering that most airplanes of the time landed at a slow trot and scarcely needed high-lift devices. The precise origins and dates of many advances in early aviation are disputed, but by some accounts the trailing-edge flap was invented in 1908, as a control device, by Henri Farman, a Frenchman born in Paris of British parents. Farman may have hoped to make an end run around the extremely vigilant and litigious Wrights, who had patented their idea of wing warping and claimed, rather grandiosely, to own the whole idea of roll control of any sort. Hinged trailing-edge flaps deflecting differentially -- what we now call ailerons -- were more practical than wing warping for roll control -- even the Wrights adopted them in due course -- and they evolved rapidly during the First World War. By the war's end the ailerons, and indeed the entire wing, of one design, the Fokker D.VIII, were scarcely distinguishable from modern ones.

The idea of deflecting flaps downward on both sides of the airplane in order to increase lift and reduce landing speed originated with Britain's Royal Aircraft Factory in 1914. It did not gain much traction at first, though one British manufacturer, Fairey, began incorporating flaps in its production airplanes in 1916. As wing loading and landing speeds increased, however, lift-augmenting devices came to be of greater and greater interest.

The idea of a slot as a lift-enhancer dates to around 1920, when a German pilot named Gustav Lachmann, recuperating in a hospital from a stalling accident, conceived the idea that a nozzle-shaped leading-edge slot might squirt a sheet of high-velocity air back along the upper surface of the wing, stemming the forward march of flow separation. Amusingly, Lachmann's first patent application was denied on the not illogical grounds that a slot in the leading edge of a wing ought to diminish, not increase, its lift. Some people have a better grasp of the behavior of moving air than others.

In the meantime, the Englishman Frederick Handley Page had had the same idea as Lachmann, and had tested it in a wind tunnel. Lachmann learned of Handley Page's results and prevailed upon the godlike German aerodynamicist Ludwig Prandtl to test a slotted wing at his laboratory at Göttingen. The result -- a 60 percent increase in maximum lift -- matched the British ones, and Lachmann obtained his patent.

Perceiving that if a slot delayed separation at the leading edge of a wing it might do the same at the leading edge of a deflected flap, Handley Page continued to experiment, and discovered, among other things, that the greatest gains in lift from slotted flaps were obtained with thick wings. Thick airfoils sections were then a relative novelty. It had always been believed, in part, I guess, because the wings of birds are thin and in part because it just seemed intuitively obvious, that thick wings must have more drag than thin ones. Until you get up to transonic speeds, this is not in fact the case. Prandtl had demonstrated the superiority of thicker airfoils in 1917, and, in an instance of knowledge flowing from the laboratory to the field, Anthony Fokker immediately incorporated them in his triplane of Red Baron fame.

It was in this context that Harlan D. Fowler invented his flap. "Have you ever thought," he once wrote in the course of his protracted effort to gain acceptance for his invention, "of the powerful advantage the birds of the air possess by their ability to expand or contract their wing spread and to alter the curvature of the feathered ribs? … Man has conquered the air, an achievement, perhaps, of far greater consequence than any invention produced by him … The airplane, as we know it today, is probably in principle correct … But the demand of progress in carrying larger loads and to obtain higher cruising speeds is leading towards two serious factors. Increased power, representing an uneconomic development, and high stalling speed, representing an unsound safety development … The development and perfection of the Fowler Variable Area Wing presents a very sound and practical solution …" Fowler's intention was to combine two established lift-enhancing techniques, camber change from flap deflection and the stall-delaying slot, with a third, variable wing area.

What was novel about Fowler's flap was that it emerged from the wing and traveled aft a considerable distance -- usually all the way to the trailing edge, or nearly so -- before deflecting. It thereby increased not only the camber of the wing but also its area, and provided a greater increase in lift than any other type of high-lift device. The Fowler flap had another advantage as well. Most of the drag of any flap is due to its deflection. The lift of the Fowler flap was due in part to area increase and in part to deflection, and these occurred in sequence, not simultaneously. The flap deflected very little while traveling aft, and so it initially increased the wing area while producing little extra drag and nose-down moment. It was therefore very well suited for takeoff, when the lift associated with flaps was wanted but the drag wasn't. There was a downside to this low drag, on the other hand; to the extent that the drag of a flap can be used to make an approach steeper, the Fowler flap was less effective than, for instance, the split flap, a sort of lower-surface spoiler that has now gone out of fashion.

Today the term "Fowler flap" is tossed around imprecisely. You sometimes see it -- for instance if you search online for "Fowler flap" images -- applied to slotted flaps that provide no more aft travel than rotation about an external hinge is bound to produce. A true Fowler flap does not simply rotate about an external hinge; it always slides or rolls on tracks or is carried by some sort of complicated parallelogram linkage, and its hallmark, during deployment, is an initial period of moving more or less straight aft with little change of incidence. The upper surface of the wing is unbroken to the trailing edge, or close to it, and the flap does not begin to deflect in earnest until its own leading edge is under the trailing edge of the wing.

Fowler eventually managed to get his idea adopted by Lockheed, which incorporated it on its 14 Super Electra, a twin-engine airliner that was fated to be eclipsed by the split-flap-equipped DC-3, and on many subsequent designs, including the seemingly immortal C-130 Hercules. Several recent Gulfstream models use Fowler flaps, as do a few small airplanes, including at least one light sport aircraft, the Czech-designed Kappa KP-5. Robertson STOL mods were developed for a number of Cessna twins that replaced their split flaps with Fowlers. Landing distance is inversely proportional to maximum lift coefficient, and the maximum lift coefficient of a Fowler flap is between 10 and 20 percent higher than that of most other flap types.

It turns out that if one slot is good, two are better, and so on. The double-slotted Fowler flap, on which the flap itself has its own small leading edge device called a fore flap or vane, seemed like the last word in high lift until Boeing's 727 arrived on the scene with triple-slotted flaps. Subsequent Boeing flap systems have been similar (737, 747, 777) or somewhat simpler (757, 767). Most of these Boeing confections do not travel aft all the way to the trailing edge, as Fowler flaps on smaller aircraft do; but they nevertheless achieve a large wing area increase because they consist of a cascade of three flaps that nest within one another. During extension, each one executes its own Fowler action with respect to the one ahead of it, with the result that the chord of the flap when deployed is much larger than when it is nested within the wing.

Despite their acknowledged effectiveness, Fowler flaps pose certain problems that discourage their widespread use. They are structurally inconvenient. Unless the wing airfoil is quite thick, the flap and the long overhanging upper-surface lip must be thin. The actuating hardware -- jackscrews, hydraulic cylinders or mechanical linkages -- is bulky because of the long distance the flap must travel. A very thin flap requires more closely-spaced supports, and the cantilever supports themselves, because of the long flap travel, are highly loaded and must be quite stout. (I'm not sure whether the enormous "canoe" fairings on Boeing flap supports are absolutely necessary, or whether they also have some area-ruling function, like the "shock bodies" on the old Convair 990; but external tracks can be avoided, and Gulfstream has managed to conceal the entire actuation and support hardware for its Fowler flaps within a pristine wing.) The flap pitching moment -- the nose-down force associated with increased camber -- tends to be large.

There can also be operational problems due to the sheer effectiveness of large Fowler flaps. My first homebuilt design had a double-slotted Fowler flap that extended over less that half of the wingspan and increased its wing area by only about 10 percent. On the principle that more must be better, I provided its successor with a Fowler that has only a single slot, but extends across two-thirds of the wingspan and increases wing area by 20 percent. I was very slow to put its actuation system, which is rather complex, into operation, and flew for six years without using the flaps at all. I have recently gotten them working, and have begun learning about their quirks.

Recently I was landing into a fresh breeze of 15 knots or so, and when I stepped on the brakes the expected deceleration wasn't there. The tires squealed, but that was about it. This took me by surprise, and I realized only later that the flaps were providing so much lift, even with the airplane in a level attitude, that the expected weight on the main wheels wasn't there. That, I suspect, is the main reason for the immediate deployment of the spoilers of heavier aircraft when they touch down. It's not just the aerodynamic drag they provide; they are dumping lift that would otherwise enfeeble the wheel brakes.

Well, live and learn. My short-field landing technique will have to include immediately retracting the flaps after touching down. Just have to be sure to grab the right handle.