"I always use flaps for climb. I get more lift that way."
Some would call this statement perfectly logical, because flaps do increase lift and increased lift certainly ought to make an airplane climb faster. Others would say that the reasoning is fallacious, and that flaps, by increasing drag, reduce rate of climb rather than increase it.
Who's right?Flaps were, at first, an invention without a requirement. Aviation historian John Anderson (whose account in his own A History of Aerodynamics is based on that by Miller and Sawers in their book The Technical Development of Modern Aviation) describes the landing flap as an aileron that has wandered away from home. The aileron-the word is borrowed from French, where its primary meaning is the fin of a fish-was the brainchild of a French-born Englishman, Henry Farman, who wanted a roll control that would not violate the wing-warping patents of the vigilant and litigious Wrights. Separate moveable surfaces, external to the wings, had already been tried, but it was Farman who in 1908 came up with the idea of simply hinging the trailing edge of the wing and moving it up and down. This worked so well that it quickly became standard practice, making the Wrights' jealously guarded patent irrelevant.
In 1914 the Royal Aircraft Factory equipped both wings of its beautifully streamlined S.E.4 biplane with full-span ailerons that could be moved up or down together-in other words, with what we now call flaperons. The S.E.4 did not get very far; it had engine problems, the prototype was destroyed in a crash and the remarkable design was abandoned. Probably its flaperons, though prescient, had little effect upon its performance.
After 1916 all Fairey aircraft had flaps. But Anderson points out that the airplanes of the era were so slow anyway that flaps had scant practical use. As speeds increased and wing areas diminished during the '20s and '30s, however, their value became more evident. Today,
an airplane-other than a light-sport airplane-without flaps is a rare exception. A flap whose hinge lies on or within the wing surface is called a plain flap. Two other species of flap have been widely used: slotted and split. Split flaps, which are a sort of lower-surface spoiler that descends while the upper surface remains in place, were common before and after World War II. The DC-3 had them. But they are now extinct; the last airplane I can think of to use them was the Cessna 310-the classic twin with the tuna-shaped tip tanks. Slotted flaps predominate today. These consist of a separate airfoil-shaped surface that nests in the trailing edge of the wing and deflects either by traveling along tracks or by pivoting on hinges set below the wing. A subset of slotted flaps is the area-increasing type, invented by Harlan Fowler in 1924, which travel aft in order to increase wing area at the same time as they deflect. There is no clear line of demarcation between Fowler and non-Fowler flaps, since any flap that is hinged below the wing surface is bound to increase wing area somewhat. Real Fowler action is like obscenity-you know it when you see it.
The purpose of a slot is to take high-pressure air from below the wing and squirt it, through a nozzle-shaped passage, over the upper surface. The first application of the idea, around 1920, was at the leading edge of the wing and initially met with skepticism because it was supposed that a slot would destroy lift, not enhance it. In fact, as practical tests eventually showed, a leading-edge slot can increase maximum lift by more than 60 percent. It does so by sweeping dead and dying air backward along the airfoil, and thereby delaying the stall to a much higher angle of attack. Its action falls under the broad heading of boundary-layer control-that is, direct action upon the thin layer of disturbed air close to the surface of the airplane. Slotted leading edges have been used to prevent tip stall (Globe Swift), to enhance STOL performance (Helio Courier) and to increase safety (Socata Rallye); they are present in one form or another on most airliners.
Leading edge slats (the small vane ahead of the wing is called a slat; the gap between it and the wing is the slot) tend to mechanical complexity and to dragginess unless very carefully fitted. Slotted flaps, on the other hand, increase the drag of a wing only slightly when retracted (drag may come from external hinges or imperfect gap seals); but they increase maximum lift and reduce landing speed significantly. Slots can be multiple-the old Boeing 727 has triple-slotted flaps, with a partial-span fourth slot at the nose-creating, in effect, a cascade of airfoils, each one directing high-speed air onto the upper surface of the next.
Trailing edge flaps represent a way to increase the camber of a wing-that is, its curvature as seen in profile-while slots are a way to ensure that airflow follows the curvature rather than separating from it. Flaps can more than double the lift available from a plain airfoil, and thereby reduce landing speed by as much as 30 percent.
