Can Flap Deflection Help You Climb?


"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.

It's difficult to generalize about the drag that flaps produce. Increased camber usually reduces drag at higher lift coefficients; in other words, more camber is for airplanes that go slower, and less camber is for airplanes that go faster. With a well-faired plain flap, a few degrees of downward deflection can at least in theory reduce wing drag at the lift coefficient-typically around .6-used for climb. But leakage through the slot and discontinuities at the ends of the flap exact a drag penalty that increases rapidly with greater flap deflection. That number .6-the climbing lift coefficient-is important. The maximum lift coefficient of an unflapped wing is usually somewhere around 1.3 to 1.5; so the climbing lift coefficient is way below the maximum. Deflecting a flap increases the maximum lift coefficient, meaning that the airplane stalls at a lower speed, but it does not affect lift at higher speeds. Lift coefficient still varies at the same rate-around 0.1 per degree of angle of attack-but the angle of attack at which a given lift coefficient, .6 for example, is developed is lower. In other words, putting the flap down a bit makes the airplane fly in a more nose-down attitude.

Because airplanes are designed to cruise with the fuselage in the least draggy attitude, it follows that there may be some extra fuselage drag when the airplane is, say, five degrees nose-up for climb. Therefore, some flap deflection, by lowering the nose, may not only reduce wing drag, but may reduce fuselage drag as well. But the effect will be minimal; the incremental drag due to tilting a streamlined body a few degrees is small. For a sailplane, the speed called Vy, the best rate of climb speed for a powered airplane is, instead, the minimum sink speed. Many high-performance sailplanes rely on variable camber, usually achieved by means of sealed plain flaps, to adjust the location of the "drag bucket"-the range of speeds within which laminar flow occurs. The effect of variable camber is quite real, but it is significant only in airplanes with large, very smooth and well-contoured wings able to sustain large areas of laminar flow, and with flaps whose deflection does not create gaps and discontinuities that add drag. Few powered airplanes meet these criteria.

Now, why all this talk about drag, when we're interested in climbing and therefore presumably in lift? Because it's not true that excess lift makes an airplane climb. An airplane in a steady climb has essentially the same lift as one in level flight-just one or two percent more, at most, because in a climb the lift vector is at a slight angle to the vertical.

Some excess lift-the amount depends on how abruptly the pilot transitions into the climb-is required to establish a vertical velocity. Once in a steady climb, the airplane is raised higher and higher by power, not by lift. Therefore, the higher maximum lift coefficient obtained by deflecting the flaps has nothing whatever to do with climbing. The only thing that matters is the drag of the complete airplane.

Well, not the only one. There is another factor that is usually ignored: the vertical component of thrust. In a typical climbing attitude, say five degrees nose up, up to eight percent of thrust acts along the vertical axis. This does not amount to much-maybe one or two percent of the weight of the airplane-but it slightly reduces the induced drag of the wing. In other words, at the same time as a nose-up attitude increases fuselage drag, it also reduces the lift-dependent induced drag.

While rate of climb may or may not be affected by a small flap deflection, angle of climb is. Angle of climb is essentially a matter of how slowly you can fly, other things being equal; and so a flap setting that lowers stalling speed without disproportionately increasing drag will allow an airplane to climb at a steeper angle. The deal-breaker here is that proviso about not increasing drag. Flaps do increase drag, and the more you deflect them the more drag you get. Some airplanes can't climb at all with their flaps fully deflected.

In the final analysis-a phrase often used to make a collection of uncertainties seem more informative than they really are-the effect of flaps on rate of climb depends on so many factors that no general claim can be made. In my opinion, it is unlikely that any flap deflection has a discernible positive effect upon rate of climb, but I would be interested in seeing evidence-from actual flight test, not anecdote-to the contrary.