What Flaps on an Airplane Do: They Don’t Just Push Upward

The purpose of increasing drag is not so much to steepen the approach as to require you to use some power to maintain a desired approach angle. If you carry a moderate amount of power during the approach, you can adjust the aim point with small throttle movements. Airplanes that approach at idle power, as ones with low-wing loadings like old taildraggers do, have to use the somewhat less delicate forward slip to steepen the approach.

There are other ways to increase an airplane’s drag that have fewer side effects than trailing edge flaps do, and so the major reason for flaps is lift, not drag. Just how they produce that extra lift, however, is not obvious. 

It’s a perennial embarrassment to high school physics teachers that cheap balsa model airplanes—to say nothing of folded-up pieces of paper, or butterflies—fly well. After all, it says right here in the textbook that airplanes fly because air has to go a longer distance over the top of the wing than  the bottom. And so (because some guy named Bernoulli who lived hundreds of years ago said so) the air has to go faster, and therefore the pressure on the top of the wing is lower than the pressure on the bottom.

This Bernoulli fellow was stating a simple fact of the physics of fluids that he considered almost self-evident, and he would be no better known to pilots than d’Alembert or Torricelli had his name not come to be associated with an appealing, though flawed, explanation of lift. In fact, air particles marching past a wing are not like a phalanx of soldiers—the Bernoulli Brigade—who part at the leading edge and rejoin their neighbors at the other end. There is nothing to cause the upper-surface flow to arrive at the trailing edge at the same time as the lower-surface flow, and in fact it doesn’t. (Surprise—it gets there sooner.)

But it’s not even necessary that the distance along the upper surface be greater than that along the lower, as the folded paper airplane and the simple balsa glider show. If the angle of attack is small enough, it’s not the airfoil shape but the fact that the wing is at a nose-high angle to the passing air that is fundamentally responsible for generating lift. Shaping the wing like an airfoil reduces its drag and allows it to reach a greater angle of attack without stalling, but the airfoil shape is not indispensable—it’s a refinement.

Angle of attack works by shifting the location of the so-called stagnation point, which is the place on the wing surface where air going over the top of the wing parts company with air going under the bottom.

When a wing’s angle of attack increases, the underside of the wing presents an obstacle to the airflow. Pressure beneath the wing increases and propagates through the air in all directions at the speed of sound. The air ahead of the approaching wing “feels” that the pressure is greater below the leading edge than above it, and so it veers upward even before reaching the wing. As a result, the stagnation point, where the flow splits, moves aft along the underside of the wing.

Counterintuitively, air approaching the wing above the stagnation point flows slightly forward before rounding the leading edge and flowing aft from there. As it squirts around the leading edge it momentarily accelerates to a high speed—up to several times the free stream velocity—and that acceleration produces a suction peak that accounts for the center of lift of an airfoil not being at its midpoint, as you might suppose, but somewhere between the midpoint and the leading edge.

Between the 1930s and the ’50s, the so-called split flap was popular with airplane designers. DC-3s had split flaps, as did Cessna 310s. The aft 20 percent or so of the lower surface of the wing deflected downward; the upper surface stayed put. A split flap resembled a spoiler but upside down. It was hard to see how this arrangement, which left behind a huge turbulent wake, could yield anything but lots of drag, until you realized that by erecting an obstacle to lower-surface flow it moved the stagnation point aft and forced more air over the upper surface. With more air passing around the leading edge per unit of time, its velocity had to be greater, and suction over the entire upper surface of the wing increased.

A plain flap, such as an aileron, acts similarly, but without the turbulent wake. Moving the aileron shifts the stagnation point forward or aft, adjusting the shape of the airflow at the leading edge. The lift-altering effect of the aileron is not due just to the impact of air on the aileron itself. It’s due to a change in the flow conditions over the entire wing.

What separates an airfoil from a flat sheet of paper or balsa wood is the shape of its nose. Once a flat sheet’s angle of attack exceeds a few degrees, it stalls. The flow tears away from the upper surface, and the resulting void is filled by air flowing forward from the trailing edge. An airfoil’s smoothly rounded leading edge helps the air to make the high-velocity change of direction at the stagnation point up to much higher angles of attack. “High lift” airfoils—ones designed to delay stalling as long as possible—have thick, gently curving leading edges, just as high-speed roads have gradual, sweeping curves.

Slotted flaps, hinged or tracked, add a separate source of lift. They not only pull more air over the upper surface of the wing, but they also create an additional leading edge (or several, if the flap is double- or triple-slotted) with its own stagnation point and suction peak. The deflected flap becomes an additional mini-wing, and the center of lift of the wing and flap together shifts aft, contributing to the nose-down trim change associated with the deflection of a flap.

That the nose-down pitching effect can be easily trimmed out by the elevator shows that the flap’s lift is not concentrated on the flap alone but is spread out over the entire flapped portion of the wing. A powerful slotted flap may double a wing’s lifting ability. If that added lift—equal to the weight of the airplane—were confined to the flap alone, the pitching moment that resulted would be enormous and the nose-down pitch would be uncontrollable. But flap deflection influences the entire wing, and so the pitching effect can be canceled by an elevator of normal dimensions.

This column first appeared in the January Issue 954 of the FLYING print edition.