Airflow Control

Aerodynamic tricks to improve handling

October 1, 1995

When a bird approaches to land, it spreads its feathers and changes the shape of its wings, which is not unlike what a pilot does to an airplane before landing. But instead of rearranging feathers, the human pilot extends a set of flaps. Each method accomplishes the same basic function; it increases wing camber and usually increases wing area. The purpose, of course, is to allow an airplane (or a bird) to fly safely at a relatively low speed. This allows for reduced takeoff and landing distances, and improves crash survivability.

There are several types of wing flaps, the four most common being plain, split, slotted, and Fowler flaps.

Aircraft designers discovered a long time ago that similar improvements in slow-speed flight could be obtained by adding leading-edge devices.

The simplest of these is the leading-edge flap, also known as a Krueger flap. It is essentially a flat or curved plate that, when retracted, lies flush with the underside of the leading edge of the wing. When such a flap is extended, wing area and camber are increased, which improves lift capability at large angles of attack. One can think of a leading-edge flap as the blade of a bulldozer; it forces more of the oncoming air to flow up and over the wing.

Leading-edge flaps almost always extend simultaneously with deployment of trailing-edge flaps. Unlike trailing-edge flaps, however, they have no intermediate position. Leading-edge flaps are either fully retracted or fully extended. They can be installed along the entire leading edge or — for reasons explained later — along only a portion of a wing.

The drooped, circular-radius, leading-edge cuff found on various lightplanes also is a leading-edge device and is roughly analogous to permanently installed and extended leading-edge flaps. Because they usually have such short radii, they exact little or no drag penalty in cruise flight. They do, however, improve airflow above the wing and help to preserve high lift coefficients at large angles of attack. (Some early aircraft had controllable leading edges or "drooped snoots" that were extended only for takeoff and landing.)

The slat is more effective than a leading-edge flap. It is essentially a small auxiliary airfoil that — when retracted — forms the wing's leading edge. When deployed, the slat moves forward and down. This usually (but not always) opens a slot between the slat and the fixed leading edge of the wing.

The slat serves three purposes. First, it increases wing area and camber like a conventional flap. Second, the slat — being essentially a small wing — is an independent lifting surface. And finally, the slot formed behind the slat is similar to the one(s) formed when slotted, trailing-edge flaps are deployed. High- pressure air from beneath the leading edge of the wing is ducted through the slot and into the boundary layer above the wing. This delays airflow separation (stall) to some larger angle of attack and lift coefficient. Slats also can improve aileron effectiveness and therefore roll control during low-speed flight. (Unlike trailing-edge flaps, slats do not create pitching moments when deployed.)

Slats first appeared on piston-powered airplanes designed for STOL (short takeoff and landing) operations. The Helio Courier, for example, incorporates slats that are held open by aerodynamic forces at large angles of attack. As the angle of attack is reduced, however, the force of the relative wind comes more from ahead of the wing and pushes the slats into their retracted positions against the leading edge. Other than controlling angle of attack with the elevator, the pilot of such an airplane has no direct control of slat operation.

Leading-edge devices did not come into vogue, however, until the advent of swept-wing, turbojet airplanes. Such wings have a particularly nasty trait: the wingtip stall. Instead of the stall's occurring near the wing root and propagating outboard (as with straight wings), the stall of a swept wing begins near the wingtip. This can result in strong nose-up pitching moments, sharp and uncommanded roll rates, and reduced aileron authority. Aircraft designers obviously go to great length to preclude the possibility of such a stall.

One solution is to install slats or — as in the case of the Boeing 727 — a combination of slats and leading-edge flaps. It is natural to wonder why Boeing uses inboard flaps and outboard slats on the "three-holer." The answer is both simple and clever. Because the leading-edge flap is less effective in delaying a stall than a slat, the inboard section of the wing is forced to stall first, thus preventing the possibility of a tip stall.

There are two other popular ways to ensure that the root stalls before the tip. One is to wash out the wing (reduce the wingtip's angle of incidence). The other is to vary the airfoils used along the span of the wing. In other words, an airfoil more prone to stalling at a given angle of attack is used inboard while an airfoil less prone to stalling is used outboard.

The Dassault Falcon 900 also incorporates a clever scheme to prevent tip stalling. If a pilot attempts to stall this airplane in a clean configuration (slats and flaps retracted), the outboard slats deploy automatically. The inboard slats remain retracted, which forces the stall to occur near the wing root.

On the forthcoming Sino-Swearingen L.P. SJ30, the slats are designed to extend automatically whenever the angle of attack reaches approximately 20 degrees.

Most business jets do not have leading-edge devices because their wings do not have sufficient sweep to justify their use. The break-even point appears to be about 25 degrees of sweep. Aircraft with the greatest amount of wing sweep typically are the most efficient (in terms of fuel flow and powerplant requirements) because slats and leading-edge flaps enable a designer to employ more highly swept wings that otherwise appear too small to lift the airplane (such as is the case with the McDonnell Douglas MD-80).

Another relatively recent development found on many turbofan airplanes is the winglet, which is used to reduce drag.

On a typical wing, high-pressure air from beneath the wing tends to curl around the tip and combine with the relatively low- pressure air above the wing to form a vortex, which is intricately tied to downwash and the development of induced drag. After beginning a study in 1970 about what could be done to reduce induced drag, NASA aerodynamicist Richard Whitcomb and his team developed an effective solution: vortex diffusers, or winglets, that control airflow curling at the wing tip.

Winglets are essentially small wings mounted almost vertically on the wing tips. Often, there is a smaller, secondary winglet installed below the wingtip, as on some Airbus airliners. The size of the secondary surface, however, often is limited by ground-clearance requirements.

A winglet's installation angle and cant is critical. For optimum effectiveness, a winglet must be finely tuned to the wing, a procedure that can require exhaustive flight testing. Simply installing winglets on an airplane can result in a totally ineffective system and, as some homebuilders have discovered, measurably greater drag when done incorrectly.

The principle of a winglet is roughly analogous to the dynamics of sailing that allow a boat to tack into the wind. During flight, the relative wind (which comes from directly ahead of the winglet) combines with the curling action of the air (which comes from beneath the wingtip with a sideways component). This produces a resultant relative wind that approaches the winglet from ahead and abeam, which is why the winglet is canted (not aligned exactly fore and aft). As a result, the winglet creates lift in a direction that is both inboard and slightly forward. In effect, the forward vector of lift creates enough of a forward force to more than overcome the drag of the winglet itself.

Most important, however, is that the presence of the winglet weakens the curling of air around the wingtip, which reduces vorticity and induced drag.

According to Whitcomb, winglets are more than twice as effective as wing tip extensions of equal size in improving the lift- to-drag ratio of the airplane. In other words, they effectively increase the aspect ratio (span-to-chord ratio) of the wing. Theoretically, they can reduce induced drag during high-altitude cruise flight by as much as 15 to 25 percent, which can equate to a 10-percent reduction in total drag. It is little wonder that they are installed on long-range aircraft.

Whitcomb claims that winglets are most beneficial on highly loaded, swept wings during high-altitude flight and are of dubious value on general aviation airplanes.

If the slat and the winglet can be regarded as small wings, then the vortex generator (VG) is an even smaller one. But instead of reducing vorticity the way a winglet does, the purpose of a VG — as its name implies — is just the opposite. It creates a vortex.

As air flows over a wing, the thin layer of air immediately adjacent to the wing surface does not move relative to that surface. This explains why a layer of dust on a wing before takeoff does not get blown off during flight.

The next extremely thin layer of air slides very slowly over the stationary layer, and so on, until the layer of air at some small distance above the wing has the same speed as the free airstream. Consequently, the boundary layer has relatively little energy and is susceptible to separating from the surface at large angles of attack.

Introduce the seemingly innocuous VG, which is little more than an extremely thin, stubby wing. It is installed vertically so that the tip of this miniature "wing" extends above the boundary layer and into the relatively high-energy airstream above the real wing. It also is mounted at a 15- to 20-degree angle to the relative wind, which gives it a relatively large angle of attack. The result is that this "low-aspect-ratio wing" creates (for its size) a very powerful "wingtip" vortex just as an actual stubby wing would at a similarly large angle of attack.

The miniature tornadoes created by the VGs stir the air along the surface behind the VGs. In this manner, the vortices displace the stagnant boundary layer with high-energy air that normally is found only above the boundary layer. The result is that the air immediately above the wing becomes energized and more resistant to separation during high- and low-speed flight. For similar reasons, VGs installed ahead of control surfaces can make them more effective.

The art and science of airflow control also has been applied to the design of artificial hearts and valves. And why not? The only difference between blood and air are color and viscosity. Both are fluids.

So if a problem relates to fluid flow — whether through your heart or over your wings — an aerodynamicist probably can find the answer.