The behavior of the air around an aircraft makes a big difference.
By John Likakis
Modern military transports sure look sleek and slippery. And designers do their utmost to reduce drag wherever possible and practical. Drag, after all, cuts airspeed, while increasing fuel burn. But one source of drag has been the special target of engineers, researchers, and airframe designers: the aircraft’s boundary layer.
The boundary layer is the layer of air that lies right against the skin of an aircraft. It can vary in thickness, depending on a number of factors, including airspeed and surface finish; it generally will be thicker on relatively rough surfaces and much thinner on highly polished areas.
In flight, this layer of air experiences so much friction with the surface of the aircraft, that, in effect, it “sticks” to the skin. This effectively slows the air enough that the close-lying layer of air molecules can remain almost stationary in relation to the moving aircraft. Those almost stationary molecules dramatically slow down the next layer of air molecules as well. This effect continues for some distance from the exterior of the aircraft, diminishing as you move farther away, until you finally have air moving freely past at whatever speed the aircraft is flying.
The presence of the boundary layer prevents what is called “laminar flow,” which is air flowing smoothly as a continuous sheet across a wing or other airframe structure. Without getting into too much technical detail, laminar flow is highly desirable, because it translates directly into low drag and high efficiency — two qualities that are of paramount importance, particularly for large aircraft, such as transports.
The boundary layer also causes undesirable secondary effects. Because the boundary layer gets thicker as air flows toward the trailing edge of a wing, a design that does not control the boundary layer can result in poor flight control response. This may include phenomena such as aileron “deadbanding” (where the aileron becomes unresponsive until it deflects beyond a certain point) or even, in extreme instances, aileron “snatching” (when it moves out of control).
Given the importance of the boundary layer, engineers have spent decades pondering various ways to reduce it, control it, and compensate for it. Some methods have worked well, while others had potential but proved impractical. Today’s modern transports use a combination of the tried-and-true and more recently developed methods. At the same time, manufacturers are looking at some truly amazing technologies that may more effectively control the boundary layer.
Stick To It
Controlling the boundary layer on wings not only pays the biggest dividends, it also can solve a host of control and performance problems. So it is not surprising that so much effort has been focused on this area.
Some of the earliest efforts concentrated on preserving laminar flow over as much of the wing as possible. The physics of the boundary layer typically causes the airflow over a wing to separate and become turbulent just past the thickest part of the airfoil. So the goal was keeping the airflow contiguous and moving together smoothly along the surface right to the trailing edge of the wing.
Back in the 1940s, at the outset of World War II, the first efforts to streamline military aircraft began with trying to make the wing skin as smooth as possible. Flush rivets, extra-slick paint polished to a high shine, lapped wing skins with almost nonexistent joining lines, and other tricks were used to try to keep the unbroken airflow moving along.
Some 70 years later, much theoretical progress has been made in developing airfoils that have natural laminar flow. (There is actually a series of airfoil shapes carrying the designation NLF.) However, there are still no production aircraft featuring full natural laminar flow wings.
Other schemes to achieve laminar flow have included sucking the boundary layer into the wing interior through thousands of tiny holes in the wing’s upper surface. That works, but the weight and complexity of such systems currently makes them impractical.
The opposite also has been tried — blowing high-speed compressed air across the top of the wing from specially designed slits or ports along the upper surface. Again, the equipment involved in this “active blowing” is too heavy and complex for practical use.
Yet another scheme involves specially designed “gloves” to tailor the wing surface shape and smoothness. By carefully selecting the wing surface-finish characteristics, engineers can determine where and how the boundary layer breaks into turbulent flow. As envisioned, this could solve two aerodynamic problems at once: providing precise control of the boundary layer, and limiting the spanwise airflow found on swept-back wings. (The common way to control spanwise airflow is with winglets at the wing tips.) Laminar-flow gloves have been tried on a number of aircraft, but success has been limited, and adaptability to existing aircraft also is, at best, limited. So, as promising as the concept is, researchers have yet to design a practical laminar flow wing.
A Disturbance in the Force
If undesirable boundary layer transitions cannot be defeated by making the air flow smoothly over the entire wing, an alternative is breaking up the boundary layer to keep the airflow moving along the aircraft’s skin. This process is called “energizing” the boundary layer, and the most common method to accomplish this is a deceptively simple part called a vortex generator.
Vortex generators are small plates that poke through the boundary layer into the fast-moving, free-stream airflow. As the name implies, these plates generate tiny vortices that pull this airflow down to the wing surface. This breaks up the boundary layer, reducing the stagnation that would otherwise build up toward the trailing edge of the wing. If you have ever looked out the window of a modern commercial airliner such as a Boeing 737, you have no doubt seen rows of vortex generators along the top of the wing.
Vortex generators are efficient and effective. They have no moving parts, and they do the job of controlling and re-energizing the boundary layer. The result is that the airflow moves smoothly past the wing, and control surfaces, such as ailerons and flaps, work much better.
As a bonus, vortex generators lower the aircraft’s stall speed by keeping the airflow energized and smooth at higher angles of attack, which, in turn, means that it can land at a lower speed and thus can use shorter runways — a particularly desirable trait for transports.
Not Just Wings
Boundary layer problems are not limited to the wings of an aircraft. Poor boundary layer conditions anywhere on the airframe can be a significant contributor to overall drag. This can reduce control, potentially requiring larger tail surfaces, with all the extra weight and drag those bring.
The aft-most part of an aircraft fuselage is where boundary layer problems are typically the most intense — particularly where the fuselage narrows to the tail cone or where it turns abruptly for an aft cargo ramp and door area. The Lockheed Martin C-130 Hercules is a prime example of an aircraft design that experienced this kind of problem. Lockheed Martin’s engineers were able to tame the boundary layer in this area by adding microvanes along the sides of the aft fuselage. These vanes change the airflow around the aft cargo doors to reduce overall airframe drag, helping the Hercules fly farther on less fuel.
The aft end of Boeing’s C-17 also has received attention in adjusting the boundary layer. As delivered in conventional configuration, the C-17 features a pair of specially designed strakes (long metal strips) attached to the aft fuselage to improve stability. The U.S. Air Force Research Laboratory (AFRL) has been conducting flight testing of a number of different devices to further improve airflow around the aft fuselage under the vertical stabilizer.
The first set of tests used “finlets” (small fins) produced by Vortex Control Technologies (VCT), of Kennesaw, Georgia. These finlets initially were attached to the fuselage in sets of three on each side; subsequent flight testing doubled the number. Another set of tests has used Lockheed Martin’s microvanes, arranged in the same area.
In each case, the objective was to re-energize the boundary layer, thus cutting drag and decreasing fuel consumption. At the time of this writing, flight test results from the program were expected to be released before the end of the year.
The Active Future
Vortex generators are considered a form of passive boundary layer control. Active boundary layer control includes the approaches mentioned above, sucking the layer into the wing skin or blowing compressed air across its surface. For the most part, these approaches have added too much weight and complexity to the airframe, thus rendering them impractical.
But Boeing and the National Aeronautics and Space Administration (NASA) have been looking at other applications of actively blown boundary layer control. Working together, they have conducted wind tunnel and flight experiments investigating an actively blown rudder for the Boeing 757. The system uses compressed bleed air from the aircraft’s engines. The air is cooled in a heat exchanger and then released through a set of slit-like nozzles, just ahead of the rudder hinge line. Blowing across both sides of the rudder, this cooled airflow greatly increases the rudder’s effectiveness.
The long-term aim of the research is to develop design and control parameters that allow the vertical stabilizer and rudder of transport aircraft to be much smaller than these components are in existing aircraft. This approach has the potential of cutting both weight and drag, while maintaining control authority and stability.
Another approach to actively controlling the boundary layer that has been accumulating wind-tunnel data is the use of synthetic jet actuators. These use piezoelectric devices to oscillate a patch on top of the wing. As the wing skin flexes up and down, it alternately pulls down a small amount of air and then expels the air upward. The first part also pulls the boundary layer down, moving more energetic air closer to the wing skin. When the skin flexes back, the little puff of expelled air disrupts the boundary layer, causing a tiny vortex to form that energizes the boundary layer downstream from the actuator.
Wind-tunnel testing has shown that synthetic jet actuators can enable aircraft to fly at much higher angles of attack and fly more slowly without danger of stalling. At this point, such synthetic jet systems have not progressed beyond the laboratory. As with some other active approaches, it may turn out that the additional weight and complexity of dozens or hundreds of piezoactuators on each wing will render the system impractical for most aircraft.
Another system being tried uses plasma to control the boundary layer. These systems are known as di-electric barrier discharge (DBD), and they work by creating an intense electric field that ionizes the air passing over the wing. Depending on the configuration and operating mode, a DBD system either can be used to maintain laminar flow much farther back on the wing (operating in a steady state), or it can be used to create micro disturbances in the boundary layer that turn into vortices to energize the boundary layer.
Of all the future-tech systems under investigation, the DBD/plasma system shows the greatest promise for active control of the boundary layer. Researchers in Germany have used DBD systems to replace the conventional flight controls on a small flying model. The 6-inch span flying wing has proven quite maneuverable, even though it lacks movable control surfaces.
Researchers in China also have been conducting wind-tunnel tests of advanced di-electric systems. Reportedly, these systems are aimed at controlling boundary layer flow in the trans-sonic and supersonic region.
The Quest Continues
Precise control of the boundary layer has been a top priority for engineers for decades. For most of this time, the dynamics of boundary layers have been very well understood. But controlling the boundary layer with any kind of precision has proven to be much more difficult. Nevertheless, given the benefits such exacting control would produce, engineers continue the quest undaunted.
Image #1 – Controlling boundary layer at the tail of the aircraft is crucial to both reducing drag and maintaining good handling capabilities. This Rockwell B-1 bomber tail has vortex generators to keep the boundary layer energized around the tail cone. (Photo by Sam King, Jr., courtesy U.S. Air Force)
Image #2 - For large transport aircraft, such as the Boeing C-17, even small reductions in drag can translate into big fuel savings. Controlling the boundary layer flow pays big dividends. (Photo by Sam King, Jr., courtesy of the U.S. Air Force.)
Image #3 - This wind-tunnel photo shows streamlines of smoke flowing over an airfoil at three different angles of attack. At low angles of attack (top image), the smoke flows smoothly around the airfoil. At high angles of attack (bottom), the air cannot follow the airfoil and separates into turbulent flow — a classic stalled condition. (Image courtesy of NASA)
Image #4 - Even in supersonic flight, boundary layer control is critical to reducing drag and aerodynamic heating. This shadowgraph of a pointed spike at Mach 3 clearly shows the transition between laminar flow and turbulent flow. (Image courtesy of NASA)
Image #5 - Sucking the boundary layer into the wing has been a favorite method for producing laminar flow. While very effective, the weight and complexity of such systems currently renders them impractical. (Image courtesy of NASA)
Image #6 - Vortex generators have been around for a long time. This Republic F-84 Thunderchief fighter was used by the National Advisory Committee for Aeronautics (NACA) in the early 1950s to test vortex generators. Boeing used vortex generators on the B-47 bomber of the same vintage to overcome boundary layer problems. (Image courtesy of NASA)
Image # 7 - Another way to handle the boundary layer is to blow high-speed air along a surface. This Boeing 757 tailfin and rudder are being used in a test applying active flow control to improve rudder effectiveness. This approach may allow smaller vertical stabilizers to be used on future aircraft. (Image courtesy of NASA)
Image #8 – The U.S. Air Force is currently testing a number of schemes to improve boundary layer flow around the aft end of Boeing’s C-17 Globemaster. Pictured here are “finlets” used to improve air flow around the tail cone. (Image courtesy of the U.S. Air Force.)