The Aerodynamicist’s Pot of Gold—Laminar Flow Control

Laminar flow control has been an elusive and alluring quest that has tempted aeronautics engineers for nearly 80 years. According to histo­rian James Hansen. “Nothing that aerodynamicists could to do to improve the aerodynamic efficiency of the airplane in the late twentieth century matched the promise of laminar flow control.”[306] Richard Wagner, the head of Langley’s Laminar Flow Control program, said that of all the ACEE programs, it offered "by far. the biggest payoff.”[307] Engineers knew that, if it could be perfected, laminar flow control could improve fuel efficiency by 30 percent or more and decrease drag by 25 percent. Using 2004 esti­mates, if the United States airlines could reduce drag by just 10 percent and fuel economy by 12 percent, it would result in a savings of SI billion per year. Albert L. Braslow, who spent his career working in the laminar control field, argued that it was the “only aeronautical technology” that would enable a transport airplane to fly nonstop to any point in the world and to stay aloft for 24 straight hours. 1 le concluded that the incredible fuel savings was the ‘“pot of gold at the end of the rainbow” for aeronautical researchers."[308] This allusion was perhaps more appropriate than Braslow realized, or would have liked. Though the lure of the rainbow’s gold and laminar flow control are undeniable, to this day, neither exists, though the commercial potential for laminar flow remains in sight.

The fundamentals of laminar flow’ are as follows. When a solid (such as an aircraft wing) moves through air. it encounters friction. The thin layer of air that interacts with the solid’s surface is called the boundary layer. Within this layer, two conditions can occur: a laminar condition, where the airflow is uniform in nonintersecting layers, and a turbulent, where the airflow within the boundary layer is characterized by turbulent eddies that cause additional drag. At lower speeds, conditions are relatively favorable for an aircraft to enjoy the smooth laminar flow over its wing surfaces, tail, and fuselage. But as the speed increases, it becomes more difficult to maintain laminar flow, and a more turbulent boundary layer takes over.11 For example, a transport plane flying at subsonic speeds spends half of its fuel to maintain normal cruise speeds while attempting to counter the friction and turbulence found in this boundary layer.

Attaining ideal laminar flow is possible in two main wrays. Natural laminar flow (also known as “passive”) can occur over the leading edge of an airplane’s wing by contouring the airfoil to a particular shape. To achieve laminar flow’ rearward from the leading edge of the wing requires an “active" approach, known as laminar flow’ control. One of the best approaches is a suction method in which holes or slots in the w’ing draw some of the boundary layer air through it. Pumps suck the air down through the surface, w here ducts vent it back out into the atmosphere. In this way, the w ing or airfoil appears to “breathe."

The earliest laminar flow investigations began in the 1930s, when German engineers first developed stability analysis methods. In 1939, Langley engineers began performing wind tunnel tests to study turbulence and laminar flow. The NACA became increasingly interested in studying this phenomenon, and 2 years later. Langley was able to flight-test a B-19 with 17 suction slots in a special test section mounted on one wing panel. During World War II. active laminar flow’ control work was suspended in order for research to take place on natural laminar flow for aircraft such as the P-51 Mustang, while Germany and Switzerland continued their active approaches. After the war. Langley (aided by the release of confi­dential German research after World War II to the aeronautics community) returned to suction studies in wind tunnels and provided theoretical sup­port that this approach wras indeed possible.[309] [310] The Air Force also became interested in laminar flow and contracted w ith Northrop Corporation to

Early laminar flow tesLs on a blunted 15-degree cone cylinder in free flight at high Reynolds number (July 23. 1956). (NASA Glenn Research Center [NASA GRC|.)

investigate suction through slots and holes. The NACA concluded that the main impediment to achieving laminar How control was the difficulty in creating smooth surfaces on the airplane. Even factors such as bugs or ice crystals could cause the loss of a laminar flow.

Research continued and tremendous optimism surged in the early 1960s over the Air Force’s work with laminar flow. In 1963, the New York Times announced an “aviation landmark” and a “new aeronautical milestone” with the flight of an X-21. a reconnaissance-bomber research aircraft, and a “revolutionary air-inhalation system."1′ Under the direction of Wener Pfenninger at Northrop, a slot-based laminar flow control system was suc­cessfully flight-tested, and some observers called it the most promising development in flight since the jet engine. Even though the Air Force viewed laminar flow as the most “prominent” and “promising” of its leading aerodynamic projects, further research was delayed for another decade.[311] [312]

The center section of each wing of this business jet was modified for tests of laminar flow control (October 15.1984). (NASA Langley Research Center {NASA LaRC].)

From the mid-1960s to the mid-1970s, laminar flow studies were sus­pended. in large part because of the commitment of military resources to the war in Vietnam. Also, the low cost of jet fuel completely offset the savings when compared with manufacturing and maintenance costs for aircraft with active laminar flow control.

This economic situation changed with the rise in fuel prices and the end of the war. When NASA began looking at technologies to include in the ACEE program, laminar flow was an early favorite. Langley research­ers had resumed studies on it, and in 1973. Albert Braslow wrote a white paper arguing that it had “by far the largest potential for fuel conservation of any discipline.”1′ While many were enthusiastic about it. Braslow noted that some managers at NASA Headquarters and Langley were “luke­warm” to the idea. Detractors thought the technological barriers were so [313] steep that it would be throwing away limited aeronautics funding to pursue the research.

As fuel costs continued to rise, the promise of laminar flow became more and more attractive. In March 1974, the AIAA held a conference with 91 of its members to discuss aircraft fuel-conservation methods, and they concluded that laminar flow deserved attention. Their ideas were sup­ported by the ACEE task force, and in September 1975, Edgar Cortright, the Langley Director, initiated the Laminar-Flow-Control Working Group. Cortright announced that Langley had accepted the responsibility of imple­menting a research and technology program focused on the “development and demonstration of economically feasible, reliable, and maintainable laminar flow control.”[314] One of the primary new focuses was a change from military to commercial applications.

There seemed to be as many staunch proponents of laminar flow’ as there were detractors. The optimists believed that a laminar flow wing could be developed using existing manufacturing techniques and known materials and implemented in a reasonable timeframe: by the 1990s. The laminar flow pessimists argued that even if all these achievements were possible (and many believed they were not), the costs and efforts required to keep the airfoil surfaces smooth, clean, and in flight-ready condition would make the entire system prohibitive. The airline industry sum­marized its concerns in four main areas: manufacturability, operational sensitivity, maintainability, and methodology.[315] Hans Mark, the Director of Ames Research Center, was one detractor. He said that the laminar flow program under ACEE should be “given low priority due to the low probability of success, and because benefits are not likely to be realized for many years, if ever”[316]

The laminar flow’ group w ithin ACEE had a difficult mission in front of it: to provide data to support or refute assumptions by both the optimis­tic and pessimistic camps so that industry could make “objective decisions on the feasibility of laminar flow control for application to commercial transports of the 1990s time period.”141 Despite the uncertainties, laminar flow was included in ACEE for two main reasons: first, it offered the prom­ise of dramatic fuel-efficiency improvement, and second, the work in com­posites might directly contribute to developing materials more operationally and economically suited for achieving laminar flow control.

The program, “involved a major change in Agency philosophy regard­ing aeronautical research," according to Albert Braslow. It included an extension of the traditional NACA role in research to include a “demon­stration of technological maturity in order to stimulate the application of technology by industry.”’0 This was also a risky proposition, made even more so during the political environment of the Reagan years. Project man­agers accepted the high level of risk in taking on this program because it was such a revolutionary idea with such great potential. Because NASA had to produce flight research results in several areas, it decided that a phased approach—by breaking down the problems into smaller units—would offer the best chances of success. Phase one involved developing methods for ana­lyzing boundary layers with new computer codes. Also included were studies of surface materials and how to best maintain them. Phase two would move to basic fabrication of test pieces and subject them to wind tunnel testing. This would include subsystems such as pumps for suctioning. Phase three included actual flight-testing, with laminar flow control over a wing or a tail. Braslow was extremely enthusiastic about the potential for the program but was also aware of the risk. He said, “Everybody agrees that you have a hell of a payoff, but the question is, ‘Can you do it on a day-to-day basis?””1

As phases one and two progressed, several key problems were over­come. Insect contamination was thought by many to be a critical issue in preventing program success. Although the insect remains on the wings were small, they were nonetheless large enough to disrupt laminar flow’. That an insect represented the margin of success or failure suggests how difficult the project was. Engineers tested washing systems and nonstick surface materials and concluded that it was best to keep the wings wet [317] [318] [319] so the insects they encountered wouldn’t stick." The potential impact of engine-generated noise waves disrupting laminar How on wings was another area of concern, and a NASA contract with Boeing investigated the laminar flow acoustic environment on a 757. Engine noise, it was found, did not cause the laminar flow to become turbulent. Research went beyond suction laminar flow control. Natural laminar flow investigations were carried out on F-l 11 and F-14 jets at Dryden Flight Research Center.

With success in these first two phases building confidence, phase three began by selecting a vehicle for flight-testing. The airlines wanted an aircraft similar in size to their commercial transports, while NASA pushed for a smaller plane to reduce costs. A compromise was eventually made using a larger plane but restricting experiments to the leading edge of a laminar flow wing, the most technically difficult area to overcome. The leading edges had to be smoother than other areas and had to withstand rain, insects, corro­sion. icing, etc. Langley eventually used a JetStar plane, similar in size to a DC-9. NASA contracted with three industry leaders—Douglas. Lockheed, and Boeing—with NASA assuming 90 percent of the cost.

The Lockheed studies used a composite (graphite epoxy) wing covered by a very thin titanium sheet. The ducting was achieved through slots, and compressors induced the suction. However, it forced the wing to maintain the entire weight of the system, which became problematic. Douglas engi­neers used a different approach, opting for perforated holes instead of slots for the ducting, and explored using a glass fiber material for the suctioning. Boeing came to the laminar flow studies later than the other two companies, preferring to focus all its early attention on near-term fuel efficiency endeav­ors, as opposed to the uncertain future of laminar flow control.2J

After 4 years of flight tests (1983 to 1987), all results were extremely positive.[320] [321] [322] Laminar flow control had been achieved for this leading edge area of the wing in a variety of test conditions, including cold. heat, rain.

Laminar flow test aircraft in flight (November 15. 1984). (NASA Langley Research Center (NASA LaRCJ.)

freezing rain, ice, moderate turbulence, and insects. Pilots had no diffi­culty adjusting to the new system. The titanium surface did not corrode over time. Enthusiasm soared higher after a series of test flights with the C-140 JetStar at Ames-Dryden Flight Research Facility, which simulated a commercial airline service operating in a variety of weather condi­tions and achieved 22-percent fuel efficiency at cruise speed. Roy Lange, the Laminar Flow Control program manager at Lockheed-Georgia. was pleased with the initial results, though more work still awaited completion. “The only question we have now,” he said in 1985,“is whether the systems can handle a day-by-day flight schedule. … I think we could get there for a 1995 aircraft.”’1 In addition, Langley engineers also investigated hybrid laminar flow control, a combination of the suction and natural laminar flow techniques. Boeing began research on a 757.[323] [324] Braslow recalled that “results were very encouraging. … All necessary systems required for practical [hybrid laminar flow control] were successfully installed into a commercial transport wing."[325] Calculated benefits for a 300-person trans­port predicted a 15-percent savings in fuel.

Despite the successful outcomes, laminar flow control is not currently used in any commercial transport. While the concept was proved in theory and flight-tested, it was never put into service nor put through the rigors of a day-to-day operational environment. It fell victim to the drop in fuel prices in the late 1980s, as there was no economic incentive for pushing through the remaining technological obstacles and actually incorporating laminar flow control into a commercial airlines’ service.

There has been some continued laminar flow research that has yielded positive results since the end of ACEE. including the NASA-Boeing-Air Force B-757 Hybrid Laminar Flow Control (HLFC) flight experiments. As one Langley press release noted in August 1990. the “aerodynamic effi­ciency of future aircraft may improve sharply due to better-than-expected findings from a joint-government-industry flight test program.” Laminar flow was achieved over 65 percent of the modified 757 wing, and engi­neers speculated that if the entire span of both of the wings were modified, the airplane drag would decrease by 10 percent. This would save roughly $100 million annually for the U. S. airline industry.[326] Despite the progress, the technology was not perfected. In 2004, aeronautical engineers William S. Saric and Helen L. Reed presented a paper on the remaining challenges in achieving practical laminar flow. They concluded that “crossflow insta­bility” remained the most significant challenge.[327] [328] [329]

Richard Wagner, the head of the program, lamented the fact that the lam­inar technology is still unused. He said,“I really was disappointed that we didn’t see, or haven’t seen an application of… laminar flow control because… the stuff was ready. I guess it’s just going to take some time to where the fuel price makes it so attractive that they can’t turn their back on it.”4′ Despite its lack of industry acceptance, the ACEE program made major advances in understanding the potential of laminar flow. As James Hansen argued, “all of the promising research indicated that its time might yet come.’’51