Advanced Aerodynamics—Energy Efficient Transport

Another important ACEE airframe technology program was one Langley engineers Robert W. Leonard and Richard D. Wagner called a “somewhat arbitrarily termed ‘Energy Efficient Transport/”6* Like the composites program, this ACEE project was also to be managed by Langley, and it promised to be of great importance to industry. Unlike the composites pro­gram. whose objective was to focus on a single technology development that promised significant fuel savings, the Energy Efficient Transport proj­ect planned to achieve fuel efficiency through a number of aerodynamics advances. These included the following areas of research: supercritical wings, winglets, nacelle aerodynamic and inertial loads, wing and tail sur­face coatings, laminar flow control, and active controls. NASA’s Langley Research Center partnered with Boeing Commercial Airplane, Douglas Aircraft, and Lockheed-Califomia to analyze, design, test, and assess these advanced aerodynamic concepts.[151] [152] [153]

Most commercial airplanes fly at what is known as transonic speeds. This is an aeronautics term for velocities just below and above the speed of sound (Mach 0.8 to 1.2). “The transonic regime,” as Roger Bilstein said, “had beguiled aerodynamicists for years." Despite being the speed of choice for modern aircraft, those cruise speeds present numerous aero­dynamic challenges.[154] At these speeds, both subsonic (less than the speed

Lockheed L-1011 EET model used in testing of the Energy Efficient Transport project at Langley Research Center (January 5.1982). (NASA Ames Research Center |NASA ARC].)

of sound) and supersonic (more than the speed of sound) airflow patterns exist over the aircraft simultaneously, so even if an airplane is flying at subsonic speeds, airflow over certain sections of the wing might reach supersonic levels, forming strong shock waves on the upper surfaces of the wings and resulting in a dramatic increase in drag. This problem is known as the “sound barrier." As one observer said, “The barrier was con­quered (in 19471 with brute force, but the trick now is to subdue it qui­etly and efficiently."6* Solving the challenge, known as the “supercritical" Mach number, was an important problem.64 Engineers knew that if they could solve it. they would significantly improve cruise performance and increase fuel efficiency. It was to this task that Langley engineer Richard Whitcomb first applied himself in the 1960s. After several years of [155] [156]

Richard Whitcomb looks over a model of the Chance Vought F-8 aircraft incorporating his supercritical wing (July 1.1970). (NASA Langley Research Center (NASA LaRC].)

research and extensive wind tunnel studies, he redesigned the wing shape with a flatter upper surface, which reduced the strength of shock waves. A dow nward sloping curve at the wing’s trailing edge increased the lift. Because the supercritical w ing could be thicker than a conventional wing, the aspect ratio of the wing could be increased to reduce the drag, and the wing sweep could be decreased for more efficient cruise. The “supercriti­cal w ing” was born. In 1972. after 12 test flights. Whitcomb said. “I feel confident we’ve reached a milestone in the program.”[157]

To take advantage of Whitcomb’s work. NASA needed an incentive to perform further flight tests and incorporate it into a commercial transport that would be both aerodynamically and structurally sound. This incentive came with the fuel crisis, and the supercritical w ing became part of the EET project. Langley engineers began generating a database of w’ing vari­ables in wind tunnels that tested the various effects of thickness, camber

NASA selected a Vought F-8A Crusader as the testbed for an experimental supercritical wing. (January 1.1972). (NASA Dryden Flight Research Center [NASA DFRC] Photo Collection.)

(the wing’s curvature), sweep, and aspect ratio (a measure of the wing’s ratio of span to area). The results of their research led to the adoption of this wing in a variety of aircraft. Industry followed Whitcomb’s lead with its own supercritical wing designs. Boeing incorporated a version of the wing in the Boeing 767 in 1981 and the Boeing 777 in 1995. James Hansen has called the 777’s wing the “most aerodynamically efficient airfoil ever developed for subsonic commercial aviation.”’1 The wing’s success can be traced directly back to the pioneering work performed by Whitcomb and the ACEE engineers.

The supercritical wing was not Whitcomb’s only inspiration. In 1974, he developed a new idea, known as winglets. While the supercritical wing promised fuel efficiency in the future when new aircraft were built, the winglets were important because they could be immediately retrofitted.

Looking and acting like a vertical sail, they took advantage of the swirling vortex of airflow around the tip of the wing. Whitcomb published [158] the results of his study in July 1976 and promised a 4- to 8-percent drag reduction. He confidently predicted, “Just as sure as the sun rises, the next new commercial transport aircraft will have winglets.”[159] Since the whirlpool of air around the wingtip was different for every airplane, it was left to the aircraft manufactures to design and test specific winglets for their planes. To encourage their adoption, NASA and the EET pro­gram cosponsored industry flight tests on aircraft. The first, between 1978 and 1979. included research with Douglas Aircraft on its DC-10. The suc­cess in reducing fuel consumption was quickly apparent: Robert Leonard. Langley’s ACEE Project Manager, said. “Frankly, the winglet looks very promising on the DC-10.”” Douglas designers incorporated winglets into their new MD-11 development in 1986.[160] [161] [162] Very quickly, the entire industry realized the importance of the winglet.[163]

Other issues caused by structural reinforcement for flutter and gust loads required solutions.6 One approach to providing structural weight reduction while maintaining safety margins was a computer-assisted advance called Active Controls Technology (ACT), also known as a Control-Configured Vehicle (CCV). While ACT technology had been investigated prior to ACEE, this program served to increase dramatically confidence and industry acceptance. The ACT system used an onboard computer system to control surfaces on the trailing edges of the wings and on the tail sections of the aircraft. The computer acted independently from the pilot, working to minimize the aircraft’s structural loads when it encountered turbulence or making a tight turn while maintaining a suffi­cient flutter margin. To achieve this, sensors on the surfaces of the aircraft sent feedback to the computer, which in turn could send compensating signals to the control surface actuators. Computers, not pilots, were best suited to handle these controls because turbulence is a random, time-

А КС-135 with winglets in flight over the San Gabriel Mountains, south of Edwards. (January 1.1979). (NASA Dryden Flight Research Center [NASA DFRC| Photo Collection.)

dependent phenomenon, and the electronic system can react much faster than a human pilot can. The sensors and the computer were able to com­municate to rudders, elevators, and ailerons within a split second to adjust correctly for these disturbances.[164] [165] Hindering the development of such a system were a lack of confidence that the design was possible and a belief that they were not cost effective. Langley engineers worked with counter­parts at Douglas, Lockheed. and Boeing to solve these problems and install active controls on several types of specific airplanes. The results of these studies proved that an ACT airplane required an investment of $600,000, with a 25-percent return on investment (based on fuel prices in the early 1980s). The FA A also concluded that they were flightworthy, and that no single failure in the system would result in the loss of control of the aircraft. Pan American World Airways purchased the first aircraft with active controls (L-1011-500) and then began to retrofit active controls to all planes of this type in its fleet.’* Another important aerodynamics advance explored under the EET program focused on airframe/propulsion integration. The main effort in this area was the Nacelle Aerodynamic and Inertial Loads (NAIL) program directed jointly by Langley and Lewis Research Center. Engineers knew that the most critical period of deterioration for aircraft engine efficiency occurs during the initial period of its life. After the engine reaches approxi­mately 1,000 flights, this deterioration levels off substantially. The goal of the NAIL program was to provide as much data as possible on the early life of a jet engine to determine the causes for the decreases in efficiency. The Centers partnered with Boeing and Pratt & Whitney, and a NAIL engine was constructed, flown, and then disassembled and inspected. The test flights revealed that the highest “flight loads,” or wear, occurred at low speeds, high angles of attack, and high engine airflow, conditions most typ­ically occurring at takeoff.74 The conclusions served as the basis for future nacelle redesigns that would have a greater ability to withstand flight-load wear and tear, specifically during these periods of flight.

There were two areas of the EET program that overlapped with other ongoing ACEE investigations at Langley. Much like the composites pro­gram. the Aircraft Surface Coatings program explored the use of new mate­rials that would improve the surface smoothness of aircraft. The Apollo spacecraft had used Kapton. a film polyimide, as a coating, which reduced drag, decreased maintenance, and offered increased protection. Similar advantages were sought for aircraft surfaces. Langley engineers identified elastomeric polyurethane coatings such as CAAPCO and Chemglaze and tested them on a Continental Airlines Boeing 727 used by Air Micronesia. Micronesia was selected because of its high rainfall environment, which typically degrades surface coatings. The engineers found that these mate­rials produced a small decrease in drag and at the same time increased pro­tection from corrosion.1“ One question the EET program left unanswered was whether the polyurethane would work equally well to reduce drag on larger winglike surfaces with curvatures.

A second EET program with similarities to another ACEE program was laminar flow (see chapter 5 for a complete description of laminar flow). EET engineers performed natural laminar flow studies that resulted in some [166] [167] successes. When analyzed, a 757 achieved a significant natural laminar flow, improving fuel efficiency on a Mach 0.8 flight over 2,400 miles.[168]

The EET programs—supercritical wings, winglets, nacelle aero­dynamic and inertial loads, wing and tail surface coatings, laminar flow control, and active controls—were successful in reaching the goals of the ACEE program. EET was the focus of nearly 150 technical reports, which serve as a comprehensive database describing the new ideas that were evaluated and proved viable. These reports expressed an overall confi­dence that EET would result in the production of new airplanes that would attain at least 15 to 20 percent more fuel efficiency than those currently in production[169] Of these. James Kramer, who initially headed the ACEE Committee, said “the major visible EET results" of this program were the winglets, supercritical wings, and the active control technologies. The advanced aerodynamics investigations of ACEE were a success.®[170]

Ironically, some of these fuel-saving technologies diffused more quickly among European nations. In the early 1980s, Richard Wagner, a Langley ACEE manager, said he was flying a French-made Airbus A310 to Israel and, to his great surprise, when he looked out his window while the plane was still on the tarmac, on the tip of the wing he saw a winglet. It was actually the Israelis who were the first to apply winglets on the Westwind. Although they had been in use on smaller business jets in the United States, this was the first time Wagner had seen winglets on a com­mercial transport, where the winglets had their greatest advantage. Wagner concluded with some remorse, “So it seems like the Europeans, in my own personal observation, may have capitalized more upon the ACEE program results than our own American companies.”[171]

A further concern that Langley managers articulated at the start of the ACEE program was the threat to American dominance of aircraft manufacturing on the world stage. By 1982, Eastern Air Lines had pur­chased 34 Airbus A300 transports. This moved Airbus into second place internationally in terms of aircraft manufacturing, putting it ahead of the American Douglas and Lockheed companies. The international challenge was growing, but because the Energy Efficient Transport and the com­posites program were already showing important fuel-efficiency returns, many believed the United States would remain competitive despite the growing challenge from Airbus and European governmental support. Robert Leonard, of Langley, believed one major reason was that “fuel efficiency will continue to dominate purchase decisions by the world’s airlines”81 Assisting in this effort were the NASA engineers at Lewis Research Center in Cleveland, OIL who focused on innovative propulsion project to further improve fuel efficiency. [172]