[107] In one sense, the X-15 was a true spacecraft, for it could reach altitudes of 67 miles where 99.999 percent of the Earth's atmosphere lay below. Its main mission was hypersonic research, not setting altitude records. The X-15 was conceived to put into practice what engineers had learned from theory and from hypersonic wind tunnels with small scale models. It was a rocket propelled plane that was released at 45 000 feet by a B-52 jet. Safely separated from its carrier plane, the pilot ignited the rocket engine which sent the craft climbing to the fringes of the atmosphere. The X-15 first flew on September 17, 1959. Although NASA was already in existence at this time, no one believed that man would orbit the Earth in less than 2 years, or that the rather ungainly X-15 was blazing a trail for the reusable Space Shuttle.
A dozen different NASA wind tunnels at Langley and Ames contributed to the development of the X-15. The major portion of the high Mach development work fell on the Langley 11-inch hypersonic tunnel, which could reach Mach 6.8-the approximate speed goal of the X-15. Tunnel logs verify that fully 50 percent of the runs at the 11-inch tunnel during the life of the facility were in support of the X-15. Here the early aerodynamic heating measurements were made along with the stability and control tests and the loading distribution studies. The distinctive wedge shaped vertical tail of the X-15 emerged from the hypersonic stability work.
Some gun launched models of the X-15 were fired in the Ames free flight tunnels to obtain shadowgraphs of the shock wave patterns between Mach 3.5 and Mach 6.0. At lower supersonic speeds the Langley Unitary Plan supersonic tunnel generated the huge mass of data on aerodynamic forces and heat transfer needed for X-15 design. Lewis Research Center carried out jet plume and rocket nozzle studies in its supersonic propulsion facilities. In the subsonic realm, where the delicate and dangerous X-15/B-52 separation occurred, exhaustive tests were carried out in the Langley 7 x 10-foot high speed wind tunnel. From these tests came the precise combinations of X-15 control settings and release attitudes that assured safe and clean separation.
Various versions of the X-15 aircraft flew over a 10 year period. It has been called the most successful of all research aircraft. Few would quarrel with this judgment. The X-15 program ended in 1968, but its direct descendant, the Space Shuttle, follows in its wake.
The ideal airplane should operate at high efficiency at all speeds and altitudes. Fixed wing aircraft do not permit this "Garden of Eden." The large span, straight winged craft that has good cruise efficiency at low to moderate subsonic speeds sprouts intense shock waves on the wings in the transonic and supersonic ranges. Drag rises and performance falls. Sweeping the wings back into a V shape reduces the...
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[109] drag considerably. For supersonic operation on the deck (a few hundred feet off the ground), to sneak in under radar coverage, the wings should be folded even farther back until they nearly disappear. The ideal all around airplane, therefore, flaunts a pair of variable sweep wings, like those of a falcon, that permit it to soar and swoop with the same equipment.
The value of variable sweep was recognized in the 1940s when the Bell X-5 was conceived. NACA had tested the X-5 at its High Speed Flight Station in California, beginning in 1951. It was a promising design, fulfilling the performance expectations of the variable sweep proponents, but for one problem: In addition to pivoting the wings they also had to be moved fore and aft along the fuselage as the sweep angle changed. This was an awkward motion to mechanize, but wing translation was necessary to keep the center of the lift close to the center of gravity and to keep the craft stable and controllable.
Working with variable sweep wings in various wind tunnels, NASA engineers found a way to eliminate wing translation altogether. They simply moved the wing pivots out on the wings instead of close to the fuselage. The inner sections of the wings remained fixed, but the outboard panels swung back and forth. The new configuration was stable and performed well at all sweep angles. This breakthrough was translated into the General Dynamics F-111 long range fighter bomber, the Grumman F-14, and the North American/Rockwell B-1 supersonic bomber.
During the development of variable sweep wing aircraft, the integrated family of NASA wind tunnels worked together at all flight regimes to iron out problems and to answer those unexpected questions that...
[110] always arise when proving out radically new designs. At one time, the adequacy of the Area Rule was questioned in predicting the drag of various variable sweep models. To dispel this doubt, four different models were designed, built, and tested by NASA in just 13 days. Such quick response illustrates the value of wind tunnels in keeping high priority programs on schedule by wringing answers out of small models rather than full scale flight testing. Another area of great concern was the response of the craft to wind gusts while flying supersonic, on the deck missions. The variable sweep aircraft is like a bullet at this time, with its wings folded as far back as possible and the fuselage providing most of the lift. Would a sudden wind gust or maneuver send the plane tumbling like a rock? The answer from the wind tunnels was no. In fact, with the wings swept fully back the pilot would have the smoothest ride of all on the deck and still be able to maneuver quickly enough to follow the terrain contours-an intuitively surprising finding.
Commercial supersonic flight differs from military supersonic flight in several important ways. The supersonic transport (SST) must first of all cruise efficiently and economically at supersonic speeds over intercontinental distances; military planners cannot rank cost factors as high as commercial operators must. In addition, the supersonic transport must be able to fly into metropolitan airports on a routine basis under the same safety and environmental restrictions as subsonic aircraft. Wind tunnels have helped the SST approach these goals, but complete success has been elusive. These differences account for the fact that the United States does not yet fly supersonic transports, whereas American supersonic fighters and bombers have been in operation for over two decades.
The NACA wind tunnels that NASA absorbed in 1958 were already attacking the special problems posed by the SST. The design of transports for the supersonic cruise phase moved along quickly with the help of advances in computers and new techniques in aerodynamic computation. But in the "off design" areas, such as takeoff and landing, transonic acceleration, stability and control, and inlet performance, mathematics faltered and wind tunnels bore the main load. By 1962 the NASA centers at Langley and Ames had evolved two basic SST configurations that looked promising: a variable sweep wing craft and a canard delta wing configuration. NASA was thus well prepared when in dune of the next year (1963), President Kennedy announced a National Supersonic Transport Program to develop an economically attractive American SST. With many outside contractors brought in to accelerate the SST program, NASA began running some of its wind tunnels 24 hours a day to keep up with design evaluation and to discover effective solutions to the many new problems arising.
The SST program was vulnerable on several counts. The requirement to show profitable operation was, in the early 1960s, difficult to meet. More than anything else, though, the problems of noise and possible atmospheric pollution scuttled the American supersonic transport. The general tenor of the times militated against costly, environmentally questionable, technological enterprises; the national SST effort was terminated in 1971. Now, roughly two decades later, technical advances by NASA and others make the SST look more attractive economically, environmentally, and from the standpoint of safe operation.
When North American Aviation started dive tests of their new F 86 fighter in 1949, residents of southern California began reporting many mysterious explosions. Thus arose the first sonic boom complaints. For North American, the solution was simple: move the dive tests out over the Pacific. Even today, supersonic flight of the Concorde transport is generally restricted....
...to the sky over the ocean. What causes these annoying and sometimes window cracking booms? Can they be muffled?
Aerodynamicists immediately recognized the sonic boom as a groundward extension of a supersonic aircraft's shock wave system. The myriad shock waves set up near the aircraft coalesce at great distances into sharply defined bow and tail waves, producing double booms when they pass over a ground based observer. Given the vicissitudes of the atmosphere, sonic booms were much like the weather-hard to predict and practically impossible to change. Scientists could identify sonic booms, but they did not understand them well in theory or practice.
A start was made in 1952 when G. B. Whitham, from the United Kingdom, presented a theory that satisfactorily described the generation of shock waves around a supersonic aircraft and their attenuation through the atmosphere. This gave aerodynamicists a model to check out in supersonic wind tunnels. An unusual impasse arose at this point because most supersonic wind tunnels had small test sections. The typical wind tunnel aircraft model practically filled the test section. How could shock wave attenuation [112] with distance be measured under such crowded conditions? Since the wind tunnels could not be greatly enlarged because of cost, the models had to shrink. Absurdly tiny models-0.25 to 1 inch in size-were tested in the Ames and Langley supersonic wind tunnels. With such miniaturization, the tunnel walls were up to 150 body lengths away from the models. The Lilliputian models generated shock waves all right, but they were so weak that new pressure sensors had to be conceived. Further, tunnel conditions had to be held more nearly uniform because slight changes in humidity or compressor speed would create transient flow conditions that confused the shock wave data. By taking great care, Whitham's theory of sonic booms was verified in the idealized environment of the wind tunnel.
Outside the wind tunnels, confirmations of the theory were plagued by the same factors that made tunnel testing difficult. Variations in atmospheric pressure and temperatures-on top of ever present turbulence-upset expected results time and time again. New instrumentation and better flight test techniques ultimately led to great improvements in sonic boom measurement and prediction. Theory finally coincided reasonably well with experiment, but the manipulation of aircraft parameters has led to only modest suppression of the sonic boom under cruise conditions. Until some sort of aircraft " silencer " can be devised, commercial supersonic flight will probably remain over water.
Most commercial jet transports cruise between Mach 0.7 and Mach 0.8. Since speed is the airlines' main selling point, why not push cruising speeds closer to Mach 1? The local shock waves that form over the wing surfaces close to Mach 1 are the culprits. Airflow separates beyond them, creating precipitous increases in drag and buffeting. All the tricks of the trade-thinner wings, more sweepback, new wing silhouettes-generated increases in subsonic cruise speeds but only with unacceptable increases in structural weight. It seemed as if subsonic flight performance had maximized.
Richard T. Whitcomb, of Area Rule fame, thought otherwise. In the late 1960s, drawing on wind tunnel experience and his own unique understanding of transonic airflow, Whitcomb shattered the fetters of conventional wisdom by subtly reshaping wing cross....
....sections into what was termed a "supercritical" airfoil. These new airfoils displayed (1) well rounded noses rather than the sharp edges intuition would suggest for higher speeds, (2) relatively flat upper surfaces that weakened the shock waves and pushed them farther back on the wings, and (3) a sharply down curved trailing edge that increased lift. Tests in Langley's 8 foot transonic pressure tunnel suggested that the supercritical wing might allow a 10 percent increase in cruise speed before flow separation became serious.
The tests of the new airfoil in the Langley tunnel were greatly compromised by the small sizes of the models. Small models mean low Reynolds numbers and tests that are characterized by premature flow separation, which tends to mask the predicted improvements of the supercritical airfoil. Only with very elaborate and careful experiments were the experimenters able to demonstrate the potential of the new wing. Unfortunately, wind tunnel results were just not convincing enough for aircraft manufacturers to risk billions of dollars on a revolutionary new wing design. The only recourse was flight testing the new wing full scale on the actual aircraft. The Navy...
Vought F-8U fighter was selected as the test aircraft. It flew with supercritical wings in March 1971. The flight tests completely confirmed the wind tunnel results.
Now convinced of the great future utility of the supercritical wing, NASA presented the wind tunnel and flight test results to U.S. industry at a special conference in 1972. Aircraft manufacturers went back to their drawing boards and computers only to emerge with a surprising discovery. Don't use the supercritical wing to increase cruise speed (which had been the goal all along); rather, hold current cruise speeds at Mach 0.8 and increase wing thickness using the supercritical shapes. A thicker wing could be made strong enough with less structural weight (the major payoff) and allow aircraft to carry considerably more fuel and thereby increase range.
The new supercritical wings are found in new subsonic transports, business jets, STOL aircraft, and remotely piloted vehicles. The blunt leading edge of the supercritical wing leads to better takeoff, landing, and maneuvering performance. Consequently, even aircraft way down in the subsonic range, such as crop duster planes and small private aircraft, are adopting the new airfoil shapes.
Above all, the story of the supercritical wing is one of individual vision, perseverance, and intimate knowledge-all in the face of a general conviction that aeronautical research had reached a plateau Whitcomb, helped by NASA's sophisticated wind tunnels and their capable staffs, was able to shake the aeronautical community from its lethargy.
The layman sees vortices in the bathtub drain water and in the small whirlwinds of leaves on sun warmed hillsides. Unseen are the vortices spawned by all lifting surfaces. A subsonic aircraft deposits long trails of vortices from its wing tips. These invisible whirlwinds persist for several miles behind a large plane. A light plane following a large jet in an airport landing pattern may be flipped over on its back by the larger plane's vortices. Landing pattern separation distances are largely dictated by these vortices. Obviously the...
....traffic handling capacity of an airport could be increased if trailing vortices could be subdued.
The Langley V/STOL and the Ames 40 x 80-foot tunnels bore the brunt of vortex research. Airspeed was slow, and these tunnels were big enough to accommodate large models. All manner of schemes were tried to attenuate the vortices: propellers at the wing tips, trailing tip parachutes, air injection into the vortex cores, and other stratagems. All succeeded to some degree but brought with them unacceptable aircraft performance penalties. However, tunnel tests also demonstrated that modest modifications of normal aircraft equipment also suppressed vortex formation. Landing gear doors, landing flap deployment, and changes in wing spoiler deflection showed promise.
Pursuing these leads, NASA equipped a Boeing 747 research plane with smoke generators and began flight tests at its Flight Research Center in California. The selective deflection of the B-747's spoilers and wing flaps effectively pulled the teeth of the strong vortices. A Cessna T-37 light plane flying behind the modified B-747 was able to approach up to 1.5 miles without undue tossing about. Compare this to the usual separation distance in a standard landing pattern of about 7 miles. The vortex reduction program is not yet complete, but the flight tests and wind tunnel results are most encouraging.
The earliest attempts at flight featured mechanical contraptions that emulated the wings of birds. Dismal failures they were, and interest shifted to fixed airfoils and separate thrust makers. However, photographs of birds in flight, particularly soaring birds like the eagles and vultures, kept showing wing tips bent nearly straight up. Did the birds know something aircraft designers did not?
Airflow in the vicinity of a plane's wing tip is complex. Here the higher pressure air beneath the wing flows out and up to mix with the lower pressure air from above the lift producing wing. A swirling motion ensues, and a trailing vortex forms. Not only do these trailing or wake vortices endanger closely....
.....following aircraft, they induce extra drag. This so called drag due to lift may represent 40 to 50 percent of the total aircraft drag. Suppressing these tip vortices could significantly increase cruise performance. Aerodynamicists surmised that the birds' bent wing tips somehow suppressed this type of drag.
As early as 1897, Lanchester, in England, obtained a patent on vertical surfaces installed on wing tips. More vertical surfaces were tried down the years with scant success. In 1974 Richard Whitcomb of NASA started on a different tack. Instead of simple, flat endplates, he tried small vertical airfoils dubbed "wingless." When properly curved and aligned with the local airflow, the lateral forces created by the wingless tended to oppose vortex circulation around the wing tip and, in turn, reduce the lift induced drag. The idea sounded attractive.
Wind tunnel tests were in order. A long series of experiments in Langley's 8 foot transonic pressure tunnel confirmed Whitcomb's intuition and computations. Vortex drag was reduced by 10 to 20 percent and the drag of the entire aircraft by 4 to 8 percent. Small though the numbers seem, the overall impact on fuel consumption is large for a thirsty jet.
Winglets had a competitor. By simply extending the span of the wing, aircraft designers could also reduce vortex drag. Which was better, longer wings or wingless? Longer wing spans made aircraft more difficult to handle at terminals-a minus for long wings. Both wingless and longer wings tended to bend the wings at their roots, necessitating more structural weight. Halved aircraft models split lengthwise were subjected to wind tunnel tests to compare the bending moments created by the competing approaches. The tests favored wingless.
Even though the winglet concept is very new, some business jets have already adopted them and reported increased range and cruise altitude. The greatest potential value of the wingless may lie in retrofitting 600 plus KC 135 Air Force jet cargo/tankers. Equipped with wingless 9 feet tall, the aerial refueling range could be increased up to 400 miles. The cumulative fuel savings of the entire KC 135 fleet might reach 25 million gallons annually, which over the next 20 years would amount to over $500 million savings at 1980 fuel prices. Apparently, evolution carried the soaring birds in a cost effective direction millions of years ago when it gave them feathered wingless.
