Category NASA’S CONTRIBUTIONS TO AERONAUTICS

The history of composite development reveals at least as many false starts and technological blind alleys as genuine progress. Leo Baekeland, an American inventor of Dutch descent, started a revolution in mate­rials science in 1907. Forming a new polymer of phenol and formal­dehyde, Baekeland had succeeded in inventing the first thermosetting plastic, called Bakelite. Although various types of plastic had been developed in previous decades, Bakelite was the first commercial success. Baekeland’s true breakthrough was inventing a process that allowed the mass production of a thermosetting plastic to be done cheaply enough to serve the mechanical and fiscal needs of a huge cross section of prod­ucts, from industrial equipment to consumer goods.

It is no small irony that powered flight and thermosetting plas­tics were invented within a few years of each other. William F. Durand, the first Chairman of the NACA, the forerunner of NASA, in 1918 summarized the key structural issue facing any aircraft designer. Delivering the sixth Wilbur Wright Memorial Lecture to the Royal Aeronautical Society, the former naval officer and mechanical engineer said, "Broadly speaking, the fundamental problem in all airplane construction is adequate strength or function on minimum weight.” [648] A second major structural concern, which NACA officials would soon come to fully appreciate, was the effect of corrosion on first wood, then metal, structures. Thermosetting plastics, one of two major forms of composite materials, present a tantalizing solution to both problems. The challenge has been to develop composite matrices and production processes that can mass-produce materials strong enough to replace wood and metal, yet affordable enough to meet commercial interests.

While Baekeland’s grand innovation in 1907 immediately made strides in other sectors, aviation would be slow to realize the benefit of thermosetting plastics.

The substance was too brittle and too week in tensional strength to be used immediately in contemporary aircraft structures. But Bakelite eventually found its place by 1912, when some aircraft manufacturers started using the substance as a less corrosive glue to bind the joints between wooden structures.[649] The material shortages of World War I, how­ever, would force the Government and its fledgling NACA organization to start considering alternative sources to wood for primary structures. In 1917, in fact, the NACA began what would become a decades-long effort to investigate and develop alternatives to wood, beginning with metal. As a very young bureaucracy with few resources for staffing or research, the NACA would not gain its own facilities to conduct research until the Langley laboratory in Virginia was opened in 1920. Instead, the NACA committee formed to investigate potential solutions to mate­rials problems, such as a shortage of wood for war production of air­craft, and recommended that the Army and the Bureau of Standards study commercially available aluminum alloys and steels for their suit­ability as wing spars.[650]

Even by this time, Bakelite could be found inside cockpits for instru­ments and other surfaces, but it was not yet considered as a primary or secondary load-bearing structure, even for the relatively lightweight aircraft of this age. Perhaps the first evidence that Bakelite could serve as an instrumental component in aircraft came in 1924. With fund­ing provided by the NACA, two early aircraft materials scientists— Frank W. Caldwell and N. S. Clay—ran tests on propellers made of Micarta material. The material was a generational improvement upon the phe­nolic resin introduced by Baekeland. Micarta is a laminated fabric—in this case cotton duck, or canvas—impregnated with the Bakelite resin.[651] Caldwell was the Government’s chief propeller engineer through 1928 and later served as chief engineer for Hamilton Standard. Caldwell is cred­ited with the invention of variable pitch propellers during the interwar period, which would eventually enable the Boeing Model 247 to achieve altitudes greater than 6,000 feet, thus clearing the Rocky Mountains and becoming a truly intercontinental aircraft. Micarta had already served

as a material for fixed-pitch blades in World War I engines, including the Liberty and the 300-horsepower Wright.[652] Fixed-pitch blades were optimized neither for takeoff or cruise. Caldwell wanted to allow the pilot to change the pitch of the blade as the airplane climbed, allow­ing the pitch to remain efficient in all phases of flight. Using the same technique, the pilot could also reverse the pitch of the blade after land­ing. The propeller blades now functioned as a brake, allowing the air­craft to operate on shorter runways. Finding the right material to use for the blades was foremost among the challenges for Caldwell and Clay. It had to be strong enough to survive the stronger aerodynamic forces as the blade changed its pitch. The extra strength had to be balanced with the weight of the material, and metal alloys had not yet advanced far enough in the early 1920s. However, Caldwell and Clay found that Micarta was suitable. In an NACA technical report, they concluded: "The reversible and adjustable propeller with micarta blades. . . is one of the most practical devices yet worked out for this purpose. It is quite strong in all details, weighs very little more than the fixed pitch propeller and operates so easily that the pitch may be adjusted with two fingers on the control level when the engine is running.” The authors had performed flight tests comparing the same aircraft and engine using both Micarta and wooden propeller blades. The former exceeded the top speed of the wooden propeller by 2 miles per hour (mph), while turning the engine at about 120 fewer revolutions per minute (rpm) and maintaining a simi­lar rate of climb. The Micarta propeller was not only faster, it was also 7 percent more fuel efficient.[653]

The propeller work on Micarta showed that even if full-up plastics remained too weak for load-bearing applications, laminating wood with plastic glues provided a suitable alternative for that era’s demands for structural strength in aircraft designs. While American developers continued to make advances, critical research also was occurring over­seas. By the late 1920s, Otto Kraemer—a research scientist at Deutsche Versuchsanstalt fur Luftfahrt (DVL), the NACA’s equivalent body in Germany—had started combining phenolic resins with paper or cloth. When this fiber-reinforced resin failed to yield a material with a struc­tural stiffness superior to wood, Kraemer in 1933 started to investigate

birch veneers instead as a filler. Thin sheets of birch veneer impreg­nated with the phenolic resin were laminated into a stack 1 centimeter thick. The material proved stronger than wood and offered the capabil­ity of being molded into complex shapes, finally making plastic a viable option for aircraft production.[654] Kraemer also got the aviation industry’s attention by testing the durability of fiber-reinforced plastic resins. He exposed 1 – millimeter-thick sheets of the material to outdoor exposure for 15 months. His results showed that although the material frayed at the edges, its strength had eroded by only 14 percent. In comparison to other contemporary materials, these results were observed as "practically no loss of strength.”[655] In the late 1930s, European designers also fabri­cated propellers using a wood veneer impregnated with a resin varnish.[656]

A critical date in aircraft structural history is March 31, 1931, the day a Fokker F-10A Trimotor crashed in Kansas, with Notre Dame foot­ball coach Knute Rockne among the eight passengers killed. Crash inves­tigators determined that the glues joining the wing strut to the F-10A’s fuselage had been seriously deteriorated by exposure to moisture. The cumulative weakening of the joint caused the wing to break off in flight. The crash triggered a surge of nationwide negative publicity about the weaknesses of wood materials used in aircraft structures. This caused the aviation industry and passengers to embrace the transition from wood to metal for airplane materials, even as progress in synthetic mate­rials, especially involving wood impregnated with phenolic resins, had started to develop in earnest.[657]

In his landmark text on the aviation industry’s transition from wood to metal, Eric Schatzberg sharply criticizes the ambivalence of the NACAs leadership toward nonmetal alternatives as shortsightedness. For exam­ple, "In the case of the NACA, this neglect involved more than passive ignorance,” Schatzberg argues, "but rather an active rejection of research on the new adhesives.” However, with the military, airlines, and the trav­eling public all "voting with their feet,” or, more precisely, their bank accounts, in favor of the metal option, it is not difficult to understand the NACA leadership’s reluctance to invest scarce resources to develop
wood-based synthetic aircraft materials. The specimens developed during this period clearly lacked the popular support devoted to metal. Indeed, given the dominant role that metal structures were to play in aircraft and aerospace technology for most of the next 70 years, the priority placed on metal by the NACAs experts could be viewed as strategically prescient.

That is not to say that synthetic materials, such as plastic resins, were ignored by the aerospace industry in the 1930s. The technology of phenol- and formaldehyde-based resins had already grown beyond functioning as an adhesive with superior properties for resisting corro­sion. The next step was using these highly moisture-resistant mixtures to form plywood and other laminated wood parts.[658] Ultimately, the same resins could be used as an impregnant that could be reinforced by wood,[659] essentially a carbon-based material. These early researchers had discovered the building blocks for what would become the carbon – fiber-reinforced plastic material that dominates the composite structures market for aircraft. Of course, there were also plenty of early applica­tions, albeit with few commercial successes. A host of early attempts to bypass the era of metal aircraft, with its armies of riveters and con­cerns over corrosion and metal fatigue, would begin in the mid-1930s.

Clarence Chamberlin, who missed his chance by a few weeks to beat Charles Lindbergh across the Atlantic in 1927, flew an all-composite airplane. Called the Airmobile, it was designed by Harry Atwood, once a pupil of the Wright brothers, who flew from Boston to Washington, DC, in 1910, landing on the White House lawn.[660] Unfortunately, the full story of the Airmobile would expose Atwood as a charlatan and fraud. However, even if Atwood’s dubious financing schemes ultimately hurt his reputation, his design for the Airmobile was legitimate; for its day, it was a major achievement. With a 22-foot wingspan and a 16-foot- long cabin, the Airmobile weighed only 800 pounds. Its low weight was achieved by constructing the wings, fuselage, tail surfaces, and aile­rons with a new material called Duply, a thin veneer from a birch tree impregnated with a cellulose acetate.[661]

Writing a technical note for the NACA in 1937, G. M. Kline, work­ing for the Bureau of Standards, described the Airmobile’s construction: "The wings and fuselage were each molded in one piece of extremely thin films of wood and cellulose acetate.”[662] To raise money and attract public attention, however, Atwood oversold his ability to manufacture the air­craft cheaply and reliably. According to his farfetched publicity claims, 10 workers starting at 8 a. m. could build a new Airmobile from a sin­gle, 6-inch-diameter birch tree and have the airplane flying by dinner.

After a 12-minute first flight before 2,000 gawkers at the Nashua, NH, airport, Chamberlin complained that the aircraft was "nose heavy” but otherwise flew well. But any chance of pursuing full-scale manufacturing of the Airmobile would be short-lived. To develop the Airmobile, Atwood had accumulated more than 200 impatient creditors and a staggering debt greater than $100,000. The Airmobile’s manufac­turing process needed a long time to mature, and the Duply material was not nearly as easy to fabricate as advertised. The Airmobile idea was dropped as Atwood’s converted furniture factory fell into insolvency.[663]

Also in the late 1930s, two early aviation legends—Eugene Vidal and Virginius Clark—pursued separate paths to manufacture an air­craft made of a laminated wood. Despite the military’s focus on devel­oping and buying all-metal aircraft, Vidal secured a contract in 1938 to provide a wing assembly molded from a thermoplastic resin. Vidal also received a small contract to deliver a static test model for a basic trainer designated the BT-11. Schatzberg writes: "A significant innova­tion in the Vidal process was the molding of stiffeners and the skin in a single step.” Clark, meanwhile, partnered with Fairchild and Haskelite to build the F-46, the first airliner type made of all-synthetic materi­als. Haskelite reported that only nine men built the first half-shell of the fuselage within 2 hours. The F-46 first flew in 1937 and generated a great amount of interest. However, the estimated costs to develop the molds necessary to build Clark’s proposed production system (greater than $230,000) exceeded the amount private or military investors were willing to spend. Clark’s duramold technology was later acquired by Howard Hughes and put to use on the HK-1 flying boat (famously nick­named—inaccurately—the "Spruce Goose”).[664]

The February 16, 1939, issue of the U. K.-based Flight magazine offers a fascinating contemporary account of Clark’s progress:

Recent reports from America paint in glowing terms a new process said to have been invented by Col Virginius Clark (of Clark Y wing section fame) by which aero­plane fuselages and wings can, it is claimed, be built of plastic materials in two hours by nine men. . . . There is little doubt that Col Clark and his associates of the Bakelite Corporation and the Haskelite Manufacturing Corporation have evolved a method of production which is rapid and cheap. Exactly how rapid and how cheap time will show. In the meantime, it is well to remember that we are not standing still in this country. Dr. Norman de Bruyne has been doing excellent work on plastics at Duxford, and the Airscrew Company of Weybridge is doing some very interesting and promising experimen­tal and development work with reinforced wood.[665]

The NACA first moved to undertake research in plastics for aircraft in 1936, tasking Kline to conduct a review of the technical research already completed.[666] Kline conducted a survey of "reinforced phenol – formaldehyde resin” as a structural material for aircraft. The survey was made with the "cooperation and financial support” of the NACA. Kline also summarized the industry’s dilemma in an NACA technical note:

In the fabrication of aircraft today the labor costs are high relative to the costs of tools. If large sections could be molded in one piece, the labor costs would be reduced but the cost of the molds and presses would be very high. Such a change in type construction would be economically practicable excepting the mass produc­tion of aircraft of a standard design. Langley suggests, therefore, that progress in the utilization of plastics in aircraft construction will be made by the gradual intro­duction of these materials into an otherwise orthodox
structure, and that the early stages of this development will involve the molding of such small units as fins and rudders and the fabrication of the larger units from reinforced sheets and molded sections by conventional methods of jointing.[667]

Kline essentially was predicting the focus of a massive NASA research program that would not get started for nearly four more decades. The subsequent effort was conducted along the lines that Kline prescribed and will be discussed later in this essay. Kline also seemed to under­stand how far ahead the age of composite structure would be for the aviation industry, especially as aircraft would quickly grow larger and more capable than he probably imagined. "It is very difficult to outline specific problems on this subject,” Kline wrote, "because the explora­tion of the potential applications of reinforced plastics to aircraft con­struction is in its infancy, and is still uncharted.”[668]

In 1939, an NACA technical report noted that synthetic materials had already started making an impact in aircraft construction of that era. The technology was still unsuited for supporting the weight of the aircraft in flight or on the ground, but the relative lightness and durabil­ity of synthetics made them popular for a range of accessories. Inside a wood or metal cockpit, a pilot scanned instruments with dials and casings made of synthetics and looked out a synthetic windshield. Synthetics also were employed for cabin soundproofing, lights encasings, pulleys, and the streamlined housings around loop antennas. The 1939 NACA paper concludes: "It is realized, at present, that the use of synthetic resin mate­rials in the aircraft industry have been limited to miscellaneous accesso­ries. The future is promising, however, for with continued development, resin materials suitable for aircraft structures will be produced.”[669]

In the mid-1970s, coincident with the beginning of the fuel and litiga­tion crises that would nearly destroy GA, production of homebuilt and kit-built aircraft greatly accelerated, reflecting the maturity of light air­craft design technology, the widespread availability of quality engineer­ing and technical education, and the frustration of would-be aircraft owners with rising aircraft prices. Indeed, by the early 1990s, kit sales would outnumber sales of production GA aircraft by more than four to one.[869] Today, in a far-different post-GARA era, kit sales remain strong. As well, new manufacturers appeared, some wedded to particular ideas or concepts, but many also showing a broader (and thus generally more successful) approach to light aircraft design.

Exemplifying this resurgence of individual creativity and insight was Burt Rutan of Mojave, CA. An accomplished engineer and flight – tester, Rutan designed a small two-seat canard light aircraft, the VariEze, powered by a 100-hp Continental engine. Futuristic in look, the VariEze embodied very advanced thinking, including a GA(W)-1 wing section and Whitcomb winglets. The implications of applying the configuration to other civil and military aircraft of far greater performance were obvious, and NASA studied his work both in the tunnel and via flight tests of the VariEze itself.[870] Rutan’s influence upon advanced general aviation air­craft thinking was immediate. Beech adopted a canard configuration for a proposed King Air replacement, the Starship, and Rutan built a subscale demonstrator of the aircraft.[871] Rutan subsequently expanded his range of work, becoming a noted designer of remarkable flying machines capable of performance—such as flying nonstop around the world or rocketing into the upper atmosphere—many would have held impossible to attain.

NASA followed Rutan’s work with interest, for the canard config­uration was one that had great applicability across the range of air­craft design, from light aircraft to supersonic military and civil designs. Langley tunnel tests in 1984 confirmed that with a forward center of gravity location, the canard configuration was extremely stall-resistant. Conversely, at an aft center of gravity location, and with high power, the canard had reduced longitudinal stability and a tendency to enter a high – angle-attack, deep-stall trim condition.[872] NASA researchers undertook a second series of tests, comparing the canard with other wing planforms including closely coupled dual wings, swept forward-swept rearward wings, joined wings, and conventional wing-tail configurations, evaluat­ing their application to a hypothetical 350-mph, 1,500-mile-range 6- or 12-passenger aircraft operating at 30,000 to 40,000 feet. In these tests, the dual wing configuration prevailed, due to greater structural weight efficiencies than other approaches.[873]

Seeking optimal structural efficiency has always been an important aspect of aircraft design, and the balance between configuration choice and structural design is a fine one. The advent of composite structures enabled a revolution in structural and aerodynamic design fully as sig­nificant as that at the time of the transformation of the airplane from wood to metal. As designers then had initially simply replaced wooden components with metal ones, so, too, in the earliest stage of the com­posite revolution, designers had initially simply replaced metal com­ponents with composite ones. In many of their own GA proposals and studies, NASA researchers repeatedly stressed the importance of getting away from such a "metal replacement” approach and, instead, adopt­ing composite structures for their own inherent merit.[874]

The blend of research strains coming from NASA’s diverse work in structures, propulsion, controls, and aerodynamics, joined to the cre­ative impact of outside sources in industry and academia—not least of which were student study projects, many reflecting an insight and expertise belying the relative inexperience of their creators—informed NASA’s next steps beyond AGATE. Student design competitions offered a valuable means of both "growing” a knowledgeable future aerospace workforce and seeking fresh approaches and insight. Beginning in 1994, NASA joined with the FAA and the Air Force Research Laboratory to sponsor a yearly National General Aviation Design Competition estab­lishing design baselines for single-pilot, 2- to 6-passenger vehicles, tur­bine or piston-powered, capable of 150 to 400 knots airspeed, and with a range of 800 to 1,000 miles. The Virginia Space Grant Consortium at Old Dominion University Peninsula Center, near Langley Research Center, coordinated the competition. Competing teams had to address "design challenges” in such technical areas as integrated cockpit sys­tems; propulsion, noise, and emissions; integrated design and manu­facturing; aerodynamics; operating infrastructure; and unconventional designs (such as roadable aircraft).[875] In cascading fashion, other oppor­tunities existed for teams to take their designs to ever-more-advanced levels, even, ultimately, to building and test-flying them. Through these competitions, study teams explored integrating such diverse technical elements as advanced fiber optic flight control systems, laminar flow design, swept-forward wings, HITS cockpit technology, coupled with advanced Heads-up Displays (HUD) and sidestick flight control, and advanced composite materials to achieve increased efficiencies in per­formance and economic advantage over existing designs.[876]

Succeeding AGATE was SATS—the NASA Small Aircraft Transportation System Project. SATS (another Holmes initiative) sought to take the integrated products of this diverse research and form from it a distributed public airport network, with small aircraft flying on demand as users saw fit, thereby taking advantage of the ramp space capacity at over 5,000 public airports located around the country.[877] SATS would benefit as well by a Glenn Research Center initiative, the GAP (General Aviation Propulsion) program, seeking new propulsive effi­ciencies beyond those already obtained by previous NASA research.[878] In 2005, SATS concluded with a 3-day "Transformation of Air Travel” held at Danville Airport, VA, showcasing new aviation technologies with six air­craft equipped with advanced cockpit displays enabling them to operate from airports lacking radar or air traffic control services. Complementing SATS and GAP was PAV—a Langley initiative for Personal Air Vehicles, a reincarnation of an old dream of flight dating to the small ultralight aircraft and airships found at the dawn of flight, such as Alberto Santos – Dumont’s little one-person dirigibles and his Demoiselle light aircraft. Like many such studies through the years, PAV studies in the 2002-2005 period generated many innovative and imaginative concepts, but the

Agency did not support such studies afterwards, turning instead towards good stewardship and environmental responsibility, seeking to reduce emissions, noise, and improve economic efficiencies by reducing air­port delays and fuel consumption. These are not innocuous challenges: in 2005, airspace system capacity limitations generated fully $5.9 bil­lion in economic impact through airline delays, and the next year, fuel consumption constituted a full 26 percent of airline operating costs.[879]

The history of the NACA-NASA support of General Aviation is one of mutual endeavor and benefit. Examining that history reveals a surpris­ing interdependency between the technologies of air transport, military, and general aviation. Developments such as the supercritical wing, elec­tronic flight controls, turbofan propulsion, composite structures, syn­thetic vision systems, and heads-up displays that were first exploited for one have migrated and diffused more broadly across the entire aeronau­tical field. Once again, the lesson is clear: the many streams of NASA research form a rich and broad confluence that nourishes and invigorates the entire American aeronautical enterprise, ever renewing our nature as an aerospace nation.

The Low-Observable Flight Test Experiment (LoFLYTE) program was a joint effort among researchers at NASA Langley and the Air Force Research Laboratory with support from NASA Dryden and the 445th Flight Test Squadron at Edwards Air Force Base. Accurate Automation, Corp., of Chattanooga, TN, received a contract under NASA’s Small Business Innovation Research program to explore concepts for a stealthy hypersonic wave rider aircraft. The Navy and the National Science Foundation provided additional funding. A wave rider derives lift and experiences reduced drag because of the effects of riding its bow shock wave. Applications for wave rider technology include transatmospheric vehicles, high-speed passenger transports, missiles, and military aircraft.

The LoFLYTE vehicle was designed to serve as a testbed for a vari­ety of emerging aerospace technologies. These included rapid prototyp­ing, instrumentation, fault diagnosis and isolation techniques, real-time data acquisition and control, miniature telemetry systems, optimum antenna placement, electromagnetic interference minimization, advanced exhaust nozzle concepts, trajectory control techniques, advanced land­ing concepts, free-floating wingtip ailerons (called tiperons), and adap­tive compensation for pilot-induced oscillations.[1008] Most important of all, LoFLYTE was eventually to be equipped with neural network flight controls. Such a system employs a network of control nodes that inter­act in a similar fashion to neurons in the human brain. The network "learns,” altering the aircraft’s flight controls to optimize performance and take pilot responses into consideration. This would be particularly useful in situations in which a pilot needed to make decisions quickly and land a damaged aircraft safely, even if its controls are partially destroyed. Researchers also expected that neural network controls would be useful for flying unstable configurations, such as those necessary for efficient hypersonic-flight vehicles. The computing power of Accurate Automation’s neural network was provided by 16,000 parallel neurons making 1 billion decisions per second, giving it the capability to adjust to changing flight conditions faster than could a human pilot.[1009] The LoFLYTE model was just 100 inches long, with a span of 62 inches and a height of 24 inches. It weighed 80 pounds and was configured as a narrow delta planform with two vertical stabilizer fins. The shell of the model, made from fiberglass, foam, and balsa wood, was constructed at Mississippi State University’s Raspet Flight Research Laboratory and then shipped to SWB Turbines in Appleton, WI, for installation of radio control equipment and a 42-pound-thrust microturbine engine.[1010] The first flight took place at Mojave Airport, CA, on December 16, 1996. The vehicle was not yet equipped with a neural network and relied instead on conventional computerized stabilization and control systems. All went well as the LoFLYTE climbed to an altitude of about 150 feet and the pilot began a 180-degree turn. At that point—about 34 seconds into the flight—the ground pilot was forced to land the craft wheels-up in the sand beside the runway because of control difficulties. The model suffered only minor damage, and researchers generally considered the flight a suc­cess because it was the first time a wave-rider-concept vehicle had taken off under its own power.[1011] Testing resumed in June 1997 with several flights from the Edwards North Base runway. This gave researchers the opportunity to verify the subsonic airworthiness of the wave rider shape and analyze basic handling characteristics. The results showed that a full – scale vehicle would be capable of taking off and landing at normal speeds (i. e., those comparable to such high-speed aircraft as the SR-71). Flight tests of the neural network control system began in December 1997 and continued into 1998. These included experiments to verify the system’s ability to handle changes in airframe configuration (such as removal of vertical stabilizers) and simulated damage to control surfaces.[1012]

The American involvement in combat operations in Vietnam escalated by the late 1960s to something that was not called a war by the Government but actually was. The public turned against the war as casualties and costs escalated; by 1968, a sense of distrust of the Government and all its programs also affected a significant portion of the populace. The Apollo program was about to achieve President Kennedy’s goal of landing on the Moon, but people were beginning to question its value. A youth – oriented cultural shift had not only a pro-peace stance but also an anti­technology bent, and environmental movements such as the Sierra Club were becoming increasingly influential. Nuclear powerplants and nuclear weapons were increasingly cited as being harmful to the environment, and many people wanted them limited or banned. The United States SST program was a high-visibility target and opportunity for environmental movements. Initially, the arguments focused on the sonic booms to be produced by an SST fleet. Dr. William Shurcliff, a Harvard University physicist, formed the Citizens League against the Sonic Boom to argue against the SST. As the SST and Sonic Boom Handbook (published in 1970 by an environmental activist organization) stated on its jacket, "This book demonstrates that the SST is an incredible, unnecessary insult to
the living environment, and an albatross around the neck of whatever nations seek to promote it.”[1094]

American legislators were not deaf to this increasing clamor. The sonic boom problem remained real and apparently technically unsolv­able. An operational solution was to ban overland supersonic flights by the SST. This had a serious impact on the economic case for an air­plane that would have to fly at off-design cruise speeds for significant amounts of time if flying on anything other than transatlantic or trans­pacific flights. Transcontinental flights in the United States had always been a prime revenue generator for airlines. A further noise problem was represented by New York City airports surrounded by densely pop­ulated communities. Subsonic jets with 10,000-pound thrust engines had difficulty meeting local noise standards; engines with 30,000 and even 60,000 pounds of thrust promised to be even more intractable. But even solving these problems would not satisfy some later environmen­tal concerns. Water vapor from SST exhaust at high altitude having the potential to damage the protective ozone layer was a major concern, which later proved to be unwarranted, even if 500 SSTs had been built (although this was not so for fluorocarbons in spray cans at sea level). At a hearing before the United States Senate in May 1970, a member of the President’s Environmental Quality Council referred to the SST as "the most significant unresolved environmental problem.”[1095] By late 1970, polls showed that American voters were 85 percent in favor of ending Federal funding for the SST program. In May 1971, the Senate voted to withhold further Government funding. Boeing, already developing the 737 and 747 at its own expense, said it could not proceed without $500 million in Federal funding. The American SST program of the 1960s was over.

The international SST race ended with only one horse crossing the finish line, albeit at a walk rather than a gallop. Although the Soviet SST flew first at the end of 1968, it required redesign and never was commer­cially successful. Two were lost in crashes during trials, and it only flew limited cargo flights from Moscow to Siberia over the sparsely populated Russian landmass. The SST field was left to the Anglo-French Concorde, which finally entered service on the transatlantic run in 1976 and flew for over 25 years. Only 13 aircraft entered service with the British and French national airlines, and most of their traffic was on the transatlantic run
for which the aircraft had initially been sized. The fuel price increase that started in 1974 and the de facto ban on overland supersonic flight meant that there was no move to improve or expand the Concorde fleet. It essen­tially remained a limited capacity first-class-only means of quickly getting from the United States to Paris or London while experiencing supersonic flight. Boeing’s privately financed gamble on the 747 jumbo jet turned out to be the winning hand, as it revolutionized air traveling habits, especially after airline deregulation began in the United States in 1978.[1096]

As quickly as the hazards of aircraft icing became known in the early days of aviation, inventive spirits applied themselves to coming up with ways to remove the hazard and allow the airplane to keep flying. These ideas at first took the form of understanding where and when icing occurs and then simply not flying through such conditions, then ways to prevent ice from forming in the first place—proactive anti-icing— were considered, and at the same time options for removing ice once it

had formed—reactive de-icing—were suggested and tested in the field, in the air, and in the wind tunnel. Of all the options available, the three major ones are the pneumatic boot, spraying chemicals onto the air­craft, and channeling hot bleed air.[1224]

The oldest of the de-icing methods in use is the pneumatic boot sys­tem, invented in 1923 by the B. F. Goodrich Corporation in Akron, OH. The general idea behind the boot has not changed nearly a century later: a thick rubber membrane is attached to the leading edge of a wing air­foil. Small holes in the wing behind the boot allow compressed air to blow through, ever so slightly expanding the boot’s volume like a bal­loon. Any time that ice builds up on the wing, the system is activated, and when the boot expands, it essentially breaks the ice into pieces, which are quickly blown away by the relative wind of the moving air­craft. Again, although the general design of the boot system has not
changed, there have been improvements in materials science and sen­sor technology, as well as changes in the shape of wings used in vari­ous sizes and types of aircraft. In this manner, NASA researchers have been very active in coming up with new and inventive ways to enhance the original boot concept and operation.[1225]

One way to ensure there is no ice on an aircraft is to remove it before the flight gets off the ground. The most common method for doing this is to spray some type of de-icing fluid onto the aircraft surface as close to takeoff as possible. The idea was first proposed by Joseph Halbert and used by the United Kingdom Royal Air Force in 1937 on the large flying boats then operated by Imperial Airways.[1226] Today, the chemicals used in these fluids usually use a propylene glycol or ethylene glycol base and may include other ingredients that might thicken the fluid, help inhibit corrosion on the aircraft, or add a color to the mixture for easier iden­tification. Often water is added to the mixture, which although counter­intuitive makes the liquid more effective. Of the two glycols, propylene is more environmentally friendly.[1227]

The industry standard for this fluid is set by the aeronautics division of the Society of Automotive Engineers, which has published standards for four types of de-icing fluids, each with slightly different properties and intentions for use. Type I has a low viscosity and is usually heated and sprayed on aircraft at high pressure to remove any snow, ice, or frost. Due to its viscosity, it runs off the aircraft very quickly and provides lit­tle to no protection as an anti-icing agent as the aircraft is exposed to snowy or icy conditions before takeoff. Its color is usually orange.[1228] Type II fluid has a thickening agent to prevent it from running very quickly off the aircraft, leaving a film behind that acts as an anti-icing agent until the aircraft reaches a speed of 100 knots, when the fluid breaks down from aerodynamic stress. The fluid is usually light yellow. Type III flu­id’s properties fall in between Type I and II, and it is intended for smaller,

slower aircraft. It is popular in the regional and business aviation mar­kets and is usually dyed light yellow. Type IV fluids are only applied after a Type I fluid is sprayed on to remove all snow, ice, and frost. The Type IV fluid is designed to leave a film on the aircraft that will remain for 30 to 80 minutes, serving as a strong anti-icing agent. It is usually green.[1229]

NASA researchers have worked with these fluids for many years and found uses in other programs, including the International Space Station. And during the late 1990s, a team of engineers from the Ames Research Center (ARC) at Moffett Field, CA, came up with an anti-icing fluid that was nontoxic—so much so that it was deemed "food grade” because its ingredients were approved by the U. S. Government for use in food— namely ice cream—and promised to last longer as an anti-icing agent for aircraft, as well as work as an effective de-icing agent. Although it
has not found wide use in the aviation industry, NASA did issue a license to a commercial firm who now sells the product to consumers as "Ice Free,” a spray for automobile windshields that can provide protection from snow or ice forming on a windshield in temperatures down to 20 degrees Fahrenheit (-7 degrees Celsius).[1230]

The third common technique for dealing with ice accretion is the hot bleed air method. In this scheme, hot air is channeled away from the air­craft engines and fed into tubes that run throughout the aircraft near the areas where ice is most likely to form and do the most damage. The hot air warms the aircraft skin, melting away any ice that is there and discour­aging any ice from forming. The hot gas can also be used as the source of pressurized air that inflates a rubber boot, if one is present. While the idea of using hot bleed air became most practical with the introduction of jet engines, the basic concept itself dates back to the 1930s, when NACA engineers proposed the idea and tested it in an open-air-cockpit, bi-wing airplane. The in-flight experiments showed that "a vapor-heating system which extracts heat from the exhaust and distributes it to the wings is an entirely practical and efficient method for preventing ice formation.”[1231]

As for melting ice that can accrete on or in other parts of an air­craft, such as windshields, protruding Pitot tubes, antennas, and car­buretors on piston engines, electrically powered heaters of one kind or another are employed. The problem of carburetor ice is especially important and the one form of icing most prevalent and dangerous for thousands of General Aviation pilots. NASA has studied carburetor ice for engines and aircraft of various configurations through the years[1232] and in 1975 surveyed the accident database and found that between 65 and 90 accidents each year involve carburetor icing as the probable cause. And when there are known carburetor icing conditions, between 50 and 70 percent of engine failure accidents are due to carburetor icing. Researchers found the problem to be particularly acute for pilots
with less than 1,000 hours of total flying time and overall exposed about 144 persons to death or injury each year.[1233]

NASA’s role in high-angle-of-attack technology rapidly accelerated begin­ning in 1971. Extensive research was conducted with generic models, simulator techniques for assessing high-alpha behavior were developed, and test techniques were upgraded. Active participation in the F-14, F-15, and B-1 development programs was quickly followed by similar research for the YF-16 and YF-17 Lightweight Fighter prototypes, as
well as later efforts for the F-16, F-16XL, F/A-18, X-29, EA-6B, and X-31 programs. Summaries of Langley’s contributions in those programs have been documented, and equally valuable contributions from Dryden and Ames will be described herein.[1290] Brief highlights of a few NASA contri­butions and their technical impacts follow.

One of the more intriguing forms of aircraft propulsion is the ducted fan: the fan enclosed within a ring and powered by a drive train from an engine typically located elsewhere in the aircraft. Researchers inter­ested in V/STOL flight expended great effort on ducted-fan approaches, and indeed, such an approach is incorporated on the Joint Strike Fighter, the Lockheed Martin F-35 Lightning II. Though ducted-fan propulsion for Conventional Take-Off and Landing aircraft had enjoyed at best a mixed record, those advocating it for V/STOL applications were hope­ful it would prove more successful. Ducted-fan options included piv­oting fans that could furnish direct lift, like tilt rotors, then pivot to furnish power for wing-borne forward flight, or rely on horizontal fans in a wing or fuselage to generate vertical lift, or combinations of these.

Ducted-fan aircraft intended for conventional flight were tried in many nations. Likewise, ducted-fan V/STOL adherents in various coun­tries had proposed concepts for such craft. But the first two American ducted-fan V/STOL airplanes—that is, ducted-fan aircraft with wings, as opposed to various Hiller and Piasecki flying platforms—were the Doak Model 16, designated the VZ-4, and the Vanguard Omniplane. Each rep­resented a different approach, though, of the two, only the Doak flew.[1401]

The Doak began as an Army research project, first flying in February 1956. A pleasing and imaginative design of conventional straight wing aerodynamic layout, it had a single 860-horsepower Lycoming YT53 turboshaft engine driving two pivoted ducted fans on the wingtips. During hover, variable inlet guide vanes in the ducts furnished roll con­trol, with pitch and yaw control provided by vectored jet exhaust from

a variable tail nozzle. The Doak proved that its design approach worked, readily making vertical descents, transitions to conventional flight, and transition back to hover and landing, accelerating in just 17 seconds from 0 to 200 knots. It arrived at the NACA’s Langley laboratory in September 1957. Testing revealing a mix of undesirable handling qualities across its flight envelope, with one subsequent NASA study concluding that it

suffered from low inherent control power about all axes, sensitivity to ground-effect disturbances, large side forces associated with the large ducts, and a large (positive) dihe­dral effect which restricted operation to calm-air conditions and no crosswinds. No large STOL performance benefit was evident with this design.[1402]

During vertical descents, it buffeted with alternate left-and-right wing­dropping. This became so severe as the plane approached stall angle that "roll control was not adequate to keep the aircraft upright,” noted pilot Jack Reeder. Large nose-up pitching moments required careful speed and duct-angle management to prevent duct-lip airflow stalling. In hover, the inlet guide vanes were "very inadequate.” In ground effect, Reeder noted:

if lifted clear of the ground by several feet, uncontrollable yawing and persistent lateral upsetting tendencies have been encountered. With the weak yaw control and, particularly the weak roll control, the unindoctrinated pilot may find himself unable to control the aircraft.[1403]

The Doak Company closed in 1960, and NASA retired the airplane in 1972. By that time, a ducted-fan successor, the Bell Aerospace Textron X-22A, was already flying. Far more unconventional in appearance than was the Doak, it nevertheless owed a technical debt to the earlier design. As NASA researchers had concluded from the VZ-4’s testing, the little Doak had "indicated the feasibility as well as the inherent problems of the tilt-duct concept, which helped the X-22 design which followed.”[1404]

Bell Aerospace Textron’s X-22A grew out of company studies in the mid-1950s. Successive examination of Bell-proposed military concepts led to increasing service interest by the Navy, Air Force, Marine Corps, and Army for a range of transport, rescue, and counterinsurgency appli­cations. In late 1962, the U. S. Navy signed a development contract with Bell for a half-scale flying testbed of one of the company’s proposed designs. This became the X-22A, two of which were built. Because the loss of any one fan would spell disaster, the X-22A used four General Electric T58 turboshaft engines, interconnected to the ducted fans so that an engine failure would not result in a loss fan power. NASA sup­ported Bell’s development with extensive wind tunnel studies at Ames and Langley. Control was exercised by changing the pitch of each of the four propellers and by moving the four elevons. These eight variables were used to control the X-22 in normal flight with ducts horizontal and in hover with ducts vertical. In horizontal flight, pitch and roll were con­trolled by the elevons and yaw by differential variation of propeller pitch. For hovering flight, propeller pitch adjustments controlled pitch and roll, while elevon movements controlled yaw. During transition, control func­tions were phased in gradually as a function of duct tilt angle. The pilot was provided with artificial "feel” in yaw during forward flight, but this was removed during transition to hover. Pitch and roll "feel” were provided by a hydroelectric system that applied stick reactions proportional to g-forces. For hover, transition, and low-speed flight, a Stability Augmentation System was used to improve aircraft stability and handling characteristics. The X-22A was equipped with a sophisticated Variable Stability and Control System (VSCS) developed by the Calspan Corp. This allowed it to be pro­grammed to behave like other existing or projected VTOL aircraft for assessment of flight characteristics. The VSCS interacted with the Smiths Industries head-up display (HUD) and the Kaiser Electronics head-down display (HDD). Data inputs to the VSCS included those from a low-speed airspeed sensor, the Linear Omnidirectional Resolving Airspeed System (LORAS), invented by Calspan’s Jack Beilman.[1405]

The first X-22A flew March 17, 1966, but a hydraulic system fail­ure led to the loss of the aircraft in August of that year during an emer­gency vertical landing, fortunately without injury to the crew. The second X-22A became one of the more successful research aircraft ever flown,

completing over 500 flights and over 1,300 transitions, between com­mencement of its flight-test program at the end of January 1967 through its retirement from flight-testing 17 years later. It hovered at over 8,000 feet altitude and achieved a forward speed of 315 mph, proving conclu­sively that a tilt duct vehicle could fly faster than could a conventional helicopter. In May 1969, it was turned over to the Navy, which appointed Calspan to continue the flight-test program and fly it as a variable stabil­ity research and training aircraft. Eventually, three NASA test pilots flew it, two of whom were formerly at Calspan Corporation—Rogers Smith and G. Warren Hall—and Ron Gerdes from Ames. Assessing the X-22A’s place in V/STOL history, NASA researchers concluded:

Hover operation Out of Ground Effect (OGE) in no wind was rated excellent, with no perceptible hot-gas ingestion. A 12% positive thrust increase was generated In Ground Effect (IGE) by the favorable fountain. Airframe shaking and buffeting occurred at wheel heights up to about 15 ft, and cross-wind effects were quite noticeable because of large side forces gen­erated by the ducts. Vertical cross-wind landings required an excessive bank angle to avoid lateral drift. STOL performance was rated good by virtue of the increased duct-lifting forces. Highspeed performance was limited by inherent high drag associated with the four large ducts. Transition to conven­tional flight could be made easily because of a wide transition
corridor; however, inherent damping was low. Deceleration and descent at low engine powers caused undesirable duct "buzz” as a result of flow separation on the lower duct lips. Vortex gen­erators appreciably improved this flow-separation problem.[1406]

The X-22A proved to be a successful and versatile research tool, flying for at least 17 years and providing much valuable information on ducted VTOL systems and the larger operational issues of VTOL and STOL aircraft.

NASA expended a great deal of study effort examining the benefits of lift-fan technology and various design approaches that might be taken in design of a practical military and civil lift-fan aircraft. In the course of these trials, involving model tests, tests of candidate fan technolo­gies, and examinations of lift-fan aircraft (such as the ill-fated Vanguard Omniplane), researchers studied an experimental Army-Ryan program, the XV-5A Vertifan. The XV-5A was an ill-fated program, like its con­temporary, the Army-Lockheed XV-4 Hummingbird, which is discussed subsequently. Between them, the aircraft built of these two types killed three test pilots and nearly a fourth. Powered by two General Electric J85 engines driving in-wing fans and a nose pitch-control fan (like the Vanguard Omniplane) and used for conventional propulsion, the first of two XV-5As flew in 1964 but crashed during a public demonstration at Edwards AFB in August 1965, killing Ryan test pilot Lou Everett. The sec­ond fared little better, crashing in October 1966 at Edwards after one lift fan ingested a rescue hoist deployed from the aircraft, causing an asym­metric loss of lift. Air Force test pilot Maj. David Tittle perished while ejecting from the ailing aircraft, which, in sad irony, impacted with sur­prisingly little damage. Rebuilt as the XV-5B with some changes to its avi­onics, cockpit layout, ejection seat, and landing gear, it flew again in 1968, flying afterward at Ames Research Center on a variety of NASA investi­gations led by David Hickey, until its retirement in 1974.[1407]

Among these studies were tests in and out of ground effect of var­ious wing and inlet configurations, exit-vane designs, nose fans, and control devices. The research studies focused on problems of transi­tion from vertical to horizontal flight, and on improvements of the lift fans to provide quieter, smaller fans with greater thrust. These studies were funded in part by the U. S. Army Aeronautical Research Laboratory, reflecting the Army’s interest in V/STOL aircraft technology and matu­ration. Ames researchers found that the XV-5B could take off and land vertically from an area the size of a tennis court; hover in midair for several minutes like a helicopter; and fly straight up, down, backward, or to either side at speeds up to 25 mph. As well, it could operate like a conventional jet airplane using a runway, flying up to 525 mph. However, though a NASA summary report on V/STOL concepts concluded, "The lift-fan concept proved to be relatively free of mechanical problems,” tests revealed that the XV-4B was still far from a practical vehicle. Hot – gas ingestion degraded engine performance while in ground effect, drag

from the fan installations limited STOL performance, combinations of fan overspeed and a nose-up tendency complicated conversions, and the design layout hinted at a potential deep-stall problem characteristic of many T-tail aircraft. NASA concluded: "This configuration has limited high-speed potential because of the relatively thick wing section needed to house the lift fans and vectoring hardware.”[1408]

As part of an Advanced Short Take-Off and Vertical Landing (ASTOVL) study program that began in 1980, NASA continued detailed studies of ducted lift fans, among other propulsion concepts intended for a supersonic successor to the vectored-thrust AV-8B Harrier II. The outcome of that development effort was validated in the successful flight­testing of a lift fan on the STOVL variant of the Lockheed Martin X-35, the experimental proof-of-concept demonstrator for the F-35 Joint Strike Fighter a quarter century later.[1409] In this regard, lessons learned from NASA’s various lift-fan programs, particularly the XV-5A and XV-5B, com­piled by Ames test pilot and distinguished V/STOL researcher Ronald

M. Gerdes, are included as an appendix to this study.

Proving the Tilt Rotor: From XV-3 and X-100 to XV-15 and on to V-22

One V/STOL concept that proved to have enduring appeal was the tilt rotor, which entered production and operational service with the joint – service Bell-Boeing V-22 Osprey. The tilt rotor functioned like a twin-rotor helicopter during lift-off, hover, and landing. But for cruising flight, it tilted forward to operate as high aspect ratio propellers. Such a concept meant that the tilt rotor would necessarily have its rotors pod-mounted on the tips of conventional wings.[1410]

Though various designers across the globe envisioned tilt rotor con – vertiplanes, the first successful one was the Bell Model 200, produced by Bell Helicopter for the Air Force and Army as the XV-3 under a joint Army-USAF "convertiplane” program started in August 1950. Relatively

streamlined and looking more like an airplane than did many early V/STOL testbeds, the XV-3 had an empty weight of 3,600 pounds and a normal gross weight of just 4,800 pounds, as it was relatively under­powered. A single Pratt & Whitney R985 radial piston engine producing 450 horsepower drove two three-bladed rotors via drive shafts. With this propulsion system, the XV-3 completed its first hover in August 1955, piloted by Floyd Carlson.[1411]

Flight-testing over the next year demonstrated flight at progressive levels of rotor tilt, though it had not made a full 90-degree conversion of its rotors to level position before it crashed while landing in October 1956 from a rotor instability. Bell test pilot Dick Stansbury survived but was seriously injured. Afterward, the second XV-3 was equipped with stiffer two-bladed rotors. On December 18, 1958, Bell test pilot Bill Quinlan achieved a full conversion from a helicopter-like ascent to for­ward flight like an airplane. During the XV-3 flight-test program, the lack of engine power prevented it from hovering out of ground effect. When
it did hover in ground effect, reflected rotor wash caused unpredictable darting, something the tilt rotor V-22 experienced four decades later during its testing. The lack of an SAS further exacerbated pilot hover challenges, and in gusty air, high pilot workload was required to hover. The XV3 transited rapidly from hover to conventional flight, requiring only small pitch changes across the range of speed and angle of attack encompassed by the transition corridor. Pitch and yaw dynamic insta­bility triggered by side forces as blade angle was increased limited maxi­mum cruise speed to 140 knots and pointed to rotor dynamics and flight control challenges that future tilt rotors would have to overcome. In all of this research, Bell blended extensive analytical studies and scale model experiments with tests in the Ames 40-foot by 80-foot wind tunnel.[1412]

In May 1959, the surviving XV-3 was delivered to the Air Force Flight Test Center at Edwards Air Force Base (AFB), where it under­went a 3-month Air Force evaluation before being delivered for more extensive testing and research to the Ames Research Center. During Edwards’s testing, Maj. Robert Ferry successfully demonstrated a power – off reconversion to a vertical autorotation descent and landing, an impor­tant milestone. At Ames, Hervey Quigley carried out the research, and Don Heinle and Fred Drinkwater conducted most of the test flying, in the course of which the XV-3 was modified with a large ventral fin to improve its directional stability. In the Ames tests, flapping of the teeter­ing rotors during maneuvers introduced moments that reduced damp­ing of the longitudinal and lateral-directional oscillations to near zero at speeds approaching 140 knots. Despite these problems and despite being underpowered and limited in payload, the XV-3 proved the capa­bility of the tilt rotor to perform in-flight conversions between the heli­copter and the airplane modes, though much work on understanding rotor dynamics and flight control issues needed to be done. The XV-3 flew at Ames until summer 1962, when it began an extensive series of wind tunnel studies in the 40-foot by 80-foot tunnel. In November 1968, during 200-mph tunnel trials, fatigue failure in one wingtip led to sep­aration of the rotors and their pylons from the aircraft, bringing its 13-year test career, at last, to an end. By that time, it had validated the tilt rotor concept, thus influencing—as discussed subsequently—the next step forward in experimental tilt rotor design, the XV-15. That vehicle,

of course, would exert an even greater influence upon development of its operational successor, the V-22 Osprey.[1413]

Before settlement on the tilt rotor as exemplified by Bell’s design approach with the XV-3, researchers considered another seemingly closely related concept: the tilt prop. However, the tilt prop idea was different. Researchers had long known that rotating propellers gener­ate a powerful side force, and Curtiss-Wright Corporation engineers envisioned taking advantage of this property by using smaller diame­ter and lower aspect ratio propellers than tilt rotors that could use this "radial lift force” as a means of lifting a V/STOL airplane vertically. Such a design, they hoped, would have higher top-end speed after conversion than an XV-3-like tilt rotor approach.[1414]

The result was the X-100, a small testbed whose twin broad-chord propellers were driven by a single Lycoming YT53-L-1 turboshaft engine. Its jet exhaust vented through an omnidirectional tail nozzle, furnishing low-speed pitch and yaw control. Differential propeller operation fur­nished roll control during hover. The X-100 underwent testing in Ames 40-foot by 80-foot tunnel and extensive ground trials before making its first flight in March 1960. In August, it underwent a NASA flight evalu­ation, after which it went to Langley Research Center for further test­ing, including downwash effects on various kinds of ground surfaces.[1415] Langley pilot Jack Reeder found it longitudinally unstable during con­versions, something "very undesirable during landing approaches, par­ticularly under instrument conditions.” During hover it demonstrated "erratic wing dropping and yawing,” necessitating "noticeably large” cor­rective control inputs to correct, and "weak” yaw control that prevented holding a desired heading. It "settled rapidly toward the ground when upset in bank or pitch attitude” while in ground effect, again, something he found "very undesirable.” On the positive side, he found that "The X-100 aircraft suffers no apparent stall problems.”[1416]

In October 1961, the X-100 was seriously damaged in a hovering accident that, fortunately, did not result in injury to its pilot. Despite its mediocre performance, it had demonstrated the feasibility of the radial – lift propeller concept. Thus, Curtiss-Wright continued pursuing the tilt prop approach but now chose to make a four-propeller craft with equal span fore and aft wings, rather than an X-100-like twin-rotor design. The company subsequently received an Air Force developmental contract for this larger and more powerful design, which became the experimen­tal X-19. Of the two that were built, only the first flew, and it had a brief and troubled flight-test program before crashing in August 1965 at the FAA’s National Aviation Facilities Experimental Center (NAFEC) after experiencing a catastrophic gearbox failure. Fortunately, its crew ejected from the now-propless testbed before it plunged to Earth. At the compa­ny’s request, the X-19 program was terminated the following December. The accident, one NASA authority concluded, "exemplified an inherent deficiency of this VTOL (lift) arrangement: to safely transmit power to the extremities of the planform, very strong (and fatigue-resistant)

structures must be incorporated with an obvious weight penalty.”[1417] The future belonged to the tilt rotor, not tilt prop.

Though tests with the XV-3 had identified numerous challenges in stability and control, handling qualities, and the dynamics of the combined wing-pylon-rotor interactions, the program encouraged tilt rotor proponents to continue their studies. So promising did the tilt rotor appear that the Army and NASA formed a joint project office at Ames to study tilt rotor technology and undertook a number of sim­ulations of such systems to refine project goals and efficiencies.[1418] In 1971, Dr. Leonard Roberts of Ames’s Aeronautics and Flight Mechanics Directorate established a V/STOL Projects Office under Woody Cook to develop and flight-test new V/STOL aircraft. That same year, in partner­ship with the Army, NASA launched a competitive development program for the design and fabrication of two tilt rotor research aircraft. Four companies responded, and Boeing and Bell received study contracts in October 1972. After evaluating each proposal, NASA selected Bell’s Model D301 for development, issuing Bell a contract at the end of July 1973.[1419]

As developed, the XV-15 was an elegant and streamlined technol­ogy demonstrator, a two-pilot testbed powered by twin Lycoming T53 turboshafts rated at 1,550 horsepower each, driving 25-foot-diameter three-bladed prop rotors. Bell completed the first XV-15 in October 1976 and, after ground tie-down testing, undertook its first preliminary hover

trials in May 1977, piloted by Ron Erhart and Dorman Cannon. In May 1978, before flight envelope expansion, it went into the Ames 40-foot by 80-foot wind tunnel for extensive stability, performance, and loads tests. Ames’s aeronautical facilities greatly influenced the XV-15’s development, particularly simulations of anticipated behavior and operational nuances, and tilt rotor performance and dynamic tests in the wind tunnel.[1420]

The second XV-15 went to Dryden for contractor flight tests, con­ducted between April 1979 and July 1980, and was delivered to NASA for research in August.[1421] By that time, NASA, Army, and contractor researchers had already concluded:

The XV-15 tilt rotor has exhibited excellent handling quali­ties in all modes of flight. In the helicopter mode it is a sta­ble platform that allows precision hover and agility with low pilot workload. Vibration levels are low as are both internal and external noise levels. The conversion procedure is uncom­plicated by schedules, and it is easy to perform. During the
conversion or reconversion, acceleration or deceleration are impressive and make it difficult for conventional helicopters or airplanes to stay with the XV-15. Handling qualities are excellent within the airplane mode envelope investigated to date; however, gust response is unusual. Although internal noise levels are up somewhat in the airplane mode, external noise levels are very low. Overall the XV-15 is a versatile and unique aircraft which is demonstrating technology that has the potential for widespread civil and military application.[1422]

Such belief in the aircraft led to its participation in the 1981 Paris Air Show, the first time NASA had demonstrated one of its research vehicles in an international venue. It was an important vote of confidence in tilt rotor technology, made more evident still by the XV-15’s stopover at the Royal Aircraft Establishment at Farnborough, where it demonstrated its capabilities before British aeronautical authorities. In 1995, 14 years after its first Paris appearance, the XV-15 would again fly at Le Bourget, this time in company with its successor, the Bell-Boeing V-22 Osprey.[1423]

Over its two-decade test program, the XV-15 was not immune to var­ious mishaps, though fortunately, no one was seriously injured. Both air­craft experienced various emergencies, including forced landings after engine failures, a close call from a bird strike that cracked a wing spar, a tree strike, near-structural failure caused by an unsuitable form of tita­nium alloy fortuitously discovered before it could do harm, intergran­ular corrosion that caused potentially dangerous hairline blade cracks, and even one major accident. In August 1991, the first XV-15 crashed while landing after an improperly secured nut separated from a linkage controlling one of the prop rotors. Pilots Ron Erhart and Guy Dabadie were not seriously injured, though the accident destroyed the aircraft.[1424]

In retrospect, the XV-15 was the most influential demonstrator air­craft program that Ames ever pursued. For a cost to taxpayers of $50.4 million, NASA and its partners significantly advanced the technology and capability of tilt rotor technology. In over two decades of flight operations, more than 300 guest pilots would fly in the XV-15. As well, it would operate from the New York Port Authority heliport, fly abroad,

and go to sea, demonstrating its ability to operate from amphibious assault ships. Among the many at Ames who contributed to making the program a success were NASA’s Wally Deckert, Mark Kelly, and Demo Giulianetti, and the Army’s Paul Yaggy, Dean Borgman, and Kipling Edenborough, who furnished critical guidance and oversight as the project was being established. Dave Few, Army Lt. Col. James Brown, and John Magee served as Program Managers. Principal investigators were Laurel Schroers, Gary Churchill, Marty Maisel, and Jim Weiberg. The project pilots were Daniel Dugan, Ronald Gerdes, George Tucker, Lt. Col. Grady Wilson, and Lt. Col. Rick Simmons. They shepherded the XV-15 through two decades of research on flying qualities and stability and control evaluations, control law development, side stick controller tests, performance evaluations in all flight modes, acoustics tests, flow surveys, and documentation of its loads, structural dynamics, and aero – elastic stability characteristics, generating a useful database that was digitized by Ames and made available to industry and military custom­ers. In sum, the XV-15 did much to advance the V/STOL cause, partic­ularly that of the tilt rotor concept.[1425]

In particular, flight experience with the XV-15 contributed greatly to the development of the joint-service V-22 Osprey tilt rotor.[1426] This Bell – Boeing aircraft, now in service with the U. S. Marines, the U. S. Navy, and the U. S. Air Force, fulfills a variety of roles, including combat assault, insertion and support of special operations forces (SOF), combat search and rescue (CSAR), and logistical support. Time will tell whether the V-22 Osprey will come to enjoy the longevity and ubiquity attendant to conventional joint-service fixed and rotary wing transports, such as the legendary Douglas C-47, Lockheed C130, Bell UH-1, and Sikorsky H-53.

In Phase I, typical flights involved a climb and acceleration to supersonic speeds and cruise altitudes, 15 minutes of stable supersonic cruise, a descent and deceleration to subsonic cruise conditions for subsonic test points, and finally, approach and landing work.[1507] All 19 Phase I flights were accomplished by Tupolev crews. Flights 20 through 23 incorporated the NASA pilot evaluations at the beginning of Phase II. The descrip­tion of the Handling Qualities Assessment Experiment 2.4/2.4A will cen­ter on these flights, because they are of more special interest to NASA.[1508]

Working with Tupolev chief test pilot Sergei Borisov and project engi­neer Vladimir Sysoev, USPET developed a set of efficient handling qual­ities maneuvers to be used on these flights. These maneuver sets were derived from the consensus reached among USPET members regard­ing the highest-priority tasks from Mach 2 to approach and landing. To assist the pilots, specifically defined maneuvers were repeated for dif­ferent flight conditions and aircraft configurations. These maneuver sets included

• Integrated test block (ITB): The ITB was a standard block of maneuvers consisting of pitch attitude captures, bank captures, heading captures, steady heading sideslips, and a level acceleration/deceleration.

• Parameter identification (PID) maneuvers: The PID maneuvers generated either a sinusoidal frequency sweep or a timed pulse train in the axis of interest and contributed to the dataset needed for the LOES analysis.

• Simulated engine failure: This consisted of retarding an outboard throttle to minimum setting, stabilizing on a trimmed condition, and performing a heading capture.

• Slow flight: Accomplished in both level and turning flight, this maneuver was flown at minimum airspeed.

•

Structural excitation maneuvers: These maneuvers con­sisted of sharp raps on each control inceptor to excite and observe any aeroservoelastic response of the aircraft.

• Approaches and landings: Different configurations were specified to include canard retracted, lateral off­set, manual throttle, nose retracted (zero forward vis­ibility), simulated engine out, visual, and Instrument Landing System (ILS) approaches.52

Flight 20 was flown by an all-Russian crew but was observed from a control room at the Gromov Russian Federation State Scientific Center at the Zhukovsky Air Development Center.

This flight provided USPET with an excellent opportunity to observe Tu-144 planning and operations and prepared the team for the NASA piloted flights. With a better sense of Tupolev operations, USPET was able to develop English checklists and procedures to complement the Russian ones. Fullerton and Rivers learned all of the Russian-labeled
switches and controls and procedural calls. USPET made bound check­lists from the cardboard backs of engineering tablets, because office mate­rial was in short supply at that time in Russia. Flight 20 also allowed USPET engineers Jackson, Cox, and Princen and pilots Fullerton and Rivers to develop a working relationship with Tupolev project engineer Sysoev in developing the test cards for the U. S. flights. The stage was set for the first flight of a Tu-144 by a United States pilot.

Flight 21 was scheduled for September 15, 1998. Fullerton and Rivers agreed that Fullerton would pilot this flight and Rivers would observe from the cockpit, taking notes, timing maneuvers, and assisting with the crew coordination. As it turned out, Fullerton’s communications failed during the flight, and Rivers had to relay Tupolev pilot Sergei Borisov’s comments to Fullerton. Borisov sat in the left seat and Fullerton in the right; Victor Pedos occupied the navigator’s seat and Anatoli Kriulin the flight engineer’s station. Rivers stood behind Borisov and next to Pedos. Jackson and Cox had seats in the Gromov control room. Flight 21 was to be a subsonic flight with handling qualities maneuvers completed by Fullerton during the climb, Mach 0.9 cruise, descent, low-altitude slow – flight maneuvering, and approach and landing tasks. Because of the shortage of tires, each flight was allowed only one landing. The multiple approaches flown were to low approach (less than 200-feet altitude) only.

The flight is best described by the flight test summary contained in a NASA report titled "A Qualitative Piloted Evaluation of the Tu-144”:

Shortly after take-off a series of ITBs were conducted for the take-off and the clean configurations at 2 km altitude. Acceleration to 700 km/hr was initiated followed by a climb to the subsonic cruise condition of Mach 0.9, altitude 9 km. Another ITB was performed followed by evaluations of a simu­lated engine failure and slow speed flight. After descent to 2 km, evaluations of slow speed flight in the take-off and landing con­figurations were conducted as well as an ITB and a simulated engine failure in the landing configuration. Following a descent to pattern altitude three approaches to 60 m altitude were con­ducted with the following configurations: a canard retracted configuration using the ILS localizer, a nominal configuration with a 100 m offset correction at 140 m altitude, and a nom­inal configuration using visual cues. The flight ended with a visual approach to touchdown in the nominal configuration.

However, due to unusually high winds the plane landed right at its crosswind limit, necessitating the Russian pilot in com­mand to take control during the landing. Total flight time was approximately 2 hours 40 minutes. The maximum speed and altitude was 0.9 Mach and 9 km.53

The flight completed all test objectives. Thorough debriefs ensued, the obligatory postflight party sponsored by Tupolev was held, and USPET began intensive training and planning for the first supersonic flight, to be flown just 3 days later.

September 18 opened cool, clear, and much less gusty than the pre­ceding days. Flight 22 would be a Mach 2 mission to an altitude of 60,000 feet, with at least 20 minutes flight at twice the speed of sound. Rob Rivers was the NASA pilot for this flight. Pukhov’s only requirement for Rivers was that he no longer need his crutches by flight day. Two nights before, Bruce Jackson had helped Rivers practice using a cane for over an hour until Rivers was comfortable. At the next day’s preflight party, Rivers demonstrated to Pukhov his abilities without crutches, and his approval for the flight was assured. At 11:08 a. m. local time, the Tu-144 became airborne.

The flight is described below in Rivers’s original flight test report:

Flight Profile. The flight profile included takeoff and accelera­tion to 700 kilometers per hour (km/hr) to intercept the climb schedule to 16.5 kilometers (km) and Mach 2.0. The flight direction was southeast toward the city of Samara on the Volga River at a distance of 700 km from Zhukovsky. Approximately 20 minutes were spent at Mach 2.0 cruise which included an approximately 190 degree course reversal and a cruise climb up to a maximum altitude of 17.3 km. A descent and decel­eration to 9 km and Mach 0.9 was followed by a brief cruise period at that altitude and airspeed prior to descent to the traffic pattern at Zhukovsky Airfield for multiple approaches followed by a full stop landing on Runway 30.

Flight Summary. After all preflight checklists had been com­pleted, the evaluation pilot taxied Tu-144LL Serial Number

77144 onto Runway 12, and the brake burn-in process was accomplished. At 11:08 brakes were released for takeoff, power was set at 98° PLA (partial afterburner), the start brake was released, and after a 30 sec takeoff roll, the aircraft lifted off at approximately 355 km/hr. The landing gear was raised with a positive rate of climb, the canard was retracted out of 120 m altitude, and the nose was raised out of 1000 m altitude. The speed was initially allowed to increase to 600 km/hr and then to 700 km/hr as the Vertical Regime Indicator (VRI) profile was intercepted. Power remained at 72° PLA (maximum dry power) for the climb until Mach 0.95 and CG of 47.5% at which point the throttles were advanced to maximum power, 115° PLA. The climb task was a high work­load task due to the sensitivity of the head up pitch refer­ence indicator, the sensitivity of the pitch axis, and the continual change in CG requiring almost continuous lon­gitudinal trim inputs. Also, since the instantaneous center of rotation is located at the pilot station, there are no cock­pit motion cues available to the pilot for pitch rate or atti­tude changes. Significant pitch rates can be observed on the pitch attitude reference indicator that are not sensed by the pilot. During the climb passing 4 km, the first of a repeating series of bank angle captures (±15°) and control raps in all three axes (to excite any aircraft structural modes) was com­pleted. These maneuvers were repeated at 6 km and when accelerating through Mach 0.7, 0.9, 1.1, 1.4, and 1.8. The bank angle captures demonstrated rather high roll forces and relatively large displacements required for small roll angles. A well damped (almost deadbeat) roll mode at all airspeeds up to Mach 2.0 was noted. The control raps showed in gen­eral a higher magnitude lower frequency response in all three axes at subsonic speeds and lower magnitude, higher frequency responses at supersonic speeds. The pitch response was in general of lower amplitude and frequency with fewer over­shoots (2-3) than the lateral and directional responses (4-5 overshoots) at all speeds. Also of interest was that the axis exhibiting the flexible response was the axis that was perturbed, i. e., pitch raps resulted in essentially only pitch responses. The motions definitely seemed to be aeroservoelastic in nature,

and with the strong damping in the lateral and directional axes, normal control inputs resulted in well damped responses. Level off at 16.5 km and Mach 1.95 occurred 19 minutes after takeoff. The aircraft was allowed to accelerate to Mach 2.0 IMN as the throttles were reduced to 98° PLA, and a series of control raps was accomplished. Following this, a portion of the Integrated Test Block set of maneuvers consisting of pitch captures, steady heading sideslips, and a level decel­eration was completed. The pitch captures resulted in slight overshoots and indicated a moderate delay between pitch atti­tude changes and flight path angle changes. The steady head­ing sideslips showed a slight positive dihedral effect, but no more than approximately 5° angle of bank was required to maintain a constant heading. No unpleasant characteristics were noted. At this point the first set of three longitudinal and lateral/directional parameter identification (PID) maneuvers were completed with no unusual results. By this time a course reversal was necessary, and the bank angle and heading cap­ture portions of the ITB were completed during the over 180° turn which took approximately 7 min to complete at Mach

1. 95. During the inbound supersonic leg, two more sets of PID maneuvers with higher amplitude (double the first set) con­trol inputs were completed as were several more sets of con­trol raps. Maximum altitude achieved during the supersonic maneuvering was 17.3 km.

The descent and deceleration from Mach 2.0 and 17 km began with a power reduction from the nominal 98° PLA to 59° and a deceleration to 800 km/hr. During the descent bank angle captures (±30°) and control raps were accomplished at or about Mach 1.8, 1.4, 1.1, and 0.9 with similar results as reported above. The aircraft demonstrated increased pitch sensitivity in the transonic region decelerating through Mach

1.0. The pitch task during descent in following the VRI guid­ance was fairly high in workload, and the head-up pitch ref­erence indicator was very sensitive and indicated fairly large pitch responses from very small pitch inputs. Since the CG is being transferred aft during supersonic descent, frequent pitch trimming is required. A level off at 9 km at Mach 0.9 was accomplished without difficulty, and an ITB (as described

above) was completed. Further descents as directed by air traffic control placed the aircraft in the landing pattern with 32 metric tons of fuel, 6 tons above the planned amount.

Five total approaches including the final full stop land­ing were completed. These included a straight-in localizer only approach with the canard retracted; an offset approach with the nose raised until on final; a manual throttle off­set approach; a manual throttle straight-in approach; and a straight-in visual approach to a full stop landing. The first approach with the canard retracted was flown at 360 km/hr due to the loss of about 12 tons of lift from the retracted canards. Pitch control was not as precise in this configuration. There was also a learning curve effect as the evaluation pilot gained experience in making very small, precise pitch inputs which is necessary to properly fly the aircraft on approach and to properly use the pitch reference indicator. After terminating the approach at 60 m, a canard retracted, gear down low pass up the runway at 30-40 m was completed in accordance with a ground effects experiment requirement. The nose-up approach demonstrated the capability to land this aircraft with the nose retracted providing an angling approach with some sideslip is used. The offset approaches were not representative of the normal offset approaches flown in the HSR program since they are to low approach only and do not tax the pilot with the high gain spot landing task out of the corrective turn. No untow­ard pitch/roll coupling or tendency to overcontrol the pitch or roll axes was noted. The manual approaches were very interesting in that the Tu-144LL, though a back-sided air­plane on approach, was not difficult to control even with the high level of throttle friction present. The engine time con­stant appears reasonable. It was noted that a large pitching moment results from moderate or greater throttle inputs which can lead to overcontrolling the pitch axis if the speed is not tightly controlled and large throttle inputs are required. The full stop landing was not difficult with light braking required due to the decelerating effects of the drag parachutes. The flight terminated with the evaluation pilot taxiing the aircraft clear of the runway to the parking area. 16 tons of fuel remained.

All test points were accomplished, and several additional optional test points were completed since the flight remained ahead of the planned fuel burn. One additional approach was completed. The planned flight profile was matched very closely, and all flight objectives were achieved.54

Onboard recording was used to gather all of the data, because the flight profiles took the Tu-144LL far out of telemetry range. Subsequent to each flight, Tupolev would produce a data time history plot, including over a dozen measured parameters plotted on the vertical axis versus time on the horizontal axis. On one plot, the entire flight could quickly be viewed. From the plotted time histories, much additional data could be ascertained. By comparing fuel quantity expended versus time, for example, fuel flows could be determined. This contrasts with the meth­ods in NASA in which, with paper supplies not of concern, the practice

The Tupolev and NASA flightcrews after the completion of the last U. S. piloted evaluation flight, with Tu-144LL “Moscow" in the background. NASA.

is often to plot individual time histories. USPET members felt that this straightforward Tupolev method showed great merit.

Flight 23 was completed September 24, after several days of weather delays. Gordon Fullerton was the NASA evaluation pilot for this flight, which was very similar to flight 22. The only differences occurred at Mach 2, at which Fullerton simulated an engine failure at the beginning of descent from just over 10-mile altitude and in the landing pattern in which a clean pass was flown for a photographic opportunity, and two simulated engine failure approaches and an additional ILS approach were accomplished. All test objectives were achieved.

The USPET team was feted to a final postflight party and, jokingly, according to Professor Pukhov, was not allowed to leave until a prelim­inary report was completed. The U. S. team completed the report and departed September 26, with a mutual exchange of best wishes with the Tupolev Tu-144 project staff. Four more Phase II flights were completed with the Tupolev crew to gather more handling qualities data and data for the other six experiments. After Sergei Borisov shut down the engines following the last flight in winter 1999, the Tu-144 never flew again.

NASA TM-2000-209850 thoroughly describes the operational qual­ities of the Tu-144LL. A brief description will be presented here. The Tu-144 taxied much like a Boeing 747 with mild cockpit accelerations and nominal cockpit overshoots while turning. Throttle friction was extremely high because of the rerouted throttle cables for the retrofit­ted NK-321 engines. The engines had operational limits and restrictions, some peculiar to a specific engine, but they performed well through­out the flight envelope, were robust and forgiving at Mach 2 cruise, and responded well in the landing pattern. Takeoff acceleration was very rapid, and the takeoff speeds were quite high, as expected with unstick occurring at 220 mph after 30 seconds of ground roll. A very high ambi­ent noise level and moderate buffet were experienced, with the nose drooped to the 11-degree takeoff position and the canard extended. With the nose retracted, the forward view was blocked, and the view through the somewhat distorted and crazed side windows was poor. Because the rate dampers were required to be engaged at all times, the unaugmented characteristics of the aircraft were not investigated. Pitch forces were moderately heavy, and small pitch inputs resulted in significant longi­tudinal motion, creating a tendency to overcontrol the pitch axis. The lateral forces were high, and large displacements were necessary for small roll rates, resulting in poor pitch-roll harmony. Roll inputs would
often couple into undesired pitch inputs. With poor pitch cues because of the visibility issues mentioned earlier, the pilot relied on the Sensitive Pitch Angle Indicator for pitch control. The pitch axis was the high workload axis, and this was exacerbated by the rapid center-of-gravity changes because of fuel transfer balancing in the transonic range. Roll response was very well-damped, with no proverse or adverse yaw, even with large lateral inputs. Precise bank angle captures were easy to accomplish. The aircraft demonstrated positive speed stability. Rudder inputs produced a positive dihedral effect and were well-damped/ deadbeat, but rudder pedal forces were very high. Full pedal deflection required 250-300 pounds of force. All of these characteristics were invariant with speed and configuration, except for the slightly degraded handling qualities near Mach 1. With the exception of the heavy con­trol forces (typical of Russian airplanes), the Tu-144 possessed adequate to desirable handling qualities. This result disputed the data taken in Phase I and led engineers to uncover the artificial 0.25-second time delay in the Damien DAS that produced such questionable handling qualities data.

During that same month, the findings were released of what the FAAs offi­cial historical record details as its first joint research project with NASA.[188]

A year earlier, during May and June 1964, two series of flight tests were conducted using FAA aircraft with NASA pilots to study the haz­ards of light to moderate air turbulence to jet aircraft from several per­spectives. The effort was called Project Taper, short for Turbulent Air Pilot Environment Research.[189] In conjunction with ground-based wind tunnel runs and early use of simulator programs, FAA Convair 880 and

Boeing 720 airliners were flown to define the handling qualities of air­craft as they encountered turbulence and determine the best methods for the pilot to recover from the upset. Another part of the study was to determine how turbulence upset the pilots themselves and if any changes to cockpit displays or controls would be helpful. Results of the project presented at a 1965 NASA Conference on Aircraft Operating Problems indicated that in terms of aircraft control, retrimming the stabilizer and deploying the spoilers were "valuable tools,” but if those devices were to be safely used, an accurate g-meter should be added to the cockpit to assist the pilot in applying the correct amount of control force. The pilots also observed that initially encountering turbulence often cre­ated such a jolt that it disrupted their ability to scan the instrument dials (which remained reliable despite the added vibrations) and rec­ommended improvements in their seat cushions and restraint system.[190]

But the true value of Project Taper to making safer skyways may have been the realization that although aircraft and pilots under con­trolled conditions and specialized training could safely penetrate areas of turbulence—even if severe—the better course of action was to find ways to avoid the threat altogether. This required further research and improvements in turbulence detection and forecasting, along with the ability to integrate that data in a timely manner to the ATC system and cockpit instrumentation.[191]

Another NASA air traffic simulation tool, the Future ATM Concepts Evaluation Tool (FACET), was created to allow researchers to explore, develop, and evaluate advanced traffic control concepts. The system can operate in several modes: playback, simulation, live, or in a sort of hybrid mode that connects it with the FAAs Enhanced Traffic Management System (ETMS). ETMS is an operational FAA program that monitors and reacts to air traffic congestion, and it can also predict when and where conges­tion might happen. (The ETMS is responsible, for example, for keeping a plane grounded in Orlando because of traffic congestion in Atlanta.) Streaming the ETMS live data into a run of FACET makes the simula­tion of a new advanced traffic control concept more accurate. Moreover, FACET is able to model airspace operations on a national level, processing the movements of more than 5,000 aircraft on a single desktop computer, taking into account aircraft performance, weather, and other variables.[257]

Some of the advanced concepts tested in FACET include allowing aircraft to have greater freedom in maintaining separation on their own,[258] integrating space launch vehicle and aircraft operations into the

airspace, and monitoring how efficiently aircraft comply with ATC instructions when their flights are rerouted.[259] In fact, the last of these concepts was so successful that it was deployed into the FAA’s operational ETMS. NASA reports that the success of FACET has lead to its use as a simulation tool not only with the FAA, but also with sev­eral airlines, universities, and private companies. For example, Flight Dimensions International—the world’s leading vendor of aircraft sit­uational displays—recently integrated FACET with its already popu­lar Flight Explorer product. FACET won NASA’s 2006 Software of the Year Award.[260]