Category NASA’S CONTRIBUTIONS TO AERONAUTICS

Much of NASA’s aerospace research overlaps various fields. For exam­ple, improving EMP tolerance of space-based systems involves studying plasma interactions in a high-voltage system operated in the ionosphere. But a related subject is establishing design practices that may have pre­viously increased adverse plasma interactions and recommending means of eliminating or mitigating such reactions in future platforms.

Standards for lightning protection tests were developed in the 1950s, under FAA and Department of Defense (DOD) auspices. Those studies mainly addressed electrical bonding of aircraft components and protec­tion of fuel systems. However, in the next decade, dramatic events such as the in-flight destruction of a Boeing 707 and the triggered responses of Apollo 12 clearly demonstrated the need for greater research. With advent of the Space Shuttle, NASA required further means of lightning protection, a process that began in the 1970s and continued well beyond the Shuttle’s inaugural flight, in 1981.

Greater interagency cooperation led to new research programs in the 1980s involving NASA, the Air Force, the FAA, and the government of France. The goal was to develop a lightning-protection design phi­losophy, which in turn required standards and guidelines for various aerospace vehicles.

NASA’s approach to lightning research has emphasized detection and avoidance, predicated on minimizing the risk of strikes, but then, if strikes occur nevertheless, ameliorating their damaging effects. Because early detection enhances avoidance, the two approaches work hand in glove. Translating those related philosophies into research and thence to design practices contains obvious benefits. The relationship between lightning research and protective design was noted by researchers for Lightning Technologies, Inc., in evaluating lightning protection for digi­tal engine control systems. They emphasized, "The coordination between the airframe manufacturer and system supplies in this process is fun­damental to adequate protection.”[176] Because it is usually impractical to perform full-threat tests on fully configured aircraft, lightning protec­tion depends upon accurate simulation using complete aircraft with full systems aboard. NASA, and other Federal agencies and military services, has undertaken such studies, dating to its work on the F-8 DFBW test­bed of the early 1970s, as discussed subsequently.

In their Storm Hazards Research Program (SHRP) from 1980 to 1986, Langley researchers found that multiple lightning strikes inject random electric currents into an airframe, causing rapidly changing magnetic fields that can lead to erroneous responses, faulty commands, or other "upsets” in electronic systems. In 1987, the FAA (and other nations’ avi­ation authorities) required that aircraft electronic systems perform­ing flight-critical functions be protected from multiple-burst lightning.

At least from the 1970s, NASA recognized that vacuum tube electron­ics were inherently more resistant to lightning-induced voltage surges than were solid-state avionics. (The same was true for EMP effects. When researchers in the late 1970s were able to examine the avionics of the Soviet MiG-25 Foxbat, after defection of a Foxbat pilot to Japan, they were surprised to discover that much of its avionics were tube-based, clearly with EMP considerations in mind.) While new microcircuitry obviously was more vulnerable to upset or damage, many new-generation aircraft would have critical electronic systems such as fly-by-wire control systems.

Therefore, lightning represented a serious potential hazard to safety of flight for aircraft employing first-generation electronic flight control architectures and systems. A partial solution was redundancy of flight controls and other airborne systems, but in 1978, there were few if any standards addressing indirect effects of lightning. That time, however, was one of intensive interest in electronic flight controls. New fly-by-wire aircraft such as the F-16 were on the verge of entering squadron service. Even more radical designs—notably highly unstable early stealth aircraft such as the Lockheed XST Have Blue testbed, the Northrop Tacit Blue, the Lockheed F-117, and the NASA-Rockwell Space Shuttle orbiter— were either already flying or well underway down the development path.

NASA’s digital fly-by-wire (DFBW) F-8C Crusader afforded a ready means of evaluating lightning-induced voltages, via ground simulation and evaluation of electrodynamic effects upon its flight control computer. Dryden’s subsequent research represented the first experimental investi­gation of lightning-induced effects on any FBW system, digital or analog.

A summary concluded:

Results are significant, both for this particular aircraft and for future generations of aircraft and other aero­space vehicles such as the Space Shuttle, which will employ digital FBW FCSs. Particular conclusions are: Equipment bays in a typical metallic airframe are poorly shielded and permit substantial voltages to be induced in unshielded electrical cabling. Lightning-induced volt­ages in a typical a/c cabling system pose a serious haz­ard to modern electronics, and positive steps must be taken to minimize the impact of these voltages on sys­tem operation. Induced voltages of similar magnitudes will appear simultaneously in all channels of a redun­dant system. A single-point ground does not eliminate lightning-induced voltages. It reduces the amount of diffusion-flux induced and structural IR voltage but per­mits significant aperture-flux induced voltages. Cable shielding, surge suppression, grounding and interface modifications offer means of protection, but successful design will require a coordinated sharing of responsibil­ity among those who design the interconnecting cabling and those who design the electronics. A set of transient control levels for system cabling and transient design levels for electronics, separated by a margin of safety, should be established as design criteria.[177]

The F-8 DFBW program is the subject of a companion study on electronic flight controls and so is not treated in greater detail here. In brief, a Navy Ling-Temco-Vought F-8 Crusader jet fighter was modi­fied with a digital electronic flight control system and test-flown at the NASA Flight Research Center (later the NASA Dryden Flight Research Center). When the F-8 DFBW program ended in 1985, it had made 210 flights, with direct benefits to aircraft as varied as the F-16, the F/A-18, the Boeing 777, and the Space Shuttle. It constituted an excellent exam­ple of how NASA research can prove and refine design concepts, which are then translated into design practice.[178]

The versatile F-106B program also yielded useful information on protection of digital computers and other airborne systems that trans­lated into later design concepts. As NASA engineer-historian Joseph Chambers subsequently wrote: "These findings are now reflected in lightning environment and test standards used to verify adequacy of protection for electrical and avionics systems against lightning hazards. They are also used to demonstrate compliance with regulations issued by airworthiness certifying authorities worldwide that require lightning strikes not adversely affect the aircraft systems performing critical and essential functions.”[179]

Similarly, NASA experience at lightning-prone Florida launch sites provided an obvious basis for identifying and implementing design practices for future use. A 1999 lessons-learned study identified design considerations for lightning-strike survivability. Seeking to avoid nat­ural or triggered lightning in future launches, NASA sought improve­ments in electromagnetic compatibility (EMC) for launch sites used by the Shuttle and other launch systems. They included proper grounding of vehicle and ground-support equipment, bonding requirements, and circuit protection. Those aims were achieved mainly via wire shielding and transient limiters.

In conclusion, it is difficult to improve upon D. L. Johnson and W. W. Vaughn’s blunt assessment that "Lightning protection assessment and design consideration are critical functions in the design and develop­ment of an aerospace vehicle. The project’s engineer responsible for lightning must be involved in preliminary design and remain an inte­gral member of the design and development team throughout vehi­cle construction and verification tests.”[180] This lesson is applicable to many aerospace technical disciplines and reflects the decades of experience embedded within NASA and its predecessor, the NACA, involving high-technology (and often high-risk) research, testing, and evaluation. Lightning will continue to draw the interest of the Agency’s researchers, for there is still much that remains to be learned about this beautiful and inherently dangerous electrodynamic phenomenon and its interactions with those who fly.

Recommended Additional Reading

In 2006, NASA’s Aeronautics Research Mission Directorate (ARMD) was reorganized. As a result, the projects that fell under ARMD’s Aviation Safety Program were restructured as well, with more of a focus on

aircraft safety than on the skies they fly through. Air traffic improvements in the new plan now fall almost exclusively within the Airspace Systems Program. The Aviation Safety Program is now dedicated to developing the principles, guidelines, concepts, tools, methods, and technologies to address four project areas: the Integrated Vehicle Health Management Project,[245] the Integrated Intelligent Flight Deck Technologies Project,[246] the Integrated Resilient Aircraft Control Project,[247] and the Aircraft Aging and Durability Project.[248]

The consideration of human factors in technology has existed since the first man shaped a wooden spear with a sharp rock to help him grasp it more firmly. It therefore stands to reason that the dimension of human factors has changed over time with advancing technology—a trend that has accelerated throughout the 20th century and into the current one.[296]

Man’s earliest requirements for using his primitive tools and weapons gave way during the Industrial Revolution to more refined needs in oper­ating more complicated tools and machines. During this period, the emer­gence of more complex machinery necessitated increased consideration of the needs of the humans who were to operate this machinery—even

if it was nothing more complicated than providing a place for the oper­ator to sit, or a handle or step to help this person access instruments and controls. In the years after the Industrial Revolution, human fac­tors concerns became increasingly important.[297]

As is apparent from the foregoing discussions, a recurring theme in NASA’s human factors research has been its dedication to improving aviation safety. The Agency’s many human factors research initiatives have contributed to such safety issues as crash survival, weather knowl­edge and information, improved cockpit systems and displays, security, management of air traffic, and aircraft control.[402]

Though NASA’s involvement with aviation safety has been an impor­tant focus of its research activities since its earliest days, this involve­ment was formalized in 1997. In response to a report by the White House Commission on Aviation Safety and Security, NASA created its Aviation Safety Program (AvSP).[403] As NASA’s primary safety program, AvSP dedi­cated itself and $500 million to researching and developing technologies that would reduce the fatal aircraft accident rate 80 percent by 2007.[404]

In pursuit of this goal, NASA researchers at Langley, Ames, Dryden, and Glenn Research Centers teamed with the FAA, DOD, the aviation industry, and various aviation employee groups—including the Air Line Pilots Association (ALPA), Allied Pilots Association (APA), Air Transport Association (ATA), and National Air Traffic Controllers Association

(NATCA)—to form the Commercial Aviation Safety Team (CAST) in 1998. The purpose of this all-inclusive consortium was to develop an integrated and data-driven strategy to make commercial aviation safer.[405]

As highlighted by the White House Commission report, statistics had shown that the overwhelming majority of the aviation accidents and fatalities in previous years had been caused by human error—specifically, loss of control in flight and so-called controlled flight into terrain (CFIT).[406] NASA—along with the FAA, DOD, the aviation industry, and human factors experts—had previously formed a National Aviation Human Factors Plan to develop strategies to decrease these human- caused mishaps.[407] Consequently, NASA joined with the FAA and DOD to further develop a human performance research plan, based on the NASA-FAA publication Toward a Safer 21st Century—Aviation Safety Research Baseline and Future Challenges.[408] The new AvSP thus incor­porated many of the existing human factors initiatives, such as crew fatigue, resource management, and training. Human factors concerns were also emphasized by the program’s focus on developing more sophis­ticated human-assisting aviation technology.

To accomplish its goals, AvSP focused not only on preventing accidents, but also minimizing injuries and loss of life when they did occur. The program also emphasized collection of data to find and address problems. The comprehensive nature of AvSP is beyond the scope of this case study, but some aspects of the program (which, in 2005, became the Aviation Safety & Security Program, or AvSSP) with the greatest human factors implications include accident mitiga­tion, synthetic vision systems, system wide accident prevention, and aviation system monitoring and modeling.[409]

• Accident mitigation: The goal of this research is to find ways to make accidents more survivable to aircraft

occupants. This includes a range of activities, some of which have been discussed, to include impact tests, in­flight and postimpact fire prevention studies, improved restraint systems, and the creation of airframes better able to withstand crashes.

• Synthetic vision systems: Unrestricted vision is vital for a pilot’s situational awareness and essential for him to control his aircraft safely. Limited visibility contributes to more fatal air accidents than any other single factor; since 1990, more than 1,750 deaths have been attrib­uted to CFIT—crashing into the ground—not to men­tion numerous runway incursion accidents that have taken even more lives.[410]

• The traditional approach to this problem has been the development of sensor-based enhanced vision systems to improve pilot awareness. In 2000, however, NASA Langley researchers initiated a different approach. They began developing cockpit displays, termed Synthetic Vision Systems, which incorporate such technologies as Global Positioning System (GPS) and photo-realistic terrain databases to allow pilots to "see” a synthetically derived 3-D digital reproduction of what is outside the cockpit, regardless of the meteorological visibility. Even in zero visibility, these systems allow pilots to synthet­ically visualize runways and ground obstacles in their path. At the same time, this reduces their workload and decreases the disorientation they experience during low – visibility flying. Such systems would be useful in avoid­ing CFIT crashes, loss of aircraft control, and approach and landing errors that can occur amid low visibility.[411] Such technology could also be of use in decreasing the risk of runway incursions. For example, the Taxiway

Navigation and Situation Awareness System (T-NASA) was developed to help pilots taxiing in conditions of decreased visibility to "see” what is in front of them. This system allows them to visualize the runway by present­ing them with a head-up display (HUD) of a computer­generated representation of the taxi route ahead of them.[412]

• One of the most important synthetic vision sys­tems initiatives arose from the Advanced General Aviation Transport Experiments (AGATE) program, which NASA formed in the mid-1990s to help revi­talize the lagging general-aviation industry. NASA joined with the FAA and some 80 industry mem­bers, in part to develop an affordable Highway in the Sky (HITS) cockpit display that would enhance safety and pilot situational awareness. In 2000, such a system was installed and demonstrated in a small production aircraft.[413] Today, nearly every aviation manufacturer has a Synthetic Vision System either in use or in the planning stages.[414]

• System wide accident prevention: This research, which focuses on the human causes of accidents, is involved with improving the training of aviation professionals and in developing models that would help predict human error before it occurs. Many of the programs address­ing this issue were discussed earlier in greater detail.[415]

• Aviation system monitoring and modeling (ASMM) proj­ect: This program, which was in existence from 1999 to 2005, involved helping personnel in the aviation indus­try to preemptively identify aviation system risk. This included using data collection and improved monitoring of equipment to predict problems before they occur.[416] One important element of the ASMM project is the Aviation Performance Measuring System (APMS).[417] In 1995, NASA and the FAA coordinated with the airlines to develop this program, which utilizes large amounts of information taken from flight data recorders to improve flight safety. The techniques developed are designed to use the data collected to formulate a situational aware­ness feedback process that improves flight performance and safety.[418]

• Yet another spinoff of ASMM is the National Aviation Operational Monitoring Service (NAOMS). This system­wide survey mechanism serves to quantitatively assess the safety of the National Airspace System and evaluate the effects of technologies and procedures introduced into the system. It uses input from pilots, controllers, mechanics, technicians, and flight attendants. NAOMS therefore serves to assess flight safety risks and the effec­tiveness of initiatives to decrease these risks.[419] APMS impacts air carrier operations by making routine mon­itoring of flight data possible, which in turn can allow evaluators to identify risks and develop changes that will improve quality and safety of air operations.[420]

• A similar program originating from ASMM is the Performance Data Analysis and Report and System (PDARS). This joint FAA-NASA initiative provides a

way to monitor daily operations in the NAS and to eval­uate the effectiveness of air traffic control (ATC) services. This innovative system, which provides daily analysis of huge volumes of real-time information, including radar flight tracks, has been instituted throughout the conti­nental U. S.136

The highly successful AvSP ended in 2005, when it became the Aviation Safety & Security Program. AvSSP exceeded its target goal of reducing air­craft fatalities 80 percent by 2007. In 2008, NASA shared with the other members of CAST the prestigious Robert J. Collier Trophy for its role in helping produce "the safest commercial aviation system in the world.”137 AvSSP continues to move forward with its goal of identifying and develop­ing by 2016 "tools, methods, and technologies for improving overall air­craft safety of new and legacy vehicles operating in the Next Generation Air Transportation System.”138 NASA estimates that the combined efforts of the ongoing safety-oriented programs it has initiated or in which it has participated will decrease general-aviation fatalities by as much as another 90 percent from today’s levels over the next 10-15 years.139

Spurred on by postwar interests in the variable-wing-sweep concept as a means to optimize mission performance at both low and high speeds, the NACA at Langley initiated a broad research program to identify the potential benefits and problems associated with the concept. The disap­pointing experiences of the Bell X-5 research aircraft, which used a sin­gle wing pivot to achieve variable sweep in the early 1950s, had clearly identified the unacceptable weight penalties associated with the con­cept of translating the wing along the fuselage centerline to maintain satisfactory levels of longitudinal stability while the wing sweep angle was varied from forward to aft sweep. After the X-5 experience, military interest in variable sweep quickly diminished while aerodynamicists at

Langley continued to explore alternate concepts that might permit vari­ations in wing sweep without moving the wing pivot location and with­out serious degradation in longitudinal stability and control.

After years of intense research and wind tunnel testing, Langley researchers conceived a promising concept known as the outboard pivot.[486] The basic principle involved in the NASA solution was to pivot the mov­able wing panels at two outboard pivot locations on a fixed inner wing and share the lift between the fixed portion of the wing and the movable outer wing panel, thereby minimizing the longitudinal movement of the aerodynamic center of lift for various flight speeds. As the concept was matured in configuration studies and supporting tests, refined designs were continually submitted to intense evaluations in tunnels across the speed range from supersonic cruise conditions to subsonic takeoff and landing.[487]

The use of dynamically scaled free-flight models to evaluate the sta­bility and control characteristics of variable-sweep configurations was an ideal application of the testing technique. Since variable-sweep designs are capable of an infinite number of wing sweep angles between the for­ward and aft positions, the number of conventional wind tunnel force tests required to completely document stability and control variations with wing sweep for every sweep angle could quickly become unacceptable. In contrast, a free-flight model with continually variable wing sweep angles could be used to quickly examine qualitative characteristics as its geome­try changed, resulting in rapid identification of significant problems. Free – flight model investigations of a configuration based on a proposed Navy combat air patrol (CAP) mission in the Full-Scale Tunnel provided a con­vincing demonstration that the outboard pivot was ready for applications.

The oblique wing concept (sometimes referred to as the "switch­blade wing” or "skewed wing”) had originated in the German design studies of the Blohm & Voss P202 jet aircraft during World War II and was pursued at Langley by R. T. Jones. Oblique wing designs use a single­pivot, all-moving wing to achieve variable sweep in an asymmetrical fashion. The wing is positioned in the conventional unswept position for takeoff and landings, and it is rotated about its single pivot point for high-speed flight. As part of a general research effort that included

theoretical aerodynamic studies and conventional wind tunnel tests, a free-flight investigation of the dynamic stability and control of a sim­plified model was conducted in the Free-Flight Tunnel in 1946.[488] This research on the asymmetric swept wing actually predated NACA wind tunnel research on symmetrical variable sweep concepts with a research model of the Bell X-1.[489] The test objectives were to determine whether such a radical aircraft configuration would exhibit satisfactory stability characteristics and remain controllable in the swept wing asymmetric state at low-speed flight conditions. The results of the flight tests, which were the first U. S. flight studies of oblique wings ever conducted, showed that the wing could be swept as much as 40 degrees without significant degradation in behavior. However, when the sweep angle was increased to 60 degrees, an unacceptable longitudinal trim change was experienced, and a severe reduction in lateral control occurred at moderate and high angles of attack. Nonetheless, the results obtained with the simple free – flight model provided optimism that the unconventional oblique wing concept might be feasible from a perspective of stability and control.

R. T. Jones transferred to the NACA Ames Aeronautical Laboratory in 1947 and continued his brilliant career there, which included his continuing interest in the application of oblique wing technology. In the early 1970s, the scope of NASA studies on potential civil supersonic transport configurations included an effort by an Ames team headed by Jones that examined a possible oblique wing version of the super­sonic transport. Although wind tunnel testing was conducted at Ames, the demise and cancellation of the American SST program in the early 1970s terminated this activity. Wind tunnel and computational studies of oblique wing designs continued at Ames throughout the 1970s for subsonic, transonic, and supersonic flight applications.[490] Jones stim­ulated and participated in flight tests of several oblique wing radio – controlled models, and a joint Ames-Dryden project was initiated to use a remotely piloted research aircraft known as the Oblique Wing Research Aircraft (OWRA) for studies of the aerodynamic characteris­tics and control requirements to achieve satisfactory handling qualities.

Growing interest in the oblique wing and the success of the OWRA remotely piloted vehicle project led to the design and low-speed flight demonstrations of a full-scale research aircraft known as the AD-1 in the late 1970s. Designed as a low-cost demonstrator, the radical AD-1 proved to be a showstopper during air shows and generated consider­able public interest.[491] The flight characteristics of the AD-1 were quite satisfactory for wing-sweep angles of less than about 45 degrees, but the handling qualities degraded for higher values of sweep, in agreement with the earlier Langley exploratory free-flight model study.

After his retirement, Jones continued his interest in supersonic oblique wing transport configurations. When the NASA High-Speed Research program to develop technologies necessary for a viable super­sonic transport began in the 1990s, several industry teams revisited the oblique wing for potential applications. Ames sponsored free-flight radio – controlled model studies of oblique wing configurations at Stanford University in the early 1990s. As a result of free-flight model contribu­tions from Langley, Ames, Dryden, and academia, major issues regarding potential dynamic stability and control problems for oblique wing con­figurations have been addressed for low-speed conditions. Unfortunately, funding for transonic and supersonic model flight studies has not been forthcoming, and high-speed studies have not yet been accomplished.

Efforts by the NACA and NASA over the last 80 years with applications of free-flying dynamic model test techniques have resulted in signifi­cant contributions to the civil and military aerospace communities. The results of the investigations have documented the testing techniques and lessons learned, and they have been especially valuable in defining critical characteristics of radical new configurations. With the passing of each decade, the free-flight techniques have become more sophis­ticated, and the accumulation of correlation between model and full – scale results has rapidly increased. In view of this technical progress, it

is appropriate to reflect on the state of the art in free-flight technology and the challenges and opportunities of the future.

NASA has always justified its existence by making itself available for outside research. In an effort to advertise the services and capabilities of Langley’s wind tunnels, NASA published the technical memorandum, "Characteristics of Major Active Wind Tunnels at the Langley Research Center,” by William T. Shaefer, Jr., in July 1965. Unlike the NACA’s goal of assisting industry through the use of its pioneering wind tunnels at a time when there were few facilities to rely upon, NASA’s wind tunnels first and foremost met the needs of the Agency’s fundamental research and development. Secondary to that priority were projects that were important to other Government agencies. Two specific committees han­dled U. S. Army, Navy, and Air Force requests concerning aircraft and missiles and propulsion projects. Finally, the aerospace industry had access to NASA facilities, primarily the Unitary Plan Wind Tunnels, on a fee basis for the evaluation of proprietary designs. No NASA wind tun­nel was to be used for testing that could be done at a commercial facil­ity, and all projects had to be "clearly in the national interest.”[625]

NASA continued to "sell” its tunnels on through the following decades. In 1992, the Agency confidently announced:

NASA’s wind tunnels are a national technological resource. They have provided vast knowledge that has contributed to the development and advancement of the nation’s aviation industry, space program, economy and the national security. Amid today’s increasingly fierce international, commercial and technological competi­tion, NASA’s wind tunnels are crucial tools for helping the United States retain its global leadership in aviation and space flight.[626]

According to this rhetoric, NASA’s wind tunnels were central to the continued leadership of the United States in aerospace.

As part of the selling of the tunnels, NASA initiated the Technology Opportunities Showcase (TOPS) in the early 1990s. The program distrib­uted to the aerospace industry a catalog of available facilities similar to a real estate sampler. A prospective user could check a box marked "Please Send More Information” or "Would Like To Discuss Facility Usage” as part of the process. NASA wind tunnels were used on a space-available basis. If the research was of interest to NASA, there would be no facility charge, and the Agency would publish the results. If a manufacturing concern had a proprietary interest and the client did not want the test results to be public, then it had to bear all costs, primarily the use of the facility.[627]

The TOPS evolved into the NASA Aeronautics Test Program (ATP) in the early 21st century to include all four Research Centers at Langley, Ames, Glenn, and Dryden.[628] The ATP offered Government, corpora­tions, and institutions the opportunity to contract 14 facilities, which included a "nationwide team of highly trained and certified staff, whose backgrounds and education encompass every aspect of aerospace test­ing and engineering,” for a "wide range” of experimental test services that reflected "sixty years of unmatched aerospace test history.” The ATP

and, by extension, NASA maintained that they could provide clients test results of "unparalleled superiority.”[629]

The 1970s inauguration of widebody jumbo jets posed special prob­lems for smaller aircraft because of the powerful streaming wake vor­tices generated by aircraft such as the Boeing 747, Douglas DC-10, and Lockheed L-1011. After several unexplained accidents caused by aircraft
upset, and urged by organizations such as the Flight Safety Foundation and the Aircraft Owners and Pilots Association, the Federal Aviation Administration (FAA) asked NASA and the U. S. Air Force to initiate a flight-test program to evaluate the effect of the wingtip vortex wake gen­erated by large jet transport airplanes on a variety of smaller airplanes. The program began in December 1969 and, though initially ended in April 1970, was subsequently expanded and continued over the next decade. Operations were performed at Edwards Air Force Base, CA, under the supervision of the NASA Flight Research Center in cooperation with the Ames Research Center and the U. S. Air Force, using a range of research aircraft including 747, 727, and L-1011 airliners, and smaller test sub­jects such as the T-37 trainer and QF-86 drones, supported by extensive wind tunnel and water channel research.[848]

Subsequently, in 1972, NASA intensified its wake vortex research to seek reducing vortex formation via aerodynamic modification and addition of wind devices. By the beginning of 1974, Alfred Gessow, the Chief of Fluid and Flight Dynamics at NASA Headquarters, announced the Agency was optimistic that wake vortex could be eliminated "as a constraint to airport operations by new aerodynamic designs or by ret­rofit modifications to large transport aircraft.”[849] Overall, the tests, and ones that followed, had clearly demonstrated the power of wake vortices to constrain the operations GA aircraft; light jet trainers and business aircraft such as the Lear Jet were buffeted and rolled, and researchers found that the vortices maintained significant strength up to 10 miles behind a widebody. As a result of NASA’s studies, the FAA introduced a requirement for wake turbulence awareness training for all pilots, increased separation distances between aircraft, and mandated verbal warnings to pilots during the landing approach at control-towered air­ports when appropriate. NASA has continued its wake turbulence studies since that time, adding further to the understanding of this fascinating, if potentially dangerous, phenomenon.[850]

Low-cost RPRVs have contributed to the development of hypersonic vehi­cle concepts and advanced cruise-missile technology. The first such proj­ect undertaken at Dryden originated with the Sandia Winged Energetic Reentry Vehicle (SWERVE).

Sandia National Laboratories developed the SWERVE under an exploratory tactical nuclear weapon program. With a slender cone-shaped body and small triangular fins that provided steering, the SWERVE was capable of maneuvering in the range from Mach 2 to Mach 14. Several flight tests in the late 1970s and early 1980s demonstrated maneuver­ability at high speeds and high angles of attack. Three SWERVE vehi­cles of two sizes were lofted to altitudes of 400,00 to 600,000 feet on a Strypi rocket and reentered over the Pacific Ocean. The SWERVE 3 test in 1985 included a level flight-profile segment to extend the vehicle’s range. Because technologies demonstrated on SWERVE were applica­ble to development of such hypersonic vehicles as the proposed X-30 National Aero-Space Plane (NASP), Sandia offered to make a SWERVE – derived vehicle available to defense contractors and Government agen­cies for use as a hypersonic testbed.[972] During the early 1980s, NASA’s Office of Aeronautics and Space Technology (OAST) began studying technologies that would enable development of efficient hypersonic aircraft and aerospace vehicles. As part of the program, OAST officials explored the possibility of a joint NASA-Sandia flight program using a SWERVE-derived vehicle to provide hypersonic entry and flight data. Planners wanted to use the capabilities of both NASA and Sandia to refine the existing SWERVE configuration to enable data measurement in specific flight regimes of interest to NASA engineers.[973] The SWERVE shape was optimized for hypersonic performance, but for a transatmo­spheric vehicle to be practical, it had to be capable of subsonic opera­tion during the approach and landing phases of flight. In 1986, Sandia and NASA officials agreed to participate in a joint project involving an unpowered, radio-controlled model called Avocet. Based on the SWERVE shape, the model retained the slender conical fuselage but featured the addition of narrow-span delta wings. It was approximately 9 feet long and weighed about 85 pounds, including instrumentation. For flight tests, the Avocet vehicle was dropped from a Piper PA-18-150 Super Cub owned by Larry G. Barrett of Tehachapi, CA. The test plan called for 30 to 40 flights to collect data on low-speed performance, handling qualities, and stability and control characteristics.[974] Dryden engineers Henry Arnaiz and Robert Baron managed the Avocet project. R. Dale Reed worked with Dan Garrabrant and Ralph Sawyer to design and build the model. Principal investigators included Ken Iliff, Alex Sim, and Al Bowers. Larry Schilling developed a simulation for pilot training. James B. Craft, Jr., and William Albrecht served as systems and operations engineers, respec­tively. Robert Kempel and Bruce Powers developed the flight control sys­tem. Eloy Fuentes provided safety and quality assurance. Ed Schneider served as primary project pilot, with Einar Enevoldson as backup.[975] All tests were conducted at the China Lake Naval Weapons Center, about 40 miles northeast of Edwards. The model was carried to an altitude of about

8,0 feet beneath the wing of the Super Cub and released above a small dry lakebed. Schneider piloted the vehicle from a ground station, using visual information from an onboard television camera. After accomplishing all test points on the flight plan, Schneider deployed a parachute to bring the vehicle gently to Earth. Testing began in spring 1986 and concluded November 2. Results indicated the configuration had an extremely low lift-to-drag ratio, probably unacceptable for the planned National Aero­Space Plane then being considered in beginning development studies.[976] In 1988, Sandia officials proposed a follow-on project to study the Avocet configuration’s cruise and landing characteristics. Primary objectives included demonstration of powered flight and landing characteristics, determination of the long-range cruise capabilities of a SWERVE-type vehicle, and the use of Avocet flight data to determine the feasibility of maneuvering and landing such a vehicle following a hypersonic research flight. The new vehicle, called Avocet II, was a lightweight, radio – controlled model weighing just 20 pounds. Significant weight reduction was made possible, in part, through the use of an advanced miniature instrumentation system weighing 3 pounds—one-tenth the weight of the instrumentation used in Avocet I. Powered by two ducted-fan engines, the Avocet II was capable of taking off and landing under its own power.

NASA Dryden officials saw several potential benefits to the projects. First was the opportunity to flight-test an advanced hypersonic config­uration that had potential research and military applications. Second, continued work with Sandia offered access to a wealth of hypersonic experience and quality information. Third, Avocet II expanded the NASA – Sandia SWERVE program that had become the heart of NASA’s Generic Hypersonic Program, a research project initiated at Dryden and managed by Dr. Isaiah Blankson at NASA Headquarters. Finally, the small-scale R/C model effort served as an excellent training project for young Dryden engineers and technicians. Moreover, total costs for vehicle, instrumen­tation, flight-test operations, miscellaneous equipment, data analysis, and travel were estimated to be $237,000, truly a bargain by aeronauti­cal research standards.[977] In 1989, a team of researchers at Dryden began work on Avocet II under the direction of Robert Baron. Many of the orig­inal team members were back, including William Albrecht, Henry Arnaiz, R. Dale Reed, Alex Sim, Eloy Fuentes, and Al Bowers. They were joined by engineers Gerald Budd, Mark Collard, James Murray, Greg Noffz, and James Yamanaka. Charles Baker provided additional project man­agement oversight. Others included ground pilot Ronald Gilman, crew chief David Neufeld, model builder Robert Violett, and instrumentation engineer Phil Hamory. James Akkerman built and supplied twin ducted – fan engines for the model.[978] For flight operations, the team traveled to the remote test site in a travel trailer equipped with all tools and supplies necessary for onsite maintenance and repair of the model. After setting up camp on the edge of a dry lakebed, technicians unloaded, preflighted, and fueled the model. If the configuration had been changed since the pre­vious flight, an engineer performed a weight-and-balance survey prior to takeoff. When the crew chief was satisfied that the vehicle was ready, the flight-test engineer reviewed all pertinent test cards to ensure that each crewmember was aware of his responsibilities during each phase of flight. The ground pilot followed a structured sequence of events outlined in the test cards in order to optimize the time available for research maneuvers.

Typically, the pilot flew a figure-eight ground track that produced the longest-possible steady, straight-line flight segment between turns at each end of the test range. The ground pilot controlled the Avocet II using a commercially available nine-channel, digital pulse-code modu­lation radio-control system. Since loss of the vehicle was considered an acceptable risk, there was no redundant control system. Software per­mitted preprogrammed mixing of several different control functions, greatly simplifying vehicle operation. After landing, recorded test data were downloaded to a personal computer for later analysis.[979] Initial taxi tests revealed that the model lacked sufficient thrust to achieve takeoff. Modifications to the inlet solved the problem, but the model had a very low lift-to-drag ratio, which made it difficult to maneuver. The turn­ing radius was so large that it was nearly impossible to keep the model within visual range of the ground pilot, so the flight-test engineer pro­vided verbal cues regarding heading and attitude while observing the model through binoculars. The pilot executed each research maneu­ver several times to ensure data quality.[980] The first flight took place November 18, 1989, and lasted just 2 minutes. Ron Gilman lost sight of the model in the final moments of its steep descent, resulting in a hard landing. Over the course of 10 additional flights through February 1991, Gilman determined the vehicle’s handling qualities and longitudinal sta­bility, while engineers attempted to define local flow-interference areas using tufts and ground-based high-speed film.[981] The instrumentation system in the Avocet II vehicle, consisting of a Tattletale Model 4 data logger with 32 kilobytes of onboard memory, provided research-quality quantitative analysis data on such performance parameters as lift-curve slope, lift-to-drag ratio, and trim curve. An 11-channel, 10-bit analog – to-digital converter capable of operating at up to 600 samples per sec­ond measured analog signals. The 2.2-ounce device, measuring just 3.73 by 2.25 by 0.8 inches, also featured a 128-kilobyte memory expansion board to increase data-storage capability.

The pilot quantified aircraft performance by executing a quasistatic pushover/pull-up (POPU) maneuver. Properly executed, a single POPU maneuver could simultaneously characterize all three of the desired flight-test parameters over a wide angle-of-attack range. Structural vibra­tion at high-power settings—such as those necessary to execute a POPU maneuver—caused interference with onboard instrumentation. Attempts to use different mounting techniques and locations for both engines and accelerometers failed to alleviate the problem. Eventually, engineers developed a POPU maneuver that could be flown in a steep dive with the engines at an idle setting. In this condition, the accelerometers pro­vided usable data.[982] Researchers at Dryden teamed up with Sandia again for the Royal Amber Model (RAM) project, later renamed the Sandia Hybrid Inlet Research Program (SHIRP). This project included tests of subscale and full-scale radio-controlled models of an advanced cruise missile shape designed by Sandia under the Standoff Bomb Program. The goal of the SHIRP experiments was to provide flight-test data on an experimental inlet configuration for use in future weapons, such as the Joint Air-to-Surface Standoff Missile, then under development. Sandia engineers designed an engine inlet to be "stealthy”—not detect­able by radar—yet still capable of providing good performance charac­teristics such as a uniform airflow with no separation. Airflow exiting the inlet and entering the turbine had to be uniform as well. The design of the new inlet was complex. Instead of a standard rectangular chan­nel, the cross-sectional area of the inlet varied from a high aspect ratio V-shape at the front to an almost circular outlet at the back end.[983] Sandia funded Phase I flight tests of a 40-percent-scale RAM from August 1990 through August 1991. Because the project was classified at the time, flight operations could not take place at Dryden. Instead, the test team used secure range areas at Edwards Air Force Base North Base and China Lake Naval Weapons Center.[984] The first flight took place in August 1990 at China Lake. Typically, the model was released from the R/C mother ship at an altitude of about 600 feet. The ground pilot performed a series of gliding and turning maneuvers, followed by a controlled pullup prior to impact. Results from the first four flights indicated good longitudinal and directional stability and neutral lateral stability.

The next three flights took place in February 1991 at North Base, just a few miles northeast of Dryden. During the first of these, a recov­ery parachute deployed at 150 feet but came loose from the vehicle. The ground pilot made a horizontal landing on the runway centerline. On the next flight, the vehicle exhibited good controllability and stability in both pitch and yaw axes at airspeeds between 35 and 80 miles per hour (mph). The pilot elected to land on the runway rather than use the recovery parachute. The final 10 flights took place at China Lake, ending July 13, 1991.[985] During fall 1991 and early 1992, researchers proposed tasks and milestones for the second phase of testing, and in February 1992, RAM Phase II was reorganized as the unclassified SHIRP proj­ect. During spring 1992, however, conditions arose at both Sandia and Dryden that required modification of the proposed schedule.

In support of a Sandia initiative to conduct a prototype flight dem­onstration program, the stabilizing and lifting surfaces for the baseline Standoff Bomb were reevaluated based on the most recent wind tunnel data and taking into account the current mass properties and flight pro­files. This revised geometry was used for the definition of wind tunnel models to collect data on static aerodynamics, diffuser distortion, and total pressure loss. In order to use the revised definition for the SHIRP flight-test models, the schedule had to be compromised.[986] An initial flight-test series in December 1992 involved launching a subscale model called Mini-SHIRP from the R/C Mothership. The team also constructed two full-scale vehicles, each 14 feet long and weighing about 52 pounds. SHIRP-1 was uninstrumented, unpowered, and lacked inlets. SHIRP-2 featured the experimental inlet configuration and was pow­ered by two electric ducted-fan engines to extend the glide range and provide short periods of level flight (10-15 seconds). The ground pilot controlled the vehicle through a fail-safe pulse-code modulation radio­uplink system. The test vehicles were equipped with deployable wings and pneumatically deployable recovery parachutes. The two full-scale vehicles, tested in 1993, were launched from the modified Rans S-12 (also known as "Ye Better Duck”) remotely piloted ultralight aircraft.

Flight operations began with takeoff of the mother ship from North Base followed by launch and landing of the test article in the vicinity of Runway 23 on the northern part of Rogers Dry Lake. The SHIRP flights demonstrated satisfactory lateral, longitudinal, and directional static and dynamic stability. The vehicle had reasonable control authority, required only minimal rudder deflection, and had encouraging wing-stall char­acteristics.[987] NASA project personnel included Don Bacon, Jerry Budd, Bob Curry, Alex Sim, and Tony Whitmore. Contractors from PRC, Inc., included Dave Eichstedt, Ronald Gilman, R. Dale Reed, B. McCain, and Dave Richwine. Todd M. Sterk, Walt Rutledge, Walter Gutierrez, and Hank Fell of Sandia worked with NASA and PRC personnel to analyze and document the various test data. In a September 1992 memoran­dum, Gutierrez noted that Sandia personnel recognized the SHIRP effort as "an opportunity to learn from the vast flight-test experience avail­able at Dryden in the areas of experimental testing and data analysis.”

In acknowledging the excellent teaming opportunity for both Sandia and NASA, he added that, "Dryden has an outstanding rep­utation for parameter estimation of aerodynamic characteristics of flight-test vehicles.”[988]

A byproduct of this and other incidents was that Ship 1 was eventually limited to Mach 2.5 because of flight safety concerns of the skin shed­ding. But Ship 2 made its first flight July 17, 1965, and it had numerous improvements. Skin bonding had been improved, an automated air inlet control system had been installed, wing dihedral had been increased to 5 degrees to improve lateral directional stability, and fuel tank No. 5 could now be filled. NASA planned to use Ship 2 for its research program; an extensive instrumentation package recording over 1,000 parameters such as temperature, pressure, and accelerations was installed in the weapons bay for use when NASA took over the direction of the flight – test program. Ship 2 still had some of the gremlins that seemed to haunt the XB-70, mainly connected to the complex landing gear. Flight 37 on AV-2 resulted in the pilots having to do some in-flight maintenance when the nose gear door position prevented proper retraction or extension of the nose gear. The activity was widely advertised as the pilot using "a paperclip” to short an electrical circuit to allow exten­sion (actually, there were no paperclips on board; USAF pilot Joseph Cotton fashioned the device from a wire on his oxygen mask). But AV-2 showed that the high-speed skin-shedding problem had indeed been solved. Beginning in March 1966, AV-2 routinely spent 50 minutes to 1 hour at speeds from Mach 2.5 to Mach 2.9. And on May 19, AV-2 reached the (contractual) holy grail of 32 minutes at Mach 3 (actually up to 3.06). Skin stagnation temperature was over 600 °F. With accom­plishment of that goal, NASA moved to put a new pilot in the program.

NASA X-15 veteran test pilot Joe Walker had been undergoing delta wing training and preparation to fly the B-70 as the program moved to the second stage of flight test. National Sonic Boom Program (NSBP) tests were flown June 6, 1966, to prepare for the official change over to NASA on June 15, but on June 8, disaster struck, dramatically chang­ing the program.

That day, AV-2 took off on a planned flight-test mission that would include a photo session at the end of the sortie with a number of other aircraft powered by engines made by General Electric.[1083] One of the air­craft was a Lockheed F-104N Starfighter flown by Joe Walker, who was observing the mission as he prepared to fly the B-70 on the next sor­tie. During the photo shoot, which required close formation flight, his F-104 was seen to fly within 30-50 feet of the Valkyrie’s right wingtip, which had been lowered to the 20-degree intermediate droop position. As the photo session ended, the F-104 tail struck the XB-70 wingtip, causing the F-104 to roll violently to the left and pass inverted over the top of the bomber, shearing off most of the twin vertical tails and caus­ing the Starfighter to erupt in flames, killing Walker. The XB-70 subse­quently entered an inverted spin, from which recovery was impossible. Company test pilot Joe Cotton ejected using the complex encapsulated ejection seat and survived; USAF copilot Carl Cross did not eject and died in the ensuing crash. The accident was not related to the Valkyrie design itself; nevertheless, the loss of the improved Ship 2 and its com­prehensive instrumentation package meant that AV-1 would now have to become the NASA research aircraft. A new instrumentation package was installed in AV-1, but the Mach 2.5 speed limit imposed on AV-1 for the skin shedding problem and the workload-intensive manual inlets meant the program orientation could be less of an analog for the national SST program, which was now approaching the awarding of contracts for an SST with speeds of Mach 2.7 to 3.