Category NASA’S CONTRIBUTIONS TO AERONAUTICS

As the 1970s came to an end, the U. S. military fleet of high-performance fighter aircraft had been transformed from departure-prone designs to new configurations with outstanding stability and departure resistance at high angles of attack. Thanks to the national research and develop­ment efforts of industry and Government following the Dayton sympo­sium in 1971, the F-14, F-15, F-16, and F/A-18 demonstrated that the peril of high-angle-of-attack departure exhibited by the previous gener­ation of fighters was no longer a critical concern. Rather, the pilot could exploit high angles of attack under certain tactical conditions without fear of nose slice or pitch-up. At air shows and public demonstrations, the new "supermaneuverable” fighters wowed the crowds with high – angle-of-attack flybys, and more importantly, the high-alpha capabilities provided pilots with new options for air combat. High-angle-of-attack
technology had progressed from concerns over stall characteristics to demonstrated spin resistance and was moving into a focus on poststall agility and precision maneuverability.

Reflecting on the advances in high-angle-of-attack technology of the 1970s and concepts yet to be developed, technical managers at Langley, Dryden, and Ames began to advocate for a cohesive, integrated research program focused on technologies and innovative ideas. The Agency was in an excellent position to initiate such a program thanks to the unique ground – and flight-testing capabilities that had been developed and the expertise that had been gathered by interactions of the NASA researchers with the real-world challenges of specific aircraft programs. At Langley, for example, researchers had been intimately involved in high-angle-of – attack/departure/spin activities in the development of all the new fight­ers and had accumulated in-depth knowledge of the characteristics of the configurations, including aerodynamics, flight control architecture, and handling characteristics at high angles of attack. Technical exper­tise and facilities at Langley included subscale static and dynamic free – flight model wind tunnel testing, advanced control-law synthesis, and computational aerodynamics. In addition, extensive peer contacts had been made within industry teams and DOD aircraft development offices.

At Dryden, the world-class flight-test facilities and technical expertise for high-performance fighter aircraft had been continually demonstrated in highly successful flight-test programs in which potentially hazardous testing had been handled in a professional manner. The Dryden staff was famous for its can-do attitude and accomplishments, including the conception, development, and routine operation of experimental air­craft; advanced flight instrumentation; and data extraction techniques.

Meanwhile, at Ames, the aeronautical research staff had aggressively led developments in high-performance computing facilities and com­putational aerodynamics. Computational fluid dynamics (CFD) codes developed at Ames and Langley had shown powerful analysis capabil­ity during applications to traditional aerodynamic predictions such as cruise performance and the analysis of flow-field phenomena. In addi­tion to computational expertise, Ames had extensive wind tunnel facili­ties, including the huge 80-by 120-Foot Tunnel, which had the capability of testing a full-scale fighter aircraft as large as the F/A-18.

From the perspective of the three technical managers, the time was right to bring together the NASA capabilities into a focused program directed toward some of the more critical challenges in
high-angle-of-attack technology.[1308] The research program that evolved from the planning meetings grew into one of the more remarkable efforts ever undertaken by NASA. The planning, advocacy, and conduct of the program was initiated at the grassroots level and was managed in a most remarkable manner for the duration of the program. Within NASA’s aero­nautics activities, the program brought an enthusiastic environment of cooperation—not competition—that fostered a deep commitment to team spirit and accomplishments so badly needed in research endeav­ors. The personal satisfaction of the participants was widely known, and the program has become a model for NASA intercenter relationships and joint programs.[1309]

The first task in planning the program was to identify major techni­cal issues facing the high-angle-of-attack community. Foremost among these was the understanding, prediction, and control of aerodynamic phenomena at high-angle-of-attack conditions, especially for aircraft configurations with strong vortical flows. Achieving this goal involved detailed studies of separated flow characteristics; measurement of static and dynamic phenomena in ground-test facilities as well as flight; cal­ibration of flow predictions from CFD methodology, wind tunnels, and flight; and the development of CFD codes for high-angle-of-attack condi­tions. In addition, the analysis and prediction of aerodynamic phenom­ena associated with structural fatigue issues for vertical tails immersed in violently fluctuating separated flows at high-angle-of-attack condi­tions became a major element in the program.

The second research thrust in the proposed program was directed toward an exciting new technology that offered unprecedented levels of controllability at high angles of attack—thrust vectoring. The thrust­vectoring concept had been developed in early rocket control applica­tions by placing vanes in the exhaust of the rocket vehicle, and extensive NASA-industry-DOD studies had been conducted to develop movable nozzle vectoring concepts for aircraft applications. The introduction of the superb fighters of the 1970s had demonstrated a new level of design achievement in stability at high angles of attack, but another nemesis remained—inadequate control at high-angle-of-attack conditions at
which conventional aerodynamic control surfaces lose effectiveness because of separated flow. The problem was particularly critical in the lack of ability to create crisp, precise roll control for "nose pointing” at high angles of attack. For such conditions, the ability to roll is dependent on providing high levels of yaw control, which creates sideslip and roll­ing motion because of dihedral effect. Unfortunately, conventional rud­ders mounted on vertical tails become ineffective at high angles of attack.

During the early 1980s, researchers at the Navy David Taylor Research Center pursued the application of simple jet-exit vanes to the F-14 for improved yaw control.[1310] Teaming with Langley in a joint study in the Langley Differential Maneuvering Simulator, the researchers found that the increased yaw control provided by the vanes resulted in a dramatic improvement in high-angle-of-attack maneuverability and dominance in simulated close-in air combat. Inspired by these results, Langley researchers evaluated the effectiveness of similar vanes on a variety of configurations during free-flight model testing in the Langley Full Scale Tunnel. Following investigations of modified models of the F-16, F/A-18, X-29, and X-31, the researchers concluded that thrust vec­toring in yaw provided unprecedented levels of maneuverability and control at high angles of attack. In addition, providing feedback from flight sensors to the vane control system enhanced dynamic stability for the test conditions.

Another technology that had matured to the point of research appli­cations was the control of strong vortical flow shed from the long pointed forebodies of contemporary fighters at high angles of attack. As previ­ously mentioned, Dryden and the Air Force Flight Dynamics Laboratory had conducted a joint program to evaluate the effects of blowing on the nose of the X-29A for enhanced control. Competing concepts for vorti­cal flow control had also received attention during NASA and industry research programs, including investigations at Langley of deflectable forebody strakes that could be used to control flow separation on the forebody for enhanced yaw control.

Perhaps the most contentious issue in planning the integrated NASA high-angle-of-attack program was whether a research air­craft was required and, if so, which aircraft would make the best testbed for research studies. Following prolonged discussions (the Ames
representative did not initially endorse the concept of flight-testing), the planning team agreed that flight-testing was mandatory for the program to be relevant, coordinated, and focused. Consideration was given to the F-15, F-16, X-29, and F/A-18 as potential testbeds, and after discussions, the team unanimously chose the F/A-18, for several reasons. The ear­lier Navy F/A-18 development program had included extensive support from Langley; therefore, its characteristics were well known to NASA (especially aerodynamic and aeroelastic phenomena, such as vortical flow and vertical tail buffet). During spin-testing for the development program, the aircraft had displayed reliable, stall-free engine operations at high angles of attack and excellent spin recovery characteristics. The F/A-18 was equipped with an advanced digital flight control system that offered the potential for modifications for research flight tests. Finally, the aircraft exhibited a remarkably high-angle-of-attack capability (up to 60 degrees in trimmed low-speed flight)—ideal for aerodynamic tests at extreme angles of attack.

The intercenter planning team presented its integrated research program plan to NASA Headquarters, seeking approval to pursue the acquisition of an F/A-18 from the Navy and for program go-ahead. After Agency approval, the Navy transferred the preproduction F/A-18A Ship 6, which had been used for spin testing at Patuxent River, MD, to NASA Dryden, where it arrived in October 1984. This particular F/A-18A had been stripped of several major airframe and instrument components following the completion of its spin program at Patuxent River, but it was still equipped with a multi-million-dollar emergency spin recov­ery parachute system and a programmable digital flight control com­puter ideally suited to NASA’s research interests. The derelict aircraft was shipped overland to Dryden and reassembled by a team of NASA and Navy technicians into a unique high-angle-of-attack research air­plane known as the F/A-18A High-Alpha Research Vehicle (HARV).[1311] The HARV was equipped with several unique research systems, includ­ing flow visualization equipment, a thrust-vectoring system using exter­nal postexit vanes around axisymmetric nozzles, and deployable nose strakes on a modified fuselage of forebody. Additional aircraft systems included extensive instrumentation, integrated flight research con­trols with special flight control hardware and software for the thrust-

vectoring system, interface controls for the forebody strakes, and safety backup systems including a spin recovery parachute.

The High-Angle-of-Attack Technology program (HATP) was funded and managed under an arrangement that was different from other NASA programs but was extremely efficient and productive. Headquarters pro­vided program management oversight, but recommendations for day – to-day technical planning, distribution of funds, and technical thrusts were provided by an intercenter steering committee consisting of mem­bers from each of the participating Centers. In recognition of its techni­cal expertise and accomplishments in high-angle-of-attack technology, Langley was designated the technology lead Center. Dryden was desig­nated the lead Center for flight research and operations of the HARV, and Ames and Langley shared the technical leadership for CFD and experi­mental aerodynamics. In subsequent years, the NASA Lewis Research Center (now the NASA Glenn Research Center) joined the HATP for experiments on engine inlet aerodynamics for high-angle-of-attack con­ditions. The HATP included aerodynamics, flight controls, handling qual­ities, stability and control, propulsion, structures, and thrust vectoring.[1312]

The HATP program was conducted in three sequential phases, centering on high-angle-of-attack aerodynamic studies (1987-1989), evaluation of thrust vectoring effects on maneuverability (1990-1994), and forebody flow control (1995-1996), with 383 research flights. In the first activities, aerodynamic characteristics obtained from flight-test results for the baseline HARV (no vectoring) were correlated with wind tunnel and CFD predictions, with emphasis on flow separation predic­tions and vortical flow behavior on the fuselage forebody and wing-body leading-edge extension (LEX).[1313]

The first HARV research flight was April 17, 1987. Flown in its base­line configuration, the HARV provided maximum angles of attack on the order of 55 degrees, limited by aerodynamic control. At the time the flight studies were conducted, CFD had not yet been applied to real
aircraft shapes at high angles of attack. Rather, researchers had used computational methods to predict flow over simple shapes such as pro­late spheroids, and the computation of flow fields, streamlines, and sep­aration phenomena for a modern fighter was extremely challenging. Many leaders in the NASA and industry CFD communities were pessi­mistic regarding the success of such a venture at the time.

The experimental wind tunnel community was also facing issues on how (or whether) to modify models to better simulate high-angle – of-attack aerodynamics at flight values of Reynolds numbers and to understand the basic characteristics of vortical flows and techniques for the prediction of flow interactions with aircraft structures. Using a highly innovative, Dryden-developed propylene glycol monomethyl ether (PGME) dye flow-visualization technique that emitted colored dye tracers from ports for visualization of surface flows over the HARV forebody and LEX, the team was able to directly compare results, ana­lyze separation phenomena, and modify CFD codes for a valid predic­tion of the observed on-surface flow characteristics. The ports for the PGME were later modified for pressure instrumentation to provide even more detailed information on flow fields. Additional instrumentation for aerodynamic measurements was initially provided by a nose boom, but evidence of aerodynamic interference from the nose boom caused the team to remove it and replace the boom with wingtip air-data probes. A rotating, foldout flow rake was also used to measure vortical flows shed by the LEX surfaces.[1314]

The results of the HATP flight – and ground-based aerodynamic stud­ies provided a detailed perspective of the relative accuracy of compu­tational flow dynamics and wind tunnel testing techniques to predict critical flow phenomena such as surface pressures, separation con­tours, vortex interaction patterns, and laminar separation bubbles.[1315] The scope of correlation included assessments of the impact of Mach and Reynolds numbers on forebody and LEX vortexes as observed in flight with the HARV, the Langley 7- by 10-Foot High-Speed Tunnel, the Langley

30- by 60-Foot Tunnel, the Navy David Taylor Research Center 7- by 10-Foot Transonic Tunnel, and the Ames 80- by 120-Foot Wind Tunnel.

A wide variety of subscale models of the HARV configuration was tested in the various wind tunnels, and a full-scale F/A-18 aircraft was used for testing in the Ames 80- by 120-Foot Tunnel. The test article was an ex-Blue Angel flight demonstrator, whose life had been exceeded,

that had been bailed to NASA for the tests. When the tunnel tests were conducted in 1991 and 1993, the aircraft had both engines, flowthrough inlets, and the wingtip missile launchers removed.

Using extensive instrumentation that had been carefully coordi­nated between ground and flight researchers gathered an unprecedented wealth of detail on aerodynamic characteristics of a modern fighter at high angles of attack. The effort was successful particularly because it had been planned with common instrumentation locations for pressure ports and flow visualization stations between wind tunnel tests and the flight article. More importantly, the high value of the data obtained was the result of one of the most successful aspects of the program—close communications and working relationships between the flight, wind tunnel, and CFD technical communities.

As NASA neared the end of the aerodynamic phase of testing for the HARV, growing concerns over buffeting of the vertical tail surfaces for military fleet F/A-18 aircraft led the Navy and McDonnell-Douglas to develop vertical longitudinal fences on the upper surfaces of the LEX to
extend the service life of the tails of fleet F/A-18s. Although the fences were not installed on HARV during the early aerodynamic studies, they were added during the second and third phases of the program, when extensive wind tunnel and HARV flight studies of the tail buffet phenom­enon were conducted. Resulting data were transmitted to the appropri­ate industry and service organizations for analysis of the F/A-18 specific phenomena as well as for other twin-tail fighter aircraft.

As the second phase of the HATP began, Dryden accepted major program responsibilities for the implementation of a relatively simple and cheap thrust-vectoring system for the HARV aircraft. The objec­tive of NASA’s research was not to develop a production-type thrust­vectoring engine/nozzle system, but rather to evaluate the impact of vectoring for high-angle-of-attack maneuvers, assess control-law require­ments for high-angle-of-attack applications, and use the control augmen­tation provided by vectoring to stabilize the aircraft at extreme angles of attack for additional aerodynamic studies. With this philosophy in mind, the program contracted with McDonnell-Douglas to modify the HARV with deflectable external vanes mounted behind the aircraft’s two F-404 engines, similar in many respects to the installations used by the Navy F-14 mentioned earlier and the Rockwell X-31 research aircraft.

For the installation, the exhaust nozzle divergent flaps were removed from the engines and replaced with a set of three vanes for each engine, thereby providing both pitch and yaw vectoring capability. The research teams at Dryden and Langley thoroughly studied the specific vane con­figuration, structural design, and control system modifications required for the project. The scope of activities included measurements of thrust – vane effectiveness for many powered model configurations at Langley, simulator studies of the effectiveness of vectoring on maneuverability and controllability at Langley, and hardware and software development— as well as the integration, checkout, and operations of the system—at Dryden. The implementation of the HARV thrust-vectoring hardware and software modifications proved to be relatively difficult, requiring the NASA research team to participate in the final design of the thrust­vectoring system. The HARV vectoring system followed the HATP objec­tive of providing thrust-vectoring research capability at minimal cost through external airframe modifications rather than a new production – type vectoring engine. With the massive external thrust-vectoring vane actuation system and the emergency spin recovery parachute system both mounted on the rear of the aircraft and necessary ballast added

to the nose of the aircraft to maintain balance, the weight of the HARV was increased by about 4,000 pounds.

In the thrust-vectoring phase of the HATP project, the conventional flight control system of the HARV was modified to include a research flight control system (RFCS) to influence control laws. The conventional F/A-18 control laws were used for takeoff, for landing, and as a backup in case of failure of the RFCS, whereas the second set of control laws were for high-angle-of-attack research flights. The design and implementation of the RFCS system was one of the more complex changes to the F/A-18 digital flight control system undertaken at that time.

First flight of the HARV with vectoring engaged occurred in July 1991, a few weeks after the X-31 research aircraft demonstrated pitch­vectoring capability at Edwards, but the HARV conducted the first mul­tiaxis vectoring flights shortly thereafter. Research flight-testing of the HARV equipped with thrust vectoring vividly demonstrated the anticipated benefits at high angles of attack that had been predicted by earlier free – flight model tests and piloted simulator studies. The precision and angu­lar rates available to the pilot were remarkable, and the enhanced stability and control at extreme angles of attack permitted precision aerodynamic
studies that had previously been impossible. Angles of attack as high as 70 degrees were flown with complete control in aerodynamic experiments.

During the late 1980s, three NASA-industry-DOD programs had been initiated to explore thrust-vectoring systems for high-angle-of – attack conditions. Each program had different objectives and focused on separate technologies. NASA’s HARV aircraft was designed to evaluate fundamental thrust-vectoring system control-law synthesis and use vec­toring to stabilize the aircraft at high angles of attack for aerodynamic experiments. The DARPA X-31A aircraft was conceived to demonstrate enhanced fighter maneuverability at poststall angles of attack under simulated tactical conditions. In addition, the Air Force F-16 Variable – Stability In-Flight Simulator Test Aircraft (VISTA) was modified into the F-16 Multi-Axis Thrust Vectoring (MATV) project with an objective of demonstrating the effectiveness of a production-type thrust-vectoring system. All three programs had different goals, and the three research aircraft underwent flight-testing at Edwards in the same time period.

The HATP participants conceived, developed, and assessed several control-law schemes, which included special configurations for longi­tudinal control at high angles of attack, lateral and directional control mixing strategies, automatic spin prevention, and spin recovery modes. Seventy-five spin attempts (at low power conditions) resulted in 70 fully developed spins with satisfactory recoveries, and the emergency spin recovery parachute was never fired in flight.

As the HARV conducted its thrust-vectoring research program, a critical issue emerged within the advanced fighter design community. With new configurations under consideration having extreme angle-of – attack capability and reduced longitudinal stability for performance and maneuverability enhancements, the issue of providing sufficient nose – down control effectiveness for recovery from high-angle-of-attack excur­sions became significant. NASA-DOD technical meetings had been held to discuss studies to assess the adequacy of theoretical and wind tunnel predictions, and it appeared that using the HARV flight capability with thrust vectoring would provide highly desirable data for design criteria for future fighters. In view of the urgency of the situation, Langley led a HATP element known as High-Alpha Nosedown Guidelines (HANG), which included extensive simulator studies and flights with the HARV.[1316]

Although the main objective of the HARV thrust-vectoring experi­ments was not air-to-air combat maneuvering, Dryden conducted flight tests to provide validation data for a proposed high-angle-of-attack flying qualities requirement MIL-STD-1797A by using basic fighter maneuvers and limited air combat maneuvering. Six NASA research test pilots from Dryden and Langley provided the major expertise and guidance for the HATP simulator and HARV flight-testing. Other guest pilots from NASA, the Navy, the Canadian Air Force, the United Kingdom, McDonnell-Douglas, and Calspan also participated in flight-test evaluations of the HARV vectoring capabilities.

The third and final phase of the HATP was directed to in-flight assessments of the effectiveness of controlling the powerful vortex flows shed by the fuselage forebody for augmentation of yaw control at high angles of attack. Ground-based research in NASA wind tunnels and sim­ulators had indicated that the most effective method for rolling an air­craft about its flight path for nose pointing at high angles of attack was through the use of yaw control. Unfortunately, conventional rudders suffer a severe degradation and control effectiveness at high angles of attack because of the impingement of low-energy stalled flows only ver­tical tail surfaces. Years of NASA research had demonstrated that the use of deployable fuselage forebody strakes was a potentially viable con­cept for yaw control augmentation. With a vast amount of wind tunnel data and pilot opinions derived from air combat simulation, the strake concept was ready for realistic evaluations in flight. Once again, the cohesive nature of the HATP was demonstrated when the strake hard­ware was designed and fabricated on a special F/A-18 forebody radome in machine shops at Langley and the control laws were developed at Langley and delivered to Dryden, where the flight computer interface and instrumentation were accomplished by the Dryden staff. The proj­ect, known as actuated nose strakes for enhanced rolling (ANSER), was evaluated independently and in combination with thrust vectoring.[1317]

Implementation of the ANSER concept on the thrust-vectoring – equipped HARV provided three control combinations. The aircraft could be flown with thrust vectoring only, thrust vectoring in longitudinal con­trol with a thrust-vectored and strake-blended mode for lateral control,
and a strake mode with thrust-vectoring control longitudinally and strakes controlling the lateral mode. As was the case for thrust vector­ing, the forebody strake flight results demonstrated that a significant enhancement of high-angle-of-attack rolling capability was obtained, particularly at higher subsonic speeds. In fact, at those speeds, the effec­tiveness of the strakes was comparable to that of thrust vectoring.

Several other subsystems were implemented on the HARV, including an instrumented inlet rake, extensive pressure instrumentation, aero – servoelastic accelerometers, thrust-vectoring vane loads and tempera­tures, and an emergency power backup system. Notably, although the power backup system was implemented to continue aircraft systems operation in the event of a dual-engine flameout or unrecoverable dual­engine stalls, it was removed later in the program when testing showed excellent high-angle-of-attack engine operations. In fact, 383 high-angle – of-attack flights were made without experiencing an engine stall.

Throughout the HATP program, NASA ensured that results were widely disseminated within industry and DOD. Major HATP technical conferences were held, with at least 200 attendees at Langley in 1990, at Dryden in 1992 and 1994, and a wrap-up conference at Langley in 1996.[1318] Hundreds of reports and presentations resulted from the program, and the $74 million (1995 dollars) activity produced cutting-edge tech­nical results that were absorbed into the Nation’s latest aircraft, includ­ing the F-22, F-35 and F/A-18E.

The XV-5 was a proof-of-concept lift-fan aircraft and thus employed a completely "manual” powered-lift flight control system. The lack of an integrated powered-lift system required the pilot to manually control the aircraft flight-path through independent manipulation of stick, engine power, thrust vector angle and collective lift. This lack of an integrated powered-lift management system (and in particular, the conversion controls) was responsible for most of the adverse handling qualities of the aircraft. An advanced digital fly­by-wire control system must provide level one handling qualities, especially for integrated powered-lift management.

CONVERSION SYSTEM DESIGN

The manually operated conversion system was the most exacting, interesting and potentially hazardous flight opera­tion associated with the XV-5. This type of "bang-bang” con­version system should not be considered for the SSTOVLF. Ideally, the conversion should consist of a fully reversible and continuously controllable process. That is, the pilot must be able to continuously control the conversion process. Good examples are the XV-15 Tilt Rotor, the X-22A and the AV-8 Harrier. Furthermore, the conversion of the SSTOVLF with an advanced digital flight control system should be fully decou­pled so that the pilot would not have to compensate for lift, attitude or speed changes. The conversion controller should be a single lever or beeper-switch that is safety-interlocked against inadvertent actuation. The conversion airspeed limit corridor must be wide enough to allow for operational flexi­bility and compensate for single-pilot operation where mis­sion demands can compete for pilot attention.

The conditions if not the mechanics that generate lightning are now well known. In essence, this atmospheric fire is started by rubbing particles together. But there is still no agreement on which processes ignite lightning. Current hypotheses focus on the separation of electric charge and generation of an electric field within a thunderstorm. Recent studies further suggest that lightning initiation requires ice, hail, and semifrozen water droplets, called "graupel.” Storms that do not pro­duce large quantities of ice usually do not develop lightning.[116] Graupel forms when super-cooled water droplets condense around a snowflake nucleus into a sphere of rime, from 2 to 5 millimeters across. Scientific debate continues as experts grapple with the mysteries of graupel, but the stages of lightning creation in thunderstorms are clear, as outlined by the National Weather Service of the National Oceanic and Atmospheric Administration (NOAA).

First comes charge separation. Thunderstorms are turbulent, with strong updrafts and downdrafts regularly occurring close to one another. The updrafts lift water droplets from warmer lower layers to heights between 35,000 and 70,000 feet, miles above the freezing level. Simultaneously, downdrafts drag hail and ice from colder upper layers. When the opposing air currents meet, water droplets freeze, releasing heat, which keeps hail and ice surfaces slightly warmer than the sur­rounding environment, so that graupel, a "soft hail,” forms.

Electrons carry a negative charge. As newly formed graupel collides with more water droplets and ice particles, electrons are sheared off the ascending particles, charging them positively. The stripped electrons col­lect on descending bits, charging them negatively. The process results in a storm cloud with a negatively charged base and positively charged top.

Once that charge separation has been established, the second step is generation of an electrical field within the cloud and, somewhat like a mirror image, an electrical field below the storm cloud. Electrical opposites attract, and insulators inhibit current flow. The separation of positive and negative charges within a thundercloud generates an electric field between its top and base. This field strengthens with fur­ther separation of these charges into positive and negative pools. But the atmosphere acts as an insulator, inhibiting electric flow, so an enor­mous charge must build up before lightning can occur. When that high charge threshold is finally crossed, the strength of the electric field over­powers atmospheric insulation, unleashing lightning. Another electrical field develops with Earth’s surface below negatively charged storm base, where positively charged particles begin to pool on land or sea. Whither the storm goes, the positively charged field—responsible for cloud – to-ground lightning—will follow it. Because the electric field within the storm is much stronger than the shadowing positive charge pool, most lightning (about 75 to 80 percent) remains within the clouds and is thus not attracted groundward.

The third phase is the building of the initial stroke that shoots between the cloud and the ground. As a thunderstorm moves, the pool of positively charged particles traveling with it along the ground gath­ers strength. The difference in charge between the base of the clouds and ground grows, leading positively charged particles to climb up taller objects like houses, trees, and telephone poles. Eventually a "stepped leader,” a channel of negative charge, descends from the bottom of the storm toward the ground. Invisible to humans, it shoots to the ground in a series of rapid steps, each happening quicker than the blink of an eye. While this negative leader works its way toward Earth, a positive charge collects in the ground and in objects resting upon it. This accumulation of positive charge "reaches out” to the approaching negative charge with its own channel, called a "streamer.” When these channels connect, the resulting electrical transfer appears to the observer as lightning.

Finally, a return stroke of lightning flows along a charge channel about 0.39 inches wide between the ground and the cloud. After the ini­tial lightning stroke, if enough charge is left over, additional strokes will flow along the same channel, giving the bolt its flickering appearance.

Land struck by a bolt may reach more than 3,300 °F, hot enough to almost instantly melt the silica in conductive soil or sand, fusing the grains together. Within about a second, the fused grains cool into ful­gurites, or normally hollow glass tubes that can extend some distance into the ground, showing the path of the lightning and its dispersion over the surface.

The tops of trees, skyscrapers, and mountains lie closer to the base of storm clouds than does low-lying ground, so such objects are commonly struck by lightning. The less atmospheric insulation that lightning must burn through, the easier falls its strike. The tallest object beneath a storm will not necessarily suffer a hit, however, because the opposite charges may not accumulate around the highest local point or in the clouds above it. Lightning can strike an open field rather than a nearby line of trees.

Lightning leader development depends not only upon the electrical breakdown of air, which requires about 3 million volts per meter, but on prior channel carving. Ambient electric fields required for lightning leader propagation can be one or two orders of magnitude less than the electrical breakdown strength. The potential gradient inside a developed return stroke channel is on the order of hundreds of volts per meter because of intense channel ionization, resulting in a power output on the order of a megawatt per meter for a vigorous return stroke current of 100,000 amperes (100 kiloamperes, kA).

In June 1995, the FAA announced its plans for a joint FAA-DOD-NASA initiative called the National Plan for Civil Aviation Human Factors. The plan detailed a national effort to reduce and eliminate human error as the cause of aviation accidents. The plan called for projects that would iden­tify needs and problems related to human performance, guide research programs that addressed the human element, involve the Nation’s top scientists and aviation professionals, and report the results of these efforts to the aviation community.[221]

NASA’s extensive involvement in human factors issues is detailed in another case study of this volume.

From new navigation instruments to updated air traffic control proce­dures, none of the developments intended to make safer skyways that was produced by NASA could be deployed into the real world until it had been thoroughly tested in simulated environments and certified as ready for use by the FAA. Among the many facilities and aircraft available to NASA to conduct such exercises, the Langley-based Boeing 737 and Ames-based complement of air traffic control simulators stand out as major contrib­utors to the effort of improving the National Airspace System.

NASA initiated and implemented this important human-based safety program in 1976 at the request of the FAA. Its importance can best be judged by the fact it is still in full operation—funded by the FAA and managed by NASA. The Aviation Safety Reporting System (ASRS) col­lects information voluntarily and confidentially submitted by pilots, controllers, and other aviation professionals. This information is used to identify deficiencies in the National Aviation System (NAS), some of which include those of the human participants themselves. The ASRS analyzes these data and refers them in the form of an "alerting message” to the appropriate agencies so that problems can be corrected. To date, nearly 5,000 alert messages have been issued.[377] The ASRS also educates through its operational issues bulletins, its newsletter CALLBACK and its journal ASRS Directline, as well as through the more than 60 research studies it has published.[378] The massive database that the ASRS main­tains benefits not only NASA and the FAA, but also other agencies world­wide involved in the study and promotion of flight safety. Perhaps most importantly, this system serves to foster further aviation human fac­tors safety research designed to prevent aviation accidents.[379] After more than 30 years in operation, the ASRS has been an unqualified success. During this period, pilots, air traffic controllers, and others have pro­vided more than 800,000 reports.[380] The many types of ASRS responses to the data it has collected have triggered a variety of safety-oriented actions, including modifications to the Federal Aviation Regulations.[381]

It is impossible to quantify the number of lives saved by this impor­tant long-running human-based program, but there is little dispute that its wide-ranging effect on the spectrum of flight safety has benefitted all areas of aviation.

Wind tunnel free-flight testing facilities provide unique and very valuable information regarding the flying characteristics of advanced aerospace vehicles. However, they are inherently limited or unsuit­able for certain types of investigations in flight dynamics. For example, vehicle motions involving large maneuvers at elevated g’s, out-of­control conditions, and poststall gyrations result in significant changes in flight trajectories and altitude, which can only be studied in the expanded spaces provided by outdoor facilities. In addition, critical studies associated with high-speed flight could not be conducted in Langley’s low-speed wind tunnels. Outdoor testing of dynamically scaled powered and unpowered free-flight models was therefore developed and applied in many research activities. Although outdoor test techniques are more expensive than wind tunnel free-flight tests, are subject to limitations because of weather conditions, and have inherently slower turnaround time than tunnel tests, the results obtained are unique and especially valuable for certain types of flight dynamics studies.

One of the most important outdoor free-flight test techniques developed by NASA is used in the study of aircraft spin entry motions, which includes investigations of spin resistance, poststall gyrations, and recovery controls. A significant void of information exists between the prestall and stall-departure results produced by the wind tunnel free-flight test technique in the Full-Scale Tunnel discussed earlier and the results of fully developed spin evaluations obtained in the Spin Tunnel. The lack of information in this area can be critically mis­leading for some aircraft designs. For example, some free-flight mod­els exhibit severe instabilities in pitch, yaw, or roll at stall during wind tunnel free-flight tests, and they may also exhibit potentially danger­ous spins from which recovery is impossible during spin tunnel tests. However, a combination of aerodynamic, control, and inertial prop­erties can result in this same configuration exhibiting a high degree of resistance to enter the dangerous spin following a departure, despite forced spin entry attempts by a pilot. On the other hand, some configurations easily enter developed spins despite recovery controls applied by the pilot.

To evaluate the resistance of aircraft to spins, in 1950 Langley revisited the catapult techniques of the 1930s and experimented with

an indoor catapult-launching technique.[454] Once again, however, the cat­apult technique proved to be unsatisfactory, and other approaches to study spin entry were pursued.[455] Disappointed by the inherent limita­tions of the catapult-launched technique, the Langley researchers began to explore the feasibility of an outdoor drop-model technique in which unpowered models would be launched from a helicopter at higher alti­tudes, permitting more time to study the spin entry and the effects of recovery controls. The technique would use much larger models than those used in the Spin Tunnel, resulting in a desirable increase in the test Reynolds number. After encouraging feasibility experiments were conducted at Langley Air Force Base, a search was conducted to locate a test site for research operations. A suitable low-traffic airport was iden­tified near West Point, VA, about 40 miles from Langley, and research operations began in 1958.[456]

As testing progressed at West Point, the technique evolved into an operation consisting of launching the unpowered model at an altitude of about 2,000 feet and evaluating its spin resistance with separately located, ground-based pilots who attempted to promote spins by var­ious combinations of control inputs and maneuvers. At the end of the test, an onboard recovery parachute was deployed and used to recover the model and lower it to a ground landing. This approach proved to be the prototype of the extremely successful drop-model testing technique that was continually updated and applied by NASA for over 50 years.

Initially, two separate tracking units consisting of modified power – driven antiaircraft gun trailer mounts were used by two pilots and two tracking operators to track and control the model. One pilot and tracker were to the side of the model’s flight path, where they could control the longitudinal motions following launch, while the other pilot and tracker were about 1,000 feet away, behind the model, to control lateral- directional motions. However, as the technique was refined in later

years, both pilots used a single dual gun mount arrangement with a single tracker operator.

Researchers continued their search for a test site nearer to Langley, and in 1959, Langley requested and was granted approval by the Air Force to conduct drop tests at the abandoned Plum Tree bombing range near Poquoson, VA, about 5 miles from Langley. The marshy area under con­sideration had been cleared by the Air Force of depleted bombs and muni­tions left from the First and Second World War eras. A temporary building and concrete landing pad for the launch helicopter were added for opera­tions at Plum Tree, and a surge of request jobs for U. S. high-performance military aircraft in the mid – to-late 1960s (F-14, F-15, B-1, F/A-18, etc.) brought a flurry of test activities that continued until the early 1990s.[457]

During operations at Plum Tree, the sophistication of the drop-model technique dramatically increased.[458] High-resolution video cameras were

used for tracking the model, and graphic displays were presented to a remote pilot control station, including images of the model in flight and the model’s location within the range. A high-resolution video image of the model was centrally located in front of a pilot station within a build­ing. In addition, digital displays of parameters such as angle of attack, angle of sideslip, altitude, yaw rate, and normal acceleration were also in the pilot’s view. The centerpiece of operational capability was a digital flight control computer programmed with variable research flight con­trol laws and a flight operations computer with telemetry downlinks and uplinks within the temporary building. NASA operations at Plum Tree lasted about 30 years and included a broad scope of free-flight model investigations of military aircraft, general aviation aircraft, parawings, gliding parachutes, and reentry vehicles. In the early 1990s, however, sev­eral issues regarding environmental protection forced NASA to close its research activities at Plum Tree and remove all its facilities. After consid­erable searching and consideration of several candidate sites, the NASA Wallops Flight Facility was chosen for Langley’s drop-model activities.

The last NASA drop-model tests of a military fighter for poststall studies began in 1996 and ended in 2000.[459] This project, which evalu­ated the spin resistance of a 22-percent-scale model of the U. S. Navy F/A-18E Super Hornet, was the final evolution of drop-model technol­ogy for Langley. Launched from a helicopter at an altitude of about

15,0 feet in the vicinity of Wallops, the Super Hornet model weighed about 1,000 pounds. Recovery of the model at the end of the flight test was again initiated with the deployment of onboard parachutes. The model used a flotation bag after water impact and was retrieved from the Atlantic Ocean by a recovery boat.

Outdoor free-flight model testing has also flourished at NASA Dryden Flight Research Center. Dryden’s primary advocate and highly success­ful user of free-flight models for low-speed research on advanced aero­space vehicles was the late Robert Dale Reed. An avid model builder, pilot, and researcher, Reed was inspired by his perceived need for a sub­scale free-flight model demonstrator of an emerging lifting body reen­try configuration created by NASA Ames in 1962.[460] After initial testing of gliders of the Ames M2-F1 lifting body concept, he progressed into

the technique of using radio-controlled model tow planes to tow and release M2-F1 models. In the late 1960s, the launching technique for the unpowered models evolved with a powered radio-controlled mother ship, and by 1968, Reed’s mother ship had conducted over 120 launches. Dale Reed’s innovation and approach to using radio-controlled mother ships for launching drop models of radical configurations have endured to this day as the preferred method for small-scale free-flight activities at Dryden.

In the early 1970s, Reed’s work at Dryden expanded into a series of flight tests of powered and unpowered remotely piloted research vehicles (RPRVs). These activities, which included remote-control evaluations of subscale and full-scale test subjects, used a ground-based cockpit equipped with flight instruments and sensors typical of a representative

full-scale airplane. These projects included the Hyper III lifting body and a three-eighths-scale dynamically scaled model of the F-15. The technique used for the F-15 model consisted of air launches of the test article from a B-52 and control by a pilot in a ground cockpit outfit­ted with a sophisticated control system.[461] The setup featured a digital uplink capability, a ground computer, a television monitor, and a telem­etry system. Initially, the F-15 model was recovered on its parachute in flight by helicopter midair snatch, but in later flights, it was landed on skids by the evaluation pilot.

NASA Ames also conducted and sponsored outdoor free-flight pow­ered model testing in the 1970s as a result of interest in the oblique wing concept championed by Robert T. Jones. The progression of sophistica­tion in these studies started with simple unpowered catapult-launched models at Ames, followed by cooperative powered model tests at Dryden in the 1970s and piloted flight tests of the AD-1 oblique wing demonstra­tor aircraft in the 1980s.[462] In the 1990s, Ames and Stanford University collaborated on potential designs for oblique wing supersonic transport designs, which led to flight tests of two free-flight models by Stanford.

Yet another historic high-speed outdoor free-flight facility was spun off Langley’s interests. In 1945, a proposal was made to develop a new NACA high-speed test range known as the Pilotless Aircraft Research Station, which would use rocket-boosted models to explore the transonic and supersonic flight regimes. The facility ultimately became known as the NACA Wallops Island Flight Test Range.[463] From 1945 through 1959, Wallops served as a rocket-model "flying wind tunnel” for researchers in Langley’s Pilotless Aircraft Research Division (PARD), which con­ducted vital investigations for the Nation’s emerging supersonic aircraft, especially the Century series of advanced fighters in the 1950s. Rocket – boosted models were used by the Pilotless Aircraft Research Division of the NACA’s Langley Laboratory in flight tests at Wallops to obtain valu­able information on aerodynamic drag, dynamic stability, and control effectiveness at transonic conditions.

External stores have been found to have large effects on spin and recovery, especially for asymmetric loadings in which stores are located asymmetrically along the wing, resulting in a lateral displace­ment of the center of gravity of the configuration. For example, some aircraft may not spin in the direction of the "heavy” wing but will spin fast and flat into the "light” wing. In most cases, model tests in which the shapes of the external stores were replaced with equivalent weight ballast indicated that the effects of asymmetric loadings were primarily due to a mass effect, with little or no aerodynamic effect detected. However, very large stores such as fuel tanks were found, on occasion, to have unexpected effects because of aerodynamic char­acteristics of the component. During the aircraft development phase, spin characteristics of high-performance military aircraft must be assessed for all loadings proposed, including symmetric and asymmet­ric configurations. Spin tunnel tests can therefore be extensive for some aircraft, especially those with variable-sweep wing capabilities. Testing

of the General Dynamics F-111, for example, required several months of test time to determine spin and recovery characteristics for all poten­tial conditions of wing-sweep angles, center-of-gravity positions, and symmetric and asymmetric store loadings.[513]

The example of the Langley Transonic Dynamics Tunnel (TDT) illustrates how the NACA and NASA took an unsatisfactory tunnel and converted it into one capable of contributing to longstanding aerospace research. The Transonic Dynamics Tunnel began operations as the 19-Foot Pressure Tunnel in June 1939. The NACA design team, which included Smith J. DeFrance and John F. Parsons, wanted to address continued prob­lems with scale effects. Their solution resulted in the first large-scale high-pressure tunnel. Primarily, the tunnel was to evaluate propellers and wings at high Reynolds numbers. Researchers were to use it to study the stability and control characteristics of aircraft models as well. Only able to generate a speed of 330 mph in the closed-throat test sec­tion, the NACA shifted the high-speed propeller work to another new facility, the 500 mph 16-Foot High-Speed Tunnel. The slower 19-Foot Pressure Tunnel pressed on in the utilitarian work of testing models at high Reynolds numbers.[572]

Dissatisfied with the performance of the 19-Foot Pressure Tunnel, the NACA converted it into a closed-circuit, continual flow, variable pressure Mach 1.2 wind tunnel to evaluate such dynamic flight char­acteristics as aeroelasticity, flutter, buffeting, vortex shedding, and gust loads. From 1955 to 1959, the conversion involved the installation of new components, including a slotted test section, mounts, a quick-stop drive system, an airflow oscillator (or "gust maker”), and a system that

generated natural air or a refrigerant (Freon-12 and later R-134a) test medium. The use of gas improved full-scale aircraft simulation.[573] It produced higher Reynolds numbers, eased fabrication of scaled mod­els, reduced tunnel power requirements, and, in the case of rotary wing models, reduced model power requirements.[574]

After 8 years of design, calibration, and conversion, the TDT became the world’s first aeroelastic testing tunnel, becoming operational in 1960. The tunnel was ready for its first challenge: the mysterious crashes of the first American turboprop airliner, the Lockheed L-188 Electra II. The Electra entered commercial service with American Airlines in December 1958. Powered by 4 Allison 501 turboprop engines, the $2.4-million Electra carried approximately 100 passengers while cruising at 400 mph. On September 29, 1959, Braniff Airways Flight 542 crashed near Buffalo, TX, with the loss of all 34 people aboard the new Electra air­liner. A witness saw what appeared to be lightning followed by a ball of fire and a shrieking explosion. The 2.5- by 1-mile debris field included the left wing, which settled over a mile away from the main wreckage. The initial Civil Aeronautics Board crash investigation revealed that failure of the left wing about a foot from the fuselage in flight led to the destruction of the airplane.[575]

There was no indication of the exact cause of the wing failure. The prevailing theories were sabotage or pilot and crew error. The crash of a Northwest Orient Airlines Electra near Tell City, IN, on March 17, 1960, with a loss of 63 people provided an important clue. The right wing landed 2 miles from the crash site. Federal and Lockheed investigators believed that violent flutter ripped the wings off both Electras, but they did not know the specific cause.[576]

The future of the new American jet airliner fleet was a stake. While the tragic story of the Electra unfolded, the Langley Transonic Dynamics Tunnel became operational in early 1960. NASA quickly prepared a one-eighth-scale model of an Electra that featured rotating propellers, simulated fuel load changes, and different engine-mount structural configurations. Those features would be important to the wind tunnel tests because a Lockheed engineer believed that the Electra experienced propeller-whirl flutter, a phenomenon stimulated by engine gyroscopic torques, propeller forces and moments, and the aerodynamic loads acting on the wings. Basically, a design flaw, weakened engine mounts, allowed the engine nacelles and the wings to oscillate at the same frequency, which led to catastrophic failure. Reinforced engine mounts ensured that the Electra continued operations through the 1960s and 1970s.[577]

Flutter has been a consistent problem for aircraft since the 1960s, and the Transonic Dynamics Tunnel contributed to the refinement of many aircraft, including frontline military transports and fighters.

The Lockheed C-141 Starlifter transport experienced tail flutter in its original configuration. The horizontal tail of the McDonnell-Douglas F-15 Eagle all-weather air superiority fighter-bomber fluttered.[578] The inclusion of air-to-air and air-to-ground missiles, bombs, electronic countermeasures pods, and fuel tanks produced wing flutter on the General Dynamics F-16 Fighting Falcon lightweight fighter. NASA and General Dynamics underwent a combined computational, wind tun­nel, and flight program from June 1975 to March 1977. The TDT tests sought to minimize expensive flight-testing. They verified analytical methods in determining flutter and determined practical operational methods in which portions of fuel tanks needed to be emptied first to delay the onset of flutter.[579]

The TDT offered versatility beyond the investigation of flutter on fixed wing aircraft. Tunnel personnel also conducted performance, load, and stability tests of helicopter and tilt rotor configurations. Researchers in the space program used the tunnel to determine the effects of ground – wind loads on launch vehicles. Whether it is for a fixed or rotary wing airplane or a spacecraft, the TDT was used to evaluate the effect of wind gusts on flying vehicles.[580]

The National Advisory Committee for Aeronautics (NACA) was formed on March 3, 1915, to provide advice and carry out much of cutting-edge research in aeronautics in the United States. This organization was modeled on the British Advisory Committee for Aeronautics. President Woodrow Wilson created the advisory committee in an effort to orga­nize American aeronautical research and raise it to the level of European aviation. Its charter and $5,000 initial appropriation (low even in 1915) were appended to a naval appropriations bill and passed with little fan­fare. The committee’s mission was "to supervise and direct the scientific study of the problems of flight, with a view to their practical solution,” and to "direct and conduct research and experiment in aeronautics.”[775] Thus, from its outset, it was far more than simply a bureaucratic panel distanced from design-shop, laboratory, and flight line.

The NACA soon involved itself across the field of American aero­nautics, advising the Government and industry on a wide range of issues including establishing the national air mail service, along with its night mail operations, and brokering a solution—the cross­licensing of aeronautics patents—to the enervating Wright-Curtiss patent feud that had hampered American aviation development in the pre-World War I era and that continued to do so even as American forces were fighting overseas. The NACA proposed establishing a Bureau of Aeronautics in the Commerce Department, granting funds to the Weather Bureau to promote safety in aerial navigation, licensing of pilots, air­craft inspections, and expanding airmail. It also made recommenda­tions in 1925 to President Calvin Coolidge’s Morrow Board that led to passage of the Air Commerce Act of 1926, the first Federal legislation regulating civil aeronautics. It continued to provide policy recommen­dations on the Nation’s aviation until its incorporation in the National Aeronautics and Space Administration (NASA) in 1958.[776]

The NACA started working in the field of GA almost as soon as it was established. Its first research airplane programs, undertaken pri­marily by F. H. Norton, involved studying the flight performance, sta­bility and control, and handling qualities of Curtiss JN-4H, America’s iconic "Jenny” of the "Great War” time period, and one that became first great American GA airplane as well.[777] The initial aerodynamic and performance studies of Dr. Max M. Munk, a towering figure in the his­tory of fluid mechanics, profoundly influenced the Agency’s subsequent approach to aerodynamic research. Munk, the inventor of the variable – density wind tunnel (which put NACA aerodynamics research at the forefront of the world standard) and architect of American aerodynamic research methodology, dramatically transformed the Agency’s approach to airfoil design by introducing the methods of the "Prandtl school” at Gottingen and by designing and supervising the construction of a rad­ical new form of wind tunnel, the so-called "variable density tunnel.” His GA influence began with a detailed study of the airflow around and through a biplane wing cellule (the upper and lower wings, connected with struts and wires, considered as a single design element). He pro­duced a report in which the variation of the section, chord, gap, stag­ger, and decalage (the angle of incidence of the respective chords of the upper and lower wings) and their influence upon the available wing cell space for engines, cockpits, passenger, and luggage, were investigated with a great number of calculated examples in which all of the numer­ical results were given in tables. Munk’s report was in some respects a prototypical example of subsequent NACA-NASA research reports that, over the years, would prove beneficial to the development of GA by investigating a number of areas of particular concern, such as air­craft aerodynamic design, flight safety, spin prevention and recoveries, and handling qualities.[778] Arguably these reports that conveyed Agency research results to a public audience were the most influential product of NACA-NASA research. They influenced not only the practice of engi­neering within the various aircraft manufacturers, but provided the latest information incorporated in many aeronautical engineering text­books used in engineering schools.

Though light aircraft are often seen as the by-product of the air trans­port revolution, in fact, they led, not followed, the expansion of com­mercial aviation, particularly in the United States. The interwar years saw an explosive growth in American aeronautics, particularly private flying and GA. It is fair to state that the roots of the American air trans­port revolution were nurtured by individual entrepreneurs manufactur­ing light aircraft and beginning air mail and air transport services, rather than (as in Europe) largely by "top-down” government direction. As early as 1923, American fixed-base operators "carried 80,888 passengers and 208,302 pounds of freight.”[779] In 1926, there were a total of 41 private airplanes registered with the Federal Government. Just three years later, there were 1,454. The Depression severely curtailed private ownership, but although the number of private airplanes plummeted to 241 in 1932, it rose steadily thereafter to 1,473 in 1938, with Wichita, KS, emerging as the Nation’s center of GA production, a distinction it still holds.[780]

Two of the many notable NACA-NASA engineers who were influ­enced by their exposure to Max Munk and had a special interest in GA, and who in turn greatly influenced subsequent aircraft design, were Fred E. Weick and Robert T. Jones. Weick arrived at NACA Langley Field, VA, in the 1920s after first working for the U. S. Navy’s Bureau of Aeronautics.[781] Weick subsequently conceived the NACA cowling that became a feature of radial-piston-engine civil and military aircraft design. The cowling both improved the cooling of such engines and streamlined the engine installation, reducing drag and enabling aircraft to fly higher and faster.

In late fall of 1934, Robert T. Jones, then 23 years old, started a tem­porary, 9-month job at Langley as a scientific aide. He would remain with the Agency and NASA afterwards for the next half-century, being particularly known for having independently discovered the benefits of wing sweep for transonic and supersonic flight. Despite his youth, Jones already had greater mathematical ability than any other of his coworkers, who soon sought his expertise for various theoretical anal­yses. Jones was a former Capitol Hill elevator operator and had previ­ously been a designer for the Nicholas Beazley Company in Marshall, MO. The Great Depression collapsed the company and forced him to seek other employment. His work as an elevator operator allowed him to hone his mathematical abilities gaining him the patronage of senior officials who arranged for his employment by the NACA.[782]

Jones and Weick formed a fruitful collaboration, exemplified by a joint report they prepared on the status of NACA lateral control research. Two things were considered of primary importance in judging the effec­tiveness of different control devices: the calculated banking and yaw­ing motion of a typical small airplane caused by control deflection, and the stick force required to produce this control deflection. The report included a table in which a number of different lateral control devices
were compared.[783] Unlike Jones, Weick eventually left the NACA to con­tinue his work in the GA field, producing a succession of designs empha­sizing inherent stability and stall resistance. His research mirrored Federal interest in developing cheap, yet safe, GA aircraft, an effort that resulted in a well-publicized design competition by the Department of Commerce that was won by the innovative Stearman-Hammond Model Y of 1936. Weick had designed a contender himself, the W-1, and though he did not win, his continued research led him to soon develop one of the most distinctive and iconic "safe” aircraft of all time, his twin-fin and single-engine Ercoupe. It is perhaps a telling comment that Jones, one of aeronautics’ most profound scientists, himself maintained and flew an Ercoupe into the 1980s.[784]

The NACA-NASA contributions to GA have come from research, development, test, and evaluation within the classic disciplines of aero­dynamics, structures, propulsion, and controls but have also involved functional areas such as aircraft handling qualities and aircrew

performance, aviation safety, aviation meteorology, air traffic control, and education and training. The following are selected examples of such work, and how it has influenced and been adapted, applied, and exploited by the GA community.