Category AERONAUTICS

As described earlier in this article, the Navier-Stokes equations are the full equations that govern a viscous flow. Solutions of the Navier-Stokes equations are the ultimate in fluid dynamics. To date, no general analyt­ical solutions of these highly nonlinear equations have been obtained. Yet they are the equations that reflect the real world of fluid dynamics. The only way to obtain useful solutions for the Navier-Stokes equations is by means of CFD. And even here such solutions have been slow in coming. The problem has been the very fine grids that are necessary to define certain regions of a viscous flow (in boundary layers, shear layers, separated flows, etc.), thus demanding huge numbers of grid point in the flow field. Practical solutions of the Navier-Stokes equations had to wait for supercomputers such as the Cray X-MP and Cyber 205 to come on the scene. NASA became a recognized and emulated leader in CFD solu­tions of Navier-Stokes equations, its professionalism evident by its hav­ing established the Institute for Computer Applications in Science and Engineering (ICASE) at Langley Research Center, though other Centers as well, particularly Ames, shared this interest in burgeoning CFD. [778] In particular, NASA researcher Robert MacCormack was responsible for the development of a Navier-Stokes CFD code that, by far, became the most popular and most widely used Navier-Stokes CFD algorithm in the last quarter of the 20th century. MacCormack, an applied math­ematician at NASA Ames (and now a professor at Stanford), conceived a straightforward algorithm for the solution of the Navier-Stokes equa­tions, simply identified everywhere as "MacCormack’s method.”

To understand the significance of MacCormack’s method, one must understand the concept of numerical accuracy. Whenever the derivatives in a partial differential equation are replaced by algebraic difference
quotients, there is always a truncation error that introduces a degree of inaccuracy in the numerical calculations. The simplest finite differences, usually involving only two distinct grid points in their formulation, are identified as "first-order” accurate (the least accurate formulation). The next step up, using a more sophisticated finite difference reaching to three grid points, is identified as second-order accurate. For the numer­ical solution of most fluid flow problems, first-order accuracy is not sufficient; not only is the accuracy compromised, but such algorithms frequently blow up on the computer. (The author’s experience, however, has shown that second-order accuracy is usually sufficient for many types of flows.) On the other hand, some of the early second-order algo­rithms required a large computation effort to obtain this second-order accuracy, requiring many pages of paper to write the algorithm and a lot of computations to execute the solution. MacCormack developed a predictor-corrector two-step scheme that was second-order accurate but required much less effort to program and many fewer calculations to execute. He introduced this scheme in an imaginative paper on hyper­velocity impact cratering published in 1969.[779]

MacCormack’s method broke open the field of Navier-Stokes solu­tions, allowing calculation of myriad viscous flow problems, beginning in the 1970s and continuing to the present time, as was as well (in this author’s opinion) the most "graduate-student friendly” CFD scheme in existence. Many graduate students have cut their CFD teeth on this method and have been able to solve many viscous flow problems that otherwise could not have attempted. Today, MacCormack’s method has been supplanted by several very sophisticated modern CFD algorithms, but even so, MacCormack’s method goes down in history as one of NASA’s finest contributions to the aeronautical sciences.

Goddard Space Flight Center was established in 1959, absorbing the U. S. Navy Vanguard satellite project and, with it, the mission of devel­oping, launching, and tracking unpiloted satellites. Since that time, its roles and responsibilities have expanded to consider space science, Earth observation from space, and unpiloted satellite systems more broadly.

Structural analysis problems studied at Goddard included definition of operating environments and loads applicable to vehicles, subsystems, and payloads; modeling and analysis of complete launch vehicle/payload sys­tems (generic and for specific planned missions); thermally induced loads and deformation; and problems associated with lightweight, deployable structures such as antennas. Control-structural interactions and multi­body dynamics are other related areas of interest.

Goddard’s greatest contribution to computer structural analysis was, of course, the NASTRAN program. With public release of NASTRAN, management responsibility shifted to Langley. However, Goddard remained extremely active in the early application of NASTRAN to practical problems, in the evaluation of NASTRAN, and in the ongoing improvement and addition of new capabilities to NASTRAN: thermal analysis (part of a larger Structural-Thermal-Optical [STOP] program, which is discussed below), hydroelastic analysis, automated cyclic sym­metry, and substructuring techniques, to name a few.[885]

Structural-Thermal-Optical analysis predicts the impact on the per­formance of a (typically satellite-based) sensor system due to the defor­mation of the sensors and their supporting structure(s) under thermal and mechanical loads. After NASTRAN was developed, a major effort began at GSFC to achieve better integration of the thermal and optical analysis components with NASTRAN as the structural analysis compo­nent. The first major product of this effort was the NASTRAN Thermal Analyzer. The program was based on NASTRAN and thereby inherited a great deal of modeling capability and flexibility. But, most impor­tantly, the resulting inputs and outputs were fully compatible with NASTRAN: "Prior to the existence of the NASTRAN Thermal Analyzer, available general purpose thermal analysis computer programs were designed on the basis of the lumped-node thermal balance method.

. . . They were not only limited in capacity but seriously handicapped by incompatibilities arising from the model representations [lumped – node versus finite-element]. The intermodal transfer of temperature data was found to necessitate extensive interpolation and extrapolation. This extra work proved not only a tedious and time-consuming process but also resulted in compromised solution accuracy. To minimize such an interface obstacle, the STOP project undertook the development of a general purpose finite-element heat transfer computer program.”[886] The capability was developed by the MacNeal Schwendler Corporation under subcontract from Bell Aerospace. "It must be stressed, however, that a cooperative financial and technical effort between [Goddard and Langley] made possible the emergence of this capability.”[887]

Another element of the STOP effort was the computation of "view factors” for radiation between elements: "In an in-house STOP proj­ect effort, GSFC has developed an IBM-360 program named ‘VIEW’ which computes the view factors and the required exchange coefficients between radiating boundary elements.”[888] VIEW was based on an ear­lier view factor program, RAVFAC, but was modified principally for compatibility with NASTRAN and eventual incorporation as a subrou­tine in NASTRAN.[889] STOP is still an important part of the analysis of many of the satellite packages that Goddard manages, and work contin­ues toward better performance with complex models, multidisciplinary design, and optimization capability, as well as analysis.

COBSTRAN was a preprocessor for NASTRAN, designed to generate finite element models of composite blades. While developed specifically

for advanced turboprop blades under the Advanced Turboprop (ATP) project, it was subsequently applied to compressor blades and tur­bine blades. It could be used with both COSMIC NASTRAN and MSC/ NASTRAN, and was subsequently extended to work as a preprocessor for the MARC nonlinear finite element code.[984]

1) BLAde SIMulation (BLASIM), 1992

BLASIM calculates dynamic characteristics of engine blades before and after an ice impact event. BLASIM could accept input geometry in the form of airfoil coordinates or as a NASTRAN-format finite ele­ment model. BLASIM could also utilize the ICAN program (discussed separately) to generate ply properties of composite blades.[985] "The ice impacts the leading edge of the blade causing severe local damage. The local structural response of the blade due to the ice impact is pre­dicted via a transient response analysis by modeling only a local patch around the impact region. After ice impact, the global geometry of the blade is updated using deformations of the local patch and a free vibra­tion analysis is performed. The effects of ice impact location, ice size and ice velocity on the blade mode shapes and natural frequencies are investigated.”[986]

The fabrication of turbine blades represents a related topic. No blade has indefinite life, for blades are highly stressed and must resist creep while operating under continuous high temperatures. Table 3 is taken from the journal Metallurgia and summarizes the stress to cause rupture in both wrought – and investment-case nickel – base superalloy.[1092]

An important development involved the introduction of directionally solidified (d. s.) castings. Their advent, into military engines in 1969 and commercial engines in 1974, brought significant increases in allowable metal temperatures and rotor speeds. D. s. blades and vanes were fab­ricated by pouring molten superalloy into a ceramic mold seated on a water-cooled copper chill plate. Grains nucleate on the chill surface and grow in a columnar manner parallel to a temperature gradient. These columnar grains fill the mold and solidify to form the casting.[1093]

A further development involved single-crystal blades. More was required here than development of a solidification technique; it was nec­essary to consider as well the entire superalloy. It was to achieve a high melting temperature by containing no grain boundary-strengthening elements such as boron, carbon, hafnium, and zirconium. It would achieve high creep strength with a high gamma-prime temperature. A high temper­ature for solution heat treatment would also provide improved properties.

The specialized Alloy 454 had the best properties. It showed a com­plete absence of all grain boundary-strengthening elements and made significant use of tantalum, which suppressed a serious casting defect known as "freckling.” Chromium and aluminum were included to protect against oxidation and hot corrosion. It had a composition of 12Ta+4W+10Cr+5Al+1.5Ti+5Co, balance Ni.

Single-crystal blades were fabricated using a variant of the cited d. s. arrangement. Instead of having the ceramic mold rest directly on the chill

plate, it was separated from this plate by a helical single-crystal selector. A number of grains nucleated at the bottom of the selector, but most of them had their growth cut off by its walls, and only one grain emerged at the top. This grain was then allowed to grow and fill the entire mold cavity.

Creep-rupture tests showed that Alloy 454 had a temperature advan­tage of 75 to 125 °F over d. s. MAR-M200 + Hf, the strongest produc­tion-blade alloy. A 75 °F improvement in metal temperature capability corresponds to threefold improvement in life. Single-crystal Alloy 454 thus was chosen as the material for the first-stage turbine blades of the JT9D-7R4 series of engines that were to power the Boeing 767 and the Airbus A310 aircraft, with engine certification and initial production shipments occurring in July 1980.[1094]

In addition to the NASA F-8 DFBW program, several other highly note­worthy efforts involving the use of computer-controlled fly-by-wire flight control technology occurred during the 1970s. The Air Force had initi­ated the Lightweight Fighter program in early 1972. Its purpose was "to determine the feasibility of developing a small, light-weight, low-cost fighter, to establish what such an aircraft can do, and to evaluate its pos­sible operational feasibility.”[1167] The LWF effort was focused on demon­strating technologies that provided a direct contribution to performance, were of moderate risk (but sufficiently advanced to require prototyping to reduce risk), and helped hold both procurement and operating costs down. Two companies, General Dynamics (GD) and Northrop, were selected, and each was given a contract to build two flight-test prototypes. These would be known as the YF-16 and the YF-17. In its YF-16 design, GD chose to use an analog-computer-based quadruplex fly-by-wire flight control system with no mechanical backup. The aircraft had been designed with a negative longitudinal static stability margin of between 7 percent and 10 percent in subsonic flight—this indicated that its center of gravity was aft of the aerodynamic center by a distance of 7 to 10 per­cent of the mean aerodynamic chord of the wing. A high-speed, com­puter-controlled fly-by-wire flight control system was essential to provide the artificial stability that made the YF-16 flyable. The aircraft also incor­porated electronically activated and electrically actuated leading edge maneuvering laps that were automatically configured by the flight con­trol system to optimize lift-to-drag ratio based on angle of attack, Mach number, and aircraft pitch rate. A side stick controller was used in place of a conventional control column.[1168]

Following an exceptionally rapid development effort, the first of the two highly maneuverable YF-16 technology demonstrator aircraft (USAF serial No. 72-1567) had officially first flown in February 1974, piloted by General Dynamics test pilot Phil Oestricher. However, an unin­tentional first flight had actually occurred several weeks earlier, an event that is discussed in a following section as it relates to developmental issues with the YF-16 fly-by-wire flight control system. During its devel­opment, NASA had provided major assistance to GD and the Air Force on the YF-16 in many technical areas. Fly-by-wire technology and the side stick controller concept originally developed by NASA were incor­porated in the YF-16 design. The NASA Dryden DFBW F-8 was used as a flight testbed to validate the YF-16 side stick controller design. NASA Langley also helped solve numerous developmental challenges involving aerodynamics and control laws for the fly-by-wire flight control system. The aerodynamic configuration had been in development by GD since 1968. Initially, a sharp-edged strake fuselage forebody had been elim­inated from consideration because it led to flow separation; however, rounded forward fuselage cross sections caused significant directional instability at high angles of attack. NASA aerodynamicists conducted wind tunnel tests at NASA Langley that showed the vortexes generated by sharp forebody strakes produced a more stable flow pattern with increased lift and improved directional stability. This and NASA research into leading – and trailing-edge flaps were used by GD in the develop­ment of the final YF-16 configuration, which was intensively tested in the Langley Full-Scale Wind Tunnel at high angle-of-attack conditions.[1169]

During NASA wind tunnel tests, deficiencies in stability and control, deep stall, and spin recovery were identified even though GD had pre­dicted the configuration to be controllable at angles of attack up to 36 degrees. NASA wind tunnel testing revealed serious loss of directional stability at angles of attack higher than 25 degrees. As a result, an auto­matic angle of attack limiter was incorporated into the YF-16 flight con­trol system along with other changes designed to address deep stall and spin issues. Ensuring adequate controllability at higher angles of attack also required further research on the ability of the YF-16’s fly-by-wire flight control system to automatically limit certain other flight param­eters during energetic air combat maneuvering. The YF-16’s all-moving

horizontal tails provided pitch control and also were designed to oper­ate differentially to assist the wing flaperons in rolling the aircraft. The ability of the horizontal tails and longitudinal control system to limit the aircraft’s angle of attack during maneuvers with high roll rates at low airspeeds was critically important. Rapid rolling maneuvers at low airspeeds and high angles of attack were found to create large nose-up trim changes because of inertial effects at the same time that the aero­dynamic effectiveness of the horizontal tails was reduced.[1170]

An important aspect of NASA’s support to the YF-16 flight control sys­tem development involved piloted simulator studies in the NASA Langley Differential Maneuvering Simulator (DMS). The DMS provided a real­istic means of simulating two aircraft or spacecraft operating with (or against) each other (for example, spacecraft conducting docking maneu­vers or fighters engaged in aerial combat against each other). The DMS consisted of two identical fixed-base cockpits and projection systems, each housed inside a 40-foot-diameter spherical projection screen. Each projection system consisted of a sky-Earth projector to provide a hori­zon reference and a system for target-image generation and projection. The projectors and image generators were gimbaled to allow visual sim­ulation with completely unrestricted freedom of motion. The cockpits contained typical fighter cockpit instruments, a programmable buffet mechanism, and programmable control forces, plus a g-suit that acti­vated automatically during maneuvering.[1171] Extensive evaluations of the YF-16 flight control system were conducted in the DMS using pilots from NASA, GD, and the Air Force, including those who would later fly the aircraft. These studies verified the effectiveness of the YF-16 fly-by­wire flight control system and helped to identify critical flight control system components, timing schedules, and feedback gains necessary to stabilize the aircraft during high angle-of-attack maneuvering. As a result, gains in the flight control system were modified, and new con-

trol elements—such as a yaw rate limiter, a rudder command fadeout, and a roll rate limiter—were developed and evaluated.[1172]

Despite the use of the DMS and the somewhat similar GD Fort Worth domed simulator to develop and refine the YF-16 flight control system, nearly all flight control functions, including roll stick force gra­dient, were initially too sensitive. This contributed to the unintentional YF-16 first flight by Phil Oestricher at Edwards AFB on January 20, 1974. The intent of the scheduled test mission on that day was to evalu­ate the aircraft’s pretakeoff handling characteristics. Oestricher rotated the YF-16 to a nose-up attitude of about 10 degrees when he reached 130 knots, with the airplane still accelerating slightly. He made small lateral stick inputs to get a feel for the roll response but initially got no response, presumably because the main gear were still on the ground. At that point, he slightly increased angle of attack, and the YF-16 lifted off the ground. The left wing then dropped rather rapidly. After a right roll command was applied, it went into a high-frequency pilot-induced oscillation. Before the roll oscillation could be stopped, the aft fin of the inert AIM-9 missile on the left wingtip lightly touched the runway, the right horizontal tail struck the ground, and the aircraft bounced on its landing gear several times, resulting in the YF-16 heading toward the edge of the runway. Oestricher decided to take off, believing it impossible to stay on the runway. He touched down 6 minutes later and reported: "The roll control was too sensitive, too much roll rate as a function of stick force. Every time I tried to correct the oscillation, I got full-rate roll.” The roll control sensitivity problem was corrected with adjustments to the control gain logic. Stick force gradients and control gains continued to be refined during the flight-test program, with the YF-16 subsequently demonstrating generally excellent control characteristics. Oestricher later said that the YF-16 control problem would have been discovered before the first flight if better visual displays had been available for flight simulators in the early 1970s.[1173] Lessons from the YF-16 and DFBW F-8 simulation experiences helped NASA, the Air Force, and industry refine the way that preflight simulation was structured to support new fly-by-wire flight control systems development. Another flight control issue that arose during

the YF-16 flight-test program involved an instability caused by inter­action of the active fly-by-wire flight control system with the aeroelas – tic properties of the airframe. Flutter analysis had not accounted for the effects of active flight control. Closed loop control systems test­ing on the ground had used simulated aircraft dynamics based on a rigid airframe modeling assumption. In flight, the roll sensors detected aeroelastic vibrations in the wings, and the active flight control system attempted to apply corrective roll commands. However, at times these actually amplified the airframe vibrations. This problem was corrected by reducing the gain in the roll control loop and adding a filter in the feedback patch that suppressed the high-frequency signals from struc­tural vibrations. The fact that this problem was also rapidly corrected added confidence in the ability of the fly-by-wire flight control system to be reconfigured. Another change made as a result of flight test was to fit a modified side stick controller that provided the pilot with some small degree of motion (although the control inputs to the flight con­trol system were still determined by the amount of force being exerted on the side stick, not by its position).[1174]

Three days after its first official flight on February 2, 1974, the YF-16 demonstrated supersonic windup turns at Mach 1.2. By March 11, it had flown 20 times and achieved Mach 2.0 in an outstanding demon­stration of the high systems reliability and excellent performance that could be achieved with a fly-by-wire flight control system. By the time the 12-month flight-test program ended January 31, 1975, the two YF-16s had flown a total of 439 flight hours in 347 flights, with the YF-16 Joint Test Force averaging over 30 sorties per month. Open communications between NASA, the Air Force, and GD had been critical to the success of the YF-16 development program. In highlighting this success, Harry J. Hillaker, GD Vice President and Deputy Program Director for the F-16, noted the vital importance of the "free exchange of experience from the

U. S. Air Force Laboratories and McDonnell-Douglas 680J projects on the F-4 and from NASA’s F-8 fly-by-wire research program.”[1175] The YF-16 would serve as the basis for the extremely successful family of F-16 mul­
tinational fighters; over 4,400 were delivered from assembly lines in five countries by 2009, and production is expected to continue to 2015. While initial versions of the production F-16 (the A and B models) used analog computers, later versions (starting with the F-16C) incorporated digital computers in their flight control systems.[1176] Fly-by-wire and relaxed static stability gave the F-16 a major advantage in air combat capabil­ity over conventional fighters when it was introduced, and this technol­ogy still makes it a viable competitor today, 35 years after its first flight.

The F-16’s main international competition for sales at the time was another statically unstable full fly-by-wire fighter, the French Dassault Mirage 2000, which first flew in 1978. Despite the F-16 being selected for European coproduction, over 600 Mirage 2000s would also eventu­ally be built and operated by a number of foreign air forces. The other technology demonstrator developed under the LWF program was the Northrop YF-17. It featured a conventional mechanical/hydraulic flight control system and was statically stable. When the Navy decided to build the McDonnell-Douglas F/A-18, the YF-17 was scaled up to meet fleet requirements. Positive longitudinal static stability was retained, and a pri­mary fly-by-wire flight control system was incorporated into the F/A-18’s design. The flight control system also had an electric backup that enabled the pilot to transmit control inputs directly to the control surfaces, bypass­ing the flight control computer but using electrical rather than mechan­ical transmission of signals. A second backup provided a mechanical linkage to the horizontal tails only. These backup systems were possible because the F/A-18, like the YF-17, was statically stable about all axes.[1177]

In Japan, the CCV approach that was taken involved modification of a Mitsubishi T-2 jet training aircraft. Horizontal canards were fitted to reduce static stability, and an all-movable vertical surface was added to the forward fuselage to enable direct side force control investiga­tions. The existing wing-mounted flaps were modified to enable direct lift control and maneuver load control studies. A triply redundant dig­ital fly-by-wire flight control system was installed with quadruplex pilot force sensors used to sense stick and rudder pedal forces and air­craft motion sensors. Aircraft motion sensors (such as pitch, roll, and yaw rate gyros, and vertical and lateral acceleration sensors) were also quadruplex. The original mechanical flight control system was retained as a backup mode. Three identical digital computers processed sensor signals, and the resultant command signals were used to con­trol the horizontal stabilizer, leading and trailing edge flaps, rudder, and vertical canard. Electrohydraulic actuators converted electrical
signals into mechanical inputs for the control surface actuators. The CCV T-2 first flew in August 1983. After 24 flights by Mitsubishi, the aircraft was delivered to the Japanese Technical Research Development Institute (TRDI) at Gifu Air Base in March 1984 for government flight­testing, which was completed in March 1986.[1220]

These research programs (along with the Soviet Projekt 100LDU testbed discussed earlier) provided invaluable hands-on experience with state – of-the-art flight control technologies. Data from the Jaguar ACT and the CCV F-104G supported the Experimental Aircraft Program (EAP) and contributed to the technology base for the Anglo-German-Italian – Spanish Eurofighter multirole fighter, now known as the Typhoon. Many other advanced aircraft development programs, including the French Rafale, the Mitsubishi F-2 fighter, the Russian Su-27 family of fighters and attack aircraft, and the entire family of Airbus airliners, were the beneficiaries of these research efforts. In addition, the importance of the infusion of technology made possible by open dissemination of NASA technical publications should not be underestimated.

A second wave of engine-improvement programs was initiated in 1969 and continued throughout the 1970s, as the noise around airports con­tinued to be a social and political issue and the FAA tightened its environ­mental regulations. Moreover, with the oil crisis and energy shortage later in the decade adding to the forces requiring change, the airline indus­try once again turned to NASA for help in identifying new technology.

At the same time, the airline industry was studying the feasibility of introducing a new generation of commuter airliners to fly between cities along the Northeast corridor of the United States. To make these routes attractive to potential passengers, new airports would have to be built as close to the center of cities such as Boston, New York, and Philadelphia. For aircraft to fly into such airports, which would have shorter runways and strict noise requirements, the airliners would have to be capable of making steep climbs after takeoff, quick turns without losing control, and steep descents on approach to landing, accommodat­ing short runways and meeting the standards for Stage 2 noise levels.[1301]

In terms of advancing propulsion technology, NASA’s answer to all of these requirements was the Quiet Clean Short Haul Experimental Engine. Contracts were awarded to GE to design, build, and test two types of high-bypass fanjet engines: an over-the-wing engine and an under-the-wing engine. Self-descriptive as to their place on the airplane, both turbofans were based on the same engine core used in the military F-101 fighter jet. Improvements to the design included noise-reduction features evolved from the Quiet Engine program; a drive-reduction gear to make the fan spin slower than the central shaft; a low-pressure tur­bine; advanced composite construction for the inlet, fan frame, and fan exhaust duct; and a new digital control system that allowed flight com­puters to monitor and control the jet engine’s operation with more pre­cision and quicker response than a pilot could.[1302]

In addition to those "standard” features on each engine, the under – the-wing engine tried out a variable pitch composite low-pressure fan with a 12 to 1 ratio—both features were thought to be valuable in reduc­ing noise, although the variable pitch proved challenging for the GE

team leading the research. Two pitch change mechanisms were tested, one by GE and the other by Hamilton Standard. Both worked well in controlled test conditions but would need a lot of work before they could go into production.[1303]

The over-the-wing engine incorporated a higher fan pressure and a 10 to 1 bypass ratio, a fixed pitch fan, a variable area D-shaped fan exhaust nozzle, and low tip speeds on the fans. Both engines directed their exhaust along the surface of the wing, which required modifica­tions to handle the hot gas and increase lift performance.[1304]

The under-the-wing engine was test-fired for 153 hours before it was delivered to NASA in August of 1978, while the over-the-wing engine received 58 hours of testing and was received by NASA during July of 1977. Results of the tests proved that the technology was sound and, when configured to generate 40,000 pounds of thrust, showed a reduction in
noise of 8 to 12 decibels, or about 60- to 75-percent quieter than the quietest engines flying on commercial airliners at that time. The new technologies also resulted in sharp reductions in emissions of carbon monoxide and unburned hydrocarbons.[1305]

Unfortunately, the new generation of Short Take-Off and Landing (STOL) commuter airliners and small airports near city centers never materialized, so the new engine technology research managed and paid for by NASA but conducted mostly by its industry partners never found a direct commercial application. But there were many valuable lessons learned about the large-diameter turbofans and their nacelles, informa­tion that was put to good use by GE years later in the design and fabri­cation of the GE90 engine that powers the Boeing 777 aircraft.[1306]

While refinements in engine design have been the cornerstone of NASA’s efforts to improve fuel efficiency, the Agency has also sought to improve airframe structures and materials. The ACEE included not only propulsion improvement programs but also efforts to develop light­weight composite airframe materials and new aerodynamic structures that would increase fuel efficiency. Composite materials, which consist of a strong fiber such as glass and a resin that binds the fibers together, hold the potential to dramatically reduce the weight—and therefore the fuel efficiency—of aircraft.

Initially, Boeing began to investigate composite materials, using f iberglass for major parts such as the radome on the 707 and 747 com­mercial airliners.[1450] Starting around 1962, composite sandwich parts comprised of fiberglass-epoxy materials were applied to aircraft such as the Boeing 727 in a highly labor-intensive process.[1451] The next advance in composites was the use of graphite composite secondary aircraft struc­tures, such as wing control surfaces, wing trailing and leading edges, vertical fin and stabilizer control surfaces, and landing gear doors.[1452]

NASA research on composite materials began to gain momentum in 1972, when NASA and the Air Force undertook a study known as Long Range Planning Study for Composites (RECAST) to examine the state of existing composites research. The RECAST study found two major obstacles to the use of composites: high costs and lack of confi­dence in the materials.[1453]

However, by 1976, interest in composite materials had picked up steam because they are lighter than aluminum and therefore have the potential to increase aircraft fuel efficiency. Research on composites was formally wrapped into ACEE in the form of the Composite Primary Aircraft Structures program. NASA hoped that research on composites would yield a fuel savings for large aircraft of 15 percent by the 1990s.

NASA’s efforts under ACEE ultimately led the aircraft manufactur­ing industry to normalize the use of composites in its manufacturing
processes, driving down costs and making composites far more com­mon in aircraft structures. "Ever since the ACEE program has existed, manufacturers have been encouraged by the leap forward they have been able to make in composites,” Jeffrey Ethell, the late aviation author and analyst, wrote in his 1983 account NASA’s fuel-efficiency programs. "They have moved from what were expensive, exotic materials to routine manufacture by workers inexperienced in composite structures.”[1454] Today, composite materials have widely replaced metallic materials on parts of an aircraft’s tail, wings, fuselage, engine cowlings, and landing gear doors.[1455]

NASA research under ACEE also led to the development of improved aerodynamic structures and active controls. This aspect of ACEE was known as the Energy Efficient Transport (EET) program. Aerodynamic structures can improve the way that the aircraft’s geometry affects the airflow over its entire surface. Active controls are flight control systems that can use computers and sensors to move aircraft surfaces to limit unwanted motion or aerodynamic loads on the aircraft structure and to increase stability. Active controls lighten the weight of the aircraft, because they replace heavy hydraulic lines, rods, and hinges. They also allow for reductions in the size and weight of the wing and tail. Both aerodynamic structures and active controls can increase fuel efficiency because they reduce weight and drag.[1456]

One highly significant aerodynamic structure that was explored under ACEE was the supercritical wing. During the 1960s and 1970s, Richard Whitcomb, an aeronautical engineer at NASA Langley Research Center, led the development of the new airfoil shape, which has a flat­tened top surface to reduce drag and tends to be rounder on the bot­tom, with a downward curve at the trailing edge to increase lift. ACEE research at NASA Dryden led to the finding that the supercritical wing could lead to increased cruising speed and flight range, as well as an
increase in fuel efficiency of about 15 percent over conventional-wing aircraft. Supercritical wings are now in widespread use on modern subsonic commercial transport aircraft.[1457]

Whitcomb also conducted research on winglets, which are verti­cal extensions of wingtips that can improve an aircraft’s fuel efficiency and range. He predicted that adding winglets to transport-size aircraft would lead to improved cruising efficiencies between 6 and 9 percent. In 1979 and 1980, flight tests involving a U. S. Air Force KC-135 aerial refueling tanker demonstrated an increased mileage rate of 6.5 percent.[1458] The first big commercial aircraft to feature winglets was the MD-11, built by McDonnell-Douglas, which is now a part of Boeing. Today, winglets can be are commonly found on many U. S.- and foreign-made commercial airliners.[1459]

Laminar flow is another important fuel-saving aircraft concept spear­headed by NASA. Aircraft designed to maximize laminar flow offer the potential for as much as a 30-percent decrease in fuel usage, a benefit that can be traded for increases in range and endurance. The idea behind laminar flow is to minimize turbulence in the boundary layer—a layer of air that skims over the aircraft’s surface. The amount of turbulence in the boundary layer increases along with the speed of the aircraft’s sur­face and the distance air travels along that surface. The more turbulence, the more frictional drag the aircraft will experience. In a subsonic trans­port aircraft, about half the fuel required to maintain level flight in cruise results from the necessity to overcome frictional drag in the boundary layer.[1460]

There are two types of methods used to achieve laminar flow: active and passive. Active Laminar Flow Control (LFC) seeks to reduce turbu­lence in the boundary layer by removing a small amount of fluid (air) from the boundary layer. Active LFC test sections on an aircraft wing contain tiny holes or slots that siphon off the most turbulent air by using an internal suction system. Passive laminar flow does not involve a suc-

An F-1 6XL flow visualization test. This F-1 6 Scamp model was tested in the NASA Langley Research Center Basic Aerodynamics Research Tunnel. This was a basic flow visualization test using a laser light sheet to illuminate the smoke. NASA.

tion system to remove turbulent air; instead, it relies on careful contour­ing of the wing’s surface to reduce turbulence.[1461]

In 1990, NASA and Boeing sponsored flight tests of a Boeing 757 that used a hybrid of both active and passive LFC. The holes or slots used in active LFC can get clogged with bugs. As a result, NASA and Boeing used
a hybrid LFC system on the 757 that limited the air extraction system to the leading edge of the wing, followed by a run of the natural lami­nar flow.[1462] Based on the flight tests, engineers calculated that the appli­cation of hybrid LFC on a 300-passenger, long-range subsonic transport could provide a 15-percent reduction in fuel burned, compared with a conventional equivalent.[1463]

NASA laminar flow research continued to evolve, with NASA Dryden conducting flight tests on two F-16 test aircraft known as the F-16XL-1 and F-16XL-2 in the early and mid-1990s. The purpose was to test the application of active and passive laminar flow at supersonic speeds. Technical data from the tests are available to inform the development of future high-speed aircraft, including commercial transports.[1464]

Today, laminar flow research continues, although active LFC, required for large transport aircraft, has not yet made its way into widespread use on commercial aircraft. However, NASA is continuing work in this area. NASA’s subsonic fixed wing project, the largest of its four aeronautics programs, is working on projects to reduce noise, emissions, and fuel burn on commercial-transport-size aircraft by employing several tech­nology concepts, including laminar flow control. The Agency is hoping to develop technology to reduce fuel burn for both a next generation of narrow-body aircraft (N+1) and a next generation of hybrid wing/body aircraft (N+2).[1465] NASA is expected to conduct wind tunnel tests of two hybrid wing body (also known as blended wing body) aircraft known as N2A and N2B in 2011. Those aircraft, which will incorporate hybrid LFC, are expected to reduce fuel burn by as much as 40 percent.[1466]

Together with this research on emissions and fuel burn has come a heightened awareness on reducing aircraft noise. One example of a very
beneficial technical "fix” to the noise problem is the chevron exhaust nozzle, so called because it has a serrated edge resembling a circular saw blade, or a series of interlinked chevrons. The exhaust nozzle chev­ron has become a feature of recent aircraft design, though how to best configure chevron shapes to achieve maximum noise-reduction bene­fit without losing important propulsive efficiencies is not yet a refined science. The takeoff noise reduction benefits, when "traded off” against potential losses in cruise efficiency, clearly required continued study, in much the same fashion that, in the piston-engine era, earlier NACA engineers grappled with assessing the benefits of the controllable-pitch propeller and the best way to configure early radial engine cowlings. As that resulted in the emergence of the NACA cowling as a staple and indeed, design standard, for future aircraft design, so too, presumably, will NASA’s work lead to better understanding of the benefits and design tradeoffs that must be made for chevron design.[1467]

The ERAST program was organized pursuant to a unique arrange­ment known as a Joint Sponsored Research Agreement (JSRA).[1524] This type of agreement was authorized by the National Aeronautics and Space Act of 1958, and the specific ERAST agreement was authorized under the NASA Administrator’s delegations of March 31, 1992, and February 25, 1994. The purpose of the agreement was to: (1) develop and demonstrate UAV flight capability at altitudes up to 100,000 feet and up to 4 days’ duration; (2) further develop payload integration capabil­ities responsive to the data collection and measurement requirements of the atmospheric community; (3) research activity toward further resolution of UAV certification and civil operational issues; (4) fur­ther demonstrate UAV viability to scientific, Government, and civil users, leading to increased applications for UAVs; and (5) effect tech­nology transfers to the parties to develop a robust United States UAV industry capable of asserting the lead as the premier provider of UAVs for government and civil users worldwide. The agreement, which became effective in September 1994, provided for the terms and conditions of the arrangement, various participation categories,

preliminary budgets for the first 5 years, and operational and reporting requirements.[1525]

The agreement established an Alliance Council, which was to meet at least twice a year, to coordinate with NASA Dryden’s ERAST Project Office on planning the research and development and flight-testing to be performed. The joint agreement provided for ERAST manage­ment through a NASA program manager and a NASA ERAST project manager. Required reports included an annual report, R&D/technical reports, monthly progress reports, intellectual property reports, com­mercialization reports, and management and financial reports. While actual program expenditures could and did vary, NASA’s projected finan­cial commitments from 1994 through 2000 were $2.8 million for 1994, $5.75 million for 1995, $6.05 million for 1996, $6.35 million for 1997, $6.70 million for 1998, $7.25 million for 1999, and $7.25 million for 2000. Finally, the terms and conditions of the agreement provided for extensions through December 31, 2000.[1526]

The above arrangement, however, actually remained in effect after 2000. In 2002, NASA entered into a followup joint agreement with AeroVironment, Inc., of Simi Valley, CA, including its SkyTower sub­sidiary. The new agreement was intended to streamline existing efforts to merge solar-powered UAV development into a single solar-electric plat­form program with the goal of developing multiple aircraft. This collab­orative effort included continued development of the Helios Prototype.[1527]

Industry partners that participated in the JSRA program included four primary companies—AeroVironment, Inc. (builder of the four solar prototype UAV s), Aurora Flight Sciences (manufacture of the Perseus B), General Atomics Aeronautical Systems (builder of the Altus 2), and Scaled Composites (developer of the Proteus). American Technology Alliances (AmTech) served as facilitator for the alliance, and Karen Risa Robbins, a founder of AmTech, played a primary role in development and acceptance of the ERAST JSRA. Of the above companies, AeroVironment was the primary one involved in the solar-powered part of the ERAST pro­
gram. There were up to 28 participants in the alliance, including small businesses, universities, and nonprofit organizations. NASA also worked closely with the Federal Aviation Administration to address a program goal of resolving issues related to operation of UAVs in the National Airspace System, including development of "see and avoid” sensors and "over-the-horizon” communications equipment. Under the joint agree­ment, NASA was able to provide program management and oversight, flight-test facilities, operational support, and project funding. The fund­ing aspect of the joint NASA-industry effort was facilitated because the program was permitted to use Federal Acquisition Regulations as guide­lines rather than as rules. Furthermore, NASA safety regulations were not required to be specifically followed. [1528] As ERAST project manager, NASA Dryden was responsible for the setting of priorities, determina­tion of technical approaches toward meeting project objectives, proj­ect funding and oversight, coordination of facilities for UAV operations, development and coordination of payloads for test flights, and foresight to ensure that actions taken by ERAST alliance partners satisfied NASA’s future needs for UAVs. Each company in the alliance made contributions to the project through combinations of money and services. The ERAST program, however, required only nominal funding by the companies, and, in order to further commercial development of HALE UAVs, NASA offered the companies ownership of all hardware developed by the program.[1529]

Jenny Baer-Riedhart, NASA Dryden ERAST Program Manager for the first 4 years, described the NASA-industry working relationship under the joint agreement as follows:

NASA and the companies agreed on business plans at the annual alliance meeting. Each year at this meeting, I laid out the requirements for the program, based on input from all of the parties. Together, we evaluated our working business plan against these requirements. We set programmatic milestones, as well as milestones for each of the companies.[1530]

Baer-Riedhart added that NASA and the companies put funding into a shared bank account from which AmTech, acting as the go-between for the companies and NASA, distributed the funds to the parties. She noted that, at first, the companies wanted to get their own money, build their own aircraft, and have a flyoff, but that NASA’s vision from the start of the alliance was for the companies to get together to build one aircraft. Jeffrey Bauer, who was the Chief Engineer at Dryden and later served as the last ERAST Program Manager, credits the success of the program to the structure and partnerships that formed the alliance, not­ing: "One of the major attributes of the program is the alliance of gov­ernment and industry. ERAST is not a contract. We work collectively to develop what’s best for the group and community.”[1531] John Del Frate, Dryden solar-power aircraft manager, commenting on the alliance, stated: "The technology early on was immature. We knew there would be prob­lems, but the foundation of the program was built on the premise that we were allowed to take risks, and that made it very successful.”[1532] In addition to Baer-Riedhart, Del Frate, and Bauer, other NASA Dryden senior ERAST program/project managers included James Stewart, John Sharkey, and John Hicks.

Adding an industry perspective to the working relationship between the ERAST alliance partners, Ray Morgan, then vice president of AeroVironment, a company that had over 13 years of experience devel­oping UAVs, noted: "Like most new relationships, the alliance went through an initial courtship phase, followed by a few spats, before it set­tled into an ongoing relationship that worked, more or less, for the good of all.” Morgan added that NASA brought considerable expertise to the program, including vast experience in developing and testing unique air vehicles at high altitudes.[1533]

One area of NASA expertise that Morgan specifically noted was the advice that NASA provided AeroVironment regarding how best to implement redundant systems for critical components, especially where the systems must automatically determine which sensors are work­ing properly and which ones are not. AeroVironment had used "single thread” systems across major components for the first Pathfinder pro­totype, meaning that failure of one component would likely cause fail­
ure of the UAV.[1534] The utilization of redundant systems also extended to other components of vehicle operation. For example, the control sys­tems for the solar-powered UAVs were remotely piloted through a dual radio frequency data link with the vehicle’s automatic control system, which likewise achieved redundancy through the use of two identical flight computers, uplink receivers, and downlink transmitters. In addi­tion, there was a triple set of airspeed sensors and dual Global Positioning System (GPS) receivers.[1535] Even the fuel cells were originally to be com­pletely redundant, but this plan was abandoned because of budget lim­itations and fuel cell development problems. This need for redundant systems in UAVs was reinforced by NASA Dryden’s experience with test­ing UAVs, including a number of program mishaps. The NASA team real­ized that the chance of mission success was greatly improved through the use of redundant systems for the UAVs.

As part of the broad scope of aeronautics research, the rotary wing efforts spanned the full range of research activity, including theoretical study, wind tunnel testing, and ground-based simulation. Flight-test NACA rotary wing research began in the early 1920s with exploratory wind tun­nel tests of simple rotor models as the precursor to the basic research undertaken in the 1930s. The Langley Memorial Aeronautical Laboratory, established at Hampton, VA, in 1917, purchased a Pitcairn PCA-2 auto­giro in 1931 for research use.[269] The National Advisory Committee for Aeronautics had been formed in 1915 to "supervise and direct scien­tific study the problems of flight, with a view to their practical solution.” Rotary wing research at Langley proceeded under the direction of the Committee with annual inspection meetings by the full Committee to review aeronautical research progress. In the early 1940s, the Ames Aeronautical Laboratory, now known as the Ames Research Center, opened for research at Moffett Field in Sunnyvale, CA. Soon after, the Aircraft Engine Research Laboratory, known for many years as the Lewis Research Center and now known as the Glenn Research Center, opened in Cleveland, OH. Each NACA Center had unique facilities that accom­modated rotary wing research needs. Langley Research Center played a major role in NACA-NASA rotary wing research until 1976, when Ames Research Center was assigned the lead role.

The rotary wing research is carried out by a staff of research engi­neers, scientists, technical support specialists, senior management, and administrative personnel. The rotary wing research staff draws on the expertise of the technical discipline organizations in areas such as aero­dynamics, structures and materials, propulsion, dynamics, acoustics, and human factors. Key support functions include such activities as test apparatus design and fabrication, instrumentation research and development (R&D), and research computation support. The constant instrumentation challenge is to adapt the latest technology available to acquiring reliable research data. Over the years, the related challenge for computation tasks is to perform data reduction and analysis for the

increasing sophistication and scope of theoretical investigations and test projects. In the NACA environment, the word "computers” actu­ally referred to a large cadre of female mathematicians. They managed the test measurement recordings, extracted the raw data, analyzed the data using desktop electromechanical calculators, and hand-plotted the results. The NASA era transformed this work from a tedious enterprise into managing the application of the ever-increasing power of modern electronic data recording and computing systems.

The dissemination of the rotary wing research results, which form the basis of NACA-NASA contributions over the years, takes a number of forms. The effectiveness of the contributions depends on making the research results and staff expertise readily available to the Nation’s Government and industry users. The primary method has tradition­ally been the formal publication of technical reports, studies, and com­pilations that are available for exploitation and use by practitioners. Another method that fosters immediate dialogue with research peers and potential users is the presentation of technical papers at confer­ences and technical meetings. These papers are published in the con­ference proceedings and are frequently selected for broader publication as papers or journal articles by technical societies such as the Society of Automotive Engineers (SAE)-Aerospace and the American Institute of Aeronautics and Astronautics (AIAA). Since 1945, NACA-NASA rotary wing research results have been regularly published in the Proceedings of the American Helicopter Society Annual Forum and the Journal of the AHS. During this time, 30 honorary awards have been presented to NACA and NASA researchers at the Annual Forum Honors Night cere­monies. These awards were given to individual researchers and to tech­nical teams for significant contributions to the advancement of rotary wing technology.

Over the years, the technical expertise of the personnel conducting the ongoing rotary wing research at NACA-NASA has represented a valu­able national resource at the disposal of other Government organizations and industry. Until the Second World War, small groups of rotary wing specialists were the prime source of long-term, fundamental research. In the late 1940s, the United States helicopter industry emerged and estab­lished technical teams focused on more near-term research in support of their design departments. In turn, the military recognized the need to build an in-house research and development capability to guide their major investments in new rotary wing fleets. The Korean war marked

the beginning of the U. S. Army’s long-term commitment to the utiliza­tion of rotary wing aircraft. In 1962, Gen. Hamilton H. Howze, the first Director of Army Aviation, convened the U. S. Army Tactical Mobility Requirements Board (Howze Board).[270] This milestone launched the emer­gence of the Air Mobile Airborne Division concept and thereby the steady growth in U. S. military helicopter R&D and production. The working relationship among Government agencies and industry R&D organiza­tions has been close. In particular, the availability of unique facilities and the existence of a pool of experienced rotary wing researchers at NASA led to the United States Army’s establishing a "special relation­ship” with NASA and an initial research presence at the Ames Research Center in 1965. This was followed by the creation of co-located and inte­grated research organizations at the Ames, Langley, and Glenn Research Centers in the early 1970s. The Army organizations were staffed by spe­cialists in key disciplines such as unsteady aerodynamics, aeroelastic- ity, acoustics, flight mechanics, and advanced design. In addition, Army civilian and military engineering and support personnel were assigned to work full time in appropriate NASA research facilities and theoretical analysis groups. These assignments included placing active duty mili­tary test pilots in the NASA flight research organizations. Over the long term, this teaming arrangement facilitated significant research activity. In addition to Research and Technology Base projects, it made it possi­ble to perform major jointly funded and managed rotary wing Systems Technology and Experimental Aircraft programs. The United States Army partnership was augmented by other research teaming agreements with the United States Navy, FAA, the Defense Advanced Research Projects Agency (DARPA), academia, and industry.