Category AERONAUTICS

While most of NASA’s aircraft fuel-efficiency research grew out of the reality jolt of the 1970s oil crisis and environmental concerns, there is at least one notable exception. Researchers first began to investigate liq­uid hydrogen as an alternative to hydrocarbon-based fuel in the mid – 1950s because they suspected major performance efficiencies could be gained.[1468] The Lewis Flight Propulsion Laboratory issued a seminal report in April 1955—although it was not declassified until September 1962—suggesting that liquid hydrogen might have a positive impact on the performance of high-altitude military aircraft (subsonic and super­sonic bombers, fighters, and reconnaissance aircraft flying at 75,000 to 85,000 feet).[1469] While the report raised the aviation community’s aware­ness of the potential for hydrogen as a fuel source, it did not lead to widespread use in aircraft because of technical problems with using

hydrogen inside an aircraft. Again, this early interest in hydrogen did not reflect environmental or conservation concerns, but rather, a desire to achieve much higher flight vehicle performance.

In the report, two NACA researchers—Abe Silverstein and Eldon Hall—argued the case for using liquid hydrogen, noting it has a spe­cial advantage as an aviation fuel: a high heating value. This means that it takes less hydrogen fuel than hydrocarbon fuel to achieve the same thrust. That advantage could prove particularly important at high alti­tudes, the researchers noted, where maximizing fuel efficiency is criti­cal to make up for other penalties associated with flying high.

Indeed, one of the downsides of a high-altitude flight regime—in which atmospheric pressure is low—is that it generally requires heavy, high-pressure-ratio engines to ensure combustion is sustained and thrust levels are adequate. The NACA report speculated that it might be possi­ble to use lighter engines—albeit less-efficient ones, with lower pressure ratios—if liquid hydrogen were used for fuel. Liquid hydrogen requires less combustion volume than hydrocarbons do, making shorter and lighter engines feasible. And, with its high heating value, liquid hydro­gen fuel generates more thrust per pound than hydrocarbons do, even if it’s being used in a light engine running at a lower pressure ratio. The report posited that if every pound of weight saved by using a lighter

engine could be replaced by a pound of liquid hydrogen fuel, an aircraft could be over twice as effective in extending its range at high altitudes.

After Silverstein and Hall issued their report, the NACA conducted experiments showing that hydrogen had a high combustion efficiency in a turbojet combustor even in low-pressure conditions. In 1956, NACA researchers at Lewis made "three completely successful flights” using liquid hydrogen in one engine of a modified Martin B-57B jet bomber, thereby effectively demonstrating that liquid hydrogen could be handled and jettisoned safely and was feasible for use in aircraft.[1470] Meanwhile, from 1956 to 1958, the U. S. Air Force began work on a secret project, known as Suntan, to develop a high-altitude, hydrogen-fueled aircraft with performance superior to the secret U-2 spy plane of the Counter Intelligence Agency (CIA).

The use of liquid hydrogen in aircraft would have marked a major breakthrough in terms of high altitude flight because engine weight is

"the single most important variable determining the height at which an airplane can fly.”[1471] Liquid hydrogen offered the potential to fly at high altitudes at an extended range. Despite its potential, however, nei­ther the NACA nor the Air Force was able to convince enough stake­holders in Government and industry that liquid hydrogen was a viable candidate for aviation fuel.

Hydrogen’s excellent combustion qualities raised questions about whether it could be safely transported or carried inside aircraft. To be sure, NACA flight tests demonstrated that safe handling of hydrogen fuel on the ground and in the air was possible. Also, the Air Force conducted tests in which liquid hydrogen tanks under pressure were ruptured, and it found that in many cases the hydrogen quickly escaped without igni­tion. Yet concerns about safety persisted, and, in a tight budget climate, hydrogen-fuelled aircraft lost out to other priorities. After receiving a full briefing on Suntan, Gen. Curtis E. Lemay, the former head of Strategic Air Command who had moved up to Vice Chief of Staff in July 1957, raised concerns about safety. "What,” he said, "put my pilots up there with a bomb?”[1472]

Others questioned whether using liquid hydrogen would truly yield big gains in aircraft range at high altitudes. Hydrogen has a high vol­ume—10 times that of hydrocarbons—which means that the aircraft fuselage has to be bigger and weigh more to accommodate the fuel. Silverstein and Hall argued that there would be more room for hydro­gen fuel tanks in high-altitude aircraft, which would need larger wings and therefore a bigger fuselage to provide lift in the thin air of the upper atmosphere (a bigger fuselage would mean more room for hydrogen fuel tanks). But while it might have been possible to extend the range of air­craft because of the increased efficiency of liquid hydrogen, others ques­tioned whether hydrogen-fuelled aircraft would still be fairly limited by the tremendous amount of fuel storage capacity that hydrogen requires. Kelly Johnson, the Lockheed Martin engineer who designed the U-2 and the hydrogen-fueled CL-400 for the Air Force’s Suntan project, said he could see a range growth of only 3 percent from adding more hydrogen fuel storage capacity to his CL-400 design. "We have crammed the max­
imum amount of hydrogen in the fuselage that it can hold. You do not carry hydrogen in the flat surfaces of the wing,” he said.[1473]

While liquid hydrogen is highly energetic, it has far less energy density than hydrocarbon fuels. Thus, to get an equivalent amount of energy from hydrogen requires a much greater volume. Accordingly, a hydrogen airplane would have extremely large fuel tanks, which, hav­ing to be supercold as well, pose significant technical challenges to air­craft designers. Researchers have not yet found a way to overcome the challenges associated with hydrogen’s large volume, which forces air­craft design compromises and requires complex ground transportation, storage distribution, and vent capture system. Moreover, hydrogen is not a viable source of energy in itself; producing it requires the use of other sources of energy—such as electric power produced by nuclear fusion as well as a large source of clean water. However, in one respect, hydrogen could "pay back” this "debt,” for it could be used to enrich the production process of synthetic fuel, achieving similar production efficiencies while reducing the amount of water and coal traditionally required for enrichment.[1474]

Despite these technical challenges, NASA’s research on the use of hydrogen to power aircraft did lead to some important findings: namely, that hydrogen is a potentially promising turbojet fuel in a high-altitude, low-speed flight regime. These conditions favor a fuel that can operate efficiently in low-pressure conditions. High altitudes also favor a large – volume aircraft, helping to offset the disadvantage of hydrogen’s low den­sity. Given these attractive characteristics, the prospect of using hydrogen as an aircraft propellant has continued to resurface in the past decade, especially when the cost of hydrocarbon-based fuel rises. For example, NASA’s Zero CO2 research project sought to eliminate carbon dioxide and lower NOx emissions by converting propulsion systems to hydro­gen fuel.[1475] One new propulsion technology that NASA engineers consid­ered as part of Zero CO2 was the use of fuel cells, which are discussed below. A NASA Glenn Web page updated as recently as 2008 says that the Combustion Branch of NASA’s Propulsion Division is still studying
hydrogen combustion to demonstrate that hydrogen can be used as an aviation fuel to minimize emissions.[1476]

The NACA’s early research on hydrogen-fuelled aircraft also cre­ated an awareness of hydrogen as a potential fuel source that did not exist before Silverstein and Hall embarked on their study. This aware­ness helped lead to important breakthroughs in rocketry and fuel cell research. In particular, research on the use of hydrogen in air-breath­ing aircraft laid the groundwork for the successful development of hydrogen-fueled rockets in the mid-1950s. In fact, Silverstein and Hall’s research helped to inform NASA’s decision in 1959 to use liquid hydro­gen as a propellant in the upper stage of the Saturn launch vehicle. That decision was one of the keys to the success of the Apollo Moon landing missions of the 1960s and 1970s.[1477]

The NACA’s early efforts to draw attention to hydrogen as a power source also led to the development of fuel cells for the Apollo and Gemini capsules. Apollo employed the world’s first fuel cells, which used hydro­gen and oxygen to generate onboard power for Apollo command and ser­vice modules. Fuel cells are essentially plastic membranes treated with a special catalyst; hydrogen seeps into the membrane and meets up with the oxygen inside to generate electricity and water. The fuel cells used on the Apollo proved so successful that they were once again employed on the Space Shuttle orbiter.

The first-generation ERAST solar-test program HALE vehicle was the Pathfinder, which was designed and built by AeroVironment. AeroVironment’s earlier solar aircraft projects included the building of the piloted Gossamer Albatross and a scaled-down version known as the Gossamer Penguin. This experience assisted the company in building the Pathfinder UAV. In addition to Ray Morgan, who was vice president of AeroVironment, the company team included a number of experienced engineers and technicians, including William Parks, who was the com­pany’s chief engineer for the Centurion and Helios Prototype UAVs, and Robert Curtin and Kirk Flittie, who both served as project and later as program managers. The program brought honors for Bob Curtin and Ray Morgan, who both received the Aviation Week Laurel Award in 1996.[1536]

Pathfinder, which initially was battery-powered, was a remotely piloted flying wing that demonstrated a number of technologies, includ­ing lightweight composite structures, low wing loading flying-wing configuration, redundant and fault tolerant flight control systems,

high-efficiency electric motors, thermal control systems for high-alti­tude flight, and a high specific power solar array. The remotely piloted Pathfinder had 6 electric motors that each weighed 13 pounds and con­sisted of a fixed-pitch 79-inch propeller and a solid-state motor with inter­nal power electronics, nacelle, and cooling fins. Differential power to two wingtip motors on either side was used for lateral control. Wing dihedral (upsweep) provided roll stability, and 26 elevator control surfaces were attached to the wing’s trailing edge for pitch control. Pathfinder’s solar array generated approximately 8,000 watts near solar noon. The solar UAV could obtain an airspeed of between 15 and 25 mph and a cruising speed of between 17 to 20 mph. The vehicle had a length of 12 feet, a wingspan of 98.4 feet, a wing chord (front to rear distance) of 8 feet, a gross weight of approximately 560 pounds, a payload capacity of up to 100 pounds, a wing aspect ratio (the ratio between the wingspan and the wing chord) of 12 to 1, and a power-off glide ratio of 18 to 1. Pathfinder had a maxi­mum bank rate of 5 degrees and a maximum turn rate of 3 degrees per second at sea level and 1.7 degrees at 60,000 feet.[1537]

To gain some introductory understanding and experience with the challenges and nuances of solar cell operation, prior to the official start of the ERAST program and the transfer of Pathfinder from BMDO, the project team had arranged for some solar cell flight tests on local sorties over the Edwards dry lake. Pathfinder itself was not equipped with solar arrays on early test flights because the ERAST alliance did not want to risk any damage to the expensive solar cell arrays until Pathfinder’s fly­ing capabilities could be tested. Pathfinder’s gross weight of 560 pounds produced a very low wing-loading load distribution of less than 0.64 pounds per square foot that significantly increased sensitivity to winds during takeoff and landing. This necessitated special training for the ground controllers, especially during takeoff and landing of the UAV. Pathfinder’s first foray to high altitude took it to 50,500 feet and proved immensely productive. "We learned tons from that flight,” John Del Frate recalled afterward, noting, "There were a lot of naysayers that were qui­eted after that flight.”[1538] Unfortunately, afterward the vehicle dramati­cally demonstrated its sensitivity to wind, being seriously damaged in its hangar when ground crews opened both hangar doors, thus creat­ing a draft that blew Pathfinder into a jet that was in the same hangar.

After a number of developmental flights and further modifications at Dryden, Pathfinder was transported to the Island of Kauai, which offered a more favorable wind environment and a greater operational area with less competing air traffic. From testing at Edwards Air Force Base, the NASA-industry alliance learned that weather factors—including wind, turbulence, cloud cover, humidity, temperature, and pressure—were crit­ical in attempting to fly the wing-loaded Pathfinder at high altitudes. In addition, the team noted that the UAV s would probably not be flying in the same conditions as found in standard atmosphere reference tables because testing indicated a surprising variance in actual temperature in comparison with the tables. The team also noted that the higher a solar UAV flies, the greater the downwind drift distance if activation of a flight termination system (FTS) is required.[1539] These factors required careful study of historical weather patterns to determine the optimum

site to attempt to set a world record UAV altitude flight. Accordingly, NASA selected the Navy’s Pacific Missile Range Facility in Hawaii as the location to test the high-altitude capabilities of the solar UAVs.[1540]

In Hawaii, Pathfinder was flown for seven additional flights, one of which in 1997 set a world record of over 71,500 feet for a high-altitude flight by a propeller-driven aircraft. This broke a 1995 Pathfinder test alti­tude flight record of 50,500 feet, which had earned NASA recognition as 1 of the 10 most memorable record flights of 1995. Pathfinder test flights also demonstrated solar-powered HALE vehicles’ potential as platforms for environmental monitoring and technical demonstration missions by gaining additional information relating to the Island of Kauai’s terrestrial and coastal ecosystems. These science missions, which employed specially build lightweight sensor systems (see below), included detection of for­est nutrient status, forest regrowth after damage from Hurricane Iniki in 1992, sediment concentrations in coastal waters, and assessment of coral reef conditions. Experience from Pathfinder test flights, in combination with other UAV testing, also yielded a number of lessons regarding hard­ware reliability, including the following recommended procedures: (1) testing the airframe structure as much as possible before flight, particu­larly the composite airframe joint bondings; (2) testing the vehicle’s sys­tems in an altitude chamber because of the extreme cold and low-pressure conditions encountered by high-altitude science aircraft; (3) recognizing that UAVs, like aircraft, have a tendency to gain weight; (4) maintaining strict configuration control; and (5) ensuring that a redundant system is functional before switching from the primary system.[1541]

The transformation of the NACA into NASA in 1958 was marked by an inevitable subordination of the NACA’s aeronautical research char­ter to NASA’s mandated space mission work. The assigned aeronau­tics staff dropped over 80 percent, from 7,100 to 1,400, as the space program gained momentum in the early 1960s. In the new space – focused environment, aeronautics needed to be product-oriented to attract budget allocation support. In these circumstances, helicop­ter research lost ground as the focus shifted to new nonrotor Vertical Take-Off and Landing (VTOL) and Short Take-Off and Landing air­craft. In many cases, the rotary wing work formed the base for VTOL investigations. In the case of NACA-NASA rotor-flow studies, exper­imental and theoretical studies on rotor-time-averaged inflow led to extensive work on establishing wind tunnel jet-boundary layer (wall interference) correction methodology for other VTOL, as well as rotor – borne, lifting systems.[283]

In a sense, it became the U. S. Army’s turn to bolster NASA rotary wing endeavors in support of the Army’s need for continued helicop­ter development. In 1965, the Army was granted permission to reacti­vate, staff, and utilize the Ames 7- by 10-foot Tunnel No. 2. In addition, the Army provided personnel to assist Ames in carrying out projects of interest to the Army. A group of about 45 people was established by the Army and identified as the Army Aeronautical Activity at Ames (AAA – A).[284] In 1970, the working relationship between NASA and the Army was significantly enhanced. Co-located Army research organizations were established at Ames, Langley, and Lewis (now Glenn) Research Centers. They focused on the respective Center’s specialty of aeroflight dynamics, structures, and propulsion. This teaming laid the solid groundwork for major rotary wing programs that NASA and the Army jointly planned, executed, and funded in the 1970s and 1980s that influenced both mil­itary and civilian rotary wing aircraft development.

One of the unique research facilities authorized in 1939 and oper­ated by the NACA, and then NASA, was the 40- by 80-foot Full-Scale Tunnel at Ames. This research facility also provided the opportunity to work directly with industry on vehicle development programs. In the case of rotary wing aircraft, the tunnel was utilized for investigating new vehicle and rotor system concepts and for thoroughly documenting the basic aerodynamic behavior of prototype and production articles. By the 1960s, numerous in-house and industry full-scale rotary wing hardware were tested. Examples include the Bell XV-1 "convertiplane” in 1953­1954, followed by many other projects, including a modified production rotor incorporating leading edge camber and boundary-layer control; the Bell UH-1 "Huey” helicopter (tested to assist in the development of a high-performance flight-test helicopter); a folded rotor with test data obtained in start-stop and folding conditions at forward speeds; and a four-bladed rotor investigation with extensive rotor-blade pressure mea­surements taken as a followup to prior flight test measurements made at Langley Research Center.[285]

The pressure-instrumented blade used in the latter tests had an extremely limited operating life of only 10 hours. This was because of the installation of nearly 50 miniature differential pressure transduc­ers inside the rotor blade. This required that a total of almost 100 small holes be drilled in the upper and lower surface of the primary structure D-spar—normally an absolute "safety of flight” violation. The conserva­tive 10-hour limit was based upon conservative crack-growth-rate limits determined from blade specimen cyclic load tests. The earlier flight test investigation of blade pressure distributions produced a very significant contribution as a primary database for the understanding of basic rotor unsteady aerodynamics. The tabulated pressure data provided time his­tories of individual differential pressures and simultaneous blade bend­ing moments around the rotor azimuth in a wide assortment of steady and maneuvering flight conditions.[286] This database became the standard experimental data reference source for advancing theoretical comparison work for many years. As an aside, in working with the original flight data to hand-digitize the detailed recordings of differential pressure time-his­tory traces, it became possible, in time, to visually recognize the specific flight-test condition by the periodic pressure trace signature.[287] It was pos­sible to identify the rotor’s actual flight condition relative to the surround­ing airmass. This still raises the question of the possibility of applying modern signal recognition technology to provide on-board safety-of-flight and noise abatement operating boundary displays for the pilot.

Flying qualities flight investigations emphasized the importance of ample damping of angular velocity and of control power (rotor-gener­ated aircraft pitch and roll control moments) and their interaction. This work at Langley and similar work at Ames provided a significant portion of the helicopter flying qualities criteria. This early work was extended to the use of in-flight simulation using Langley’s YHC-1A tandem rotor helicopter with special onboard computing and recording equipment.[288]

The flight operations of most interest were terminal area instrument flight on steep approaches to vertical touchdown landings. The results of this work were initially oriented to nonrotor VTOL operations, but the results were found to be equally applicable to helicopters.

In the area of structural dynamics, investigations addressing the problems of aeroelastic stability of rotor-powered aircraft were con­ducted utilizing new analytical methods and experimental studies by Langley and Ames researchers. Emphasis was placed on tilt rotor and tilt propeller (i. e., tilt wing) aircraft concepts. Two-degree-of-freedom "air resonance” (akin to rotor-fuselage "ground resonance”) and prop – rotor/propeller whirl instability were among the problems investigated.[289] Rotor-pylon-wing aeroelastic instability problems for tilt rotor designs were explored in the Ames 40 by 80 Full-Scale Tunnel in this period. The aeroelastic stability problems of the tilt rotor and tilt-stopped rotor designs were also investigated at model scale in the unique Freon atmo­sphere of the Langley Transonic Dynamics Tunnel, which provided full – scale Mach number and Reynolds number scaling.[290] These research

investigations resulted in significant contributions to the development of the validated design tools for advanced rotorcraft.

With the increased interest in hingeless rotor concepts, NASA obtained and quickly accomplished flight research with a copy of an experimental Bell Helicopter three-bladed hingeless rotor installed on an H-13 helicopter.[291] Early experience with "rigid” rotors had led the NACA to encourage interest in exploring the possibilities of removing conventional blade-root hinges and substituting instead blade struc­tural flexibility. Another manufacturer, Lockheed Aircraft, made a major commitment to the hingeless rotor concept coupled to a mast-mounted mechanical gyro introduced into the pitch control linkage.[292] The root regions of the blades in this innovative design were "matched stiffness” or "soft in-plane,” which meant that the blade chord-wise, or horizon­tal, structural bending stiffness was matched to the flap-wise, or vertical, bending stiffness. Dynamic model tests of this concept were conducted in the Langley 30 by 60 Full-Scale Tunnel and in the Freon atmosphere of the Langley Transonic Dynamics Tunnel. The use of Freon gas facilitated the testing of the 10-foot-diameter rotor model at full-scale Reynolds number and Mach numbers. This work began the establishment of a documented database for hingeless rotor design. These dynamic model tests were part of a cooperative NASA-Army AVLABS program.

To further explore the problems and practical means for realizing the potential of the hingeless rotor concept, Langley Research Center purchased the Lockheed XH-51N, a high-speed research helicopter. The flight investigation focused on the tendency for hingeless rotors to encounter high in-plane blade loads in roll maneuvers, coupling between the response to longitudinal and lateral control input, ride quality, and pilot handling qualities. In general, it was demonstrated with the flight tests and model tests that the hingeless rotor system was different from the conventional hinged systems. Inherently, the hingeless designs pro­duced increased control moments, quicker response to pilot input and superior handling qualities. It turned out that later rotor designs incor­porating elastomeric bearings to replace conventional hinges could pro­vide a practical option to some of the fully hingeless designs.

During the Second World War, Germany held global leadership in high­speed aerodynamics. The most impressive expression of its technical interest and competence in high-speed aircraft and missile design was the V-2 terror weapon, which introduced the age of the long-range rocket. It had a range of over 200 miles at a speed of approximately Mach 5.[547] A longer-range experimental variant tested in 1945, the A-4b, sported swept wings and flew at 2,700 mph, reentering and leveling off in the upper atmosphere for a supersonic glide to its target. In its one semi­successful flight, it completed a launch and reentry, though one wing broke off during its terminal Mach 4+ glide.[548] One appreciates the ambi­tious nature and technical magnitude of the German achievement given that the far wealthier and more technically advantaged United States pursued a vigorous program in piloted rocket planes all through the 1950s without matching the basic performance sought with the A-4b.

Key to the German success was a strong academic-industry part­nership and, particularly, a highly advanced complex of supersonic wind tunnels. The noted tunnel designer Carl Wieselsberger (who died of cancer during the war) introduced a blow-down design that initially operated at Mach 3.3 and later reached Mach 4.4. The latter instrument supported supersonic aerodynamic and dynamic stability studies of var­ious craft, including the A-4b. German researchers had ambitious plans for even more advanced tunnels, including an Alpine complex capable of attaining Mach 10. This tunnel work inspired American emulation after the war and, in particular, stimulated establishment of the Air Force’s Arnold Engineering Development Center at Tullahoma, TN.[549]

At war’s end, America had nothing comparable to the investment Germany had made in high-speed flight, either in rockets or in wind tun­nels and other specialized research facilities. The best American wartime tunnel only reached Mach 2.5. As a stopgap, the Navy seized a German facility, transported it to the United States, and ran it at Mach 5.18, but

HIGH PRESSURE TANK	HIGH VACUUM -PUMP
LOW PRESSURE TANK

DIFFUSER

The layout of the Langley 11-inch hypersonic tunnel advocated by John V. Becker. NASA.

it did this only beginning in 1948.[550] Even so, aerodynamicist John Becker, a young and gifted engineer working at the National Advisory Committee for Aeronautics (NACA) Langley Laboratory, took the initiative in intro­ducing Agency research in hypersonics. He used the V-2 as his rationale. In an August 1945 memo to Langley’s chief of research, written 3 days before the United States atom-bombed Hiroshima, he noted that planned NACA facilities were to reach no higher than Mach 3. With the V-2 having already flown at Mach 5, he declared, this capability was clearly inadequate.

He outlined an alternative design concept for "a supersonic tunnel having a test section four-foot square and a maximum test Mach number of 7.0.”[551] A preliminary estimate indicated a cost of $350,000. This was no mean sum. It was equivalent six decades later to approximately $4.2 mil­lion. Becker sweetened his proposal’s appeal by suggesting that Langley

begin modestly with a small demonstration wind tunnel. It could be built for roughly one-tenth of this sum and would operate in the blow-down mode, passing flow through a 1-foot-square test section. If it proved suc­cessful and useful, a larger tunnel could follow. His reasoned idea received approval from the NACA’s Washington office later in 1945, and out of this emerged the Langley 11-Inch Hypersonic Tunnel. Slightly later, Alfred J. Eggers began designing a hypersonic tunnel at the NACAs West Coast Ames Aeronautical Laboratory, though this tunnel, with a 10-inch by 14-inch test section, used continuous, not blow-down, flow. Langley’s was first. When the 11-inch tunnel first demonstrated successful operation (to Mach 6.9) on November 26, 1947, American aeronautical science entered the hyper­sonic era. This was slightly over a month after Air Force test pilot Capt. Charles E. Yeager first flew faster than sound in the Bell XS-1 rocket plane.[552]

Though ostensibly a simple demonstration model for a larger tun­nel, the 11-inch tunnel itself became an important training and research tool that served to study a wide range of topics, including nozzle devel­opment and hypersonic flow visualization. It made practical contribu­tions to aircraft development as well. Research with the 11-inch tunnel led to a key discovery incorporated on the X-15, namely that a wedge­shaped vertical tail markedly increased directional stability, eliminat­ing the need for very large stabilizing surfaces. So useful was it that it remained in service until 1973, staying active even with a successor, the larger Continuous Flow Hypersonic Tunnel (CFHT), which entered ser­vice in 1962. The CFHT had a 31-inch test section and reached Mach 10 but took a long time to become operational. Even after entering ser­vice, it operated much of the time in a blow-down mode rather than in continuous flow.[553]

Nearly all high-speed airplanes use hydraulic actuators to operate the control surfaces. This provides a significant boost to the pilot’s ability to move a large control surface, which is experiencing very high aero­dynamic loads. The computers and other electronic devices mentioned above merely provided signals to servos, which in turn commanded movement of hydraulic actuators. The hydraulic system provided the real muscle to move the surfaces. When Lockheed Martin’s "Skunk Works” was designing the planned X-33 Research Vehicle (intended to explore one possible design for a single-stage-to-orbit logistical spacecraft), keeping gross lift-off weight (GLOW) as low as possible was a crucial design goal. Because the hydraulic system would have been employed only during boost and entry, the entire hydraulic system would have been dead weight while the vehicle was in the space environment. Thus, control system designers elected to use electro-mechanical actuators to move the control surfaces, eliminating any need for a hydraulic system. Though X-33 was canceled for a variety of other reasons, its provision for electrical actuators clearly pointed toward future design practice.

Following up on this were a series of three flight-test projects dur­ing 1997-1998 as part of the Electrically Powered Actuator Design (EPAD) program sponsored by the Naval Air Warfare Center and Air Force Research Laboratory. Each project tested a different advanced flight control actuator for the left aileron of NASA Dryden’s F/A-18 Systems Research Aircraft (SRA). The first was the "smart actuator” that used fiber optics instead of the normal fly-by-wire system to con-

trol an otherwise conventional hydraulic actuator.[708] The next project flew an electro-hydrostatic actuator that used an electric motor to drive a small hydraulic pump that actuated the left aileron; the actuator was independent of the normal aircraft hydraulic system.[709] The third project used an electro-mechanical actuator (EMA) that used electrical power generated by the F/A-18’s engines to power the left aileron actuator. A fiber-optic controller, self-contained control-surface actuator promises a significant reduction in weight and complexity over conventional actu­ation systems for future advanced air and space vehicles.[710]

Prior to the X-15 flight-test program, there were several theories pre­dicting the amount of friction heat that would transfer to the surface of a winged aircraft, with substantial differences in the theories. Wind tunnels, ballistic ranges, and high-temperature facilities such as arc-jets were unable to adequately duplicate the flight environment necessitating

full-scale flight test to determine which theory was correct. The X-15 was that test aircraft. The design needed to be robust in order to survive the worst-case heating predictions if the theories proved to be correct.

A heat-sink structure was selected as the safest and simplest method for providing thermal protection. Inconel X, a nickel alloy normally used for jet engine exhaust pipes, was selected as the primary structural mate­rial. It maintained adequate structural strength to about 1,200 °F. The design proceeded by first defining the size of each structural member based on the air loads anticipated during entry, then increasing the size of each member to absorb the expected heat load that would occur dur­ing the short exposure time of an X-15 flight.

As with most of the first missile and aircraft explorations, early hyper­sonic flights in the X-15 showed that none of the prediction methods was completely accurate, although each method showed some validity in a cer­tain Mach range. In general, the measured heat transfer was less than pre­dicted. Thus, one of the most significant flight-test results from the X-15 program was development of more accurate prediction methods based upon real-world data for the thermal protection of future hypersonic and entry vehicles.[750] The majority of aerodynamic heating issues that required attention during the X-15 flight-test program were associated with local­ized heating: that is, unexpected hot spots that required modification. Some typical examples included loss of cockpit pressurization because of a burned canopy seal (resolved by installing a protective shield in front of the canopy gap), cockpit glass cracked because of deformation of the glass retainer ring (resolved by increasing the clearance around the glass), wing skin buckling behind the slot in the leading edge expansion joint (resolved by installing a thin cover over the expansion joint), thermal expansion of the fuselage triggering nose gear door deployment with resulting damage to internal instrumentation (resolved by increasing the slack in the deploy­ment cable), and buckling of skin on side tunnel fairings because of large temperature difference between outer skin and liquid oxygen (LOX) tank (resolved by adding expansion joints along the side tunnels).

Most of these issues were discovered and resolved fairly easily since the flight envelope was expanded gradually on successive flights with small increases in Mach number on each flight. Had the airplane been exposed to the design entry environment on its very first flight, the

combined results of these local heating problems would probably have been catastrophic.

Not only has NASA played a strong role in the development of new CFD algorithms, it has delivered these contributions to the technical pub­lic in the form of highly developed computer codes for the user. In the context of this survey, it would be remiss not to underscore the impor­tance of these codes, three in particular, which this author (and his stu­dents) have used as numerical tools for carrying out research: LAURA, OVERFLOW, and CFL3D.

The LAURA code was developed principally by Dr. Peter Gnoffo at the NASA Langley Research Laboratory.[783] This code solves the three­dimensional Euler or Navier-Stokes equations for high-speed super­sonic and hypersonic flow fields. It is particularly noteworthy because
it deals with very detailed nonequilibrium and equilibrium chemically reacting flows pertaining to hypersonic reentry vehicles in Earth’s and foreign planetary atmospheres. Some applications involve flow-field temperatures so high that radiation becomes a dominant physical fea­ture. The LAURA program readily handles radiative gas dynamics, and, to this author’s knowledge, it is the only existing standard code to do so. The LAURA code has been used for the design and analysis of all NASA entry bodies in recent experience and is the most powerful and useful code in existence for high-temperature flow fields.

Of particular use for computing lower speed subsonic and transonic flows is OVERFLOW. This code was developed in the early 1990s by Pieter Burning and Dennis Jesperson as a collaborative effort between NASA Johnson Space Center and the NASA Ames Research Center. It solves the compressible three-dimensional Reynolds-averaged Navier – Stokes equations by means of a time-marching algorithm. OVERFLOW is widely used for the calculation of three-dimensional subsonic and tran­sonic flows, and it proved particularly valuable for computing subsonic viscous flows over airfoils in a recent graduate study of innovative new airfoil shapes for high lift undertaken at the University of Maryland at College Park’s Department of Aerospace Engineering.[784]

In the mid-1980s, Dr. Jim Thomas and his colleagues at the NASA Langley Research Center recognized the need for a code that contained the latest advancements in CFD methodology being developed by the applied mathematics community. Out of their interest sprang CFL3D, one of the earliest (yet still most powerful) CFD codes developed by NASA.[785]

This code is applicable across the whole flight spectrum, from low-speed subsonic flow to hypersonic flow. Not only does it handle steady flows, but it calculates time-accurate unsteady flows as well. Much effort was invested in the development of detailed grids so that it readily handles flows over complex three-dimensional bodies. An appreciation of the power, usefulness, and widespread acceptance of CFL3D can be gained by noting that it is used by over 100 researchers in 22 companies, 13 universities, NASA, and the military services.

LAURA, OVERFLOW, and CFL3D are just three of the CFD codes NASA researchers have generated. Most importantly, because they are the product of taxpayer-supported research, all are readily available, free of charge, to the general public, making NASA unique among other organi­zations working in the field of CFD. NASA’s commitment to making sci­entific and technical information of the highest quality available to the public—a legacy of its predecessor, the National Advisory Committee for Aeronautics—has influenced its approach to CFD code development and may be counted one of the Agency’s most valuable contributions to the whole discipline of computational fluid dynamics. When students and professional practitioners alike need viable computer codes for complex fluid dynamic applications, they have ready access to such codes and the extremely competent individuals who develop them. This is perhaps the highest accolade one can pronounce upon NASA’s computational fluid dynamics efforts.

In closing, a proper history of CFD would require a lengthy book and a greater perspective of the past: something yet impossible, for the history of this rather young discipline is still evolving. The challenge is akin to what one might have expected trying to write a history of the balloon in the early 1800s, or a history of flight in 1914. In this case, I have tried to share my perspective in an accessible format, based in part on my own experiences and on my familiarity with the work of many colleagues, especially those within NASA. I have had to leave out so many others and so much great work in CFD just to tell a short story in a limited amount of pages that I feel compelled to apologize to those many others that I have not men­tioned. To them I would say that their absence from this case certainly does not mean their contributions were any less important. But this has been an effort to paint a broad-stroke picture, and, like any such picture, it is somewhat subjective. My best wishes go out to all those researchers, pres­ent and future, who have and will continue to make computational fluid dynamics a vital, essential, and lasting tool for the study of fluid dynamics.

Langley was the first NACA laboratory, established in 1917. As such, it is the oldest and most distinguished of NASA aeronautics Centers, with a pedigree that dates to meetings held prior to the First World War to determine the future aeronautical laboratory structure of the Nation. Since the earliest days of American aviation, Langley has constantly anticipated, reacted, and adapted as necessary to meet the Nation’s aeronautical research needs, reflecting its broad technical capabilities and expertise in areas such as aerodynamics, aircraft and spacecraft structures, flight dynamics, crew systems, space environmental phys­ics, and life sciences.

Among the very earliest NACA technical reports were several con­cerning loads calculation and structural analysis, some of which are cited in the introduction to this paper. These papers, and others that fol­lowed throughout the era of the NACA, were widely used in the aircraft industry. By the time NASA was founded, Langley had become a major Center for all forms of aeronautics research, engineering, and analysis.

Through the 1980s and1990s, Langley had approximately 150 tech­nical professionals in the structural disciplines (not including Materials), covering both aircraft and spacecraft applications. This work was orga­nized primarily in two divisions, Structural Mechanics (static prob­lems) and Structural Dynamics, plus a separate Optimization Methods group of approximately 15 members.[908] Structural Mechanics included Composites, Computational Structural Mechanics, Thermal Structures, Structural Concepts, and AeroThermal Loads.[909] Structural Dynamics included Aeroelasticity, Unsteady Aerodynamics, Aeroservoelasticity, Landing and Impact Dynamics, Spacecraft Dynamics, and Interdisciplinary Research.[910] (Reorganizations sometimes changed the specific delineation of responsibilities.) Langley researchers pur-

sued many separate computational structural analysis studies and efforts, but overall, the Center was particularly (and intimately) involved with NASTRAN, the Design Analysis Methods for Vibration (DAMVIBS) rotorcraft structural dynamics modeling program, and efforts at integration and optimization.

After NASTRAN was developed during the period from 1965 to 1970, management of it was transferred from Goddard to Langley. Accordingly, a major emphasis at Langley through the 1970s was the maintenance and continuing improvement of NASTRAN. The first four Users’ Colloquia were held at Langley. While COSMIC handled the administrative aspects of NASTRAN distribution, the NSMO was responsible for technical management and coordinating NASTRAN development efforts across all Centers and many contractors. The program itself is discussed in greater detail elsewhere in this case.

The DAMVIBS research program, conducted from 1984 to 1991, reflected Langley’s long-standing heritage of research on rotorcraft struc­tural dynamics. DAMVIBS achieved concrete advances in the industry state of the art in helicopter structural dynamic modeling, analysis-to-test matching, and, perhaps most importantly, acceptance of and confidence in modeling as a useful tool in designing helicopter rotor-airframe systems for low vibration. Key NASA program personnel were William C. Walton, Jr., who spearheaded program concept and initial direction (he retired in 1984); Raymond G. Kvaternik, who furnished program direction after 1984; and Eugene C. Naumann, who supplied critical technical guidance. The industry participants were Bell Helicopter Textron, Boeing Helicopters, McDonnell-Douglas Helicopter Company, and Sikorsky Aircraft. The participants developed rotor-airframe finite element models, conducted ground vibration tests, made test/analysis comparisons, improved their models, and conducted further study into the "difficult components” that current state of the art rotorcraft analysis could not adequately model.[911]

Modeling "guides”—documented procedures—were identified from the start as a key element to the program:

This program emphasized the planning of the modeling. . .

the NASA Technical Monitor insisted on a well thought out

plan of attack, accompanied by detailed preplanned instruc­tions. . . . The plan was reviewed by other industry representa­tives prior to undertaking the actual modeling. Another unique feature was that at the end of the modeling, deviations from the planned guides due to cause were reported.[912]

All of the participants reported that finite element modeling could predict vibrations more accurately than previously realized but required more attention to detail in the modeling, with finer meshes and the inclu­sion of secondary components not normally modeled for static strength and stiffness analysis. The participants further reported on specific improvements to dynamic modeling practice resulting from the exer­cise and on the increased use and acceptance of such modeling in the design phase at each respective company.[913] As a result of DAMVIBS:

• Bell and Boeing incorporated DAMVIBS lessons into the modeling of their respective portions of the V-22.[914]

• Boeing made improvements the NASTRAN dynamic model of the CH47D, which was still in production, achieving greatly improved correlation to test data. Boeing

credited Eugene Naumann of Langley with identifying many of the needed changes.[915]

• McDonnell-Douglas improved its dynamic models of exist­ing and newly developed products, achieving improved correlation with test results.[916]

• Sikorsky developed an FEM model of the UH60A air­frame "having a marked improvement in vibration-pre-

8 dicting ability.”[917]

• Sikorsky also developed a new program (PAREDYM, programmed in NASTRAN DMAP language) that could automatically adjust an FEM model so that its modal characteristics would match test values.[918] PAREDYM then found use as a design tool: having the ability to modify a model of an existing design to better match test data, it also had the ability to modify a model of a new design not yet tested, to a set of desired modal char­acteristics. Designers could now specify a target (low) level of vibration response and let PAREDYM tune its model—essentially designing the airframe—to meet the goal. (The improvements would not be "free,” however, as the program could add weight in the process.) After discovering this usage mode, the developers then added facilities for minimizing the weight impact to achieve a desired level of vibration improvement.[919]

DAMVIBS ended in 1991, though this did not mark an end to Langley’s work on rotorcraft structural dynamics.[920] Rather, it reflected a shift in emphasis away from the traditional helicopter to other aeronautics and

astronautics research ventures as well.[921] As basic analysis capability had become relatively mature by around 1990, attention turned toward the integration of design, analysis, and optimization; to the integration of structural analysis with other disciplines; and to nondeterministic meth­ods and the modeling of uncertainty.[922] Projects included further work on rotorcraft, aircraft aerostructural optimization, control-structural optimi­zation for space structures, and nondeterministic or "fuzzy” structures, to name a few.[923] Many optimization projects at Langley used the CONMIN constrained function minimization program, developed at Ames, as the optimization driver, interfaced with various discipline-specific analysis codes developed at Langley or elsewhere.

In the 1970s, NASA Langley began what would prove to be some very significant multidisciplinary optimization (MDO) studies. Jaroslaw Sobieszczanski-Sobieski pioneered the Bi-Level Integrated System Synthesis (BLISS), a general approach that is applicable to design opti­mization in any set of disciplines and of any system, aircraft, or otherwise. His work at Langley, spanning from the 1970s to the present, is recognized throughout the aerospace industry and the MDO community. BLISS and related methods constitute one of the major classes of MDO techniques in widespread use today. Some of the early work on BLISS was concerned with improving the structural design process and addressing aerodynamic and structural problems concurrently. For example, in the late 1970s, Sobieszczanski-Sobieski developed methods for designing metal and/or composite wing structures of supersonic transports for minimum weight, including the effect of structural deformations on aeroelastic loads.[924]

This Langley work continued into the 1980s, when Langley research­ers moved forward to apply the knowledge gained with BLISS to space­craft, generating two other systems: the Integrated Design and Evaluation of Advanced Spacecraft (IDEAS) and Programming Structural Synthesis (PROSS). IDEAS did not perform optimization per se, but it did pro­vide integration of design with analysis in multiple disciplines, includ-

ing structures and structural dynamics.[925] PROSS combined the Ames CONMIN optimizer with the SPAR structural analysis program (developed at NASA Lewis). PROSS was publicly released in 1983.[926] Several subsequent releases incorporated either new optimization strategies and/or improved finite element analysis.[927]

One of these was ST-SIZE, which started as a hypersonic vehicle structural-thermal design code. In 1996, Collier Research Corporation obtained an exclusive license from Langley for the ST-SIZE program. Under a new model for NASA technology transfer, Collier agreed to pay NASA royalties from sales of Collier’s commercialized version of the code. This version, called HyperSizer (trademark of Collier Research Corporation), was intended to be applicable to a wide variety of uses, including office design and construction, marine systems, cargo contain­ers, aircraft, and railcars. The program performed design, weight buildup, system-level performance assessments, structural analysis, and struc­tural design optimization.[928] In 2003, Spinoff reported that this model had worked well and that Collier and NASA were still working together to enhance the program, specifically by incorporating further analysis codes from NASA Glenn Research Center: Micromechanics Analysis Code with Generalized Method Cells (MAC/GMC) and higher-order theory for functionally graded materials (HOTGFM). Both of these were developed collaboratively between Glenn, University of Virginia, Ohio Aerospace Institute, and Tel Aviv University.[929]

JPL operates both ground-based and space-borne antennas. These pose their own array (no pun intended) of structural challenges because of their large size, the need to maintain precision alignment, and, in the case of space-borne antennas, the need for extremely light weight. JPL was one of the early users of NASTRAN, using it to calculate gravity defor-

mation effects on ground-based 210-foot-diameter antennas in 1971.[993] In 1976, JPL developed a simplified stiffness formulation (translational degrees of freedom only at the nodes) coupled with a structural member sizing capability for design optimization: "Computation times to exe­cute several design/analysis cycles are comparable to the times required by general-purpose programs for a single analysis cycle.”[994] In the late 1980s, JPL upgraded a set of 64-meter antennas to a new diameter of 70 meters. This project afforded "the rare opportunity to collect field data to compare with predictions of the finite-element analytical models.” Static and dynamic tests were performed while the antenna structures were in a stripped-down configuration during the retrofit process. The data provided insight into the accuracy of the models that were used to optimize the original structural designs.[995]

3) TRASYS Radiative Heat Transfer (Johnson, 1980s-1990s)

TRASYS was developed by Martin Marietta for Johnson to calculate inter­node radiative heat transfer as well as heat transfer to a model from the surroundings. It was used extensively through the 1990s. Applications included propulsion analysis at Glenn Research Center and Structural – Thermal-Optical analysis (when integrated with NASTRAN for structural calculations and MACOS/IMOS and POPOS optical codes) at Marshall Space Center and JPL.[996] BUCKY was developed in-house. BUCKY was initially introduced as a basic plane stress and plate buckling program but was extensively developed during the 1990s to include plate bend­ing, varying element thickness, varying edge and pressure loads, edge moments, plasticity, output formatting for visualization in I-DEAS (a CAD program developed by Structural Dynamics Research Corporation), three-dimensional axisymmetric capability, and improvements in execution time.[997]

Early aviation pioneers gradually came to realize that an effective flight control system was necessary to control the forces and moments act­ing on an aircraft. The creation of such a system of flight control was one of the great accomplishments of the Wright brothers, who used a
combination of elevator, rudder, and wing warping to achieve effective three-axis flight control in their 1903 Flyer. As aircraft became larger and faster, wing warping was replaced by movable ailerons to control motion around the roll axis. This basic three-axis flight control system is still used today. It enables the pilot to maneuver the aircraft about its pitch, yaw, and roll axes and, in conjunction with engine power adjust­ments, to control velocity and acceleration as well. For many genera­tions after the dawn of manned, controllable powered flight, a system of direct mechanical linkages between the pilot’s cockpit controls and the aircraft’s control surfaces was used to both assist in stabilizing the aircraft as well as to change its flight path or maneuver. The pilot’s abil­ity to "feel” the forces being transmitted to his flight controls, especially during rapid maneuvering, was critically important, because many early aircraft were statically unstable in pitch with the pilot having to exert a constant stabilizing influence with his elevator control. Well into the First World War, many aircraft on both sides of the conflict had poor stability and control characteristics, issues that would continue to chal­lenge aircraft designers well into the jet age. Wartime experience showed that adequate stability and positive aircraft handling qualities, coupled with high performance (as exemplified by parameters such as low wing loading, high power-to-weight ratio, and good speed, turn, and climb rates) played a major role in success in combat between fighters.

By the Second World War, electrically operated trim tabs located on aerodynamic control surfaces and other applications of electrical con­trol and actuation were emerging.[1101] However, as aircraft performance increased and airframes grew larger and heavier, it became increas­ingly harder for pilots to maneuver their aircraft because of high aero-

dynamic forces on the control surfaces. World War II piston engine fighters were extremely difficult to maneuver in pitch and roll and often became uncontrollable as compressibility effects were encountered as speeds approached about Mach 0.8. The introduction of jet propulsion toward the later stages of the Second World War further exacerbated this controllability problem. Hydraulically actuated fight control sur­faces were introduced to assist the pilot in moving the control surfaces at higher speeds. These "boosted” control surface actuators were con­nected to the pilot’s flight controls through a system consisting of cables, pulleys, and cranks, with hydraulic lines now also being routed through the airframe to power the control surface actuators.[1102]

With a boosted flight control system, the pilot’s movements of the cockpit flight controls opens or closes servo valves in the hydraulic sys­tem, increasing or decreasing the hydraulic pressure powering the actu­ators that move the aircraft control surfaces. Initially, hydraulic boost augmented the force transmitted to the control surfaces by the pilot; such an approach is referred to as partial boost. However, fully boosted flight controls quickly became standard on larger aircraft as well as on those aircraft requiring high maneuverability at high indicated airspeeds and Mach numbers. The first operational U. S. jet fighter, the Lockheed P-80 Shooting Star, used electric pitch and roll trim and had hydraulically boosted ailerons to provide roll effectiveness at higher airspeeds. The first jet-powered U. S. bomber to enter production, the four-engine North American B-45, flew for the first time in March 1947. It had hydraulically boosted flight control surfaces and an electrically actuated trim tab on the elevator that was used to maintain longitudinal trim. Despite their undeniable benefits, boosted flight control systems could also produce unanticipated hazards. The chief test pilot for the Langley Aeronautical Laboratory of what was then the National Advisory Committee for Aeronautics (NACA), Herbert "Herb” Hoover, was killed in the crash of a B-45 on August 14, 1952, when the aircraft disintegrated during a test mission near Barrowsville, VA.[1103]

As a NASA report noted: "The aerodynamic power of the trim-tab – elevator combination [on the B-45] was so great that, in the event of an inadvertent maximum tab deflection, the pilot’s strength was insuf­ficient to overcome the resulting large elevator hinge moments if the hydraulic boost system failed or was turned off. Total in-flight destruc­tion of at least one B-45, the aircraft operated by NACA, was probably caused by this combination of circumstances that resulted in a normal load factor far greater than the design value.”[1104]

The Air Force/NACA Bell XS-1 rocket-powered research aircraft was equipped with an electrically trimmed adjustable horizontal stabilizer. It enabled the pilot to maintain pitch control as the conventional eleva­tor lost effectiveness as the speed of sound was approached.[1105] Using this capability, U. S. Air Force (USAF) Capt. Chuck Yeager exceeded Mach 1 in level flight in the XS-1 in October 1947, followed soon after by a North American XP-86 Sabre (although in a dive). Hydraulically boosted controls and fully movable horizontal tails were rapidly implemented on operational high-performance jet aircraft, an early example being the North American F-100.[1106] To compensate for the loss of natural feed­back to the pilot with fully boosted flight controls, various devices such as springs and bob weights were integrated into the flight control sys­tem. These "artificial feel” devices provided force feedback to the pilot’s controls that was proportional to changes in airspeed and acceleration. Industry efforts to develop boosted fight control surfaces directly ben­efited from NACA flight-test efforts of the immediate postwar period.