Category NASA’S CONTRIBUTIONS TO AERONAUTICS

Most lightning forms in the negatively charged region under the base of a thunderstorm, whence negative charge is transferred from the cloud to the ground. This so-called "negative lightning” accounts for over 95 percent of strikes. An average bolt of negative lightning carries an elec­tric current of 30 kA, transferring a charge of 5 coulombs, with energy of 500 megajoules (MJ). Large lightning bolts can carry up to 120 kA and 350 coulombs. The voltage is proportional to the length of the bolt.[117]

Some lightning originates near the top of the thunderstorm in its cirrus anvil, a region of high positive charge. Lightning formed in the upper area behaves similarly to discharges in the negatively charged storm base, except that the descending stepped leader carries a posi­tive charge, while its subsequent ground streamers are negative. Bolts thus created are called "positive lightning,” because they deliver a net positive charge from the cloud to the ground. Positive lightning usually consists of a single stroke, while negative lightning typically comprises two or more strokes. Though less than 5 percent of all strikes consist of positive lightning, it is particularly dangerous. Because it originates in the upper levels of a storm, the amount of air it must burn through to reach the ground is usually much greater. Therefore, its electric field typically is much stronger than a negative strike would be and generates enormous amounts of extremely low frequency (ELF) and VLF waves. Its flash duration is longer, and its peak charge and potential are 6 to 10 times greater than a negative strike, as much as 300 kA and 1 billion volts!

Some positive lightning happens within the parent thunderstorm and hits the ground beneath the cloud. However, many positive strikes occur near the edge of the cloud or may even land more than 10 miles away, where perhaps no one would recognize risk or hear thunder.

Such positive lightning strikes are called "bolts from the blue.” Positive lightning may be the main type of cloud-to-ground during winter months or develop in the late stages of a thunderstorm. It is believed to be responsible for a large percentage of forest fires and power-line damage, and poses a threat to high-flying aircraft. Scientists believe that recently discovered high-altitude discharges called "sprites” and "elves” result from positive lightning. These phenomena occur well above parent thunderstorms, at heights from 18 to 60 miles, in some cases reaching heights traversed only by transatmospheric systems such as the Space Shuttle.

Lightning is by no means a uniformly damaging force. For exam­ple, fires started by lightning are necessary in the life cycles of some plants, including economically valuable tree species. It is probable that, thanks to the evolution and spread of land plants, oxygen concentra­tions achieved the 13-percent level required for wildfires before 420 mil­lion years ago, in the Paleozoic Era, as evinced by fossil charcoal, itself proof of lightning-caused range fires.

In 2003, NASA-funded scientists learned that lightning produces ozone, a molecule composed of three oxygen atoms. High up in the stratosphere (about 6 miles above sea level at midlatitudes), ozone shields the surface of Earth from harmful ultraviolet radiation and makes the land hospitable to life, but low in the troposphere, where most weather occurs, it’s an unwelcome byproduct of manmade pollutants. NASA’s researchers were surprised to find that more low-altitude ozone devel­ops naturally over the tropical Atlantic because of lightning than from the burning of fossil fuels or vegetation to clear land for agriculture.

Outdoors, humans can be injured or killed by lightning directly or indirectly. No place outside is truly safe, although some locations are more exposed and dangerous than others. Lightning has harmed vic­tims in improvised shelters or sheds. An enclosure of conductive mate­rial does, however, offer refuge. An automobile is an example of such an elementary Faraday cage.

Property damage is more common than injuries or death. Around a third of all electric power-line failures and many wildfires result from lightning. (Fires started by lightning are, however, significant in the natural life cycle of forests.) Electrical and electronic devices, such as telephones, computers, and modems, also may be harmed by lightning, when overcurrent surges fritz them out via plug-in outlets, phone jacks, or Ethernet cables.

The Lightning Hazard in Aeronautics and Astronautics: A Brief Synopsis

Since only about one-fourth of discharges reach Earth’s surface, lightning presents a disproportionate hazard to aviation and rocketry. Commercial aircraft are frequently struck by lightning, but airliners are built to reduce the hazard, thanks in large part to decades of NASA research. Nevertheless, almost every type of aircraft has been destroyed or severely damaged by lightning, ranging from gliders to jet airliners. The follow­ing is a partial listing of aircraft losses related to lightning:

• August 1940: a Pennsylvania Central Airlines Douglas DC-3A dove into the ground near Lovettsville, VA, kill­ing all 25 aboard (including Senator Ernest Lundeen of Minnesota), after "disabling of the pilots by a severe lightning discharge in the immediate neighborhood of the airplane, with resulting loss of control.”[118]

• June 1959: a Trans World Airlines (TWA) four-engine Lockheed Starliner with 68 passengers and crew was destroyed near Milan, Italy.

• August 1963: a turboprop Air Inter Vickers Viscount crashed on approach to Lyon, France, killing all 20 on board plus 1 person on the ground.

• December 1963: a Pan American Airlines Boeing 707 crashed at night when struck by lightning over Maryland.

All 82 aboard perished.

• April 1966: Abdul Salam Arif, President of Iraq, died in a helicopter accident, reportedly in a thunderstorm that could have involved lightning.

• April 1967: an Iranian Air Force C-130B was destroyed by lightning near Mamuniyeh. The 23 passengers and crew all died.

• Christmas Eve 1971: a Lockheed Electra of Lmeas Aereas Nacionales Sociedad Anonima (LANSA) was destroyed over Peru with 1 survivor among 92 souls on board.

• May 1976: an Iranian Air Force Boeing 747 was hit during descent to Madrid, Spain, killing all 17 aboard.

• November 1978: a U. S. Air Force (USAF) C-130E was struck by lightning near Charleston, SC, and fatally crashed, with six aboard.

• September 1980: a Kuwaiti C-130 crashed after a light­ning strike near Montelimar, France. The eight-man crew was killed.

• February 1988: a Swearingen Metro operated by Nurnberger Flugdienst was hit near Mulheim, Germany, with all 21 aboard killed.

• January 1995: a Super Puma helicopter en route to a North Sea oil platform was struck in the tail rotor, but the pilot autorotated to a water landing. All 16 people aboard were safely recovered.

• April 1999: a British glider was struck, forcing both pilots to bail; they landed safely.

Additionally, lightning posed a persistent threat to rocket-launch operations, forcing extensive use of protective systems such as light­ning rods and "tripwire” devices. These devices included small rockets trailing conductive wires that can trigger premature cloud-to-ground strokes, reducing the risk of more powerful lightning strokes. The clas­sic example was the launch of Apollo 12, on November 14, 1969. "The flight of Apollo 12,” NASA historian Roger E. Bilstein has written, "was electrifying, to say the least.”[119]

During its ascent, it built up a massive static electricity charge that abruptly discharged, causing a brief loss of power. It had been an excep­tionally close call. Earlier, the launch had been delayed while technicians dealt with a liquid hydrogen leak. Had a discharge struck the fuel-air mix of the leak, the conflagration would have been disastrous. Of course, three decades earlier, a form of lightning (a brush discharge, commonly called "St. Elmo’s fire”) that ignited a hydrogen gas-air mix was blamed by investigators for the loss of the German airship Hindenburg in 1937 at Lakehurst, NJ.[120]

Flight Research on Lightning

Benjamin Franklin’s famous kite experiments in the 1750s constituted the first application of lightning’s effect upon "air vehicles.” Though it is uncertain that Franklin personally conducted such tests, they certainly were done by others who were influenced by him. But nearly 200 years passed before empirical data were assembled for airplanes.[121]

Probably the first systematic study of lightning effects on aircraft was conducted in Germany in 1933 and was immediately translated by NASA’s predecessor, the National Advisory Committee on Aeronautics (NACA). German researcher Heinrich Koppe noted diverse opinions on the subject. He cited the belief that any aircraft struck by lightning "would be immediately destroyed or at least set on fire,” and, contrarily, that because there was no direct connection between the aircraft and the ground, "there could be no force of attraction and, consequently, no danger.”[122]

Koppe began his survey detailing three incidents in which "the con­sequences for the airplanes were happily trivial.” However, he expanded the database to 32 occasions in 6 European nations over 8 years. (He searched for reports from America but found none at the time.) By dis­counting incidents of St. Elmo’s fire and a glider episode, Koppe had 29 lightning strikes to evaluate. All but 3 of the aircraft struck had extended trailing antennas at the moment of impact. His conclusion was that wood and fabric aircraft were more susceptible to damage than were metal airframes, "though all-metal types are not immune.” Propellers frequently attracted lightning, with metal-tipped wooden blades being more susceptible than all-metal props. While no fatalities occurred with the cases in Koppe’s studies, he did note disturbing effects upon aircrew, including temporary blindness, short-term stunning, and brief paraly­sis; in each case, fortunately, no lingering effects occurred.[123]

Koppe called for measures to mitigate the effects of lightning strikes, including housing of electrical wires in metal tubes in wood airframes and "lightning protection plates” on the external surfaces. He said radio masts and the sets themselves should be protected. One occasionally overlooked result was "electrostriction,” which the author defined as "very heavy air pressure effect.” It involved mutual attraction of parallel tracks into the area of the current’s main path. Koppe suggested a shield on the bottom of the aircraft to attract ionized air. He concluded: "airplanes are not ‘hit’ by lightning, neither do they ‘accidentally’ get into the path of a stroke. The hits to airplanes are rather the result of a release of more or less heavy electrostatic discharges whereby the airplane itself forms a part of the current path.”[124]

American studies during World War II expanded upon prewar exam­inations in the United States and elsewhere. A 1943 National Bureau of Standards (NBS, now the National Institute for Standards and Technology, NIST) analysis concluded that the power of a lightning bolt was so enormous—from 100 million to 1 billion volts—that there was "no possibility of interposing any insulating barrier that can effectively resist it.” Therefore, aircraft designers needed to provide alternate paths for the discharge via "lightning conductors.”[125] Postwar evaluation reinforced Koppe’s 1933 observations, especially regarding lightning effects upon airmen: temporary blindness (from seconds to 10 minutes), momentary loss of hearing, observation of electrical effects ranging from sparks to "a blinding blue flash,” and psychological effects. The latter were often caused more by the violent sensations attending the entrance of a tur­bulent storm front rather than a direct result of lightning.[126]

Drawing upon British data, the NACA’s 1946 study further detailed atmospheric discharges by altitude bands from roughly 6,500 to 20,500 feet, with the maximum horizontal gradient at around 8,500 feet. Size and configuration of aircraft became recognized factors in lightning, owing to the greater surface area exposed to the atmosphere. Moisture and dust particles clinging to the airframe had greater potential for drawing a light­ning bolt than on a smaller aircraft. Aircraft speed also was considered, because the ram-air effect naturally forced particles closer together.[127]

A Weather Bureau survey of more than 150 strikes from 1935 to 1944 defined a clear "danger zone”: aircraft flying at or near freezing temper­atures and roughly at 1,000 to 2,000 feet above ground level (AGL). The most common factors were 28-34 °F and between 5,000 and 8,000 feet AGL. Only 15 percent of strikes occurred above 10,000 feet.[128]

On February 19, 1971, a Beechcraft B90 King Air twin-turboprop business aircraft owned by Marathon Oil was struck by a bolt of light­ning while descending through 9,000 feet preparatory to landing at Jackson, MI. The strike caused "widespread, rather severe, and unusual” damage. The plane suffered "the usual melted metal and cracked nonmetallic materials at the attachments points” but in addition suffered a local structural implosion on the inboard portions of the lower right wing between the fuselage and right engine nacelle, damage to both flaps, impact-and-crush-type damage to one wingtip at an attachment point, elec­trical arc pitting of flap support and control rod bearings, a hole burned in a ventral fin, missing rivets, and a brief loss of power. "Metal skins were distorted,” NASA inspectors noted, "due to the ‘magnetic pinch effect’ as the lightning current flowed through them.” Pilots J. R. Day and J. W. Maxie recovered and landed the aircraft safely. Marathon received a NASA com­mendation for taking numerous photographs of record and contacting NASA so that a much more detailed examination could be performed.[129]

The jet age brought greater exposure to lightning, prompting further investigation by NOAA (created in 1970 to succeed the Environmental Science Services Administration, which had replaced the Weather Bureau in 1965). The National Severe Storms Laboratory conducted Project Rough Rider, measuring the physical characteristics and effects of thunderstorms, including lightning. The project employed two-seat F-100F and T-33A jets to record the intensity of lightning strikes over Florida and Oklahoma in the mid-1960s and later. The results of the research flights were studied and disseminated to airlines, providing safety guidelines for flight in the areas of thunderstorms.[130]

In December 1978, two Convair F-106A Delta Dart interceptors were struck within a few minutes near Castle Air Force Base (AFB), CA. Both had lightning protection kits, which the Air Force had installed beginning in early 1976. One Dart was struck twice, with both jets sustaining "severe” damage to the Pitot booms and area around the radomes. The protection kits prevented damage to the electrical sys­tems, though subsequent tests determined that the lightning currents well exceeded norms, in the area of 225 kA. One pilot reported that the strike involved a large flash, and that the impact felt "like someone hit the side of the aircraft with a sledgehammer.” The second strike a few minutes later exceeded the first. The report concluded that absent the protection kits, damage to electrical and avionic systems might have been extensive.[131]

Though rare, other examples of dual aircraft strikes have been recorded. In January 1982, a Grumman F-14A Tomcat was en route to the Grumman factory at Calverton, NY, flown by CDR Lonny K. McClung from Naval Air Station (NAS) Miramar, CA, when it was struck by light­ning. The incident offered a dramatic example of how a modern, highly sophisticated aircraft could be damaged, and its safety compromised, by a lightning strike. As CDR McClung graphically recalled:

We were holding over Calverton at 18,000 waiting for a rainstorm to pass. A lightning bolt went down about half a mile in front of us. An arm reached out and zapped the Pitot probe on the nose. I saw the lightning bolt go down and almost as if a time warp, freeze frame, an arm of that lightning came horizontal to the nose of our plane.

It shocked me, but not badly, though it fried every com­puter in the airplane—Grumman had to replace every­thing. Calverton did not open in time for us to recover immediately so we had to go to McGuire AFB (112 miles southwest) and back on the "peanut gyro” since all our displays were fried. With the computers zapped, we had a bit of an adventure getting the plane going again so we could go to Grumman and get it fixed. When we got back to Calverton, one of the linemen told us that the same lightning strike hit a news helo below us. Based on the time, we were convinced it was the same strike that got us. An eerie feeling.[132]

The 1978 F-106 Castle AFB F-106 strikes stimulated further research on the potential danger of lightning strikes on military aircraft, particularly as the Castle incidents involved currents beyond the strength usually encountered.

Coincidentally, the previous year, the National Transportation Safety Board had urged cooperative studies among academics, the aviation community, and Government researchers to address the dangers posed to aircraft operations by thunderstorms. Joseph Stickle and Norman Crabill of the NASA Langley Research Center, strongly supported by Allen Tobiason and John Enders at NASA Headquarters, structured a compre­hensive program in thunderstorm research that the Center could pur­sue. The next year, Langley researchers evaluated a lightning location detector installed on an Agency light research aircraft, a de Havilland of Canada DHC-6 Twin Otter. But the most extensive and prolonged study NASA undertook involved, coincidentally, the very sort of aircraft that had figured so prominently in the Castle AFB strikes: a two-seat NF-106B Delta Dart, lent from the Air Force to NASA for research purposes.[133]

The NASA Langley NF-106B lightning research program began in 1980 and continued into 1986. Extensive aerial investigations were under­taken after ground testing, modeling, and simulation.[134] Employing the NF-106B, Langley researchers studied two subjects in particular: the mech­anisms influencing lightning-strike attachments on aircraft and the elec­trical and physical effects of those strikes. Therefore, the Dart was fitted with sensors in 14 locations: 9 in the fuselage plus 3 in the wings and 2 in the vertical stabilizer. In all, the NF-106B sustained 714 strikes during 1,496 storm penetrations at altitudes from 5,000 to 50,000 feet, typically flying within a 150-mile radius of its operating base at Langley.[135] One NASA pilot—Bruce Fisher—experienced 216 lightning strikes in the two – seat Dart. Many test missions involved multiple strikes; during one 1984 research flight at an altitude of 38,000 feet through a thunderstorm, the NF-106B was struck 72 times within 45 minutes, and the peak recorded on that particular test mission was an astounding 9 strikes per minute.[136]

NASA’s NF-106B lightning research program constituted the sin­gle most influential flight research investigation undertaken in atmo­spheric electromagnetic phenomena by any nation. The aircraft, now preserved in an aviation museum, proved one of the longest-lived and most productive of all NASA research airplanes, retiring in 1991. As a team composed of Langley Research Center, Old Dominion University, and Electromagnetic Applications, Inc., researchers reported in 1987:

This research effort has resulted in the first statistical quantification of the electromagnetic threat to aircraft based on in situ measurements. Previous estimates of the in-flight lightning hazard to aircraft were inferred from ground-based measurements. The electromagnetic measurements made on the F-106 aircraft during these strikes have established a statistical basis for determi­nation of quantiles and "worst-case” amplitudes of elec­tromagnetic parameters of rate of change of current and the rate of change of electric flux density. The 99.3 percentile of the peak rate of change of current on the F-106 aircraft struck by lightning is about two and a half times that of previously accepted airworthiness cri­teria. The findings are at present being included in new criteria concerning protection of aircraft electrical and electronic systems against lightning. Since there are at present no criteria on the rate of change of electric flux density, the new data can be used as the basis for new criteria on the electric characteristics of lightning – aircraft electrodynamics. In addition to there being no criteria on the rate of change of electric flux density, there are also no criteria on the temporal durations of this rate of change or rate of change of electric current exceeding a prescribed value. Results on pulse char­acteristics presented herein can provide the basis for this development. The newly proposed lightning crite­ria and standards are the first which reflect actual air­craft responses to lightning measured at flight altitudes.[137]

The data helped shape international certification and design stan­dards governing how aircraft should be shielded or hardened to minimize damage from lightning. Recognizing its contributions to understanding the lightning phenomena, its influence upon design standards, and its ability to focus the attention of lightning researchers across the Federal Government, the Flight Safety Foundation accorded the NF-106B pro­gram recognition as an Outstanding Contribution to Flight Safety for 1989. This did not mark the end of the NF-106B’s electromagnetic research, however, for it was extensively tested at the Air Force Weapons Laboratory at Kirtland AFB, NM, in a cooperative Air Force-NASA study comparing lightning effects with electromagnetic pulses produced by nuclear explosions.[138]

As well, the information developed in F-106B flights led to exten­sion of "triggered” (aircraft-induced) lightning models applied to other aircraft. Based on scaling laws for triggering field levels of differing air­frame sizes and configurations, data were compiled for types as diverse as Lockheed C-130 airlifters and light, business aircraft, such as the Gates (now Bombardier) Learjet. The Air Force operated a Lockheed WC-130 during 1981, collecting data to characterize airborne light­ning. Operating in Florida, the Hercules flew at altitudes between 1,500

and 18,000 feet, using 11 sensors to monitor nearby thunderstorms. The flights were especially helpful in gathering data on intercloud and cloud-to-ground strokes. More than 1,000 flashes were recorded by ana­log and 500 digitally.[139]

High-altitude research flights were conducted in 1982 with instru­mented Lockheed U-2s carrying the research of the NF-106B and the WC-130 at lower altitudes well into the stratosphere. After a smaller 1979 project, the Thunderstorm Overflight Program was cooperatively spon­sored by NASA, NOAA, and various universities to develop criteria for a lightning mapping satellite system and to study the physics of light­ning. Sensors included a wide-angle optical pulse detector, electric field change meter, optical array sensor, broadband and high-resolution Ebert spectrometers, cameras, and tape recorders. Flights recorded data from Topeka, KS, in May and from Moffett Field, CA, in August. The project col­lected some 6,400 data samples of visible pulses, which were analyzed by NASA and university researchers.[140] NASA expanded the studies to include

flights by an Agency Lockheed ER-2, an Earth-resources research aircraft derived from the TR-2, itself a scaled-up outgrowth of the original U-2.[141]

Complementing NASA’s lightning research program was a coop­erative program of continuing studies at lower altitudes undertaken by a joint American-French study team. The American team consisted of technical experts and aircrew from NASA, the Federal Aviation Administration (FAA), the USAF, the United States Navy (USN), and NOAA, using a specially instrumented American Convair CV-580 twin – engine medium transport. The French team was overseen by the Offices Nationales des Etudes et Recherches Aerospatiales (National Office for Aerospace Studies and Research, ONERA) and consisted of experts and aircrew from the Centre d’Essais Aeronautique de Toulouse (Toulouse Aeronautical Test Center, CEAT) and the l’Armee de l’Air (French Air Force) flying a twin-engine medium airlifter, the C-160 Transall. The Convair was fitted with a variety of external sensors and flown into thunderstorms over Florida in 1984 to 1985 and 1987. Approximately 60 strikes were received, while flying between 2,000 and 18,000 feet. The hits were categorized as lightning, lightning attachment, direct strike, triggered strike, intercepted strike, and electromagnetic pulse. Flight tests revealed a high proportion of strikes initiated by the aircraft itself. Thirty-five of thirty-nine hits on the CV-580 were determined to be aircraft-induced. Further data were obtained by the C-160 with high­speed video recordings of channel formation, which reinforced the opinion that aircraft initiate the lightning. The Transall operated over southern France (mainly near the Pyrenees Mountains) in 1986-1988, and CEAT furnished reports from its strike data to the FAA, and thence to other agencies and industry.[142]

Electrodynamic Research Using UAVs

Reflecting their growing acceptance for a variety of military missions, unmanned ("uninhabited”) aerial vehicles (UAVs) are being increasingly used for atmospheric research. In 1997, a Goddard Space Flight Center space sciences team consisting of Richard Goldberg, Michael Desch, and William Farrell proposed using UAVs for electrodynamic studies. Much research in electrodynamics centered upon the direct-current (DC) Global Electric Circuit (GEC) concept, but Goldberg and his colleagues wished to study the potential upward electrodynamic flow from thunderstorms. "We were convinced there was an upward flow,” he recalled over a decade later, "and [that] it was AC.”[143] To study upward flows, Goldberg and his colleagues decided that a slow-flying, high-altitude UAV had advantages of proximity and duration that an orbiting spacecraft did not. They con­tacted Richard Blakeslee at Marshall Space Flight Center, who had a great interest in Earth sciences research. The Goddard-Marshall part-

nership quickly secured Agency support for an electrodynamic UAV research program to be undertaken by the National Space Science and Technology Center (NSSTC) at Huntsville, AL. The outcome was Altus, a modification of the basic General Atomics Predator UAV, leased from the manufacturer and modified to carry a NASA electrodynamic research package. Altus could fly as slow as 70 knots and as high as 55,000 feet, cruising around and above (but never into) Florida’s formidable and highly energetic thunderstorms. First flown in 2002, Altus constituted the first time that UAV technology had been applied to study electrody­namic phenomena.[144] Initially, NASA wished to operate the UAV from Patrick AFB near Cape Canaveral, but concerns about the potential dan­gers of flying a UAV over a heavily populated area resulted in switching its operational location to the more remote Key West Naval Air Station. Altus flights confirmed the suppositions of Goldberg and his colleagues, and it complemented other research methodologies that took electric, magnetic, and optical measurements of thunderstorms, gauging lightning

activity and associated electrical phenomena, including using ground – based radars to furnish broader coverage for comparative purposes.[145]

While not exposing humans to thunderstorms, the Altus Cumulus Electrification Study (ACES) used UAVs to collect data on cloud prop­erties throughout a 3- or 4-hour thunderstorm cycle—not always possible with piloted aircraft. ACES further gathered material for three­dimensional storm models to develop more-accurate weather predictions.

With the Aviation Safety Reporting System fully operational for two decades, NASA in 1996 once again found itself working with the FAA to gather raw data, process it, and make reports—all in the name of identi­fying potential problems and finding solutions. In this case, as part of a Flight Operations Quality Assurance program that the FAA was working with industry on, the agency partnered with NASA to test a new Aviation Performance Measuring System (APMS). The new system was designed to convert digital data taken from the flight data recorders of participat­ing airlines into a format that could easily be analyzed.[222]

More specifically, the objectives of the NASA-FAA APMS research project was to establish an objective, scientifically and technically sound basis for performing flight data analysis; identify a flight data analysis system that featured an open and flexible architecture, so that it could easily be modified as necessary; and define and articulate guidelines that would be used in creating a standardized database structure that would form the basis for future flight data analysis programs. This stan­dardized database structure would help ensure that no matter which data-crunching software an airline might choose, it would be compat­ible with the APMS dataset. Although APMS was not intended to be a nationwide flight data collection system, it was intended to make avail­able the technical tools necessary to more easily enable a large-scale implementation of flight data analysis.[223]

At that time, commercially available software development was not far enough advanced to meet the needs of the APMS, which sought identification and analysis of trends and patterns in large-scale data­bases involving an entire airline. Software then was primarily written with the needs of flight crews in mind and was more capable of spotting single events rather than trends. For example, if a pilot threw a series of switches out of order, the onboard computer could sound an alarm. But that computer, or any other, would not know how frequently pilots made the same mistake on other flights.[224]

A particularly interesting result of this work was featured in the 1998 edition of NASA’s annual Spinoff publication, which highlights successful NASA technology that has found a new home in the commercial sector:

A flight data visualization system called FlightViz™ has been created for NASA’s Aviation Performance Measuring System (APMS), resulting in a comprehensive flight visualization and

analysis system. The visualization software is now capable of very high-fidelity reproduction of the complete dynamic flight environment, including airport/airspace, aircraft, and cock­pit instrumentation. The APMS program calls for analytic methods, algorithms, statistical techniques, and software for extracting useful information from digitally-recorded flight data. APMS is oriented toward the evaluation of performance in aviation systems, particularly human performance. . . . In fulfilling certain goals of the APMS effort and related Space Act Agreements, SimAuthor delivered to United Airlines in 1997, a state-of-the-art, high-fidelity, reconfigurable flight data replay system. The software is specifically designed to improve airline safety as part of Flight Operations Quality Assurance (FOQA) initiatives underway at United Airlines. . . . Pilots, instructors, human factors researchers, incident investigators, mainte­nance personnel, flight operations quality assurance staff, and others can utilize the software product to replay flight data from a flight data recorder or other data sources, such as a training simulator. The software can be customized to pre­cisely represent an aircraft of interest. Even weather, time of day and special effects can be simulated.[225]

While by no means a complete list of every project NASA and the FAA have collaborated on, the examples detailed so far represent the diverse range of research conducted by the agencies. Much of the same kind of work continued as improved technology, updated systems, and fresh approaches were applied to address a constantly evolving set of challenges.

The first Boeing 737 ever built was acquired by NASA in 1974 and modi­fied to become the Agency’s Boeing 737-100 Transport Systems Research Vehicle. During the next 20 years, it flew 702 missions to help NASA advance aeronautical technology in every discipline possible, first as a NASA tool for specific programs and then more generally as a national airborne research facility. Its contributions to the growth in capabil­ity and safety of the National Airspace System included the testing of hardware and procedures using new technology, most notably in the cockpit. Earning its title as an airborne trailblazer, it was the Langley 737 that tried out and won acceptance for new ideas such as the glass

cockpit. Those flat panel displays enabled other capabilities tested by the 737, such as data links for air traffic control communications, the microwave landing system, and satellite-based navigation using the rev­olutionary Global Positioning System.[273]

With plans to retire the 737, NASA Langley in 1994 acquired a Boeing 757-200 to be the new flying laboratory, earning the designa­tion Airborne Research Integrated Experiments System (ARIES). In 2006, NASA decided to retire the 757.[274]

NASA Ames Research Center began the Fatigue Countermeasures pro­gram in the 1980s in response to a congressional request to determine if there existed a safety problem "due to transmeridian flying and a poten­tial problem due to fatigue in association with various factors found in air transport operations.”[382] Originally termed the NASA Ames Fatigue/ Jet Lag program, this ongoing program, jointly funded by the FAA, was created to study such issues as fatigue, sleep, flight operations perfor­mance, and the biological clock—otherwise known as circadian rhythms. This research was focused on (1) determining the level of fatigue, sleep loss, and circadian rhythm disruption that exists during flight opera­tions, (2) finding out how these factors affect crew performance, and (3) developing ways to counteract these factors to improve crew alert­ness and proficiency. Many of the findings from this series of field stud­ies, which included such fatigue countermeasures as regular flightcrew naps, breaks, and better scheduling practices, were subsequently adopted by the airlines and the military.[383] This research also resulted in Federal Aviation Regulations that are still in effect, which specify the amount of rest flightcrews must have during a 24-hour period.[384]

Free-flight models are complementary to other tools used in aeronauti­cal engineering. In the absence of adverse scale effects, the aerodynamic characteristics of the models have been found to agree very well with data obtained from other types of wind tunnel tests and theoretical analyses. By providing insight into the impact of aerodynamics on vehicle dynam­ics, the free-flight results help build the necessary understanding of crit­ical aerodynamic parameters and the impact of modifications to resolve problems. The ability to conduct free-flight tests and aerodynamic mea­surements with the same model is a powerful advantage for the testing technique. When coupled with more sophisticated static wind tunnel tests, computational fluid dynamics methods, and piloted simulator technology, these tests are extremely informative. Finally, even the very visual results of free-flight tests are impressive, whether they demonstrate to critics and naysayers that radical and unconventional designs can be flown or identify a critical flight problem and potential solutions for a new configuration.

The most appropriate applications of free-flight models involve eval­uations of unconventional designs for which no experience base exists and the analysis of aircraft behavior for flight conditions that are not easily studied with other methods because of complex aerodynamic phe­nomena that cannot be modeled at the present time.[464] Examples include flight in which separated flows, nonlinear aerodynamic behavior, and large dynamic motions are typically encountered.

The following discussion presents a brief overview of the historical applications and technological impacts of the use of free-flight models for studies of flight dynamics by the NACA and NASA in selected areas.

The most important applications have been in

• Dynamic stability and control.

• Flight at high angles of attack.[465]

• Spinning and spin recovery.

• Spin entry and poststall motions.

The use of tail-mounted parachutes for emergency spin recovery has been common practice from the earliest days of flight to the present day. Properly designed and deployed parachutes have proven to be relatively reliable spin recovery device, always providing an antispin moment, regardless of the orientation of the aircraft or the disorientation or confu­sion of the pilot. Almost every military aircraft spin program conducted in the Spin Tunnel includes a parachute investigation. Free-spinning model tests are used to determine the critical geometric variables for parachute systems. Paramount among these variables is the minimum size of parachute required for recovery from the most dangerous spin modes. As would be expected, the size of the parachute is constrained by issues regarding system weight and the opening shock loads transmitted to the rear of the aircraft. In addition to parachute size, the length of parachute riser (attachment) lines and the attachment point location on the rear of the aircraft are also critical design parameters.

The importance of parachute riser line length can be especially crit­ical to the inflation and effectiveness of the parachute for spin recov­ery. Results of free-spin tests of hundreds of models in the Spin Tunnel has shown that if the riser length is too short, the parachute will be immersed in the low-energy wake of the spinning airplane and will not inflate. On the other hand, if the towline length is too long, the parachute will inflate but will drift inward and align itself with the axis of rotation, thereby providing no antispin contribution. The design and operational implementation of emergency spin recovery para­chutes are a stringent process that begins with spin tunnel tests and proceeds through the design and qualification of the parachute system, including the deployment and release mechanisms. By participation in each of these segments of the process, Langley researchers have amassed tremendous amount of knowledge regarding parachute systems and are called upon frequently by the aviation community for consultation

before designing and fabricating parachute systems for spin tests of full-scale aircraft.[514]

In 1958, NASA was on a firm foundation for hypersonic and space research. Throughout the 1950s, NACA researchers first addressed the challenge of atmospheric reentry with their work on intercontinen­tal ballistic missiles (ICBMs) for the military. The same fundamental design problems existed for ICBMs, spacecraft, interplanetary probes, and hypersonic aircraft. Each of the NASA Centers specialized in a spe­cific aspect of hypersonic and hypervelocity research that resulted from their heritage as NACA laboratories. Langley’s emphasis was in the cre­ation of facilities applicable to hypersonic cruise aircraft and reentry vehicles—including winged reentry. Ames explored the extreme tem­peratures and the design shapes that could withstand them as vehicles

returned to Earth from space. Researchers at Lewis focused on propul­sion systems for these new craft. With the impetus of the space race, each Center worked with a growing collection of hypersonic and hyper­velocity wind tunnels that ranged from conventional aerodynamic facil­ities to radically different configurations such as shock tubes, arc-jets, and new tunnels designed for the evaluation of aerodynamic heating on spacecraft structures.[581]

In the early 1930s, largely thanks to the work of Munk, the NACA had risen to world prominence in airfoil design, such status evident when, in 1933, the Agency released a report cataloging its airfoil research and presenting a definitive guide to the performance and characteristics of a wide range of airfoil shapes and concepts. Prepared by Eastman

N. Jacobs, Kenneth E. Ward, and Robert M. Pinkerton, this document, TR-460, became a standard industry reference both in America and abroad.[785] The Agency, of course, continued its airfoil research in the 1930s, making notable advances in the development of high-speed air­foil sections and low-drag and laminar sections as well. By 1945, as valuable as TR-460 had been, it was now outdated. And so, one of the
most useful of all NACA reports, and one that likewise became a stan­dard reference for use by designers and other aeronautical engineers in airplane airfoil/wing design, was its effective replacement prepared in 1945 by Ira H. Abbott, Albert E. von Doenhoff, and Louis S. Stivers, Jr. This study, TR-824, was likewise effectively a catalog of NACA airfoil research, its authors noting (with justifiable pride) that

Recent information of the aerodynamic characteristics of NACA airfoils is presented. The historical develop­ment of NACA airfoils is briefly reviewed. New data are presented that permit the rapid of the approximate pres­sure distribution for the older NACA four-digital and five­digit airfoils, by the same methods used for the NACA 6-series airfoils. The general methods used to derive the basic thickness forms for NACA 6 and 7 series air­foils together with their corresponding pressure distri­butions are presented. Detailed data necessary for the application of the airfoils to wing design are presented in supplementary figures placed at the end of the paper.

This report includes an analysis of the lift, drag, pitch­ing moment, and critical-speed characteristics of the air­foils, together with a discussion of the effects of surface conditions available data on high-lift devices. Problems associated with the later-control devices, leading edge air intakes, and interference is briefly discussed, together with aerodynamic problems of application.[786]

While much of this is best remembered because of its association with the advanced high-speed aircraft of the transonic and supersonic era, much was as well applicable to new, more capable civil transport and GA designs produced after the war.

Two key contributions to the jet-age expansion of GA were the super­critical wing and the wingtip winglet, both developments conceived by Richard Travis Whitcomb, a legendary NACA-NASA Langley aerody – namicist who was, overall, the finest aeronautical scientist of the post­Second World War era. More comfortable working in the wind tunnel than sitting at a desk, Whitcomb first gained fame by experimentally investigating the zero lift drag of wing-body combinations through the transonic flow regime based on analyses by W. D. Hayes.[787] His result­ing "Area Rule” for transonic flow represented a significant contribu­tion to the aerodynamics of high-speed aircraft, first manifested by its application to the so-called "Century series” of Air Force jet fighters.[788] Whitcomb followed area rule a decade later in the 1960s and derived the supercritical wing. It delayed the sharp drag rise associated with shock wave formation by having a flattened top with pronounced curva­ture towards its trailing edge. First tested on a modified T-2C jet trainer, and then on a modified transonic F-8 jet fighter, the supercritical wing proved in actual flight that Whitcomb’s concept was sound. This distinc­tive profile would become a key design element for both jet transports and high-speed GA aircraft in the 1980s and 1990s, offering a benefi­cial combination of lower drag, better fuel economy, greater range, and higher cruise speed exemplified by its application on GA aircraft such as the Cessna Citation X, the world’s first business jet to routinely fly faster than Mach 0.90.[789]

The application of Whitcomb’s supercritical wing to General Aviation began with the GA community itself, whose representatives approached Whitcomb after a Langley briefing, enthusiastically endorsing his concept. In response, Whitcomb launched a new Langley program, the Low-and – Medium-Speed Airfoil Program, in 1972. This effort, blending 2-D com­puter analysis and tests in the Langley Low-Turbulence Pressure Tunnel, led to development of the GA(W)-1 airfoil.[790] The GA(W)-1 employed a

Low-and-Medium-Speed variants of the GA(W)-1 and -2 airfoil family. From NASA CP – 2046 (1979).

17-percent-thickness-chord ratio low-speed airfoil, offering a beneficial mix of low cruise drag, high lift-to-drag ratios during climbs, high max­imum lift properties, and docile stall behavior.[791] Whitcomb’s team gen­erated thinner and thicker variations of the GA(W)-1 that underwent its initial flight test validation in 1974 on NASA Langley’s Advanced

The Advanced Technology Light Twin-Engine airplane undergoing tests in the Langley 30 ft x 60 ft Full Scale Tunnel. NASA.

Technology Light Twin (ATLIT) engine airplane, a Piper PA-34 Seneca twin-engine aircraft modified to employ a high-aspect-ratio wing with a GA(W)-1 airfoil with winglets. Testing on ATLIT proved the practical advantages of the design, as did subsequent follow-on ground tests of the ATLIT in the Langley 30 ft x 60 ft Full-Scale-Tunnel.[792]

Subsequently, the NASA-sponsored General Aviation Airfoil Design and Analysis Center (GA/ADAC) at the Ohio State University, led by Dr. Gerald M. Gregorek, modified a single-engine Beech Sundowner light aircraft to undertake a further series of tests of a thinner variant, the GA(W)-2. GA/ADAC flight tests of the Sundowner from 1976-1977 con­firmed that the Langley results were not merely fortuitous, paving the way for derivatives of the GA(W) family to be applied to a range of new aircraft designs starting with the Beech Skipper, the Piper Tomahawk, and the Rutan VariEze.[793]

Following on the derivation of the GA(W) family, NASA Langley researchers, in concert with industry and academic partners, contin­ued refinement of airfoil development, exploring natural laminar flow (NLF) airfoils, previously largely restricted to exotic, smoothly finished sailplanes, but now possible thanks to the revolutionary development of smooth composite structures with easily manufactured complex shapes tailored to the specific aerodynamic needs of the aircraft under devel­opment.[794] Langley researchers subsequently blended their own concep­tual and tunnel research with a computational design code developed at the University of Stuttgart to generate a new natural laminar flow airfoil section, the NLF(1).[795] Like the GA(W) before it, it served as the basis for various derivative sections. After flight testing on various test­beds, it was transitioned into mainstream GA design beginning with a derivative of the Cessna Citation II in 1990. Thereafter, it has become a standard feature of many subsequent aircraft.[796]

The second Whitcomb-rooted development that offered great prom­ise in the 1970s was the so-called winglet.[797] The winglet promised to dra­matically reduce energy consumption and reduce drag by minimizing the wasteful tip losses caused by vortex flow off the wingtip of the air­craft. Though reminiscent of tip plates, which had long been tried over the years without much success, the winglet was a more refined and

The Gates Learjet 28 Longhorn, which pioneered the application of Whitcomb winglets to a General Aviation aircraft. NASA.

better-thought-out concept, which could actually take advantage of the strong flow-field at the wingtip to generate a small forward lift compo­nent, much as a sail does. Primarily, however, it altered the span-wise distribution of circulation along the wing, reducing the magnitude and energy of the trailing tip vortex. First to use it was the Gates Learjet Model 28, aptly named the "Longhorn,” which completed its first flight in August 1977. The Longhorn had 6 to 8 percent better range than pre­vious Lears.[798]

The winglet was experimentally verified for large aircraft applica­tion by being mounted on the wing tips of a first-generation jet transport, the Boeing KC-135 Stratotanker, progenitor of the civil 707 jetliner, and tested at Dryden from 1979-1980. The winglets, designed with a general – purpose airfoil that retained the same airfoil cross-section from root to tip, could be adjusted to seven different cant and incidence angles to enable a variety of research options and configurations. Tests revealed the winglets increased the KC-135’s range by 6.5 percent—a measure of both aerodynamic and fuel efficiency—better than the 6 percent projected by Langley wind tunnel studies and consistent with results obtained with the Learjet Longhorn. With this experience in hand, the winglet was swiftly applied to GA aircraft and airliners, and today, most airlin­ers, and many GA aircraft, use them.[799]

Aeronautical researchers have long known that low wing load­ing contributes to poor ride quality in turbulence. This problem is compounded by the fact that lightweight aircraft, such as general aviation airplanes, spend a great deal of their flight time at lower altitudes, where measurable turbulence is most likely to occur. One way to improve gust alleviation is through the use of a torsionally free wing, also known as a free wing.

The free-wing concept involves unconventional attachment of a wing to an airplane’s fuselage in such a way that the airfoil is free to pivot about its spanwise axis, subject to aerodynamic pitching moments but otherwise unrestricted by mechanical constraints. To provide static pitch stability, the axis of rotation is located forward of the chordwise aerodynamic center of the wing panel. Angle-of-attack equilibrium is established through the use of a trimming control surface and natural torque from lift and drag. Gust alleviation, and thus improved ride qual­ity, results from the fact that a stable lifting surface tends to maintain a prescribed lift coefficient by responding to natural pitching moments that accompany changes in airflow direction.[923] Use of a free wing offers other advantages as well. Use of full-span flaps permits operation at a higher lift coefficient, thus allowing lower minimum-speed capability. A free sta­bilizer helps eliminate stalls. Use of differentially movable wings instead of ailerons permits improved roll control at low speeds. During take­off, the wing rotates for lift-off, eliminating pitching movements caused by landing-gear geometry issues. Lift changes are accommodated without body-axis rotation. Because of independent attitude control, fuselage pitch can be trimmed for optimum visibility during landing approach. Negative lift can be applied to increase deceleration during

landing roll. Fuselage drag can be reduced through attitude trim. Finally, large changes in the center of gravity do not result in changes to longi­tudinal static stability.[924] To explore this concept, researchers at NASA Dryden, led by Shu Gee, proposed testing a radio-controlled model air­plane with a free-wing/free-canard configuration. Quantitative and qual­itative flight-test data would provide proof of the free-wing concept and allow comparison with analytical models. The research team included engineers Gee and Chester Wolowicz of Dryden. Dr. Joe H. Brown, Jr., served as principal investigator for Battelle Columbus Laboratories of Columbus, OH. Professor Gerald Gregorek of Ohio State University’s Aeronautical Engineering Department, along with Battelle’s Richard F. Porter and Richard G. Ollila, calculated aerodynamics and equations of motion. Battelle’s Professor David W. Hall, formerly of Iowa State University, assisted with vehicle layout and sizing.[925] Technicians at Dryden modified a radio-controlled airplane with a 6-foot wingspan to the test configuration. A small free-wing airfoil was rigidly mounted on twin booms forward of the primary flying surface. The ground pilot could change wing lift by actuating a flap on the free wing for longitudinal

control. Elevators provided pitch attitude control, while full-span ailerons were used for roll control.

For data acquisition, the Free-Wing RPRV was flown at low alti­tude in a pacing formation with a ground vehicle. Observers noted the positions of protractors on the sides of the aircraft to indicate wing and canard position relative to the fuselage. Instrumentation in the vehi­cle, along with motion picture film, allowed researchers to record wing angle, control-surface positions, velocity, and fuselage angle relative to the ground. Another airplane model with a standard wing configuration was flown under similar conditions to collect baseline data for comparison with the Free-Wing RPRV performance.[926]

Researchers conducted eight flights at Dryden during spring 1977. They found that the test vehicle exhibited normal stability and con­trol characteristics throughout the flight envelope for all maneuvers performed. Pitch response appeared to be faster than that of a con­ventional airplane, apparently because the inertia of the free-wing assem­bly was lower than that of the complete airplane. Handling qualities appeared to be as good or better than those of the baseline fixed-wing airplane. The investigators noted that separate control of the decoupled fuselage enhanced vehicle performance by acting as pseudo-thrust vectoring. The Free-Wing RPRV had excellent stall/spin characteristics, and the pilot was able to control the aircraft easily under gusty condi­tions. As predicted, center of gravity changes had little or no effect on longitudinal stability.[927] Some unique and unexpected problems were also encountered. When the canard encountered a mechanical trailing – edge position limit, it became aerodynamically locked, resulting in an irreversible stall and hard landing. Increased deflection limits for the free canard eliminated this problem. Researchers had difficulty matching the wing-hinge margin (the distance from the wing’s aerody­namic center to the pivot) and canard control effectiveness. Designers improved handling qualities by increasing the wing hinge margin, the canard area aft of the pivot, and the canard flap area. Canard pivot friction caused some destabilizing effects during taxi, but these abated during takeoff. The ground pilot experienced control difficulty

because wing-fuselage decoupling made it difficult to visually judge approach and landing speeds, but it was concluded that this would not be a problem for a pilot flying a full-scale airplane equipped with con­ventional flight instruments.[928]

In the 1950s and into the 1960s, the USAF and Navy demanded super­sonic performance from fighters in level flight. The Second World War experience had shown that higher speed was productive in achieving superiority in fighter-to-fighter combat, as well as allowing a fighter to intercept a bomber from the rear. The first jet age fighter combat over Korea with fighters having swept wings had resulted in American air superiority, but the lighter MiG-15 had a higher ceiling and better climb rate and could avoid combat by diving away. When aircraft designers interviewed American fighter pilots in Korea, they specified, "I want to go faster than the enemy and outclimb him.”[1060] The advent of nuclear­armed jet bombers meant that destruction of the bomber by an intercep­tor before weapon release was critical and put a premium on top speed, even if that speed would only be achievable for a short time.

Similarly, bomber experience in World War II had shown that loss rates were significantly lower for very fast bombers, such as the Martin B-26 and the de Havilland Mosquito. The prewar concept of the slow, heavy-gun-studded "flying fortress,” fighting its way to a target with no fighter escort, had been proven fallacious in the long run. The use of B-29s in the Korean war in the MiG-15 jet fighter environment had resulted in high B-29 losses, and the team switched to night bombing, where the MiG-15s were less effective. Hence, the ideal jet bomber would be one capable of flying a long distance, carrying a large payload, and capable of increased speed when in a high-threat zone. The length of the high-speed (and probably supersonic) dash might vary on the threat, combat radius, and fuel capacity of the long-range bomber, but it would likely be a longer distance than the short-legged fighter was capable of at supersonic flight. The USAF relied on the long-range bomber as a primary reason for its independent status and existence; hence, it was
interested in using the turbojet to improve bomber performance and survivability. But supersonic speeds seemed out of the question with the early turbojets, and the main effort was on wringing long range from a jet bomber. Swept thin wings promised higher subsonic cruise speed and increased fuel efficiency, and the Boeing Company took advantage of NACA swept wing research initiated by Langley’s R. T. Jones in 1945 to produce the B-47 and B-52, which were not supersonic but did have the long range and large payloads.[1061]

The development of more fuel-efficient axial-flow turbojets such as the General Electric J47 and Pratt & Whitney J57 (the first mass – produced jet engine to develop over 10,000 pounds static sea level non­afterburning thrust) were another needed element. Aerial refueling had been tried on an experimental basis in the Second World War, but for jet bombers, it became a priority as the USAF sought the goal of a large-payload jet bomber with intercontinental range to fight the pro­jected atomic third World War. The USAF began to look at a supersonic dash jet bomber now that supersonic flight was an established capabil­ity being used in the fighters of the day. Just as the medium-range B-47 had served as an interim design for the definitive heavy B-52, the ini­tial result was the delta wing Convair B-58 Hustler. The initial designs had struggled with carrying enough fuel to provide a worthwhile super­sonic speed and range; the fuel tanks were so large, especially for low supersonic speeds with their high normal shock drag, that the airplane was huge with limited range and was rejected. Convair adopted a new approach, one that took advantage of its experience with the area rule redesign of the F-102. The airplane carried a majority of its fuel and its atomic payload in a large, jettisonable shape beneath the fuselage, allow­ing the actual fuselage to be extremely thin. The fuselage and the fuse – lage/tank combination were designed in accordance with the area rule. The aircraft employed four of the revolutionary J79 engines being devel­oped for Mach 2 fighters, but it was discovered that with the increased fuel capacity, high installed thrust, and reduced drag at low supersonic Mach numbers, the aircraft could sustain Mach 2 for up to 30 minutes, giving it a supersonic range over 1,000 miles, even retaining the cen­terline store. It could be said that the B-58, although intended to be a

supersonic dash aircraft, became the first practical supersonic cruise aircraft. The B-58 remained in USAF service for less than 10 years for budgetary reasons and its notoriously unreliable avionics. The safety record was not good either, in part because of the difficulty in train­ing pilots to change over from the decidedly subsonic (and huge) B-52 with a crew of six to a "hot ship” delta wing, high-landing-speed aircraft with a crew of three (but only one pilot). Nevertheless, the B-58 fleet amassed thousands of hours of Mach 2 time and set numerous world speed records for transcontinental and intercontinental distances, most averaging 1,000 mph or higher, including the times for slowing for aer­ial refueling. Examples included 4 hours 45 minutes for Los Angeles to New York and back, averaging 1,045 mph, and Los Angeles to New York 1 way in 2 hours 1 minute, at an average speed of 1,214 mph, with 1 refueling over Kansas.

The later record flight illustrated one of the problems of a supersonic cruise aircraft: heat.[1062] The handbook skin temperature flight limit on the B-58 was 240 degrees Fahrenheit (°F). For the speed run, the limit was raised to 260 degrees to allow Mach 2+, but it was a strict limit; there was concern the aluminum honeycomb skin would debond above that temperature. Extended supersonic flight duration meant that the air­craft structure temperature would rise and eventually stabilize as the heat added from the boundary layer balanced with radiated heat from the hot airplane. The stabilization point was typically reached 20-30 minutes after attaining the cruise speed. The B-58’s Mach 2 speed at 45,000-50,000 feet had reached a structural limit for its aluminum mate­rial; the barrier now was "the thermal thicket”—a heat limit rather the sound barrier.