NASA's Contributions to Aeronautics, Volume 1 by National Aeronautics & Space Administration. - HTML preview

CASE

10

Case-10 Cover Image: The AFTI/F-16, shown here on a 1991 test flight, flew 15 years and over 700 research flights at NASA Dryden. NASA.

The evolution of advanced aircraft equipped with computerized flight and propulsion control systems goes back a long way and involved many players, both in the United States and internationally. During the Second World War, use of electronic sensors and subsystems began to become pervasive, adding new mission capabilities as well as increasing complexity. Autopilots were coupled to flight control systems, and electric trim was introduced. Hydraulically or electrically boosted flight control surfaces, along with artificial feel and stability augmentation systems, soon followed as did electronic engine control systems. Most significantly, the first uses of airborne computers emerged, a trend that soon resulted in their use in aircraft and missiles for their flight and mission control systems. Very early on, there was a realization that traditional mechanical linkages between the cockpit flight controls and the flight control surfaces could be replaced by a computer-controlled fly-by-wire (FBW) approach in which electric signals were transmitted from the pilot’s controls to the control surface actuators by wire. This approach was understood to have the potential to reduce mechanical complexity, lower weight, and increase safety and reliability. In addition, the processing power of the computer could be harnessed to enable unstable aircraft designs to be controlled. Properly tailored, these unstable designs could enable new aircraft concepts to be implemented that could fully exploit the advantages of active flight control. Such aircraft would be more maneuverable and lighter, have better range, and also allow for the fully integrated control of aircraft, propulsion, navigation, and mission systems to optimize overall capability.

Two significant events unfolded during the 1960s that fostered the move to electronic flight control systems. The space race was a major influence, with most space systems relying on computerized fly-by-wire control systems for safe and effective operation. The Vietnam war provided a strong impetus for the development of more survivable aircraft systems as well for new aircraft with advanced performance features. The National Aeronautics and Space Administration (NASA) and the Air Force aggressively responded to these challenges and opportunities, resulting in the rapid transition of digital computer technology from the space program into aircraft fly-by-wire applications as exemplified by the Digital Fly-By-Wire F-8 and the AFTI/F-16 research programs. Very quickly after, a variety of flight research programs were implemented. These programs provided the basis for development and fielding of numerous military and civil aircraft equipped with advanced digital fly-by-wire flight control systems. On the civil aviation side, safety has been improved by preventing aircraft flight envelope limitations from being exceeded. Operating efficiency has been greatly enhanced and major weight savings achieved from fly-by-wire and related electronic flight control system components. Integrated flight and propulsion control systems precisely adjust throttles and fuel tank selections. Rudder trim drag because of unbalanced engine thrust is reduced. Fuel is automatically transferred between tanks throughout the aircraft to optimize center of gravity during cruise flight, minimizing elevator trim drag. In the case of advanced military aircraft, electronically controlled active flight, propulsion, and mission systems have been fully integrated, providing revolutionary improvements in capabilities. Significantly, new highly unstable aircraft configurations are providing unprecedented levels of mission performance along with very low radar signatures, capabilities that have been enabled by exploiting digital fly-by-wire flight and propulsion control systems pioneered in NASA.

Aircraft Flight Control: Beginnings to the 1950s

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.[1] However, as aircraft performance increased and airframes grew larger and heavier, it became increasingly harder for pilots to maneuver their aircraft because of high aerodynamic 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.[2]

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.[3]

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.”[4]

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.[5] 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.[6] 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.

Fly-By-Wire: The Beginnings

The Second World War witnessed the first applications of computer-controlled fly-by-wire flight control systems. With fly-by-wire, primary control surface movements were directed via electrical signals transmitted by wires rather than by the use of mechanical linkages. The German Army’s A-4 rocket (the famous V-2 that postwar was the basis for both U.S. and Soviet efforts to move into space) used an electronic analog computer that modeled the differential equations governing the missile’s flight control laws. The computer-generated electronic signals were transmitted by wire to direct movement of the actuators that drove graphite vanes located in the rocket motor exhaust. The thrust of the rocket engine was thus vectored as required to stabilize the V-2 missile at lower airspeeds until the aerodynamic control surfaces on the fins became effective.[7] Postwar, a similar analog computer-controlled fly-by-wire thrust vectoring approach was used in the U.S. Army Redstone missile, perhaps not surprisingly, because Redstone was predominantly designed by a team of German engineers headed by Wernher von Braun of V-2 fame. The Redstone would be used to launch the Mercury space capsule that carried Alan Shepard (the first American into space) in 1961.

The German Mistle (Mistletoe) composite aircraft of late World War II was probably the first example of the use of fly-by-wire for flight control in a manned aircraft application. Mistle consisted of a fighter (usually a Focke-Wulf FW 190) mounted on a support structure on a Junkers Ju 88 bomber.[8] The Ju 88 was equipped with a 3,500-pound warhead and was intended to be flown to the vicinity of its target by the FW 190 pilot, at which time he would separate from the bomber and evade enemy defenses while the Ju 88 flew into its target. Potentiometers at the base of the FW 190 pilot’s control stick generated electrical commands that were transmitted via wire through the support structure to the bomber. These electrical commands activated electric motors that moved the system of pushrods leading to the Ju 88 control surfaces.[9]

Another electronic flight control system innovation related to the fly-by-wire concept had its origins in electronic feedback flight control research that began in Germany in the late 1930s and was published by Ernst Heinkel and Eduard Fischel in 1940. Their research was used in the 1944 development of a directional stability augmentation system for the Luftwaffe’s heavily armed and armored Henschel Hs 129 ground attack aircraft to compensate for an inherent Dutch roll[10] instability that affected strafing accuracy with its large-caliber, low-rate-of-fire antitank cannon.[11] This consisted of modifying the rudder portion of the flight control system for dual mode operation. The rudder was split into two sections, with the lower portion directly linked to the pilot’s flight controls. The upper section was electromechanically linked to a gyroscopic yaw rate sensor that automatically provided rudder corrections as yawing motions were detected.[12] This was the first practical aircraft yaw damper. Northrop incorporated electronic stability augmentation devices into its YB-49 flying wing bomber that first flew in late 1947 in an attempt to compensate for serious directional stability problems. After the war, the NACA Ames Aeronautical Laboratory conducted extensive flight research into artificial stability. An NACA-operated Grumman F6F-3 Hellcat was modified to incorporate roll and yaw rate servos that provided stability augmentation, with flight tests beginning in 1948. In the following years, a number of other aircraft were modified by the NACA at Ames for variable stability research, including several variants of the North American F-86.[13] By the 1950s, most high-performance swept wing jet-powered aircraft were designed with electronic stability augmentation devices.

Early Aircraft Fly-By-Wire Applications

By the 1950s, fully boosted flight controls were common, and the potential benefits of fly-by-wire were becoming increasingly apparent. Beginning during the Second World War and continuing postwar, fly-by-wire and power-by-wire flight control systems had been fielded in various target drones and early guided missiles.[14] However, most aircraft designers were reluctant to completely abandon mechanical linkages to flight control surfaces in piloted aircraft, an attitude that would undergo an evolutionary change over the next two decades as a result of a broad range of NACA–NASA, Air Force, and foreign research efforts.

Beginning in 1952, the NACA Langley Aeronautical Laboratory began an effort oriented to exploring various aspects of fly-by-wire, including the use of a side stick controller.[15] By 1954, flight-testing began with what was perhaps the first jet-powered fly-by-wire research aircraft, a modified former U.S. Navy Grumman F9F-2 Panther carrier-based jet fighter used as an NACA research aircraft. The primary objective of the NACA effort was to evaluate various automatic flight control systems, including those based on rate and normal acceleration feedback. Secondary objectives were to evaluate use of fly-by-wire with a side stick controller for pilot inputs. The existing F9F-2 hydraulic flight control system, with its mechanical linkages, was retained with the NACA designing an auxiliary flight control system based on a fly-by-wire analog concept. A small, 4-inch-tall side stick controller was mounted at the end of the right ejection seat armrest. The controller was pivoted at the bottom and was used for both lateral (roll) and longitudinal (pitch) control. Only 4 pounds of force were required for full stick deflection. The control friction normally present in a hydromechanical system was completely eliminated by the electrically powered system. Additionally, the aircraft’s fuel system was modified to enable fuel to be pumped aft to destabilize the aircraft by moving the center of gravity rearward. Another modification was the addition of a steel container mounted on the lower aft fuselage. This carried 250 pounds of lead shot to further destabilize the aircraft. In an emergency, the shot could be rapidly jettisoned to restabilize the aircraft. Fourteen pilots flew the modified F9F-2, including NACA test pilots William Alford[16] and Donald L. Mallick.[17] Using only the side stick controller, the pilots conducted takeoffs, stall approaches, acrobatics, and rapid precision maneuvers that included air-to-air target tracking, ground strafing runs, and precision approaches and landings. The test pilots quickly became used to flying with the side stick and found it comfortable and natural to use.[18]

In mid-1956, after interviewing aircraft flight control experts from the Air Force Wright Air Development Center’s Flight Control Laboratory, Aviation Week magazine concluded:

The time may not be far away when the complex mechanical linkage between the pilot’s control stick and the airplane’s control surface (or booster valve system) is replaced with an electrical servo system. It has long been recognized that this“fly-by-wire” approach offered attractive possibilities for reducing weight and complexity. However, airplane designers and pilots have been reluctant to entrust such a vital function to electronics whose reliability record leaves much to be desired.[19]

Even as the Aviation Week article was published, several noteworthy aircraft were under development that would incorporate various fly-by-wire approaches in their flight control systems. In 1956, the British Avro Vulcan B.2 bomber flew with a partial fly-by-wire system that operated in conjunction with hydraulically boosted, mechanically activated flight controls. The supersonic North American A-5 Vigilante Navy carrier-based attack bomber flew in 1958 with a pseudo-fly-by-wire flight control system. The Vigilante served the fleet for many years, but its highly complex design proved very difficult to maintain and operate in an aircraft carrier environment. By the mid-1960s, the General Dynamics F-111 was flying with triple-redundant, large-authority stability and command augmentation systems and fly-by-wire-controlled wing-mounted spoilers.[20]

On the basic research side, the delta winged British Short S.C.1, first flown in 1957, was a very small, single-seat Vertical Take-Off and Landing (VTOL) aircraft. It incorporated a triply redundant fly-by-wire flight control system with a mechanical backup capability. The outputs from the three independent fly-by-wire channels were compared, and a failure in a single channel was overridden by the other two. A single channel failure was relayed to the pilot as a warning, enabling him to switch to the direct (mechanical) control system. The S.C.1 had three flight control modes, as described below, with the first two only being selectable prior to takeoff.[21]

• Full fly-by-wire mode with aerodynamic surfaces and nozzles controlled electrically via three independent servo motors with triplex fail-safe operation in conjunction with three analog autostabilizer control systems.
• A hybrid mode in which the reaction nozzles were servo/autostabilizer (fly-by-wire) controlled and the aerodynamic surfaces were linked directly to the pilot’s manual controls.
• A direct mode in which all controls were mechanically linked to the pilot control stick.

The S.C.1 weighed about 8,000 pounds and was powered by four vertically mounted Rolls-Royce RB.108 lift engines, providing a total vertical thrust of 8,600 pounds. One RB.108 engine mounted horizontally in the rear fuselage provided thrust for forward flight. The lift engines were mounted vertically in side-by-side pairs in a central engine bay and could be swiveled to produce vectored thrust (up to 23 degrees forward for acceleration or –12 degrees for deceleration). Variable thrust nose, tail, and wingtip jet nozzles (powered by bleed air from the four lift engines) provided pitch, roll, and yaw control in hover and at low speeds during which the conventional aerodynamic controls were ineffective. The S.C.1 made its first flight (a conventional takeoff and landing) on April 2, 1957. It demonstrated tethered vertical flight on May 26, 1958, and free vertical flight on October 25, 1958. The first transition from vertical flight to conventional flight was made April 6, 1960.[22]

During 10 years of flight-testing, the two S.C.1 aircraft made hundreds of flights and were flown by British, French, and NASA test pilots. A Royal Aircraft Establishment (RAE) report summarizing flight-test experience with the S.C.1 noted: “Of the visiting pilots, those from NASA [Langley’s John P. “Jack” Reeder and Fred Drinkwater from Ames] flew the aircraft 6 or 7 times each. They were pilots of very wide experience, including flight in other VTOL aircraft and variable stability helicopters, which was of obvious assistance to them in assessing the S.C.1.”[23] On October 2, 1963, while hovering at an altitude of 30 feet, a gyro input malfunction in the flight control system produced uncontrollable pitch and roll oscillations that caused the second S.C.1 test aircraft (XG 905) to roll inverted and crash, killing Shorts test pilot J.R. Green. The aircraft was then rebuilt for additional flight-testing. The first S.C.1 (XG 900) was used for VTOL research until 1971 and is now part of the Science Museum aircraft collection at South Kensington, London. The second S.C.1 (XG 905) is in the Flight Experience exhibit at the Ulster Folk and Transport Museum in Northern Ireland, near where the aircraft was originally built by Short Brothers.

The Canadian Avro CF-105 Arrow supersonic interceptor flew for the first time in 1958. Revolutionary in many ways, it featured a dual channel, three-axis fly-by-wire flight control system designed without any mechanical backup flight control capability. In the CF-105, the pilot’s control inputs were detected by pressure-sensitive transducers mounted in the pilot’s control column. Electrical signals were sent from the transducers to an electronic control servo that operated the valves in the hydraulic system to move the various flight control surfaces. The CF-105 also incorporated artificial feel and stability augmentation systems.[24] In a highly controversial decision, the Canadian government canceled the Arrow program in 1959 after five aircraft had been built and flown. Although only about 50 flight test hours had been accumulated, the Arrow had reached Mach 2.0 at an altitude of 50,000 feet. During its development, NACA Langley Aeronautical Laboratory assisted the CF-105 design team in a number of areas, including aerodynamics, performance, stability, and control. After the program was terminated, many Avro Canada engineers accepted jobs with NASA and British or American aircraft companies.[25] Although it never entered production and details of its pioneering flight control system design were reportedly little known at the time, the CF-105 presaged later fly-by-wire applications.

NACA test data derived from the F9F-2 fly-by-wire experiment were used in development of the side stick controllers in the North American X-15 rocket research plane, with its adaptive flight control system.[26] First flown in 1959, the X-15 eventually achieved a speed of Mach 6.7 and reached a peak altitude of 354,200 feet. One of the two side stick controllers in the X-15 cockpit (on the left console) operated the reaction thruster control system, critical to maintaining proper attitude control at high Mach numbers and extreme altitudes during descent back into the higher-density lower atmosphere. The other controller (on the right cockpit console) operated the conventional aerodynamic flight control surfaces. A CALSPAN NT-33 variable stability test aircraft equipped with a side stick controller and an NACA-operated North American F-107A (ex-USAF serial No. 55-5120), modified by NACA engineers with a side stick flight control system, were flown by X-15 test pilots during 1958–1959 to gain side stick control experience prior to flying the X-15.[27]

Interestingly, the British VC10 jet transport, which first flew in 1962, has a quad channel flight control system that transmits electrical signals directly from the pilot’s flight controls or the aircraft’s autopilot via electrical wiring to self-contained electrohydraulic Powered Flight Control Units (PFCUs) in the wings and tail of the aircraft, adjacent to the flight control surfaces. Each VC10 PFCU consists of an individual small self-contained hydraulic system with an electrical pump and small reservoir. The PFCUs move the control surfaces based on electrical signals provided to the servo valves that are electrically connected to the cockpit flying controls.[28] There are no mechanical linkages or hydraulic lines between the pilot and the PFCUs. The PFCUs drive the primary flight control surfaces that consist of split rudders, ailerons, and elevators on separate electrical circuits. Thus, the VC10 has many of the attributes of fly-by-wire and power-by-wire flight control systems. It also features a backup capability that allows it to be flown using the hydraulically boosted variable incidence tail plane and differential spoilers that are operated via conventional mechanical linkages and separate hydraulic systems.[29] The VC10K air refueling tanker was still in Royal Air Force (RAF) service as of 2009, and the latest Airbus airliner, the A380, uses the PFCU concept in its fly-by-wire flight control system.

The Anglo-French Concorde supersonic transport first flew in 1969 and was capable of transatlantic sustained supercruise speeds of Mach 2.0 at cruising altitudes well above 50,000 feet. In support of the Concorde development effort, a two-seat Avro 707C delta winged flight research aircraft was modified as a fly-by-wire technology testbed with a side stick controller. It flew 200 hours on fly-by-wire flight trails at the U.K. at Farnborough until September 1966.[30] Concorde had a dual channel analog fly-by-wire flight control system with a backup mechanical capability. The mechanical system served in a follower role unless problems developed with the fly-by-wire control elements of the system, in which case it was automatically connected. Pilot movements of the cockpit controls operated signal transducers that generated commands to the flight control system. These commands were processed by an analog electrical controller that included the aircraft autopilot. Mechanically operated servo valves were replaced by electrically controlled ones. Much as with the CF-105, artificial feel forces were electrically provided to the Concorde pilots based on information generated by the electronic controller.[31]

Space Race and the War in Vietnam: Emphasis on FBW Accelerates

During the 1960s, two major events would unfold in the United States that had very strong influence on the development and eventual introduction into operational service of advanced computer-controlled fly-by-wire flight control systems. Early in his administration, President John F. Kennedy had initiated the NASA Apollo program with the goal of placing a man on the Moon and safely bringing him back to Earth by the end of the decade. The space program, and Apollo in particular, would lead to major strides in the application of the digital computer to manage and control sensors, systems, and advanced fly-by-wire vehicles (eventually including piloted aircraft). During the same period, America became increasingly involved in the expanding conflict in South Vietnam, an involvement that rapidly escalated as the war expanded into a conventional conflict with dimensions far beyond what was originally foreseen. As combat operations intensified in Southeast Asia, large-scale U.S. strike missions began to be flown against North Vietnam. In response, the Soviet Union equipped North Vietnamese forces with improved air defense weapons, including advanced fighters, air-to-air and surface-to-air missiles, and massive quantities of conventional