Partners in Freedom

		McDonnell Douglas F-4 Phantom II


Specifications Manufacturer McDonnell Douglas Date in service December 29, 1960 Type Fighter Crew Two Engine General Electric J79-GE-17 Users U.S. Navy, U.S. Marine Corps, U.S. Air Force, Egypt, Israel, Iran, Greece, Spain, Turkey, South Korea, West Germany, Australia, Japan, and Great Britain Dimensions Wingspan . . . . . . . . . . . .38.4 ft Length . . . . . . . . . . . . . . 63.0 ft Height . . . . . . . . . . . . . . 16.6 ft Wing area . . . . . . . . . . 530 sq ft Weight Empty . . . . . . . . . . . .31,360 lb Gross . . . . . . . . . . . . .58,000 lb Performance Max speed . . . . . Mach number of 2.0

		Highlights of Research by Langley for the F-4 Studies in the Langley Unitary Plan Wind Tunnel revealed that the configuration exhibited lateral-directional stability problems at supersonic speeds, which resulted in McDonnell redesigning the wing and vertical tail. Langley conducted studies of the stall departure and flat spin and identified causes of the poor stall-spin behavior of the F-4. Langley, McDonnell Douglas, the Air Force, and the Navy investigated wing modifications to improve the maneuverability and handling characteristics, which resulted in incorporation of leading-edge slats on later versions of the aircraft. Langley provided a solution to a severe nose buffet problem for the reconnaissance version. Langley and Air Force braking performance tests conducted with an F-4 aircraft established runway grooving as an effective solution to hydroplaning accidents. The McDonnell Douglas (now Boeing) F-4 was an exceptional fighter aircraft, which was used for air superiority, interdiction, and close air support. Originally developed by the Navy as a supersonic fleet defense fighter, the F-4 was also put into service by the Air Force and served a variety of roles in the Vietnam conflict. The final application of the F-4 by the U.S. was in the “Wild Weasel” role for suppressing enemy air defense systems. F-4 production ended in 1979 after over 5,000 had been built—more than 2,600 for the U.S. Air Force, about 1,200 for the U.S. Navy and U.S. Marine Corps, and the rest for friendly foreign nations. Later versions of the aircraft were in the U.S. Air Force inventory until December 1995. They are still flown by other nations. NASA Langley Research Center contributions to the F-4 program began during the design stages when supersonic wind-tunnel tests at Langley identified stability issues that required redesign of the airframe, including adding dihedral (angle between horizon and wing panels from frontal perspective) to the outer wing panels and increasing the size of the vertical fin. Another contribution from Langley resulted in the development and incorporation of wing leading-edge slats for enhanced maneuverability and increased safety for high-angle-of-attack conditions. Langley facilities used in the development and support of the F-4 program included the Langley Unitary Plan Wind Tunnel, the 30- by 60-Foot (Full-Scale) Tunnel, the 20-Foot Vertical Spin Tunnel, the 16-Foot Transonic Dynamics Tunnel, the 7- by 10-Foot High-Speed Tunnel, piloted simulators, and radio-controlled drop models.

		Langley Contributions to the F-4

Early Configuration Development		In 1954, the Navy selected McDonnell Aircraft to design and develop an all-weather supersonic fighter. The fighter, designated the F4H, was to be a fleet defense fighter that could take off from an aircraft carrier, have a cruise distance of 250 mi, intercept intruders, and then return to the carrier 3 hr after takeoff. The aircraft was to be armed with missiles and would not carry guns. It would operate as a high-speed (Mach number of 2), standoff missile launcher that would not engage in close-in combat. In mid-1955 the full-scale engineering mock-up of the twin-engine aircraft featured a swept wing with no dihedral, and the horizontal tails drooped down at an angle of 15 deg. At the request of the Navy, tests began in the Langley Unitary Plan Wind Tunnel under the direction of Melvin M. Carmel to determine the supersonic performance and stability and control characteristics of the original configuration. Three separate entries in the Unitary Tunnel were made to evaluate the early F4H design. Results of the first phase of tunnel tests indicated that the F4H exhibited serious deficiencies in lateral-directional stability characteristics at supersonic speeds, including unstable dihedral effect and marginal directional stability. To cure these problems, McDonnell introduced 12 deg of geometric dihedral into the outer wing panels (which were foldable for carrier operations) and increased the size of the vertical tail. Analysis had indicated that only 3 deg of geometric dihedral across the entire wing would solve the problem, but the same effect was achieved with less redesign and developmental effort by changing the outboard panels. Additional tests in other wind tunnels had indicated an undesirable pitch-up characteristic at transonic and low speeds, which was solved by adding chord to the outer wing panels (producing a leading-edge snag or “dog tooth”) and by increasing the droop of the horizontal tail to 23 deg. The final phases of the F4H study in the Unitary Tunnel were directed to the integration of external stores at supersonic speeds. Engineering mock-up of the original F4H Phantom without dihedral in outer wings and 15-deg droop of the horizontal tail. F4H model after modifications with dihedral in outer wings and increased vertical tail. After the development process, the F4H was placed into operations with the U.S. Navy and Marines under the new designation F-4 and set several speed and altitude records. The Air Force began to acquire F-4’s in 1962, and this famous fighter became a mainstay in the air forces of friendly nations, with several variants produced by the McDonnell Douglas Corporation.
Maneuverability		During the first few years of the Vietnam conflict, the U.S. found itself engaging enemy aircraft such as the MiG-17 and MiG-19 that were relatively agile and could easily outmaneuver the heavier U.S. aircraft (F-4 and F-105) that had been designed without requirements for close dogfighting or close weapons such as a gun. Initial tactics used by U.S. pilots to try and turn with enemy aircraft had been relatively unsuccessful, and it had become apparent that missiles in use at that time were relatively unreliable at long ranges. Pilot training and revised tactics were ultimately employed to blunt the threat and use U.S. aircraft to an advantage, but the lack of maneuverability and a gun for close-in combat became issues for the Air Force. A new Air Force version known as the F-4E was equipped with a nose mounted M61 cannon, and additional deliveries to the Air Force began in October 1967. McDonnell Douglas became interested in wing modifications for the F-4 that would improve buffet onset and increase lift and turning performance, while retaining satisfactory characteristics for approach and landing. Langley researcher Edward J. Ray led a cooperative Langley, McDonnell Douglas, and Air Force study of F-4 maneuver and buffet characteristics in the Langley 7- by 10-Foot High-Speed Tunnel in 1969. Candidate configurations included the use of wing leading-edge flaps, leading-edge camber, trailing-edge flaps, and other devices; however, the most effective modification was a two-position leading-edge slat. Two slats were mounted on the leading edge of each wing panel in place of the earlier leading-edge flap. The inner slat was fully retractable at high speeds, but the outer slat remained deployed in both the cruise and high-lift configurations. With the slats deployed, the F-4 could make tighter turns, and approach speeds were also reduced by a significant amount. Another benefit of this modification was a dramatic improvement in the lateral-directional handling characteristics and spin resistance at high angles of attack. F-4E model in the Langley 7- by 10-Foot High-Speed Tunnel for evaluation of wing leading-edge modifications for enhanced maneuverability. The slat configuration was evaluated during flight tests (known as Project Agile Eagle) of a modified F-4 test aircraft with extremely impressive results. The wing leading-edge slats were incorporated on all F-4E aircraft built during and after 1972. Later, the Navy received a slat equipped version of the aircraft known as the F-4S. The F-4G Advanced Wild Weasel, which inherited most of the features of the F-4E including the wing slats, was one of the last versions of the F-4. Working in “hunter-killer” teams of two aircraft, such as F-4G and F-16C, the F-4G hunter could detect, identify, and locate enemy radar then direct weapons to ensure destruction or suppression of the radar. The technique was effectively used during Operation Desert Storm against enemy surface-to-air missile batteries.
High-Angle-of-Attack Stability		Military strategists had procured the F-4 design as a standoff missile launcher with no requirement for strenuous maneuvering at high angles of attack. At that time, the need for close-in air combat maneuvering was thought to be obsolete, due to the advent of air-to-air missiles. Operations prior to Vietnam had stressed the supersonic mission for the F-4, and the initial safety record for the aircraft was extremely good. The return of close-in air-to-air combat during Vietnam unfortunately exposed a deficiency in the flying characteristics of the F-4. During hard turns to engage or escape enemy aircraft, pilots began to fly the F-4 at high angles of attack where they experienced a marked deterioration in lateral-directional stability and control characteristics. Inadvertent loss of lateral-directional control and spin entries occurred, with an alarming number of accidents and losses of crew and aircraft during training and combat. The Navy, Marine Corps, and Air Force suffered over 100 high-angle-of-attack and stall-spin accidents over the lifetime of the F-4 aircraft. In 1967, representatives from the Air Force Aeronautical Systems Division met with Langley researchers Edward C. Polhamus and Joseph R. Chambers to discuss the growing concern over the high-angle-of-attack and stall-spin accidents (at that time, about 10 aircraft had been lost). Analysis of the problem by the Air Force was particularly difficult because the limited interest in high-angle-of-attack maneuvers had resulted in no wind-tunnel tests (other than spin tunnel tests) of the F-4 configuration for such conditions. Langley agreed to conduct diagnostic high-angle-of-attack wind-tunnel tests of the F-4. Tests in the Langley 12-Foot Low-Speed Tunnel and the Langley 30- by 60-Foot (Full-Scale) Tunnel thoroughly documented the aerodynamic factors that produced the loss of lateral-directional stability at high angles of attack. Chambers and Sue B. Grafton conducted two free-flight model investigations of the F-4 for high-angle-of-attack conditions in the Full-Scale Tunnel. In the first study, flight tests of a model of the Air Force F-4C demonstrated an abrupt loss of directional stability (nose slice) near wing stall, and most of the flights ended with the model going out of control. Analysis of wind-tunnel data indicated that massive flow separation on the swept wing caused adverse flow fields at the tail, thereby degrading the stabilizing influence of the vertical fin at high angles of attack. No simple fixes could be found for the problem, short of a redesign of the wing. When interest in wing leading-edge modifications for enhanced maneuverability became known, Langley conducted free-flight tests of a model of the later F-4E configuration. The test program, which was conducted by Edward Ray in the 7- by 10-Foot High-Speed Tunnel, was closely monitored. The results obtained from the Full-Scale Tunnel by Chambers and Grafton for high-angle-of-attack characteristics were fed back to Ray and others interested in maneuver performance. The results of the free-flight model test indicated that the incorporation of wing leading-edge slats markedly improved the high-angle-of-attack behavior of the F-4 by eliminating the severe nose slice tendency of the basic configuration. Researcher Sue Grafton with the slatted F-4E model used for free-flight tests in the Langley Full-Scale Tunnel. F-4 drop model mounted on the Langley helicopter drop rig for a spin entry test. Langley also conducted studies on the high-angle-of-attack characteristics of the F-4 with a radio-controlled model and a piloted simulator. The radio-controlled model was used by Charles E. Libbey to determine aircraft motions after loss of control at high angles of attack and to demonstrate the beneficial effects of the leading-edge slat modification. The simulator study of the F-4 by Frederick L. Moore was one of the first attempts to develop a simulator to study high-angle-of-attack behavior. The application of piloted simulators to this area is now a routine development tool.
The Flat Spin		Tests of an F-4 model by James S. Bowman, Jr. in the Langley 20-Foot Vertical Spin Tunnel in the early 1960’s indicated that the F-4 configuration would exhibit several types of spins. In addition, the recovery characteristics from these various types of spins differed greatly. For example, one spin involved a relatively steep nose-down fuselage attitude with large oscillations in roll during the spin motion. Recovery from this spin was relatively easy with normal pilot control inputs. In contrast to this spin recovery, the F-4 also exhibited a relatively fast, flat spin in which the aircraft descended vertically in a rapid spin motion with an almost horizontal (flat) fuselage attitude. Recovery from the flat spin with normal aircraft controls was found to be impossible. During these spin tests, Langley determined that a 30-ft diameter emergency spin recovery parachute would be required for the test aircraft. Spin tunnel tests are conducted by hand launching a model into a vertically rising airstream with an artificial spinning motion. The question remains as to whether the aircraft could enter such spinning conditions from conventional flight. Langley, therefore, conducts outdoor radio-controlled drop-model tests to evaluate this tendency. Drop tests were considered mandatory to confirm the potential for flat spins for the F-4. Fortunately, the results of the radio-controlled F-4 drop-model tests by Libbey indicated that the aircraft would have a dominant tendency to enter a steeper, recoverable spin rather than the flat, unrecoverable spin. In fact, the flat spin was only obtained once in over 20 aggressive attempts to spin the drop model. The Navy and Air Force both conducted full-scale aircraft spin tests of the F-4, with aggressive control inputs to seek out the various spin modes. The Navy had expressed doubts over the existence of the flat spin at the beginning of their F-4 spin program. The agreement of the precursor Langley Spin Tunnel tests with the flight tests was remarkable. The steep, oscillatory spin mode was normally encountered and recovery was satisfactory as predicted by the Spin Tunnel tests. Unfortunately, the fast, flat unrecoverable spin was encountered in both test programs. The respective test aircraft were lost in crashes at Patuxent River Naval Air Station, Maryland, and Edwards Air Force Base, California, when the emergency spin parachute systems failed to operate properly. In the case of the Navy tests, the parachute failed to inflate properly, while in the Air Force tests, the mechanical system malfunctioned and released the parachute prematurely. Although accident statistics for the F-4 indicated that the accidents involved about an equal number of approach-to-landing and up-and-away maneuvers, the Air Force and Navy expressed great interest in determining what features or factors of the F-4 were responsible for the unrecoverable flat spin mode. A request was made to Langley for assistance in determining these factors, and a contract was given to McDonnell Douglas to analytically study the flat spin. During tests of the F-4 in the Full-Scale Tunnel, Chambers noted that a prospin yawing moment existed for the configuration at the attitudes associated with the fast, flat spin. Results from these tests also indicated that the prospin tendencies were produced by the aft end of the aircraft. An innovative, inexpensive test was used by Langley to identify the responsible aircraft components and the primary mechanism of the flat spin. Chambers mounted a commercially available 1/48-scale plastic hobby model of the F-4 on a spindle assembly with a shaft through the top of the fuselage. The model was free to rotate as it would in a flat spin. When the hobby model was tested in the wind tunnel in a simulated flat spin attitude, researchers found that it would immediately spin up to the angular rates exhibited by the spin tunnel model and that the hobby model would do this to the right or left when released from a nonrotating condition. The researchers found that the driving mechanism for the flat spin tendency was adverse aerodynamic interactions between the drooped horizontal tail and the vertical fin. When the drooped tails were inverted (that is, the tails had 23 deg of positive geometric dihedral) the model would no longer accelerate into the spin condition and would stabilize without rotating. The results of the hobby model tests were checked out in the Langley Spin Tunnel with the dynamically scaled F-4 model previously used for spin tests. The initial tests were conducted with the baseline F-4 configuration, and as expected, the model exhibited the fast flat spin. However, when the horizontal tails were inverted, the model would no longer spin flat, even though it was hand launched with conditions associated with the fast flat spin. Instead, the model would nose down into the steeper, recoverable spin. It was also found that increasing the deflection angle of the horizontal tail to 55 deg or greater eliminated the prospin aerodynamic interference effect for the basic configuration, which would no longer spin flat. The results of these tests to identify the cause of the flat spin and other concepts identified by Langley (including an automatic spin prevention control system) were evaluated by the military. The recommended modifications to the F-4 fleet were considered too drastic at that time in the operational life of the aircraft, but the important adverse effect of the tail interference phenomenon was noted for subsequent aircraft development programs. In the mid-1980’s, the Navy approached Langley with reports that Marine pilots were recovering from flat spins by lowering the landing gear. The Navy requested spin tunnel tests and funded an F-4S model to determine the effects of operating the landing gear during the flat spin. Spin tunnel tests by Bowman showed that lowering the gear actually increased the spin rate in the flat spin! Based on these tests, the Navy did not incorporate lowering the gear as a spin recovery technique in the pilot handbook. Perhaps the most significant result from this test was that it alerted researchers to the potential adverse effects of components such as landing gear, gear doors, and weapons bay doors on spin recovery. The effects of such components had not been routinely tested in decades. This experience later proved very valuable during other programs. The contributions and participation of Langley in the F-4 high-angle-of-attack and stall-spin efforts are also noteworthy because they formed the impetus within Langley to form a cohesive suite of test techniques designed to extract the maximum amount of information in this difficult and complex area. Following the F-4 program, Langley greatly increased commitments to this area and embarked on a research program with industry and DOD that has benefited the development of high-performance U.S. aircraft to this day.
Nose Vibrations		In late November 1965, the staff of the Langley 16-Foot Transonic Dynamics Tunnel (TDT) was urgently requested to assist the Air Force with a buffet problem that was experienced by the RF-4 reconnaissance version of the aircraft during combat operations in Vietnam. When pilots attempted to perform high-speed pre- and post-strike photographic reconnaissance missions, they found that the nose mounted cameras in the RF-4 were being literally shaken to pieces, seriously degrading the clarity of the photographic information and ruining the mechanical operations and operational lifetime of the cameras. Langley researchers Robert V. Doggett and Perry Hanson conducted wind-tunnel tests of the front fuselage of an RF-4 model in the TDT in early 1966 to measure fluctuating pressures in the area near the camera lens and housing. The results of the study indicated that aerodynamic flow separation on the lower forebody over the camera installation produced large vibratory loads that were the source of the problem. Doggett and Hanson, working with McDonnell Douglas and the Air Force, developed a modified camera ramp angle and revised camera enclosure fairing that eliminated the problem and was incorporated on later models of the RF-4. The fairing, which rounded the flat lower surface of the baseline nose, also resulted in increased internal nose volume and allowed larger cameras to be utilized. The RF-4 reconnaissance version of the F-4 with camera fairing under the forward nose section. Simplified model of RF-4 nose tested in the Langley 16-Foot Transonic Dynamics Tunnel.
Runway Grooving		In the 1950’s, Langley researchers at the Aircraft Landing Dynamics Facility focused their research on aircraft braking and directional control performance on wet runway surfaces. This research, led by Langley researchers Walter Horne and Thomas J. Yager, included ground handling tests of an F-4 test aircraft. Comparative tests of the effects of runway grooving on braking and handling characteristics provided clear demonstrations of the effectiveness of the grooving concept.