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Quest for Performance: The Evolution of Modern Aircraft


Part II: THE JET AGE


Chapter 11: Early Jet Fighters


The Swept Wing Emerges



[291] Described in chapter 10 are the advantages of wing sweepback as applied to aircraft designed for flight at high-subsonic or supersonic Mach numbers. At subsonic speeds, increasing sweepback angle increases the wing critical Mach number, that is, the Mach number at which the adverse effects of compressibility first begin to appear. The effect of sweepback on the critical Mach number was first pointed out in the United States by Robert T. Jones of NACA in 1945. German engineers under the leadership of A. Busemann were aware of the importance of wing sweep in high-speed-aircraft design at an earlier date; and, following the end of World War II, much experimental information that had accumulated in Germany became available in the United [292] States. Together with data obtained in NACA's wind tunnels, this information served as the basis for the first swept-wing fighters designed in this country. The early swept-wing fighters were strictly subsonic aircraft. Discussed in the following paragraphs are the first USAF and Navy fighters to incorporate wing sweepback. Both aircraft had long and distinguished careers, and both will find an important place in any account of the development of jet fighters.

First of the swept-wing subsonic jet fighters to serve in the USAF was the North American F-86 Sabre, which made its first flight on October 1, 1947. Before production ended, nearly 10 000 Sabres had been produced in 20 different variants (including the Navy FJ series known as the Fury), with five different engines. During its long service life, the F-86 formed a part of the air forces of 24 different countries. As late as 1980, eight Third World nations still included a number of F-86 fighters in their inventories (ref. 177). Production lines were established in four foreign countries, with the last aircraft coming from the Japanese line in 1961. The Sabre saw extensive service with the USAF during the Korean war, in which it achieved an outstanding exchange ratio of nearly 14 to I in combat with the Soviet-built MiG-15. Surely the F-86 must be ranked, along with its illustrious World War 11 ancestor the P-51 Mustang, as one of the great fighter aircraft of all time.

Originally designed as a fair-weather air-superiority fighter, the F-86 later appeared in all-weather interceptor and ground-attack versions. Some of these variants had major design differences; consequently, the F-86 must be considered as a whole family of related aircraft. Included here is a brief description of the F-86E air-superiority version of the aircraft. Data for this version are given in table V, and an F-86F in flight is shown in figure 11.7.

Identifying features of the F-86 are the graceful sweptback wing and the nose inlet located in the fuselage. According to the comprehensive paper by Blair contained in reference 155, the 4.78-aspect-ratio wing of 35° sweepback was derived from captured German data for the advanced Messerschmitt fighter under design study at the time hostilities ended. Streamwise airfoil-section thickness ratios varied from 9.5 percent at the root to 8.5 percent at the tip. (These thickness ratios are based on data contained in the paper by Blair in ref. 155; other sources, e.g., ref. 126, give larger values for the thickness ratios. The apparent discrepancy is largely resolved, however, if the higher thickness ratios cited in ref. 126 are assumed to be based on a wing chord length measured in a direction perpendicular to the quarter-chord line...




[293] Figure 11. 7 - North American F-86F Sabre single-engine jet fighter. [mfr via Martin COPP]


...rather than in the streamwise direction.) Pitch-up was prevented on many versions of the aircraft by full-span leading-edge slats. As on the Messerschmitt Me 262, deployment of the slats was automatically initiated at the correct angle of attack by aerodynamic loads acting at the leading edge of the wings. On some versions of the aircraft, the slats were replaced by a sharp, extended-chord, cambered leading edge. Single-slotted high-lift flaps and outboard ailerons were incorporated in the trailing-edge portions of the wing. The ailerons were hydraulically actuated, as were the horizontal-tail surfaces, which, on the F-86E, consisted of a movable stabilizer with linked elevator. Some versions of the F-86 had an all-moving, slab-type horizontal tail with no elevator. Greater control effectiveness is possible at high-subsonic and supersonic Mach numbers with the all-moving horizontal tail, and this arrangement was to become standard on future transonic/supersonic fighters. The hydraulically actuated controls of the F-86E were of the fully powered, irreversible type with artificial control feel provided for the pilot. Fully powered, irreversible controls aid in eliminating such instabilities as aileron and rudder buzz, in addition to permitting maximum deflection of the control surfaces without requiring excess physical effort on the part of the pilot. These controls differ from the hydraulically boosted [294] controls used on some early versions of the F-86, as well as on other aircraft. In a boosted control system, the pilot is still directly linked to the aerodynamic control surfaces, but his strength is augmented by a hydraulic booster. Dive brakes were mounted on either side of the fuselage behind the wing.

As mentioned, another identifying feature of many versions of the F-86 was the fuselage nose-inlet installation. Inlet air was ducted under the cockpit and delivered to the turbojet engine located behind the pilot; the exhaust nozzle was at the rear end of the fuselage. To minimize the depth of the fuselage in the cockpit area, the shape of the duct leading from the inlet to the engine was changed from a circular to an elliptical shape with the long axis being in the horizontal plane. In the all-weather interceptor versions of the aircraft, notably the F-86D, K, and L models, the distinctive nose inlet was replaced by a chin installation to provide space in the nose for the necessary radar gear. In contrast to other F-86 variants, the all-weather interceptor models were equipped with afterburning engines to provide the high rates of climb and high-altitude capability necessary to execute interception missions.

Armament of the fighter versions of the aircraft consisted of 3 .50caliber machine guns buried in each side of the fuselage near the nose and provisions for carrying 2 1000-pound bombs or 16 5-inch rockets on the wings. Interceptor versions of the aircraft carried 24 2.75-inch rockets mounted on a retractable tray contained in the bottom of the fuselage. The tray extended only long enough to launch the rockets. Environmental control in the cockpit consisted of air-conditioning, heating, and pressurization; in addition, the pilot was equipped with an ejection seat.

The data in table V show that the thrust-to-weight ratio of the F-86E was about the same as that of the P-59A. Yet, as compared with the earlier aircraft, the Sabre showed a speed advantage of nearly 300 miles per hour at sea level. A smaller wing area, wing sweepback, and thinner airfoil sections, together with careful attention to aerodynamic design, were responsible for the large increment in maximum speed between the two types. Also, improved engine performance, not reflected in the values of static thrust given in the table, no doubt played a role in the superior performance of the F-86. Drag area was a little greater for the F-86 than for the P-80 by an amount that corresponds closely to the difference in wing area of the two aircraft. As would be expected, the zero-lift drag coefficients were about the same for both aircraft. Comparison of values of the maximum lift-drag ratio shows the [295] P-80 to have had the advantage by about 17 percent; this difference is primarily due to the lower wing aspect ratio of the F-86. Although the Sabre was strictly a subsonic aircraft, low-supersonic speeds could be achieved in a shallow dive. Flight through Mach 1.0 first took place on April 26, 1948.

Sea-level rate of climb was 7250 feet per minute, and 6.3 minutes were required to reach an altitude of 30 000 feet; service ceiling was 47 200 feet. For the afterburning F-86D interceptor, the sea-level rate of climb was 12 000 feet a minute, and 6.9 minutes were required to reach 40 000 feet; service ceiling was 49 750 feet. Ferry range for the F-86E is given in table V as 1022 miles. According to reference 162, the combat radius of action with internal fuel was only 321 miles; with drop tanks the radius of action was increased to 424 miles.

Surpassed in performance in the early 1950's by the Century Series fighters, the F-86 has long been retired from the USAF operational inventory. A number are still in use as target drones and for various flight-test purposes, and at least one manufacturer uses an F-86 as a chase plane.

While recognizing the high-speed performance advantages of the swept wing, there was skepticism within the Navy regarding the carrier compatibility of a swept-wing fighter. Low maximum lift coefficients, poor stability and control characteristics at low speeds, and high angles of attack during the landing approach were cited as serious deficiencies that mitigated against the use of sweepback on Navy fighters. Nevertheless, swept-wing Navy fighters were under development in the late 1940's; and one of these, the tailless Vought F7U Cutlass, made its maiden flight in 1948. Because of engine and/or airframe problems, however, neither the Cutlass nor any of the new Navy fighters were in operational use when the Korean war began in June 1950. During the early months of the conflict, however, an urgent need developed for a Navy fighter with a higher performance than then available with straight-wing Navy jet fighters.

The first operational swept-wing Navy fighter was a product of the urgent Korean war need and consisted of a hasty albeit skillful and highly successful modification of an existing straight-wing Navy fighter. Since its first flight in 1947, the Grumman F9F Panther straight-wing jet fighter had been developed into a highly capable aircraft that first entered operational service in 1949. In the incredibly short time period of about 6 months, the Panther was converted into an effective sweptwing fighter, which made its first flight on September 20, 1951. Named the Cougar, the new aircraft was designated the F9F-6; later versions [296] were the F9F-7 and the F9F-8. Operational service of the Cougar began in November 1952, and so successful was the aircraft that over 1500 were built. The last of these, a trainer version, was finally withdrawn from service in 1974. During its active lifetime, it was employed as a fighter as well as for ground-attack and photoreconnaissance duties and, with an added second seat, as a trainer.

Three-quarter front and rear views of an F9F-7 are shown in figures 11.8 and 11.9, and the approximate shape of the wing planform of an F9F-8 is depicted in figure 11.10. This wing-planform sketch, as well as those presented later for several other aircraft, was based on information contained in references 162 and 171.

The Cougar was a midwing monoplane with leading-edge wingroot inlets feeding the single 7250-pound-thrust Pratt & Whitney turbojet engine located behind a large fuel tank immediately to the rear of the pilot. In a somewhat unusual arrangement, the vertical-tall surfaces extended beyond the end of the fuselage, which contained the engine exhaust nozzle. The horizontal tall was positioned part way between the top and bottom of the fixed portion of the vertical tail. Advantages offered by this configuration design are a reduction in tailpipe length and associated internal losses and external fuselage drag while providing at the same time a satisfactorily long tail moment arm.

Figure 11.10 shows the unusual planform shape of the 35° sweptback wing of the F9F-8. A distinctive feature of the wing is the large increase in wing chord between the inboard end of the flap and the....




Figure 11. 8 - Grumman F9F- 7 Cougar single-engine jet fighter. [NASA]



[297] Figure 11. 9 - Rear view of Grumman F9F-7 Cougar single-engine jet fighter. [NASA]


....side of the fuselage. An acceptable airfoil thickness chord ratio, while permitting the large physical thickness required to accommodate the inlet and internal flow duct, is the reason for the large increase in wing chord. Hydraulically actuated spoilers on the upper surface of the wing just forward of the flaps provided the sole source of roll control - no ailerons were used. Space was accordingly available for large trailing edge flaps. On the F9F-6, the trailing-edge flaps operated in conjunction with full-span leading-edge slats to provide the high-lift capability needed for carrier operation. Later versions of the aircraft incorporated a wing of larger chord, without slats, to decrease the wing thickness ratio and thus increase the critical Mach number; the corresponding 12-percent increase in wing area also increased the lifting capability of the wing and no doubt compensated to some degree for the removal of the leading-edge slats. The relatively sharp wing leading edge together with the fence and leading-edge snag, or dogtooth, provided the necessary wing flow control to avoid serious pitch-up problems.

The longitudinal control system on the aircraft consisted of a fully powered stabilizer with a linked elevator; an interconnect between the flaps and the stabilizer provided automatic pitch-trim compensation with flap deflection. The rudder was operated manually since this control was little used in the high-speed regime where hinge moments are high. As mentioned, roll control was accomplished by wing spoilers. An ejection seat was provided for the pilot, and the cockpit was heated,....




[298] Figure 11.10 - Approximate wing-planform shape of Grumman F9F-8 carrier-based jet fighter.

....air-conditioned, and pressurized. Armament on the fighter version of the F9F consisted of four 20-mm cannons and two Sidewinder missiles. For ground-attack missions, the aircraft could carry up to four 500pound bombs. A glance at the data in table V indicates that the maximum speed of the Cougar was nearly the same as that of the F-86 at an altitude of 35 000 feet; near sea level, the F-86 was about 30 miles per hour faster than the Cougar. Data in reference 200 show that the Cougar was 40-plus miles per hour faster than its straight-wing cousin the Panther. The higher performance of the swept-wing Cougar was achieved with about the same thrust as the straight-wing Panther in spite of a drag-producing increase of 35 percent in wing area. Together with the highlift devices on the Cougar, the larger wing area resulted in a stall speed of about 140 miles per hour for both aircraft (ref. 200).

The North American Sabre and the Grumman Cougar, both good aircraft, have been described here as being the first swept-wing jet fighters operated by the USAF and the Navy. Both services operated other subsonic swept-wing fighters representative of the same level of [299] technology as the two aircraft. Details of these various aircraft can be found in references 162, 163, and 200.