FROM THE ENGINEERS DESK McDonnell XF-88 Voodoo The USAAF had found by painful experience in World War II that fighter escort was absolutely vital for the survival of bombers in enemy airspace. Unfortunately, the first jet aircraft were notorious fuel hogs and lacked the range and endurance of their piston-engined counterparts, and would be unable to escort long-range bombers such as the B-29, B-50, and B-36 all the way to their targets. In an attempt to solve this problem, the USAF considered all sorts of proposals for markedly increasing the range of jet fighter escorts, some of which bordered on the bizarre. In early 1946, the USAAF informally requested proposals for a penetration fighter with a combat radius of at least 900 miles and a performance capable of meeting all opposing fighters on more than equal terms. In addition, the USAAF wanted to keep the gross weight of the aircraft below 15,000 pounds. Spurred on by the USAAF request, on April 1, 1946 the McDonnell Aircraft Corporation of St. Louis, Missouri initiated work on the Model 36. a large twin-jet aircraft powered by a pair of 3000 lb. Westinghouse J34 engines. Originally, McDonnell had considered installing the engines in the wing roots, but this proved infeasible and the engines were moved to a side-by-side location in the lower central fuselage where they could be more easily reached for maintenance. The engines were fed by straight-through air intakes mounted in the wing roots. The jet exhausts were underneath the rear fuselage. This configuration, it was hoped, would leave enough space in the fuselage for the fuel needed for the long-range penetration mission. A 35-degree sweptback wing was fitted, and a V-tail was to be used. The V-tail arrangement was selected because of the desire to reduce compressibility effects, which was thought would be helped by cutting the number of tail intersections from three to two. A set of perforated dive brakes was mounted on the rear fuselage, hinged at the rear. The pilots cockpit was situated well forward of the wing. The armament was to be six 20-mm cannon. The USAAF was sufficiently interested in the proposal that a Letter of Intent was awarded to McDonnell on May 7, 1946. On June 20, 1946, the USAAF awarded McDonnell a contract for two XP-88 prototypes. Serials were 46-525 and 46-526. A mockup was ready by the summer of 1946. Following inspection of the mockup in August, some changes were made. The wing root intakes were given 40 degrees of sweep and a boundary layer ramp was added on the intakes to improve pressure recovery. Early wind tunnel tests had indicated that the V-tail arrangement would result in adverse rolling moments due to rudder action and would produce insufficient longitudinal stability near the stall. Consequently, the V-tail was replaced by conventional swept surfaces, with the tailplane mounted partway up the fin to keep it in undisturbed airflow. The USAAF was satisfied with the changes, and confirmed the order for two prototypes on February 14, 1947. On June 11, 1948, the designation XP-88 was changed to XF-88 when the P designation was replaced by F. The name *Voodoo* was assigned, consistent with McDonnells tradition of choosing the names of spirit-like apparitions for its aircraft. The first prototype (46-525) was rolled out on August 11, 1948. It was taken out to Muroc Dry Lake for testing. It made its maiden flight on October 20, 1948, piloted by McDonnell chief test pilot Robert M. Edholm. The powerplant was a pair of 3000 lb.st. Westinghouse XJ34-WE-13 axial-flow turbojets mounted side-by-side in the lower center fuselage. No armament was fitted. Some minor problems were encountered during the test flight program. Some loss of thrust was encountered during takeoff due to choking in the S-shaped air ducts. This was solved by fitting spring-loaded blow-in doors in the wheel well section of the ducts. The rolling rate was found to be insufficient. It was improved to a certain extent by increasing the aileron chord by 26 percent, but the full cure for this problem required an increase in the torsional rigidity of the wing. When the dive brakes were fully open, there was excessive buffeting. This problem was eliminated by perforating the brakes and restricting their opening angle to 45 degrees. In order to correct problems with inadequately damped directional oscillations and objectionable roll coupling, artificial means of stabilization had to be devised, and a yaw damper system was adopted using a yaw rate gyro which controlled the rudder. With these changes, handling characteristics were generally satisfactory. However, performance was rather disappointing, due primarily to the demanding range requirements and to the weight increases which had taken place since the initial design phase. Maximum speed at sea level was only 641 mph, slower than that of the F-86 Sabre which was already in production. It took approximately six minutes for the XF-88 to reach an altitude of 30,000 feet. Initial estimates of a combat weight of only 16,500 pounds had proven to be overly optimistic, and the installation of additional equipment as requested by the USAF and the various modifications required as a result of flight testing were estimated to push the combat weight to over 20,000 pounds, making matters even worse. In order to satisfy the range requirements, the 734 US gallon internal fuel capacity was to be supplemented by 350-gallon wingtip tanks. However, wind tunnel testing indicated that serious stalling problems would be encountered when these tip tanks were mounted. It would appear that the XF-88 needed more power. McDonnell proposed to fit afterburners to the J34 engines that powered the second prototype, and the production F-88s were to be powered by 6000 lb.st. Westinghouse J46 afterburning turbojets. The USAF did not choose to fund the J46 installation, but they did approve the adaptation of the second prototype to the afterburning J34 engines. However, this adaptation proved more difficult than expected. Westinghouse found it difficult to achieve the additional thrust that was required with an afterburner only 52 inches long (the maximum which could be allowed due to ground clearance considerations). McDonnell was forced to develop the afterburner on its own. The length of the McDonnell-devised afterburner turned out to be only 30 inches and weighed only 218 pounds. It boosted the thrust of each Westinghouse XJ34-WE-15 engine from 3600 to 4825 pounds. The second prototype (46-526) was redesignated XF-88A, and was fitted with the afterburning XJ34-WE-15 engines. It was also fitted with bladder fuel cells in the wings to increase internal fuel capacity to 834 gallons. The XF-88A made its first flight on April 26, 1949. The performance improvement was apparent --- maximum speed at sea level was almost 700 mph, time to climb to 30,000 feet was cut to 4 minutes, and takeoff run was reduced by 20 percent. The XF-88A was faced with some stiff competition for the USAF penetration fighter order. The Lockheed XF-90 and the North American XF-93A had also been entered as contenders for the USAF penetration fighter order. The XF-90 was a twin-jet design which first flew in June of 1949. The XF-93A was a beefed-up derivative of the F-86 Sabre, initially ordered under the designation of F-86C. In the penetration fighter competition, the USAF initially favored the North American design because of its commonality with other Sabre variants, and in June of 1948 they supplemented the contract for the two F-86Cs with a contract for 118 production aircraft. In December of 1948, McDonnell was instructed to stop design and development work on the F-88, but was permitted to continue flight testing on the two prototypes. It would seem that the curtain had been brought down on the XF-88. However, the F-93A production contract contract was suddenly cancelled in February of 1949. Several reasons were given. Perhaps the most important reason was a severe reduction in the military budget for that year. With limited funds available, it was decided to give priority to interceptors and to strategic bombers. In addition, a Senior Officers Board felt that no production order for any penetrator fighters should be awarded until a competitive fly-off between the three contenders could be carried out. The fly-off between the Lockheed XF-90, the McDonnell XF-88, and the North American YF-93A took place between June 30 and July 8 of 1950. On August 15, 1950, the Evaluation Board declared the McDonnell XF-88A to be the winner of the contest. However, McDonnells victory was rather hollow since no penetration fighters were every actually manufactured or placed in service because wartime pressures mandated that priority be given to the procurement of existing types for use in Korea. In addition, the development of long-range, high-speed jet bombers such as the B-47 and the B-52 eliminated any real need for penetration fighters. Although the F-88 never attained production status, it was the inspiration for the later supersonic F-101 Voodoo, which performed successfully in any number of roles, including penetration, escort, tactical strike, nuclear strike, photographic reconnaissance, interceptor, and conversion training. Following the completion of its testing, the XF-88A sat in a junkyard at Langley AFB in Virginia for several years. The XF-88A was eventually scrapped. XF-88 specification: Engines: Two 3000 lb.st. Westinghouse XJ34-WE-13 axial-flow turbojets. Dimensions: wingspan 39 feet 8 inches, length 54 feet 1 1/2 inches, height 17 feet 3 inches, wing area 350 square feet. Weights: 12,140 pounds empty, 18,500 pounds loaded, 23,100 pounds maximum. Maximum speed: 641 mph at sea level. Climb to 35,000 feet in 14.5 minutes, service ceiling 36,000 feet, range 1737 miles. The planned armament of 6 20-mm cannon was not installed. XF-88A specification: Two Westinghouse J34-WE-15 turbojets, 3600 lb.st. dry, 4825 lb.st with afterburning. Dimensions: wingspan 39 feet 8 inches, length 54 feet 1 1/2 inches, height 17 feet 3 inches, wing area 350 square feet. Weights: 12,140 pounds empty, 18,500 pounds loaded, 23,100 pounds maximum. Maximum speed: 641 mph at sea level. 706 mph at 20,000 feet. Initial climb rate 8000 feet per minute. climb to 35,000 feet in 14.5 minutes, service ceiling 39,400 feet, range 1737 miles. Armament consisted of six 20-mm cannon in the nose. r/Jose
Posted on: Mon, 20 Oct 2014 13:00:00 +0000
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