As a development of the F-84B, the F-84D introduced a number of new features. These included a thicker skin gauge on wings and ailerons, winterized fuel system suitable for JP4, and mechanical linkages instead of hydraulic in the landing gear to shorten the shock strut during retraction. The F-84D was fitted with the J-35A-13 engine, first used on the F-84C.
The Air Force accepted one aircraft in November 1948, and 36 others in December. The first 4 months of 1949 saw the delivery of 117 additional F-84Ds.
Since the early F-84s were less than satisfactory maintenancewise, development changes, geared toward some kind of improvement, accompanied each production group of F-84D airplanes.
Two months before taking delivery of the first F-84D and 2 months after procurement of the aircraft's subsequent model series had been tentatively approved, the Air Force undertook a complete review of the entire F-84 program. Results of the study that ensued were baffling. The 7 F-84 of the B and C series did not satisfactorily meet "any phase of the missions of the major commands," and only a major retrofit program could make the aircraft operational. Although 571 F-84s of the B, C, and D series had been purchased on four previous procurement programs, amounting to a total of some $80 million, production was a year behind schedule. Theoretically, cancellation of the F-84D production would save the government close to $20 million, but in actuality, production of the D had progressed to the point that if cancelled, 'more than half the cost of the 154 F-84D aircraft would be spent without anything in return." Too, the resultant adverse effect upon Republic's financial status might jeopardize the F-84E production, should it be finally approved.
To solve its dilemma, the Air Force directed special tests from 2 February-6 March 1949. Specific purposes were to determine if discrepancies in the F-84 prototypes had been corrected on the D type, and which of the F-84 or F-80 aircraft was the more suitable for fighter operation. Results of the tests conducted early in 1949 at both Wright Patterson AFB, Ohio, and Eglin AFB in Florida, were encouraging. They indicated that many of the deficiencies of previous types of F-84 aircraft had been eliminated in the D model. The Air Proving Ground (APG) tests also concluded that "the F-84 range, acceleration, versatility, load carrying ability, high altitude climb, and level flight speed exceeded that of the F-80. Not all comments were favorable, however. The F-84 was inferior to the F-80 in shortness of takeoff roll, low altitude climb, and maneuverability. Furthermore, it was the opinion of maintenance personnel at both air bases that the maintenance improvements made in the F-84D airplanes were partially offset by the additional time required to change accessories on the front end of the engine.
Despite other minor discrepancies uncovered during the APG tests; the Air Force reached a final decision in favor of the F-84 program. Specifically, the F-84Ds would be accepted for standard use, but no further procurement beyond the current contract would be made. Additional funds in the amount of $3.3 million would be secured for design improvements of the programmed F-84E, and $8 million would be spent to modernize the 382 F-84B and C aircraft remaining in the operational inventory. In May 1949, implementation of the $8 million modernization program received Presidential approval.
The F-84D entered operational service in 1949. The F-84D was the first version. of the Thunderjet to arrive in Korea (December 1950).
A total of 154 F-84Ds were accepted. One F-84D was accepted in November 1948, 36 in December of the same year. Thirty were delivered during each of the first 3 months of 1949, and the last 27 aircraft were delivered to the Air Force in April.
Flyaway Cost Per Production Aircraft was $212,241-airframe, $139,863; engine (installed), $41,654; electronics, $7,165; armament, $23,559.
Republic, at a cost of about $2.9 million, modified the leading edge of all F-84D wings and made other engineering changes. Attempts also were made to correct some of the additional discrepancies uncovered during the APG tests. Efforts centered on improvement of the A-1B gunsight, and reduction of the tailpipe's excessive temperature caused by the aircraft's high thrust J-35-A-13 engine.
The F-84B and C aircraft were not assigned to overseas units because early versions of the J-35 engine allowed only 40 hours of operation between overhauls. Although also not earmarked for oversea use, modified F-84Ds were deployed to the Korean war theater where they began serving with the 27th Fighter Escort Wing. In the spring of 1952, as the Fifth Air Force's fighterbomber strength had been seriously depleted by logistical causes and excessive losses during the railway interdiction campaign, additional F-84Ds were sent overseas. Headquarters USAF decided that the Fifth Air Force would for 5 months receive a total of 102 F-84Ds as attrition replacements. Most of these aircraft were assigned to the 136th Wing, a former Air National Guard organization whose period of authorized service was running out.
Receipt of new F-84 models during August and September 1952 accelerated phaseout of the F-84Ds, which had created many combat logistical and operational problems. In mid 1957 the Guard likewise phased out the last of its Ds.
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