The XP-84 design proposal from Republic Aviation Corporation was submitted in response to a General Operation Requirement (GOR) issued in September, 1944. The design group was headed by Alexander Kartveli, who earlier headed the design of the Seversky P-35 and the Republic P-47. The original Thunderjet proposal was based on a design which was evolutionary from the P-47, specifically by attempting to fit the specified GE TC-180 axial-flow engine into a modified P-47J. This design proposal was soon revised, and in November, 1944, Republic submitted the revised design, which, upon acceptance by the Army Air Force, became the XP-84. Three XP-84 aircraft were ordered, power plant was GE's J35-GE-7 producing 3,750 pounds thrust. The first flight of the prototype XP-84, serial no. 45-59475, was done at Muroc Dry Lake in California, on 28 February, 1946, after the disassembled plane was flown in the new Boeing XC-97 from the Republic facility at Farmingdale on Long Island to Muroc for the flight tests. Major Wallace Lein was at the controls for the first flight. The second prototype arrived at Muroc in August, and on 7 September, in an attempt to better the World Speed Record set the day before in a Gloster Meteor, it was flown at 611 mph, which, though just short of the Meteor's speed of over 615 mph, was still a new U.S. National Speed Record. The third aircraft, because of numerous evolutionary changes and being fitted with an Allison J-35-A-15 of 4,000 pounds thrust, was redesignated from XP-84 to XP-84A, and was the only Thunderjet so designated.
The XP-84 featured a very clean design as compared to other jet aircraft of the time. The P-59 had the low-slung appendages, comprising intakes, engine bays, and exhausts; the P-80 had the bulging intakes on each side to feed its centrifugal flow TG-160 engine; the British jets had either twin engine-pods or twin booms; the Russian jets still tended to look like pregnant guppies; the Me-262, albeit as clean as a twin-pod engine arrangement could be made, still did not have the simplicity of line of the Thunderjet. In contrast, the XP-84 had a slim round to ovoid cross-section fuselage with clean wings and horizontal stabilizers, the wings featuring an airfoil, which, being thicker to accommodate fuel tanks and gear wells, was more of a limit to the top speed of the Thunderjet than was the fuselage. As with many aircraft, operational needs, especially for armament, the auxiliary fuel tanks, and eventually, the auxiliary intakes and canopy bracing, cluttered the clean lines of the prototypes. Notwithstanding these later accoutrements, the Thunderjet was and is an unusually clean-lined and attractive aircraft.
The XP-84, as the prototype, had several features unique to that model. None of the XP-84s had auxiliary tanks fitted, their navigation lights were midpoint at the extreme wing tips, and the pitot tube was midpoint on the leading edge of the left wing. As with the P-80, it was thought that paint would help airflow, and the first XP-84, PS-475 (559475), was painted gray. Practice showed the paint was of no benefit, and subsequent aircraft were generally left in bare aluminum, although a number of NATO aircraft were camouflage painted.
Pitot tube on mid left wing; navigation lights centered on wingtips.
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