<p>In general, if a combat aircraft has an unofficial name, even a derogatory one, it cannot be all that bad. The famed P-47 Thunderbolt of World War II, made in greater numbers than any other US fighter in history, was to its pilots universally called the 'Jug', short for 'Thunderjug'. This did not necessarily set Republic Aviation firmly on the road of monster overweight fighters, but the fact remains that the P-47's jet successor, the Republic F-84 Thunderjet, simply had nothing like enough 'poke', and its protracted (to the point of being dangerous) take-off gave it the name 'Ground Hog'. Natural development resulted in the more powerful F-84F, but this had short-span swept wings and much greater weight, and instead of being the agile fighter everyone wanted it gained the name 'Super Hog'. It also became known as 'The Thud's Mother', because it was the direct ancestor of the great F-105 Thunderchief. The latter masqueraded as a fighter, but was such a great all-round machine it became 'The Thud', or 'The Lead Sled' or even 'The Ultra Hog'.
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<p>What emerges from all these names is that, when a pilot pushed open the throttle on any of these aircraft, he was hardly likely to be electrified by subsequent events. Many hundreds of pilots never made it, running clean out of runway before the unwilling combination of thrust and lift managed to overcome the inbuilt mass of metal. So how can the F-84 be described as one of the world's greatest aircraft? It is simply that for 20 years from the end of World War II the F-84 was, despite its shortcomings, one of the few truly great aircraft. A critic could say that it simply happened to be made in large numbers and therefore inevitably filled an important niche in the order of battle of the NATO air forces. A fairer assessment would be that the F-84, in all its many quite different versions, has a secure place in history because, over a long and crisis-ridden period, it was ready for battle as a known and trusted (even if in some ways marginal) quantity. Put another way, no listing of the world's greatest aircraft could possibly omit it.
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<p>The man who began the company was an emigre from Georgia, USSR, Major Alex de Seversky. His vice-president engineering (in British terms, the chief designer) was a fellow Georgian, Alex Kartveli. In World War II they tended to produce fighters that nullified the great power of their engines by their sheer size and weight. The P-47 just managed to remain a great fighter, but the XP-69 was fortunately cancelled and the XP-72 was just a P-47 with a bigger engine. By mid-1944 Kartveli's project team had drawn a Jet Thunderbolt with parts of the airframe little changed, but with a General Electric TG180 axial turbojet in the belly, fed by a nose inlet and with the jetpipe extended right to the tail. This was straightforward, and access to the engine was easily arranged through large doors covering the belly. The Jet Thunderbolt might have been a good answer to the urgent wartime need, but, like the parallel Jet Mustang at North American Aviation, it was inferior to the best 'clean sheet of paper' design. In November 1944 it looked as if the war would soon be won, and Republic was authorized to start with a fresh design.
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<hi style=hdr1>Roll-Out From Farmingdale</hi>
<p>The first XP-84 was completed at the Farmingdale, Long Island, plant in December 1945 and flown to Muroc Lake in a Boeing C-97. The XP-84 was a fine-looking machine, much sleeker than the P-47 derivatives but still with an unswept wing of so-called laminar profile. The TG-180 had by this time matured as the J35, initially made in pilot numbers by Chevrolet, but in September 1946 passed to another General Motors division, Allison, for quantity production at a rating of 4,000 lb (17.79 kN). The slim engine was attached behind the rear spar of the wing and fed by ducts passing on each side of the pressurized cockpit from the plain nose inlet. From the compressor to the jetpipe the engine was cantilevered without any rear support, so that by pulling off the entire rear fuselage the whole engine was exposed to view. Jet propulsion had been expected to make possible very short landing gears, but in fact the legs of the XP-84's tricycle gear were exceptionally long to give the correct ground angle and prevent the tail scraping on take-off and landing (a steel bumper skin was still needed). The castoring nose gear retracted to the rear and the main gears, with great cunning, were made to shorten as they folded inwards so that there was just room for the wheels between the spars of the thin wing. The landing gear and split flaps were hydraulically actuated, and there was adjustable hydraulic boost on the ailerons, but not on the rudder (usually ignored) or elevators on the tailplane which was mounted well up the fin with 5 degrees dihedral. The canopy was electrically sliding to the rear, and in emergency could be blown clear by a cartridge. Production machines introduced a cordite-gun seat triggered by levers on each side of the pan. A speed brake was added in the belly, the subject of several modifications.
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<p>Three prototypes (45-59475/59477) had been ordered, but only the first two were completed. Ship 475, with the fashionably large buzz-number PS-475 on the nose, made its maiden flight at Muroc on 28 February 1946. Remarkably, this was also the first flight of theJ35 engine (two more J35s flew in the XB-43 three months later). The second XP-84 flew in August, and in September set a national speed record at Muroc at 611 mph (983 km/h). By this time Republic was well advanced with 16 YP-84As, numbered from 45-59482, with the planned armament of six 0.5-in (12.7-mm) guns, each with 300 rounds, four in the upper part of the nose and the other pair in the roots of the wings. Some of the YP-84s had only four guns, while others had improved wheel brakes and provision for 230-US gal (871-liter) tip tanks. The latter were greatly needed, because there was very limited internal fuel, almost all of it in the wings.
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<p>In 1946 Republic had received a remarkably large order for 500 production aircraft, later styled F-84B (F for fighter instead of P for pursuit), numbered from 45-59497. These had the 4,000-lb (17.79-kN) J35-15C engine, tip tanks, six guns, ejection seat and, from the 85th aircraft (F-84B-20-RE), retractable zero-length launchers for 32 rockets. Except for the first few, all had the faster-firing M3 gun and cockpit air-conditioning. Production did not go on to 500, because at no. 226 it switched to the F-84C with revised fuel, electrical and hydraulic systems, and the Dash-13C engine (still rated at 4,000 lb/17.79 kN thrust) with shorter jetpipe. The pitot head was moved from the fin to the splitter plate in the center of the nose. Again, the order, for 409, was cut short by switching at the 191st example to the F-84D. This was a major improvement, with the Dash-17D engine rated at 5,000 lb (22.24 kN) thrust, enabling gross weight to increase and requiring thicker skin on wings and ailerons. It also had a hinged gun deck, winterized fuel system, simple mechanical leg shortening during retraction (instead of using hydraulic pressure at the lower end of the shock struts) and numerous small improvements.
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<p>Yet again, production was cut short by a further improvement, in this case the F-84E which took over after 154 of the F-84D model had been built. The F-84E had a 12-in (30.5-cm) longer fuselage giving more fuel capacity and more leg room for tall pilots, pylons under the wing gun bays for two 1,000-lb (454-kg) bombs, 230-US gal (871-liter) tanks (same as wing tip) or other loads, and numerous other changes including multi-pin quick-disconnect electrical plugs and a Sperry radar-ranging gunsight. Provision was also made for two 1,000-lb (4.45-kN) JATO rocket units to assist take-off, because the incidence of write-offs (occasionally fatal) as a result of aircraft failing to become airborne had reached serious proportions. The F-84E was the first real quantity-production type, 843 being delivered, not only to the USAF but also to most of the growing NATO air forces which badly needed real muscle. The F-84E also served in large numbers in Korea, where Thunderjets, as the straight-wing F-84 was named, dropped 50,000 tons of bombs and napalm in 86,408 sorties, both a record for a USAF type, sustaining 153 losses in action.
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<p>There was still to be one more straight-wing model, and it far outnumbered all the others. No fewer than 3,025 were built of the F-84G, developed because of long delays with the swept-wing F-84F. Powered by the Dash-29 engine, rated at 5,600 lb (24.91 kN) thrust, the F-84G was distinguished by its multi-framed canopy, although more important changes were the Boeing flying boom receptacle in the left wing (a curious location which spoiled air flow over the whole inner wing on that side) and an autopilot to enable long inflight-refueled missions to be undertaken without severe pilot fatigue. At least 1,900 F-84Gs were assigned to NATO nations, and many others were exported after USAF service.
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<p>With JATO bottles, up to 4,000 lb (1,814 kg) of bombs could be carried by the F-84G, but the test pilots who cleared this loading did not have to contend with rough PSP (pierced steel planking) airstrips on hot days, when—if everything happened to go exactly right, the J35 turbine blades did not shake loose, and the engine gave full power—the aircraft just staggered into the air after using every inch of runway. Without the rocket bottles the take-off would be impossible, but their use resulted in a trail of dense white smoke which forced the rest of the squadron to take off on instruments! Off the far end of the strip it was just possible to coax the aircraft to a speed of above 160 mph (257 km/h), at which the flaps could very carefully be retracted in small steps. Once the spent JATO bottles had been dropped acceleration was better, and in its element the 'plank-wing' F-84 was quite a good ground-attack aircraft, although its seat was uncomfortable, there was no room for maps, maneuverability was almost nonexistent (conversely, this had the advantage of making the aircraft a steady air-to-ground gun platform) and any attempt to exceed Mach 0.82 suddenly resulted in pitch-up violent enough to cause structural failure.
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<p>Against this could be set the F-84's great toughness, good array of weapons, stability (so that bad-weather instrument flying was no problem) and general ease of maintenance. It was the fact that in Korea it rudely came up against the MiG-15, which was in a different class as an air-combat fighter, that prevented the F-84 from ever becoming famed or popular.
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<p>What the original F-84 wanted was sweepback, plus an engine of much greater power. Prior to the Korean War defense money was tight, and all the USAF could afford was a single XF-84F prototype (49-2430), originally known as the YF-96A. It was basically an F-84E with a long low-drag canopy and a new wing, only nine per cent thick and swept back at 38.5 degrees, as was the horizontal tail. Flown on 3 June 1950 the new prototype was actually worse than before in climb and ceiling, but it went 80 mph (129 km/h) faster, at 693 mph (1,152 km/h). Clearly it had to have a more powerful engine, and the Korean War, which began later in June 1950, opened the floodgates of defense dollars. On 6 August Republic was told to go ahead with a new F-84F to use the J65 engine, the British Sapphire made under licence by Wright and sub-licenced to Buick. Installing this engine meant that the inlet duct had to be considerably enlarged, and this was done by splicing in an extra 7 in (17.8 cm) of depth, starting at an oval inlet. In fact, the entire fuselage was redesigned, one of the more obvious changes being a short steel-framed cockpit hood opened on two hinged side arms and a third at the rear to swing up and back in a level attitude. (After several fatal ejections the arms were drilled to provide weak links, and when this failed the separation of the canopy was ensured by explosive squibs.) Behind the hood a large rear fairing with side windows tapered off to meet the new swept fin.
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<p>Despite the deeper air duct there was room for considerably more fuel in the fuselage, as well as more in the very broad wings, which retained a straight leading edge but added large slats as well as perforated lift dumpers ahead of the slotted flaps on the upper surface. Wheel track was much greater even than in the straight-wing models at 20 ft 5 in (6.25 m), and although there was still no nosewheel steering the power-boosted toe-operated brakes made taxiing simple. Speed brakes comprised large perforated panels just behind the wing, and a braking parachute (not used on most normal landings) was housed at the rear of a large box under the tail which incorporated a steel undershoe. Guns were unchanged, but extra pylons outboard of the main gears enabled a bomb load of 6,000 lb (2,722 kg) to be carried even with two tanks inboard. With tanks on, total fuel amounted to 9,525 lb (4,275 kg) and again there was a boom receptacle in the top of the left wing to top up the tanks on long overseas deployments (the idea of the Rapid Deployment Force, as far as Tactical Air Command is concerned, dates not from 1980 but from 1956).
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<p>Wright had terrible trouble 'Americanizing' the J65, and although the first F-84F Thunderstreak (51-1344) flew on 14 February 1951, no F-84F reached TAC until 1954, and during most of the intervening period the Farmingdale airfield was packed with new F-84Fs waiting for usable engines. Even then the installation took away a lot of thrust, so that instead of the advertised 7,800 lb (34.7 kN) it was more like 7,100 lb (31.59 kN), and a good deal less on a hot day. Without an afterburner this made the new all-swept model no more lively than its predecessors, although at least there was no limitation to fly under Mach 0.82. On the other hand, pulling tight g soon resulted in accelerated stall pitch-up so violent that, if airspeed was high enough, the wings came off. At lower speeds this violent pitch-up caused such rapid decay in speed that it became an accepted way of getting an enemy off one's tail. In combat, however, the F-84F was extremely unimpressive, and the 2,713 that were built served basically in the attack role. Temptation to introduce improvements was resisted, although from the F-84F-25-RE block the horizontal tail was made a single powered 'slab' surface. Of the total, 237 were F-84F-GKs made at the Fisher Body Division of General Motors. These tough but rather disappointing machines served with 12 air forces, remaining in the inventory of Turkey until the late 1980s.
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<p>The second F-84F (51-1345) had wingroot inlets, and it was planned at that time to make this arrangement standard. In fact it was used only for the RF-84F Thunderflash reconnaissance aircraft whose nose was occupied by six cameras, defensive armament comprising four machine guns in the upper part of the inlet ducts. No fewer than 715 were built, of which 386 were supplied to nine friendly air forces.
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<p>Two unusual variants of the swept-wing F-84 were the GRF-84K and XF-84H. The latter designation covered two F-84Fs reworked with T-tails and an XT40 coupled turboprop in the nose driving a supersonic three-bladed propeller, while the GRF-84F (later RF-84K) was the service designation of the 25 RF-84Fs converted for the FICON program. Modified with a nose attachment hook and a sharply anhedralled tailplane, the GRF-84F was carried in the cut-out bomb bay of a GRB-36F, and transported to near its target before releasing and performing its reconnaissance mission. The FICON combination saw very limited service with the 91st Strategic Reconnaissance Squadron at Malmstrom AFB.
