The Republic XF-84H was an experimental airframe based on the jet-powered, single-seat Republic F-84F Thunderstreak fighter. The XF-84H development attempted to mate the existing Republic fighter airframe to a turbine engine arrangement utilizing a supersonic propeller system to produce a speedy fighting platform as an alternative to the fuel-hungry, underpowered turbojet breeds of the time. However, issues abounded in such an endeavor and the program resulted in just two troublesome prototypes being built and none were ever tested by USAF test pilots before the complicated program was cancelled. The noise generated from the powerful turboprop engine - reportedly heard as far away as 25 miles - earned the XF-84H its nickname of "Thunderscreech", staying consistent with the "Thunder" naming classification of the original Republic fighter series.
Republic had garnered a fine name for itself in World War 2 thanks to their development of the P-47 Thunderbolt. The aircraft proved her worth as both a competent dogfighter (making many aces of her pilots) and a stable gunnery platform when charged with attacking ground targets across Europe and the Pacific (trains proved a favorite in the European Theater). As great as she was, the aircraft was dropped from service immediately after the war during huge military purges in peacetime America. By the time of the Korean War, Republic had put forth their F-84 "Thunderstreak" - a single-seat fighter powered by a turbojet engine and sporting straight monoplane wings. The aircraft was quickly outclassed by the arrival of the Soviet Mikoyan-Gurevich MiG-15 "Fagot" single-seat, swept-wing jet fighter and attempts at improving the Republic F-84 resulted in a swept-wing version all their own - the F-84F. Approximately 7,500 F-84s were eventually built despite their limitations and these were fielded in great numbers as a fighter-bombers during the war over the Korean Peninsula.
Origins of the XF-84H itself emerged from a United States Navy requirement for a carrier-based fighter aircraft that could take off under its own power without the need for catapult-assisted launches. Republic attempted to produce such a product with their "Project 3347" demonstrator of which the United States Air Force's Wright Air Development Center became a supporter of. The aircraft took the existing F-84F Thunderstreak airframe and fitted the experimental Allison XT40-A-1 turboprop engine (powered by a pair of coupled jet turbines spinning a supersonic, three-bladed propeller) in an amidships fitting directly behind the cockpit. From the engine ran an extension shaft under the cockpit floor to the propeller fitted to the nose of the aircraft. The tail was revised by raising the elevators high along the vertical tail fin and away from the powerful prop wash to be developed by the blades. The development, as a whole, had the potential to supply the airframe with the speed and performance of a turbojet-powered aircraft but, at the same time, could offer the improved ranges and low-fuel consumption of a propeller-driven airframe.
Project 3347 evolved to become the "XF-106" but this was later redesignated under the more familiar "XF-84H" to keep her F-84 Thunderstreak roots in check. The United States Navy had three such prototypes on order but eventually cancelled them, leaving two completed prototypes to be used as research aircraft for the USAF Propeller Laboratory based out of Wright-Patterson AFB. The XF-84H was constructed at Republic's fabled Farmingdale facility before being delivered to Edwards AFB by train. First flight was recorded on July 22nd, 1955.
The Allison XT40-A-1 series was a early-form turboprop engine that was made up by coupling Allison's T38 engines, these operating a single gearbox. The T38 was developed in the late 1940s and, itself, went on to become the successful line of T56 turboprop (powering such mounts as the Lockheed C-130 Hercules family). The XT40 powerplant was mated to the aforementioned three-bladed propeller system located at the front of the aircraft, each bade noticeably large in area though relatively stocky in appearance. This engine was noted for many things beyond its proposed inherent power (said to be capable of knocking down a standing man caught in her prop wash), not the least of these being the level of noise that the powerplant generated even when on idle. The XF-84H's propeller ran at supersonic speeds and created its own visible sonic booms in the process. The noise generated by the XF-84H engines was such that Republic was forced by the USAF Flight Test Center to run the remainder of their evaluations out of Rogers Dry lake in the Mojave Desert of California - such was the disruptive power of the Allison XT40-A-1 to USAF operations at Edwards.
Power was derived from the single Allison XT-40-A-1 turboprop engine delivering an impressive 5,850 horsepower. This supplied the aircraft with a top speed of 520 miles per hour, a range of about 2,000 miles and a service ceiling equal to about 40,000 feet. Rate-of-climb was measured at 5,000 feet per minute. A ram air turbine was also fitted to the airframe (aft near the fuselage spine, ahead of the vertical tail fin) on a retractable arm to be used in the event of an in-flight engine failure. However, as problems persisted with the Allison powerplant (ten of the eleven flights by the first prototype ended in forced landings), the ram air turbine was most always kept extended and ready for use in further test flights. Torque - the natural resulting force generated by a spinning object - from the massive power output eventually led to several revisions in the XF-84H airframe design, among these being the relocation of the left-side intake some 12 inches forward. Individual operating flaps were also trialed. Theoretically, the XF-84H could also gain beneficial thrust output from the jet wash exhausting at the rear of the fuselage and utilization of afterburner (raw fuel pumped directly into the engine for a boost in thrust) could push output upwards of 7,200 horsepower. However, the afterburner capability was never used during flight testing despite its installation.
In the whole of the program, the XF-84H was only produced in two prototypes - these being s/n 51-17059 (FS-059) and s/n 51-17060 (FS-060). The first prototype netted eleven total flights but at least ten of these ended in forced landings of the aircraft. At one point, even a Republic test pilot refused to take the XF-84H airborne ever again. The second prototype managed only a single flight. If doing the math, this resulted in just twelve total flights being reported between the pair. The inherent instability dangers of the airframe - coupled with the complicated and temperamental engine - meant that the XF-84H was doomed to a short lifespan. The program was officially cancelled by the USAF in September of 1956.
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