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.
Following the cancellation of the Thunderscreech program, prototype 51-17059 spent her days as a "gate guardian" outside of the Meadows Field Airport in Bakersfield, California. She was fitted to a suspension rod and was showcased dramatically above the ground as if in flight - her propeller was actually being controlled by an installed electric motor for effect. In 1992, this aircraft was removed from her mounting and came under the attention of volunteers from the Ohio Air National Guard who saw it fit to restore her to museum display form. After some 3,000 hours of reconstruction, the XF-84H was handed over to its new owners for final display at the National Museum of the United States Air Force at Wright-Patterson AFB in Dayton, Ohio where she resides even today. Prototype 51-17060 is thought to have been scrapped at project's end, her Allison XT40-A-1 turboprop engine reportedly used during development of the Douglas A2D Skyshark.
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