When selecting topics for this column, I try to stay true to the spirit of Plane & Pilot and focus on single- and multi-engine propeller-driven aircraft that have a great story to tell.
We have covered everything from the first X-plane, the 1909 Wright Model R, also known as “Baby Grand,” to the exotic Pond Racer, and more recently, the extraordinary 400 mph Nemesis NXT. However, one single-engine propeller-driven aircraft stands head and shoulders above the rest for the imagination it took to develop—and the courage it took to fly it—the Republic XF-84H “Thunderscreech.”
The Thunderscreech was a product of the fertile creativity of aircraft development present between the end of World War II and the swinging ’60s. Unlike today, when a new jet fighter may take a decade or more to develop, in the 1950s new and unique ideas were designed, constructed, and tested in a matter of months. This golden age birthed the fabled Century Series jet fighters, the X-planes that set and reset speed and altitude records, and the incredible U-2 and SR-71 spy planes.
However, some of the concepts developed, while incredibly imaginative, simply stretched the technology a bit too far.
The XF-84H was the product of a joint U.S. Navy and Air Force project managed by the Wright Air Development Center in Dayton, Ohio. The Navy wanted a turbine-powered, swept-wing fighter that could launch off the deck of an aircraft carrier without a catapult. The Air Force, saddled with several thirsty pure jet fighter projects, needed a long-range bomber escort fighter. A turboprop fighter that could fly long-distance bomber escort missions and take off from the short deck of a carrier seemed the perfect solution.
The aircraft chosen for the radical modification program was the Republic F-84F Thunderstreak. Richard Bach writes beautifully about this sturdy and reliable stablemate to the more famous North American F-86 Sabre in his classic book Stranger to the Ground.
The F-84 family of aircraft may go down in history as one of the most flexible jet designs in history. Beginning with the straight-wing 1946 F-84 Thunderjet, itself a jet-powered iteration of the famed P-47 Thunderbolt, it evolved into the sleek, swept-wing F-84F Thunderstreak and the reconnaissance variant, aptly titled the Thunderflash. Additionally, numerous F-84 airframes were modified for the various experimental programs, some more successful than others.
Republic Aircraft developed the F-84H Thunderscreech—an unofficial name earned during flight testing—in short order. The first order of business was replacing the F-84F’s Wright J65 engine with a 5,850 hp Allison XT40-A-1 turboprop engine. The powerplant, which actually consisted of two smaller jet turbine engines mounted behind the cockpit, turned two driveshafts that ran along each side of the pilot at nearly 40,000 rpm. These two shafts drove a reduction gearbox mounted behind the propeller.
The propeller, a 12-foot-diameter, square-bladed design, featured a tip speed of Mach 1.18. This supersonic tip speed remained constant from start to shutdown. Thrust was increased or decreased by varying the pitch of the propeller. Notably, contra-rotating propellers were not considered for the design.
In order to handle the enormous P-factor, and the turbulence caused by the propeller, a T-tail and a small triangular aerovane just behind the pilot’s canopy were added to the original F-84F airframe. Due to concerns about the impact of this torquey engine and propeller combination on other aircraft systems, a ram air turbine (RAT) was installed. As a precaution, it was deployed before takeoff during each of the Thunderscreech’s 12 test flights.
Before flight tests could be conducted, several ground test runs were required to ensure that the novel engine, propeller, and flight control system functioned properly. These tests are likely where the Thunderscreech earned its unofficial name. Imagine the sound generated by the outer 24-30 inches of the propeller tips traveling faster than the speed of sound.
The combination of the endless sonic booms emanating from the propeller caused frequent nausea and headaches among the ground crews, and is said to have caused a Republic engineer to suffer a seizure. The noise was so loud that the Edwards Air Force Base tower in California asked the team to tow the Thunderscreech several miles out to Rogers Dry Lake before starting the engine.
Despite these issues, on July 22, 1955, the XF-84H Thunderscreech took to the air, with Republic test pilot Hank Baird at the controls. During its 12 test flights, the XF-84H achieved an unofficial top speed of 520 mph, earning the unofficial Guinness Book of World Records recognition as the fastest propeller-driven aircraft. This record stood until Rare Bear, a modified F8F Bearcat, surpassed it in 1989.
However, everything was not going well. Eleven of the 12 test flights ended with in-flight emergencies. Baird noted that at nearly 450 knots the propeller governor would begin to surge, and the rotational force of it would begin to rotate the aircraft forcibly to the left. To his credit, Baird flew 11 of the 12 test flights. Republic test pilot Lin Hendrix flew the aircraft once, and famously declared: “You aren’t big enough and there aren’t enough of you to get me in that thing again.”
Two XF-84H prototypes were produced and flown. Aircraft No. 17060 was scrapped when the program was canceled after only four flights. However, aircraft No. 17059 was saved from the smelter and served as a front-gate display at Meadows Field in Bakersfield, California, with an electric motor turning its propeller. It was rescued in the mid-1990s and restored by the 178th Fighter Wing of the Ohio National Guard.
Today, you can see this incredible plane in the research and development gallery of the National Museum of the U.S. Air Force in Dayton. While the program was ultimately canceled, the XF-84H Thunderscreech serves as an example of the magnificent creativity and dedication of “Right Stuff” test pilots during the fabulous ’50s.
