02 January 2010

Jet engine technology in the late 1940s was still immature enough that a jet-engined fighter often still lacked comparable range to a piston-engined fighter. As a result, both the USAF and US Navy saw the turboprop as the potential answer- the USAF hoped a turboprop fighter would offer a longer range and the US Navy was interested in the power of turboprop fighter to make takeoff from a carrier deck possible without a catapult. To evaluate the possibility, both branches agreed to fund an experimental turboprop fighter based on Republic's AP-46 design. The USAF would buy three and the US Navy would acquire a single example.

By 1951 the arrangement had changed to with only the USAF getting two AP-46 aircraft designated XF-84H. Although a development of the F-84F Thunderstreak, the XF-84H only had wing and canopy of the Thunderstreak with an all new fuselage and tail unit. A 5,580-horsepower Allison XT40 turboprop that drove via an 18-foot shaft a 12-foot three-bladed constant speed propeller. The Aeroproducts prop rotated at 3,000 rpm which meant that the blade tips traveled at Mach 1.18 and as such, created an immense amount of noise, leading to the XF-84H's name, the Thunderscreech. As the prop was constant speed, thrust was effected by adjusting the pitch of the prop.

With its first flight at Edwards AFB on 22 July 1954, it was immediately apparent the XF-84H had some serious flaws. Maximum deflection of the rudder was insufficient to counter the massive torque and p-factor of the supersonic prop. In addition, there were constant hydraulic problems, the elevator was limited in its pitch authority and the prop gearbox kept overheating.

But most famously, the XF-84H Thunderscreech was noisy. The supersonic prop blades were so loud that residents over 20 miles away from Edwards AFB filed noise complaints with the USAF! When the aircraft first taxied out under its own power and took off, the control tower crew often had to hide under their consoles shielding their ears. Any ground crew standing nearby often felt nausea and dizziness even with ear protection due to the Mach shockwaves from the prop blades.

As a result, Republic was banned from doing run ups on the flightline and it would have to be towed 7 miles away from the main ramp behind a ridge called "Rocket Hill" as it had rocket engine test stands in the area.

The second Thunderscreech arrived in May 1956 and was temporarily bailed to the US Navy in support of the VTOL tail sitting convoy fighter program as both the Lockheed XFV-1 and Convair XFY-1 used the same Allison XT40 turboprop engine. The last Thunderscreech flight was made on 9 October 1956 with cancellation following, much to the relief of the pilots, ground crew and most everyone at Edwards AFB!

Source: International Air Power Review, Volume 24 (Summer 2002). AIRtime Publishing, 2002. "Warplane Classic: Republic F-84, Thunderjet, Thundersteak, and Thunderflash" by David Willis, p124.

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