Pratt & Whitney’s latest geared turbofan engine – the PW1200G for the Japanese MRJ project – is now starting test flights on the company’s recently-inaugurated Boeing 747SP flying testbed. The high-altitude, long endurance and wide speed range capability of the SP make it ideal as a testbed particularly since the unusual-looking stub wing, which houses the test unit, can be adapted to take everything from small GTFs and business jet engines to turboprops. Sister company Pratt & Whitney Canada will use this testbed predominantly, while Pratt & Whitney will use the sister 747SP for testing larger engines. The second 747SP mounts the test engine in the No.2 inboard position under the wing in place of one of the aircraft’s standard JT9D powerplants.
The aircraft, now Canadian-registered with the appropriate identity C-GTFF, was originally built for Korean Airlines. First flown at Everett in January 1981, it entered service with Korean in March 1981 and remained in service with the carrier for the next 18 years. Following stints with The Air Medical Foundation and Transatlantic International Airlines, the 747SP spent several years in storage after being acquired by P&W in the mid-2000s. As with GE’s ex-Pan Am 747-100 flying testbed, Pratt’s 747SP gives away its origins in the first class area in the nose where the original seats and KAL décor can still be seen.
The unusual mounting configuration of the testbed engine is emphasized in this view from the forward main cabin entry door.
Heat rejection from the test engine and systems on-board is handled through these load bank and heat exchanger devices built into the aft lower lobe of the belly area.
From the ground the relative downward pitch of the stub wing and test engine can be clearly seen. This angle helps minimize the impact of the stub and test engine thrust on the overall stability characteristics of the aircraft, and reduces excess trimming requirements.
Inside the top of the upper deck hump are the reinforcing structural members, put together by modification specialists L3, which support the stub wing.