Pratt & Whitney has conducted the first test flight of the PW1217G geared turbofan. This model is destined to power the Mitsubishi Regional Jet (MRJ), now in development. But, to me, the real interesting bit isn’t the engine being tested, but the unusual test aircraft it is flying on.
Most new engines are flight tested by hanging them under the wing of an aircraft in place of one of the normal engines. P&W has a Boeing 747SP flying testbed that is used in just this way, with test engines carried in the #2 position (inboard on the left wing). Since the B747 is a four engine aircraft it still has three standard engines to rely on. There are other aircraft used in the same way, including Airbus A340 & A380 flying testbeds. Pratt’s B747SP replaced their earlier B720 testbed. (The Boeing 720 was a variant of the B707.) The B747 is extremely popular as a flying testbed as it is large, providing adequate ground clearance under the wing for larger turbofans, and four engined, allowing for safe flight while testing the new engine – which requires in-flight shut down and restarts, icing tests, etc.
But the testbed being used for the PW1217G, while also a B747SP, is different. See for yourself:
Yes, they’ve mounted a stub wing to the upper deck of the B747 to carry the test engine, instead of mounting it under the wing. It makes the testbed look quite strange, but it gets the job done. This isn’t the first time something like this has been done. Way back the B707 prototype, the 367-80 aka Dash 80, tested tail mounted engines for the B727 development program. And Honeywell has a B757 flying testbed for their small jet engines with a similar stub wing.
Still, it is quite an unusual site. I think the asymmetry is what really makes it look strange. We’re used to aircraft being symmetrical, and this looks like it either has something missing on the other side, or the stub wing is the result of some mishap. But it makes sense for what it does. When testing smaller engines, the under wing mount just isn’t appropriate. Form definitely follows function here.