Chase Plane 101: Need that speed

In early May, Gulfstream found itself in a good kind of quandary - it's new G650 ultra long range business jet began probing to its maximum speed of boundaries, but in doing so, it blew away its Gulfstream V photo chase plane, a fast machine, yet slow by comparison.

"We were chasing 6001 [the first G650 prototype] with 501 [the GV chase plane]," Pres Henne, Gulfstream senior vice president of programs, engineering and test told Flight International in late May. "We started at Mach 0.85. The GV does that great since its MMO [maximum operating speed] is M0.885."

"We got to M.875, and the GV was able to keep up. When 6001 pushed up to M0.9, the GV pilots tried everything to make it go faster, even switching to "alternative mode" on FADEC to redline the engines."

The solution for Gulfstream? Chase speed with speed - a second G650.

Last week Sikorsky faced a similar issue with its new X2 compound helicopter demonstrator, a coaxial twin counter-rotating rotor fly-by-wire vehicle that will use a variable pitch propulsor on the aft end to speed it to 250kt or greater. Click the picture at left to see a larger image of the X2 on a recent test fligth in West Palm Beach, Florida. 

The company was using an S-76 helicopter for the chase job, a twin-engine helicopter that maxes out at 155kt, the top speed of just about all present day civil helicopters. On last week's envelop expansion flight, X2 test pilot Kevin Bredenbeck "bumped" the X2's propulsor pitch mechanism to push the compound helicopter to 168kt. 

Sikorsky business development manager Jim Kagdis says the S-76 (pictured at left during an earlier flight in NY) tried its best to keep up, going to a "heavy" nose-down attitude (pitching the lift vector as far forward as possible), and "going as fast as it could". The X2 just slid on by however.

Next week, Sikorsky plans to take the X2 to 180kt, finishing the third of four test phases in the program, and introducing a fixed wing chase plane - A Cessna Conquest, which can fly at speeds in the 250kt regime. Kagdis says Sikorsky had considered an OV-10, but its top speed of 210kt killed the deal. 

Assuming all goes well, phase 4, which will see the X2 flying at 250kt or more, should commence by mid-June.