GlobalFlyer flight-test progressing

 - December 18, 2007, 6:17 AM

In preparation for his solo nonstop around-the-world (ATW) flight tentatively scheduled for January, adventurer and solo ATW balloonist Steve Fossett has begun familiarization flights in the single-engine Virgin Atlantic GlobalFlyer in Mojave, Calif., following a number of envelope expansion flights by Scaled Composites project engineer and test pilot Jon Karkow.

Designed by Burt Rutan and built by his crew at Mojave-based Scaled Composites, now most famous for the SpaceShipOne commercial manned spacecraft that won the $10 million Ansari X Prize last month, the GlobalFlyer is reminiscent of Rutan’s earlier nonstop ATW airplane, Voyager, in which his brother Dick and copilot Jeana Yeager circumnavigated the earth in nine days in 1986. Unlike Voyager, which had two piston engines, Global-Flyer uses a single Williams FJ44-3 jet engine as its powerplant. It will, however, require substantially more fuel for the expected 80-hour ATW trip than Voyager, 18,000 pounds compared with 7,010 pounds. At 81.8 percent, GlobalFlyer’s estimated ATW fuel-to-takeoff-weight ratio (also known as the fuel fraction) is much higher than Voyager’s 72.2 percent.

“GlobalFlyer is about 2.3 times heavier (than Voyager) with full fuel,” said Karkow. “So the wings are much stronger than Voyager’s. They deflect about 100 inches in flight, but only 20 inches on the ground.” Since the unloaded wingtips sit about 48 inches off the ground, Scaled staff are confident that the fully loaded GlobalFlyer will escape the wingtip damage Voyager suffered on its two-minute takeoff roll that ground off the winglet bottoms, even though GlobalFlyer has not yet been tested with full fuel.

Envelope-expansion flights Karkow has flown GlobalFlyer on 13 test flights, including its initial flight on March 5 this year, which lasted approximately 90 minutes and reached an altitude of 12,000 feet for stability tests at speeds ranging from near stall (54 knots) to 110 knots. Takeoff weight on GlobalFlyer’s first flight was 5,272 pounds, less than a quarter of the expected takeoff weight of approximately 22,000 pounds for the circumnavigation attempt.

“At the light weights, Global-Flyer is very glider-like,” Karkow told AIN. “It has a good roll rate and good aileron response. The tail surfaces are mounted far aft, resulting in good pitch and yaw stability.”

By Karkow’s 13th flight in early October, the company was flight testing the airplane at 16,035 pounds, with a fuel weight at takeoff of 12,429 pounds (fuel fraction of 77.5 percent) or nearly three-quarters of the expected maximum takeoff weight. Karkow compared flying the heavily loaded GlobalFlyer to maneuvering a barge with small tugboats, requiring the operator to anticipate the turn. “As [GlobalFlyer] gets heavier, roll and yaw inertias increase, and it’s harder to maneuver. Although the aerodynamic damping is actually the same, the perceived damping is less due to the extra weight, which requires additional forces on the stick to move at similar rates.”

Karkow also indicated that GlobalFlyer has a significant dihedral effect (roll due to yaw) incurred by the increased bending in the wing at heavier loads. Stall speeds vary from 50 knots at the lighter weights to 115 knots at heavy weights. Ultimate takeoff speed with full fuel is estimated at 120 knots, with average cruise speed at approximately 250 knots.

On approach to landing, the pilot of the GlobalFlyer engages one or both of its drag chutes to decrease speed and increase the descent rate. Approaches are generally flown at 1.2 times the stall speed for the weight. “Landing without any drag chute is like landing a glider with no spoiler,” Karkow said. “With both chutes out and gear down, descent angles are typical of those of a small general aviation aircraft. Airspeeds are relatively low, so the perceived maneuverability is good.” Karkow said he generally deploys just one drag chute on landing, which results in a three-degree glide path. Drag chutes cannot be retracted or released in flight.

At press time, Fossett had conducted three familiarization flights in GlobalFlyer, on October 13, 15 and 21, all at relatively low weight. (The heaviest flight was at a takeoff weight of 10,000 pounds.) According to Karkow, plans are to continue test flights up to 100 percent takeoff weight, with Fossett likely at the controls in preparation for the around-the-world flight, tentatively scheduled for January.