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Globalflyer AdventureScaled Composites' GlobalFlyer Readied for Record Flight
Aviation Week & Space Technology
01/03/2005, page 46
Michael A. Dornheim
Circling the globe requires difficult combination of light weight and low drag. Flight tests show this has been achieved.
Adventurer Steve Fossett plans to take off from Salina, Kan.--perhaps as soon as early February--to be the first man to fly solo around the world, unrefueled.
He will be piloting the Virgin Atlantic "GlobalFlyer" built by Burt Rutan's Scaled Composites company here. Rutan also built Voyager, the first aircraft to fly around the world unrefueled, in a project put together by his brother Dick and piloted by him and Jeana Yeager in 1986 (AW&ST Jan. 5, 1987, p. 22). In contrast, GlobalFlyer has only a single crewman, and is jet-powered to speed and simplify the trip. Voyager took 9 days and GlobalFlyer could take as little as 2 days 17 hr.
While trials so far have gone well, engineers are still anxious about the record flight attempt because it will be heavier than any test. The fraction of weight that's structure is extremely low. The scariest part is being heavy on fuel at takeoff and climbing through turbulence, says Clint Nichols, the flight test and performance engineer. Handling, which so far has been reasonable, could take a dramatic turn with the final 15% increase in takeoff weight. And there's concern about whether the aircraft can descend quickly enough from high altitude, even with drogue chutes deployed, for the pilot to breathe oxygen adequately if the engine dies, causing a quick loss of cabin pressure.
GlobalFlyer has made 23 test flights so far to iron out the bugs and get the aircraft's systems into proper working order. Measured range performance indicates it can make the long trip. "It's in pretty good shape for the round-the-world flight," says Scaled project engineer and test pilot Jon Karkow. Fossett expects to fly it to the Salina mission base this month, possibly as soon as this week, then make a few flights of his own before the record attempt. He has already flown it several times from Scaled's facility here.
Mission sponsorship was originally by Fossett, and then picked up by Virgin Atlantic in October 2003 (AW&ST Jan. 12, 2004, p. 50).
The aircraft has a maximum takeoff weight (MTOW) of 22,000 lb. and a zero fuel weight with pilot of 3,700 lb., meaning that 83% of the MTOW is fuel, or conversely only 17% of it is not fuel. The structural fraction is lower than probably all other airplanes, and is especially impressive with a spindly wing aspect ratio of 32.5:1. Lift-to-drag ratio is estimated at 37.
The composite-material Voyager has been the exemplar of combined structural and aerodynamic efficiency, and had a zero fuel weight fraction of 28%. GlobalFlyer is a remarkable 39% less than that. It is also made mainly of graphite-epoxy composite, mostly in honeycomb form. One reason that GlobalFlyer is relatively lighter is the turbofan engine, which has higher thrust-to-weight ratio than Voyager's piston powerplants. Another reason is that much of Voyager's structure was thicker than needed because of minimum gauge limits; the bigger GlobalFlyer doesn't carry this extra weight.
The powerplant is a single 2,300-lbf. Williams FJ44-3 ATW turbofan. The fuel system has 13 tanks--six per side and one 31-gal. header tank in the aft fuselage that supplies the engine (see drawing, p. 49). Each side has a forward and aft boom tank with jet pumps that keep the header filled. An automated fuel transfer system switches among the boom tanks to maintain lateral and longitudinal balance. The boom tanks include sections inside the inboard wing. Each outboard wing has four fuel tanks that gravity-feed into the boom tanks by pilot-controlled valves.
Rare JP-4 fuel is being used because of its low freezing point to handle a three-day cold soak. The tanks were designed for a total of 2,915 gal. of heavier Jet A fuel but have measured a few percent small. With JP-4 at 6.35 lb./gal., they hold 18,000 lb., and full-tanks takeoff weight is 21,700 lb., 300 lb. shy of MTOW.
A fuel dump system was added during flight test to reduce the time to achieve a light landing weight. It's just a 1/2-in. pipe and valve added to the pressurized header tank and together with the engine can reduce fuel load by 1,200 lb./hr. If Fossett finds the aircraft difficult to fly at MTOW, it will take 5 hr. to dump down to the 16,000-lb. maximum landing weight.
A tough decision at the start of the record flight is how much fuel to load. On one hand, full tanks are best for range, but engineers are concerned about long takeoff length and unknown handling at high weight. The heaviest takeoff made so far was 19,164 lb. from the 15,000-ft. (plus overrun) runway at Edwards AFB, Calif. MTOW is 15% higher--an amount that can have large effects. The runway at Salina Municipal Airport is 12,300 ft. plus 1,300 ft. of overrun. The flight will head east around the world.
The fastest the aircraft has flown is 150 kt., and that serves as a placard because airframe flutter has not been calculated or tested. At MTOW Karkow would like to fly faster, at the 170-kt. best-rate-of-climb speed.
To set an around-the-world record, an aircraft must have a ground track no less than a tropic line, or 19,864 naut. mi. Nichols expects the ground track will be about 20,000 naut. mi., which should take about 65 hr. with a 50%-probability tail wind of 64 kt. GlobalFlyer's estimated still-air range capability of 19,000 naut. mi. combined with a tail wind of 58 kt. (one standard deviation low) translates into a ground range of about 23,000 naut. mi., he says.
It's hard to measure fuel mileage in flight because the glider-like nature of GlobalFlyer throws it off the test condition if there's wind shift. Once when flying a low-speed point at high altitude, the airplane got too slow and took a 2,000-ft. drop to recover. But data taken in late November at the highest weight yet, 19,000 lb., "looked good," says Nichols.
Ideally the aircraft would fly a Breguet cruise profile, constantly climbing as weight burned off, reducing airspeed to stay at a constant lift coefficient while keeping a high throttle setting for good engine efficiency. GlobalFlyer is so slow that Mach number is not a concern.
But two constraints cut into range--the desire to not exceed 45,000 ft. so Fossett can survive if the cabin loses pressure; and the desire to stay above the minimum speed on the engine's certification envelope. The engine is certified to 51,000 ft. There also is concern that higher altitudes increase the chance of engine flameout and airframe flutter.
The 45,000-ft. limit means the engine will have to be throttled back prematurely to less-efficient thrust, and the minimum speed--100 kt. at 45,000 ft.--means the aircraft will have to fly faster than desired. Going 10 kt. too fast over the entire trip costs 1,700 naut. mi. of range, says Nichols. Similarly, removing the altitude and speed constraints would add range.
Other problems addressed during flight test include:
*Flight controls. At high weights, Karkow found the stick forces getting high enough that he added an extension to the sidestick controller to gain more leverage. However, the difficulties turned out to be mechanical and not purely aerodynamic. In pitch, the elevator autopilot servo was hitting its stop prematurely due to cable stretch, interfering with manual control on takeoff rotation. The fix was to change the stop. In roll, the cold of high altitude made the cables shrink more than the carbon airframe, causing large tension that increased friction. Also, the high wing bending that goes with full boom tanks was causing friction along the cable run. The effect was that the ailerons wouldn't return all the way to neutral by themselves. The fix was to reduce cable tension so it's a little loose when warm, then shrinks to a good fit at cruise. The sidestick extension may no longer be necessary.
*Cabin noise. Noise coming out the front of the engine strikes the top of the cabin, and on early flights it was an ear-piercing 125 dBA. inside the cabin. Several types of sound insulation were added, dropping noise to a tolerable 95 dBA. The insulation increased weight by roughly 30 lb.
*Cabin heat. Flights for a few hours at 45,000 ft. with the outside temperature pushing -70F revealed problems: The bubble canopy frosted over, heat was poorly distributed, and at low throttle settings there was not enough heat. Heat primarily comes from the engine bleed air that pressurizes the cabin. The original plan was to have the bare-walled cabin that's adequate for Scaled's high-altitude workhorses--the twin-engine Proteus and White Knight. But leaky Proteus supplies 12 lb./min. of warm air to the cabin, while GlobalFlyer is restricted to 1 lb./min. for high engine efficiency, and White Knight flies only a few hours instead of a few days.
Most of the meager heat supply is needed to keep a few spots clear on the canopy. This results in poor distribution, where the pilot's head is cooking and his toes are freezing. The main fixes, implemented last month, were to line the cockpit with heat insulation and add electric fan heaters at the back and in the footwell. There may also be a thermal curtain placed at the canopy entrance to isolate it from the cabin when not in use.
*Fuel system. There was too much pressure drop across a number of solenoid valves and they were replaced with motor-driven ball valves.
*Fuel migration. Several leaks between adjacent fuel tanks were found and fixed. The final one remained mysterious until after the 23rd flight on Dec. 16, when all outboard wing tanks were full and no fuel was burned from them or dumped, which simplified detective work. A good post-flight topoff and weighing revealed that 32 gal. had leaked. "Five percent of the fuel used was unaccounted for--that's not good," Karkow said. The crew concluded that one outboard tank was leaking into a more inboard one, causing it to overflow through its vent line. Using educated guesswork, they cut holes in the wing skin to find and repair the leak. The repair seems to have worked but has not been flight-tested yet.
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