GlobalFlyer: Son of Voyager, Mother of risky ventures

 - February 1, 2007, 6:33 AM

 “LAST BIG PLUM” REVISITED–Burt Rutan and the Voyager team in 1986 described their quest to fly around the world nonstop and unrefueled as “the last big plum” in the orchard of aviation records. In 1999 Messrs. Piccard and Jones, similarly, characterized their ultimately successful circumnavigation of the world in the Breitling Orbiter balloon as “the last great aeronautical adventure.”

Against this backdrop of dwindling ultimates, pilot Steve Fossett and sponsor Sir Richard Branson are labeling their quest to make the first solo nonstop and unrefueled flight around the world as “the first great aviation feat of the 21st century.”

The airplane with which they plan to establish the record was rolled out just downramp from designer-builder Rutan’s Scaled Composites facility in Mojave, Calif., last month. Called the Virgin Atlantic GlobalFlyer, the airplane is powered by a single Williams FJ44-3 ATW turbofan producing 2,300 pounds of thrust and is likely to fly for the first time this month. Its creators expect the slender airplane to complete the 22,874-mile circumnavigation of the globe in less than 80 hours. The ATW
in the engine designation stands for “around the world.”

Just as the extraordinary demands of flying around the world unrefueled produced an extraordinary airplane in Voyager, so too is the GlobalFlyer immediately recognizable as shooting for the same, nigh-impossible goal. Like Voyager, it houses a massive fuel load in twin booms and, to a lesser extent, in slender wings of huge span. Unlike Voyager, powered by a pair of piston engines and carrying a crew of two, GlobalFlyer will have just one engine and one pilot. And one has to hope that GlobalFlyer lands with more fuel than Voyager, which had just 106 pounds remaining of the 3.5 tons it took off with.

Pilot Fossett
Steve Fossett, born in 1944, is no stranger to the lonely pursuit of solo round-the-world flights, having persevered and finally succeeded at floating around the globe solo in a balloon in 2002–on his sixth attempt, after eight years of trying. Fossett’s exploits in his Citation X have also garnered numerous speed records, including U.S. transcontinental (two hours 56 minutes at an average groundspeed of 726.83 mph), Australia transcontinental (705.06 mph) and round-the-world westbound (500.56 mph). His training for and completion of such endurance events as the Iditarod, Ironman Triathlon and English Channel Swim have helped him deal with the physical demands of his record flights.

Sir Richard Branson, never one to shrink from a challenge involving balloons, boats or taking on leviathans in business, has eagerly undertaken to sponsor the project.
GlobalFlyer’s mission will test its pilot’s mettle sorely, as did Voyager. On the piston-engine airplane in 1986, having two pilots aboard at least offered the possibility of their taking turns. In reality, however, Jeana Yeager did not get to take the controls until Dick Rutan (Burt’s brother) was exhausted to the point of hallucinating. At least on GlobalFlyer, the mission is being designed around the endurance of one pilot. Two digital three-axis autopilots made by TruTrak Flight Systems, a supplier in the homebuilt world, will keep a grip on what promises in the early stages of the flight to be a major handful of airplane. The challenge facing these electronic helping hands will be severe, as the airplane’s weight will change six-fold between takeoff and landing. Fixed gains will likely be unable to cope with such a wide control loading envelope, but the team’s decision to proceed with the project was driven to a significant extent by the availability of these autopilots.

Just looking at the dimensions and layout of the airplane tells a pilot that GlobalFlyer will likely not fly like most airplanes even before the numbers are examined. Flight testing, due to begin imminently, will reveal just how different it is. The wingspan is 114 feet (about the same as a Boeing Business Jet’s), and the wing area of 400 square feet (a little more than that of a Hawker 800XP) provides an aspect ratio of 32 and a wing loading of 55 pounds per square foot at the anticipated circumglobal takeoff weight of approximately 22,000 pounds. Of that, 18,000 pounds
will be fuel, meaning that the entire structure (airframe, powerplant, avionics, systems and so on), the pilot and his life-support systems will account for just 4,000 pounds.

To the casual observer, Global-Flyer might look like a repeat of the Voyager exercise–“make a big enough collection of gas tanks stay aloft long enough to make it around the world”–but such a verdict would overlook the enormous challenges presented by such a goal. Flying Voyager in the early, heavily loaded stages of the mission was apparently akin to stuffing a snake into a sack. Former USAF pilot Dick Rutan has often said that maintaining control took almost his entire powers of concentration.

Brother Burt, faced with the need for weight-saving during the design and building phase on a scale he had never encountered before, was equally taxed, and on takeoff day in December 1986 he had to confront the blunt fact that nobody–not even he, the designer–knew if the airplane would hold together. (When the airplane was later disassembled to take its place in the Smithsonian National Air and Space Museum, the plates that had held the wing spar together were found to be cracked, revealing just how close the structure came to failing.)

Weighty Issues
Seventy-two percent of Voyager’s circumglobal takeoff weight was fuel; 83 percent of Global-Flyer’s mission takeoff weight will be fuel (JP-4, for its low freezing temperature). These numbers are much more different than they might at first appear to be. Turning the equations on their heads reveals that the percentages of takeoff weight devoted to empty weight are 28 percent for Voyager and only 17 percent for Global-Flyer–a major difference showing that GlobalFlyer’s empty weight as a percentage of takeoff weight is a massive 39 percent less than Voyager’s was.

Burt Rutan told AIN that he finds this figure of 39 percent to be uniquely revealing about the design challenges he faced when creating the jet-powered airplane. Only a little more than 1,700 pounds of the 22,000-pound mtow will be composite airframe structure, so in effect the bare-bones airframe will be lifting almost 13 times its own weight. Of that 1,700 pounds, the 550-pound solid graphite-epoxy I-beam that
is the one-piece main wing spar is not only the largest single component of GlobalFlyer but also the largest single piece of airplane structure Scaled has ever made. The whole structure is designed to handle two Gs at high weights and four Gs at low weights.

Between the births of the two airplanes, technology has advanced. Where Rutan relied mostly on long-hand calculations to solve structural questions with Voyager in the mid-1980s, finite-element analysis refined the process of designing GlobalFlyer. Only Fossett on his round-the-world attempt, however, will be able to answer, for sure, the question of whether or not such a major reduction in the ratio of empty weight to max takeoff weight has been successful.

This is going to be a very risky flight.

There will be no weather radar on board–at 45,000 feet the airplane should be above all weather except that large enough to show up on satellite images–and it will carry no ice protection other than pitot heat. In addition to the TruTrak autopilots, the airplane’s avionics suite will include an Inmarsat C satellite datalink (for providing automated position reports to the mission control center, which will ease Fossett’s workload by handling most ATC calls), an Iridium satphone and a 20W Wulfsberg VHF com.

Voyager was storm-tossed into attitudes that would be a challenge for any airplane, including 90-degree banks in fierce weather in the intertropical convergence zone in Africa, and the poor roll authority that stemmed from its ungainly yet graceful dimensions was barely adequate for the conditions encountered. The ailerons on GlobalFlyer will be split into six segments to handle the distortions of wing bending, and Fossett’s experience in high-performance sailplanes should serve as at least a primer on GlobalFlyer’s handling characteristics in roll.

As the tense two-minutes-and-six-seconds takeoff run from Edwards AFB in December 1986 clearly showed, Voyager exhibited alarming flexibility. For much of
the takeoff run, the tips of its fuel-laden wings dragged on the runway, grinding away the roots of the winglets and exposing the fuel-system vents that ran to the tops of the winglets. Eventually, pilot Dick Rutan raised the nose enough to coax the wings off the concrete, and when Voyager finally lifted off, just 800 feet of the 15,000-foot runway remained. In flight, the airplane flexed severely on occasion, making for queasiness both in those aboard and in those not on board but familiar with the structure.

Despite the technological advances in the intervening 17 years, there will be equally tense questions about its ability to succeed when GlobalFlyer launches its assault on the globe. Known within Scaled Composites as Capricorn, after the tropic of that name, GlobalFlyer must fly a distance of at least the 19,864 nm defined by a tropic line if it is to meet the Fédération Aéronautique Internationale’s definition of a round-the-world flight.

In the field of more conventional jet-powered aviation, engine supplier Dr. Sam Williams has been reluctant to become involved with any single-engine applications until he is confident in the reliability of the powerplant. For him, the magic number has been one million hours of flight experience, a milestone the FJ44 (with 1.7 million hours logged) has long since passed. At the public presentation of GlobalFlyer last month the Williams International name and logo featured prominently on the engine nacelle of this decidedly unconventional application. The small turbofan will run nonstop for about 3.3 days if all goes to plan.

While reliable and smooth-running, a jet engine has its disadvantages in this application. For sufficient margin above compressor stall, the airplane will have to maintain at least Mach 0.4 at its ceiling of 45,000 feet, but as weight burns off and the airplane can no longer fly at the ever diminishing speed for best lift/drag ratio, cruise efficiency will erode.

Initial flight trials will be handled by Scaled Composites test pilots, but they will likely take the airplane to only 70 percent of mission weight. Fossett says that it is for him alone to take the push-the-boat-out step to full mission weight–a 40-percent increase in takeoff weight that could radically change the airplane’s handling.