Bank On It
In the propeller age, airplanes seldom climbed much above 20,000 feet/6,096 meters. The air at those altitudes is still fairly dense, and airplanes behave largely the same as they do near sea level.
The introduction of the jet engine allowed airplanes to climb above 30,000 feet/9,144 meters, and a few could even make it above 40,000 feet/12,192 meters. At those flight levels, only a small fraction of the Earth’s surface atmospheric density remains, and engineers and pilots soon discovered that airplanes—actually the airplane wing—performed very differently in thin air.
Flying at altitudes in the upper 30s, and especially above 40,000 feet/12,192 meters, the less dense air means there are far fewer air molecules available to generate lift. So much so that if a jet slowed even a few knots below its target airspeed in the early days of jet aviation, the wing could stall and quit flying. Air at higher altitudes is thin and cold so it responds much differently to the rapidly advancing wing than it would at lower altitudes. Aerodynamicists call the phenomenon compressibility. Many of us think of it as the “sound barrier” that holds nearly all airplanes below Mach 1, the speed of sound.
One need not be an aeronautical engineer to see the difference between a Gulfstream wing and all others; simply look at any other large-cabin or super-midsize business jet and see for yourself.
Because of compressibility—also called Mach effect—the wing on a jet can buffet and lose lift by flying too fast or too slow. In some jets a pilot flying at high altitude has to be careful not to slow down because the wing could stall, but conversely, not fly so fast that the wing will buffet. That narrow range of speed between low-speed stall and high-speed buffet was once called the “coffin corner,” and early jet pilots avoided getting caught near the edge of that unforgiving envelope.
An even greater concern for jet pilots today is a change in wing loading at high altitude. Wing loading changes during maneuvering, but also when turbulence is encountered. Some jets simply cannot maneuver promptly at high altitude without the risk of flying into a “coffin corner.” Worse yet, unexpected turbulence, particularly clear air turbulence, could suddenly push the airplane beyond the “coffin corner.”
Fortunately Gulfstream pilots simply don’t have a “coffin corner” to worry about, even when cruising at 51,000 feet/15,545 meters, the highest altitude authorized for any business jet. The safety margin between the high-speed and low-speed buffet in a Gulfstream is so large it could never be called a “corner,” more like a gently sloping curve that can easily be accommodated. No surprises, no ragged edge that quickly drops off the performance charts, just a superior wing in all respects.
The primary reason Gulfstream wings perform so flawlessly and safely at extreme altitudes is their large wing area and proprietary airfoil. There is simply no substitute for having more wing available to do the heavy lifting during climb and cruise at very high altitudes.
But the challenge is to make a large wing that flies fast with low drag. A typical large wing produces tons of lift. But the penalty for that lift is drag. High drag slows an airplane down, requiring more thrust to push the airplane forward, and thus burning more fuel and thereby shortening range. To design a wing with the necessary wing area and airfoil to avoid any “coffin corner” without creating additional drag is the magic. And Gulfstream Flight Sciences developed wing technology that solved the elusive riddle.
The design of Gulfstream wings is, understandably, proprietary. But there are several observable design factors that contribute to Gulfstream’s unmatched wing design. One factor is sweep angle. A swept wing behaves as though it were flying more slowly. Mach effects are minimized even though the Gulfstream G650 and Gulfstream G650ER, for example, are flying at 90 percent of the speed of sound, or even a bit faster. That is a feat no other large-cabin airplane can claim.
Another unique characteristic of Gulfstream wings is their totally smooth exterior. Unlike lesser wings, there is nothing hanging below a Gulfstream wing to create drag and degrade performance. While other manufacturers need to cover external flap mechanisms with large “canoe-like” fairings, Gulfstream engineers located all flap and control mechanisms within the wing. One need not be an aeronautical engineer to see the difference between a Gulfstream wing and all others; simply look at any other large-cabin or super-midsize business jet to see for yourself. It’s no surprise that a perfectly clean wing will have lower drag than one with a bunch of fat fairings hanging off the lower surface. The highly swept, clean wing also gives the G650 and G650ER the best fuel efficiency in the ultralong-range class at Mach 0.85.
Because Gulfstream wing technology is able to create a large wing with low drag, there is plenty of room to carry the fuel that helps give Gulfstream the legendary range at a speed that sets the standard for others to chase. Speed and range are what everyone wants in an airplane because that equals time savings. It’s only possible with patented wing technology that provides the lift for a rapid climb to very high cruise altitudes and then the ability to safely fly as fast as possible at those altitudes.
Jet pilots new to Gulfstream may be understandably cautious about maneuvering heavy aircraft at very high altitudes. And that’s good, and that is the procedure Gulfstream and FlightSafety International teach. But as part of Gulfstream’s special PlaneAdvantage advanced training course, I was able to aggressively bank a Gulfstream G450 to 45 degrees in a steep turn at 41,000 feet/12,497 meters to demonstrate that the wing behaved perfectly thanks to its superior design. That’s something I wouldn’t think of trying in other jets, but it’s the convincing proof that Gulfstream engineers have conquered what others find elusive—the ability to design wings for a diverse fleet of aircraft that balance low-speed maneuverability with high-speed efficiency, sea-level performance up to the thin air at 51,000 feet/15,545 meters, and the range to maximize the benefits of cruise near the speed of sound.
While the same laws of physics bind us all, Gulfstream engineers have slipped the surly bonds that still taunt lesser wing designs.
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