In many ways the Cessna Citation X is an extreme airplane. Its radically swept wing, huge wing-to-fuselage fairing and enormous vertical fin are unique in the business jet world. The X looks like it does because that's what it took to reach the goal of being the fastest civilian airplane now in service.
With its limit speed of Mach .92 the X can cruise up to about 528 knots true airspeed. A more typical fast jet cruise speed is around Mach .82, which equals about 470 knots. And lots of jets, particularly the extremely popular Boeing 737, cruise well under Mach .80, which is 459 knots. The Citation X does stand out.
The Citation X hits its top speeds, like any jet does, at lighter cruise weights and at altitudes below maximum fuel efficiency. But the X has the range to cross the country at top speed, and that's what many passengers want. When you buy the fastest airplane available, why throttle back? But there are trips that require longer range than the X's typical 3,000 miles. And there are warm temperatures aloft that rob cruise efficiency from any jet. As many jet makers have shown, the way to improve on both of those situations is with carefully designed winglets, and now the X can have them.
A winglet is actually a very clever way to make a wing behave as though it has greater span without adding all of the structural loads the longer span requires. Winglets are not a free lunch, but they can pay for themselves in terms of fuel savings and avoiding en route fuel stops.
In general, the longer the span of a wing the more efficient it is in climb and high-altitude cruise. Think of gliders with their fundamental requirement for efficiency since they have no engine. A glider wing is extremely long and thin to extract the most lift from the air for the lowest drag penalty.
The modern jet wing is much like a glider's, with long span and narrow chord. Huge trailing-edge flaps and often leading-edge slats transform the jet wing into a different shape and add wing area for the low speeds needed for takeoff and landing. But for climb and cruise, a long slender wing dominates jet design.
A successful winglet captures some of the energy of the high-pressure air under a wing that is escaping at the tip. The shape of the winglet is, of course, critical, and so is its angle from the vertical. The more a winglet tilts outward away from the fuselage, the more lift it can generally add. But that tilt transfers bending loads back into the wing, so more structure and weight are required to carry them. The optimum tilt of a winglet is the angle that gives the most added lift for the least structural load on the wing.
Why Mach Matters
When an airplane moves through the air at speeds below Mach 1, the air ahead of it is not disturbed because pressure disturbances can't move fast enough. But at Mach 1 the airplane's pressure field is pushing air molecules faster than they can get out of the way, and a sharp pressure wave forms on the leading edges of the airplane. This wave creates tremendous drag, and the airplane must have sufficient power to push the wave along.
It is, however, wrong to think of Mach 1, the "sound barrier," as being an on-off switch because the local airflow over parts of the airplane are always moving faster than the airplane itself is. Air must accelerate to pass over the nose and fuselage, and over and under the wing. At some airspeed this "local airflow" will accelerate to Mach 1 while the entire airplane is moving much more slowly. This speed where local airflow hits Mach 1 is called the critical Mach because there is a big jump in drag.
Up to about 300 knots true airspeed there are no issues with Mach, but at higher airspeeds it takes careful design of the airplane to defeat, or delay, the drag rise of Mach effects. It is delaying, or controlling, the Mach effects of local airflow that the design of the Citation X is all about.
The large sweep-back angle of the wings and tail, and the shapes of the airfoils, control the drag rise of local airflow as the airplane exceeds 90 percent of the speed of sound, Mach .90. The very long engine nacelles and extremely long wing-to-fuselage fairing also contribute by increasing the fineness ratio. A long, slender object will suffer less Mach effect drag than a fat blunt shape will — think of an arrow, or the Concorde.
Despite the excellent efforts of Cessna engineers, there is still an economic penalty to cruising ever closer to Mach 1 because drag increase is inevitable and more power is required, thus there's higher fuel consumption. So by slowing down just .06 Mach from the X's maximum speed of Mach .92 to Mach .86 — about 34 knots — range jumps way up.
Mach 1 is no longer the "sound barrier" as was believed in the 1940s, but it remains an economic barrier to low-drag cruise efficiency. But with the Citation X leading the way, airplane designers are creeping ever closer to routine cruise speeds very close to Mach 1.
Another key issue in winglet design is reducing the induced drag — the drag caused by generating lift — while minimizing overall drag of the extra area. If a winglet increases lift substantially so that a jet can climb to higher, more efficient altitudes faster, that's good. But if in the process it adds so much drag that top speed is reduced at lower altitudes, that's not a good trade.
So the challenge for Cessna and its partner Winglet Technology was to create a winglet for the Citation X that allowed the airplane to climb more quickly to very high cruise altitudes without robbing any of the top speed in the middle altitudes. That wasn't easy, and the project was in the works for several years.