The controls felt like a Citation CJ, but the numbers on the new Collins Pro Line 21 displays didn't belong to any CJ I had ever flown. Level at 45,000 feet, the true airspeed was 425 knots. And it had taken only 23 minutes to reach that rarified altitude after a near-maximum weight takeoff. How had Cessna's entry-level light jet grown into such a performer? Well, it took about 17 years and four big steps to grow the original CitationJet into the CJ4 I was flying.
In the early 1990s, the cost of the Pratt & Whitney JT15 engine that powered the light Citations had increased to the point that the company could no longer continue to build its Citation 501 entry-level airplane at a price that made sense. But then Williams International entered the market with its all-new FJ44 series of turbofan engines with thrust ratings and prices well-suited for entry-level jets.
Cessna engineers realized that the new engine also provided an opportunity to improve the aerodynamic efficiency of the Citation 500 series, and the CitationJet — later shortened to just CJ — was born. The CJ had a cabin and cockpit very similar in size and shape to the original Citation 500, but it flew on an all-new wing that achieved a significant amount of laminar flow.
Laminar flow is, of course, the smooth, undisturbed passing of air over a surface, and it creates far less drag than the turbulent flow that is typical over most wings. The new wing was so efficient that the CJ, with about the same thrust, cabin size and weight as the Citation 500, cruised 30 to 35 knots faster.
Williams created ever more powerful versions of the FJ44 engine, and Cessna stretched the CJ cabin and increased the performance with the CJ2 and CJ3 models. The wingspan grew along with the fuselage length, but the same basic wing design, primary systems and avionics flowed into the new models.
Still, pilots and owners wanted more speed and range than the CJ3 could provide but didn't necessarily want to move up into the midsize cabin category of jet. So Cessna's mission became to preserve the excellent flying qualities of the CJ family, and its low operating costs, while giving owners an airplane with more speed, range and cabin room to move up to, and the CJ4 was born.
It's easy to focus on the cockpit and cabin of an airplane because that is where we sit and work, but all successful airplanes start with a wing specifically tailored for the mission. Cessna enjoyed huge success with the unique wing it created for the long-range midsize cabin of its Sovereign several years ago, so it was natural to start with that basic design to create the performance necessary for the CJ4.
The wing is unusual because it has a very noticeable sweep — 12.5 degrees — of the leading edge but a straight trailing edge. Wing sweep reduces drag when an airplane is flying at Mach .70 or faster because the sweep makes the slipstream behave as though the wing were thinner than it actually is. In general, a swept wing gives up some low-speed lift and can produce unpredictable behavior at the stall. The CJ series of jets is rightly noted for low speeds on takeoff and landing, and docile stall behavior, and Cessna didn't want to give any of that up, so it split the difference by sweeping only the leading edge.
However, the performance of the new wing is more complicated than just its leading-edge sweep angle. The airfoils are proprietary to Cessna and continuously change shape along the span of the wing to optimize performance at each station. The trailing edge of the wing is absolutely blunt and squared off. The edge itself is about half an inch thick and seems to defy what is logical about how a low-drag wing should look. When you run your fingers under the trailing edge, you can feel a subtle concave hollow ahead of the trailing edge, a design that Cessna has sought to protect. The unusual shape of the trailing edge cut overall drag in testing but was most effective in drag reduction during climb. I am constantly amazed by how such seemingly insignificant shape changes can alter the performance of a wing.
To qualify for the maximum Mach operating limit (MMo) of .77, Cessna dived the CJ4 to Mach .84, and the experimental test pilots reported that the dive test was one of the smoothest they had conducted. The wing seemed ready for even more speed, but the trim forces trying to push the nose back up reached the test limits.
The CJ4 wing is also significantly larger than those on the earlier CJs. The new wing has 330 square feet of area compared with 294 square feet for the CJ3 wing. More wing area is effective both at high-altitude cruise and on takeoff and landing. And a bigger wing provides space for more fuel, 1,118 pounds more than the CJ3. The new wing is a home run with the capability for faster climb, low-drag cruise at high altitude and higher airspeed, and more fuel capacity. The result is a 2,002 nm — no kidding, flight testing has demonstrated that the extra two miles are really there — IFR range at the high-speed cruise of 425 knots.
The CJ4 wing has two other features new to the light Citations, ground spoilers and modulated speed brakes. Others in the family have only two speed-brake panels per wing, one on top and one below, and they can only be either fully deployed or fully retracted. On the CJ4, the speed brakes are controlled by a lever next to the throttles and can be extended as much or as little as needed to meet descent requirements.
The three spoiler panels per side are for ground use after touchdown and, unlike the speed brakes, which are drag devices, the spoilers effectively kill residual lift to plant the CJ4 on the pavement for improved braking after landing, or during a rejected takeoff. Ground spoilers are the norm on all larger jets but are new to the light Citations.
When designing the new wing, Cessna engineers were able to move the main landing gear inboard, reducing the track. A narrower track makes all aspects of ground handling smoother, and the long-stroke, trailing-link landing gear that has been on all CJs smoothes any reasonable landing into a greaser.
When you first see a CJ4, I think you will find it to be a more sleek and stylish airplane than others in the Citation 500 family, but you may not immediately know why. The swept wing certainly adds a look of speed, but the real change is to the windshields and canopy. The new windshields and cockpit side windows are heated glass and sweep back gracefully into the canopy, while the windshields on the other CJs are high-strength plastic and are much more upright.
When the first Citation was developed about 40 years ago, the technology to make curved and heated glass windshields was in its infancy, so Cessna, like Learjet, used acrylic material and then blew hot engine bleed air over the outside of the windshield to prevent icing. Those plastic windshields and bleed-air heat systems have served well on thousands of Citations over the decades, but the bleed-air blast is noisy and can require fiddling with the manual control valves to get it just right.
The heated glass windshields in the CJ4 are on all of the time, so fogging and ice are simply not an issue. The glass area is somewhat smaller than on the earlier models, but I found the visibility in flight and on the ground to be great. And with less windshield and side window wrapping up into the overhead, there is less space for the sun to beat down on you.
Cockpit Redesign
The CJ4 cockpit is what I would call current in terms of human factors design. Except for a half-dozen switches on the avionics and electrical power control panel, all toggle switches are gone, replaced by on-off push buttons that light up to indicate their position. Toggle switches are OK in daylight, where it's easy to see if the switch is up or down, right or left, to indicate the state of the system it controls, but at night toggles are hard to see, and thus it's difficult to know what has been selected off or on.
