Iron Bird: SkyCourier’s ‘iron’ testbed

Ice protection system components are chilled—well, frozen—to minus 100 degrees Fahrenheit to confirm they’ll work under temperature extremes. Another test rig subjects various parts to violent shaking to quantify how much abuse they can tolerate. Then there’s the HIRF (high intensity radiation field) chamber, with its ceiling-mounted antennas aimed at wiring bundles, electrical control units, and other electrical components. This is where their shielding has to prove its ability to withstand damaging currents. The chamber’s plated, three-foot-thick walls and entry door contain the huge electromagnetic fields broadcast by those evil-looking antennas, preventing them from harming the building’s employees. As I walked past another test site, I saw a windshield with a spidery network of cracks. It had been subjected to repeated extremes of hot and cold conditions. While full of cracks, it held its integrity.

This is all very interesting, but for pilots the star of L22 is its array of “iron birds.” Most are not familiar with the iron bird concept. While iron birds can’t fly in the strict sense, they’re central as testbeds, “fly” quite a bit, and are essential in refining the links in the design, prototype, systems integration, flight test, and refinement tasks in building a new airplane. They are used by many turbine aircraft manufacturers.

My escort in L22, Aasiri Fernando, director of engineering, mechanical systems, simulation and test lab, takes me to the newest iron bird—the one for the new Cessna SkyCourier, a high-wing, fixed-gear, multi-role turboprop twin. The front of the bird looks like a simulator cab, complete with Garmin G1000 NXi avionics, wrap-around visual displays, engine and flight controls, a two-seat cockpit, and, behind them, workstations for the engineers who monitor the iron bird’s flights.

It’s the iron bird’s aft section—the part behind the cockpit’s aft wall—that grabs your attention. Parts of it look like an erector set gone amok, with a maze of control cables, pulleys, actuators, and computers. Then there’s the aluminum scaffolding that supports this complicated apparatus—including the twin black boxes that house computers that exactly duplicate the behavior of the SkyCourier’s twin 1,100-shaft horsepower Pratt & Whitney PT6A-65SC turboprop engines.

Stand back, throw your eyes out of focus, and the shape of the entire structure sort of resembles that of a real airplane. There’s the vertical capstan (that’s the post with the rudder cable windings); the two wings (more cables, and the Garmin autopilot’s wing actuators, one on each side); and those engine computer boxes on either side. A closer inspection shows angle of attack vanes, and two large pulleys that carry the aileron cables. Unlike some iron birds, the control cable runs are not as long as those of the real airplane’s, so the whole assembly is small (about 10 by 20 feet, and 10 feet high) compared to the iron birds used on, say, Airbuses or Gulfstreams. But Fernando assures me that the SkyCourier iron bird’s (and the six others in the stable—including the Citation X+, Mustang, and Longitude) functions are scalable, and adjusted to make the iron bird’s simulator fly like the prototype and flight test SkyCouriers “with 95 percent accuracy.”

The design process begins with management’s goals for the airplane, then moves on to realization by computer-aided design, manufacturing, and modeling using Dassault Systèmes’ computer-aided three-dimensional interactive application (CATIA) software. The end product is fed into scale models that fly in wind tunnels and generate data on flight behavior. This wind tunnel data is adapted to run on the iron bird so that it can help answer a big question: Will the iron bird fly the way the software says the airplane should? Or are modifications needed?

When the prototype flies, it will be instrumented to generate even more data on control forces, center of gravity influences, and other dynamics—data that can also be fed into the iron bird for analysis and modifications. So, the iron bird acts as a judge of sorts between the model predictions and the prototype and flight test airplanes’s real-world behavior. That, and the avionics’ integration with the airplane’s systems and flight guidance components. Compared to old-school design methods, using an iron bird in finalizing the airplane design makes flight testing safer, more efficient, and saves time.

This wind tunnel data is adapted to run on the iron bird so that it can help answer a big question: Will the iron bird fly the way the software says the airplane should?Now I’m in the iron bird’s pilot’s seat, with Todd Dafforn, Textron engineering flight test pilot, riding shotgun. The panel is identical to the SkyCourier’s, and engine starts and other preflight tasks—like setting the trims and flaps for takeoff and filing a short flight plan—flowed easily. However, the SkyCourier doesn’t have full authority digital engine control (FADEC) so you have to monitor starts for interstage turbine temperature excursions, signs of a hung start, and any other issues as the engines spool up. Even though Dafforn warned me that the bird’s ground modeling wasn’t exactly ideal—he said it would feel like the airplane was moving on ice—I managed to keep the airplane on the centerline, saw a V1 of 90 knots and VR of 93 knots go by, and lifted off into a climb of 600 fpm or so.

I should mention that the iron bird was set at a weight of 17,000 pounds, which is 2,000 pounds lower than the airplane’s planned maximum takeoff weight of 19,000 pounds. Its maximum payload is three tons in its cargo configuration, and 5,000 pounds in the 19-seat passenger version. Oh, and its wingspan is 72 feet. So, if you’re thinking that the SkyCourier is some sort of twin-Caravan clone, banish the thought: The Caravan’s maximum takeoff weight is 8,000 pounds, and its wingspan is 52 feet.

As you might suspect, the SkyCourier’s mass makes for some truck-like handling. This showed up mostly in steep turns, but it was nothing obtrusive. Besides, the airplane has envelope protection to help ensure that inadvertent overbanking doesn’t occur. If you try to bank much past 45 degrees, corrective control forces fight back. In all, the airplane behaves the way you’d expect of a freight/passenger hauler. That means a “steady as she goes,” feel, which makes for a stable platform in instrument conditions, especially in single-pilot operations. The airplane will be certified for single-pilot operations in the commuter category under the revised FAR Part 23. For FAR Part 135 operations, the 19-seat variant will require two pilots.

After a stall demonstration (the SkyCourier has a stick pusher, but no shaker), some engine-out work, and two ILS approaches, one of them coupled to the Garmin GFC 700 autopilot, I landed out of a VREF of 95 or so knots. My hour flying the iron bird was over all too soon, but it showed that the SkyCourier has straightforward handling and procedures. Sure, your arms can get a workout at times, but what do you expect from such a huge heavy-hauler? As this goes to press the SkyCourier was targeted for certification in the near future. After that, here’s hoping we get to fly the real thing. We’ll keep you posted. [email protected]