Falcon 7X fly-by-wire system progressing in simulator mode

 - January 18, 2008, 11:01 AM

Dassault pilots and engineers have been “flying” a Falcon 7X simulator since December at the French company’s Saint-Cloud headquarters near Paris. The Falcon simulation testbed (FST) has been built to develop the control laws of the new trijet’s fly-by-wire (FBW) system. Next year it will be fitted with all of the 7X’s control-related systems, Dassault technical managers told AIN. The Falcon 7X is scheduled to make its maiden flight in early 2005.

The FST made its first simulated flight on that most significant of dates for aviation–December 17. Test pilots Philippe Deleume and Yves “Bill” Kerhervé were at the controls. The FST allows full simulated flights, from takeoff to landing.

However, the FST’s layout is in a test configuration, said Pascal Vallat, head of the testbed team. For example, the display icons are simplified, providing just the basics for the test pilots.

The handling characteristics of the simulated Falcon result from wind-tunnel tests and Dassault’s experience in modelization, Jeannine Lafont, deputy senior vice president for civil aircraft, told AIN. A monitoring room is located next to the FST. Screens display what the pilot sees, while others show additional data, mainly software internal parameters.

The 7X’s FST was inspired by the Rafale and the Mirage 2000 testbeds. As Vallat put it, the role of the FST is to study how a pilot’s input translates into a control-surface movement. As in a real airplane, “we are progressively opening the flight envelope,” he said. Between December 17 and February 7, Dassault engineers had logged 60 test hours on the testbed.

When AIN visited the facility housing the FST recently, Dassault engineers were working on the side-stick controllers. Dassault considered using Airbus-designed side-stick controllers, but it appears the whole system (including sensors and force-feedback devices) was too bulky for the 7X cockpit. Both pilots and test engineers are involved in this development.

So far, the FST involves simulation computers that emulate the flight control-related equipment. “Later, we will connect the flight controls to servos and actual hydraulic systems,” Olivier Villa, head of Falcon programs, told AIN. This will translate next year into a complete testbed–something close to an iron bird, but limited to flight controls.

This testbed will allow engineers to ensure that the software works as expected in a real environment. The complete 7X testbed will closely mimic that of the Rafale fighter. “We will perform a lot of failure and degraded configuration tests on this testbed,” Vallat explained.

A first-flight standard of the flight controls will be available next year, and an intermediate standard will be developed in 2005. A certification standard will be available in 2006. The airframer will reap benefits of developing FBW controls on the 7X when derivatives are designed. “We will not have to redesign the FBW system’s architecture,” Villa stressed. Both longer-range and twinjet versions are already planned.

The principle of fly-by-wire is to replace the mechanical control system with a computer and associated software. The latter calculates, from pilot input, the movement of the control surface. The computer uses feedback from the aircraft’s movement to further adapt control-surface movement to the pilot’s command. This process is totally transparent to the pilot.

FBW controls were born on combat aircraft so that the fighter would behave the same regardless of its load. In addition, designing a more agile fighter implies making it unsteady. This translates into difficult handling characteristics with conventional controls, such that when the pilot pulls up and then pushes the stick back into the neutral position, the aircraft’s pitch angle continues to increase. Only FBW controls allow the pilot to handle a cutting-edge fighter safely and predictably.

On civil aircraft, FBW controls retain the advantage of homogenizing the aircraft’s behavior throughout the entire flight envelope. “The aircraft is safer and more pleasant to fly,” Jean-François Georges, Dassault’s senior vice president for civil aircraft, told AIN. Designers can also adopt an aerodynamically “pure” wing, even if it makes the aircraft sometimes tricky to control because the FBW system will adjust automatically for the wing’s quirks. In addition, FBW is lighter and more easily maintained than a conventional control system, according to Dassault.

On the 7X, FBW will include flight-envelope protection. In other words, the controls will prevent the aircraft from entering a dangerous attitude. “I can’t see the point in stalling a civil aircraft,” Georges said. This philosophy is similar to that of Airbus.

Trims are unnecessary on FBW-equipped aircraft. For example, to start a three-degree pitch down with FBW, the pilot pushes the stick and then releases. The stick will go back to its neutral position and the aircraft will continue the three-degree descent.

The 7X’s controls should be “consistent” with the EASy flight deck. EASy uses HUD-like symbology based on the climb rate rather than the attitude, “and FBW controls fly a flightpath,” Kerhervé pointed out. Dassault engineers will study the interface between FBW controls and the EASy flight deck on the FST.

Whether the two side sticks will be synchronized (the right one moves in sync with the left one when it is being moved, and vice-versa) is still under study. Most likely “the two side-stick controllers will remain independent, as in Airbuses,” Kerhervé told AIN. But Dassault will still test synchronization as a way to avoid pilot miscommunication about who is flying the aircraft.

In the current design, side-stick inputs are added algebraically (in fact, the resulting input is half the sum). When both sticks receive an input, a “dual input” warning is activated. A “priority switch” is also available to make one stick inoperative. Synchronization is a better solution, but “it is complex, heavy and expensive,” Kerhervé emphasized. The possibility of synchronization failure must be taken into account. And regulations also call for an emergency cutoff mechanism if a pilot collapses onto his stick.