Preparing for Flight
Bell/Agusta’s BA609 looks nothing like the finished article in the VMSIL. In place of a fuselage and wings, the tiltrotor’s systems, interfaced with an aircraft flight-simulation host computer, are spread across three separate areas in the lab. Although a separate cockpit rig features a dual set of flight controls and instrument displays facing a pair of 50-inch LCD (liquid crystal display) flat-screen panels, “flying” the VMSIL is not a simulated affair–at least, not in the conventional sense.
When a pilot lifts the power lever, trims the cyclic or alters nacelle position, he activates the same transducers, flight-control computers, avionics displays, electrical power systems and hydraulic actuators found in the actual aircraft. Unseen by the pilot, the adjacent lab rooms become filled with movements both big and small.
The VMS on the BA609 employs a triplex fly-by-wire flight and engine control system, redundant electrical and hydraulic power-generating system and a dual-pilot glass cockpit with integrated avionics functions.
In the VMSIL, this equipment operates in a closed-loop mode, by integrating a high-fidelity mathematical model of the aircraft’s flight characteristics. The equipment senses actuator positions and feeds the information into aircraft, rotor and propulsion system dynamic equations. Another mathematical model then computes the aerodynamic loads acting on the rotor and control surface actuators, and a hydraulic rig applies the loads to the actuator rod end. Aircraft state variables such as engine power settings are output from the model and fed back into the flight-control computers through sensor models.
Incorporating this real hardware-in-the-loop simulation (HILS) into the test program was the final phase before first flight. VMSIL combines four systems benches–electrical, hydraulic, avionics and flight controls–to provide the required onboard system monitoring and data-collection capability.
Flight controls lead Cliff Harrell said, “The more you can make real and the less you have to simulate, the easier you make your job.” During HILS testing, the only systems simulated in the VMSIL are the engines, rotors and inertial and air-data sensors. The largest area in the lab is dedicated to the actual conversion and swashplate actuator rigs, where actual interconnect drives, hydraulic power drive units and ball screw actuators drive heavy steel weights representing the engines and rotors. Aircraft state information from the various rigs is fed back to the aircraft flight displays and to a recently upgraded visual system, which displays local area topography and man-made structures.