Above: the Mitsubishi Regional Jet
The sky’s the limit for this structures grad
By Steve Stowe (MS 70)
My current project is the new Mitsubishi Regional Jet, “MRJ” for short, a twin engine jet airliner seating 70 to 90 passengers. MRJs are developed by the Mitsubishi Aircraft Corporation in Nagoya, Japan. Due to the limited airspace for flight testing in Japan, Mitsubishi Aircraft teamed with Seattle-based AeroTEC to conduct flight testing in the U.S. at the Moses Lake Flight Test Center, Grant County Airport (KMWH), Wash. Four “FTAs” (Flight Test Aircraft) have been ferried to KMWH and are currently conducting development and certification flight test. My role is Senior Experimental Test Pilot for envelope expansion on FTAs 1 and 2.
Test pilots and engineers based at Moses Lake Flight Test Center are indeed a culturally diverse group, coming from Japan, the USA, England, Canada, France and many other countries. The initial structural testing was recently completed and consisted of the two structural engineering disciplines critical for all new aircraft designs: Flutter and Loads Survey.
“Flutter" refers to a dynamic structural failure mode similar to the Tacoma Narrows Bridge collapse that occurred in gale force winds. Aerodynamic forces can cause the wings or other aircraft flight control surfaces to self-destruct at high airspeed due to undamped aeroelastic bending response and vibration. The JCAB, FAA and EASA airworthiness regulations require us to verify the MRJ is flutter-free up to “Vd” and “Md”: the maximum structurally designed Calibrated Airspeed (CAS) and Mach Number (MN). For such testing, FTA 1 and 2 are extensively instrumented with accelerometers, strain gauges and other sensors. Thousands of parameters are digitally processed and recorded onboard. For structural testing, critical flight data for the maneuvers are transmitted via telemetry equipment to engineers at a ground station where aeroelastic response and damping are viewed and analyzed real-time. This “TM Room” has about 30 engineer desks and maintains VHF Radio contact at all times with the test aircraft. Before flight testing, structural engineers developed a model of MRJ aeroelastic response for all configurations and load distributions from wind tunnel tests, ground vibration tests and computer tools. These models were incorporated into the TM Room displays so engineers can visually compare actual response to predictions. Flutter testing utilizing the TM Room capabilities reduces the onboard flight crew required and increases safety and efficiency.
Test points are sequenced from well damped CAS and MN conditions in progressive increments to Vd/Md in a methodical “build up” fashion. At the higher speeds, increments may be only a few knots or hundredths of a Mach. Control surface oscillations were excited manually by pilot control column or rudder pedal pulses, or mechanically by computer controlled flutter exciter devices mounted on the wingtips and empennage. Pilot pulses were sharp step inputs one axis at a time: pitch, roll or yaw. The Flutter Exciters generated pre-programmed sinusoidal frequency sweeps. Following the input, if the test pilot observed any signs of divergence or anything unusual, the aircraft would be immediately slowed down well below the test point speed. At the same time, the TM engineers could transmit “Knock It Off” over the radio – code word to slow down – if they observed any unpredicted behavior on their screens. If the TM Room engineers were satisfied with the aeroelastic response and damping, the FTA crew were approved for the next test point with a “Cleared Next” radio call. MRJ Flutter testing showed that damping was positive up to Vd/Md and agreed well with predictions.
Whereas Flutter testing was flown with the aircraft in steady unaccelerated flight, Loads Survey testing involved maneuvering the aircraft in turns, pull ups and push overs while progressively increasing the normal load factor (Nz) up to plus 2.0 and down to 0.5 “g’s” (airworthiness regulations require a safety factor to ultimate limit of 1.5 similar to civil engineering applications). In addition, side loads were tested up to full rudder pedal sideslips. MRJ structural loads were verified satisfactory similar to Flutter with prediction models, TM Room utilization and the test point sequence “build up” approach to the load factor limits.
With the structural testing successfully completed, all MRJ flight test aircraft are now at the level of design speed range to conduct all kinds of tests required to achieve type certification. If you ever fly on the MRJ as a passenger, you can sit back and relax knowing the aircraft’s structure has been flight tested and proven by an Illini CEE grad test pilot.