|(c) STEVE DOUGLASS|
The Bell 525 Relentless is an American medium-lift helicopter, under development by Bell Helicopter. The Bell 525 was unveiled at the 2012 Heli-Expo in Dallas, Texas in February 2012. The helicopter first flew on 1 July 2015. It is designed to transport up to 19 passengers.
On July 6, 2016, the prototype crashed during a test flight, killing the two occupants.The aircraft broke up in flight while traveling about 229 mph at an altitude of about 2,000 feet. In January 2018, the US National Transportation Safety Board released its findings, saying that the aircraft had suffered from severe inflight vibrations, which resulted in a loss of rotor RPM, subsequent rotor flapping and rotor impact with the tailboom, causing the inflight break-up. Contributing causes were collective biomechanical feedback which caused the tailcone to pulsate at 6 cycles/second, plus the attitude and heading reference system response, "both of which occurred due to the lack of protections in the flight-control laws against the sustainment and growth of adverse feedback loops when the 6-hertz airframe vibration initiated." Further causes included the lack of software safeguards designed in and the lack of a low rotor RPM indicator. The investigation was hampered by Bell not recording cockpit audio or imagery during the flight.
After the accident, Bell amended the control paradigm, improving the filter on side-stick controller inputs to block transmission of stick vibrations to the rotor system. Filtering was also added to the control system to account for gusts and maneuver loads.
The crash delayed certification from 2017 to 2018. In February 2018, Bell predicted certification to be completed by late 2018 or early 2019. In December 2018, 1,300h of turn time and 900h of flight were accumulated, towards a 2019 US type certification. In early 2019, two helicopters will be tested in cold weather in Yellowknife, Canada, as a third prototype will validate performance in snowy north continental USA.
First flight is expected within the next two weeks.
PHOTO BY STEVE DOUGLASS