Guy Norris/LOS ANGELES
McDONNELL DOUGLAS Helicopter Systems (MDHS) has begun flight testing a series of modifications to the MD600N eight-seat civil helicopter following the crash of a prototype earlier this year.
The changes are aimed at increasing the clearance between the main rotor blades and the tail boom. The outcome of flight tests will dictate the final design, according to MDHS.
The main options include increasing the minimum rotor blade RPM in autorotation, from 86% to 90%; making a minor adjustment to the forward centre-of-gravity envelope; or moving the empennage control cable from the top of the boom to the side, "-and out of any possible immediate contact with the main rotor", says the US helicopter maker.
Another option includes changing the down-force on the tail-boom by adjusting the angle of attack of the horizontal stabilizer, or by reversing the NOTAR (no-tail-rotor) anti-torque-system thruster, or a combination of both. This would make the aircraft fly with a slightly more nose-up attitude.
The crash of the second prototype occurred on 28 May when the main rotor struck and partially severed the tail boom as the pilot was performing control reversals in the longitudinal axis at maximum speed and minimum rotor RPM.
"During this extreme manoeuvre, designed to determine the maximum flight envelope for the aircraft, the main-rotor blade contacted the NOTAR tail-boom and cable controlling the thruster and vertical stabilisers," says MDHS. The pilot escaped unhurt after he began an auto-rotative descent, but lost rotor RPM and landed the aircraft heavily. The helicopter was destroyed by fire.
The third MD600N was flown on 9 August and, although originally destined to be the first production version, has replaced the destroyed helicopter as the primary-development test vehicle. MDHS says: "The new test flight was the culmination of an intensive effort to complete the aircraft, add design modifications to increase main-rotor-blade-to-tail-boom clearance, and install the instrumentation suite required for the resumption of US Federal Aviation Administration testing."
The company says that the FAA is "happy" with the results of the accident investigation and has approved the company's plan for continuation of the MD600N certification programme.
FAA flight testing is expected to be completed by November and MDHS hopes to achieve certification by the end of the year. Some 75% of the flight and development testing has been completed, most of which will not be repeated.
The company is believed to be planning a mass hand over ceremony of the first 14 aircraft in December, following receipt of FAA certification. The MD600N is a stretched derivative of the MD520N.
Source: Flight International
