US manufacturer makes design changes to reduce Model 429’s noise signature

Bell Helicopter is designing its Model 429 to meet stringent noise limits that would allow the light twin-turbine helicopter to be used for sightseeing flights over US national parks. The drive to reduce noise has resulted in the change to an X-type tailrotor for the helicopter, which will fly next year.

“We aim to be a couple of decibels below ICAO [International Civil Aviation Organisation] Stage 3 in quiet cruise mode,” says Bill Stromberg, executive director, commercial programme management. “We will get close to or below US Park Service noise standards – below is the goal.” Main and tailrotor rpm is reduced in quiet cruise mode.

The 429 is the first in Bell’s Modular Affordable Product Line (MAPL) family of light single- and twin-turbine helicopters, and incorporates 10 of the 13 technologies being developed under the MAPL initiative. These include what Stromberg describes as the “MAPL cabin”, which seats seven in the 429 and which will be “deplugged” for a five-place single-engine helicopter by taking out a row of seats.

The 429 is the only light twin that can accommodate two litters and two attendants in an emergency medical-service layout, Stromberg says. Flight controls are routed under the floor and up the lift frames and windshield centrepost to provide an unobstructed cabin with flat floor.

The helicopter also features clamshell rear doors that allow stretchers to be loaded and unloaded ambulance-style.

Bell has selected large Rogerson Kratos flat-panel displays for the 429 cockpit, with two screens in single-pilot layout and a third added for two-pilot operation or to display images from a forward-looking infrared sensor. A 90h flight data recorder is embedded in the displays, says Stromberg. Garmin GNS 430 GPS-based navigation will be standard.

Other key MAPL technologies in the 429 include main rotor blades featuring new composite construction and aerodynamics; a two-piece supercritical tailrotor driveshaft to eliminate hanger bearings; and the tailrotor, made up of two stacked Model 407 two-blade rotors with swept tips. The tailrotor will turn slower than the that of the 407 to reduce noise.

The $2.8 million visual-flight-rules-only Model 427 will continue in production. “It is a nice high-low mix,” says Stromberg.

Certification of the 429 is scheduled for the first half of 2007, and Bell has orders for 110 of the $3.95 million helicopters.


Source: Flight International