Airframe and Systems
The basic airframe remains the same as that of the legacy S-76. However, the new model is being offered with some significant improvements, which resulted from discussions with the owners, pilots and maintainers, Hunter said.
“They had a lot of input," he continued, "and we made a lot of changes based on their saying, 'This is really a pain, you did not think this out well.'” Discussions were held with users during 2005/2006, both in the U.S. and abroad, with “approximately 200 suggestions collected and used as input to the S-76D design,” according to Hunter.
The key areas addressed were reliability and maintenance, “with a number of items that were formerly options now included as baseline installations,” the executive said. “Most of the aircraft lighting has been upgraded to LED, and the remaining two non-LED lights are planned to be replaced as an in-line baseline change. LED lighting has increased reliability with lower power usage.”
Hunter also noted that a lot of the product improvements came from the S-92 program. “As they developed them, we stole them.”
Several changes have already been incorporated into the S-76D, while others are planned for the near future. The major upgrades include new engines, new rotor blades and a new all-glass cockpit designed to simplify single-pilot operations. Other changes now in the D model include a health usage monitoring system (HUMS), active vibration control and an improved autopilot. The HUMS had been available for the C++, but as an option. It is now standard on the D.
Improvements to come include a payload increase, SAR autopilot modes, improved crashworthiness of seats and floor as well as Type IV egress windows. A combined flight data recorder (FDR) and cockpit voice recorder (CVR) will be standard equipment on the aircraft, along with an integrated emergency flotation system. An automatic flotation deployment system will be available as an option.
One of the biggest changes involved replacing the Turbomeca Arriel 2S2 engines on the S-76C++ with the Pratt & Whitney Canada PW210S, giving the D model not only more power, but greater fuel economy. (See “Power for a Gen-8 Helicopter” sidebar.)
The main rotor hub system is the same as that on the C++, but Sikorsky took the “Advanced Growth Blade” developed for the S-92 and newer versions of the UH-60 Black Hawk, and adapted it to the S-76D. These are all-composite blades as opposed to the metal blades on previous models and have a wider chord and reconfigured tips, although their length remains the same. The Advanced Growth Blade uses two geometric parameters — sweep and taper — to reduce main-rotor blade tip vortex. This gives the blade tip a swept-back angle, improving lift, while decreasing noise.
The tail rotor also received a tapered blade tip and its chord was increased by “about an inch,” Hunter said, adding, “This allows us to slow the tail-rotor tip speeds down through changing the gear ratio in the intermediate gearbox. This gives us the same performance because it's an increased efficiency rotor [while] substantially lowering the acoustics of the aircraft.”