An executive favorite gets a makeover — again.
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 theArriel 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++, buttook the “Advanced Growth Blade” developed for the S-92 and newer versions of the 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.”
Sikorsky rates the external noise levels of the D model at 86 dBA and internal at 83 dBA, compared to 92 dBA and 87 dBA, respectively, for the C series.
Both the main- and tail-rotor blades are built with heating mats in them for the rotor ice protection system (RIPS), whether a new owner wants RIPS installed or not. RIPS has not yet been certified, but all the provisions for the system can be optionally built into the aircraft. Certification is expected in the 2014/15 time frame, at which point the system boxes can be added. The hardwiring put into the airframe weighs about 100 lb., while the boxes weigh around 200 lb. However, the operator can take the boxes out during the summer months to save that weight.
Perhaps the most significant product improvement to the D model, at least as far as pilots are concerned, is the fully digital glass cockpit.
The very earliest S-76s had virtually no digital instruments, while the C++ had a limited glass cockpit provided by Parker-Gull of Long Island, N.Y.
“For the avionics suite, we started with about eight vendors who responded to the RFP. Then we down selected to three and scheduled a customer's conference, with all three presenting their systems to the customers. Then the customers voted on it,” Hunter explained.
won that election. With the notable exceptions being the model's Pro Line nav/com radios and 's Mark XXII EGPWS, all of the avionics are from Thales' TopDeck avionics suite, powered by the DC generator. Thales offered a com radio with its system, but the customers clearly preferred the Pro Line unit.
Thales also developed the four-axis autopilot, of which the S-76D has two. That way, if one fails, the other automatically takes over with no pilot input, although the pilot is alerted to the fact.
Thales says the TopDeck integrated modular avionics suite represents the company's “most advanced avionics suite for rotor-wing aircraft.” For the S-76D, Thales specifically created a new program that uses a “click to fly” concept. The Toulouse, France-based company says that the new concept is based on “functionalities that improve intuitivity, interactivity, integration and safety,” or “Icube-S.”
In its instructions to Thales to develop a totally pilot-friendly avionics suite, Sikorsky said that it wanted virtually all of the commands from the pilot to require no more than two pushes of a button, significantly reducing pilot workload.
“We did not meet that [two-push criteria] everywhere,” Hunter said. “There are some places that require more than two button presses because there is no way around it. But if they didn't design the software with that in mind, they had to come to me and ask permission to do it another way.”
The Thales system is fully coupled to the helicopter's flight control system so that by punching the appropriate button, most flight commands are automatically recognized and initiated at the appropriate time, such as flying to an ILS localizer and coupling with the ILS to fly the approach to touchdown.
The overall system is designed to provide everything the pilot needs in a single-pilot operation, requiring the absolute minimum amount of effort. It basically does everything but brew coffee, something to consider for a future E model, perhaps. (See “Avionics Suite as Computer” sidebar.)
Also new to the S-76D is aVigor HUMS as baseline equipage. The system provides the operator with a detailed analysis of the health of the aircraft, feeding the information to the operator and, at the request of the user, allowing that information to be fed to Sikorsky's Fleet Management Operations Center (FMOC) where it is monitored on a 24/7 basis and analyzed for trends in the aircraft's maintenance and usage. The FMOC personnel look for and evaluate any inconsistencies in performance against fleet trends to alert operators to potential maintenance problems.
The use of HUMS also allows the FMOC to recommend changes in maintenance programs. Hunter noted that the S-76D HUMS comes from the S-92 program, and those being used on that aircraft allowed operators to go from 4,700-hr. TBO to 9,400 hr. on the swash plate alone.
Hunter said that at this time the HUMS data are accessed through a USB port in the baggage compartment. However, several formats that operators can use are in the planning stage, including a card in the cockpit to allow the pilots to access the information, as well as eventual options for remote wireless access.
The D model is also equipped with a Moog active vibration suppression system, using force generators that measure the vibration frequencies and produce exact opposite frequencies to counter them. Operators can order up to six of these force generators, although each one adds weight and cost. While each operator will determine how much vibration is acceptable to the mission, Hunter said Sikorsky expects three to four to be the average.
The anti-vibration system is one of only three systems using AC power, although Sikorsky is looking at a program that may change the anti-vibration system to DC.
Sikorsky went with a totally redesigned electrical system for the S-76D “to address some customer issues and to maximize compliance with theregulations,” Hunter said. He noted that the systems in the legacy aircraft were not satisfying D model requirements, such as providing additional torque to start the PW210S engine, requiring an increase from the legacy 200-amp rated generator to a 300-amp unit. The redesign put all the systems except anti-vibration, windshield heat and RIPS on a 28-volt DC electrical system newly developed by Sikorsky just for the S-76D.
Two AC generators were installed on the aircraft for the RIPS, with each capable of powering the whole system. These generators were put on the engines rather than the gearbox since the Pratts are able to produce plenty of power during cold weather when RIPS is needed, Hunter said. “All it does is raise the temperature of an engine a little bit. It doesn't rob torque out of the box. In the winter we have boatloads of power, boatloads of ITT margin. I want to use that torque that is going to the main gearbox to fly the aircraft, not to power the RIPS.”
The aircraft has also been designed to carry more fuel, which, combined with the more efficient engines, should result in greater range. However, the added components, such as HUMS and vibration control units, add weight which, in turn, threaten such gains, “and you never, ever want to deliver an aircraft with less range than its predecessor,” Hunter said. So to counter that, Hunter says Sikorsky plans to increase the maximum gross weight from 11,700 lb. this summer to 11,875 lb. and "essentially will get us back on parity with the C++ across the spectrum.” He further notes that the weight increase will actually increase the range over the C++ since most operators of that aircraft have already added optional equipment, such as HUMS, which is standard on the D.
The new model has a total fuel capacity of 296 gal. contained in two fuel tanks with gravity fuel fillers. These are supported by two independent suction fuel systems with cross-feed capability.