's S-76 has proved popular in the medium commercial helicopter market, with more than 800 produced since deliveries began in 1979. Now the company has switched production to the S-76D, its eighth rendition of the proven design.
The aircraft receivedcertification in October 2012, and is on track to be accorded certification in April, followed by European and Japanese approval in May. First deliveries are expected by mid-2013.
The D model is targeted at the same basic markets as its predecessor—offshore, executive transport, emergency medical service and search-and-rescue. But the aircraft offers some significant improvements over earlier S-76s, based on discussions with owners, pilots and maintainers, says Dan Hunter, director of programs for Sikorsky Global Helicopters, the manufacturer's commercial division.
“They had a lot of input, and we made a lot of changes based on that,” he says, noting that many product improvements came from the larger S-92. “As they developed them, we stole them.”
Among those improvements, the principal changes involve more powerful engines, new rotor blades and an integrated glass cockpit designed to simplify single-pilot operations.
The twin Pratt & Whitney Canada PW210S turboshafts, which have dual-channel digital controls, produce 14% more takeoff power and burn 8% less fuel than theArriel 2S2 engines that power the S-76C++.
The only changes to the main rotor system are to the blades, using the Advanced Growth Blade developed for the S-92 and adapted to the S-76D. These all-composite blades have a wider chord for more lift and reshaped blade tips for less noise. Sweep and taper are used to reduce blade-tip vortex strength and minimize compressibility. The tips are swept back approximately 30 deg.
The tail rotor also a tapered blade tip, high-lift airfoil and a chord increased by “about an inch,” Hunter says. “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 has measured the noise levels of the D model at 86 dBA externally and 83 dBA internally, compared with 92 dBA and 87 dBA, respectively, for the C series.
Both the main- and tail-rotor blades have built-in electrical heating mats for the rotor ice protection system (RIPS), which will allow flight into known icing. RIPS has not yet been certified, but provisions for the system can be built into the aircraft as an option. Certification is expected in the 2014-15 timeframe.
But perhaps the most significant improvement to the D model, certainly for the pilots, is the fully digital glass cockpit. The earliest S-76s had virtually no digital instruments, while the previous C++ model had only a limited glass cockpit provided by Parker-Gull.
With the exception of thePro Line navigation/communication radios and 's Mark XXI enhanced ground-proximity warning system, all of the avionics are from , including the four-axis autopilot.
The TopDeck avionics suite involves a “click to fly” concept that has never been used on a helicopter before, according to Thales. Essentially it reduces most, but not all, of the commands required from the pilot to no more than two button pushes, significantly reducing workload.
The system is designed to provide everything that could be needed by a single pilot, particularly for instrument flight-rules flying, requiring the minimum of effort. Basically, it does everything for the pilot except brew coffee in the cockpit, something that will have to wait for a future S-76.
While the system is sophisticated and complex, it is user friendly, says Hunter. An hour-long demonstration flight showed how simple it is.
The instrument panel features two 8 X 6-in. screens for each pilot that can serve as either a primary flight display (PFD) or multifunction display (MFD). In the center of the panel are the pilots' multipurpose control and display units. Directly below each pair of displays is a PFD control panel and a Cobham 380 audio control panel.
Two changes made to the D model's instrument panel compared to the C++'s were to reduce its size, taking about an inch off the bottom and trimming the glareshield to improve visibility, and moving the landing-gear control from the center console to the upper center of the main panel for easier access.
A major factor in reducing the pilot's workload is the cursor control device (CCD), a raised housing containing a roller ball sitting toward the front of the center console. Each pilot has a CCD, and uses the trackball to move a cursor around the display screens. The pilot can rest his hand on the housing to provide stability in flight, then use his fingers to move the cursor wherever he wants on the MFD or PFD. Once the cursor is positioned, a button is pushed and the command inputted, just like a left or right click with a mouse.
Also new to the S-76D is the installation of aAerospace Systems (formerly ) Vigor health and usage monitoring system (HUMS) as a baseline item. This feeds information on the drivetrain and other components to Sikorsky's Fleet Management Operations Center, where usage is monitored and analyzed for trends and exceedances to drive maintenance. Hunter says the HUMS comes from the S-92 program, where it has allowed operators to go to 9,400 hr. between overhauls, from 4,700 hr., on the swashplate alone.
The D is also equipped with a Moog active vibration-suppression system, using force generators that measure the vibration frequencies and produce opposing forces to suppress the vibration. Operators can fit 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 its mission, Hunter expects “three to four” force generators to be the average installation.
Active vibration suppression is one of only three systems on the aircraft that use AC power. The two others are windshield heat and the RIPS. All other systems are driven by a 28-volt DC electrical system developed by Sikorsky for the S-76D. Two AC generators for the RIPS are mounted on the engines for redundancy and because the system will be used only in cold weather when the engines have power to spare.
The D model has been designed to carry more fuel, which combined with the improved engines will provide greater range. But the added components, such as HUMS and vibration suppression, add weight, which reduce range, “and you never, ever want to deliver an aircraft with less range than its predecessor,” Hunter says. So Sikorsky plans to increase the maximum gross weight to 11,875 lb. this summer, from 11,700 lb. “That essentially will get us back on parity with the C++ across the spectrum,” he says.
Sikorsky offered Aviation Week an opportunity to fly the new S-76 shortly after it received certification, so I arrived at its West Palm Beach, Fla., test facility on a beautiful day in late November, a perfect day for flying, but a terrible day for a flight evaluation as winds were light at around 17 kt., temperature was 78F, altitude was a mere 20 ft. and the aircraft weighed in at 9,494 lb., well below its 11,700-lb. maximum.
With twin PW210S engines producing 1,077 shp each, pushing the aircraft to its limits was unlikely. But what I really wanted to experience was just how much the pilot's workload was reduced by the new Thales TopDeck cockpit—and an hour's flight with Greg Barnes, project pilot for the S-76D program, more than met the need.
The initial part of the demonstration flight in aircraft N767J was fairly standard, with me sitting in the right seat and Barnes in the left. Start-up was the easiest I can remember in a helicopter. Power is controlled by two overhead “T”-shaped throttles, each with a start button at the end of one arm of the T. Push the button, the engine starts. Push the throttle forward and you have full power. The power can be applied slowly, in stages, or in one quick thrust—the pilot's choice. You probably would not want to make it one quick thrust on an icy helipad.
Taxiing from the helipad to the runway was basically as in any wheeled helicopter—put in a little power, push the cyclic forward a bit to get it started, then steer with the pedals while using just a bit of cyclic pitch input during a turn to keep the aircraft level. We taxied out to the active runway, where Barnes started by showing me that the D can do a 360-deg. turn around its own axis.
Barnes picked the aircraft up into a hover while I checked power levels. Like most of today's glass cockpits, the Thales system uses a power limit (PL) indicator to monitor torque, inter-stage turbine temperature and gas-generator speed, displaying the one closest to its limit. In the S-76D, the gauges are based on 100%. Only the rotor rpm is different, being rated to 107%. It was originally set at 100%, but rotor speed was increased to 107% of the original rpm on one of the earlier S-76s, so that is where the limit rests.
If the aircraft enters a non-standard condition, such as an engine failing, the scale drops the green line to the percentage limit for that condition, with the time limit at that condition shown in amber.
As expected, at no time during our hovering exercise did the power limit get close to 100%. The aircraft automatically computes its own weight and at 9,494 lb., a stable hover required 62% torque. The aircraft is limited to 35 kt. sideward and rearward flight, which required only 79-80% torque for each. There was no problem with tail-rotor authority with the tail stuck into the wind as the wind was minimal. There was a slight increase in vibration, but that was all.
I found the aircraft to be a bit tight on the controls during standard hover maneuvers, and Barnes recommended using the force-trim release buttons to loosen it up. That helped a lot—although it then seemed a bit too sensitive. But, like pilots, every helicopter has its idiosyncrasies. You get used to them, and it did not take long to figure out how to handle this one.
By engaging the autopilot's velocity hold (VHLD) function, hovering can be accomplished by simply “beeping” the aircraft to where you want it. Placing a green circle at a point on the runway map display—with zero airspeed dialed in—puts the aircraft in a stable, hands-off hover. Repositioning the green circle repositions the aircraft. Changing the heading while maintaining zero airspeed turns the nose of the aircraft without changing its position. If the helicopter is at a stable hover and the wind blows it off its hold point, it will recalculate and return to the original position.
Takeoffs, both normal and maximum performance, were without drama. Climb out for a normal takeoff was at 75 kt., 750 ft./min. at 59% torque. The aircraft monitors outside air temperature and weight to determine best rate of climb, for both normal and one-engine-inoperative climb outs, and indicates that rate with a white triangle on the airspeed indicator.
We climbed to 2,000 ft. holding 100 kt. in the climb, then accelerated to 130 kt. and engaged vertical and area navigation to hold us on a steady course and altitude. An assortment of charts is available to the pilot on the digital map display. Barnes pulled up a vector chart to demonstrate the aircraft's flightpath control. He simulated a large storm cell directly ahead, then used the cursor control to mark a point to the right of the storm. The aircraft automatically turned toward that mark. Remarking our original aimpoint, Barnes said when the aircraft reached the new waypoint, it would automatically track to the original point, thereby avoiding the storm. This was all done by using the trackball and pushing a single button.
The Thales system offers XM Weather for on-screen satellite weather service as an option. Also available as an option are automatic dependent surveillance-broadcast (ADS-B) and GPS precision-approach capabilities. The baseline weather radar for the D model is the Honeywell Primus 660.
To test its basic flight characteristics, we took the aircraft up to its 155-kt. never-exceed speed (Vne), pulling only 50% torque. Barnes notes that, while Vne is 155 kt., because of the efficiency of the Pratt & Whitney Canada engines, cruise speed for best range is 154 kt. He also notes that Sikorsky already has the data to increase the Vne limit “down the road.”
For steep turns, Barnes “beeped” the aircraft over into a 30-deg. angle of bank and put it on automatic hold. The aircraft held steady at 110 kt. pulling only 40% torque. The 30-deg. bank angle is the maximum for automatic hold. That can be manually overridden to allow up to a 60-deg. bank, at which point a warning voice announces that you are reaching power limits.
Virtually all aspects of the flight can be performed using the push-button controls on the autopilot control panel (ACP) located directly behind the trackball housing on the center console. Barnes programmed in a flight plan for the approach back to Sikorsky's field, then coupled in the heading, airspeed and altitude using the pushbuttons on the ACP.
The aircraft flew itself to the ILS, then automatically transitioned to ILS mode. The navigation, glideslope and deceleration commands are coupled to the primary commands in the autopilot, allowing the aircraft to fly hands-off down to the runway.
Repair work was being performed on the approach end of the runway, so we had to take over manually to land. But with VHLD engaged and the trim switch pushed in, altitude above the runway and a spot on the runway can be programmed in to take the aircraft to that altitude, fly down the runway to the designated spot and come to a hover, still hands-off. The aircraft also automatically decelerates. Once below 130 kt., the gear can be dropped and the aircraft set on the ground.
The flight lasted 1 hr. 10 min., with a fuel burn of 650 lb. Sikorsky lists a fuel burn of 578 lb. per hour in its charts, so our 557 lb./hr. was very close.
As for transitioning into the D model from an earlier S-76, Barnes says a new pilot could be using the aircraft “within a few hours” and that, after one or two flights, “they are doing things on their own.” I can see why . . . it is just a matter of learning which buttons to push.
To find out more about Sikorsky's S-76 family and see video of the latest -D model in flight, tap here in the digital edition of AW&ST, or go to AviationWeek.com/video
|2 X Pratt & Whitney|
|Takeoff (5 min.)||1,050 shp|
|OEI (30 sec.)||1,241 shp|
|Max. Gross Weight||11,700 lb.|
|Max. Cruise Speed||155 kt.|
|Max. Range||441 nm|
|HIGE Ceiling||9,700 ft.|
|HOGE Ceiling||5,000 ft.|
|OEI Service Ceiling||7,550 ft.|
|Source: Sikorsky Aircraft|