Thursday, August 1, 2013
Sikorsky S-76D: Power and Innovation
Rotor & Wing puts the latest version of Sikorsky’s S-76 line through its paces during a demo flight at the OEM’s flight test facility near West Palm Beach, Fla.
N767J is the seventh S-76D to roll off the assembly line. Still in its green primer, this aircraft is assigned to Sikorsky’s facility in Jupiter, Fla. as a platform for additional research and development.
For many years, the aviation community has treated the helicopter segment like a child that is only worthy of hand-me-downs, while multi-million dollar jet aircraft received all of the fancy new things. Those “hand-me-downs” were often the less-advanced avionics and flight control systems. But when you consider that airplanes outnumber rotorcraft by around 25 to 1, it’s no wonder engineers poured most of their efforts into the fixed-wing market. Times are changing, however, and helicopters are finally getting the brand new, top-shelf technology once reserved exclusively for jet transports. Sikorsky’s new S-76D is one of those helicopters.
The S-76D embodies an impressive list of technology that was formerly found only on the most sophisticated corporate planes and airliners. Some of those technologies were fairly easy to migrate into rotorcraft, while others caused huge problems for the avionics engineers who had to make airplane technology work aboard machines that don’t always fly where their noses are pointed.
The Thales avionics team in Piscataway, N.J. joined with Stratford, Conn.-based Sikorsky Aircraft to marry high-end electronics to the latest variant in the line of the world’s first, purpose-built, executive transport helicopter: the S-76D. And this marriage is made all the better, considering the performance improvements the D model has over earlier 76s.
The initial version of the S-76 was first flown in March 1977, with actual sales beginning in 1979. Since then, more than 815 of the medium twin-engine aircraft have either been produced or converted into the S-76A, A++, B, C, C+, or the S-76C++. As of this writing, they have all accumulated more than 6.2 million flight hours.
The assembly and completion plant, located in Coatesville, Pa., will churn out four major versions of the S-76D: offshore, VIP, emergency medical transport, and search and rescue (SAR). Eleven aircraft in the first stage of the D-model line have already been sold.
But what makes the D model – the aircraft I have been hearing about for several years now – so special? To find out, I took a trip to Sikorsky’s remote engineering and testing facility near West Palm Beach, Fla. to see. Upon my arrival at the William P. Gwinn Airport (06FA) in Jupiter, Fla. – the privately owned airport used exclusively by Sikorsky and its parent company United Technologies – I saw several UH-60 Black Hawks, and at least one CH-53 and an S-92 on the apron and in the hangars. Most had test equipment installed on them, but all were surrounded by technicians and pilots.
Inside one the buildings adjacent to the flight line was a conference room, which is where I met with Leon Silva. As the program manager for the S-76D, it is his job to shepherd that aircraft from concept to retirement. For all intents and purposes, the Sikorsky S-76D is his baby.
Also in the conference room were the chairman and several lead directors of National Helicopter Services, a half-private-, half-government-owned helicopter operation that handles offshore drilling support and emergency medical transports in Trinidad. They had expressed an interest in purchasing three D models, and came to learn more about them. We would all be learning together.
Silva began the briefing by saying that the S-76D has very few external changes when compared to its predecessors. “The outside of the aircraft is as fresh today, in my opinion, as it was back in the 70s,” he said. “It’s a beautiful aircraft.” (He got no arguments from anyone in the room.)
On the outside, the D model sports a new set of main rotor blades. The 44-inch diameter system is comprised of four, swept-tipped wings similar to the ones on the UH-60M Black Hawk and the S-92. Each blade has two trim tabs for fine tuning their track, and deicing gear to keep them clean in bad weather.
In the back is Sikorsky’s Quiet Tail Rotor technology, which greatly reduces the aircraft’s noise signature by slowing down the RPMs on the anti-torque rotor. And like the main rotor blades, the four-bladed tail rotor is manufactured with a deicing system.
Driving the rotors on the S-76D are a pair of Pratt & Whitney Canada PW210S turbine engines, which deliver a robust 966-shp per plant when cruising at maximum continuous power. (That’s a hefty 14 percent increase over previous S-76 engines.) If you need some extra takeoff power, 1,077 shp is available for five minutes. And if range is what you’re interested in, the D model will carry you 348 nm without tapping into your 30-minute fuel reserve.
|The S-76D is equipped with four, 6 x
8-inch Thales displays. The information
they provide is managed by the two
cockpit control devices (circled) located
on the center console. This technology
comes directly from the Airbus 380
All of that power and speed can introduce a lot of unwanted vibrations throughout the airframe. To combat this, the engineers developed what they call “active vibration control” or AVC, a system that introduces just the right amount of energy into the airframe to cancel out unwanted vibrations.
The S-76D is equipped with a data collection system that allows Sikorsky engineers to monitor the health of the entire D-model fleet. So, let’s say engine #1 aboard serial #0009 is experiencing slightly lower oil pressure than the day before. Sikorsky’s 360 Live Support Center will see it, notify the operator, and query its database to determine if the problem is isolated to that one powerplant, or is being experienced elsewhere in the worldwide S-76D family. Armed with that information, technicians can address a variety of issues before they become emergencies, and even identify trends.
While there is no argument that more power, speed and endurance are good things to have, and that they alone can make the S-76D an attractive airframe to any operator in need of a ship that size, it also has the latest avionics gear that was once only found in high-end airplanes.
Gone is the deep, shadowy, analog instrument panel that had been the trademark of the S-76 for decades. The Thales Advanced TopDeck avionics suite – a digital, computer-driven system that borrows several features from the cockpit of the double-decker Airbus 380 – is now standard aboard the D model, and rests in a new instrument panel that seems easier to consult.
The suite begins with four “smart” display panels. In the standard layout, there is one 6 x 8-inch LCD primary flight display (PFD) on each outboard end of the instrument panel. These show standard flight information, such as attitude, altitude, airspeed, etc. on the top half; plus navigation and component RPM information on the bottom half. Next to each of those screens is a 6 x 8-inch LCD multi-function display (MFD). The list of what can be displayed on those monitors would need several pages to cover, but can include engine data, information from the Honeywell MK XXII EGPWS, digital maps, and weather radar, just to name a few. And if you need something that’s exclusive to your own operation, like a library of oil platforms or helicopter route charts, they can be programmed into the system with little effort.
On the center console, just aft of the spot where it meets with the main instrument panel, are two odd-looking lumps, if you will, that came right out of the Airbus 380. They are called Cockpit Control Devices, or “CCDs” for short. They look like a throttle handle on a fighter jet, except they don’t move. They are cleverly designed to put a variety of buttons at the pilots’ fingertips when their hand is resting on it. If you need to expand the range on your weather radar, the +/- switch is near your index finger. Do you need to move a waypoint? Your fingertips are resting on a trackball that will do that.
I didn’t ask Silva if the engineers were thinking about this when they designed the CCDs, but it occurred to me that an additional advantage to having them is that they give my hand more stability when attempting to press a button. After all, who hasn’t had a hard time trying to zero-in on a small push button while being bounced around in turbulence? The CCD keeps your hand steady, puts needed switches nearby, and is even a comfortable alternative to holding a flight control.
I did have one concern about all of the features available in the TopDeck suite, though: Considering the number of functions and combinations of information the MFDs can provide, can a pilot find something without having to spend a lot of time looking down? Greg Barnes, the lead test pilot for the S-76D program, was way ahead of me.
“[During the initial design phase] I told Thales that I should be able to get anywhere I wanted in just two button pushes,” recalled Barnes. “A complex cockpit should not be complicated to operate.” Thales made sure to honor that.
The S-76D is equipped with one of the most sophisticated autopilots flying today. Thales calls it a “dual duplex system.”
Dual duplex means that there are two sets of two processors – command processors and monitor processors – acting independently of one another. The two sets of command processors compute the commands for the servos independently. The job of the monitor processors is to check the results of the command processors. If they agree, they send a signal over a bus to the series actuators, asking them to move a flight surface to a certain position. Inside of the series actuators are a command processor and a monitor processor, too. They receive the command, process it, and check the results with one another. If they agree that the math is correct, they honor the request, and move the flight surface to the position requested by the autopilot. They then check the position of the actuator, and report it back to the computer. If any processor disagrees with any other processor, they will re-center the actuator, and disconnect the autopilot.
The bottom line with the dual duplex autopilot system is this: If you’re in duplex mode (i.e., both autopilots engaged), the chances of the 76D getting into a hard-over attitude are less than one in a billion. And even if that outside chance should occur, you will experience a half-degree bobble in the aircraft, receive a caution light that reads “AUTOPILOT 1 DISCONNECT,” and the autopilot will decouple before you can blink.
The autopilot in the D model is also very accommodating to a pilot’s special needs. Let’s say you were becoming concerned about your fuel burn. Just tell the autopilot that you need maximum endurance. The system will compute all of the necessary conditions, bring the ship to that speed, and make the appropriate adjustments should anything change.
Like any other aircraft, maintaining proficiency in the S-76D is critical. With that in mind, Sikorsky took advantage of the engine’s technologies, and developed a more prudent way to simulate one engine inoperable (OEI) situations. Instead of retarding one engine to 91 percent, Pratt & Whitney does what they call “OEI divided by two.” That means that when you activate the OEI switch, the targeted engine will roll back, but then both it and the “good” engine will stabilize at half-power. The result is a realistic training scenario – complete with instruments that show one dead engine, and reduced performance – but an actual condition that won’t overtax either powerplant. (If you want to see your true power state, a real-world engine page can still be pulled up.)
By the way, according to Barnes, the aircraft will fly away on one engine “...if you have any kind of speed or altitude.”
But enough conference room talk. It was soon time for a quick lunch, and a flight in the Sikorsky S-76D.
N767J is the seventh D model to come off the assembly line. It was flown down to Jupiter from the Coatesville assembly line still dressed in its green primer. Because it is serving as a test bed for continued evaluation and improvements, there were no trim panels to conceal the guts of the aircraft. The instrument panel, however, had all of the systems described during the briefing, plus some orange test cables running over the surface.
As I climbed into the right seat, the cockpit seemed slightly more spacious than other 76s. Barnes told me that in an effort to increase the pilots’ field of view, the engineers relocated the landing gear knob to a higher spot on the instrument panel, then shaved 1-1/4 inch off the bottom of the panel. They also cut the size of the overhead panels in half.
While Barnes was cranking the aircraft, I spent a little time getting to know the remodeled flight deck.
As with legacy S-76 aircraft, I had no trouble finding a comfortable seating position behind the cyclic – the same cyclic they use in the S-92. All switches, especially the ones built into an around those CCD hand rest controllers, were easy to reach. The bright display screens were a snap to consult, even with my orange flight suit reflecting on the screens. Head, knee, and elbow room were more than adequate for my 70-inch frame.
Cabin noise was understandably high. But that was only because test aircraft don’t fly with interior trim panels and sound insulation. Completed 76s can be quiet.
At the time of takeoff, the winds were reported as 270 at 14, a few clouds above 5,000 feet, and unlimited visibility. The temperature was 28°C, the altimeter read 30.21, and Barnes estimated our takeoff weight to be around 10,200 lbs.
|The S-76D’s Thales displays offer a limitless
choice of information. The multi-function
display (left) can show anything from aircraft
systems data to overlays of operator-specific
information, such as company landing zones.
The primary flight display (right) shows
standard piloting information and cues.
The autopilot, as advertised, was a dream when coupled to the Thales gear, and got us to Palm Beach County Glades Airport (KPHK) 21 miles away with no trouble. It even allowed me to steer around a pretend thunderstorm by using the trackball on the CCD to reposition my waypoint.
Since I stink as an instrument pilot, I was pleased to see that the navigation system and autopilot got me back to Gwinn and started me down to the runway surface with pinpoint precision. In fact, Barnes was even able to bring up an airport diagram, and show me an icon of our ship superimposed above our exact spot after landing.
I didn’t do any emergency OEI or autorotation work in the S-76D, but I did fly the pattern a few times, and sample a variety of takeoff and landing profiles, including a run-on landing. No problems. No issues. And for once, I didn’t have to struggle to see over the glare screen like I’ve had to do in earlier models of the S-76.
It would take an entire book to cover all of the features and characteristics of the Sikorsky S-76D. But from what I was able to sample, this aircraft will help place helicopters on the same playing field as its fixed-wing corporate and airline counterparts. The D model has brains, muscle, and what can best be described as a pilot-friendly spirit.
Related: Airframe News