Bigger brother

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PETER GRAY / STRATFORD, CONNECTICUT

Having overcome problems with the S-92's design, Sikorsky has created a machine that has a high level of safety features and is also pilot friendly

US certification of the S-92 in December was a major milestone for Sikorsky, as the medium helicopter is the first new civil rotorcraft to be developed by the US manufacturer since the smaller S-76 was certificated in 1978.

We evaluated the S-92 early in the test programme (Flight International, 25-31 July 2000). Handling discrepancies noted then were an increase in vibration passing through 150kt (278km/h), nose-up pitching at around 40kt and some fishtailing. Sikorsky has since made major changes to the aircraft.

The modifications include relocating the fixed horizontal stabiliser from the left to the right side, removing a small fin that was interfering with the tailrotor airflow, and reshaping some of the tailrotor area. These changes should alleviate the pitching and fishtailing problems. Replacing the main rotor vibration absorption system with bifilars (weights on the rotor hub) should help reduce vibration.

In 2000, Flight International flew the number three S-92. This time Sikorsky made available the number four aircraft. Of the five helicopters built so far, the first two have completed their tasks and are now engineering assets. The three aircraft that are still flying are engaged on a continuing programme of development and test work designed to lead to more advanced certifications.

Other changes made since the 2000 evaluation affect the cockpit and include a revamp of the instrument panel with four Rockwell Collins flat-panel multifunction displays (MFDs).

Coupled autopilot

Improved Martin Baker pilot seats offer adjustable lumbar and thigh supports and armrests, but were not installed for this flight. Sikorsky has also completed development of the full-function "coupled" autopilot.

When the S-92 enters service next year, it will be supported by an electronic maintenance system, to be managed eventually via the internet. The Goodrich health and utilisation monitoring system tracks 125 parameters, and this information will be used by Sikorsky to keep a history of the monitored parts.

When Flight International first evaluated the S-92, Sikorsky was only 300h into the 1,400h development and certification flight-test programme. Now the company has accumulated over 1,600h.

The public-transport Category A horizontal take-off technique has been developed, with the vertical procedure still to be completed. The horizontal technique allows a maximum-weight (11,870kg [26,150lb]) take-off at sea level up to 35°C (95°F). Those not required to operate to Category A, but still wanting a safe single-engine rate of climb, can take off up to 42°C. This will give a 100ft/min (0.5m/s) climb rate.

The all-important height versus velocity chart has been produced. A training weight, altitude and temperature graph has also been developed. This allows the use of the full-authority digital engine control (FADEC) training mode to safely and realistically simulate engine failures at any temperature, altitude and weight, at any time, without going above maximum continuous power on the "good" engine.

Graphs have been produced for hover performance in and out of ground effect, but the "hot, high and heavy" trials have yet to be completed. Sikorsky says trials so far show a marked improvement over predicted performance, so the graphs have been redrawn to reflect this. The improved performance, plus reshaping of the sponson fuel tanks to accommodate an additional 227 litres (60USgal), has increased range, endurance, hover capability, maximum weight and payload.

Power available from the twin General Electric CT7-8 turboshafts has also increased - another performance enhancement. At low altitudes the power available is FADEC-limited to the gearbox maximum of 3,100kW (4,170shp). At higher altitudes the engines' full potential can be used, providing the pilot with excellent power throughout a large part of the flight envelope. At the maximum Category A gross weight of 11,870kg the aircraft will hover in ground effect on a standard temperature day at 11,320ft, and at 8,123ft on an ISA+20°C day. A hover out of ground effect can be achieved at sea level up to 45°C. Gross weight for the military S-92 will probably be 12,850kg, says Sikorsky.

Sikorsky's pilot for Flight International's evaluation was engineering test pilot Ronald Doeppner. It was a cold day, with temperatures below freezing, and the wind was a mild 8-10kt, gusting to 14kt.

Ongoing improvements

During a brief walk around, Doeppner pointed out the design changes. The existing, hard-to-see intermediate and tail-rotor gearbox oil-level sight glasses will be expanded and made more visible. There will also be a cockpit caution light.

A small window in the nose houses a camera for recording main-rotor track and balance. The pilot can activate it during a normal revenue flight and inform the technicians of the result. They can then make small adjustments on the ground and the pilot can check the results on the next flight. As well as the flotation gear at the front of the aircraft, there are two floats in the tail cone, giving a level attitude should the aircraft have to land on water.

One of the cockpit design philosophies was to create a pilot-friendly workplace, for reduced workload and safer operations, so the number of switches has been kept to a minimum. The new MFDs help, as they provide the information required in an easy-to-interpret format. The pilot can call up whatever information is required as and when it is needed.

Driven by two independent computers, the displays are interchangeable. The normal configuration for a commercial flight is for each pilot to have primary flight, multifunction navigation and engine instrument and caution/advisory system (EICAS) displays. Warning, caution and advisory panels have all gone. Instead, systems and functions are monitored electronically. If all is well, the pilot sees nothing. If there is a malfunction the pilot needs to know. For example, if external power is connected, a warning appears in a box on the EICAS.

Cockpit warnings

If the problem is serious, the master caution will illuminate. If it is very serious, a female voice will alert the crew. The design integrity of the system is such that it is almost impossible to shut down the wrong engine in the event of a problem.

The displays can be tailored to the task to be flown. The primary flight display (PFD) has round dials, instead of the previous vertical strips. Round dials give the pilot a better idea of the rate of change. Also, in the case of the vertical speed indicator, the needle is horizontal when the aircraft is straight and level. Any climb or descent can be seen at a glance.

Weather radar and flight plan information can be overlaid on the navigation display. Provision is made for a traffic collision avoidance system and enhanced ground proximity warning system. There is a lot more information available than the pilot, or indeed the technician, can access. Any exceedences are recorded.

Doeppner demonstrated the standard check of the automatic flight control system, which is comprehensive and takes just a few minutes. I strapped into the five-point harness, adjusted the seat and controls and watched Doeppner do the start-up. There was no need for a checklist, although commercial pilots should use one. He performed a neat flow around the cockpit, switched on the fuel, moved the overhead speed select lever, pressed the starter button and sat back and watched the FADEC-controlled automatic start.

Smooth take-off

The FADEC monitors and limits T4 (combustor exit temperature). As the second engine was started and the rotor wound up to normal speed, all the warnings on the EICAS gradually disappeared, leaving us a clean, uncluttered presentation. For search and rescue launches the aircraft can be airborne in less than 2min. I taxied out to the short runway remembering to steer using the pedals and not by banking the powerful main rotor, as required on some medium-size helicopters. A pull up into a steady, accurate hover was uneventful. Power required and power available were easily calculated using the small round torquemeter on the PFD. Hover, sideways and backwards manoeuvres demonstrated the excellent, crisp handling and the huge amounts of engine, main and tailrotor power available.

We climbed up into the clear blue sky for some upper air work. The efficient heater kept our feet warm, a rare event in helicopters. Maximum continuous power gave 155kt indicated airspeed (IAS), 152kt true airspeed (TAS) - few helicopters Flight International has tested have achieved more than 150kt.

Setting torque to 70% at 4,000ft produced a healthy 140kt IAS, 135kt TAS. Fuel flow was 600kg/h (1,330lb/h) (maximum internal fuel capacity is 2,329kg). Increasing torque to 74% produced 138kt TAS. The fuel section of the EICAS panel gives an excellent presentation of the fuel system. Doeppner pointed out a small arrow on the PFD showing wind direction and speed - useful information for all helicopter pilots.

The automatic flight control system has various levels of stability augmentation. While it is unlikely there will be no artificial stabilisation, as there is almost overkill in the redundancy of the system; I had no difficulty flying the "raw" aircraft accurately, with none of the pilot-induced oscillation experienced in other helicopters.

There are cyclic, lever and pedal trims, which hold the controls in position, but provide no stability augmentation. For that, there are two independent systems, each of which has 5% control authority and keeps the aircraft where it is trimmed, hands and feet off. After displacing attitude fairly abruptly in pitch and roll to test the system, the helicopter returned quickly to its datum point with no overshoots.

Autopilot stability

The basic autopilot provides some stability enhancement, and becomes a true autopilot when it is coupled to the flight director. The pilot can then use the mode select box to set the flight path, sit back and watch the aircraft fly where directed. As the coupled automatic instrument landing system (ILS) capability is still under development, Doeppner flew a manual ILS approach. The dual visual presentations of glide slope and localiser on the PFD are excellent. Following the circle on the PFD's horizontal situation indicator keeps you on the localiser.

Although we were not coupled, Doeppner flew the approach almost hands off, using the collective trim to make minor adjustments to the glide path. Once development is complete, the system will carry out an automatic ILS approach and decelerate to 60kt at decision height. Pressing the "go around" button during a 120kt descent at higher altitude caused the aircraft to climb and slow to 80kt. This could be a life saver if the pilot is flying low level in murky conditions over the sea and gets disorientated, says Doeppner. The flight-control system will be used to develop search and rescue techniques such as automatic hover and programmed search patterns.

The FADEC training mode is excellent, giving the pilot all the sensations of single-engine operation, including instrument indications and actual rotor droop, but not allowing power from the "good" engine to exceed maximum continuous. There are built-in safety systems should the pilot start to droop the rotor excessively or experience a real loss of power. If the situation gets out of hand, the training captain can restore both engines to normal by selecting the "off" switch.

The in-flight power assurance check gives the pilot an instant indication of engine state, plus a record of the last 10 checks. Our results showed a healthy margin above minimum specification, despite the hard use these engines have been subjected to during development.

The in-flight rotor track and balance check was equally simple to perform.

Power recovery autorotations were then performed, the FADECs producing impressive engine acceleration at the bottom of the autorotation. They seemed to anticipate what was going to happen and applied the power early but fast.

Problems overcome

During the previous evaluation, while accelerating through 150kt towards the 165kt never-exceed speed, there was a noticeable increase in vibration. This time around there was nothing at 150kt, and only slight vibration at 165kt. Repositioning the horizontal stabiliser and the other modifications seems to have solved the problem. I did not notice the fishtailing reported in 2000, but Doeppner said he could feel some.

Doeppner demonstrated several horizontal Category A take-offs, some with simulated engine failures using the training system, some leading to a landing back, some with a continued take-off. The technique is benign and uncomplicated. The aircraft's excellent single-engine performance helps, as does the FADEC, which will allow the pilot to temporarily overtorque the good engine to escape a potentially hazardous situation. There are generous overtorque limits.

A vertical Category A take-off is more interesting. The pilot selects "Cat A" on the PFD, which brings up an expanded radar altimeter presentation with an adjustable bug for the decision height. The aircraft is then pulled up until it is light on its undercarriage, then maximum take-off torque is applied. At decision height the aircraft is rotated. Using this technique instead of a more sedate pull-up is more efficient, Doeppner says, and will probably allow an extra passenger to be carried. The reject take-off technique has not yet been developed.

Doeppner pulled an engine back to flight idle using the speed select lever several times during our high-level work, often at high speed, but the main rotor drooped little and recovered quickly every time. We never exceeded maximum continuous power on the other engine, and could have flown on at a respectable speed at 120% torque all day.

Back at the field, a wheels-up approach demonstrated that the female voice alert is a good attention-getter. A steep approach showed that Sikorsky has given the pilot sufficient downward and forward visibility. The nose had to be pointed only slightly to the left to keep the intended hover and landing spot in view all the way down. This bodes well for the standard steep approach that is used for offshore platforms.

At this point, the master caution light came on. We were getting low on one of the two fuel tanks, but we had finished our schedule. Taxiing back, Doeppner demonstrated how the aircraft can be turned in a circle by applying enough pedal to turn the nose wheel through 90°.

The S-92 has been certificated by the US Federal Aviation Administration for visual flight rules operations, and other clearances will follow. Sikorsky is attempting the first "harmonisation" certification, meaning there should be just one inspection to clear the aircraft for FAA, Canadian and European Joint Aviation Authorities Part 29 approval.

User-friendly design

Designing for joint FAA/JAA certification means the safety features of the S-92 are some of the most all-embracing seen in a helicopter. And despite its size and complexity, with two engines, three hydraulic systems, retractable undercarriage, and complex transmission, automatic flight control and electrical systems, Sikorsky has succeeded in making the S-92 user-friendly.

Military operators may want to fly the helicopter single pilot and the ease of aircraft management, particularly in an emergency, should safely allow this. The lone pilot can reach everything from the right hand seat. Maintenance staff should appreciate the accessibility of all areas of the helicopter. Operators should be impressed with the direct operating costs, provided Sikorsky's predictions come true. Pilots should enjoy the flying characteristics, spacious cockpit and having adequate and engine and rotor power available.