Peter Gray/WEST PALM BEACH
One potential customer called Sikorsky Aircraft's S-92 helicopter "a flying box". So when Sikorsky invited Flight International to fly one of its five test S-92s, I was intrigued to see how this "box" performed.
In creating a replacement medium helicopter, Sikorsky set about designing an aircraft that could be used for military missions such as long-range search and rescue, tactical troop transport, maritime patrol and ship-based anti-submarine warfare. It was also looking at civilian operations like worldwide oil support in even the worst conditions, for example in the North Sea and northern Canada, and inter-city and VIP transport.
The result is the S-92, which can be configured for civil use with 19 seats, or as a 22-troop seat military version. The troop seats can be folded quickly and easily to make way for freight on the reinforced cabin floor, which can also be equipped with rollers, remote freight positioning control and winch.
And the company's confidence in the quality and characteristics of this aircraft - previously referred to as a Helibus - is high at this early stage in its development.
Test aircraft
Flight International flew aircraft number three of the five test aircraft. The first is a ground tiedown vehicle, numbers two and three are now flying, numbers four and five are under construction, and number six (to be completed in mid-2002) will be the first production helicopter for Cougar, a Halifax-based company that operates offshore Canada.
Just over 300h have been flown in a 1,400h programme. As well as meeting all the demands of the above operations, the design philosophy required a range of 740km (400nm) with North Sea instrument flight rules reserves (a single-engine offshore diversion); lower operating costs than its competitors; and the ability to change configuration quickly. Not least, Sikorsky set itself high reliability targets.
Direct maintenance costs are $800 an hour, lower than all its rivals, says Sikorsky. It is aiming for a direct total operating cost of $2,200 per hour.
Sikorsky used or modified several well-tried Black Hawk components and systems for the S-92, such as the main gearbox and ballistic tolerant flight control runs and spring quadrant, which allows for full tail rotor control in the event of the loss of a control cable. But the S-92 is not a derivative of the Black Hawk. The design started with a clean sheet of paper. However, it makes good sense to use previous design concepts where applicable.
My pilot was Bob Spaulding, Sikorsky's chief S-92 project pilot. The outside air temperature of 30°C gave us a density altitude of 2,200ft (670m) at our airfield pressure altitude of -150. The wind was light and variable. Despite all the telemetry equipment, ballast, full fuel and a flight test engineer on board, we were short of the normal proposed Category A maximum gross weight of 11,570kg (25,500lb) by 336kg - 35min of average fuel burn at maximum weight. The Portuguese want long distance with extra fuel tanks to increase range, so Sikorsky successfully demonstrated the aircraft at 13,610kg, for a range of 1,480km. Typical empty weight will be about 7,030kg, says Sikorsky.
Guts of the aircraft
The white-painted aircraft certainly did not resemble a box as we approached it. A remarkable and unusual feature of the S-92 is that you can climb all over this large aircraft without using external platforms.
We clambered up the side and opened all the top panels to see the guts of the aircraft. Spaulding went along the top of the tail cone - also box-shaped - to point out the retractable steps up the side of the vertical stabiliser to reach the tail rotor.
The auxiliary power unit sits behind the engines and, after using the only onboard battery to start it, is available to provide electrical and hydraulic power for full and free testing of the controls and air pressure for the engine starter motors. The General Electric CT7-8 engines, a development of the well-tried and extensively used T700 model, are easily accessible and birdstrike-tolerant. The main gearbox limits are less than the engines' power available, which means that full gearbox limits can be maintained by the engines at high outside air temperatures and altitudes.
As is now usual, the gearbox will run without oil for at least 30min. The single-block titanium, spindleless main rotor head needs no lubrication and its maintenance is on condition only or at 50,000h. The main rotor blade tips are swept aft 30° and have 20° of anhedral, reducing the compressibility of the effects at the tips of the advancing blades which, in all helicopters, travel close to supersonic speed and generate considerable drag. The anhedral tips also contribute to improved hover performance.
Empennage modification
The standard rear ramp can be lowered and raised at the ramp location or from the cockpit. This is where the baggage will go in civil operations; removable bulkheads will separate this compartment from the cabin. The tail rotor is well out of harm's way for the ramp loaders. Because of some aerodynamic anomalies such as a slight pitch up at around 40kt (75km/h) and fishtailing, the empennage will be modified. It is rare for any prototype not to have the rear end modified as a result of flight test. Helicopter design is not as precise as fixed-wing.
The two large sponsons hold all the fuel, taking it away from under the cabin floor (a good safety feature in the event of a crash landing) and will also house flotation gear for offshore operations. The fuel lines have breakaway fittings and military versions can have self-sealing, ballistic-tolerant fuel cells.
The cabin is 1.83m (6ft) high, 2m wide and, with the recent addition of a 400mm (15.75in) fuselage plug, nearly 6.1m long. This plug also allows a 1.27m wide forward cabin door, sufficient for winch operations. A second standby winch will be provided if requested. Access is enhanced by a full cabin-width rear ramp.
In the cabin there are enough windows - all jettisonable - for each passenger to have nearby access. They will have three- or four-point harnesses in crash-resistant seats. The cockpit windows are also jettisonable. In the event of a crash landing with high g forces, the undercarriage is designed to absorb most of the impact, the fuselage and seats the rest. The heavy overhead parts, such as the main gearbox, are designed to withstand 20g forward and down, 18g laterally and 10g up.
Spaulding offered me a choice of the left or right seat. I chose left because the right seat had slightly restricted visibility due to additional telemetry instruments. The comfortable seat is fully adjustable for reach and height, and the proposed new seat will have extras such as arm rests and adjustable lumbar and thigh supports. It is good to see helicopter manufacturers spend money on improved pilot seats. The pedals, too, can be swept forward and aft. So, after adjusting the five-point harness, I had no trouble finding a comfortable position.
Sikorsky has placed great emphasis on good outside visibility, so I found I could see over the nose and down by 25°. The intentionally huge windscreen, chin windows and the offset instrument panel allowed excellent look-out. The rest of the cockpit has also been ergonomically designed to be pleasingly functional. The present displays and avionics will be replaced with Rockwell Collins Pro Line equipment.
The Collins displays will revert to round-dial presentations instead of the vertical strips. The circumference of a round dial is normally greater than the length of a vertical strip, giving the pilot a better presentation of rate of change. However, the schematic layout of the whole aircraft as represented on two of the four multi-function displays (MFDs) will be retained. Instead of having caution and warning lights together on one panel, they are distributed geographically on the engine instrument and caution advisory system (EICAS) display. For example, a tail gearbox chip light will appear on the tail end of the plan representation. I liked the logic of this, as I did with many other good design features.
Part of my evaluation is to check how benign the aircraft is regarding pilot interpretation of what is happening and pilot management.
Cockpit checks
The engines are Hamilton Sundstrand FADEC (full authority digital electronic control) controlled and managed. After a brief but logical flow check through the cockpit, both engines were started and the rotor wound up to its best rpm (NR) of 105%. All the colours on the MFDs turned to green and we were ready to go. If in a hurry, all this can be achieved in about two minutes. I taxied out to the runway using the toe brakes and tail rotor pedals to steer, keeping the powerful main rotor horizontal. I was able to pivot the aircraft easily around one of the main wheels using differential braking. My first hover, with both digital automatic flight control system (AFCS) and stabilisation augmentation system (SAS) on, was uneventful.
Presentation of power used and power available was excellent, as Sikorsky says it will also be on the Collins displays.
I still had more right pedal available during sideways flight to the left up to 35kt. I ran out of left pedal during sideways flight to the right at 46kt. These manoeuvres demonstrate the large amount of tail rotor power available. At 35kt rearwards flight, the fuselage attitude remained at a constant nose up, with no tendency to tuck forwards and down. Spot turns at 30°/s in both directions were benign, so I handed over control to Spaulding to demonstrate the maximum rate of turn of 59°/s, another good demonstration of tail rotor power.
Crosswind and downwind landings were problem free and easily controllable. There are no apparent wind sectors that might limit performance, although the aircraft still has to do the hot, high and heavy trials.
While still fairly heavy, I transitioned into forward flight, climbed up rapidly to a density altitude of 3,000ft, pulled maximum continuous power and settled down to wait for the maximum speed in level flight. Our weight was now 10,900kg. We got 153kt indicated airspeed (IAS), which equated to 163kt true airspeed (TAS) at this density altitude. Of the 30-odd helicopters I have evaluated for Flight International, few have achieved more than 150kt at maximum continuous.
Vibration levels
We next went for the VNE of 165kt, which we achieved in a slight dive. Above 150kt, the vibration levels increased, but not uncomfortably so. Sikorsky is working on this and is confident it can reduce the levels. No additional vibrations were produced by 30° turns in both directions. After levelling off and slowing down slightly, we rolled quickly from 45° of bank from one direction to the other. There is lots of control power here, the rate of roll being impressive for such a large aircraft. As is usual, the vertical door pillars get in the way of the look-out in the direction of the turn, but not excessively so. I just moved my head around it.
Establishing 120kt, I asked Spaulding to close one throttle, warning him I would do nothing unless NR approached the lower limit. In reality, the remaining engine would automatically go to 30s maximum contingency power, the automatic countdown would begin and the pilot can then reduce to 2min contingency power with a button on the collective, with another automatic countdown starting, and then to 30min power.
So as not to deliberately use contingency power, Spaulding arranged our remaining engine to go to 30min power, an even more severe test of NR. We lost only 3%, a very comfortable result for all pilots. Our speed reduced to 99kt IAS (104kt TAS). We went to 2min power and the best rate of climb speed of 80kt and got a healthy rate of climb of 600ft/min (3m/s). I asked Spaulding to lower and raise the collective from maximum continuous power to fully down and back up as quickly as he dared to check the quality of NR governing. NR stayed within ±2-3%. All pilots will be happy with this.
We looked at single- and double-generator failures. There is so much redundancy that even a double failure is not a problem. The aircraft has the luxury of an APU, which can be started using the battery and will provide nearly 100% electrical power. The Collins presentations will allow the pilot to bring up systems such as the electrical layout on the MFDs to examine their state. Sikorsky has incorporated its own vibration suppression system (the Active Vibration Computer) which can be switched off. At 150kt, with the system off, I felt a slight floor vibration.
Engine fire drills, as in all the other modern multi-engined helicopters I have flown, are designed to be user-friendly and preclude the crew finishing up with both engines stopped.
Co-ordinated turns
Co-ordinated turns can be made with feet on the floor - the AFCS and SAS will hold whatever angle of bank the pilot selects or will return the aircraft to its original attitude if a non-pilot-induced upset occurs.
Sikorsky is carrying out more development work on the autopilot/flight director, but it currently has heading, height, airspeed and rate of climb and descent holds. At 3,500ft, we selected 2,500ft and down we went, hands and feet off the controls, levelling off at 2,500ft. A coupled instrument approach can be done with the pilot just trimming the indicated airspeed and rate of descent with the cyclic beeper. Instrument pilots will love this.
Spaulding progressively downgraded the whole system while I flew the aircraft, by removing attitude retention, AFCS, one SAS and then the other. At 130kt I had no trouble flying the aircraft accurately and with none of the pilot-induced oscillations that I have experienced in some other machines. The FADECs have twin-engine and single-engine training modes in which the power instruments are biased to show the pilot that he is using the maximum power settings, but in reality, to protect the engines, making sure they never go above maximum continuous. More work is being done on the training system, including a weight, altitude and temperature graph.
We went low level to treetop height and flew around for about five minutes. Visibility was excellent except during turns to the right from the left-hand seat and, presumably, turns to the left from the other seat. The top canopy gets in the way. We entered and landed in a confined area among the trees. Again, the excellent visibility helped a lot.
Loss of a hydraulic system causes no difference in feel. Loss of servo boost does, however, so I asked Spaulding to switch it off. All three controls stiffened up, but not excessively or uncomfortably so. However, I left my final verdict until I had attempted an approach to the hover over a predetermined spot (to simulate an offshore approach) and a landing.
So we returned to base and I made an approach to the box drawn on the runway. I had no difficulty coming to a good hover over the box, though off a shallow, gently decelerating approach and a landing. I felt that, with just a little practice, I would be confident enough to attempt the same on to an offshore platform using a steep approach technique.
I carried out a number of single-engine running landings and a twin-engine running take-off. There is plenty of single-engine power available, enough to carry out a single-engine landing onto an offshore deck, after burning off some fuel. Steep and vertical approaches and vertical climbs to 150ft and back were easy to achieve, given the excellent visibility. Autorotations at the best rate of descent speed of 80kt gave a rate of descent of 2,500ft/min. A lot of other helicopters come down more quickly than this.
Control of NR was easy, and Spaulding had such confidence that he did not follow me through on the collective. NR limits are a generous 90-120%. The flare has to be started quite early at the bottom to give the opportunity to wash off the 80kt before coming to a powered recovery to the hover. Engine response was excellent for this. We did a best range autorotation at 100kt. Although the rate of descent was higher, we appeared to go further.
Shutdown was straightforward and logical. Spaulding is pleased with the rotor brake design and function. Our previous power assurance checks were available for the technicians to download and compare.
Early impressions
Training WAT graphs, slope limits, Category A profiles, height velocity graph, maximum and minimum g envelope (currently +2.8 to -0.5) and other performance criteria still have to be developed or expanded, but at this early stage, I was impressed. I found the aircraft very pilot-friendly, even when failures occur. If handles very well with adequate main and tail rotor, twin- and single-engine power available.
The reliability results so far are above the criteria set. There will be many optional extras, such as a full icing clearance and health and utilisation monitoring system.
The operators should be pleased with the low running costs and all should be satisfied with the many safety features incorporated into the S-92. Its flexibility to easily change role should be attractive to military users.
Source: Flight International
