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Aviation History
1978
1978 - 0754.PDF
1382 FLIGHT International, 6 May 1978 •4- -4- page 1379 automatic start cycle raise T5 temperature to a modest 700° before settling back to 600°, and advancing the lever again to 96 per cent gas generator r.p.m. The sequence was repeated for the second engine and then both were beeped to 100 per cent NR—a typically modest Sikorsky 293 r.p.m.—on the collective thumb panel. In 18°C ambient (ISA+3°) we hovered at this high weight with 68 per cent torque in a 12kt wind, then surged forward into a l,000ft/min climb at 70kt with take-off power. Increasing to 120kt, we still held l,000ft/min as we chased the S-58T across the swamps. Then we dawdled along beside the photo ship while Tom Hamill hung perilously from the open door, determined to photograph the S-76 tail-on, the impossible angle, before we steered away towards Lake Okeechobee for a work-out. Up to max cruise and VNE The max continuous T5 of 768° was giving only 94 per cent torque, but we worked up to 155kt IAS, 159kt TAS, at 2,500ft. Fuel was down to 1,2301b, putting our weight at 9,7301b. Nick dived to 182kt, well beyond the 156kt VHE for our weight, producing a hard, small-amplitude vibra tion. At 160kt we pulled 1 • 5g with only 20 per cent push- rod load—no sweat for the controls and nothing untoward. Next Nick showed me the customer-convincing points, starting with best-range speed at 1,000ft in 13 • 5°C outside- air temperature. The required T5 of 650° gave 58 per cent torque, 377 h.p. per engine, and resulted in an indicated 126-127kt on the panel and 125kt by the boom. Fuel flow was 2501b per engine at the gross weight of 9,7001b, figures which just yield the claimed 400 n.m. plus 30min long- range cruise with normal fuel and 14 souls aboard. By the end of such a flight the fuel flow would drop to 4351b/hr. Now for max cruise: 694° T5 per engine, the warranty power, produced 75 per cent torque, less than the warranty 86 per cent, and resulted in 143kt IAS at 1,300ft in 14°C OAT. The brochure claims 145kt. I then flew the S-76 with SAS engaged. There is virtually nothing to do in the cruise with so much electronic assist ance, but it was strange to have to use plenty of rudder to co-ordinate entry into a turn. The S-76 is basically a no-problem cruising helicopter, and the stick can be released any time. In a bare cabin and wearing a crash-hat, I got no clear impression of the level of interior comfort, but the vibration—Sikorsky claims around 0-lg—was certainly up to modern standards. A born manual driver like me tends to reject electronic controls and get back to basics, but most people will soon accept the rather auto matic handling. Single-engined climb and speed We turned next to single-engined flying, whereby hang the field lengths, critical decision points, rejected take-off distances, climb rates and safety speeds related to Category A and B transport operations. Basically, Category A demands an en route single-e.ngined climb better than 150ft/min using 30min power at 1,000ft above the take-off elevation—a tough one. Up to that point, a rate of climb at VTOS (take-off safety speed) of better than lOOft/min using 212min power has to be proved. The first go/no-go mark is the critical decision point (CDP), from which the machine can climb away following engine failure without sinking below 35ft. Engine failure before the CDP means an immediate landing. Category B assumes insufficient room to land after the CDP, as would apply on an oil rig. Though the aircraft might have to ditch after an engine failure in these circum stances, the exposure time in the S-76 is only three or four seconds. Take-off and landing distances for the S-76 had not been measured at the time of my flight, but for the S-61 both distances are in the region of 900ft. Nick Lappos set the S-76 at 1,700ft and 150kt, with warranty power on both engines and SAS working. He then cut one engine with his hands off the controls. The S-76 twitched to the left and the good engine went automatically to 830° T5, giving 115 per cent torque, just above 212min power. The S-76 held height and slowed down. Lappos then demonstrated single-engined climb at 212min power at a VTOs of 75kt. Going into a descent at 75kt in order to start without any artificial advantage, he pulled the good engine to the 212min power at 820° T5, giving 107 per cent torque, and equalled or bettered 500ft/min climb at 1,500ft and beyond, handsomely exceeding the required lOOft/min. For the difficult en route single-engined climb with 30min power at 1,000ft above the take-off elevation, Nick set 798° T5, giving 100 per cent torque, at 70kt/ 2,OO0ft/ll°C OAT, and achieved 400ft/min compared with the required 150ft/min. To indicate single-engined ceiling potential, Nick set the good engine at max continuous power at 760° T5, giving 96 per cent torque, and achieved a maximum 102kt in level flight at 2,000ft. To show minimum level speed on one engine, he set the 30min power with 798° T5 at 1,300ft and 13 °C OAT and maintained height down to an IAS of 35kt. Finally, he returned to the airfield and set up a single- engined approach at 40kt. Descending at about l,200ft/min with between 50 and 70 per cent torque, he flared at 20kt into the lOkt reported wind, touched down and stopped in about two fuselage lengths. Then it was my turn again. Lift-off is comfortable with SAS engaged, and the S-76 will hover and manoeuvre with feet off the pedals without yawing. I pulled enough collec tive to make the NR sag distinctly as we sailed vertically upwards, but the S-76 still held its heading with feet off. Pressing the cyclic trim button disengages attitude hold but retains rate damping, allowing more rapid manoeuvring without complete loss of artificial assistance. At one point I was manoeuvring with cyclic and collective trim buttons and the intercom trigger squeezed—just too many buttons. The ability to make a quick start and stop with feet off the pedals and to release the controls in the hover reminded me of the highly electronic feel of the Black Hawk. Such automation is helpful to the pilot who has no reservations about that degree of artificial assistance. Flight without SAS I wanted to feel the bare, basic machine to see what lay beneath this bland exterior. Nick switched off both trims and the SAS so that I had no more than the basic powered-control system, pitch bias and viscous rudder damper. Very cautiously, I lifted off and hovered. The S-76 proved to be more responsive and more loosely damped, and twitched a bit in the ground cushion, but was basically docile and without surprises. I transitioned forward at full climb power, turned left and took my hand off the stick. The S-76 flew hands-off, but the stick dragged slowly to the right and a fair right bank developed over a period of five or ten seconds. The lib built-in stick friction prevented anything more drastic from happening. Laterally, the S-76 responded very rapidly without SAS, but the bank angle froze as soon as I centred the stick. Quick movements in, pitch did not lead to overshooting or hunting, and the handling remained perfectly comfort able up to 120kt. I made the approach at 80kt, noting that I needed little rudder as speed and power diminished. The S-76 decelerated rapidly, and came to the hover easily with a brief but moderate vibration as I applied hover power. I commented that the instrument panel and nose blocked forward view during the landing transition, and Nick said that an additional nose window was to be added. The S-76 undercarriage is the smoothest I have ever met. As I hovered, Nick switched on the stabiliser and the S-76 noticeably settled down. When he brought in attitude- hold the machine seemed almost to freeze. If the S-76 is gentlemanly without SAS, it virtually runs on rails with the system engaged. Sikorsky claims to have achieved 50kt in sideways flight at 10,3001b gross weight. I had reached about 20kt sideways to the right when Nick asked me to stop because we were moving out over the water. I had applied virtually full left rudder by then, but steadily and without snatching or reversing. The amount of rudder needed apparently decreases again beyond 20kt. At this point we had to land so that the S-76 could be prepared for its second test flight that day, and we our selves had to hurry to Miami to catch a plane westwards to LA. All the nice aeroplanes seem to be busy.
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