Graham Warwick/SAVANNAH
GULFSTREAM IS NOW officially a two-aircraft company, for the first time in its history. While flight-testing of the 12,000km (6,500nm)-range Gulfstream V gathers pace, production of the 7,800km-range Gulfstream IV-SP continues. The two aircraft will be built side-by-side into the next century. Plans call for a doubling of Gulfstream annual production, to around 40 GIVs and GVs by the end of the decade.
"The market clearly sees these aircraft as different," says Gulfstream vice-chairman Bryan Moss. He is obviously buoyed by early acceptance of the GV, which has led to a $2 billion order book, initially consisting of existing Gulfstream customers, but increasingly including operators of competing large business jets.
The two-aircraft strategy is already paying dividends for Gulfstream, with the introduction of the GV spurring GIV sales. While the GV is now sold out until the third quarter of 1998, GIV-SPs are available for delivery in the second quarter of 1996, and some customers are ordering GIVs as interim aircraft until their GVs are ready, Moss says.
Range is the GV's raison de etre, but its usefulness as a business tool requires reliability (availability and ability to complete its mission); and flexibility (to ensure that an operator can use short fields, reach high altitudes and fly shorter distances efficiently). Moss says that both these features are attracting customers to the GV.
CONCEPT EVOLVES
Gulfstream began studying its next business jet soon after certification of the GIV in 1987, investigating concepts ranging from small, to supersonic aircraft. Market studies revealed additional range to be the most pressing need, and today's GV began to evolve.
Ultimately, 36 GV configurations were studied, with 11 different wing plan-forms, five engine candidates and two fuselage diameters. Gulfstream initially unveiled a 9,250km-range GV, essentially a GIV with an extended wing housing more fuel, but the market wanted more range - and comfort.
The result was a 10,400km-range aircraft, still a GIV derivative, but with a new wing, 2.1m-stretched fuselage and 11%-uprated Rolls-Royce Tay 650+ engines mounted 1.4m further back on the fuselage. Then, in 1992, the BMW Rolls Royce BR710 turbofan became available, offering a 15% reduction in specific fuel consumption and enabling Gulfstream to offer a 11,650km-range aircraft.
The GV was launched formally in August 1992. While 11,650km range was enough, on paper, to connect key city pairs such as New York and Tokyo, operational analyses using real-world winds and routing indicated that 12,000km was needed. Gulfstream decided to use the full 18,600kg fuel capacity of the new wing and, in early 1994, announced a guaranteed 12,000km range.
Gulfstream calculates that the final GV configuration achieves a 24% increase in aero-propulsive efficiency over the present GIV-SP - the combination of a 9% improvement in cruise lift/drag ratio and a 15% reduction in specific fuel consumption. Combined with a 39% increase in fuel capacity, this gives the GV a 57% longer range.
Despite the many changes needed to achieve a 12,000km range, the GV still looks much like a GIV. While up to 7° of additional wing sweep was considered, the risk of adversely affecting low-speed handling qualities outweighed the "minimal" block-time advantage of a higher cruise speed, Gulfstream says. Similarly, a wider, 2.7m-diameter, fuselage was studied and rejected, designers deciding that the existing 2.4m cross-section is an efficient balance between cabin volume and fuselage drag.
COMFORT FACTOR
No one would describe the GIV cabin as uncomfortable, but the GV will be capable of flights exceeding 14h, with most of that time spent above 41,000ft (12,500m). To increase comfort, Gulfstream stretched the fuselage by 2.1m, but ended up creating 2.3m of additional cabin length by moving the entry door back by 1.5m and relocating all avionics forward of the door, says director, GV product qualification, Ed Flinn.
The cockpit bulkhead has been moved aft by 0.3m to enlarge the flight deck, while the aft-cabin baggage compartment is 34% bigger than that of the GIV, with a larger external door. As the walk-in compartment is within the engine-rotor-burst zone, it is separated from the cabin by a secondary pressure-bulkhead, with a door, which must be kept closed above 41,000ft.
The GV benefits from the wide-inside interior introduced on the GIV-SP, with reshaped fuselage frames, which increase cabin height (1.9m) and width (2.2m). Gulfstream's trademark windows, six large oval apertures per side, remain unchanged, despite the GV's extra fuselage length.
To combat fatigue on long flights, cabin pressure-differential has been increased to 0.7bar (10.17lb/in2), providing a 6,000ft cabin altitude at the GV's 51,000ft maximum operating altitude. The environmental-control system provides each of the eight passengers and four crew with 2.97m[3]/min (84ft[3]/min) of fresh air at a 6,000ft cabin altitude, and includes water separation, ozone filtration and three-zone temperature control.
While the cabin volume has been increased by 13%, the GV's outfitting allowance of 3,200kg represents a 23% increase over that of the GIV. This is despite that fact that several items previously installed during completion are now included as standard in the production aircraft. These include avionics such as dual global-positioning and triple inertial-reference systems, collision avoidance and ground-proximity warning.
Completion of a $29.5 million GV is expected to cost $4-4.5 million, compared with $4-4.5 million to complete a $24 million GIV. Gulfstream's two completion centres, are working on several GV-related development projects, including 16g seats, optional vacuum toilet and a membrane-technology air humidifier to reduce passenger fatigue. Moss says that the GV's flexibility to carry more people, or be flown faster, over shorter distances is simulating customer interest in reconfigurable interiors.
FLEXIBLE PERFORMER
While the GV's design mission is to carry eight passengers 12,000km at M0.8, the aircraft can perform the GIV's 7,800km/M0.8 mission at M0.861, saving some 40min, or at M0.8, saving some 1,600kg of fuel. In addition, the range remaining after a maximum-weight landing is 7,000km, compared with 5,500km for the GIV, giving increased flexibility for multiple hops.
A bigger wing and bigger flaps give the GV better field performance than the GIV, resulting in almost 60% more range from the same runway, or a 30% shorter field length for the same range. Although substantially heavier than the GIV, the GV's stall speeds are similar. At the GIV's lower take-off weight, the GV's stall speed is 12kt slower, Flinn says.
Climb performance is also improved. On the same 6,500km mission, while it takes the GIV 20min to climb to its 41,000ft initial cruise altitude, the GV can climb to 45,000ft in 17min. High-altitude handling - rated excellent on the GIV, Flinn says - is further improved: the GV has a higher buffet-free margin for manoeuvres at 51,000ft than the GIV has at its 45,000ft maximum altitude.
When compared with the GIV, the GV's range advantage translates into substantial time, and cost, savings. Flinn cites the example of one GIV operator who regularly flies from Seoul, in South Korea, to London. The 13,900km eastbound flight, via Anchorage, takes just over 17h and requires some 21,500kg of fuel. Flying westbound in the GV, a distance of around 9,300km, would take 12h 20min and consume less than 14,200kg of fuel.
RELIABILITY REQUIRED
Successful use of a business aircraft to save time and increase productivity requires reliability and availability, Gulfstream says. For this reason, the GV has been designed to the same extended-range twinjet operations (ETOPS) criteria as airliners such as the Boeing 757/767, even though ETOPS certification is not yet required for business jets.
ETOPS design has lead to some system changes, compared to the GIV, notably in the electrics and hydraulics and in the use of airline-proven components. The electrical system is modeled on that of the 757/767, with two engine-driven 40kVA integrated-drive generators replacing the GIV's variable-speed/constant-frequency generators, which are "...not as reliable as they should be", says senior vice-president, GV programme, Preston Henne.
A third 40kVA generator, is driven by the AlliedSignal Engines RE220(GV) auxiliary power unit (APU). Flinn says that the RE220 was selected for its increased capacity and reliability. Additional bleed-air was required to cool the larger cabin and to start the high-compression BR710s, he says, while ETOPS design rules required an APU which could be started at up to 43,000ft and operated up to 45,000ft.
One 10kVA hydraulic-motor generator and two 9AH sealed-cell lead-acid batteries complete the suite of back-up power sources. Hydraulic-system redundancy is also increased, with two Vickers pumps per engine, instead of one, powering the two main hydraulic systems. The GV has mechanically actuated, hydraulically powered, flight controls with manual reversion, as has the GIV.
To meet new certification requirements concerning jammed controls, the GV has independent left and right elevator and aileron control channels and a "guillotine" disconnect mechanism in the cross-tube linking the control columns. There are dual rudder actuators and quad-redundant yaw-damper computers, allowing dispatch with one channel inoperative.
The new environmental-control system features dual air-conditioning packs, with dual digital pressurisation-controls integrated with the aircraft's flight-management system. A single pack can provide 100% of the pressurisation and cooling requirements, Flinn says. Dedicated cooling air is provided for the avionics to improve reliability.
Wheel and brake improvements introduced on the GIV-SP are carried over to the GV, with Dunlop supplying the complete system, including main wheels, hydro-mechanical carbon brakes, Goodyear tyres and Hydro-Aire digital anti-skid system. A "better design" of steer-by-wire nose-wheel-steering system, supplied by Vickers/Sterer, is introduced on the GV.
Gulfstream's design rule that all fuel must be housed in the wing, for safety, results in a simple fuel system, Flinn says, with no tank management required. The GIV system is retained for the GV, with minor improvements.
SAFE DESIGN
US and European airworthiness approval of the GV, will be on the basis of an amended GIV type-certificate, but incorporating the latest FAR/JAR 25 amendments, with few exceptions. "This is basically an all-new aircraft [from a certification standpoint]," says Henne.
Where all previous Gulfstreams have been fail-safe designs, the GV meets the latest damage-tolerant requirements. "Conversion from fail-safe to damage-tolerant did not drive the design," says GV chief engineer Dick Johnson. Established Gulfstream structural philosophy - frame and stringer spacing, for example - fits the damage-tolerant approach "perfectly", he says.
Even the decision to increase the cabin pressure-differential over that of the GIV did not affect the design greatly. The GV's pressure-vessel skin is the same thickness as that of the original GI, Johnson says. "The stress levels are very low, so we could offer a 6,000ft cabin with very little change in the basic design," Johnson says.
Despite their structural similarity, however, the GV fuselage has "not one skin panel" in common with the GIV - for two reasons: corrosion and access. A major effort has been made to prevent the corrosion problems with have plagued earlier Gulfstreams. "We are a lot smarter with materials now than we were with the GII in 1965," says Johnson.
Where alloy steel was used for major joints, now stainless steel is used. Similarly, steel doublers have been replaced with titanium or stainless steel. All pressure-vessel rivets are "wet-installed" with sealant, all machined parts are double-primed and all dissimilar-metal joins are sealed. In addition, drain-valves have been improved and better access provided to known corrosion trouble spots.
The GV wing, designed and built by Northrop Grumman's Vought operation in Dallas, Texas, is structurally similar to that of the GIV, but aerodynamically different (see box). "We wanted to retain the history of the GIV wing, so the basic structure - stringer and rib spacing, access covers, materials - is the same," Johnson says.
Left and right upper and lower skins are single piece machined panels, which are formed to the complex contours required for the GV wing using a process known as age-creep forming. The panel is placed in a tool and pulled into shape by suction, then baked at 175¡C for 30h in an autoclave. The tool is more curved than the wing surface, as the skin "springs back" once removed. Gulfstream says that Northrop Grumman's first skin required only minor weighting to achieve the desired curvature.
Fokker, which, like Northrop Grumman, is a revenue-sharing partner on the GV programme, has designed and built the empennage, which is "heavily borrowed from the Fokker 100", Johnson says. Both vertical and horizontal stabilisers are produced using metal bonding, a weight-saving process new to Gulfstream but familiar to Fokker.
The Netherlands manufacturer also supplies the carbonfibre-reinforced plastic elevators. Other composite parts include the rudder (Gulfstream), ailerons, gear doors and wing/fuselage fairing (all Shinmeywa). Cabin floor boards are produced by Fokker, using a thermoplastic composite. The Hurel-Dubois thrust-reversers are composite, including the doors, which saves some 115kg per engine, Johnson says.
ENGINE FIRST
Gulfstream is the launch customer for BMW Rolls Royce's BR700 family of engines, continuing an unbroken association with R-R, which began with the Dart-powered GI and continued through the Spey-powered GII and GIII to the Tay-powered GIV. The first member of the family, the BR710-48, is being flown for the first time on the GV. Henne says that the engine is performing "better than predicted" for the initial flight configuration.
The BR710 combines wide-chord fan technology from the Tay with a core design derived from that of the International Aero Engines V2500. The -48 (the engine has a 1.2m [48in]-diameter fan) is rated at 66kN (14,750lb) -thrust on the GV. Compared with the GIV's Tay, the BR710 provides 6.5% more sea-level thrust. The complete power plant, including nacelle and reverser, is also lighter.
Initial testing of the BR710's full-authority digital engine control was conducted in an iron-bird rig. This is now part of Gulfstream's GV integration test rig (ITF), which represents an almost-$5 million investment in validating the aircraft's electrics and electronics.
The ITF, which includes a working GV cockpit, is equipped with a full suite of electronics, ranging from subsystem controllers to Honeywell's SPZ-8500 integrated avionics. The latter is based on the GIV avionics system, but with changes including new integrated avionics-computers; expanded system synoptics on the engine-indication and crew-advisory system; enhanced ground-proximity warning using a terrain database; and an airline-standard maintenance-data acquisition unit, supplied by AlliedSignal.
"The GV started out as a stretched GIV with the same avionics," says Dave Powell, technical manager, GV ITF. "By 1994 it was obvious that the aircraft was not a GIV electronically. That meant risk, so Gulfstream agreed to fund the ITF," he says. Design started in mid-1994 and the testbed was powered up for the first time in June 1995, ahead of the first flight in November. "We would not meet our 11-month flight-test goal without the ITF," Powell adds.
Four flight-test aircraft will be used in the certification programme, plus a static-test airframe and a fuselage fatigue-test specimen. The static-test airframe, aircraft 502, will be refurbished for use as a company demonstrator. US certification is scheduled for 15 October, and European certification for 15 December.
Source: Flight International
