Graham Warwick/ATLANTA
THE MAIDEN FLIGHT of the Gulfstream V is less than one month away; the first flight of the Bombardier Global Express is less than a year away. Gulfstream is underlining the reality of its GV; Bombardier is emphasising the promise of its Global Express. The competition is intense. At stake are those customers - many of them governments - which might make a purchase decision in the next 12-18 months.
Both aircraft have evolved since they were announced in October 1991, in response to market demands and competition pressures. Each now offers the same 12,000km (6,500nm) range at Mach 0.8, with eight passengers and four crew, but that was not always the case.
The GV started out with a 10,200km range, but was launched formally in August 1992 as an 11,650km aircraft. The Global Express was introduced with a 10,450km range, but this had increased to 12,000km by the launch in December 1993. Gulfstream increased the GV's range to 12,000km soon after.
Although both aircraft were unveiled at the same time, Gulfstream launched the GV more than a year ahead of the Global Express, and has maintained its lead. The GV will be flown ten months ahead of the Global Express, but will be certificated 17 months earlier than the competing aircraft, in October 1996.
Despite this, the aircraft are scheduled to enter service a little over a year apart, as Bombardier plans to begin delivering customer aircraft to completion centres in December 1997, ahead of certification (due in March 1998). Gulfstream plans to begin customer deliveries in October 1996, after certification.
The GV will be certificated as a GIV derivative, but incorporating the latest amendments with "few exceptions", the company says. Gulfstream is planning an 11-month test programme involving four flight-test aircraft and one structural-test article, leading to US certification in October 1996. European certification is scheduled for December 1996.
Although the GV will have an amended GIV-type certificate, the aircraft is designed to current damage-tolerant requirements (the GIV has a fail-safe design) and the design service life has been doubled to 40,000h. Corrosion prevention has been increased, and accessibility for inspection and servicing has been improved.
The Global Express is an all-new aircraft, designed to damage-tolerant requirements, and will be certificated to the latest amendments. Bombardier is planning an 18-month test programme involving four flight-test aircraft and two structural-test articles, leading to simultaneous Canadian, US and European certification in March 1998.
CABIN FERVOUR
Of the two cabins, the GV's is definitely smaller. The Global Express cabin is 2.49m wide, 1.91m high and 14.6m long from cockpit-divider to rear pressure-bulkhead, aft of which are the engines and the 4.96m3 (175ft3) baggage compartment. The GV cabin is 2.23m wide, 1.88m high and 15.3m long - but only 13.7m of that is usable, as the 6.4m3 baggage compartment is located in the aft cabin, behind an auxiliary pressure-bulkhead. This bulkhead is required because the aft cabin lies within the engine rotor-burst zone and the baggage-compartment door must be closed when the aircraft is above 41,000ft (12,500m).
The GV's 0.7bar (10.17lb/in2) pressure-differential, which is higher than that of the GIV, results in a cabin altitude of 6,000ft at the 51,000ft maximum operating altitude. With the Global Express' 0.67bar pressure-differential, the same cabin altitude is reached at 45,000ft, and at 51,000ft the cabin altitude is 7,200ft. A typical airliner cabin-altitude is 8,000-9,000ft. Gulfstream says that a lower cabin-altitude reduces passenger fatigue on long-range flights.
The GV's two air-conditioning packs provide each of the eight passengers and four crew with 2.97m3/min (84ft3/min) of fresh air at a 6,000ft cabin altitude, while the Global Express' two packs provide 2.33m3/min/person of mixed fresh and recirculated air under the same conditions. Bombardier says that pilot-selectable air recirculation is provided to help lower, or raise, cabin temperatures rapidly. The fresh-air-only supply, at 1.7m3/min/person, is substantially more than the airworthiness requirement for 0.35m3/min/person, the company notes.
SWEEP STAKES
Aerodynamically, the aircraft differ, mainly because the Global Express is optimised for M0.85 and the GV for M0.8. Both aircraft have wings with similar spans which house similar amounts of fuel, but the all-new Global Express wing has a 35¡ quarter-chord sweep, while the GV wing shares the 27¡ sweep and general planform of the GIV wing.
The Global Express has a third-generation supercritical wing, designed by Bombardier using its experience with the Challenger and the Regional Jet. Increased sweep, coupled with the aft-loaded aerofoil, reduces shockwave formation to give the aircraft its M0.85 cruise.
The GV wing is optimised for "virtually shock-free flight" at M0.8. A forward-loaded aerofoil - "unique to the GV", Gulfstream says - is used to avoid the pitching-moment penalties of aft-loading. This also permits manual flight-control reversion, and allows Gulfstream to stay with two hydraulic systems, while the Global Express' aft-loading requires fully powered controls and three hydraulic systems.
Compared with the GV, the Global Express has a thicker aerofoil section, which allows the same fuel to be housed in a smaller wing. To achieve the desired field performance with a smaller wing, Bombardier uses high-lift devices. The Global Express has four-section leading-edge slats and three-section trailing-edge Fowler flaps. The GV has single-section Fowler flaps and no leading-edge devices.
Gulfstream argues that the GV's larger, cleaner wing results in better climb-performance. Bombardier says that its wing design results in good high-speed and low-speed performance. In addition, the Canadian company has carefully area-ruled the rear fuselage to reduce flow velocities in the engine-pylon region. The drag saved, Bombardier says, more than offsets any additional drag caused by the Global Express' larger fuselage.
There is a major difference in the performance guarantees offered by the two manufacturers. While Gulfstream is guaranteeing a 12,000km range at M0.8, Bombardier is guaranteeing an 11,710km range at M0.85, both figures applying to an aircraft carrying eight passengers and four crew.
Both aircraft have the same M0.8 long-range cruise speed, and identical ranges at that speed, but the Global Express is designed for higher speeds: at M0.85, the GV's range is down to 10,230km; and at M0.88 the Global Express is on its own, with a range of 9,250km.
Bombardier equates higher speed with time saved, calculating that the Global Express would save 45min on a 7,400km transatlantic flight by cruising at M0.88 instead of M0.8, and 45min on an 11,100km transpacific flight by cruising at M0.85 instead of M0.8. Gulfstream calculates that, on a 9,250km flight, only 10min would be saved by cruising at M0.88 instead of M0.86, the GV's cruise capability at that range.
Bombardier is also promising better field performance, because of the high-lift devices on the Global Express wing. The guaranteed balanced take-off field length, at the 41,280kg maximum take-off weight, was recently improved, from 1,690m to 1,555m. The landing distance is 780m at the 35,650kg maximum landing weight.
Gulfstream is guaranteeing a 1,780m balanced take-off field length at the GV's 40,370kg maximum take-off weight. The landing distance is 900m at the 32,660kg maximum landing weight.
The US manufacturer says its competitor is taking advantage of rules allowing a lower V2 (take-off safety speed). While Gulfstream argues that a reduced V2 hurts climb performance, it says that a decision on whether to adopt the same criterion, which would reduce the GV's take-off field length by around 120m, will be taken early in the flight-test programme.
Enabling the Global Express to be operated from small airfields previously inaccessible to large aircraft was a design driver for Bombardier. As a result, the design relies heavily on wing high-lift devices, at the cost of additional drag early in the climb.
Gulfstream does not believe that the GV will be operated often from short runways, and has opted instead for a different balance of field and climb performance. This is evident when hot-and-high conditions are considered. Using Mexico City's Toluca Airport as a benchmark for hot-and-high performance, Gulfstream says that the GV's take-off weight from the 8,450ft-elevation airfield is unrestricted up to ISA +25¡C conditions.
Bombardier's figures show that the Global Express' range on a M0.85 mission from Toluca exceeds that of the GV at the same speed, but the 10,730km range quoted, for ISA +20¡ conditions, represents a reduction of 980km over the aircraft's maximum range capability.
WEIGHING IN
Performance guarantees are, in turn, based on empty-weight guarantees. Gulfstream has guaranteed a manufacturer's empty weight of 17,240kg (±2%) and says that it is close to that with the first aircraft. Bombardier recently guaranteed its manufacturer's empty weight of 18,460kg (±2%) and says that components produced for the first aircraft are proving to be on or under their computer-projected weights.
There is major disagreement between the two companies as to how much weight should be allowed for outfitting their aircraft with interiors and other customer options. Gulfstream bases its performance guarantees on a completion allowance of 3,175kg. Bombardier bases its guarantees on an allowance of 2,720kg.
Gulfstream, which is allowing more weight per metre of cabin length for completion of a GV that it does for a GIV, argues that Bombardier's allowance is unrealistic given the Global Express' larger cabin. The US manufacturer argues that a completion allowance of 3,175kg would reduce the Global Express' fuel load by 455kg and limit its range at M0.8, with eight passengers and four crew, to 11,600km.
Bombardier says that the allowance was "...collectively established by several leading interior-completion centres and with the input of the Global Express advisory council". The company says that a "green" (uncompleted) Global Express "includes several factory-installed items traditionally included in an aircraft-outfitting allowance".
Bombardier will have a better idea of the weight required to complete a Global Express when its Learjet completion centre finishes outfitting the first Challenger Special Edition. This modified Regional Jet has similar cabin dimensions to the Global Express and will be used by Bombardier's Middle East distributor, TAG, to demonstrate the multi-compartment cabin planned for the long-range aircraft.
Source: Flight International
