FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1984
1984 - 0761.PDF
Propfans win respectability BRISTOL Propfans were the main talk ing point at the Royal Aero nautical Society's recent symposium on "The selection of propulsion systems for future transport aircraft", reports David Velupillai. Speakers from Boeing, McDonnell Douglas, Airbus Industrie, and British Aero space all expressed enthusi asm for the concept, and there was general agreement that a twin "pusher" with contra- rotating rear-mounted prop- fans was the best layout- There was considerable debate about the timescale in which propfans would become available, however. John Morris, director of advanced engineering at Douglas Air craft, believed that a propfan could be in service by 1992. He also identified propfans as the only technological advance to promise sign ificant benefits in the near- term. Propfan would make most sense to Douglas if it were applied to a derivative MD-80, and Morris envisaged 8 per cent better seat-mile costs in this application. Morris also made the point that the major airliner market was for 150-seaters, and saw little point in developing initial propfans for the 100-seater market—estimated to be half the size of that for 150-seaters. Boeing appears less enthusiastic about an imme diate propfan application, perhaps because, unlike Douglas, it lacks a suitable derivative airframe (the 727 is no longer in production). But it was BAe's chief of future products, Paul Smith, and Airbus Industrie's general manager of technical devel opments, Doug Jagger, who were most sceptical about propfan's early service entry. Jagger suggested that propfans would require devel opments of a new core, and that overall development would take 12-20 years, but he felt that commuters of less than 80 seats would "definitely" be flying with propfans by the year 2000. The chance of an 80-130 seat regional airliner flying by then were 50:50, according to Jagger. Pratt & Whitney and General Electric are the only engine makers committed to development of propfan hard ware, with financial help from Nasa, although Rolls-Royce has clearly conducted studies. Pratt & Whitney's work centres on a propfan driven through a reduction gearbox, for which sister company Hamilton Standard is largely responsible. GE's approach is more revolutionary and elimi nates the gearbox (see follow ing story and diagram). It calls the result an "un-ducted fan". More on GE's unductedfan EVENDALE General Electric's unducted fan is arguably the most radi cal propulsion concept to emerge in recent years. The company is saying little more than was revealed in Flight, April 21, page 1055, but a few further details are available. Noise is the biggest problem to be explored by the F404-powered demonstrator according to General Electric, "because noise does not scale". Until the full-size demonstrator is built, this will remain an area of major uncertainty. Less funda mental, but no less daunting a task facing GE, is translating the unique concept into low vibration hardware with effi cient seals. Effective bypass ratio of the unducted fan is about 30:1, compared with about 6:1 for a current big fan and about 50:1 for a propeller. The pusher layout and maximum dia meter of the unducted fan are determined by airframe integration constraints. A wing-embedded tractor on a nominal Mach 0-75 airframe would induce unwanted local transonic flow. Other mount ing options , constrain the diameter, but, fortuitously the optimum disc-loading rises with cruise speed, and GE believes that the unducted fan will deliver the optimum balance between propulsive efficiency and block-time at Mach 0-75 on 1,000 n.m. sectors. GE's relevant city- pair traffic analysis predictes an 80-120 seater, which in turn leads to a propfan driven by a core engine slightly smaller than an F404, but with higher pressure ratio. The Nasa-sponsored demon stration programme using an F404 core is therefore not simply academic; it is near enough to a potentially viable commercial UDF engine to provide a genuine proof of concept. GE is keeping quiet about the innards of its unducted fan, but Flight's artists have produced this guess at its layout. Hot gases from an F404 core (left) drive the first set of fan blades (green) in one direction and the second set (yellow) in the opposite sense. Each turbine extracts energy from the gas flow, while also acting as a "stator" for the next turbine. This direct-drive arrangement eliminates the need for a reduction gearbox PROPULSION The UDF puts a premium on core efficiency, but for a given core development, GE believes it can deliver 20 per cent better s.f.c. than current high bypass ratio turbofans. That same core development applied to the turbofans themselves, would produce a gain of only 1-4 per cent. GE's 25,0001b-thrust demonstrator is due to run in the third quarter of 1985, and is scheduled for test flights aboard a Boeing 727 in late 1986. The company is confident of developing its UDF to the point of service- entry within 4-4-5 years of the end of the flight trials, which could mean airliners flying with the powerplant in the early 1990s. GE runs Gripen fan LYNN ~~ General Electric is test- running an F404 with an uprated fan destined for the Saab Gripen's 18,0001b "Tuned B" version of the engine. The Gripen engine will run in June, ready for flight clearance in September 1985 and qualification in 1987. The new fan can handle 10 per cent extra airflow, but is speed-scheduled to use only half of this capability. General Electric has em barked upon technology programmes (such as single crystal blades) to exploit the new fan, and to enable F404s to produce 20,0001b thrust while occupying the same space as their progenitors aboard F/A-18s and G ripens. A parallel weight-saving pro gramme includes a composite fan case, now chemically milled from aluminium. The first standard F404 for Dassault's ACX will be deliv ered to Snecma for bench- testing in December 1985, followed by flight-rated engines in March 1986, ready for first flight in October 1986. Two unreheated F404s have passed through the Lynn test cells. They are believed to have been delivered to Grumman for compatability tests on an A-6. A US Navy request for proposals to power A-6Fs is expected this spring. The competing engine is a re-worked version of the A-6's existing PW J52. FLIGHT International. 28 April 1984 1159
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
