FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1985
1985 - 1952.PDF
PARIS REVIEW Propfan/UDF: some answers questioned Whether propfan, unducted fan, or "a load of bananas whirling round"—a descrip tion attributed to an Airbus official—new propulsion con cepts created much of the news at Paris, and much of the confusion. There were disagreements on timing, technology, and terminology, but general agreement that "whirling bananas" will change the face of air transport—if they can be made to work. Is it propfan, UDF, or UBE? Propfan is a portmanteau term coined to describe a propulsion concept which combines the characteristics of a turboprop with some of those of a turbofan. The term was initially applied to a high speed propeller with multiple thin, swept, variable-pitch blades 80 per cent efficient at Mach 0-8, driven by a turboshaft engine via a gear box. Contra-rotation makes no fundamental difference, so the term propfan still applies. General Electric evolved its unducted fan from the single- rotation geared propfan, moving first to contra- rotation to gain efficiency, then to a pusher configuration to limit cabin noise. As propfan diameter was reduced to ease installation, rotation speeds went up until GE real ised that propeller r.p.m. was approaching that at which a contra-rotating power turbine could be made to operate effi ciently. The result was a gear- less propfan with the blades mounted directly on to a free power turbine. The gearless unducted fan is therefore a sub-species of the propfan genus. UDF, however, is a GE trademark. Seeking a blanket term to describe both geared propfans and gearless unducted fans, Boeing coined the term ultra- bypass engine (UBE). McDonnell Douglas has gone for ultra-high bypass (UHB). Instead of clearing things up, this has added to the confusion by grouping propfan/UDF with ultra- high-bypass-ratio turbofans, also known as ultra-bypass fans (UBFs), whether Rolls- Royce's direct-drive aft-fan engine or Pratt & Whitney's front-fan geared turbofan. The inner workings of General Electric's UDF unducted fan show the 12-stage contra-rotating power turbine which, says GE, produces as much work as a 24-stage single-rotation turbine, and which is the main area of difference, and disagreement, between UDF and the geared propfan . Essentially the difference is this: geared propfans and gearless unducted fans have bypass ratios of around 40:1 and no ducts. Ultra bypass turbofans have bypass ratios of 20:1 and ducts. How to compare propfan and UDF Propfan output is expressed as shaft horsepower (s.h.p.), because there is a turboshaft engine and gearbox involved. UDF output is expressed as thrust simply because there is no gearbox. At sea level, with a static engine, 1 h.p. is broadly equivalent to 21b of thrust. In the cruise, the two are roughly equal—1 s.h.p. to lib thrust. A 150-seat airliner powered by two 25,0001b- thrust turbofans, therefore, will require equivalent-sized UDFs or 12,000-13,000 s.h.p. propfans. Single- or contra- rotation? The aim of a propeller is to propel the aircraft through the air, not to push the air past the aircraft. Inevitably some power is wasted in accelerating the air, and this is manifested as swirl. By locating a second pro peller behind the first, rotating in the opposite direc tion at the same speed, most of that swirl energy is recovered. Propfan started life as a single-rotation tractor config uration. There are indications that a wing behind the propulsor will recover some of the swirl energy, but not as much as contra-rotation. Exhaust nozzle Having completed design of the single-rotation prop- fan, Hamilton Standard is now working on contra- rotation. The first large-scale flight test of the geared propfan in May 1987 will use a 9ft-diameter, eight-blade single-rotation propulsor mounted in tractor con figuration on the wing of a Gulfstream II. A production single-rotation propfan for a 160-seat airliner would be of 12ft diameter with ten blades, says Hamilton Standard. A contra-rotation propfan for a comparable aircraft would have 10 per cent less diameter and two sets of six blades. By design, UDF must use contra-rotation both in the propeller and the power turbine. GE's 25,0001b-thrust UDF demonstrator engine has a propulsor diameter of just under 12ft, while the 22,0001b-thrust GE36 UDF being designed for Boeing's 1992 150-seater is slightly less than 10ft in diameter. Pusher or tractor? Initial applications for propfan/UDF are pusher con figurations, in an effort to locate the noise source well aft of the passenger cabin. UDF will be demonstrated in this configuration, first on a Boeing 727 in 1986 and later on a McDonnell Douglas MD-80 in 1987. Propfan will be demonstrated first as a tractor, in 1987, although Allison plans to have its 578 contra-rotation pusher prop- fan engine ready for flight- test late in 1987. Propfan, with its gearbox, can be driven from the front of from the rear of a turboshaft engine, whether single-rotation or contra- rotation. A pusher configur ation presents problems, however, with turbulent flow from the wing, nacelle, and engine pylon impinging on the thin blades. The engine must exhaust on to these blades, which provides inherent deicing, but poses design problems for composite- shelled blades. Hamilton Standard's blade has an aluminium spar encased in honeycomb filler with glassfibre shell. To reduce blade temperatures, says Allison, the exhaust can be lobed and the leading propeller used to "integrate" the exhaust flow, spreading it out and reducing peak temperatures. With the roots encased in a polyimide sheath, pusher propfan blades should be able to withstand exhaust temperatures of 260°C. Contrary to popular belief, says GE, UDF does not have to be a pusher. The gas gener ator core has to be ahead of the UDF propulsor, admit tedly, but the whole engine can be mounted forward of the wing. The main engine mount is located at the join between the core and the propulsor, says GE, and the core accounts for only 25 per cent of engine weight. The UDF propulsor is largely hollow, as the diagram shows, and a pylon can be passed through the centre of FLIGHT International, 15 June 1985
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
