Propfan


A propfan, also called a propjet, an open rotor engine, or an open fan engine, is an aircraft engine combining features of turbofans and turboprops. It uses advanced, curved propeller blades without a duct. Propfans first started prototype testing in the 1970s, aiming to combine the speed capability of turbofans with the fuel efficiency of turboprops, especially at high subsonic speeds. However, they have never proceeded beyond testing, never going into commercial use. Over the decades, different efforts to perfect the concept have used names like "open rotor" and "ultra-high-bypass turbofan".

Definition

In the 1970s, Hamilton Standard described its propfan as "a small diameter, highly loaded multiple bladed variable pitch propulsor having swept blades with thin advanced airfoil sections, integrated with a nacelle contoured to retard the airflow through the blades thereby reducing compressibility losses and designed to operate with a turbine engine and using a single stage reduction gear resulting in high performance". In 1982, the weekly aviation magazine Flight International defined the propfan as a propeller with 8–10 highly swept blades that cruised at a speed of, although its definition evolved a few years later with the emergence of contra-rotating propfans.
In 1986, British engine maker Rolls-Royce used the term open rotor as a synonym for the original meaning of a propfan. This action was to delineate the propfan engine type from a number of ducted engine proposals at the time that had propfan in their names. By the 2000s, open rotor became a preferred term for propfan technology in research and news reports, with contra-rotating open rotor also occasionally being used to distinguish between single-rotation propfans. As of 2015, the European Aviation Safety Agency defined an open rotor concretely as "a turbine engine fan stage that is not enclosed within a casing"; in contrast, it had only a working definition of an open rotor engine, calling it "a turbine engine featuring contra-rotating fan stages not enclosed within a casing." The engine uses a gas turbine to drive an unshrouded contra-rotating propeller like a turboprop, but the design of the propeller itself is more tightly coupled to the turbine design, and the two are certified as a single unit.
In a 2017 Ahmed El-Sayed, a professor in the design and performance of aircraft engines, differentiated between turboprops and propfans according to 11 different criteria, including number of blades, blade shape, tip speed, bypass ratio, Mach number, and cruise altitude.

History

About a decade after German aerospace engineers began exploring the idea of using swept wings to reduce drag on transonic speed aircraft, Hamilton Standard in the 1940s attempted to apply a similar concept to aircraft propellers. It created highly swept propeller blades with supersonic tip speeds, so that engines with exposed propellers could power aircraft to speeds and cruising altitudes only attained by new turbojet and turbofan engines. Early tests of these blades revealed then-unresolvable blade flutter and blade stress problems, and high noise levels were considered another obstacle. The popularity of turbojets and turbofans curtailed research in propellers, but by the 1960s, interest increased when studies showed that an exposed propeller driven by a gas turbine could power an airliner flying at a speed of Mach 0.7–0.8 and at an altitude of. The term propfan was created during this period.
One of the earliest engines that resembled the propfan concept was the Metrovick F.5, which featured twin contra-rotating fans—14 blades in the fore fan and 12 blades in the aft fan—at the rear of the engine and was first run in 1946. The blades, however, were mostly unswept. Other contra-rotating propeller engines that featured on common aircraft included the four powerful Kuznetsov NK-12 engines on the Soviet Union's Tupolev Tu-95 high-speed military bomber and Antonov An-22 military transport aircraft, and the Armstrong Siddeley Double Mamba engines on the British Fairey Gannet anti-submarine aircraft. Both setups had four largely unswept blades in the front propeller and the back propeller.

1970s–1980s

When the 1973 oil crisis caused petroleum price spikes in the early 1970s, interest in propfans soared, and NASA-funded research began to accelerate. The propfan concept was outlined by Carl Rohrbach and Bruce Metzger of the Hamilton Standard division of United Technologies in 1975 and was patented by Rohrbach and Robert Cornell of Hamilton Standard in 1979. Later work by General Electric on similar propulsors adopted the name unducted fan, which was a modified turbofan engine, with the fan placed outside the engine nacelle on the same axis as the compressor blades.
During this era, the propeller problems became fixable. Advances were made in structural materials, such as titanium metal and graphite and glass fiber composites infused with resin. These materials replaced aluminum and steel metals in blade construction, which allowed the blades to be made thinner and stronger. Computer-aided design was also useful in refining blade characteristics. Since the blades bend and deflect with higher power loading and centrifugal force, the initial designs needed to be based on the in-motion shape. With the help of computers, the blade designers would then work backward to find the optimal unloaded shape for manufacturing purposes.

Flight test programs

Hamilton Standard, the only large American manufacturer of aircraft propellers, developed the propfan concept in the early 1970s. Hamilton Standard tested numerous variations in conjunction with NASA.
Under the Propfan Test Assessment program, Lockheed-Georgia proposed modifying a Gulfstream II to act as in-flight testbed for the propfan concept, while McDonnell Douglas proposed modifying a DC-9 for the same purpose. NASA chose the Lockheed proposal. The Gulfstream II had a nacelle added to the left wing, containing a Allison 570 turboprop engine. The engine used an eight-bladed,, single-rotation Hamilton Standard SR-7 propfan. The test engine, which was named the Allison 501-M78, had a thrust rating of. It was first operated in flight on March 28, 1987. The extensive test program, which cost about $56 million, racked up 73 flights and over 133 hours of flight time before finishing on March 25, 1988. In 1989, however, the testbed aircraft returned to the air from April 3 through April 14 to measure ground noise levels during flight. The engine was removed after that, and the aircraft was converted to a space shuttle training aircraft later that year.
File:McDonnell Douglas MD-81 McDonnell Douglas demonstrator, Farnborough UK - England, September 1988.jpg|thumb|left|The GE36 on a McDonnell Douglas MD-80 demonstrator at the 1988 Farnborough Air Show. The gearless unducted fan engine had an overall diameter of, with either eight or ten blades in front and eight blades in back.
The GE36 Unducted Fan, from American engine maker General Electric with 35-percent participation from French partner Snecma, was a variation on the original propfan concept and resembled a pusher configuration piston engine. GE's UDF had a novel direct-drive arrangement, where the reduction gearbox was replaced by a low-speed seven-stage free turbine. One set of turbine rotors drove the forward set of propellers, while the rear set was driven by the other set of rotors which rotated in the opposite direction. The turbine had 14 blade rows with seven stages. Each stage was a pair of contra-rotating rows. Airframers, who had been wary of issue-prone gearboxes since the 1950s, liked GE's gearless version of the propfan: Boeing intended to offer GE's pusher UDF engine on the 7J7 platform, and McDonnell Douglas planned to do likewise on their MD-94X airliner. The GE36 was first flight tested mounted on the #3 engine station of a Boeing 727-100 on August 20, 1986. The GE36 UDF for the 7J7 was planned to have a thrust of, but GE claimed that in general its UDF concept could cover a thrust range of, so a UDF engine could possibly match or surpass the thrust of the CF6, GE's family of widebody engines at that time.
McDonnell Douglas developed a proof-of-concept aircraft by modifying its company-owned MD-80, which is suited for propfans due to its aft fuselage-mounted engines, in preparation for the possible propfan-powered MD-91 and MD-92 derivatives and a possible MD-94X clean-sheet aircraft. They replaced the left side JT8D turbofan engine with the GE36. Test flights began in May 1987, which proved the design's airworthiness, aerodynamic characteristics, and noise signature. Following the initial tests, a first-class cabin was installed inside the aft fuselage and airline executives were offered the opportunity to experience the UDF-powered aircraft first-hand. The test and marketing flights of the GE-outfitted demonstrator aircraft concluded in 1988, exhibiting a 30% reduction in fuel consumption over turbo-fan powered MD-80, full Stage 3 noise compliance, and low levels of interior noise/vibration. The GE36 would have the same thrust on the MD-92X, but the same engine would be derated to thrust for the smaller MD-91X. The MD-80 was also successfully flight tested in April 1989 with the 578-DX propfan, which was a prototype from the Allison Engine Company that was also derived from the Allison XT701 and built with Hamilton Standard propellers. The engine program was jointly developed between Allison and another division of United Technologies, the engine maker Pratt & Whitney. Unlike the competing GE36 UDF, the 578-DX was fairly conventional, having a reduction gearbox between the LP turbine and the propfan blades. Due to jet fuel price drops and shifting marketing priorities, Douglas shelved the propfan program later that year.

Other proposed applications

Other announcements of future propfan-powered airliners included:
  • The Fokker FXX, a 100–120 seat propfan-powered aircraft that was studied in 1982
  • The MPC 75, an 80-seat, Mach 0.76 cruise speed, regional aircraft conceived by Messerschmitt-Bölkow-Blohm of West Germany and the Chinese Aero Technology Export/Import Corporation ; used as the baseline powerplant two direct-drive General Electric GE38-B5 UDF engines delivering in static thrust and cruise with a thrust-specific fuel consumption of, respectively, through an diameter propfan with 11 and 9 blades on the contra-rotating propellers; proposed as an alternative powerplant the static thrust, PW–Allison 501-M80E geared propfan engine, which was derived from the 501-M80C turboshaft that was chosen to power the United States Navy's Osprey tiltrotor aircraft; later described the propfan engine as one with the core from the T406, containing a propfan that provided of thrust in cruise with a TSFC of
  • The ATR 92, a, five- or six-abreast, 100-seat aircraft from Avions de Transport Regional and Spain's Construcciones Aeronáuticas SA, which would possibly be powered by the UDF
  • The Aerospatiale AS.100, a regional aircraft with a range of, a cruise speed of Mach 0.74–0.78 at altitude, and a capacity of 80–100 seats, that might be powered by the UDF or by a propfan version of the Allison T406 tiltrotor engine
  • The ATRA-90, an 83– to 115–seat aircraft with a range of and a cruise speed of Mach 0.8 at altitude, that was to be built by a multinational joint venture consisting of Industri Pesawat Terbang Nusantara of Indonesia, Boeing, MBB, and Fokker
  • The Tupolev Tu-334, a 126-seat aircraft that can travel with, which is powered by two Progress D-236 propfans with a specific fuel burn of 0.46 kg/kg-thrust/hour, a cruise thrust of, and a static thrust of
  • The Ilyushin Il-88, a successor to the four-turboprop Antonov An-12 tactical transporter that would be powered by two Progress D-236 propfans
  • The Ilyushin Il-118, an upgrade to the four-turboprop Ilyushin Il-18 airliner; proposed in 1984, the aircraft would instead be powered by two D-236 propfans, with the eight-bladed front propeller on each engine rotating at a speed of 1,100 rpm and the six-bladed back propeller turning at 1,000 rpm to lower noise and vibration
  • A re-engined Antonov An-124, replacing the four Progress D-18T turbofans by Kuznetsov NK-62 propfans