Propfan


A propfan, also called an open rotor engine, or unducted fan, is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. A propfan is typically designed with a large number of short, highly twisted blades, similar to a turbofan's bypass compressor. For this reason, the propfan has been variously described as an "unducted fan" or an "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.
El-Sayed differentiates between turboprops and propfans according to 11 different criteria, including number of blades, blade shape, tip speed, bypass ratio, Mach number, and cruise altitude.

Development

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 engine 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 time.
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. There were other contra-rotating propeller engines that featured on common aircraft, such as the four powerful Kuznetsov NK-12 engines on the Soviet Union's Tupolev Tu-95 Bear 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 blades in the front propeller and the back propeller, but they were also largely unswept.

1970s–1980s

When the 1973 oil crisis caused the 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 was done under 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 encountered a few decades ago 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 the 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 remaining large American manufacturer of aircraft propellers, developed the propfan concept in the early 1970s. Numerous design variations of the propfan were tested by Hamilton Standard, in conjunction with NASA in this decade. This testing led to the Propfan Test Assessment program, where 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, where the aircraft 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 as its propulsor. The test engine, which was named the Allison 501-M78, had a thrust rating of about, and 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, although most of the flight testing was done in 1987. In 1989, however, the testbed aircraft returned to the air from April 3 through April 14 to measure ground noise levels during en-route flying. The engine was removed after that, and the aircraft was converted to a space shuttle training aircraft later that year.
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 the 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 was going 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 removed the JT8D turbofan engine from the left side of the fuselage and replaced it with the GE36. Test flights began in May 1987, initially out of Mojave, California, which proved the airworthiness, aerodynamic characteristics, and noise signature of the design. 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

Aside from the aircraft mentioned above, there were several other announcements of future propfan-powered airliners, such as:
None of these projects came to fruition, however, mainly because of excessive cabin noise and low fuel prices. For General Electric, the GE36 UDF was meant to replace the CFM56 high-bypass turbofan that it produced with equal partner Snecma in their CFM International joint venture, as in the 1980s the engine was initially uncompetitive against the International Aero Engines rival offering, the IAE V2500. In December 1986, the chairman of Snecma declared that the in-development CFM56-5S2 would be the last turbofan created for the CFM56 family, and that "There is no point in spending more money on turbofans. UDF is the future." The V2500 ran into technical problems in 1987, however, and the CFM56 gained major sales momentum. General Electric became uninterested in having the GE36 cannibalize the CFM56, which also went five years before it received its first order in 1979, and while "the UDF could be made reliable by earlier standards, turbofans were getting much, much better than that." General Electric did add the UDF's blade technology directly into the GE90, the most powerful jet engine ever produced, for the Boeing 777.

1990s–current

At the beginning of the 1990s, the Soviet Union/Russia performed flight tests on the Progress D-236, a geared contra-rotating propfan engine based on the core of the Progress D-36 turbofan, with eight blades on the front propeller and six blades on the back propeller. One testbed was a propfan mounted to an Ilyushin Il-76 and flown to the Hannover ILA 90 airshow, which was intended for an unidentified four-propfan aircraft. The D-236 flew 36 times for a total of 70 flight test hours on the Il-76. The other testbed was a, mounted to a Yakovlev Yak-42E-LL and flown to the 1991 Paris Air Show, as a demonstration for the planned Yak-46 aircraft with twin propfan engines, which in its base 150-seat version would have a range of and cruise at a speed of . The Soviets claimed the D-236 had a true aerodynamic efficiency of 28 percent and a fuel savings of 30 percent over an equivalent turboprop. They also revealed plans for propfans with power ratings of.
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Like the Progress D-236, the more powerful Progress D-27 propfan engine is a contra-rotating propfan with eight front blades and six back blades, but the D-27 has advanced composite blades with a reduced thickness-to-chord ratio and a more pronounced curvature at the leading edge. An engine that was launched in 1985, the D-27 delivers of power with of thrust at takeoff. Two rear-mounted D-27 propfans propelled the Ukrainian Antonov An-180, which was scheduled for a 1995 first flight and a 1997 entry into service. In January 1994, Antonov rolled out the first prototype of the An-70 military transport aircraft, powered by four Progress D-27s attached to wings mounted to the top of the fuselage. The Russian Air Force placed an order for 164 aircraft in 2003, which was subsequently canceled. As of 2013, the An-70 was still thought to have a promising future as a freighter. However, since the propeller component of the Progress D-27 is made by Russia's SPE Aerosila, the An-70 cannot be built because of Ukraine's political conflict with Russia. Instead, Antonov began working with Turkey in 2018 to redevelop the An-70 as the rebranded An-77, so that the aircraft can comply with modern-day requirements without Russian supplier participation.
In the first decade of the 21st century, jet fuel prices began to rise again, and there was increased emphasis on engine/airframe efficiency to reduce emissions, which renewed interest in the propfan concept for jetliners that might come into service beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted contra-rotating propfans. While Rolls-Royce was lukewarm on propfan technology in the 1980s configured RB.509-11 and front, it had now developed an open rotor engine design that was thought to be a finalist for the new Irkut MS-21 narrowbody aircraft. The Rolls-Royce RB3011 engine would have a diameter of about and require a gearbox.
open rotor mockup in 2017.
The European Commission launched in 2008 an Open Rotor demonstration led by Safran within the Clean Sky program with 65 million euros funding over eight years. A demonstrator was assembled in 2015, and ground tested in May 2017 on its open-air test rig in Istres, aiming to reduce fuel consumption and associated CO2 emissions by 30% compared with current CFM56 turbofans. After the completion of ground testing at the end of 2017, Safran's geared open rotor engine had reached a technology readiness level of TRL 5. The open rotor demonstrator's twelve-blade front propeller and ten-blade back propeller have diameters of, respectively. The demonstrator, which is based on the core of the Snecma M88 military fighter engine, uses up to, provides a thrust of about, and would cruise at a speed of Mach 0.75. Safran's future open rotor engine, however, would have a maximum diameter of almost.

Limitations and solutions

Blade design

Turboprops have an optimum speed below about, because all propellers lose efficiency at high speed, due to an effect known as wave drag that occurs just below supersonic speeds. This powerful form of drag has a sudden onset, and it led to the concept of a sound barrier when first encountered in the 1940s. In the case of a propeller, this effect can happen any time the propeller is spun fast enough that the blade tips approach the speed of sound, even if the aircraft is motionless on the ground.
The most effective way to counteract this problem is by adding more blades to the propeller, allowing it to deliver more power at a lower rotational speed. This is why many World War II fighter designs started with two or three-blade propellers but by the end of the war were using up to five blades; as the engines were upgraded, new propellers were needed to more efficiently convert that power. The major downside to this approach is that adding blades makes the propeller harder to balance and maintain, and the additional blades cause minor performance penalties due to drag and efficiency issues. But even with these sorts of measures, eventually the forward speed of the plane combined with the rotational speed of the propeller blade tips will again result in wave drag problems. For most aircraft, this will occur at speeds over about.
A method of decreasing wave drag was discovered by German researchers in 1935—sweeping the wing backwards. Today, almost all aircraft designed to fly much above use a swept wing. In the 1970s, Hamilton Standard started researching propellers with similar sweep. Since the inside of the propeller is moving slower in the rotational direction than the outside, the blade is progressively more swept back toward the outside, leading to a curved shape similar to a scimitar - a practice that was first used as far back as 1909, in the Chauvière two-bladed wood propeller used on the Blériot XI. The Hamilton Standard test propfan was swept progressively to a 39-degree maximum at the blade tips, allowing the propfan to produce thrust even though the blades had a helical tip speed of about Mach 1.15.
The blades of the GE36 UDF and the 578-DX have a maximum tip speed in rotation of about, or about half the maximum tip speed for the propeller blades of a conventional turbofan. That maximum blade tip speed would be kept constant if the engine designer chooses to widen or narrow the propeller diameter.
Drag can also be reduced by making the blades thinner, which increases the speed that the blades can attain before the air ahead of them becomes compressible and causes shock waves. For example, the blades of the Hamilton Standard test propfan had a thickness-to-chord ratio that tapered from less than twenty percent at the spinner junction to two percent at the tips, with the ratio being only four percent at mid-span. Propfan blades had approximately half the thickness-to-chord ratio of the best conventional propeller blades of the era, thinned to razor-like sharpness at their edges, and weighed as little as.

Noise

Jet aircraft fly faster than conventional propeller-driven aircraft. However, they use more fuel, so that for the same fuel consumption a propeller installation produces more thrust. As fuel costs become an increasingly important aspect of commercial aviation, engine designers continue to seek ways to improve aero engine efficiency. The propfan concept was developed to deliver 35% better fuel efficiency than contemporary turbofans. In static and air tests on a modified Douglas DC-9, propfans reached a 30% improvement over the OEM turbofans. This efficiency came at a price, as one of the major problems with the propfan is noise, particularly in an era where aircraft are required to comply with increasingly strict aircraft noise regulations. The propfan research in the 1980s discovered ways to reduce noise, but at the cost of reduced fuel efficiency, mitigating some of the advantages of a propfan.
General methods for reducing noise include lowering the blade tip speeds and decreasing the blade loading, or the amount of thrust per unit of blade surface area. A concept similar to wing loading, blade loading can be reduced by lowering the thrust requirement or by increasing the amount, chord, and/or span of the blades. For contra-rotating propfans, which can be louder than turboprops or single-rotating propfans, noise can also be lowered by:
Engine makers expect propfan implementations to meet community noise regulations without sacrificing the efficiency advantage. Some think that propfan engines can potentially cause less of a community impact than turbofan engines, because the rotational speeds of a propfan are lower than that of a turbofan. Geared propfans should have an advantage over ungeared propfans for the same reason.
In 2007, the Progress D-27 was successfully modified to meet the United States Federal Aviation Administration Stage 4 regulations, which correspond to International Civil Aviation Organization Chapter 4 standards and were adopted in 2006. A 2012 trade study projected that noise from existing open rotor technology would be 10–13 decibels quieter than the maximum noise level allowed by the Stage 4 regulations; the newer Stage 5 noise limits are more restrictive than the Stage 4 requirement by only seven effective perceived noise decibels, so current propfan technology shouldn't be hindered by the Stage 5 standards. The study also projected that at existing technology levels, open rotors would be nine percent more fuel-efficient but remain 10–12 decibels louder than turbofans. Snecma, however, maintains that open-rotor tests show that its propfan engines would have about the same noise levels as its CFM LEAP turbofan engine, which entered service in 2016.
Further reductions can be achieved by redesigning the aircraft structure to shield noise from the ground. For example, another study estimated that if propfan engines were used to power a hybrid wing body aircraft instead of a conventional tube-and-wing aircraft, noise levels could be reduced by as much as 38 EPNdB compared to ICAO Chapter 4 requirements. In 2007, the British budget airline easyJet introduced its ecoJet concept, a 150-250 seat aircraft with V-mounted open rotor engines joined to the rear fuselage and shielded by a U-tail. It unsuccessfully initiated discussions with Airbus, Boeing, and Rolls-Royce to produce the aircraft.

Size

A twin-engine aircraft carrying 100–150 passengers would require propfan diameters of, and a propfan with a propeller diameter of would theoretically produce almost of thrust. These sizes achieve the desired high bypass ratios of over 30, but they are approximately twice the diameter of turbofan engines of equivalent capability. For this reason, airframers usually design the empennage with a T-tail configuration for aerodynamic purposes, and the propfans may be attached to the upper part of the rear fuselage. For the Rolls-Royce RB3011 propfan prototype, a pylon of about long would be required to connect the center of each engine to the side of the fuselage. If the propfans are mounted to the wings, the wings would be attached to the aircraft in a high wing configuration, which allows for ground clearance without requiring excessively long landing gear. For the same amount of power or thrust produced, an unducted fan requires shorter blades than a geared propfan, although the overall installation issues still apply.

Output rating

Turboprops and most propfans are rated by the amount of shaft horsepower that they produce, as opposed to turbofans and the UDF propfan type, which are rated by the amount of thrust they put out. This difference can be somewhat confusing when comparing different types of engines. The rule of thumb is that at sea level with a static engine, is roughly equivalent of thrust, but at cruise altitude, that changes to about thrust. That means a narrowbody aircraft with two engines can theoretically be replaced with a pair of propfans or with two UDF propfans.

Aircraft with propfans