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.^[11] The propfan concept was outlined by Carl Rohrbach and Bruce Metzger of the Hamilton Standard division of United Technologies in 1975^[12] and was patented by Rohrbach and Robert Cornell of Hamilton Standard in 1979.^[3] 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.^[13] 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.^[14]

Flight test programs

Hamilton Standard, the only large American manufacturer of aircraft propellers, developed the propfan concept in the early 1970s.^[15] Hamilton Standard tested numerous variations in conjunction with NASA.^[16][17]

Under the Propfan Test Assessment (PTA) 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.^[18] NASA chose the Lockheed proposal. The Gulfstream II had a nacelle added to the left wing, containing a 6,000 horsepower (4,500 kilowatts) Allison 570 turboprop engine (derived from the XT701 turboshaft developed for the Boeing Vertol XCH-62 heavy lift helicopter). The engine used an eight-bladed, 9-foot diameter (2.7-metre; 110-inch; 270-centimetre), single-rotation Hamilton Standard SR-7 propfan. The test engine, which was named the Allison 501-M78,^[19] had a thrust rating of 9,000 lbf (40 kN).^[20] It was first operated in flight on March 28, 1987.^[21] The extensive test program, which cost about $56 million,^[22] racked up 73 flights and over 133 hours of flight time before finishing on March 25, 1988.^[23] In 1989, however, the testbed aircraft returned to the air from April 3 through April 14 to measure ground noise levels during flight.^[24][25] The engine was removed after that, and the aircraft was converted to a space shuttle training aircraft later that year.^[26]

The GE36 Unducted Fan (UDF), from American engine maker General Electric (GE) with 35-percent participation from French partner Snecma (now Safran), 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.^[27] Airframers, who had been wary of issue-prone gearboxes since the 1950s, liked GE's gearless version of the propfan:^[14] Boeing intended to offer GE's pusher UDF engine on the 7J7 platform (which would have had a cruise speed of Mach 0.83),^[28] 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.^[29] The GE36 UDF for the 7J7 was planned to have a thrust of 25,000 pounds-force (110 kN), but GE claimed that in general its UDF concept could cover a thrust range of 9,000 to 75,000 lbf (40 to 334 kN),^[30] 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 (like its DC-9 ancestor), 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,^[31] 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 25,000 lbf (110 kN) thrust on the MD-92X, but the same engine would be derated to 22,000 lbf (98 kN) 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 (at that time a division of General Motors) 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.

1990s

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 10,100 hp (7,500 kW) propfan mounted to an Ilyushin Il-76 and flown to the Hannover ILA 90 airshow, which was intended for an unidentified four-propfan aircraft.^[48] The D-236 flew 36 times for a total of 70 flight test hours on the Il-76.^[49] The other testbed was a 10,990 hp (8,195 kW), 14 ft unit (4.2 m; 170 in; 420 cm) 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,^[50] which in its base 150-seat version would have a range of 1,900 nmi (2,200 mi; 3,500 km) and cruise at a speed of 460 kn (530 mph; 850 km/h; 780 ft/s; 240 m/s)^[51] (Mach 0.75).^[52] 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 14,100 and 30,200 hp (10,500 and 22,500 kW).^[48]

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,^[52] but the D-27 has advanced composite blades with a reduced thickness-to-chord ratio and a more pronounced curvature at the leading edge.^[53] An engine that was launched in 1985, the D-27^[54] delivers 14,000 hp (10,440 kW) of power with 27,000 lbf (119 kN) of thrust at takeoff.^[55] 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.^[56] 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.^[55] The Russian Air Force placed an order for 164 aircraft in 2003, later canceled. As of 2013, the An-70 was still thought to have a promising future as a freighter.^[57] Since the propeller component of the Progress D-27 is made by Russia's SPE Aerosila, however, the An-70 was stymied by the Russo-Ukrainian War. Antonov began working instead with Turkey in 2018 to redevelop the An-70 as a rebranded An-77, so that the aircraft can comply with modern-day requirements without Russian supplier participation.^[58]

Twenty-first century

In the first decade of the 21st century, rising jet fuel prices increased emphasis on engine/airframe efficiency to reduce emissions, which renewed interest in the propfan concept for jetliners beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus patented aircraft designs with twin rear-mounted contra-rotating propfans.^[59] Rolls-Royce had the rear (pusher) configured RB.509-11 and front (tractor) configured RB.509-14 geared propfan designs, which produced 15,000–25,000 lbf thrust (6,800–11,300 kgf; 67–111 kN) using the gas generator from its XG-40 engine^[60] with 13,000 hp (9,700 kW) of shaft power.^[61] It became lukewarm on propfan technology in the 1980s^[62]although it developed an open rotor design that was thought to be a finalist for the Irkut MS-21 narrowbody aircraft.^[63] The Rolls-Royce RB3011 engine would have a diameter of about 170 in (430 cm; 14 ft; 4.3 m) and require a 16,000 shaft hp (12,000 kW) gearbox.^[64]

The European Commission launched an Open Rotor demonstration in 2008 led by Safran within the Clean Sky program funded with €65 million 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.^[65] After the completion of ground testing at the end of 2017, Safran's geared open rotor engine had reached technology readiness level 5.^[66] The demonstrator's twelve-blade front propeller and ten-blade back propeller had diameters of 13.1 and 12.5 ft (4.0 and 3.8 m; 160 and 150 in; 400 and 380 cm), respectively. The demonstrator, based on the core of the Snecma M88 military fighter engine, uses up to 12,200 horsepower (9 megawatts), provides a thrust of about 22,000 lbf (100 kN), and would cruise at a speed of Mach 0.75.^[67] Safran's future open rotor engine, however, would have a maximum diameter of almost 14.8 ft (4.50 m; 177 in; 450 cm).^[68]

In 2007, the Progress D-27 was successfully modified to meet the United States Federal Aviation Administration (FAA) Stage 4 regulations, which correspond to International Civil Aviation Organization (ICAO) Chapter 4 standards.^[69] A 2012 trade study projected that propfan noise would be 10–13 decibels quieter than allowed by Stage 4 regulations.^[70] Stage 5 noise limits reduce the limits by only seven effective perceived noise decibels (EPNdB),^[71] within the propfan noise envelope. The study also projected that open rotors would be nine percent more fuel-efficient but remain 10–12 decibels louder than turbofans.^[70] Snecma claimed that its propfan engines would have about the same noise levels as its CFM LEAP turbofan engine.^[72]

In 2021, CFM International announced its Revolutionary Innovation for Sustainable Engines (RISE) development program to produce a single-stage, gear-driven propfan paired with active stators in a puller/tractor, configuration with flight tests to begin by 2025. The rotor was expected to be 12–13 ft (3.7–4.0 m) in diameter. The engine was expected to produce 20,000–35,000 lbf (9,100–15,900 kgf; 89–156 kN) of thrust, with a 20% increase in fuel efficiency. The company claimed its motivation was the global emphasis on reducing emissions. The engine was planned to support both hydrogen and sustainable aviation fuels. The engine was expected to include a compact high-pressure core and a recuperating system to preheat combustion air with exhaust heat along with ceramic matrix composites in the hot section and resin-transfer-molded composite fan blades. In addition to the rotor, the design includes a nonrotating set of variable-pitch stator blades that act as flow recovery vanes. The design increases the fan-pressure ratio and reduces rotor loading, increasing airspeed. The fan stage is to be powered by a high-speed booster compressor and a high-speed, low-pressure-shaft-driven front gearbox. The engine is slated for certification as an "integrated engine" instead of a traditional "propeller/engine" because of its airframe integration complexity.^[73] CFM planned for an aerodynamically three-dimensional rotor with 12 woven carbon-fiber composite blades. Aided by a smaller engine core, the CFM RISE engine would have a bypass ratio of 75.^[74]

Blade design

Turboprops have an optimum speed below about 450 mph (390 kn; 720 km/h),^[75] because propellers lose efficiency at high speed, due to an effect known as wave drag that occurs just below supersonic speeds. This powerful drag has a sudden onset, and it led to the concept of a sound barrier when first encountered in the 1940s. This effect can happen whenever the propeller is spun fast enough that the blade tips approach the speed of sound.

The most effective way to address this problem is by adding 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. 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 (together known as the helical tip speed) will again result in wave drag problems. For most aircraft, this will occur at speeds over about 450 mph (390 kn; 720 km/h).

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 450 mph (390 kn; 720 km/h) use a swept wing. 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. (At the blade root, the blade is actually swept forward into the rotational direction, to counter the twisting that is generated by the backward swept blade tips.)^[76] 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.^[77]

The blades of the GE36 UDF and the 578-DX have a maximum tip speed in rotation of about 750–800 ft/s (230–240 m/s; 510–550 mph; 820–880 km/h),^[78] or about half the maximum tip speed for the propeller blades of a conventional turbofan.^[79] That maximum blade tip speed would be kept constant despite wider or narrower propeller diameter (resulting in an RPM reduction or increase, respectively).^[5]

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 20% at the spinner junction to 2% at the tips, and 4% at mid-span.^[77] Propfan blades had approximately half the thickness-to-chord ratio of the best conventional propeller blades of the era,^[80] thinned to razor-like sharpness at their edges,^[14][81] and weighed as little as 20 pounds (9.1 kg).^[82] (The GE36 UDF engine that was tested on the Boeing 727 had front and back blades that weighed 22.5 and 21.5 lb (10.2 and 9.8 kg) each.)^[83]

Noise

One of the major problems with the propfan is noise. 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 tip speeds and decreasing 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, width, and/or length of the blades. For contra-rotating propfans, which can be louder than turboprops or single-rotating propfans, noise can also be lowered by:^[84]

• increasing the gap between the propellers;
• keeping back propeller blade lengths shorter than those of the front propeller, so that the back propeller blades avoid cutting through the blade tip vortices of the front propeller (blade-vortex interaction);
• using different numbers of blades on the two propellers, to avoid acoustic reinforcement; and
• turning the front propeller and back propeller at different speeds, also to prevent acoustic reinforcement.^[43]

Size

A twin-engine aircraft carrying 100–150 passengers would require propfan diameters of 120–168 inches (300–430 cm; 10.0–14.0 ft; 3.0–4.3 m),^[77] and a propfan with a propeller diameter of 236 in (600 cm; 19.7 ft; 6.0 m) would theoretically produce almost 60,000 lbf (270 kN) of thrust.^[89] These sizes achieve the desired high bypass ratios of over 30, but they are approximately twice the diameter of turbofan engines of equivalent capability.^[67] For this reason, airframers usually design the empennage with a T-tail configuration in order to avoid the turbulent propwash adversely influencing the elevators and causing vibration issues therein. The propfans may be attached to the upper part of the rear fuselage. For the Rolls-Royce RB3011 propfan prototype, a pylon of about 8.3 ft (2.54 m; 100 in; 254 cm) long would be required to connect the center of each engine to the side of the fuselage.^[90] 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,^[91] although the overall installation issues still apply.

Output rating

Turboprops and most propfans are rated by the amount of shaft horsepower (shp) that they produce, as opposed to turbofans and the UDF propfan type, which are rated by the amount of thrust they put out. The rule of thumb is that at sea level with a static engine, 1 shaft horsepower (750 watts) is roughly equivalent of 2 pounds-force (8.9 N) thrust, but at cruise altitude, that changes to about 1 pound-force (4.4 N) thrust. That means two 25,000 lbf thrust (110 kN) engines can theoretically be replaced with a pair of 12,000–13,000 shaft hp (8,900–9,700 kW) propfans or with two 25,000 lbf thrust (110 kN) UDF propfans.^[5]

Proposed aircraft with propfans

• Antonov An-180
• ATRA-90
• Boeing 7J7
• Grumman MR1
• Grumman MR2
• Grumman VSTOL^[92][93][94]
• McDonnell Douglas MD-94X
• MPC-75
• Yakovlev Yak-44
• Yakovlev Yak-46