Propfan

A propfan is typically designed with a large number of short, highly twisted blades, similar to the (ducted) fan in a turbofan engine.

When the 1973 oil crisis caused petroleum price spikes in the early 1970s, interest in propfans soared, and NASA-funded research began to accelerate.

Advances were made in structural materials, such as titanium metal and graphite and glass fiber composites infused with resin.

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.

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).

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.

Due to jet fuel price drops and shifting marketing priorities, Douglas shelved the propfan program later that year.

Other announcements of future propfan-powered airliners included: None of these projects came to fruition, however, mainly because of excessive cabin noise (compared to turbofans) and low fuel prices.

[45] 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.

General Electric lost interest in having the GE36 cannibalize the CFM56, which 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."

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.

[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).

[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.

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] 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.

[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.

[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.

[65] After the completion of ground testing at the end of 2017, Safran's geared open rotor engine had reached technology readiness level 5.

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.

[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.

In addition to the rotor, the design includes a nonrotating set of variable-pitch stator blades that act as flow recovery vanes.

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.

[79] That maximum blade tip speed would be kept constant despite wider or narrower propeller diameter (resulting in an RPM reduction or increase, respectively).

The study also projected that at existing technology levels, open rotors would be nine percent more fuel-efficient but remain 10–12 cumulative EPNdB louder than future aircraft with advanced ultra-high bypass ratio turbofans.

[87] 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.

[90] 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.

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.

For the same amount of power or thrust produced, an unducted fan requires shorter blades than a geared propfan,[92] although the overall installation issues still apply.