A propfan was first defined 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 a high performance. The propfan concept was first revealed by Carl Rohrbach and Bruce Metzger of the Hamilton Standard Division of United Technologies in 1975 and was
patented by Robert Cornell and Carl Rohrbach 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
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...

 engine, with the fan placed outside the engine nacelle on the same axis as the compressor
Axial compressor
Axial compressors are rotating, airfoil-based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have...

 blades. Propfans are also known as ultra-high bypass (UHB) engines and, most recently, open flux rotor jet engines. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop
A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear.The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...


Propeller blade tip speed limit

Turboprops have an optimum speed below about 450 mph (700 km/h). The reason is that all propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...

s lose efficiency at high speed, due to an effect known as wave drag
Wave drag
In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.- Overview :...

 that occurs just below supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...

 speeds. This powerful form of drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 has a sudden onset, and led to the concept of a sound barrier
Sound barrier
The sound barrier, in aerodynamics, is the point at which an aircraft moves from transonic to supersonic speed. The term, which occasionally has other meanings, came into use during World War II, when a number of aircraft started to encounter the effects of compressibility, a collection of several...

 when it was 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 near the speed of sound, even if the aircraft is motionless on the ground.

The most effective way to counteract this problem (to some degree) 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
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 fighter designs started with two or three-blade propellers and by the end of the war were using up to five blades in some cases as the engines were upgraded and new propellers were needed to more effectively absorb 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 at some point the forward speed of the plane combined with the rotational speed of the propeller will once again result in wave drag problems. For most aircraft this will occur at speeds over about 450 mph (700 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 (700 km/h) use a swept wing
Swept wing
A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...

. In the 1970s, Hamilton Standard
Hamilton Standard
Hamilton Standard, an aircraft propeller parts supplier, was formed in 1929 when United Aircraft and Transport Corporation consolidated Hamilton Aero Manufacturing and Standard Steel Propeller into the Hamilton Standard Propeller Corporation. Other members of the corporation included Boeing,...

 started researching propellers with similar sweep. Since the inside of the propeller is moving more slowly than the outside, the blade is progressively more swept toward the outside, leading to a curved shape similar to a scimitar
A scimitar is a backsword or sabre with a curved blade, originating in Southwest Asia .The Arabic term saif translates to "sword" in general, but is normally taken to refer to the scimitar type of curved backsword in particular.The curved sword or "scimitar" was widespread throughout the Muslim...

 - a practice that was first used as far back as 1909, in the Chauviere make of two-bladed wood propeller used on the Blériot XI
Blériot XI
The Blériot XI is the aircraft in which, on 25 July 1909, Louis Blériot made the first flight across the English Channel made in a heavier-than-air aircraft . This achievement is one of the most famous accomplishments of the early years of aviation, and not only won Blériot a lasting place in...


Jet aircraft fuel economy

Jet aircraft are well known for permitting greater thrusts and higher speeds than could be achieved by conventional propeller-driven aircraft operating within the same aerodynamic envelope. However, jet aircraft are limited in fuel economy. In fact, for the same fuel consumption, a propeller-driven aircraft can produce greater thrust. As fuel costs become an increasingly important aspect of commercial aviation, aircraft engine designers continue to seek an optimal combination of jet engine thrust ratios and propeller fuel 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
McDonnell Douglas DC-9
The McDonnell Douglas DC-9 is a twin-engine, single-aisle jet airliner. It was first manufactured in 1965 with its maiden flight later that year. The DC-9 was designed for frequent, short flights. The final DC-9 was delivered in October 1982.The DC-9 was followed in subsequent modified forms by...

, 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 Stage III and Stage IV noise requirements.

The Hamilton Standard
Hamilton Standard
Hamilton Standard, an aircraft propeller parts supplier, was formed in 1929 when United Aircraft and Transport Corporation consolidated Hamilton Aero Manufacturing and Standard Steel Propeller into the Hamilton Standard Propeller Corporation. Other members of the corporation included Boeing,...

 Division of United Technologies developed the propfan concept in the early 1970s. Numerous design variations of the propfan were tested by Hamilton Standard, in conjunction with NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 in this decade. This testing led to the Propfan Test Assessment (PTA) program, where Lockheed-Georgia proposed modifying a Gulfstream II to act as in-flight testbed for the propfan concept and McDonnell Douglas proposed modifying a DC-9 for the same purpose. NASA choose the Lockheed proposal, where the aircraft had a nacelle added to the left wing, containing a 6000 hp Allison 570 turboprop engine (derived from the XT701 turboshaft developed for the Boeing Vertol XCH-62
Boeing Vertol XCH-62
|-See also:-References:* Wilson, Michael. "". Flight International, 13 July 1972, pp. 44c–47.-External links:* * * at Friends of The Helicopter Museum...

 program), powering a 9-foot diameter Hamilton Standard SR-7 propfan. The aircraft, so configured, first flew in March 1987. After an extensive test program, the modifications were removed from the aircraft.

General Electric
GE Aviation, a subsidiary of General Electric, is headquartered in Evendale, Ohio . GE Aviation is the top supplier of aircraft engines in the world, and offers engines for the majority of commercial aircraft. GE Aviation is part of the General Electric conglomerate, which is one of the world's...

's GE36 Unducted Fan was a variation on the original propfan concept, and appears similar to a pusher configuration
Pusher configuration
In a craft with a pusher configuration the propeller are mounted behind their respective engine. According to Bill Gunston, a "pusher propeller" is one mounted behind engine so that drive shaft is in compression...

 piston engine. GE's UDF has a novel direct drive arrangement, where the reduction gearbox is replaced by a low-speed seven-stage free turbine. The turbine rotors drive one propeller, while the other propeller is connected to the free turbine stators and rotates in the opposite direction. So, in effect, the power turbine has 14 stages. Boeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...

 intended to offer GE's pusher UDF engine on the 7J7 platform, and McDonnell Douglas
McDonnell Douglas
McDonnell Douglas was a major American aerospace manufacturer and defense contractor, producing a number of famous commercial and military aircraft. It formed from a merger of McDonnell Aircraft and Douglas Aircraft in 1967. McDonnell Douglas was based at Lambert-St. Louis International Airport...

 was going to do likewise on their MD-94X airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...

. The GE36 was first flight tested mounted on the #3 engine station of a Boeing 727-100 in 1986.

McDonnell Douglas developed a proof-of-concept aircraft by modifying its company-owned MD-80. They removed the JT8D turbofan engine from the left side of the fuselage and replaced it with the GE36. A number of test flights were conducted, 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 III noise compliance, and low levels of interior noise/vibration. Due to jet fuel price drops and shifting marketing priorities, Douglas shelved the program the following year.

In the 1980s, Allison collaborated with Pratt & Whitney on demonstrating the 578-DX
Pratt & Whitney/Allison 578-DX
-External links:* * , Flight International, June 12, 2007.*...

 propfan. Unlike the competing GE36 UDF, the 578-DX was fairly conventional, having a reduction gearbox between the LP turbine and the propfan blades. The 578-DX was successfully flight tested on a McDonnell Douglas MD-80. However, none of the above projects came to fruition, mainly because of excessive cabin noise (compared to turbofans) and low fuel prices.

The Progress D-27
Progress D-27
-See also:- External links :*...

 propfan, developed in the U.S.S.R, is even more unconventional in layout, with the propfan blades at the front of the engine in a tractor configuration. Two rear-mounted D-27's propfans propelled the Antonov An-180
Antonov An-180
-External links:*...

, which was scheduled for a 1995 entry into service. Another Russian propfan application was the Yakovlev Yak-46. During the 1990s, Antonov
Antonov, or Antonov Aeronautical Scientist/Technical Complex , formerly the Antonov Design Bureau, is a Ukrainian aircraft manufacturing and services company with particular expertise in the field of very large aircraft construction. Antonov ASTC is a state-owned commercial company...

 also developed the An-70
Antonov An-70
The Antonov An-70 is a four-engine medium-range transport aircraft, and the first large aircraft to be powered by propfan engines. It is being developed by Ukraine's Antonov design bureau to replace the obsolete An-12 military transport aircraft...

, powered by four Progress D-27s in a tractor configuration
Tractor configuration
thumb|right|[[Evektor-Aerotechnik|Aerotechnik EV97A Eurostar]], a tractor configuration aircraft, being pulled into position by its pilot for refuelling....

; the Russian Air Force
Russian Air Force
The Russian Air Force is the air force of Russian Military. It is currently under the command of Colonel General Aleksandr Zelin. The Russian Navy has its own air arm, the Russian Naval Aviation, which is the former Soviet Aviatsiya Voyenno Morskogo Flota , or AV-MF).The Air Force was formed from...

 placed an order for 164 aircraft in 2003
2003 in aviation
This is a list of aviation-related events from 2003:-January:* January 8 – The Turkish Airlines Avro RJ100 Konya, operating as Flight 634, crashes in thick fog while on final approach to land at Diyarbakır Airport in Diyarbakır, Turkey...

, which was subsequently canceled. However, the An-70 remains available for further investment and production.

With the current high price for jet fuel and the emphasis on engine/airframe efficiency to reduce emissions, there is renewed interest in the propfan concept for jetliners that might come into service beyond the Boeing 787
Boeing 787
The Boeing 787 Dreamliner is a long-range, mid-size wide-body, twin-engine jet airliner developed by Boeing Commercial Airplanes. It seats 210 to 290 passengers, depending on the variant. Boeing states that it is the company's most fuel-efficient airliner and the world's first major airliner to use...

 and Airbus A350
Airbus A350
The Airbus A350 is a family of long-range, wide-body jet airliners under development by European aircraft manufacturer Airbus.A consortium originally comprising European aerospace companies from the UK, France, Spain and West Germany, Airbus is now fully owned by EADS and since 2001 has been known...

XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted counter-rotating propfans.

Aircraft with propfans

  • Antonov An-70
    Antonov An-70
    The Antonov An-70 is a four-engine medium-range transport aircraft, and the first large aircraft to be powered by propfan engines. It is being developed by Ukraine's Antonov design bureau to replace the obsolete An-12 military transport aircraft...

  • Beriev A-40
    Beriev A-40
    This article is about the amphibious jet. For Soviet tank airlift project, see Antonov A-40.This article is about the amphibious jet. For Soviet tank airlift project, see Antonov A-40.This article is about the amphibious jet...

  • EcoJet
  • McDonnell Douglas MD-94X

See also

External links

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