![578-DX (on testbed)](https://upload.wikimedia.org/wikipedia/commons/7/73/578-DX_%28on_testbed%29.png)
A propfan, also called an open rotor engine, or unducted fan (as opposed to a
ducted fan
In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller. When used in vertical takeoff and landing
(VTOL) applicatio ...
), is a type of
aircraft engine
An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many ...
related in concept to both the
turboprop
A turboprop is a turbine engine that drives an aircraft propeller.
A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle. Air enters the intake and is compressed by the compressor. Fuel ...
and
turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
, 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 the
(ducted) fan in a turbofan engine. For this reason, the propfan has been variously described as an "unducted fan" (UDF) or an "ultra-high-bypass (UHB) turbofan."
Definition
![Propulsive efficiency for different engine types and Mach numbers](https://upload.wikimedia.org/wikipedia/commons/8/87/Propulsive_efficiency_for_different_engine_types_and_Mach_numbers.png)
In the 1970s,
Hamilton Standard
Hamilton Standard was an American aircraft propeller parts supplier. It was formed in 1929 when United Aircraft and Transport Corporation consolidated Hamilton Aero Manufacturing and Standard Steel Propeller into the Hamilton Standard Propeller C ...
described its propfan as "a small diameter, highly loaded multiple bladed
variable pitch
Flight dynamics in aviation and spacecraft, is the study of the performance, stability, and control of vehicles flying through the air or in outer space. It is concerned with how forces acting on the vehicle determine its velocity and attitude w ...
propulsor having swept blades with thin advanced
airfoil
An airfoil (American English) or aerofoil (British English) is the cross-sectional shape of an object whose motion through a gas is capable of generating significant lift, such as a wing, a sail, or the blades of propeller, rotor, or turbine.
...
sections, integrated with a
nacelle
A nacelle ( ) is a "streamlined body, sized according to what it contains", such as an engine, fuel, or equipment on an aircraft. When attached by a pylon entirely outside the airframe, it is sometimes called a pod, in which case it is attached ...
contoured to retard the airflow through the blades thereby reducing
compressibility
In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a fl ...
losses and designed to operate with a turbine engine and using a single stage
reduction gear
A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission ...
resulting in high performance."
In 1982, the weekly aviation magazine
Flight International
''Flight International'' is a monthly magazine focused on aerospace. Published in the United Kingdom and founded in 1909 as "A Journal devoted to the Interests, Practice, and Progress of Aerial Locomotion and Transport", it is the world's oldes ...
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
Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque. Examples include some aircraft propellers, res ...
propfans.
In 1986, British engine maker
Rolls-Royce
Rolls-Royce (always hyphenated) may refer to:
* Rolls-Royce Limited, a British manufacturer of cars and later aero engines, founded in 1906, now defunct
Automobiles
* Rolls-Royce Motor Cars, the current car manufacturing company incorporated in ...
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'' (OR) became a preferred term for propfan technology in research and news reports, with ''contra-rotating open rotor'' (CROR) also occasionally being used to distinguish between single-rotation propfans. As of 2015, the
European Aviation Safety Agency
The European Union Aviation Safety Agency (EASA) is an agency of the European Union (EU) with responsibility for civil aviation safety. It carries out certification, regulation and standardisation and also performs investigation and monitorin ...
(EASA) defined an open rotor concretely (but broadly) 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 (the more commonly used term for propfan in the 21st century), calling it "a turbine engine featuring contra-rotating fan stages not enclosed within a casing." The engine uses a
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
to drive an unshrouded (open)
contra-rotating propeller
Aircraft equipped with contra-rotating propellers, also referred to as CRP, coaxial contra-rotating propellers, or high-speed propellers, apply the maximum power of usually a single piston engine, piston or turboprop engine to drive a pair of ...
like a turboprop, but the design of the
propeller
A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
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
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for ev ...
,
Mach number
Mach number (M or Ma) (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound.
It is named after the Moravian physicist and philosopher Ernst Mach.
: \mathrm = \frac ...
, and
cruise altitude
Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
.
History
About a decade after German
aerospace engineer
Aerospace engineering is the primary field of engineering concerned with the development of aircraft and spacecraft. It has two major and overlapping branches: aeronautical engineering and astronautical engineering. Avionics engineering is si ...
s began exploring the idea of using
swept wing
A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction.
Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigate ...
s to reduce
drag on
transonic
Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but 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
Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
tip speeds, so that engines with exposed propellers could power aircraft to speeds and cruising altitudes only attained by new
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
and
turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
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
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
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 (front) fan and 12 blades in the aft (back) 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
The Kuznetsov NK-12 is a Soviet turboprop engine of the 1950s, designed by the Kuznetsov design bureau. The NK-12 drives two large four-bladed contra-rotating propellers, diameter (NK-12MA), and diameter (NK-12MV). It is the most powerful tu ...
engines (each powering its own set of coaxial contra-rotating propellers) on the Soviet Union's
Tupolev Tu-95
The Tupolev Tu-95 (russian: Туполев Ту-95; NATO reporting name: "Bear") is a large, four-engine turboprop-powered strategic bomber and missile platform. First flown in 1952, the Tu-95 entered service with the Long-Range Aviation of the ...
high-speed military bomber and
Antonov An-22
The Antonov An-22 "Antei" (, ''An-22 Antej''; English ''Antaeus'') (NATO reporting name "Cock") is a heavy military transport aircraft designed by the Antonov Design Bureau in the Soviet Union. Powered by four turboprop engines each driving a pa ...
military transport aircraft
A military transport aircraft, military cargo aircraft or airlifter is a military aircraft, military-owned transport aircraft used to support military operations by airlifting troops and military equipment. Transport aircraft are crucial to ma ...
, and the
Armstrong Siddeley Double Mamba
The Armstrong Siddeley Double Mamba is a turboprop engine design developed in the late 1940s of around . It was used mostly on the Fairey Gannet anti-submarine aircraft developed for the Fleet Air Arm of the Royal Navy.
Design and developmen ...
(ASMD) engines (both connected to a lone set of coaxial contra-rotating propellers) on the British
Fairey Gannet
The Fairey Gannet is a carrier-borne aircraft that was designed and produced by the British aircraft manufacturer the Fairey Aviation Company. It was developed for the Royal Navy, being the first fixed-wing aircraft to combine both the search an ...
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
The 1973 oil crisis or first oil crisis began in October 1973 when the members of the Organization of Arab Petroleum Exporting Countries (OAPEC), led by Saudi Arabia, proclaimed an oil embargo. The embargo was targeted at nations that had supp ...
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
United Technologies Corporation (UTC) was an American multinational conglomerate headquartered in Farmington, Connecticut. It researched, developed, and manufactured products in numerous areas, including aircraft engines, aerospace systems ...
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
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
engine, with the fan placed outside the engine nacelle
A nacelle ( ) is a "streamlined body, sized according to what it contains", such as an engine, fuel, or equipment on an aircraft. When attached by a pylon entirely outside the airframe, it is sometimes called a pod, in which case it is attached ...
on the same axis as the compressor
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...
blades.
During this era, the propeller problems became fixable. Advances were made in structural materials, such as titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
metal and graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
and glass fiber
Glass fiber ( or glass fibre) is a material consisting of numerous extremely fine fibers of glass.
Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the inventio ...
composites infused with resin
In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...
. These materials replaced aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
and steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
metals in blade construction, which allowed the blades to be made thinner and stronger. Computer-aided design
Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve c ...
was also useful in refining blade characteristics. Since the blades bend and deflect with higher power loading and centrifugal force
In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is paralle ...
, 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
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
.
Under the Propfan Test Assessment (PTA) program, Lockheed-Georgia proposed modifying a Gulfstream II
The Gulfstream II (G-II) is an American twin engine business jet designed and built by Grumman and then in succession, Grumman American and finally Gulfstream American. Its Grumman model number is G-1159 and its US military designation is C-11A. ...
to act as in-flight testbed for the propfan concept, while McDonnell Douglas
McDonnell Douglas was a major American aerospace manufacturing corporation and defense contractor, formed by the merger of McDonnell Aircraft and the Douglas Aircraft Company in 1967. Between then and its own merger with Boeing in 1997, it produ ...
proposed modifying a DC-9
The McDonnell Douglas DC-9 is an American five-abreast single-aisle aircraft designed by the Douglas Aircraft Company. It was initially produced by the developer company as the Douglas DC-9 until August 1967 and then by McDonnell Douglas.
After ...
for the same purpose. NASA chose the Lockheed proposal. The Gulfstream II had a nacelle added to the left wing, containing a Allison
Allison may refer to:
People
* Allison (given name)
* Allison (surname) (includes a list of people with this name)
* Eugene Allison Smith (1922-1980), American politician and farmer
Companies
* Allison Engine Company, American aircraft engine ...
570 turboprop engine (derived from the XT701 turboshaft developed for the Boeing Vertol XCH-62
The Boeing Vertol XCH-62 (Model 301) was a triple-turbine, heavy-lift helicopter project designed for the United States Army by Boeing Vertol. Approved in 1971, one prototype reached 95% completion before it was canceled in 1975. The prototype wa ...
heavy lift helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by horizontally spinning rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward and laterally. These attributes ...
). 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
A testbed aircraft is an aeroplane, helicopter or other kind of aircraft intended for flight research or testing the aircraft concepts or on-board equipment. These could be specially designed or modified from serial production aircraft.
Use of te ...
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
The Shuttle Training Aircraft (STA) was a NASA training vehicle that duplicated the Space Shuttle's approach profile and handling qualities, allowing Space Shuttle pilots to simulate Shuttle landings under controlled conditions before attempting ...
later that year.
The GE36 Unducted Fan (UDF), from American engine maker General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
(GE) with 35-percent participation from French partner Snecma
Safran Aircraft Engines, previously Snecma (''Société nationale d'études et de construction de moteurs d'aviation'') or Snecma Moteurs, is a French aerospace engine manufacturer headquartered in Courcouronnes and a subsidiary of Safran. It ...
(now Safran
Safran S.A. is a French multinational company that designs, develops and manufactures aircraft engines, rocket engines as well as various aerospace and defense-related equipment or their components. It was formed by a merger between SNECMA and ...
), was a variation on the original propfan concept and resembled a pusher configuration
In an aircraft with a pusher configuration (as opposed to a tractor configuration), the propeller(s) are mounted behind their respective engine(s). Since a pusher propeller is mounted behind the engine, the drive shaft is in compression in nor ...
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
The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and product ...
intended to offer GE's pusher UDF engine on the 7J7 platform (which would have had a cruise speed of Mach 0.83), and McDonnell Douglas
McDonnell Douglas was a major American aerospace manufacturing corporation and defense contractor, formed by the merger of McDonnell Aircraft and the Douglas Aircraft Company in 1967. Between then and its own merger with Boeing in 1997, it produ ...
planned to do likewise on their MD-94X
The McDonnell Douglas MD-94X was a planned propfan-powered airliner, intended to begin production in 1994. Announced in January 1986, the aircraft was to seat between 160 and 180 passengers, possibly using a twin-aisle configuration. An all-new ...
airliner
An airliner is a type of aircraft for transporting passengers and air cargo. Such aircraft are most often operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an ...
. The GE36 was first flight tested mounted on the #3 engine station of a Boeing 727-100
The Boeing 727 is an American Narrow-body aircraft, narrow-body airliner that was developed and produced by Boeing Commercial Airplanes.
After the heavy Boeing 707, 707 quad-jet was introduced in 1958, Boeing addressed the demand for shorter f ...
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
The General Electric CF6, US military designations F103 and F138, is a family of high-bypass turbofan engines produced by GE Aviation. Based on the TF39, the first high-power high-bypass jet engine, the CF6 powers a wide variety of civilian a ...
, 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
The McDonnell Douglas MD-94X was a planned propfan-powered airliner, intended to begin production in 1994. Announced in January 1986, the aircraft was to seat between 160 and 180 passengers, possibly using a twin-aisle configuration. An all-new ...
clean-sheet aircraft. They replaced the left side JT8D
The Pratt & Whitney JT8D is a low-bypass (0.96 to 1) turbofan engine introduced by Pratt & Whitney in February 1963 with the inaugural flight of the Boeing 727. It was a modification of the Pratt & Whitney J52 turbojet engine which powered the ...
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
The Allison Engine Company was an American aircraft engine manufacturer. Shortly after the death of James A. Allison, James Allison in 1929 the company was purchased by the Fisher Body, Fisher brothers. Fisher sold the company to General Motors, ...
(at that time a division of General Motors
The General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. It is the largest automaker in the United States and ...
) 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
Pratt & Whitney is an American aerospace manufacturer with global service operations. It is a subsidiary of Raytheon Technologies. Pratt & Whitney's aircraft engines are widely used in both civil aviation (especially airlines) and military aviat ...
. 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
Fokker was a Dutch aircraft manufacturer named after its founder, Anthony Fokker. The company operated under several different names. It was founded in 1912 in Berlin, Germany, and became famous for its fighter aircraft in World War I. In 1919 ...
FXX, a 100-120 seat propfan-powered aircraft that was studied in 1982
* The MPC-75
MPC 75 was an aircraft project of the company "MPC Aircraft GmbH" a subsidiary of "Deutsche Airbus". Work on the project was done mainly between 1988 and 1992 in Hamburg, Germany. Predevelopment work was finished, however the project never got ...
, an 80-seat, Mach 0.76 cruise speed, regional aircraft conceived by Messerschmitt-Bölkow-Blohm
Messerschmitt-Bölkow-Blohm (MBB) was a West German aerospace manufacturer. It was formed during the late 1960s as the result of efforts to consolidate the West German aerospace industry; aircraft manufacturer Messerschmitt AG merged with the civi ...
(MBB) of West Germany
West Germany is the colloquial term used to indicate the Federal Republic of Germany (FRG; german: Bundesrepublik Deutschland , BRD) between its formation on 23 May 1949 and the German reunification through the accession of East Germany on 3 O ...
and the Chinese Aero Technology Export/Import Corporation (CATIC); used as the baseline powerplant two direct-drive
A direct-drive mechanism is a mechanism design where the force or torque from a prime mover is transmitted directly to the effector device (such as the drive wheels of a vehicle) without involving any intermediate couplings such as a gear train or ...
General Electric GE38
The General Electric GE38 is a gas turbine developed by GE Aviation for turboprop and turboshaft applications. It powers the Sikorsky CH-53K King Stallion as the T408.
Design and development
The GE27 was developed in the early 1980s under the " ...
-B5 UDF engines delivering in static thrust and cruise with a thrust-specific fuel consumption
Thrust-specific fuel consumption (TSFC) is the fuel efficiency of an engine design with respect to thrust output. TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust (newtons, or N), hence ''thrust-specific''. This fi ...
(TSFC) 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
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...
's Osprey
The osprey (''Pandion haliaetus''), , also called sea hawk, river hawk, and fish hawk, is a diurnal, fish-eating bird of prey with a cosmopolitan range. It is a large raptor reaching more than in length and across the wings. It is brown o ...
tiltrotor
A tiltrotor is an aircraft which generates lift and propulsion by way of one or more powered rotors (sometimes called ''proprotors'') mounted on rotating shafts or nacelles usually at the ends of a fixed wing. Almost all tiltrotors use a trans ...
aircraft; later described the propfan engine as one with the core from the T406 (the military designation for the Osprey's powerplant), 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 (ATR, a joint venture between France's Aerospatiale and Italy's Aeritalia
Aeritalia was an aerospace engineering corporation based in Italy. It was formed out of the merger of two aviation companies, Fiat Aviazione and Aerfer, in 1969.
Aeritalia continued several programs of its preceding companies, perhaps most pro ...
) and Spain's Construcciones Aeronáuticas SA
Construcciones Aeronáuticas SA (CASA) was a Spanish aircraft manufacturer that was founded in 1923 and began manufacturing aircraft the following year. In 1999 it became a subsidiary of the EADS (European Aeronautic Defence and Space Company) ...
(CASA), 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
A tiltrotor is an aircraft which generates lift and propulsion by way of one or more powered rotors (sometimes called ''proprotors'') mounted on rotating shafts or nacelles usually at the ends of a fixed wing. Almost all tiltrotors use a trans ...
engine
* The ATRA-90 (Advanced Technology Regional Aircraft), 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 (IPTN
Indonesian Aerospace (IAe) ( id, PT Dirgantara Indonesia (Persero)), is an Indonesian aerospace company involved in aircraft design and the development and manufacture of civilian and military regional commuter aircraft. The company was formerly k ...
) of Indonesia, Boeing (USA), MBB (West Germany), and Fokker (Netherlands)
* The Tupolev Tu-334
The Tupolev Tu-334 (russian: Туполев Ту-334) was a Russian short-to-medium range airliner project that was developed to replace the ageing Tu-134s and Yak-42s in service around the world. The airframe was based on a shortened Tupol ...
, a 126-seat aircraft that can travel with , which is powered by two Progress
Progress is the movement towards a refined, improved, or otherwise desired state. In the context of progressivism, it refers to the proposition that advancements in technology, science, and social organization have resulted, and by extension wi ...
(also known as Lotarev) 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
The Antonov An-12 (Russian: Антонов Ан-12; NATO reporting name: Cub) is a four-engined turboprop transport aircraft designed in the Soviet Union. It is the military version of the Antonov An-10 and has many variants. For more than thr ...
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
The Ilyushin Il-18 (russian: Илью́шин Ил-18; NATO reporting name: Coot) is a large turboprop airliner that first flew in 1957 and became one of the best known and most durable Soviet aircraft of its era. The Il-18 was one of the world ...
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
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
and the six-bladed back propeller turning at 1,000 rpm to lower noise and vibration
* A re-engined Antonov An-124
The Antonov An-124 Ruslan (; russian: Антонов Ан-124 Руслан, , Ruslan; NATO reporting name: Condor) is a large, strategic airlift, four-engined aircraft that was designed in the 1980s by the Antonov design bureau in the Ukrain ...
, replacing the four Progress D-18T
The Progress D-18T (or Lotarev D-18T) is a high-bypass turbofan that powers the Antonov An-124 Ruslan and An-225 large freighters.
Design and development
The engine was developed in the second half of the 1970s by the then Soviet Ivchenko-Pro ...
turbofans by Kuznetsov Kuznetsov, Kuznyetsov, Kuznetsoff, or Kouznetsov (masculine, russian: Кузнецов) or Kuznetsova (feminine, russian: Кузнецова) is the third most common Russian surname, an equivalent of the English "Smith" (derived from a Russian word ...
NK-62 propfans
Decline
None of these projects came to fruition, however, mainly because of excessive cabin noise (compared to turbofans) and low fuel prices. For General Electric, the GE36 UDF was meant to replace the CFM56
The CFM International CFM56 (U.S. military designation F108) series is a Franco-American family of high-bypass turbofan aircraft engines made by CFM International (CFMI), with a thrust range of . CFMI is a 50–50 joint-owned company of Safran ...
high-bypass turbofan that it produced with equal partner Snecma in their CFM International
CFM International is a 50/50 Franco-American joint venture between GE Aviation and Safran Aircraft Engines (formerly known as Snecma). It was formed to build and support the CFM56 series of turbofan engines. The company is the world’s leading s ...
joint venture. In the 1980s the engine was initially uncompetitive against the International Aero Engines
IAE International Aero Engines AG is a Zürich-registered joint venture aero-engine manufacturing company.
It was founded during 1983 specifically to develop an aircraft engine to address the 150-seat single aisle aircraft market, the IAE V2500, ...
rival offering, the IAE V2500
The IAE V2500 is a two-shaft high-bypass turbofan engine built by International Aero Engines (IAE) which powers the Airbus A320 family, the McDonnell Douglas MD-90, and the Embraer C-390 Millennium.
The engine's name is a combination of the Roma ...
. 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 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." General Electric added the UDF's blade technology directly into the GE90
The General Electric GE90 is a family of high-bypass turbofan aircraft engines built by GE Aviation for the Boeing 777, with thrust ratings from . It entered service with British Airways in November 1995. It is one of three options for the 777 ...
, the most powerful jet engine ever produced, for the Boeing 777
The Boeing 777, commonly referred to as the Triple Seven, is an American long-range wide-body airliner developed and manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet.
The 777 was designed to bridge the gap bet ...
.
1990s
At the beginning of the 1990s, the Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...
/Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
performed flight tests on the Progress D-236
Progress is the movement towards a refined, improved, or otherwise desired state. In the context of progressivism, it refers to the proposition that advancements in technology, science, and social organization have resulted, and by extension wi ...
, 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
The Ilyushin Il-76 (russian: Илью́шин Ил-76; NATO reporting name: Candid) is a multi-purpose, fixed-wing, four-engine turbofan strategic airlifter designed by the Soviet Union's Ilyushin design bureau. It was first planned as a commer ...
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-42
The Yakovlev Yak-42 (russian: Яковлев Як-42; NATO reporting name: "Clobber") is a 100/120-seat three-engined mid-range passenger jet developed in the mid 1970s to replace the technically obsolete Tupolev Tu-134. It was the first airli ...
E-LL and flown to the 1991 Paris Air Show
The Paris Air Show (french: Salon international de l'aéronautique et de l'espace de Paris-Le Bourget, Salon du Bourget) is a trade fair and air show held in odd years at Paris–Le Bourget Airport in north Paris, France. Organized by the French ...
, 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 (Mach 0.75). The Soviets claimed the D-236 had a true aerodynamic
Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
efficiency of 28 percent and a fuel savings of 30 percent over an equivalent turboprop
A turboprop is a turbine engine that drives an aircraft propeller.
A turboprop consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle. Air enters the intake and is compressed by the compressor. Fuel ...
. They also revealed plans for propfans with power ratings of .
Like the Progress D-236, the more powerful Progress D-27
The Progress D-27 is a three-shaft propfan engine developed by Ivchenko Progress. The gas generator was designed using experience from the Lotarev D-36 turbofan. The D-27 engine was designed to power more-efficient passenger aircraft such as the ...
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
In aeronautics, the thickness-to-chord ratio, sometimes simply chord ratio or thickness ratio, compares the maximum vertical thickness of a wing to its chord. It is a key measure of the performance of a wing planform when it is operating at tra ...
and a more pronounced curvature at the leading edge
The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...
. 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
Antonov State Enterprise ( uk, Державне підприємство «Антонов»), formerly the Aeronautical Scientific-Technical Complex named after Antonov (Antonov ASTC) ( uk, Авіаційний науково-технічни ...
rolled out the first prototype of the An-70
The Antonov An-70 ( uk, link=no, Антонов Ан-70) is a four-engine medium-range transport aircraft, and the first aircraft to take flight powered only by propfan engines. It was developed in the late 1980s by the Antonov Design Bureau ...
military transport aircraft, powered by four Progress D-27s attached to wings mounted to the top of the fuselage. The Russian Air Force
" Air March"
, mascot =
, anniversaries = 12 August
, equipment =
, equipment_label =
, battles =
, decorations =
, bat ...
placed an order for 164 aircraft in 2003
File:2003 Events Collage.png, From top left, clockwise: The crew of STS-107 perished when the Space Shuttle Columbia disintegrated during reentry into Earth's atmosphere; SARS became an epidemic in China, and was a precursor to SARS-CoV-2; A des ...
, later 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
SPE "Aerosila" (russian: Аэросила) is the leading Russian firm in the field of development and manufacturing of aircraft propellers, propfans, auxiliary gas-turbine engines as well as aviation aggregates of different purpose including h ...
, the An-70 was stymied by Ukraine's political conflict with Russia. Instead, Antonov began working with Turkey
Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a list of transcontinental countries, transcontinental country located mainly on the Anatolia, Anatolian Peninsula in Western Asia, with ...
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.
Twenty-first century
In the first decade of the 21st century, rising jet fuel
Jet fuel or aviation turbine fuel (ATF, also abbreviated avtur) is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is colorless to straw-colored in appearance. The most commonly used fuels for commercial a ...
prices increased emphasis on engine/airframe efficiency to reduce emissions, which renewed interest in the propfan concept for jetliners beyond the Boeing 787
The Boeing 787 Dreamliner is an American wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes.
After dropping its unconventional Sonic Cruiser project, Boeing announced the conventional 7E7 on January 29, 2003, ...
and Airbus A350
The Airbus A350 is a long-range, wide-body twin-engine jet airliner developed and produced by Airbus.
The first A350 design proposed by Airbus in 2004, in response to the Boeing 787 Dreamliner, would have been a development of the A330 wi ...
XWB. For instance, Airbus
Airbus SE (; ; ; ) is a European Multinational corporation, multinational aerospace corporation. Airbus designs, manufactures and sells civil and military aerospace manufacturer, aerospace products worldwide and manufactures aircraft througho ...
patented aircraft designs with twin rear-mounted contra-rotating propfans. Rolls-Royce
Rolls-Royce (always hyphenated) may refer to:
* Rolls-Royce Limited, a British manufacturer of cars and later aero engines, founded in 1906, now defunct
Automobiles
* Rolls-Royce Motor Cars, the current car manufacturing company incorporated in ...
had the rear (pusher) configured RB.509-11 and front (tractor) configured RB.509-14 geared propfan designs, which produced using the gas generator from its XG-40
The Eurojet EJ200 is a military low-bypass turbofan used as the powerplant of the Eurofighter Typhoon. The engine is largely based on the Rolls-Royce plc, Rolls-Royce XG-40 technology demonstrator, which was developed in the 1980s. The EJ200 i ...
engine with of shaft power. It became lukewarm on propfan technology in the 1980salthough it developed an open rotor design that was thought to be a finalist for the Irkut MS-21 narrowbody aircraft. The Rolls-Royce RB3011
The Rolls-Royce RB3011 (previously designated RB2011) is a prototype propfan engine in development by Rolls-Royce plc. The design is also known as an "open rotor" engine.
Design and development
The RB3011 is designed for the 180–300 pass ...
engine would have a diameter of about and require a gearbox
Propulsion transmission is the mode of transmitting and controlling propulsion power of a machine. The term ''transmission'' properly refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive vehicles), differe ...
.
The European Commission
The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body o ...
launched an Open Rotor demonstration in 2008 led by Safran
Safran S.A. is a French multinational company that designs, develops and manufactures aircraft engines, rocket engines as well as various aerospace and defense-related equipment or their components. It was formed by a merger between SNECMA and ...
within the Clean Sky
The Clean Sky Joint Undertaking (CSJU) is a public-private partnership between the European Commission and the European aeronautics industry that coordinates and funds research activities to deliver significantly quieter and more environmentally ...
program funded with 65 million euros over eight years. A demonstrator was assembled in 2015, and ground tested in May 2017 on its open-air test rig in Istres
Istres (; Occitan: Istre) is a commune in southern France, some 60 km (38 mi) northwest of Marseille. It is in the Provence-Alpes-Côte d'Azur region, in the Bouches-du-Rhône department, of which it is a subprefecture.
Location
Istr ...
, aiming to reduce fuel consumption
A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
and associated CO2 emissions by 30% compared with current CFM56
The CFM International CFM56 (U.S. military designation F108) series is a Franco-American family of high-bypass turbofan aircraft engines made by CFM International (CFMI), with a thrust range of . CFMI is a 50–50 joint-owned company of Safran ...
turbofans. After the completion of ground testing at the end of 2017, Safran's geared open rotor engine had reached technology readiness level
Technology readiness levels (TRLs) are a method for estimating the maturity of technologies during the acquisition phase of a program. TRLs enable consistent and uniform discussions of technical maturity across different types of technology. TR ...
5. The demonstrator's twelve-blade front propeller and ten-blade back propeller had diameters of , respectively. The demonstrator, based on the core of the Snecma M88
The Snecma M88 is a French afterburning turbofan engine developed by Snecma (now known as Safran Aircraft Engines) for the Dassault Rafale fighter.
History
The program for the M88 arose from a need for a suitable propulsion system for air-super ...
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 .
In 2007, the Progress D-27 was successfully modified to meet the United States Federal Aviation Administration
The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...
(FAA) Stage 4 regulations, which correspond to International Civil Aviation Organization
The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international sc ...
(ICAO) Chapter 4 standards. A 2012 trade study projected that propfan noise would be 10–13 decibels
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whose ...
quieter than allowed by Stage 4 regulations. Stage 5 noise limits reduce the limits by only seven effective perceived noise decibels (EPNdB Effective perceived noise in decibels (EPNdB) is a measure of the relative noisiness of an individual aircraft pass-by event. It is used for aircraft noise certification and applies to an individual aircraft, not the noise exposure from an airport. ...
), 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. Snecma claimed that its propfan engines would have about the same noise levels as its CFM LEAP
The CFM International LEAP ("Leading Edge Aviation Propulsion") is a high-bypass turbofan engine produced by CFM International, a 50–50 joint venture between American GE Aviation and French Safran Aircraft Engines (formerly Snecma). It is th ...
turbofan engine.
In 2021, CFM 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 in diameter. The engine was expected to produce 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 fuel
An aviation biofuel or bio-jet fuel Note: About">Investable Universe>About' or bio-aviation fuel (BAF) is a biofuel used to power aircraft and is said to be a sustainable aviation fuel (SAF). The International Air Transport Association (IATA) co ...
s. 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. 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.
Challenges
Blade design
Turboprops have an optimum speed below about , because propeller
A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
s lose efficiency at high speed, due to an effect known as wave drag
In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (res ...
that occurs just below supersonic
Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
speeds. This powerful drag has a sudden onset, and it led to the concept of a sound barrier
The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, th ...
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
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
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 .
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
A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction.
Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigate ...
. 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 scimitar ( or ) is a single-edged sword with a convex curved blade associated with Middle Eastern, South Asian, or North African cultures. A European term, ''scimitar'' does not refer to one specific sword type, but an assortment of different ...
- a practice that was first used as far back as 1909, in the Chauvière two-bladed wood propeller used on the Blériot XI
The Blériot XI is a French aircraft of the pioneer era of aviation. The first example was used by Louis Blériot to make the first flight across the English Channel in a heavier-than-air aircraft, on 25 July 1909. This is one of the most fam ...
. (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.) 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 despite wider or narrower propeller diameter (resulting in an RPM reduction or increase, respectively).
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
In aeronautics, the thickness-to-chord ratio, sometimes simply chord ratio or thickness ratio, compares the maximum vertical thickness of a wing to its chord. It is a key measure of the performance of a wing planform when it is operating at tra ...
that tapered from less than 20% at the spinner junction to 2% at the tips, and 4% 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 . (The GE36 UDF engine that was tested on the Boeing 727 had front and back blades that weighed each.)
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
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
per unit of blade surface area. A concept similar to wing loading
In aerodynamics, wing loading is the total mass of an aircraft or flying animal divided by the area of its wing. The stalling speed of an aircraft in straight, level flight is partly determined by its wing loading. An aircraft or animal with a ...
, 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:
* 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
A blade vortex interaction (BVI) is an unsteady phenomenon of three-dimensional nature, which occurs when a rotor blade passes within a close proximity of the shed tip vortices from a previous blade. The aerodynamic interactions represent an impo ...
);
* 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.
Community noise
Engine makers expect propfan implementations to meet community (as opposed to cabin) noise regulations without sacrificing the efficiency advantage. Some think that propfans can potentially cause less of a community impact than turbofans, given their lower rotational speeds. 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
The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...
(FAA) Stage 4 regulations, which correspond to International Civil Aviation Organization
The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international sc ...
(ICAO) 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
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whose ...
quieter than the maximum noise level allowed by the Stage 4 regulations; the newer Stage 5 noise limits (which replaced the Stage 4 regulations for larger aircraft in 2018 and mirrored the ICAO Chapter 14 noise standard established in 2014) are more restrictive than the Stage 4 requirement by only seven effective perceived noise decibels (EPNdB Effective perceived noise in decibels (EPNdB) is a measure of the relative noisiness of an individual aircraft pass-by event. It is used for aircraft noise certification and applies to an individual aircraft, not the noise exposure from an airport. ...
), 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
Safran Aircraft Engines, previously Snecma (''Société nationale d'études et de construction de moteurs d'aviation'') or Snecma Moteurs, is a French aerospace engine manufacturer headquartered in Courcouronnes and a subsidiary of Safran. It ...
, however, maintains that open-rotor tests show that its propfan engines would have about the same noise levels as its CFM LEAP
The CFM International LEAP ("Leading Edge Aviation Propulsion") is a high-bypass turbofan engine produced by CFM International, a 50–50 joint venture between American GE Aviation and French Safran Aircraft Engines (formerly Snecma). It is th ...
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
EasyJet plc (styled as easyJet) is a British multinational low-cost airline group headquartered at London Luton Airport. It operates domestic and international scheduled services on 927 routes in more than 34 countries via its affiliate airli ...
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 ratio
The bypass ratio (BPR) of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A 10:1 bypass ratio, for example, means that 10 kg of air passes through the bypass duct for ev ...
s of over 30, but they are approximately twice the diameter of turbofan engines of equivalent capability. For this reason, airframers usually design the empennage
The empennage ( or ), also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow.Crane, Dale: ''Dictionary of Aeronautical Terms, third ed ...
with a T-tail
A T-tail is an empennage configuration in which the tailplane is mounted to the top of the fin. The arrangement looks like the capital letter T, hence the name. The T-tail differs from the standard configuration in which the tailplane is ...
configuration for aerodynamic
Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
purposes, and the propfans may be attached to the upper part of the rear fuselage
The fuselage (; from the French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds crew, passengers, or cargo. In single-engine aircraft, it will usually contain an engine as well, although in some amphibious aircraft t ...
. For the Rolls-Royce RB3011
The Rolls-Royce RB3011 (previously designated RB2011) is a prototype propfan engine in development by Rolls-Royce plc. The design is also known as an "open rotor" engine.
Design and development
The RB3011 is designed for the 180–300 pass ...
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
The wing configuration of a fixed-wing aircraft (including both glider (aircraft), gliders and powered aeroplanes) is its arrangement of lifting and related surfaces.
Aircraft designs are often classified by their wing configuration. For examp ...
, which allows for ground clearance without requiring excessively long landing gear
Landing gear is the undercarriage of an aircraft or spacecraft that is used for takeoff or landing. For aircraft it is generally needed for both. It was also formerly called ''alighting gear'' by some manufacturers, such as the Glenn L. Martin ...
. 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
Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the ...
(shp) that they produce, as opposed to turbofans and the UDF propfan type, which are rated by the amount of thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
they put out. The rule of thumb
In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
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 two engines can theoretically be replaced with a pair of propfans or with two UDF propfans.
Aircraft with propfans
Proposed aircraft with propfans
See also
References
General resources
*Prop fan propulsion concepts: Technology Review, Design Methodology, State-of-the-art designs and future outlook. Raymond Scott Ciszek. University of Virginia Department of Mechanical and Aerospace Engineering. Senior Thesis Project. March 25, 2002
Bibliography
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External links
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* {{Cite web, last=Norris, first=Guy, date=June 25, 2021, title=Evolutionary Trail Of The Open-Fan Engine , url=https://aviationweek.com/air-transport/aircraft-propulsion/evolutionary-trail-open-fan-engine, access-date=2021-06-28, website=aviationweek.com
Propfan engines
ru:Винтовентиляторный двигатель