The propfan engine is designed to have the speed and performance of a turbofan, but the fuel efficiency of a turboprop. The propfan is also known as the unducted fan (UDF) engine or open rotor engine because the fan is not enclosed like that of a turbofan.
There have been several innovative designs over the years, one of which was the “Propfan” or unducted fan. Hamilton Standard, an American manufacturer of propellers, considered developing new blades in the 1940s so that its power plants could compete with the performance of the developing turbofan/jet engines. Sadly, the idea was abandoned due to concerns with flutter, blade stress, and noise levels. The team started researching installing a more fuel-efficient propeller that could function at the same speed and altitude as a jet engine under the direction of the US space agency NASA.
NASA’s inducted propfan design
Propfans were created in the 1980s as a response to increased gasoline costs brought on by a lack of fuel. The introduction of an unducted fan (UDF) in 1988 was practically invented. While the McDonnell Douglas MD-81 ultra-high bypass-powered GE36 demonstrator engine buzzed overhead at the Farnborough air show (Cross, 2022).
The Interceptor Aircraft Fuel Conservation Technology Task Force was established in February 1975 to investigate viable solutions. This led to the development of several aviation projects, including the Advanced Turboprop and the Energy Efficient Engine (E3), which would produce cutting-edge technology for the GE90 (ATP). With equivalent performance at speeds up to Mach 0.8 and altitudes up to 30,000 feet, NASA estimated that an ATP might lower fuel consumption by up to 30% compared to current turbofan engines.
Despite the fuel-saving predictions, public and industry perceptions of propellers and concerns over the technical challenges delayed the go-ahead of the ATP until 1978. NASA worked with Allison and Pratt & Whitney on a joint concept and awarded Hamilton Standard advanced blade development contracts. In 1981 Hamilton began to design a large-scale, single-rotating, composite blade set.
An unducted fan has a highly twisted blade hopped outside of an engine cowling.
In 1983, when plans were being made to start extensive demonstrations of the NASA industry team ATP idea, GE’s UDF design was presented to NASA. This was tested in 1986 and used an Allison turboshaft and gearbox together with a Hamilton Standard SR-7A propfan. In 1987, the entire engine, which included an eight-bladed unit, took flight on a modified Gulfstream II.
In 1986, the P&W/Allison propfan team was also founded. From the combined NASA-industry study, many of the technical and design components were spun off. It included a new low-pressure compressor, a new gear system and propfan module derived from the ATP work, a new nacelle, and a full authority digital electronic control system derived from PW2037. The Allison 571 power section, which was based on an industrial and marine gas generator, was also combined.
The resulting 578-DX demonstrator included two rows of 2.95m (11.6ft) diameter blades: six in the front row (counter-clockwise rotation viewed from the rear) and six in the aft row (clockwise).
GE, on the other hand, actively pursued potential applications on upcoming Boeing and McDonnell Douglas projects, including the 7J7 and the MD-90, which both had aft-engine configurations and were the only new models to which the UDF could be fitted. In 1986, GE flew its demonstrator on a 727 testbed. The MD-81 that would later fly the P&W/Allison engine was also equipped with the demonstrator engine, which was based on an F404 gas generator. It was flown in the middle of 1987. The flying experiments, which were by this point supported by NASA Lewis involvement, produced significant findings about major topics including performance and acoustic signature.
GE declared the demonstration showed that “without an acoustically attenuated duct around the fan, the community noise levels and internal cabin noise levels of such an unducted fan configuration would be acceptable and certifiable” (Hoyle, 2007).
Performance was found to be at “high propulsive efficiency levels of the 1990s,” and in a production version, like the GE36-C25 planned for a planned MD-92, would have been in the 0.5 (total fuel/payload lb) range, compared to about 0.8 for the CFM56-powered Airbus A320 and 1.3 for the JT8D-217-powered MD-80.
Airlines seemed hesitant to wager on fuel efficiency while oil prices were still low, and McDonnell Douglas later that year confirmed the adoption of the V2500, thereby killing the UHB. Around the same time, attempts to launch the MPC-75, a GE UDF-powered T-tail airplane project, also failed, leaving the different propfan ideas with nowhere to go but museums.
Boeing’s 150-seat 7J7 concept would meld prop-fan technology and lightweight composite structure to deliver big gains in fuel efficiency.
How does Propfan work?
Schematic diagram of a propfan engine
By adding external fan blades, a propfan engine can obtain a high bypass ratio, which boosts its propulsion efficiency. The fan blades are bent like scimitars to prevent shockwave generation on the outer tips of the blades, maintaining the efficiency boost even at high speeds. The increased propulsive efficiencies translate into overall propulsive efficiency gains of as much as 20-25% over turbofans (Pingstone, n.d.).
Because of the speed and power loading (the amount of power driving a propeller with a specific diameter), the propfan and UDF are exceptional among propellers. Similar to how they performed on an aviation wing, the narrow blades and sweepback boosted efficiency at transonic speeds. The plane was flying at subsonic speed, but the prop tips made a helical path at Mach 1.1.
The fuel efficiency improved by 15-20% for propfan advanced transport aircraft compared to equivalent turbofan transports.
Titanium or composite materials are used to make propfan blades to increase strength, weight, and stiffness. Compared to heavy metal blades, it is much simpler to attach many lightweight composite blades to a prop hub. With up to twelve blades being employed on a contra-prop, smaller diameter props are now more frequently found on current generation turboprop transport aircraft.
Smaller blades on a traditional prop have a lower diameter due to greater prop disc loading, which reduces efficiency. Increased blade density, as seen on a contra-Propfan, for instance, lowers prop blade loading and restores efficiency. Comparatively to a standard prop, the multi-bladed Propfan more effectively manages the unwanted compressibility issue.
Various Propfan configurations
The multiple blades of the Propfan’s airflow cascade over one another, allowing compressibility to increase gradually with the least amount of energy loss. According to the design of the Propfan, the prop disc loading (as opposed to the prop blade loading) might be on par with or even double that of a standard propeller.
Cross, L. (2022, June 29). Propfan Power: The Story of the Unducted Fan Engine. Retrieved January 17, 2023, from https://airwaysmag.com/the-unducted-fan-engine/
Hoyle, C. (2007, June 11). Whatever happened to propfans? | News | Flight Global. FlightGlobal. Retrieved January 17, 2023, from https://www.flightglobal.com/whatever-happened-to-propfans/74180.article
Pingstone, A. (n.d.). Variations of Jet Engines. SMU. Retrieved January 17, 2023, from https://s2.smu.edu/propulsion/Pages/variations.htm
Propfans. (2016, February 25). Helicopters & Aircrafts. Retrieved January 17, 2023, from http://heli-air.net/2016/02/25/propfans-2/
Sweetman, B. (n.d.). The Short, Happy Life of the Prop-fan. Smithsonian Magazine. Retrieved January 17, 2023, from https://www.smithsonianmag.com/air-space-magazine/the-short-happy-life-of-the-prop-fan-7856180/