Back to propellers

Jet engine makers think that a hybrid of the turbofan and the turboprop could make aircraft significantly more efficient - if only it can be quietened. E&T reports.

The aircraft of today owe their existence to an original piece of design and engineering by Sir Frank Whittle, nearly three-quarters of a century ago.

But in the years since Whittle developed the jet engine, the world has changed beyond all recognition. Engine noise, fuel efficiencies and emissions would all have been anathema to the pioneering, engineering spirit of the former RAF officer when he was working on his engine. But today the key challenges facing aero-engine technology are reducing fuel burn, increasing operability and lowering noise.

Modern jet engines have developed from Whittle's original design into highly efficient systems that can power huge aircraft for thousands of miles with minimal maintenance. However, this is not enough, and to meet the demand for continued, sustainable air travel, further improvements are needed.

It is becoming increasingly difficult to find ways of improving modern jet engines though. Researchers have to look closer at the details of the engine components and to try to understand more complex aspects of turbomachinery aerodynamics. For example, there is the potential for large gains in engine performance if the flow around intricate geometrical features can be controlled. At the Whittle Laboratory at Cambridge University, engine components have been simulated, both in tests and calculations, using simplified geometries. However, the real geometry in the engine is much more complex, and the performance of this can be significantly different.

Another approach is to look to more radical configurations of aero engines for the future, one of which evokes memories of a bygone era for aviation; of times when Charles Lindbergh piloted his propeller-driven Spirit of St Louis. This intriguing alternative, which is the subject of much research by Rolls-Royce and its competitors, is the 'open-rotor' or propfan engine. This is similar to a turboprop engine, which uses a gas turbine to drive a propeller, except that the propfan uses specially shaped high-speed propellers.

"We are talking about saving 10,000t of carbon dioxide per year per aircraft if you introduce an open-rotor on to a 100-200 seater aircraft," explains Mark Taylor, an engineer at Rolls-Royce. "We believe that we can produce a quiet, efficient open-rotor engine."

Propfan vs turbofan

Modern turbofan engines work by sucking in air with an enclosed fan at the front of the engine. The majority of this air is pushed out the back to produce the thrust, with the remainder used to burn the fuel to drive the fan. The more air that is pushed out rather than burned is the bypass ratio, and the higher the bypass ratio the more efficient the engine is.

Unlike traditional engines, open-rotor designs do not have a casing around the propeller. This has two main advantages. First, without this bulky housing a major source of drag is removed. Second, by making use of specially shaped counter-rotating propellers, the design avoids generating 'swirl' - the vortex of wasted energy that usually trails in the air behind aircraft propellers.

There is the potential of a 20-30 per cent reduction in the fuel consumption of aircraft powered by such devices. However, one of the major issues in making open-rotor designs acceptable is noise, and this is something that researchers are tackling.

The source of the noise is the propeller characteristics, and Rolls-Royce is addressing this by increasing the number of blades and changing their shape from the traditional elongated design to a squatter, thinner configuration. The end result is that the rotors can spin at a lower speed, which reduces noise while maintaining the high efficiency.

This route has been explored before, with both Pratt & Whitney and General Electric logging up hundreds of hours with open-rotor designs on Boeing aircraft during the early 1980s. And one of those companies, GE Aviation, is now taking another look at the technology, teaming up with NASA on a wind-tunnel test programme to evaluate counter-rotating fan blade systems.

The testing began in wind tunnel facilities at NASA's Glenn Research Centre early this year and will continue until late summer. This is not a full engine test, but a component rig test to evaluate subscale fan systems using GE's and NASA's advanced computational tools and data acquisition systems.

It is rising fuel prices that have led the partners to revisit open-rotor systems. In the 1980s, GE successfully ground-tested and flew the GE36, an open-rotor jet engine that demonstrated fuel savings of more than 30 per cent compared to similar-sized jet engines with conventional, ducted front fan systems. Since then, GE has advanced dramatically its data acquisition systems and computational tools to better understand and improve open-rotor systems.

"GE and NASA journeyed down this path 25 years ago with great technical success," David Joyce, president of GE Aviation, explains. "Today's fuel crisis greatly influences future jet engine concepts. GE and NASA will evaluate open-rotor concepts in the wind tunnel with far greater technology capability."

For the NASA tests, GE will run two rows of counter-rotating fan blades at 1/5th scale in several configurations, tested in simulated flight conditions created in Glenn Research Centre's wind tunnels.

NASA's testing rig equipment, which is being refurbished for the activities, was also used in the 1980s when NASA and GE jointly tested scale-model, counter-rotating fan systems that led to the development of the GE36 engine. The NASA test hardware is also capable of simulating aircraft installation systems with open-rotor fan systems.

The GE36, which flew on Boeing 727 and McDonnell Douglas MD-81 aircraft, featured an open-rotor fan system with two rows of counter-rotating composite blades mounted at the rear of the engine in a pusher configuration. The enormous efficiency from bypass air created by this fan system drove the GE36's fuel savings. As fuel prices fell sharply in the late 1980s and early 1990s, the GE36 was never launched commercially, though it was recognised as a technology breakthrough.

Propfan's noise problems

The upcoming rig tests will focus mostly on the acoustic characteristics of various fan configurations, as well as performance and efficiency. Engine noise is a major challenge to operating open-rotor engine systems in commercial aviation.

According to Rolls-Royce there is only one certainty regarding the next generation of circa 150-seat short range replacement aircraft - that there certainly will be a next-generation aircraft, and it is an area of the market that Rolls-Royce is determined to play a major part in

Exactly when this opportunity might be, what the airframe configuration will be, and what the requirements facing engine manufacturers will be, are all still largely unknown, but the UK-based powerplant manufacturer is already working towards providing answers for its customers.

"At this stage, it's not that we don't know the answer - it's more that we don't yet know the exact question," says Robert Nuttall, vice president strategic marketing for Rolls-Royce. "Some of the future needs of our customers are a given, such as lowest fuel burn, lowest emissions, lowest noise and lowest lifecycle operating costs, but we will not be able to meet all these needs with just one solution.

"We will combine the right product and service technologies with the right engine architectures at the right time to deliver the best overall customer solution, and this approach is called Option15-50."

This ambitious programme requires a multi-dimensional approach - looking at the total needs of this market. Rolls-Royce believes there is a continuum of options it should examine, starting with a 15 per cent improvement available as today's technology programmes mature, through to as much as 50 per cent efficiency improvements at a total aviation system level at a later date.

Option15-50 is based upon a range of technologies and engine architectures, including advanced two- and three-shaft designs and the open-rotor, all backed by lifecycle management schemes. Rolls-Royce spends around £1bn each year on R&D, and says it sees three main areas for development.

First is the three-shaft RB285, a proposed low fuel burn engine building on the Trent heritage and the proven IP (Intermediate Pressure spool) off-take, which delivers significant savings at the short ranges typical of aircraft currently operating in the 150-seat sector.

Next is the existing two-shaft RB282 family, which builds on Rolls-Royce's E3E (Environment Efficiency Economy) core technology and earlier two-shaft engines such as the V2500 and BR700, while incorporating material and component advances from the Trents.

The open-rotor is a higher risk technology, but could be game-changing, the company says, as it offers the potential for up to 30 per cent better fuel consumption than today's products. An open-rotor technology demonstrator is currently running for noise and performance validation in wind tunnels in Holland and the UK. 

Ultra-high bypass ratios

In the US, Pratt & Whitney appears to demonstrate much less enthusiasm for the open-rotor concept. The preferred option is instead to deliver greater efficiency by developing advanced ultra-high bypass ratio powered aircraft.

"Propfans or open-rotors are nothing more, or less, than modern propellers," says Alan Epstein, Pratt & Whitney's vice president of technology and environment.

"They can have one row of blades as on commuter prop planes and the C-130 airplane, or two counter-rotating blade rows as on the Antonov An-70 or the older Tupolev Tu-95 - the version with two blade rows can provide more thrust for a given prop diameter and slightly higher efficiency, but at the expense of more noise, complexity and weight.

"Modern propeller aircraft can now fly up to speeds of Mach 0.7 to 0.72, against the Mach 0.80 to 0.85 of current turbofan powered airliners. Advanced airplanes in the 2020 time frame, optimised for these propellers, can be about 30 per cent more efficient than 100-200 passenger aircraft currently in service, but fly at slower speeds.

"Pratt & Whitney believes that advanced ultra-high bypass ratio powered aircraft can deliver equivalent fuel efficiency to the open rotor in that time frame and fly at higher speeds. At all sizes, propeller transport aircraft are considerably noisier than high bypass ratio turbofan aircraft and this will be true of aircraft in that time frame as well."

The future, it seems, is all about choices. And despite the best efforts of engine manufacturers, it is likely to be the politicians that decide the future course of the aircraft engine. There are some tough choices on the horizon, chief among them being: would you rather have reduced noise, or lower fuel consumption and lower emissions? In life everything comes at a price.

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