With fuel prices at noticeably higher levels and Stage 4 noise requirements proposed to take effect next January 1, industry and government are working furiously on ways to make turbine engines even more efficient, while further reducing emissions and noise output. NASA is leading the charge, with plenty of help from the engine manufacturers themselves.
According to Rolls-Royce, the fuel efficiency of jet-powered aircraft has improved by around 70 percent since their introduction in the 1950s, and advances in engine design have contributed to more than half of this improvement. In fact, the company noted that each 1 percent improvement in fuel efficiency can save up to 80,000 gallons of fuel a year per aircraft.
Notwithstanding proposed Stage 4 rules, aircraft noise continues to be a nuisance to the general public. Increases in air traffic, along with population growth in areas surrounding airports, mean that aircraft noise is negatively affecting a larger percentage of the community and is fueling a stronger desire to cut the decibels around airports.
As important as noise is engine fuel efficiency. Airlines and other aircraft operators are feeling the pinch due to higher fuel prices, and renewed environmental concerns are leading some to point to aircraft emissions as a source of polluting greenhouse gases tied to global warming.
In response to these concerns, NASA developed new engine technologies under the Advanced Subsonic Technology (AST) program, which started in 1992 as a partnership among NASA, the U.S. aviation industry and the FAA and concluded in 1999. The goal of the AST program was to enable the development of a new generation of aircraft and engines that are both environmentally compatible and economical, some of which are already on the market.
In October 1999 the Ultra Efficient Engine Technology (UEET) program superseded the engine research component of AST. UEET, formed at the NASA Glenn Research Center in Cleveland, includes participation from three other NASA centers (Ames, Goddard and Langley), in addition to engine manufacturers GE Aircraft Engines, Pratt & Whitney, Honeywell, Allison/Rolls-Royce and Williams International. Boeing, Gulfstream and Lockheed Martin are also involved in the government-industry partnership.
NASA expects that the UEET program will result in revolutionary aircraft propulsion technologies to address local air-quality concerns by developing technologies that will reduce nitrogen oxide (NOx) emissions by 70 percent during landing and takeoff, with comparable reductions in cruise operations. UEET will also address the potential climate effects of long-term aviation growth and provide critical propulsion technologies to reduce fuel burn by 35 percent for large subsonic transports and 20 percent for high-speed and small subsonic aircraft. UEET originally was scheduled to end this year, but the program has been extended to 2007 to develop production-ready components.
The original seven technology areas researched as part of UEET were propulsion systems integration and assessment; emissions reduction; highly loaded turbomachinery; materials and structures for high performance; propulsion-airframe integration; intelligent propulsion controls; and integrated component technology demonstrations.
Propulsion systems integration and assessment employed the component technologies being developed in the other projects and integrated them into conceptual systems and assessed those systems’ potential for meeting the UEET program goals. These assessments provided overall program guidance and identified technology shortfalls. There were three key subprojects to this technology area–propulsion system evaluation, environmental impact assessment and high-fidelity system simulation.
Aircraft engine manufacturers worked with NASA on the emissions-reduction project to develop combustion technologies to significantly reduce NOx emissions smoke and unburned hydrocarbons. To do this, new combustor concepts and technologies were required to produce cleaner-burning combustors to offset the increased NOx produced by more fuel-efficient engines with higher pressure ratios and temperatures.
These new combustion concepts and technologies include lean-burning combustors with advanced controls and new high-temperature ceramic matrix composite material. According to NASA, low-emissions combustor concepts were developed and evaluated to achieve major reductions in NOx emissions in aircraft engines.
The highly loaded turbomachinery part of the UEET program provided revolutionary turbomachinery technologies for increased performance and efficiency. Under this technology area, turbomachinery technologies led to lighter-weight, reduced-stage cores, low-pressure spools and high-performing, highly efficient and environmentally compatible propulsion systems.
Specifically, concepts for significantly increased airflow loading, trailing-edge wake control and higher cooling effectiveness were developed and demonstrated through proof-of-concept tests. Fan technology development further reduced weight and increased efficiency while also lowering noise.
The goal of the materials and structures research for the high-performance side was to develop and demonstrate advanced high-temperature materials to make possible high-performance, high-efficiency and environmentally compatible propulsion systems. Technologies developed as part of this project include ceramic matrix composite combustor liners and turbine vanes, advanced disk alloys, turbine airfoil material systems, high-temperature polymer matrix composites and innovative lightweight materials and structures for static engine structures.
The result of UEET’s propulsion-airframe integration project was advanced technologies to yield lower drag propulsion system integration with the airframe. Decreasing drag improves aircraft performance and efficiency, reducing fuel burn and thus lowering carbon dioxide emissions. Essentially, this part of the research program optimized nacelle placement and shaping to minimize drag, accomplished through both computational and experimental methods.
Autonomous propulsion systems that allow the control system, independent of pilot input, to maximize performance were developed under intelligent propulsion controls research. Such control systems could also adjust system characteristics to maximize individual component life, as well as help to reduce NOx emissions during takeoff and landing.
Technology demonstration tests were a critical step in the technology-development process. Tests conducted under the integrated component technology demonstrations project provided a significant amount of risk reduction by demonstrating that the technologies are still viable when integrated into an overall system.
The results of these tests gave the aircraft engine industry the confidence necessary to incorporate the technologies in follow-on product insertion programs. This is exactly why the program was recently extended to 2007 with a new, strengthened strategic focus.
The redesigned UEET program will provide cross-cutting technology development and maturation focused on reduced NOx and improved fuel efficiency for subsonic aircraft, high-performance inlets for supersonic aircraft and advanced drive systems for rotorcraft. The new sub-projects are low-emissions combustors; systems integration and demonstration; highly loaded lightweight compressor and turbines; highly integrated inlets; advanced drive systems for heavy-lift rotorcraft; and intelligent system foundation technologies. NASA said it will release more information about these programs as they get fully under way.
Engine Noise Sources
Engines are a considerable source of aircraft noise, since turbofan engines work on the principle of sucking air into the front of the nacelle duct and pushing that same air out the back at a higher velocity. And the noise starts as soon as the air literally hits the fan. Once past the fan, the air is split down two different paths: the fan duct and core.
In the fan duct, the initial flow is swirling because of the spinning motion of the fan. Since this swirl causes loss of momentum before the air exits the nozzle, it is straightened out with a set of stator vanes. These stators are a large source of noise as the wakes of air from fan flow slap against the stators. This regular slapping takes place at the rate of blades passing by and generates a tone at what is called the blade passage frequency. Nonuniformities and nonlinearities result in many higher frequency tones, noise that is often associated with the piercing sound generated by some engines. Further, fan/stator interaction results in a rumbling sound, due to the unsteadiness in the fan flow.
In the core duct, air is further compressed through a series of smaller fans called rotors. Each of these rotor stages is separated by a set of stators to straighten the flow, which is another source of rotor/stator interaction noise. The compressed air is then mixed with fuel and burned, causing yet more noise. The hot, high-pressure combusted air is sent downstream into a turbine, which drives the fan and the compressor rotors. Since turbines tend to look and act like a set of stators, they also generate noise.
Finally, the core flow and the fan duct flow join at the engine’s exhaust outflow. Jet exhaust consists of the fan stream and the core/combustion stream. The core flow stream is typically moving at a higher speed than the fan stream. As the two flow streams mix, noise is created in the surrounding air. Adding a layer of complexity, the jet exhaust noise is actually created after the exhaust leaves the engine, so the noise cannot be reduced where it is created but must be mitigated before the exhaust leaves the engine.
NASA is studying the theory of noise generation and developing computer codes that can simulate jet engine noise. Theoretical understanding of jet noise is used to develop ideas for noise-reduction concepts that are tested in model scale. Ideas that have already been tested or will be tested include mixer devices to combine the flows quickly, which reduce the noise-generation area. According to NASA, test data has shown that a three-decibel reduction in jet noise can be achieved, but the ultimate goal is a six-decibel reduction.
Fan Noise Reduction
To make progress on fan noise reduction, it is necessary to understand and be able to predict that noise. Therefore, NASA is studying the theory of fan noise generation and developing computer codes that simulate that theory. A second approach uses the theoretical understanding of fan noise to develop a succession of ideas for testing, with each test providing both data upon which the computer codes are verified and results upon which the next test might be built.
Fortunately, NASA said, the fan thrust provides many options to explore and there are many components to alter. Besides basic geometry, there are blade-wake tailoring, boundary-layer (a thin layer of air along the duct wall that moves more slowly than the rest of the flow) effects, fan speed, number of blades and stators, among others.
A new approach to noise reduction is through active noise control. The primary principle of active noise control is to sense the noise disturbances in the engine and cancel them before they leave the engine. In effect, negative noise is made to cancel out the engine’s sound waves so that no noise is heard. This is a multidisciplinary effort involving duct acoustics, controls and actuator/sensor design.
NASA Glenn has a unique facility to test its active noise control fan demonstrator, a four-foot-diameter low-speed fan designed specifically for active noise control testing. To date, several concepts have shown successful cancellation of selected acoustic modes.
You Can Look, but You Can’t Touch
One of the challenges of wind-tunnel testing is seeing the air. Researchers need to know details about how air is flowing around a model, but placing an instrument into the flow would interfere with the processes being studied.
Over the years, NASA researchers have used and developed different measurement techniques that have little or no effect on experiments, which are called minimally intrusive or nonintrusive techniques.
One of the oldest of such techniques is the use of tufts, or strips of yarn, mounted on the floor or walls of the wind tunnel or on the model itself. As the air flows across the surface, it causes the tufts to move with the flow of the air. This technique is particularly helpful for showing the direction of airflow.
Another traditional technique uses pressure-sensitive paint dots to show how airflows are distributed. The brightness of the paint dots changes as the air pressure changes.
Although ordinary light sources and cameras cannot view the complex airflows of aircraft engine systems, several cutting-edge methods help researchers to see these complex processes in action without interfering with the results.
Lasers are playing an important role in wind-tunnel testing, NASA said. Two sheets of lasers have been used to illuminate the flow field at different heights. Water mist was often added to make the airflow visible. In the 1990s, Glenn began using fiber optics to transmit the laser light beam in wind tunnels. Fiber-optic technology brought an additional measure of safety to the test procedure, and it minimized loss in the transmitted laser light beam.
Using a thermovision system, researchers can view the hot parts of an aircraft, a simulated runway surface and the gas plume of an engine exhaust. With this concept, carbon dioxide is used to image heated surfaces and an infrared camera shows the various temperature levels in different colors.
The data gathered is processed by computers that visualize the test results. In turn, programming the computers to predict different conditions reduces the number of tests required and helps designers to solve problems faster and with greater confidence.
Researchers at NASA Glenn and other agency centers are developing minimally intrusive optical measurement systems, advanced optical instrumentation for aerospace propulsion testing facilities, new optical components for aerospace flight, advanced electro-optic circuits for space exploration and other technologies. NASA said that the agency is developing and using advanced data-processing techniques to reduce processing time.
Data from these systems will help designers to understand the basic physics of new systems and to validate computer and life models. They will lead to improved designs, increased safety and security, and reduced design times for many technologies developed at Glenn, including those for aircraft engines.
Honeywell Committed to the Environment
In its annual forecast released in October, Honeywell predicted that the business aircraft fleet will increase by 80 percent by 2013. Further, the company said that small aircraft currently account for 70 percent of all landings and takeoffs. And since Honeywell Engines Business is a leading supplier of powerplants for midsize and large business aircraft and APUs for a wide range of aircraft (including airliners), the manufacturer is concerned about the environmental effects of its products.
According to Honeywell Engines Business director of advanced technology Ron Rich, the company has been involved in several NASA programs, including AST and UEET, in addition to conducting its own advanced research and development projects to make cleaner and quieter engines. Specifically, Honeywell is focusing its R&D efforts to make more efficient and quieter fans, compressors and turbines, and improved mechanical systems, namely bearings and seals. Advanced computer analytical tools also help to refine systems before a prototype is ever built and tested.
The driving force behind the move for greener engines isn’t regulations as much as it is community perspective, Rich said. “Lower noise simply means better airport access,” he added. “Likewise for lower engine emissions.”
As proof of its efforts, Rich noted that the in-service HTF7000 engine for the Challenger 300 emits 50 to 70 percent less NOx than comparable engines, and its noise output is within Stage 4 limits. He said these accomplishments were achieved by employing air bearings, advanced nozzles and a quiet high-speed fan developed in partnership with NASA, not to mention other proprietary materials and components.
The technologies that Honeywell has developed are also finding their way into its turboshaft and turboprop engines, as well as the company’s wide array of APUs. In fact, Rich said APUs are a priority for such crossover technology at present, given that Honeywell is the leading supplier of APUs for both airliners and business aircraft and the small turbines generate “considerable noise and emissions.”
Newer APUs such as the RE220, Rich said, are being made cleaner by using new compressor designs and fuel nozzles originally developed for the HTF7000. Helping on the noise front, the RE220 employs a new inlet and exhaust design.
Looking further ahead, Honeywell has several promising new technologies brewing in its test labs. Later this year, the company will be testing a new acoustic material for APUs and engines at its Morristown, N.J. facility.
Another interesting technology being investigated at Honeywell’s Phoenix facility is advanced fuel delivery systems refined by 3-D fuel spray imaging. Rich said an advanced fuel-delivery system is undergoing the final stages of subcomponent rig testing, with full-scale testing of a new annular combustion system employing the technology to begin early this year.
Rich said the company is investigating active noise canceling systems, but for now the cost of such systems outweighs the benefits, though this could change at some point in the future.
Also farther out on the horizon are fuel-cell APUs–Honeywell contributed expertise in this area for the NASA Helios fuel-cell-powered unmanned aerial vehicle. However, Rich said, current APUs have a two kilowatt/kilogram power density, while fuel cells currently have a 0.5 to 0.6 kilowatt/kilogram power density, meaning that fuel cells will need to mature technologically before they can be seriously considered for APU use.
Rolls-Royce Thinks Long Term
Rolls-Royce has long been forward-thinking when it comes to green aircraft engines. The engine manufacturer introduced the three-shaft design in the RB211 series in the 1970s and its hollow, titanium wide-chord fan blade, which entered airline service in the 1980s, set new standards in aerodynamic efficiency and resistance to foreign object damage. Other innovations at Rolls-Royce have also led to cleaner and quieter engines over the years.
The company’s 20-year technology plans and goals are perhaps the most aggressive of those stated by any engine manufacturer. It plans to cut the current perceived average noise levels in half, reduce carbon dioxide emissions by 50 percent per passenger-mile and decrease NOx emissions by 80 percent.
Rolls-Royce’s technology programs are broken into three time frames: Vision5, Vision10 and Vision20, which describe the technologies the company is developing for future generations of aircraft engines in approximately five, 10 and 20 years.
According to the company, Vision5 refers to off-the-shelf technologies, which can be incorporated into its new products (such as the Trent 900 and Trent 500) and into its existing engines (such as the Trent 800 and Tay 211) after some modifications.
Vision10 describes the range of leading-edge technologies currently at the validation stage. Rolls-Royce said it has several technology demonstration programs aimed at delivering the technologies of Vision10, though it wouldn’t elaborate.
Finally, Vision20 technologies are currently at the strategic research stage, meaning they are emerging or as-yet-unproven technologies. This work has traditionally been beyond the scope of current business plans, though Rolls noted that this product-focused approach promotes the research and development of specific technologies through its extensive research base.
Rolls-Royce said its Vision20 goals are those of a broader initiative by the Advisory Council for Aeronautics Research in Europe (ACARE) in which the engine manufacturer plays a key part.
The engine manufacturer is currently incorporating new technologies into its affordable near-term low-emissions (ANTLE) engine demonstrator, using a Trent 500 as the platform for such testing. Some of the advanced systems include new fuel pumps, a five-stage high-pressure compressor with blisks, a lean-burn staged combustor, an oil pump with air-riding carbon and brush seals, a “novel” four-stage low-pressure turbine and a contra-rotating high-pressure turbine, among others. Testing of the ANTLE engine is slated for this spring.
Small Engines Make Big Strides
While the coming very light jets are stealing news headlines, these aircraft simply wouldn’t be possible without suitable powerplants. Williams International sparked interest in the new wave of microturbines, but Pratt & Whitney Canada and GE/Honda Aero Engines were quickly on its trail.
When asked about the technology behind his engines, Dr. Sam Williams was tight-lipped, saying only that these technologies are “the bread and butter of our designs.” However, it is known that the FJ33 and FJ44 lines employ low-noise, wide-sweep fan technology coupled with advanced high-work, high-efficiency core components to produce a high overall pressure ratio.
Williams’s new 1,941-pound-thrust FJ44-1AP turbofan for the Cessna Citation CJ1+ has a new fan and lighter interstage housing. The small turbofan also borrows the FJ44-3A’s high-pressure compressor, low- and high-pressure turbines and forced mixer.
The company’s FJ33, which was certified in September, was built on the lessons learned from the FJ44 program and incorporates new concepts proven on the EJ/ FJ22 program. Its third-generation wide-sweep fan and the low-pressure compressor are a direct result of 35 years of experience with integrally bladed fans and compressors used in Williams International civilian and military fanjet engines. The FJ33 maintains modest turbine temperatures similar to the FJ44’s and retains Williams’s low-cost, long-life fanjet technology.
Speaking of the FJ22 program, Dr. Williams told AIN that it is still in the research and development phase since there aren’t any proposed aircraft for the powerplant at present. However, he said the 770-pound-thrust engine has been run to 1,000 pounds of thrust, and the engine could be certified if an OEM commits to using the FJ22 in one of its aircraft applications.
Honda was even more reticent when asked about the technologies behind its 1,800-pound-thrust HF118 turbofan, for which it has partnered with GE to build and produce. A spokesman told AIN that the HF118 “project team is very busy at this stage, as you can imagine, and we are not conducting any interviews.”
However, the previously disclosed design goals include low acquisition cost (targeted at less than $500,000), low noise and emissions levels, improved reliability and increased fuel efficiency. Honda believes advanced materials and electronic technology borrowed from its automotive experience can improve fuel efficiency by as much as 30 percent compared with existing engines.
Pratt & Whitney Canada reached into its technology tool kit, though to create low-parts-count, affordable small engines. The result was the PW600 series, which will power the Eclipse 500 (PW610) and Citation Mustang (PW615F) very light jets.
While it shares NASA’s vision of reducing NOx emissions by 70 percent, P&WC said its small engines, by their nature, don’t emit much noise or emissions. Thus, it focused development of PW600 engines to meet customer goals, namely reliability, safety and cost (acquisition and operating).
But this doesn’t mean this series doesn’t use new technology, noted P&WC director of performance and operability Keith Morgan. The PW600 series employs an advanced shock-management fan, reduced high-pressure compressor stages, a two-stage turbine and an internal manifold.
Although currently intended for experimental aircraft, slightly larger versions of the small turboprop engines from Innodyn of Osceola Mills, Pa., could well create a market for very light turboprop twins and singles. Each of Innodyn’s four turbine engines weighs less than 190 pounds and delivers between 165 to 255 shp, at costs ranging from $26,500 to $34,500.
One of the main technologies behind the turboprops is a patented fuel-control technology called pulse width modulation. Additionally, the engines’ turbine rotor, compressor and input to the reduction gears all operate on one shaft, greatly reducing complexity and the “power lag” associated with indirect turbine designs.
There are a few cutting-edge engine technologies being developed at research labs, but while they might look good on paper these higher-risk concepts might not be feasible for civil aircraft engines as they could be prohibitively expensive to build into an engine or negatively affect reliability and/or maintainability.
In this category is GE’s revolutionary hybrid pulse-detonation engine, tentatively scheduled for testing in “2010+.” This type of engine, which would incorporate a totally new thermodynamic cycle compared with current-technology engines, would effectively replace the entire engine’s core (compressor, combustor and turbine) with a pulse-detonation combustor.
Taking a cue from fighter aircraft, researchers at the UK’s Cambridge University said last month that they are investigating a variable-area nozzle for airliner engine exhausts. It is believed that such a device could substantially reduce noise, though it would require an exit area three times the size of current core exhausts to reach the lower noise goal. However, the big unknown is how much complexity and weight such a system would add to an airliner, in addition to what possible effects it could have on reliability. Scale-model tests could start by the end of this year, according to the university.