Look Ahead Powerplants: More Efficient, More Sources

By Staff Writer | May 1, 2007
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Engine and transmission designers face customer demands for more efficient power systems capable of running on a greater variety of fuels.

CALL IT WHAT YOU LIKE — HELICOPTER, ROTORcraft, vertical-lift platform. To fly, it will need to convert fuel to kinetic energy.

For the foreseeable future, that work will be done by engines and transmissions that look familiar, or will become so. There is plenty of potential to make aircraft engines, particularly turbine ones, more efficient. For their part, transmissions are just beginning to benefit from some recent, impressive technical developments. That’s good, too, because designers and manufacturers of powerplants and transmissions will face ongoing demands for more power at higher efficiencies from helicopter makers and operators.


"The trend is bigger rather than smaller," said Alain Bellemare, president of Pratt & Whitney Canada. "Customers want bigger turboshaft machines on the civil front. Everybody wants better fuel burn, more payload, and more range. But there’s a lot we can do within the envelope."

Another demand from customers is the assurance that they will be able to find affordable fuel when they need it, which translates into the development of fuels other than those based entirely on oil and engines that can burn them.

Others in the field agree with Bellemare that the turbine engine remains the best option to produce power for aircraft. "We haven’t found a competing, high-reliability, high power-density source," said Ming Lau, chief of the Power Systems Div. of the U.S. Army’s Aviation Applied Technology Directorate. "We have quite a bit of the theoretical efficiency left in the Brayton cycle," which represents the conversion of fuel’s potential energy to thermal energy by an engine’s turbomachinery.

Tapping that theoretical efficiency will require the development of improved materials, particularly ceramics, to permit turbine engines to run at higher temperatures. That could be a constraint on powerplant advances, since the production base for ceramics is limited, particularly in the United States.

The Army aims to take advantage of that potential through the Advanced Affordable Turbine Engine program. The program’s goals are to surpass the T700 by achieving a 25-percent reduction in specific fuel consumption, an 80-percent gain in power/weight ratio, a 35-percent cut in both production and maintenance costs, and a 15-percent reduction in development costs. The engine is to have a design life of 6,000 hr.

That program comes as the Army is pushing new engine technology into the field. Honeywell earlier this year successfully completed the first engine test of the Small Heavy-Fuel Engine it developed under an Army contract. That program looked to design advanced technologies to improve engines used for light helicopters, unmanned aerial vehicles (UAVs), ground vehicles, and power generators. The program’s goals were to reduce fuel consumption by 20 percent, improve power/weight by 50 percent, and pare life-cycle costs 35 percent. "We have already applied some of these technologies in our new HTS900 turboshaft engine," said Ron Rich, Honeywell’s director of advanced technology.

The HTS900 has completed nearly 2,500 hr of engineering development testing. Honeywell hopes to win FAA certification of the engine in the third quarter.

The industry has achieved impressive gains on the transmission end. Under the Army’s Rotorcraft Drive System 21 program, Boeing and Northstar Aerospace developed a new, innovative transmission for the AH-64D Apache Longbow. The new transmission is the same size as the old design, but is capable of 25 percent more power throughput and has more than twice the endurance life. It can be applied to a variety of helicopter drive systems and used for future Army helicopter upgrades and for new helicopters.

The new transmission uses smaller, lighter-weight "face" gears that split the torque sent to the drive shaft. Traditional transmissions use a single pathway to power the aircraft’s rotor system. In the past, increasing the power meant increasing the size of the transmission.

Boeing engineers developed and designed the new technology. Northstar Aerospace developed, manufactured, assembled and tested the transmission components.

A key breakthrough in achieving that success was the development of a grinding machine capable of economically producing the face gears with their complex gear-tooth profiles in production quantities. Northstar provided much of the gear manufacturing technical expertise and the funding required to develop the advanced-technology gear-grinding machine.

Manufacturing is a key challenge in pushing the state of the art in transmissions. As power demands push the use of higher and higher shaft horsepower engines, transmission designers face a dilemma. They can handle the higher power by adding stages to the transmission, which in turn increases the complexity of the unit, its maintenance requirements, and its susceptibility to failure. Or they can increase the size of the gears in the stages to handle the extra power and confront manufacturing and reliability challenges.

"It’s one thing to develop a design that can handle all the stresses, to have all these mesh ratios that evenly distribute them," said Lau. "But to be able to reliably and efficiently manufacture those large gears is another. We’re pushing the state of the art in both design and manufacturing at the same time. We develop a new capability" with each evolution.

That’s not stopping the Army from looking for further advances in transmissions. Its Enhanced Rotorcraft Drive System program looks to develop and demonstrate critical technologies through Fiscal 2010 and to work them into product improvement programs for existing aircraft. The strategy is to conduct a technology demonstration at the component/subsystem level, with current Army aircraft as a baseline as much as possible. The program is planned for 4.5 years.

The program is focused upon developing critical performance and affordability enhancing drive system technologies for the Army’s current and future fleets of rotorcraft. The rotor drive systems of today’s rotorcraft are extremely heavy, consume a large portion of the aircraft’s empty weight, "and thus negatively impact the payload and range characteristics of the vehicles," the Army said in its notice of the program. The rotor drive system is an assembly of very high-precision mechanical components with demanding manufacturing and quality requirements. Current systems have high parts count, making the total production costs high. Once fielded, the drive systems have proven to be a major source of operating and support costs due to less than desired component durability, susceptibility to corrosion and high cost of overhaul.

The Army also wants to tackle the noise emitted by rotorcraft drive systems, which is extremely high and at a level damaging to the human ear. Noise at this level (+100 dB) also distorts voice communication, and accelerates fatigue of the crew and passengers.

The Enhanced Rotorcraft Drive System program will consist of design, fabrication, and demonstration testing of critical drive system technologies required to achieve the program goals for the Army’s current and future fleets of rotorcraft. The program goals are a 40-percent increase in drive system power/weight ratio, a 15-dB reduction in generated noise, a 30-percent reduction in production cost, a 30-percent reduction in operating and support costs, and a 75-percent automatic detection of critical mechanical component failures.

Design, fabrication and testing of these critical component technologies will be conducted to demonstrate the specific weight, durability, noise, production cost, and operating and support costs of the components.

The rising price of fuel today has everyone from the Pentagon to the single-helicopter operator asking where reliable, affordable supplies will come from in the future.

From Africa to South America, extensive research and development efforts are under way to commercialize so-called Fischer-Tropsch fuels for vehicle use. More auto manufacturers, for instance, are viewing Fischer-Tropsch liquids as a viable way to use alternative fuels in diesel engines without compromising fuel efficiency or impacting infrastructure or refueling costs.

Fischer-Tropsch technology converts coal, natural gas, and low-value refinery products into a high-value, clean-burning fuel. The resultant fuel is colorless, odorless, and low in toxicity. It is virtually interchangeable with conventional diesel fuels and can be blended with diesel at any ratio with little to no modification.

Fischer-Tropsch fuels offer important emissions benefits compared with diesel, reducing nitrogen oxide, carbon monoxide, and particulate matter. Currently, several oil companies are researching large-scale production of Fischer-Tropsch fuels. At least four major companies have announced plans to build pilot plants to produce synthetically derived Fischer-Tropsch diesel fuels.

Plants are currently planned for Indonesia, Africa, South America, and the United States.

In addition, while many alternative fuels require completely separate distribution systems, Fischer-Tropsch fuels can use the existing fuel distribution infrastructure.

This means the fuels can be transported in the same ships and pipelines as crude oil.

In the United States, the U.S. Air Force is taking the lead in that regard. In February, the Air Force Research Lab reported successful engine emissions tests to support the Defense Dept. Assured Fuels Initiative, an effort geared towards securing domestic fuel sources to meet the military’s energy needs.

To assess the performance of a manned Air Force aircraft running on alternative jet fuel, the research team measured the particulate and gaseous emissions of a TF33-PW-103 engine, comparing the results of burning conventional JP-8 fuel versus an alternative, 50/50 blend of JP-8 and Fischer-Tropsch synthetic fuel.

The tests showed that using the fuel blend produced significantly reduced particulate emissions for all engine conditions. Specifically, the researchers observed a 20-40-percent decrease in particle concentration and smoke number and a 30-60-percent reduction in particulate mass. Furthermore, the alternative fuel’s effect on most gaseous combustion products was negligible, suggesting that it had no adverse impact on TF33 engine emissions.

Part of the problem with such fuels is their purity, The Army’s Lau explained that traditional fuels continue impurities, like sulfur, that serve as lubricants and corrosion inhibitors in engines. "The energy and burn characteristics are no different," he said. "But we may need to come out with additives to keep from losing our engines to erosion and corrosion."

The next step for rotorcraft power may be variable-cycle systems, either on the engine or transmission side. "Fifteen years from now, we may be looking at different concepts than we are flying today," Lau said.

Variable-cycle engines would be designed to run at efficient settings throughout their power-production range.

A variable-speed transmission permits the main rotor to turn at least two different rpms without disengaging the engines or changing engine rpm.

The variable speed gearbox includes a clutch and a freewheel unit for each engine. A gear path drives the main gearbox in a high rotor-speed mode when the clutch is engaged to drive the main rotor at high rpm for hover flight. A reduced gear path drives the main gearbox in a low rotor-speed mode at lower rotor rpm for high-speed flight when the clutch is disengaged and power is transferred through the freewheel unit.

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