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The turbofan of tomorrow.


Advanced research is under way today at Pratt & Whitney to design an ultrahigh bypass turbofan engine, known as the advanced ducted engine, that will meet the airline's demands for higher thrust and more fuel-efficient engines, while at the same time reducing airport noise and emissions.

Ultrahigh bypass turbofans are engines that use a large fan at the front of the engine to force pressurized air around and through the engine. The bypass ratio is the ratio of air passing through the fan (thus bypassing the engine core) to the air that passes through the engine. The higher the ratio of bypassed air to air passing through the engine, the greater the fuel efficiency of the engine.

The need for such engines has been spurred by increasing airplane traffic, which raises noise, environmental, and fuel consumption issues. Airline revenue passenger miles (RPMs) are expected to increase at an average of 5 percent per year into the next century. For U.S. carriers, the Federal Aviation Administration forecasts a near doubling of RPMs by 2001, rising from 429.1 billion RPMs in fiscal year 1989 to 765.6 billion RPMs in fiscal year 2001.

Since 1985, an extensive technology readiness program has been under way to demonstrate the key technologies of a geared, variable-pitch, lightweight fan, and a thin-lip, slimline nacelle that are required to make a 10:1 to 20:1 bypass ratio advanced ducted engine a reality. Pratt & Whitney, along with Motoren-und Turbinen-Union (MTU) of Munich, West Germany, and Fiat Avio of Turin, Italy, has been addressing the key technologies of acoustics, interference drag, nacelle and variable-pitch fan design, and gear systems.

As the technology begins to mature and realistic installation effects are accounted for, it appears that the advanced ducted engine can increase thrust by over 20 percent and lower fuel consumption by 12 percent compared to existing high-technology turbofan engines. (These fuel-burn estimates are for a 150- to 200-passenger airliner operating on a nominal 500-mile flight.) These are impressive fuel savings. The goal is to achieve these savings at a selling price and maintenance cost that are competitive with existing turbofan engines, so the net overall economics would justify the investment in the new engine.

Increased Performance

The core of the advanced ducted engine (high-pressure compressor, combustor, high-pressure turbine) is identical to a conventional turbofan engine and, in fact, is fundamental to the concept of using an existing core with an ultrahigh bypass-propulsor, low spool system that includes the fan, low-pressure (LP) compressor, LP turbine, and fan drive gear. The most significant feature of the low spool is the gear-driven, variable-pitch fan. The fan is driven by a high-speed, transonic, LP turbine through a 3:1 reduction ratio planetary gear system. The high-speed low spool permits the elimination of a total of three to five stages cumulatively in the low-pressure compressor and low-pressure turbine. The variable-pitch fan system eliminates the requirement for a fan duct thrust reverser and, therefore, results in a simple slimline nacelle concept with no variable geometry. This is critically important due to the necessity of reducing the weight and drag of the large-diameter nacelles associated with ultrahigh bypass engine designs.

Any new commercial turbofan engine must consider the question of airport and community noise. The geared fan concept permits each component of the low spool system to be optimized for both noise and performance. The 3:1 speed reduction of the fan drive gear system allows the designer to select the fan tip speed for lowest noise and highest fan efficiency, while at the same time maximizing the low spool shaft speed to allow the use of fewer low-pressure compressor and turbine stages. For example, a geared fan with a bypass ratio of 14:1 can be driven by a three-stage high-speed low-pressure turbine. A 14:1 bypass ratio ungeared low spool would produce a much larger engine, with an eight-stage low-pressure turbine and a 10 percent higher fan speed. This would result in a 2 to 3 percent thrust reduction and an increase of about 2 EPNdB in takeoff noise.

In 1989, a 17-inch-diameter fan/nacelle model of the advanced ducted prop was tested at stimulated takeoff and landing conditions at the United Technologies Research Center acoustic research tunnel. The tests were conducted to assess the acoustic effects of varying fan blade pitch angle, inlet length, fan-exit-wide vane-to-blade spacing, and exit-guide vane number. Noise measurements were obtained using far-field microphone arrays and internal duct dynamic pressure transducers. The operating line characteristics, thrust, efficiency, and recovery of the fan were also calculated and then verified using static pressure measurements in the fan duct. Results of these tests are now being studied and will be factored into the design of Pratt & Whitney's advanced ducted engine.

Interference Drag

Ultrahigh bypass engines that offer the benefits of lower noise and lower fuel consumption also present a difficult challenge to the aircraft designer. Installing an engine that is as much as 50 percent larger in diameter than a conventional turbofan of the same thrust requires careful integration of the wing, pylon, and nacelle such that the performance benefits are not lost to increased interference drag.

A joint test program was conducted in 1987 by Pratt & Whitney, MTU, British Aerospace, and Messerschmitt-Bolkow-Blohm (MBB) to investigate wing, pylon, and nacelle interference drag. A series of flow-through nacelles was designed that simulated bypass ratios between 10:1 and 24:1. The test nacelles were mounted on an existing Airbus Industrie A320 wing/fuselage semispan model. High-speed cruise testing was conducted at the Aircraft Research Association in Bedford, England, and low-speed testing was conducted at MBB in Bremen, West Germany.

The test program demonstrated that ultrahigh bypass large-diameter engines can be installed under advanced wings with no increase in aerodynamic interference drag relative to a conventional turbofan installation.

Using a variable pitch fan for thrust reversing eliminates the requirement for a conventional thrust reverser installed in the nacelle. This provides a significant savings in nacelle cost, weight, and maintenance. An additional benefit is the opportunity to design a slimline nacelle to further reduce weight and drag.

One such critical condition is slow, high-angle-of-attack operation. The fan/nacelle model was tested in the wind tunnel at the United Technologies center. Low-speed testing was successfully completed up to a 30-degree angle of attack - 5 degrees higher than where a conventional inlet experiences flow separation.

Variable-Pitch Fan

The variable-pitch fan offers an opportunity to optimize the fan blade angle for takeoff, cruise, idle, windmilling, and thrust reverse. Ultrahigh bypass cycles optimize at relatively low fan-pressure ratios. Depending on the bypass ratio, fan-pressure ratios vary between 1.2 and 1.4. Low fan-pressure ratio fans are also sensitive to fan duct pressure loss. This requires careful design of the fan exit-guide vanes, fan exhaust duct, and nozzle. Minimizing fan and pylon interaction is also very important.

Reducing windmilling drag can be critically important, especially on a large twin-engined transport. Climb after takeoff with one engine out of service is usually the critical thrust-sizing condition for those types of transports. Variable pitch can be used to increase the airflow through the windmilling fan in a "feathered" position and to minimize the spillage drag.

Thrust reverse is the most technically challenging feature of the variable pitch fan. During reverse operation, the fan blade passes through the feather condition for those types of transports. Variable pitch can be used to increase the airfow through the windmilling fan in a "feathered" position and to minimize the spillage drag.

Thrust reverse is the most technically challenging feature of the variable pitch fan. During reverse operation, the fan blade passes through the feather condition and the blade becomes stalled; the trailing edge of the blade in normal flight now becomes the leading edge. The blade must become unstalled and reverse the airflow against the air that is entering the inlet at 120 to 140 knots landing speed. The reverse airflow enters the fan duct through the fan exit nozzle. Air for the core engine also enters the fan exit nozzle and is drawn into the core by the pumping action of the compressor.

In mid-1987, a 17-inch-diameter dynamic pitch-change fan rig was designed by Pratt & Whitney. Modular Engineering Inc. (Rockfall, Conn.) completed the fabrication of the rig in early 1988. A series of tests was then conducted at the United Technologies center to investigate reverse-pitch fan operation, including dynamic and steady-state blade stresses, fan pumping characteristics, and reverse thrust. The tests demonstrated acceptable dynamic blade behavior during transition from forward to reverse pitch, stable fan operation in reverse, and a reverse thrust potential of roughly 20 percent of takeoff thrust over a speed range of 25 to 120 knots. Detailed quantitative test data are now being obtained with the fan/nacelle interaction rig being tested. Additional reverse simulation testing is being conducted by MTU in West Germany to accurately measure the flow quality entering the fan nozzle and the gas turbine core.

Lightweight Fan Blade

Variable-pitch ultrahigh bypass fans must be lightweight so that the engine weight is not unduly penalized. The low fan-pressure ratio, wide chord blade has a much lower fan tip speed than a conventional fan (900 versus 1500 feet per second). This lower tip speed may permit the application of lightweight composite material and a spar/shell construction similar to the swept, counterrotating, variable-pitch blades used on the open rotor propfan engine. Hollow titanium blades are also being considered. Both concepts are being tested in Pratt & Whitney's ballistic laboratory for foreign object damage tolerance. Weight and mass distribution of the fan blade are also important considerations in the design of the variable-pitch actuation system. Heavier blades would require a stronger, heavier actuation system.

The higher-horsepower fan-drive gear system is the key component of the engine that permits the fan, low-pressure compressor, and low-pressure turbine to operate at peak performance with the fewest number of stages. Power input is through the sun gear, which drives five planet gears. The fan is driven by the planet carrier while the outer ring gear is stationary.

Fiat has prime design responsibility for the gear-drive system. Significant progress has been made in the past two years in the design of a highly reliable, low-maintenance, compact, fan drive gear system. Most significantly, the volume of the gear system has been reduced by half for the same horsepower. Gear system efficiency greater than 99 percent is required to minimize heat generation and reduce the size and volume of the heat exchanger required to cool the oil.

High reliability is absolutely essential for the gear-drive system and is being designed into each component of the system. Fan loads are carried through the gear-drive support structure and not through the gear mesh. Advanced bearing designs and materials are being tested to ensure long life. Oil system management, both distribution and heat rejection, is being optimized in test rigs at both Pratt & Whitney and Fiat. Modern, highly reliable gear systems will become a proven component of ultrahigh bypass engines in the future.

Demonstrator Engine

Pratt & Whitney, MTU, and Fiat are designing and building a full-scale demonstrator engine to examine technology concerns that cannot be verified completely in model and component rigs alone. Critical areas of interest are: thrust reverse through variable pitch, interaction of the variable-pitch fan and the core engine, full-scale acoustics, thrust increase potential of an ultrahigh bypass propulsor on a commercially certificated PW2040 core engine, as well as the associated fuel-consumption reduction and the mechanical integration compatibility of the variable-pitch mechanism and the fan-drive gear system.

Engine verification testing will take place at Pratt & Whitney's research and development facility in West Palm Beach, Fla., and in the wind tunnels of the National Full-Scale Aerodynamics Complex located at NASA Ames Research Center in Sunnyvale, Calif. If the demonstrator program is successful, it will establish technology readiness of a new generation of ultrahigh bypass geared turbofan engines.

PHOTO : Ready to roll. A model of Pratt & Whitney's new ultrahigh bypass turbofan engine undergoes tests at simulated takeoff and landing conditions in an acoustic research tunnel at the United Technologies Research Center.

PHOTO : Dual Benefits. Ultrahigh bypass engines that offer the benefits of lower noise and lower fuel consumption present a difficult challenge to the aircraft designer. This schematic highlights key design features of the new engine.

PHOTO : Multiple considerations. In designing the ultrahigh bypass turbofan, critical areas of interest include thrust reversal, interaction of the variable-pitch fan and the core engine, acoustics, fuel consumption reduction, and the mechanical integration compatibility of the variable-pitch mechanism and the fan drive gear system.
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Author:Brines, Gerald L.
Publication:Mechanical Engineering-CIME
Date:Aug 1, 1990
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