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Jetliners' future: BIG vs FAST
January 23, 1995
9 Min Read
It's crunch time in the airliner business. Positive economic trends in Asia foreshadow an imminent boom in transpacific and trans-Eurasian travel. With their long lead times, aircraft manufacturers will have to decide very soon on airplane designs to meet the demands of the next decade.
As with any other mass-flow problem, engineers face two choices: increase the flow speed or build a bigger pipeline. In aircraft design, the solutions come down to second-generation supersonic transports or 1000-passenger super-jumbo airliners.
Of the two, the subsonic Very Large Civil Transport (VLCT) seems the surer bet, since its technological hurdles appear lower. Still, with development costs for either aircraft projected to be ten billion dollars or more, it's a gamble no manufacturer takes lightly. Industry engineers are currently at work on both possibilities.
The VLCT challenge. Manufacturers' planned VLCT configurations seem to be zeroing in on a double-decker aircraft about the length of a 747. Artists renderings belie the size of the aircraft until one notices the number of emergency exits made necessary by the FAA rule requiring no more than 90 seconds to evacuate passengers in an emergency.
A VLCT's size and the need to integrate the plane into existing airport operations present significant design problems. "To compete effectively, our proposed A3XX airliner will have to offer a saving of around 15% in direct operating costs relative to the (Boeing) 747-400," explained Airbus Industrie's Adam Brown, vice president of strategic planning, in a recent speech concerning his company's plans for a super-jumbo. "Significant advances in aerodynamics, materials, systems, and manufacturing techniques will be required to meet that goal."
The aerodynamics challenge of a VLCT stems from the need to keep the wings relatively short and to minimize drag for economy's sake. "We've put emphasis on supercritical, advanced airfoils since they're more efficient at high coefficients of lift and high wing loading," says David Murphy, manager of advanced wide-body programs at Douglas Aircraft. "The result is that, in proportion to gross weight, we can fly efficiently with a smaller wing area than previous designs, and thus fit better into current airport infrastructures."
Techniques for controlling airflow over the wing may also play a part in improving the economics of a million-pound-plus airliner. Under study: laminar flow control by using engine vacuum to generate suction through thousands of laser-drilled holes on a wing's upper surface. Such a system would greatly reduce drag by forestalling the onset of turbulent airflow over the wing. Alternatively, a circulation-control system also under consideration would bleed high-pressure engine air over the leading and trailing edges of a large aircraft's wing to boost lift as needed. The concept would allow for smaller wings free of complex flap mechanisms. Nevertheless, sources say that designing these systems for low weight and simple maintenance remains a complex engineering task.
In contrast, engineers seem confident about the materials technology needed to keep down the weight of a VLCT. "We're estimating composites are to be roughly doubled in application as a percent of airframe relative to the MD-11," says Douglas' Murphy of his company's planned 600-to-800 seat MD-12 aircraft. Large composite structures such as vertical and horizontal stabilizers will require larger autoclaves and stitching machines, but there are no show stoppers, he says.
Advanced engines. Aircraft-engine price, nacelle cost, fuel use, and maintenance constitute 40% of a long-range airliner's operating costs. Thus, engines encompass all the technical challenges encountered in VLCT design.
Here, too, the design themes seem more evolutionary than revolutionary, but again, the scale is startling. The 68,000-lb-thrust high-bypass turbofan engines that manufacturers plan to use on initial versions of VLCTs feature inlet sections around 100 inches in diameter and weigh approximately 12,000 lbs each. Large as they are, even larger engines are now being developed for VLCTs that will boost fuel economy and decrease generated noise.
Pratt & Whitney calls its super-jumbo engine the Advanced Ducted Propulsor (ADP). Based on the common core of its PW4000 family of turbofans, ADPs will include new technologies giving them potential thrust in excess of 120,000 lbs. The ground-breaking feature of the ADP is an extremely efficient gearbox between the fan and low-speed rotor. Rated at 80,000 hp, the 3:1 reduction gearbox lets the engine's high-speed turbine run at an efficient 10,000 rpm while keeping fan tip speed below Mach 1.4, where efficiency drops precipitously.
ADP demonstrator engines now undergoing testing include single-crystal, transpiration-cooled, 3-D airfoil turbine blades with electron-beam, vapor-deposited ceramic coatings. Those coatings are needed to resist the 2,800F temperatures of advanced-design, float-wall-cooled combustors. With a bypass ratio of 13:1 and a jet velocity around 730 mph, the ADP should meet projected FAA "Stage IV" noise requirements and deliver better economy than any earlier design. "Right now, turbofan aircraft can move one seat 100 miles on one gallon of gas," explains David Crow, Pratt's senior vice president for high-thrust propulsion systems. "With ADP, we expect 15% fuel-economy improvements." That's music to an airline-executive's ears.
High Speed Civil Transport (HSCT). Although physical and environmental laws conspire against building an economical new breed of supersonic airliners, their potential to cut transoceanic flight times by more than 50% provides a powerful incentive for continued research and development work.
That incentive isn't just to offer passengers greater convenience: "If you can effect quick turn-around, i.e., keep it in the air, then an HSCT can carry more passengers more miles in a day than the same size subsonic aircraft," explains Douglas' Murphy. Studies show that if that greater productivity could be coupled to fares no more than 25% to 50% greater than current first-class ticket prices, passenger volume would support a fleet of 500 HSCTs.
"Baseline" industry HSCT designs center on a 250- to 300-passenger, four-engine airliner flying between Mach 2 and 3. Range must be at least 5,000 nautical miles in order to serve transpacific routes with adequate fuel reserves. Its wing will be a sharply swept delta with the fuselage pinched at the wing root to minimize trans-sonic wave drag. Sources cautiously predict an HSCT debut by 2010.
As with the super-jumbo airliners, the principle problems facing HSCT designers include increasing use of composites, improving aerodynamics, and designing more efficient engines. The similarity ends there.
Building a fleet of low-maintenance, environmentally friendly HSCTs requires enormous strides in computational fluid dynamics, extremely high-strength, high-temperature-resistant materials, better environmental modeling, and computerized flight control. The job is so daunting that long-time industry rivals around the world have established consortia to work on the problems.
Temperature/pressure extremes. Richard Hines, manager of the joint Pratt & Whitney/GE Aircraft Engines Supersonic Transport Propulsion Program has worked on supersonic engine programs for 34 of his 38 years at Pratt. An HSCT engine will look like a scaled-up version of a military turbofan. But, he says,"This will be a bigger jump in composites technology than in any previous design."
The materials problem comes down to the unprecedented levels and durations of heat and stress on aircraft components that HSCT flight entails. A subsonic airliner and its engines see their largest thermal and mechanical stresses for just a few minutes of each flight-at take off. In contrast, any HSCT will take off at relatively low engine-thrust levels to minimize noise. Once it reaches open water and cruise altitude, supercruising requires maximum throttle settings for most of the flight. No one, says Hines, knows how even advanced intermetallic composites behave at prolonged high-stress exposure to 3,000F combustion-chamber temperatures.
So reliability remains the biggest technical question surrounding an HSCT. Pratt's Crow notes that Concorde operators have to keep a second SST ready to fly in case problems develop with a scheduled aircraft. Otherwise, passengers would receive refunds for the time lost amounting to the difference between supersonic and subsonic fares. Money-making HSCT operations could not afford to be so lavish.
Will experience with supercruising military aircraft engines help answer questions about reliability? Yes, says Hines of the Pratt/GE consortium, but he cautions that military engines average around 200 flight hours per year. An HSCT engine will need to fly 3,000 hours or more in the same time and perhaps go 10,000 hours between overhauls.
"We're looking at new codings, new coatings, and new materials," explains Hines. "Cycling isn't as much of a problem as creep in an HSCT engine." A similar statement could be made about the airframe materials that will have to withstand sustained 400 degrees F temperatures during supersonic flight and last for a large portion of the plane's 20-year lifespan.
Engineers feel more sanguine about the designs needed to reduce the NOx emissions of a fleet of 500 HSCTs flying through the ozone layer at 60,000 ft. Various combustor designs, including staged combustion, show promise toward reducing emissions to the projected eight percent of current limits. More important may be reducing fuel consumption by adding high-lift devices for subsonic flight, cutting supersonic drag by careful airframe construction and minimizing control-surface movements through very fast flight-control computers. Another requirement: developing variable-cycle engines for efficient thrust at all speeds.
With all these technologies still to be mastered, and the outlook for fuel prices, as always, uncertain, the probability is that 1000 people at a time will be riding at Mach 0.8 before 300 people ride at Mach 2.5. But the opportunity for both types of aircraft is there. "The overwhelming fact is that .75 of the world's population lives within four hours flying time of Hong Kong," says Crow of Pratt & Whitney. "They're terribly under-airplaned."
Still, the enormous cost of developing either airplane makes one wonder if larger fleets of conventional aircraft wouldn't be cheaper. Asked this question, Douglas's Murphy bristles: "It's Progress. If you don't try to make improvements you guarantee that you don't get them."
HSCT design hurdles
Deliver per-seat costs no greater than 1.5 times cost of current first-class ticket
Develop advanced composites to handle increased temperature gradients, prolonged high stresses of supersonic flight
Optimize airframe and engine design trade-offs between subsonic and supersonic flight efficiencies.
Create pilot artificial vision systems for restricted-vision, aerodynamic cockpits
Reduce acoustic signatures at take-off and supercruise
Develop advanced stability and control algorithms to minimize drag-producing control-surface movements.
VLCT design hurdles
Reduce direct operating costs 15% compared to current jumbo aircraft
Expand composite construction to large flight-control surfaces, airframe
Increase engine bypass ratios to improve fuel economy while maintaining reliability and current power-to-weight standards
Meet anticipated "Stage IV" noise- level standards
Minimize necessary airport infrastructure changes and upgrades to deal with the larger aircraft
Improve wing lift/span ratios, reduce drag with supercritical airfoil, circulation-control or laminar-flow-control mechanisms.
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