(The following is from the 1987 issue of NASA's Magazine, SPINOFF. Current issues of SPINOFF can be obtained from the U.S. Government Printing Office in Washington, D.C.)
Toward Future Flight
Propfan progress highlights NASA's aeronautical research program, which is providing new technology for coming generations of more efficient aircraft.
In the oil glut of 1986, the price of jetliner fuel plummeted to 50-60 cents a gallon, less than half of its cost only three years earlier. It was a welcome respite for the world's airlines, who burn billions of gallons of fuel annually and therefore had encountered some rough financial weather in the wake of the global oil crises of the 1970s. But even at its 1986 "bottom" price, fuel still cost five times as much as it had in 1972 and constituted a significant portion of an airline's total operating costs.
Airline executives feel that fuel prices will inevitably edge upward again. Looking at the longer term, they are concerned about the uncertainties of future supply and the possibility that another crisis might send fuel costs even higher than the $1.40 a gallon peak. That's why they are noting with great interest progress in development of a new aircraft propulsion system that promises fuel savings of 30 percent, a NASA-pioneered system known variously as the propfan, unducted fan, open rotor or ultra high bypass engine. In the U.S. three different types of propfans are being flight tested or readied for flight, and two leading aircraft manufacturers have based their next generation airliner designs on propfan propulsion.
The propfan had its origin in NASA's Aircraft Energy Efficiency program, begun in 1975 to combat rising fuel costs by reducing fuel consumption in a variety of ways. Propfan work started as an investigation of combining the best features of the turbofan engine and the aircraft propeller, which has inherently better fuel consumption characteristics. Used in most modern airliners, the turbofan is a type of jet engine in which some of the air taken in is compressed, burned in a combustion chamber and expelled at high velocity as thrust, but a far greater amount of air bypasses the combustion process; pushed rearward by a large diameter, multibladed internal fan, this slower moving unburned air mixes with the hot exhaust gas. The result is a very large gain in overall thrust with minimal expenditure of fuel. Propulsion engineers use the term "bypass ratio" to indicate how much air bypasses the combustion chamber; for example, a bypass ratio of six to one means six times as much cold (unburned) air as hot. Generally speaking, the higher the bypass ratio, the greater the efficiency of the engine at subsonic airliner cruise speeds.
NASA's propfan concept of the mid 1970s envisioned use of a large external fan - in effect a reincarnation of the propeller - to move great amounts of air and thereby effect a dramatic increase in the bypass ratio. But to drive aircraft at jetliner speeds, the propfan blades would have to have supersonic tip velocity. Therefore, the new "fan" would little resemble its ancestor, the propeller; it would have to be much thinner, yet stronger, and shaped differently to allow faster rotation.
Lewis Research Center and its principal contractor - Hamilton Standard Division of United Technologies - began extensive computer design and wind tunnel testing of model propfan systems. What emerged from several years effort was a new type of rotary thruster with 8-10 extremely thin blades, made of composite material for requisite strength and "swept back" to slice through the air more efficiently. A further advantage of the propfan is that it is not encased by a large, heavy duct, as is the turbofan. The beneficial effect of the turbofan duct increases its efficiency, but the propfan can be made larger to gain back the lost efficiency of ductless operation and still provide substantial weight saving.
Research indicated that such a system, driven by an advanced engine, could provide propulsion at jetliner speeds with fuel savings of 30 percent or more. By 1980, NASA was making sufficient progress to release the results of its initial work to engine manufacturers, who subsequently started their own propfan programs.
NASA continued ground testing of propfan models in the early 1980s, but it had become apparent that flight testing of a full-scale system would be necessary to get the answers to some important questions: exactly what kind of fuel efficiency can be expected; what cabin and community noise levels will result from supersonic rotation of the blade tips; and how the shock waves generated will affect the airplane structure. Accordingly, in 1984, Lewis Research Center initiated the Propfan Test Assessment (PTA), assisted by an industry team that includes prime contractor Lockheed-Georgia Company; Hamilton Standard, which designed and built an eight-bladed, nine-feet-diameter, single rotation propfan; Allison Gas Turbine Division of General Motors, which provided a 6,000 horsepower turbine engine and gearbox; Rohr Industries, which designed the engine nacelle; Gulfstream Aerospace, which modified a Gulfstream II business jet to allow mounting of the PTA engine on a wing; and Lockheed-California Company, responsible for analysis and evaluation of noise and vibration levels.
The PTA effort began in the summer of 1986 with a successfully concluded 50-hour program of static testing to check the compatibility of the engine, fan and nacelle and to measure propulsion system performance. In late 1986, the test system was installed on the Gulfstream II and airworthiness testing of the PTA test bed, a preliminary to propfan flight testing, began in March 1987.
In flight tests on a Boeing 727 (left), the General Electric Unducted Fan was flown as the starboard engine in place of one of the regular three turbofans first flown in August 1986, the UDF is a pusher-type propfan with two counter-rotating rotors of eight blades each.
While NASA and its contractor team were pursuing propfan technology development in one direction‹the single rotation "tractor" or "puller" system - General Electric Company (U.S.A.) used the early NASA technology as a departure point for independent development of a different kind of propfan known as the Unducted Fan or UDF*. After study and component test work in 1981-82, General Electric started construction in 1983 of a proof-of-concept dual rotation pusher-type UDF. The UDF has two counter-rotating external fans, each with eight sweptback blades. Unlike the NASA PTA system, it has no gearbox; the fans are driven directly by the turbine, a weight-saving measure that eliminates the weight of the gearbox and its oil cooling system.
Propfans generate lower frequency sound pressures than do turbofans, and the supersonic blade tip speed greatly increases the acoustical energy propagated. These factors demand extensive research toward reducing community/cabin noise and designing methods of protecting against sonic metalfatigue in aircraft structures. Such research is conducted in facilities like this Douglas Aircraft anechoic chamber, where engineers use a full-scale pressurized section of a DC-9 fuselage to study structural sound transmission under conditions as close as possible to actual cruise flight.
In a cooperative General Electric/ NASA program, the UDF was extensively ground tested in 1985-86 and it demonstrated a fuel consumption rate 20 percent better than modern turbofans. Then General Electric teamed with Boeing Commercial Airplane Company to test the UDF in flight aboard a modified Boeing 727 jetliner. Flight tests began in August 1986 and continued into 1987. General Electric also built a second demonstrator engine for 1987 flight tests on a McDonnell Dougias MD-80 twinjet. The company's schedule calls for engine certification by the end of 1990 and availability for service in 1991-92.
A new propfan program, a direct offshoot of NASA propfan technology, was launched in 1986 when Allison Gas Turbine Division and Pratt & Whitney Division of United Technologies began a joint venture to pursue commercial, and possibly military, applications of the propfan. The development team includes, in addition to Allison, two other members of the NASA PTA industry group: Hamilton Standard (propfan) and Rohr Industries (nacelle).
The team built a demonstrator engine known as the Model 578-DX, a 10,000 horsepower system in which an Allison turbine drives a counterrotating pusher propfan of two six-bladed rotors. Wind tunnel tests started in 1986 and continued into 1987. The team has signed an agreement with McDonnell Douglas for flight tests on a modified MD-80 beginning in December 1987. In addition, the group is defining a production-type propfan for projected airliners of the 1990s. Targeted for a fuel consumption rate 30 percent better than the best similarly sized turbofans that will be available in the same time frame, the advanced engine will have a bypass ratio of 50-60 to one. It is expected to be ready for operational use in 1991-92.
The propfan may ultimately be applied to a wide range of airline and military transports, including retrofit installations in aircraft already in service. For now, however, developmental emphasis is on propulsion systems for medium-size airliners being designed for service introduction early in the next decade. The reason: analysts expect that airliners in the 100-180 seat bracket, and in particular the 15O-seat category, will constitute the greatest market at that time, due to projected traffic increases and the necessity for retiring more than 3,000 aircraft of that type beginning about 1990.
Boeing Commercial Airplane Company is designing an all-new, lSO-seat propfan airliner tentatively designated 7J7. Boeing is aiming for a fuel efficiency rating 60 percent better than the most efficient turbofan-powered jetliners now flying. That 60 percent is compounded of a number of technological advances in addition to propulsion, such as extralight structural materials, a new high lift/low drag wing and a highly advanced flight control system. Target date for airline operation of the 7J7 is 1992.
McDonnell Douglas Corporation's Douglas Aircraft Company is considering an all-new twin propfan; designated MD-94X, it would be a 160-180 seater available in 1994. Douglas is also planning to offer propfan retrofitting of its MD-80 series now in wide airline service, and, additionally, is designing two propfan-powered MD-80 derivatives: the 100-110 seat MD-91X that could be in production by 1991 and the 15O-seat MD-92X, planned for service beginning in 1992.
A lot of development work remains to realize the dramatic potential fuel economies and to solve problems associated with supersonic fan blade tip rotation. But barring other, unforeseen problems, the propfan airliner could be carrying passengers within five years.
*UDF is a trademark of General Electric Company (U.S.A.).