Ducted-Prop/Propfan Technologies - Archived 3/2000

Outlook

Orientation

Technical Data

Owing to the widely diverse nature of the overall propfan/ducted propeller efforts, technical data vary with the engine programs of the manufacturers. See Program Review section below.

Variants/Upgrades

Several engine/power plant designs have emerged in the past 10-15 years. See Program Review section below for a brief discussion of the major efforts worldwide.

Program Review

Propfan has come to mean much of the same to the aerospace community: highly swept, supercritical blades, either single or counter-rotating, rotating at near-supersonic or supersonic speeds, providing the potential for enormous improvements in aircraft operating economics, and at the same time allowing turbofan/turbojet-type speeds. Some propfan engine designs feature geared powerplants, while others are ungeared.

A ducted-propeller engine is much like a high-bypass turbofan, with the hub and fan or fans enlarged and placed well ahead of the inlet to the compressor section. The most visible difference from the propfan is that the fan (or fans) are housed within a large nacelle (duct). The enlarged fan(s) can either be variable-pitch to provide reverse thrust – the pitch mechanism contained in the enlarged hub – or fixed with a conventional thrust reverser on the engine. Whether a reversible pitch mechanism or thrust reverser is used is a question of weight penalties; whichever can be made lighter will likely end up being the one adopted. In all cases of ducted-prop design so far, the engine is geared for optimum efficiency.

The ducted propeller design offers a lower sfc than turbofans, though not to the degree a propfan offers. The tests have shown that the sfc can be 10 to 12 percent better than that of a contemporary turbofan. Operation is much quieter than a propfan, due to the noise-dampening shrouding of the fan section, and because the engine’s exhaust velocity is about half that of current turbofans of comparable thrust. The lower exhaust velocity is caused by the fan’s reduced pressure ratio, with the demonstrator’s fan tip speeds being reduced to roughly 900 feet/second (tip speeds of comparable turbofans are about 1,500 feet/second).

Noise should be below FAR Part 36/Stage 3 noise limits in all phases of flight. At this point, all ducted-prop efforts are being pursued as research and preliminary/exploratory development projects. Pratt & Whitney is, however, considering a production version of its ADP, the first example of which is likely to power a very large twin-engine aircraft, succeeding the Airbus A330/Boeing 777 class.

NASA Efforts. Since 1977, NASA and private industry have individually studied the various technological advantages of the advanced propeller designs, now generically designated propfan. McDonnell Douglas, Lockheed, Hamilton Standard, and Fokker of The Netherlands have done extensive research and design of advanced transports, utilizing new turboshaft or hybrid engines and transmissions turning highly swept, critical airfoils. Previous NASA funding has already demonstrated, in scale form, the inherent fuel efficiency and propulsive strengths that the propfan can provide in real life.

As part of the Aircraft Energy Efficiency Program, NASA’s Propfan, or Advanced Turboprop, includes all analytical and experimental testing of advanced propeller and drive systems. ATP is based on many USA studies conducted since 1973. In early 1976, Lockheed-California and Boeing Commercial Airplane Company conducted studies under the NASA Reduced Energy for Commercial Air Transportation (RECAT). Douglas conducted a separate study with Ames Research Center, monitoring and managing all RECAT efforts.

Lockheed and Boeing studies compared theoretical propfan performance versus turbofans. Before most of the actual wind-tunnel studies were conducted, Lockheed and Boeing projected at minimum 15 to 20 percent better fuel specifics and an 8 to 10 percent lower direct operating cost of a typical propfan versus turbofan at Mach 0.75, 35,000 feet (10,668 meters), on a 475-525 nautical-mile (880-973 km) mission. Fuel was priced at US$0.60 a gallon. Douglas compared a propfan-powered DC-9-30 with the conventionally powered transport over the typical DC-9 stage length of 275 nautical miles (509 km), and found 27 to 33 percent better fuel specifics.

Another study was conducted by the US Navy in 1978. Designated Maritime Patrol Aircraft (MPA), the study speculated on a very long endurance aircraft to eventually replace the Lockheed P-3C. At Mach 0.7, a propfan-powered MPA would use 35 to 40 percent less fuel and would have a gross weight approximately 25 percent less than the conventionally powered aircraft for the same mission.

NASA Advanced Turboprop. Since launching the ATP in FY80, NASA and its contractors have shown that improvements in power generation and fuel efficiency of the propfan are not just wishful thinking. Scale model tests of various Hamilton Standard propfan blades conducted at NASA and contractor facilities since the late 1970s culminated in scale-model tests aboard a NASA Lockheed JetStar test-bed.

Initial subscale propeller model tests verified potential propulsive efficiency of 80 percent at Mach 0.8, and measurements of noise generation and attenuation have confirmed analytical predictions in small-scale experiments. Since FY80, the ATP program activities have included investigations into integrating noise reduction, composite structures, and increased aerodynamic efficiency technologies as applicable to a wide range of general aviation and commuter/regional transports. Propulsion system aerodynamics research concentrated on analytical methods to verify turboprop-airframe integration and advanced propellers. A full-scale, nine-foot (2.74-meter) diameter propfan was contracted from Hamilton Standard, with full-scale tests begun in 1984.

In FY84, when the first of many millions was appropriated for propfan work, the Propfan Test Assessment Program was initiated, with Lockheed-Georgia given a US$40.26 million contract for installation of a complete propfan drive system, ground tests, and wind tunnel testing. Ground tests were begun late in 1984 at Rohr facilities in San Diego, CA, USA.

PTA Aircraft. In 1982, NASA selected Lockheed-Georgia to provide a testbed aircraft for a complete advanced propfan system. The aircraft was a Gulfstream II business jet modified to accept the propfan system. That system encompassed an Allison T701 (501-M78) turboshaft engine developing 6000 shp (4474 kW), a modified Allison T56-A-14 gearbox, and a Rohr nacelle. Hamilton Standard developed and manufactured the eight-blade, nine-foot diameter, single-rotation propfan for NASA under a separate, Large-Scale Advanced Propfan contract.

Actual flight testing of the G-II began in March 1987 and was completed one year later, accumulating 133.5 flight hours. The PTA propulsion system was mounted on the left wing. A 2082-pound (945-kg) static balance boom was installed on the right wingtip to compensate for the weight of the 4500-pound (2041-kg) propulsion system, while a 288-pound (130-kg) dynamic balance boom was mounted on the left wingtip.

The PTA contract, awarded to Lockheed-Georgia in August 1984, was for US$56.4 million.

NASA had also recognized that counter-rotation will provide approximately eight to ten percent better efficiency due to swirl recovery. It contracted with GE, which in 1984 unveiled its UnDucted FanT engine.

GE Aircraft Engines. In March 1984, GE Aircraft Engines received a contract from NASA to aid in development of the GE36 UnDucted Fan UDFT demonstrator engine, now a registered trademark of GE. The UDFT is an advanced aft-counter-rotating propfan driven by an F404 gas generator. The F404 drives new low-speed, two-stage power turbines that are directly connected to the propeller blades or fans. Each LPT stage consists of five turbine rows and one row of larger blade shapes that form the base of the external propfan blades. No stators are needed as the counter-rotating sections act as stators.

GE claimed that this design offered the best option for the near- to long-term in that the heavy gearbox and oil cooler were eliminated. The company countered claims that this layout posed problems of matching the high speed of the gas generator with the slow 1300 rpm of the power turbine by saying that the efficiency of the blades, the weight reduction of the system versus gear-driven units, and the ability to use smaller diameter and thus more blades negated any difficulties with matching. Indeed, with FADEC, GE claimed that there would be no such problems. GE noted that another advantage of this layout was that soft, isolated mounts could be used because no torque loads were imparted to the airframe, which incidentally lowers cabin noise appreciably.

GE ran its first GE36 in August 1985, running the unit at idle. A problem with a power turbine failure owing to turbine blade resonance near the end of 1985 resulted in a temporary program halt. GE redesigned the power turbine blades and equipped them with damper pins. In August 1986, GE tested its GE36 engine on a Boeing 727-100, thereby validating earlier testing results. It began component tests on bearing and actuation systems for the variable-pitch blades, and engine seal and combustor tests by the end of 1986.

The second engine began the fabrication process in November 1986, and was completed in January 1987. It was used for testing on a McDonnell Douglas MD-80 which began in April 1987. GE then initiated development of the 20,000-25,000 lbst GE36 production engine, with SNECMA as a 35 percent risk-sharing partner. In June 1989, GE announced that its propfan program was in a "wait and see mode," due primarily to a stabilization in the price of oil.

Allison. The Indianapolis-based manufacturer has explored small expendable propfans based on Noel Penny Turbines’ gas generators for use in Unmanned Aerial Vehicles (UAVs). Allison has shown missile configurations, featuring folding counter-rotating propfans, for the USAF/Boeing AGM-86 ALCM and the USN/McDonnell Douglas AGM-84 Harpoon antiship missile.

PW/Allison. The joint venture 17,400-lbst Model 578-DX propfan differs from GE’s configuration in its reduction gearbox which provides counter-rotating drives for the Hamilton Standard open fan. The engine, based on a modified Allison Model 571 core, first flew on an MD-80 in April 1989. Although original plans called for 50 hours of flight testing, only 20 hours had been logged when the program concluded the following month. These tests confirmed the low noise levels and reduced fuel consumption projected for the 578-DX, while the Hamilton Standard blades met all design criteria.

Rolls-Royce plc. In a program called "Aegis," Rolls-Royce plc has studied the state of the art of advanced propellers and fans. Like the team of Pratt & Whitney and Allison, Rolls-Royce felt that the use of gearing is the best way to produce the optimum combination of gas generator and propeller efficiency.

One of the chief propfan efforts explored by Rolls-Royce was the RB509 geared advanced propeller engine. The design of the RB509-11, as the latest configuration of the engine was known, would emphasize low purchase and maintenance costs as well as low fuel burn, with a simple 10-stage compressor and a three-stage, uncooled power turbine. Rolls-Royce, which had not appeared to share the enthusiasm of other firms involved in propfan research, announced at the 1988 Farnborough Show, "there is no case whatsoever for an unshrouded fan" in the long-range wide body transport market. Addressing the short-range market sector, company officials stated, "We don’t think propfan will happen very quickly."

The company has, however, continued with studies of various elements of its RB.529 "Contrafan" engine. The RB529 has a BPR of from 12 to 18:1 and an estimated sfc 15 percent lower than current large turbofans.

Pratt & Whitney. Pratt & Whitney has had a number of design studies of advanced turboprop engines including the STS589, a 9,000-15,000 shp engine, and the STS679 in the 17,500 shp class. Both come from the Pratt & Whitney Peak Performance Propfan or P3 and use Hamilton Standard pusher, counter-rotating propellers, driven through a Hamilton Standard-designed and -built gearbox.

Pratt has the ability to employ a number of core engines in any of its ducted-prop or propfan designs, including the V2500, PW2037 and PW4000. It can also use the Pratt & Whitney Canada PW100, and in the small classes, any number of Pratt & Whitney Canada small gas turbines. It may also cooperate with Rolls-Royce and Turbomeca in the RTM 322 if propfan design in that horsepower class is required.

Teledyne CAE. In early 1988, Teledyne CAE, long a manufacturer of small thrust engines for missiles, drones and other unmanned air vehicles, unveiled a mock-up of a small propfan engine for use in the next generation of cruise missiles. The engine, the Model 235, develops 200-250 lbst (0.88-1.11 kN) and weighs approximately 150 pounds (68 kg). The firm teamed with GE Aircraft Engines, with the latter contributing technology for the back end of the engine as well as basic propfan technology and experience.

Other Propfan Efforts. Besides the GE, Allison, Pratt & Whitney, Rolls-Royce, Teledyne CAE and overall NASA programming, there are a number of new and ongoing propfan propulsion efforts:

France. France has launched a project, Concept d’Helice pour Avion Rapide en vue d’une Meilleure Economie (CHARME), or Propeller concept for high-speed aircraft with improved economics. In this program, funded at some FFr40 million, sponsors include DRET, DGAC, STPA, with Aerospatiale, Ratier-Figeac and ONERA providing most of the work. Phase 2 design studies were to investigate the area of counter-rotating propfans. ONERA has already tested a one-meter, 12-blade model in the Modane wind tunnel.

Perhaps looking forward to smaller propfan engines/systems, Ratier-Figeac signed an agreement in mid-1990 with United Technologies Hamilton Standard to produce a family of advanced, lightweight, all-composite propeller systems for 1500-5000 shp engines initially for regional aircraft. Development and production will be shared approximately 60/40 in favor of the USA firm.

Japan. The Frontier Aircraft Basic Research Center (FABRC), a joint research group established by several Japanese aerospace firms, completed the concept definition of a new propfan design at the end of 1990. The engine has 18 blades arranged in two counter-rotating stages, is 150.15 inches in length and has a diameter (including fan blades) of 89.7 inches. In contrast to propfan concepts such as the Pratt/Allison 578-DX, hot-jet exhaust gases in the FABRC design are not directed onto the blades. The latter are of similar construction as those developed by General Electric for the GE36, featuring an aluminum core and composite shell.

Russia/USSR. Russia/USSR claims the most experience with counter-rotating turboprops. Thousands have reportedly been built, and that nation has produced the most powerful counter-rotating gearbox, at approximately 17,000 hp. The ZMKB Progress (in the Ukraine) Lotarev D-236T counter-rotating propfan, an experimental 16,500-lbst powerplant, was displayed at the 1989 Paris Air Show. This engine is based on the 11,400-shp D-136 turboshaft and incorporates a planetary reduction gearbox to drive two sets of fan blades - eight in the front row and six behind. The propfan has been flown, in tractor configuration, aboard Ilyushin Il-76 and Yakovlev Yak-42 testbed aircraft. A variant of the D-236T, designated D-227 and rated at between 17,635-19,840 lbst (78.5-88.25 kN), is the candidate for a proposed propfan-powered variant of the developmental Tupolev Tu.334 commercial transport.

ZMKB Progress has tested a 143,880 shp (10,440 ekW) propfan aboard an Ilyushin Il-76 testbed aircraft and the new An-70T four-engine transport. The engine, designated D-27, is also used on the Antonov An-180 transport, in an unshrouded, tractor (i.e., standard "puller" direction) configuration with contra-rotating blades. The first propeller hub holds eight blades, while the rear holds six.

The ADP ran for the first time in September 1992, concluding a 33-hour test program November 10. The ADP was run to 53,000 lbst (235.5 kN), and successfully transitioned from forward to reverse thrust. The fan has demonstrated a bypass ratio of 15:1. The engine underwent several months of low-speed wind tunnel testing at NASA-Ames Research Center, Moffett Field, CA, USA, in mid-1993. Testing of composite structures on the nacelle of the ADP began in 1995 under the US Advanced Research Project Agency’s Affordable Composites of Propulsion (ACP) program. Under the effort, Boeing is developing the pylon design, DuPont is working on a composite fan-blade containment case, and Dow UT is using resin-transfer molding techniques to produce a fan-exit case. Lockheed Martin is developing a composite core-cowl.

Large fan-cowl doors are being fabricated by Northrop Grumman’s Vought Aircraft subsidiary under a $22 million contract awarded by Pratt in May 1994. The doors are being developed to improve damage resistance to foreign objects. Production-quality doors are to be shipped to P&W in January 1997. Vought could also end up fabricating a composite inlet.

Pratt & Whitney’s former partner MTU of Germany has begun working with others in Germany on ADP design.

Called Engine 3E, the efforts are focused on environment, efficiency and economy related to ducted-prop design. MTU is aiming at an engine bypass ratio of better than 12:1, and a geared, variable-pitch fan, much like Pratt’s ADP design. The 3E’s LP section would be a high speed design along with a high exit temperature, staged low-emission combustor. The engine’s core would be a high power density type, with the engine’s overall pressure being "high". Basically, MTU is hoping to demonstrate technology which will lead to production of engines by 2010 which will have reduced engine fuel consumption by 20 percent (compared to a high-bypass turbofan), cut NOx by a whopping 85 percent from ICAO’s minimum, reduced noise by 10 db, and cut direct operating costs by 3 percent over today’s turbofans. The conceptual Engine 3E ducted-prop is the design direction which MTU feels has the best chance of demonstrating these reductions.

MTU is responsible for the compressor and turbine section development work, and BMW Rolls-Royce for the staged combustor. For its part, MTU plans to use blade-in-disk (BLISK) and blade-in-ring (BLING) technology in compressor stages to reduce weight while dealing with the transonic flow rates. Clearance control is being addressed with an attempt to match thermal response (expansion rates) of the rotors and casing.

MTU is considering a three-stage compressor rig of an LP (intermediate) compressor with four variable stator cascades to meet weight, flow and efficiency requirements. MTU is looking at a 60 percent peripheral speed increase and 10 percent blade count reduction, as well as a reduction in the number of stages and compressor length by one half. MTU will draw on commercial and military experience for the HP compressor, including their work on the EJ200 engine of the Eurofighter.

For the HP section, MTU is currently working on a six-stage unit with a 1.5:1 stage pressure ratio, higher than any attempted before, according to MTU, and their greatest challenge. MTU wants a 20 percent stage count and blade count reduction, and a reduction in length of 10 percent. MTU again sees high peripheral speeds dealt with by three-dimensionally optimized blades with low aspect ratios.

BMW Rolls-Royce is exploring rich burn, quick quench, lean burn methods of scheduling the staged combustor, which has been tested. Results showed a 57 percent reduction compared to 1995 ICAO standards, but with work still needed in fuel preparation.

MTU expects that high-strength, powder metallurgy materials and light weight intermetallic phases (TiAl) for blades and disks should result in strength and temperature improvements for the LP section. They feel fiber-reinforced ceramics will cut diffuser weight by 80 percent, but are a long ways off in practical development. They want to use a short, intermediate diffuser with an integrated inlet stator between the HP and LP turbines to provide length and weight savings. The turbine section will also experience transonic flow rates. The German Aerospace Research Establishment (DLR) an and several German universities are assisting MTU with exploration of other efficiency improving tools, such as computational methods to calculate viscous flow, and noise emission of blade cascades, to name a few.

In the 1980s, MTU, with support from partners Pratt & Whitney and FiatAvio, explored an engine designated the CRISP (Counter-Rotating Integrated Shrouded Propfan), intended for 150-seat twin-engine short/medium-haul aircraft and 250-seat four-engine long-range aircraft. The CRISP is of modular design, with a two-stage contra-rotating variable-pitch propeller (with 12 rearward-swept blades in each stage); a short, lightweight, low-resistance propeller shroud with an acoustic lining; contra-rotating in-line planetary gears; a compact accessory gearbox; and a core-engine cowling with conventional suspension. Preliminary specifications indicate that the engine would have a bypass ratio of 26:1, a pressure ratio of 38:1, and would be 14.7 feet (4.5 meters) in length, 8.86 feet (2.7 meters) in diameter, and will weigh 5290 pounds (2,400 kg). With the shrouded propfan approach, noise is more easily abated than with the UDFT. In 1991, the partners decided to focus their efforts on Pratt & Whitney’s ADP concept.

CFMI and General Electric Company USA have examined the market for ducted-propeller engines early in the next century. CFMI developed two design concepts, the M109 and M110. While no design details have been released, it is expected they would be of the fan-in-front type, like the P&W ADP. They have been studied as an alternative to Pratt’s ADP for possible use on a stretched Airbus A340 aircraft.

The Samara State Scientific & Production Enterprise of Russia has created a ducted propfan design designated NK-93; its maximum power output is 39,680 lbst (176.5 kN). The three-shaft engine features a two-stage contra-rotating shrouded propfan, with the propfan diameter being 124 inches (3,150 mm). The near-term intended applications of the NK-93 are the Ilyushin Il-96M and Tupolev Tu-204.

Funding

Owing to the widely diverse nature of the overall propfan effort worldwide, funding is not available.

Recent Contracts

No major identifiable military or commercial contracts have been issued during the past year.

Timetable

Work on propfan engines began approximately 15 years ago; the effort is currently shelved. No timetable or specific milestones have been established.

Worldwide Distribution

Owing to the diverse nature of the worldwide propfan technology effort, the exact number of pure propfan engines built, and their location, is unknown. Engines, however, have been fabricated in Russia/Ukraine, UK, USA, Germany, Japan and France.

Forecast Rationale

We have confirmed since our last review that Pratt & Whitney has now "shelved" the Advanced Ducted-Prop effort in favor of the PW6000/8000 program. The word is that the major stumbling block in the development effort was the mechanism (throw-arms) for adjusting the angle of fan blades. The adjusting was to be done to optimize air flow dependent on flight mode. Basically, the mechanism proved too heavy, and we suspect was not reliable enough for commercial service. Clearly, this a materials-type stumbling block, as a question of weight, largely. Complexity, however, appears also to be a real issue. These problems could be overcome in time, but for now, they need not be.

Pratt has gained years of very good test/run time experience with the ADP’s fan gearbox which would optimize rotation speed to air flow. The gearbox design has proved very reliable, so much so in fact that they have launched the geared-fan PW8000 engine (see PW8000 entry in "Design, Preproduction and Inactive Turbofan Engines" report), using the ADP’s gearbox design. While the conventional-fan PW6000 (which shares PW8000’s core) has been launched first, Pratt plans to forge ahead with PW8000 development without an application, confident that the engine, after full testing, will attract an application.

Not a peep has come from Boeing on Pratt’s geared-fan engine effort (though they have been briefed), but Airbus is apparently quite interested in the PW8000. Forecast International expects that they are looking to the engine for use on the eventual A320 family follow-on aircraft. Snecma is not standing still, of course, and has begun preliminary research into a successor for CFM56 (or a modernization thereof). The CFM56 powers the A320 family, and Pratt has now gotten a "foot-in-the-door" with the launch of PW6000 aboard the A318 100-seat jet. It is quite possible that PW6000/8000 will compete for sales with the CFM56 replacement/revamp on the A320 successor, or late model variants of the A320 family.

Pratt & Whitney’s former partner Motoren- und Turbinen-Union (MTU) of Germany has, with the help of BMW Rolls-Royce and academia, launched into ducted-prop engine research of its own. Their goals are the demonstration of technology which will lead to production of engines by 2010, which will have reduced fuel consumption by 20 percent (compared to a 1990’s high-bypass turbofan), cut NOx by a whopping 85 percent from ICAO’s minimum, reduced noise by 10 dB, and cut direct operating costs by 3 percent over today’s turbofans.

Called Engine 3E (see Ducted-Prop Engines section, above), the efforts are focused on environment, efficiency and economy related to ducted-prop design. MTU is aiming at an engine bypass ratio of better than 12:1, and a geared, variable-pitch fan, much like Pratt’s ADP design. The 3E’s LP section would be a high-speed design along with a high-exit temperature, staged low-emission combustor. The engine’s core would be a high-power density type, with the engine’s overall pressure being "high."

Ducted-prop designs have now become the breeding area for efficient, environmentally friendly engine design technology. MTU is taking this approach, and Pratt has learned some lessons here, too. Watch this technology appear in the next generation of turbofans, along with the IHPTET technology improvements (see report in this tab).

Ten-Year Outlook

Pratt & Whitney has discontinued its Advanced Ducted-Prop (ADP) research for the foreseeable future, concentrating instead of the geared-fan PW8000 program, which uses the ADP gearbox technology. Motoren- und Turbinen-Union (MTU) is now doing some ducted-prop engine research (3E Engine), with an eye towards technology demonstration later in the 2000-2010 period.

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