a reversed, tilted future for pratt’s geared turbofan_ _ technology content from aviation week
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A Reversed, Tilted Future for Pratt’s Geared Turbofan_ _ Technology Content From Aviation WeekTRANSCRIPT
14.04.2015 A Reversed, Tilted Future For Pratt’s Geared Turbofan? | Technology content from Aviation Week
http://aviationweek.com/technology/reversedtiltedfutureprattsgearedturbofan 1/7
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Mar 26, 2015
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A Reversed, Tilted Future For Pratt’s Geared Turbofan?Guy Norris and Graham Warwick Aviation Week & Space Technology
New Slant
As designers of future airliners look increasingly beyond traditional tubeandwingconfigurations to meet the high efficiency goals of the 2030s and beyond, new territory isbeing carved out in the critical area of airframeengine integration.
Unusual features ranging from recessed inlets to pylonmounted uppersurface engineshave become familiar sights in wind tunnels, but even seasoned researchers are surprisedby a new engine architecture proposed by Pratt & Whitney. The concept not onlyphysically separates the propulsor from the gas generator, but also mounts the corebackward and at an angle. This novel arrangement is aimed at overcoming installationchallenges in new configurations like the D8 doublebubble airliner concept under studyby NASA and the Massachusetts Institute of Technology (MIT).
Aimed at NASA’s N+3 performance goals for an airliner that could enter service around2035, the D8 is designed to burn at least 60% less fuel than the current generation ofnarrowbody airliners. The secret behind this leap in performance is a configuration thatclusters the engines together atop the wide tail of a flattened fuselage. Besides providing aclean highaspectratio wing for low drag, this enables the engines to reenergize to slowmoving boundary layer flow over the fuselage, increasing efficiency.
But such a configuration creates several issues. The engines lie so close to the uppersurface of the fuselage their fans must be sufficiently robust to cope with flow distortionfrom ingesting the boundary layer. Fan size will also be large because the enginesenvisioned for the D8 will have a bypass ratio of at least 20:1, and be targeted atextremely low noise levels of 52 EPNdb below current Stage 4 limits. Scale testsconducted at NASA of a distortiontolerant fan developed by United TechnologiesResearch Center show the boundarylayer challenge has been met, but other keyquestions remain.
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14.04.2015 A Reversed, Tilted Future For Pratt’s Geared Turbofan? | Technology content from Aviation Week
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Pratt & Whitney’s innovative reversed, separated and angled propulsion concept couldenable certification of adjacent engines. Credit: Pratt & Whitney
Because engine cores are becoming more efficient and operating at higher pressure ratios,they are also shrinking and becoming disproportionately small compared to the propulsorsection as bypass ratios increase. This leads to blade heights of 0.5 in. or less at the exit ofthe highpressure compressor. At this small scale, tip clearances not only become harderto maintain, but there is little space within the core through which to run the driveshaftconnecting the fan to the lowpressure turbine. Additionally, because the core isproportionately longer and thinner, designers face the issue of backbone bending whichfurther affects clearance control.
“So that’s when we had the breakthrough idea of turning the core backward,” says Pratt& Whitney Technology and Environment Vice President Alan Epstein. Air enters theengine through the fan as normal, but instead of continuing directly into the compressor,it is ducted around the side and back of the core to enter from the opposite direction. Inan arrangement similar to Pratt & Whitney Canada’s PT6, in which air flows forwardthrough the engine, hot gas will be discharged forward through a power (lowpressure)turbine connected to the fan via a gear system. The turbine, gearbox and fan will beconnected via “a really short shaft, and because the core is not connected to the powerside, you can take the core off easily for maintenance,” Epstein explains.
The concept also overcomes another challenge. The idea of nested engines, as in the D8,does not meet current FAA certification criteria under the “1 in 20” rule. This states thatthere should be only a 1 in 20 chance of debris from an uncontained engine failurecausing a second engine to fail. However, because the core and propulsor are no longermechanically linked, “the designers have come up with an extraordinarily cleverarrangement in which the cores are angled relative to each other,” Epstein says.
“We cant them at around 50 deg. and the exit from the core turns via a 50deg. duct togo into the power turbine. So now they are more than 90 deg. off from each other. It’ssimple geometry,” he says. “It enables you to have a large bypass ratio, and you are notturning much of the airflow if you are turning just the core flow, so pressure losses arelow.”
Clustered beneath the pitail of the NASA/MIT D8 design, the engine location presentscertification and configuration challenges. Credit: NASA
Pratt hopes to lay out a road map for future studies, possibly with NASA, to further define
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14.04.2015 A Reversed, Tilted Future For Pratt’s Geared Turbofan? | Technology content from Aviation Week
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the architecture and evaluate related elements such as shorterlength inlets that wouldalso help development of the nextgeneration geared turbofan. Other focus areas couldinclude studies of the ducting to evaluate weight and temperature requirements andwhether materials such as ceramic matrix composites might be suitable. “Then there’s thequestion of how do you convince the FAA it can be certified,” says Epstein.
For the MITled team, Pratt’s novel engine design is a key enabler of the D8configuration. Another crucial element has been validating the efficiency benefit fromboundary layer ingestion (BLI) by the clustered aftmounted engines through largescalewindtunnel testing with NASA. This has quantified the powersaving from BLI at around8% “in a realistic configuration,” says Ed Greitzer, MIT professor and D8 principalinvestigator. “This is the proof of concept for BLI in civil transports,” he told the AmericanInstitute of Aeronautics and Astronautics’ SciTech conference in Orlando in January.
In a conventional aircraft design, a significant amount of kinetic energy is lost in theslowspeed wake behind the fuselage and wasted in the highvelocity jet exhaust from theengines. This increases the power required. By ingesting and reenergizing the boundarylayer flow, “BLI reduces the wasted kinetic energy in the combined jet and wake,” saysGreitzer. With its engines mounted atop the aft fuselage, the D8 ingests about 40% of thekineticenergy deficit.
By conducting backtoback tunnel tests of BLI and conventional poddedengine “nonBLI” versions of the D8, the MIT team set out to quantify the benefit by measuring themechanical power transmitted to the flow by the propulsors to maintain the sameconditions. The measured power reduction with BLI was 8.4% when propulsor nozzlearea was held constant, increasing to 10.4% when mass flow was kept equal. “That’s asignificant benefit,” Greitzer says.
“The dominant effect is an increase in propulsive efficiency through the reduction in jetvelocity,” as a result of starting with slower flow into the propulsor, says AlejandraUranga, MIT technical lead. “The D8 engine has similar specific thrust but higherpropulsive efficiency than currentgeneration engines like the CFM567. That’s why wethink the results are applicable to full scale,” she notes.
The D8 model was tested at angles of attack up to 8 deg. and slideslip angles up to 15deg. and the flow behaved well, Uranga says. Engineout testing showed no adverseimpact on the running engine and the observed fan efficiency loss from ingestingdistorted boundarylayer flow was “an order of magnitude less” than the BLI benefit, saysGreitzer, adding that "there are no showstoppers to the D8 configuration.”
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QuestionMark on Mar 29, 2015
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BLI engines are advertised as a solution for airframe configurations that produce a thickboundary layer and a sizeable wake.Alas, for the current tubeandwing configurations with podded underwing engines this is notan issue. Yet for the D8 it was a design requirement, as it seems.
A few questions come to mind:1. What about the thick boundary layer on the lower side of the D8's fuselage?2. What do engine cores slantmounted in the fan flow and driving the fans though extensionshafts with bevel drives do to propulsive efficiency and maintenance cost?3. How would a Mach 0.70 D8 compensate for the loss in productivity compared to a currentMach 0.85 tubeandwing configuration? How would it compare to a Mach 0.70 turboprop?
Mario on Mar 29, 2015
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It seems the extension shafts and bevel drive would present similar costs/benefits to a moreconventional geared turbo fans. Are the engine cores really more accessile or modular thanpylon pods? How long will it take to carry out an engine change c.f one shift for an enginecrew for current engines?
14.04.2015 A Reversed, Tilted Future For Pratt’s Geared Turbofan? | Technology content from Aviation Week
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Darth Vader's c... on Mar 31, 2015
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1. FOD precludes the existence of an engine under the aft fuselage as it will undoubtedlyingest enough crap off a runway during its service live to cause damage, potentially seriousdamage.2. From their diagram, there is no drive shaft from the core to the fan. Instead, the power istransmitted in the fluid then extracted by a turbine local to the fan (and requiring no cvjoint).3. If you work out the time delta, and compare over total trip time (which includes airportarrival, checkin, security, boarding, taxi, flight, taxi, debarking, luggage collection), thenthe difference is quite small. Small enough that the cash price will dominate. As to how itwould compare to a turboprop, unknown depends on much more info than is known rightnow.
Another thing from back in the day when I was looking at something very similar and notoften mentioned. A wide fuselage like this permits use of widebody cargo containers. Whichcan be very useful when integrating a spoke into a global cargo network.
[email protected] on Mar 31, 2015
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I got the impression that the fan is not connected to the core by shaft or bevel drive, the corejust generates gas flow and the fan has its own coaxial turbine so the only thing makingthat turn is the exhaust gas..
[email protected] on Mar 30, 2015
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This layout is almost exactly like the Pratt PT6 turboprop engine. Like the PT6, hot highpressure gas is made in the core (gas generator) and used by the power takeoff turbine that isconnected to the prop or the ducted fan thru planetary gears. In the D8 engine the hot gas isturned 50 degrees between the core and power turbine so no bevel fan drive gears are required.
Rowboat71 on Mar 31, 2015
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...blade heights of 0.5 in. or less at the exit of the highpressure compressor.At such high pressure ratios, centrifugal compressors, which are smaller, lighter, lessexpensive and more durable, work just as well.
Rob on Apr 1, 2015
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Not quite. A centrifugal compressor will rapidly lose efficiency as the pressure ratioincreases, hence they are not generally used at ratios over 6:1. They do have a broaderoperating range though and are cheaper but they are nowhere near the efficiency required forcommercial turbofans.
carlrogg on Apr 6, 2015
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Things change and technology changes, think Silvercrest! I bet Snecma is more advancewith their patented design, they've got most of the technological bricks matured to bringsuch a revolutionary design to reality including high temp management.
[email protected] on Mar 31, 2015
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Why not simply mount both the gas generator and power turbine AHEAD of the fan? Thepower turbine would be in clear air far above the boundary layer. Admittedly, engine accessmight not be optimal...
Darth Vader's c... on Mar 31, 2015
14.04.2015 A Reversed, Tilted Future For Pratt’s Geared Turbofan? | Technology content from Aviation Week
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'cos then you lose the extra compression from the fan stage.
Pr = 1.2 (plucked from thin air) might not seem like a big pressure ratio, but when youconsider omitting that is immediately 20% off your OPR...
carlrogg on Apr 6, 2015
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Snecma has done that with their new patent application, in an all in one setup.
[email protected] on Mar 31, 2015
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neat idea. but..the cores meet the 1 in 20 rule but the fans don't. an uncontained fan failure could take out theother engine. also, i recall a nasa study several years ago involving high rear mounted engines.it was found that at high angles of attack, fuselage masking severely disturbed the airflow intothe engines. this is similar to the effects of high angles of attack on "t" tail elevators.
voyageur on Mar 31, 2015
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To maintain gasgenerator and fan gas pressure special activities will be needed. Sectionalcharging is not new idea but in the conception above eventual hot exhaust gas ingestion gonnacause compressor surge and loose of power;such a systems are complex and include fuel flowpulse pressure droop.Maintenance will be more demanding and expensive..btw this conceptionmay be a reason for some old good book rereading activities ....Thanks Pratt...
E175Pilot on Mar 31, 2015
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Perhaps it has been addressed in another article on the D8 configuration, but I'm wondering:how would snow and ice adhering to the top of the fuselage before flight be removed? I've beentrained to believe that deicing fluid is anathema for engines. Would deicing or antiicing fluidused to remove snow or other frozen contaminates from the top of the fuselage present adanger, or even just accelerated wear, when it ran back into the engines during takeoff andflight?
[email protected] on Mar 31, 2015
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RE: Darth Vader's comment: It doesn't matter. You just add an extra compressor stage to thegas generator. It will be small and insignificant... Whether you compress the air in the fan orin the gas generator matters little. Except that a dedicated stage in the gas generator can beFAR more efficient than the root section of a preceeding fan in causing air compression... :)
steven on Apr 1, 2015
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That technology along with natural laminar flow and other technologies will reshape the shorthaul aircraft. But what is happening to the long haul aircraft fly at speeds from 0.85 mach andhigher where most of the revolutionary technologies cannot work efficiently how the futurelong haul aircraft will look like and how much more efficient will be from the current aircraftlike the 777. Also can somebody answer the following question how much will hybrid laminarflow will decrease fuel consumption on long haul aircraft
TomB on Apr 1, 2015
While we await design and certification of these new P&W engines, I suggest that the aircraftdesigners could realize the benefits of Boundary Layer Ingestion with a low risk interim design:1) Mount the main engines on the wing. The Honda patent 6,102,328 and the Lockheed WingBody Airlifter design suggest that an over the wing engine mounting position is the mostefficient.2) Install electric duct fans on the rear fuselage to ingest the boundary layer and reenergize the
14.04.2015 A Reversed, Tilted Future For Pratt’s Geared Turbofan? | Technology content from Aviation Week
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wake flow. The electric duct fans may contribute 10% of the required cruise thrust.
The electric duct fans may be powerful enough to capture much of the 8% boundary layeringestion efficiency gain as identified in the D8 studies.
It will obviously take a lot of engineering design and testing to validate my conceptual idea.
Rowboat71 on Apr 1, 2015
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The aircraft needs 3 engines. Two turbofans for takeoff and climb to 25,000 feet. One turbojetrunning at 70% rpm (33% thrust) for takeoff and climb, and 100% RPM above 25,000 feet.The turbojet would have 3 cascaded centrifugal compressors, each 4:1 pressure ratio.4x4x4 = 64:1 overall pressure ratio for high altitude operation.
APEppink on Apr 2, 2015
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Re: bevel gear drives etc:
Mark, the gas turbines have no mechanical connection to the LP (power) turbine, same aspower and marine setups using light duty aircraft gas turbines, tho the heavy frame turbinesused in power generation (combined cycle) units are directly connected to the generator at thecompressor end.
Rowboat71 on Apr 3, 2015
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"BLI , for the current tubeandwing configurations with podded underwing engines this is notan issue." Yes It is. Note the huge vortex generators among the tail feathers of the 737.
carlrogg on Apr 6, 2015
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Snecma has a similar concept design (where there is separation of the gas generator/turbojet,turbine and fan) being patented for a few years now.
Such a concept is so revolutionary Airbus should seriously look at it to reconfigure an a380neoto reduce drag (with 4 engines), as many of the technologies are almost mature!
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