Ton PDF NASA était très très instructif sur ce point
Meilleurs compromis, mais d'ailleurs pas forcément le meilleurs tout court dans le cadre d'un projet neuf complet cellule + moteur
Pour un diamètre de 1.9 m ca donne moins de 300 m/s en bout de pale... ce qui est bien subsonique...
The GTF for Bombardier’s C Series will have a bypass ratio of 12.5 yet will be lighter than the equivalent engines it replaces–good for annual fuel savings of $1.5 million per aircraft at $2.50 a gallon. Compared with a 737-800, a GTF-powered aircraft will be 77 percent quieter on takeoff. P&W has spent $1 billion so far on the GTF program, and the engine has logged 12 flights aboard P&W’s 747 flying testbed and 27 flights aboard an Airbus-owned A340 flying testbed.
• The GTF’s fan speed will be less than 3000 rpm, and its tip speed will be one-third slower than that of a CFM56-5B.
Pratt & Whitney has entered early testing on the PurePower PW1524G geared turbofan, having achieved idle and full power runs.
The first of eight planned test engines, S/N001, had its first run to idle power on 25 September at the company's West Palm Beach, Florida, facility and was later run to full power, 24,000lbs (107kN) of thrust, on 8 October.
To date, the engines have conducted around 10h of testing, say Pratt.
The engine maker says assembly of the second and third test engines are underway at its Middletown, Connecticut facility.
Engine S/N002 will be run through a rigorous ground test regime, as will Engine S/N001, which is will remain on the test stand until December, at which point the engine will be inspected for wear and tear, as well as required modifications before returning to the test stand.
The first six months of the two-year development process will be devoted to establishing the conforming article for regulatory approval, which will be followed by 15-18 months of certification testing.
Engine S/N003 will be put through initial ground checks before it is fitted under the wing of Pratt & Whitney's Boeing 747SP test bed for flight trials in the second quarter 2011.
The all-new powerplant features a geared architecture allowing the front fan to spin three times slower than the core, thus optimizing the speed of each section of the engine. Pratt touts a 15% to 20% improvement in fuel burn over equivalent engines.
The PW1524G will power the first of two Bombardier CSeries aircraft, the 110 to 125-seat CS100, which is due for first flight in 2012 followed by entry into service in 2013.
The PurePower PW1000G engine has been selected by the Mitsubishi MRJ and Irkut MS-21, and is a candidate for re-engining the Airbus A320 and Boeing 737 families
These include initial tests of a production-configured fan-drive gear system as well as the first run of a CSeries-size high-pressure spool within a fully integrated engine. Until the PW1524G tests began, all GTF-related core testing has centered on either the non-production representative PW6000-based demonstrator or the slightly smaller PW800/PW1200 core aimed at executive jets and the Mitsubishi Regional Jet (MRJ).
When the third engine starts tests with a production nacelle system, the PW1524G will also become the first commercial turbofan to run with a variable-area nozzle (VAN). This is an adaptive fan duct lip that improves efficiency by exploiting the engine’s high bypass ratio of 12:1, more than double that of the CFM International CFM56 and International Aero Engines V2500 turbofans powering the Airbus A320 and Boeing 737.
If all works as planned, the certification of the PW1524G will mark a critical step for Pratt on its way to fielding the first of a new family of engines aimed at reducing fuel burn and maintenance costs on regional and mainline aircraft by up to 16% and 20%, respectively. The first will enter service with the CSeries in late 2013, shortly before a smaller PW1200G derivative is due to power the MRJ.
For the longer term, the engine maker is also developing a PW1400G version for Russia’s Irkut MS-21 airliner family, which Pratt hopes could be the basis for versions to power both re-engined A320s and 737s. The MS-21 is scheduled to enter service in 2016, corresponding roughly to provisional service-entry targets identified by Airbus for its A320 NEO (new engine option).
Engine 001 is dedicated to measurements of fan, low-pressure compressor and LP turbine stress, as well as to assessing the design integrity of the oil and fuel system. The engine also incorporates a testbed-specific variable-area nozzle mounted to the nacelle, which is being used to evaluate the fan at various pressures. “A critical test objective for this engine is to test across a very large range of pressures for a given airflow. We’ll run the engine at 50% takeoff power with the VAN in the open position, and slowly close it to characterize fan stresses as the pressure rises,” says Saia.
By altering the exit area of the bypass duct, engineers will back-pressure the fan and LP compressor well beyond the extremes of the engine’s expected operating envelope. “We’ll try to bring the fan blade up to a level of pressure well above where it would operate on the wing,” he notes. Using this method, the Pratt test team will map out the flutter margin of the swept, wide-chord fan blades. “All blades should flutter. The question is at what power and what type of flutter does it show? So that’s the testing we’ll do to establish those margins,” Saia adds.
Acting as large flapper panels, the testbed VAN is being used initially to map out pressure losses across the thrust range. “Because it’s a first of a kind, we have no knowledge of this yet. We are beginning by learning about how much we get in a fully open condition,” Saia says. Overall the VAN will operate across three times the area variation expected to be seen by a production engine using a standard-configuration variable-area system. In tests, the VAN will be opened to an area 20% larger and almost 30% smaller than the production design.
Within the normal operating range of the engine in service on the CSeries and all other GTF applications, the VAN will be fully opened for takeoff and climb, increasing fan bypass ratio and enlarging exit area by around 15%. During cruise, it will be closed to optimize fan pressure ratio, and hence propulsive efficiency. The production GTF will feature a lightweight, fully integrated VAN, which will be flight-tested on Engine 003. Unlike the proof-of-concept, rudimentary VAN flown on the PW6000-based GTF demonstrator, the production version will be incorporated directly into the aft structure of the Goodrich-made nacelle fan duct. Although conceptually similar to the actuation used in convergent-divergent nozzles in military engines, the GTF system will be “quite simple” by comparison, says Saia.
Pratt also reveals for the first time that in 2008 it quietly built and tested a second full-scale demonstrator aimed mainly at validating its design models and proving the fan-drive gear system could safely cope with a fan-blade release. Named GTF Demo 2, the PW6000-based engine with its 80-in.-dia., solid titanium fan with tested to destruction while its luckier GTF sibling was flying on the 747SP.
“The second demonstrator engine ran around 30 hours and then we did a fan blade-out,” says Saia. “We ran it up to full redline speed and released a blade. All the loads of the imbalanced fan were transmitted to the case and did not go through the gear. It was a favorable surprise.” Unlike many turbofans, which seize up after the destruction caused by the violence of a fan blade-out test, the GTF faired better. “After the test, you could go to the back of the LPT and turn it by hand and see the fan move. With this architecture the built-in gear keeps the plane of the rotor in position. So despite a lot of dynamic load, the rotor didn’t ‘chuck’ at all,” he adds.
The fan-drive gear system was then salvaged from the GTF Demo 2 engine, measured for wear and any damage, re-assembled and run for 15,000 cycles in an endurance rig. “Not one part was changed,” says Saia, who adds that the test series ran for two months in mid-2009. For the CSeries engine, which is equipped with a lighter hybrid metallic blade instead of the solid titanium of earlier tests, Pratt will conduct a fan-blade release in a horizontal test rig on a production nacelle and containment case in the third quarter of 2011, and on a full engine at the end of 2011.
The low pressure spool represents a large amount of new technology for P&W, starting with the fan. Whereas the geared turbofan demonstrator engine trials from 2008 used a titanium fan, the company moved to a lighter weight bi-metallic with a titanium-sheathed leading edge rather than a composite blade for the production geared turbofan engines. "We found it had the same weight as composite but has the significant benefit of making the blade thinner," Saia says. "It has better [aerodynamics] and still meets bird ingestion requirements."
While P&W had earlier measured fan aerodynamics on the titanium demonstrator engine and performed bird ingestion and blade out tests on the fan by itself running at red-line speed in a test rig, the ongoing S/N001 tests are the first performance testing of the new fan in the "engine environment," says Saia. "Performance looks very good" and matches that of the demonstration engine, he adds.
"We've done flutter and operability testing at elevated pressure at a given airflow," Saia says. "We're seeing double digit margins, which is above the design requirement."
German research seem to be confirming what Pratt & Whitney has been hoping for years - that the variable area nozzle (VAN), a ‘must-have’ feature of the geared turbofan design, will prove to be beneficial in more than one way.
Because the PW1000G GTF has a higher bypass ratio (12:1) and lower fan pressure ratio than previous commercial turbofans (thanks to the fan drive gear system and high-speed low-pressure turbine architecture), the engine requires a method of increasing surge margin and protecting the fan from flutter. The VAN will achieve this by actively modulating the nozzle exit area to be maintain flutter margin.
In that sense the VAN is therefore as critical a part of the GTF development effort as the advanced core or the fan drive gear system. In addition to proving the system can be designed to work as a seamless part of the engine operating cycle, and that it will not be an extra mechanical burden, Pratt also must prove the performance benefits will outweigh all other considerations.
Ulf Michel of the German Aerospace center’s Institute of Propulsion Technology says the VAN feature could provide wider operating benefits. Speaking at the recent AIAA aerospace sciences conference in Orlando, Florida, Michel says his team studied the effect of the device across a range of flight conditions from take-off and climb to cruise, holding and approach. In the study the VAN would be opened 20% for take-off, and gradually closed to around 2% during climb to cruise altitude when, at top-of-climb, it would be at the baseline 0% position.
“Top-of-climb is the most demanding working part for the fan, and defines the engine size. The engine must deliver the required thrust-to-weight ratio and, as the thrust for initial cruise is 18% smaller, this is done by reducing the jet Mach number. But that’s not the best way because it moves away from the optimum fan operating conditions,” he says. However, with a VAN, the adjustment can be made by altering the exit area rather than adjusting the Mach number, thereby maintaining the optimum fan speed.”
For cruise the throat area was opened by 5%, while for holding and approach it was pushed to its maximum 25%.
According to the study, the resulting gains in propulsive efficiency equaled 8% at take-off, 12% in climb and a dramatic 23% in the hold. In practical terms, Michel says this would mean up to 9% less power would be required for take-off, and between 6% and 2% for climb. For cruise, the power setting could be reduced by 4% for the same speed by adjusting the VAN. In addition, should the aircraft be held at lower levels for air traffic reasons, “at least part of the loss could be compensated for by opening the nozzle,” he adds.
The reduction in power associated with the VAN operation should also result in jet noise reductions says Michel. The study results indicate potential reductions of 2.5 dB for take-off, 1.5 dB for cruise (with a concurrent reduction in broadband shock noise, resulting in a quieter cabin), 5 dB in the holding pattern and 7 dB on approach.
What does all this mean for fuel burn? “There is the potential sfc benefit of up to 3% for a fan operating at its maximum efficiency, plus the wave drag of the nacelle is smaller in the cruise when the mass flow through the engine is smaller, Michel says. In addition, savings would come from lower flight idle speeds due to lighter fan loads. But exactly how much savings are possible will remain unquantifiable until flight tests of the first full-up production GTF later this year on Pratt & Whitney’s 747 flying testbed.