Pratt & Whitney Les nouveaux GTF "Purepower"

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.

Pratt & Whitney at the end of April will mate the first Bombardier CSeries prototype nacelle, built by Goodrich, to its PW1524G engine number two turbofan prototype in Mirabel for flight tests on the company's Boeing 747SP flying test bed starting mid-year.

The resulting trials will provide the first glimpse of true performance of the PW1524G's unique variable area nozzle (VAN) nacelle, a FADEC-controlled feature that P&W says is an essential ingredient in making the economic case for the geared turbofan engine (GTF). The GTF uses a fan drive gear system to both reduce fan speed and increase low spool speed, both of which benefit the propulsive efficiency of the engine.

In the "open" or widest position, the VAN protects against fan resonance, or flutter, on takeoff and reverse-thrust operations. In the closed position in cruise flight, the device is designed to cut fuel consumption by 2%; and in an intermediate position, it can in theory boost "time-to-climb" performance or cut noise, key metrics airlines must consider when operating 500nm stage length aircraft like the CSeries.

"It's a 'third knob' on an engine," says Alan Epstein, P&W's vice-president of technology and environment. "Previously we had controls for fuel flow and the variable vanes on the compressor. The GTF control system can adjust the nozzle as well".

While turbofans typically use a relatively simple fixed exit nacelle, the physics of the GTF's slow-turning, large bypass fan require the nacelle's exit to be relatively large to ensure that back-pressure does not subject the fan to potential flutter conditions at takeoff and during reverse-thrusting.

The same large-area nozzle needed to meet the high power flutter margin criteria however is inefficient for the reduced power operations at cruise, where a narrower nozzle is desired to slow the flow through the engine for optimal fan performance, hence best fuel burn.

By using a software controlled VAN, P&W says it will be able to optimise propulsive efficiency for a variety of flight conditions while maintaining fan flutter safety margins. "Now that we've got it, we'll find all sorts of other things to do," says Epstein, adding that improving climb performance is one area where the VAN will add value.

From a test standpoint, P&W must prove out not only the performance benefits of the VAN as a function of nozzle opening, but also that failure modes will not jeopardize operations.

"If there is a problem with the VAN in terms of hardware, our electronic control has failure detection and it will keep the fan in larger area and sacrifice fuel efficiency at cruise," says Bob Saia, P&W's next-generation product family vice-president.

Saia says the prototype VAN has all the functionality of the production unit, the first of which will be delivered to P&W in November and sent into flight tests with the PW1524G in early 2012.

The complaint specifically alleges that the fan stages on the Engine Alliance GP7200 engine and on UTC’s PW1000G engine (also known as the Geared Turbofan) infringe the Rolls-Royce patent. The complaint also alleges that a number of other UTC aero civil engines infringe the Rolls-Royce patent. Rolls-Royce is seeking damages and injunctions.

The dispute has its origins in the mid 1990s when Rolls-Royce and UTC filed for similar patents within five months of one another, but really got going in 2003 when UTC applied to the US Patent Office to have the Rolls-Royce swept fan blade patent, which was filed after its own, struck out. UTC’s efforts were unsuccessful and the validity of the Rolls-Royce patent was eventually upheld in the US by a Court of Appeal decision in May 2010.

Paul Foot, patent attorney and engineering sector specialist at Withers & Rogers LLP, said:

“This is a spat that has been going on for some time and so far Rolls-Royce is coming out on top.

“Rolls-Royce’s decision to proceed with their patent infringement claim against UTC is no surprise following the outcome of the US Court of Appeal earlier this year, which upheld their swept fan blade patent in the US.

“This dispute underlines the importance of having faith in the value of your intellectual property, and the courage to use it for commercial advantage. The Rolls-Royce patent is coming out on top because it was able back up the content of its application with examples of the commercial success of the invention. This attention to detail clearly impressed the US Court of Appeal and is now giving Rolls-Royce confidence to police its monopoly over the invention on UTC’s home turf.

“UTC has also challenged Rolls Royce’s equivalent European patent without success, so we may see the battle move to these shores in coming months.”

On May 5, the Federal Circuit upheld a decision by EDVA District Judge Leonie Brinkema reversing a decision of the Board of Patent Appeals and Interferences (BPAI) in an interference proceeding that has taken eight years to work its way through the courts. Rolls-Royce, PLC v. United Technologies Corp., No. 2009-1307 (Fed. Cir. May 5, 2010) (found here).
For an EDVA case, the case has a long and unusual history, and it’s not over. United Technology Corp. (UTC) filed its Request for Interference in the BPAI in July, 2002, and Judge Brinkema conducted a bench trial on the appeal from the BPAI in December, 2005. For some reason, however, Judge Brinkema did not issue a decision until more than three years later, on March 31, 2009. Rolls-Royce didn’t waste any time once the Federal Circuit upheld Judge Brinkema’s decision in its favor, however. The same day of the decision, Rolls-Royce filed suit for infringement of the patent-at-issue in the EDVA (Complaint found here). Rolls-Royce, PLC v. United Technologies Corp., Case No. 1:10CV00457 (E. D. Va.). The infringement suit has also been assigned to Judge Brinkema. Because of the long delay, however, Rolls-Royce has potentially lost several years of recoverable damages.

The patents involved in the interference proceeding involve an improved design for jet engine fan blades to reduce the amount of shockwaves produced by the engine. The BPAI ruled in favor of UTC, holding that UTC’s application covered a fan blade that had a tip that swept either forward or rearward.

Judge Brinkema reversed the BPAI’s decision. She construed the UTC claim to cover only fan blades with tips that swept rearward, and so the UTC patent did not capture Rolls-Royce’s patent on a forward swept tip. Based on her claim construction, Judge Brinkema found that the Rolls-Royce patent was not obvious over the UTC claim.

UTC appealed, and the Federal Circuit upheld both Judge Brinkema’s construction and her finding of non-obviousness. The Federal Circuit rejected UTC’s argument that a forward sweep in the tip of the fan blade would have been an easily predictable and achievable variation in view of the disclosure of the rearward sweep in UTC’s earlier application. The invention was not obvious to try, the Court held, because of the broad selection of choices for further investigation that included any degree of sweep for the fan blade tip. Further, secondary considerations, which showed that an embodiment of Rolls-Royce’s invention had become the industry standard, reinforced the finding of non-obviousness.