New nacelle designs show benefits of first steps to closer engine/airframe integration
Aerospace researchers generally agree that step changes in efficiency for future commercial aircraft designs will only be achieved through unprecedented levels of integration between propulsion and airframe.
Despite uncertainty over exactly how this will happen, some engine and nacelle manufacturers are taking the first tentative steps. Some are pressing ahead with integrated propulsion systems (IPS), while Pratt & Whitney is developing the variable-area nozzle. Though a far cry from futuristic visions of airliners with open rotors, embedded engines or distributed propulsion systems, these steps mark the start of a broader movement which ultimately will blur the distinction between engine and airframe.
As revolutions go, however, the move to an IPS is a quiet one whose changes are mostly hidden beneath the nacelle’s skin. A CFM56 incorporating baseline elements of an integrated system is undergoing tests at General Electric’s outdoor site in Peebles, Ohio, and only a trained eye would be able to differentiate it from the standard engine. Yet the concept from Nexcelle , a joint venture between GE Middle River Aircraft Systems and Safran’s Aircelle, is a first for the industry and a bellwether for a coming wave of innovations aimed at reducing weight and fuel consumption, improving performance and cutting maintenance costs.
Based on the GE-Snecma CFM alliance, Nexcelle was set up in 2008 to provide fully integrated propulsion systems for new-generation aircraft ranging from the Leap lC-powered Comae C919 airliner to the Passport 20-powered Bombardier 7000 and 8000 business jets. The potential of what could be achieved with a full IPS, however, was only publicly demonstrated for the first time when the company displayed a half-scale model at the Paris air show last year.
The model incorporated a low-drag, natural-laminar-flow forward section with a low-noise single-piece air inlet and an integrated electric anti-icing system. It also showed a fan cowl structurally integrated with the engine and a translating O-duct thrust reverser derived from the electrically actuated thrust reverser system (Etras) fitted to the inboard engines on the Airbus A380. As well as cutting maintenance costs, the concept is designed to improve fan flowpath efficiency by eliminating door links, bifurcation, steps, gaps, latches and split lines.
With the start of component tests on a real engine in 2012, Nexcelle is graduating from concept modeling to actual hardware. «For Nexcelle, this is a big next step,» says Huntley Myrie, president of the joint venture. Although not a complete IPS demonstrator, the Panache (Pylon And Nacelle Advanced Configuration for High Efficiency) test unit incorporates some key building blocks for follow-on evaluation, including a new electrically actuated thrust reverser.
The tests mark the start of a build-up process to demonstrate and evaluate a full range of advances, not all of which will be featured, depending on specific requirements. «Some are looking at the full IPS, and others at a partial solution. So they have to decide what level they want to fly,» says Myrie.
The main element Panache is validating is a one-piece composite О duct, which replaces the two-piece «D» doors on a traditional thrust reverser. When the duct moves aft to the reverse thrust position, blocker doors deploy into the engine’s secondary airflow—thereby eliminating the need for drag links. The duct’s continuous composite structure «replaces two other pieces so reduces weight, while at the same time reducing aerodynamic losses caused by the bifurcation in the flowpath,» says technical director Pierre Hurpin. Although Nexcelle has weight targets in mind for the production version, the current testing is «not about an amount in terms of weight, but to demonstrate the technology and its reliability,» he adds.
Tests at Peebles focus on repetitive thrust-reverser deployment sequences and a variety of scenarios including emergency activations for simulated rejected takeoffs. The runs will «confirm the system’s maintainability advantages and its simplicity,» says Myrie. As the core is now enclosed within a one-piece duct, part of the evaluation will look at the operation of the sliding О duct, which translates fore and aft on a pylon-mounted rail to give access to the engine core for maintenance. «To open the engine, we will need to move the О duct to get access to the engine core, and this is done through a cowl-opening system,» says Hurpin.
Panache also includes a new integrated mounting system which reduces maneuver loads on the engine, cutting down on the amount of bending and distortion transferred through the support structure to the powerplant. «The main difference between this and current engines is that in those, engine torque is taken off the aft mount, but in this it will be taken from the front,» says Hurpin.
The system shares load-bearing duties between the nacelle, strut and engine, rather than putting most of the onus on the engine, and will extend on-wing life by reducing tip-rubbing, says Nexcelle.
«We’re potentially taking loads through the О duct,» Hurpin says. Unlike conventional engines, in which the fan cowl is attached to the pylon, the Nexcelle system comprises a supporting saddle structure with an integrated fan case. This means the mount system can be smaller, lighter and generate less drag, he adds.
Panache is a work in progress, and «as we progress through IPS, we will use the cowl, inlet and other aspects. It is a broad new concept we wanted to develop and we saw we had to do a demonstrator,» says Myrie. Although there is no plan to flight-test the Panache hardware, Nexcelle ultimately plans to offer IPS for future programs and potentially for retrofit. «We are working very closely with CFM and we will develop a joint flight-test program. As for retrofit, we may look at that as part of the overall strategy to see if it is feasible, but for now we’re focusing on new platforms,» he says.
Pratt & Whitney, together with its soon-to-be sister company and nacelle-maker Goodrich, is developing another form of integrated propulsion, the variable-area fan nozzle (VAFN). Unlike current engines’ fixed-area fan nozzle, designed as a compromise between climb and cruise, the variable nozzle is designed to optimize the primary, or fan flow, for each flight condition. In basic terms, the nozzle wants to be larger for takeoff and smaller for cruise, but having it fixed in either position is inefficient and, in the case of a smaller nozzle, could even cause the engine to stall at takeoff.
The development becomes more important the larger the engine and the higher the bypass ratio. Because the bigger members of the PW1000G geared turbofan family have a higher bypass ratio (12.5:1 for the PW1500G powering Bombardier’s CSeries) and lower fan-pressure ratio, the system helps to increase surge margin and protect the fan from flutter.
For larger engines, the VAFN is therefore as key a part of the PW1000G development effort as the 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 is also out to show that the performance benefits will outweigh all other considerations.
For Pratt, the VAFN was one of three main considerations when designing the nacelle for the geared turbofan (GTF), says Graham Webb, PW1000G project chief engineer. «The nacelle generally comes along for the ride and it can be a detriment in weight and drag if you don’t pay attention, or you can take advantage of it.» Webb says. «We’ve learned a lot about how to package it. You have a giant fan, but a really small core, so you want to keep the inside as tightly wrapped as you can and it doesn’t have a lot of room for externals. There’s been a lot of work.»
First, Pratt studied the aerodynamic optimization of the nacelle structure, paying attention to protuberances and vents in the fan duct, positioning of drag links and guide vanes, and thermal management of elements in the core cowl. Second, it looked at integrating engine build-up units (EBU) with airframe external systems to avoid duplications. «For instance, we integrated the nacelle anti-ice system with the start valve, and eliminated a 12-lb. valve,» says Webb.
Third, it ensured that the VAFN movement was synchronized with the engine as a function of rotational speed and Mach number. Development focused on a closed-loop feedback system between the engine electronic control (EEC) unit and the motor controller governing the Goodrich-developed nacelle actuation system. «We’ve gone through flight tests and rig validation tests to make sure the motor drive controller talks to the EEC,» says Webb. The first PW1500G will fly on Pratt’s flying testbed with a fully functioning, production-configured nacelle in August.
In the mid-term, Pratt hopes to exploit more integration advantages from its new relationship with Goodrich. «Historically, the work has been split between the engine group, the nacelle group and the airframer, and when you begin to blur the lines,… you begin to see new opportunities for improvement,» Webb says.
«Now with Goodrich, Hamilton Sund-strand and Pratt & Whitney, we are working on the next steps. We’ve seen opportunities to take 200 lb. out just with Hamilton, and maybe there are greater opportunities with Goodrich.» The geared architecture opens up new design space in terms of reducing weight and drag as well as optimizing the thermal environment and EBUs, he notes.
«We’ve looked at a long list of nacelle technologies that will be specifically beneficial to the geared turbofan—maybe between 0.5 and 2 percent reduced fuel burn based on the current-generation architecture. We’re just now scratching the surface,» Webb adds.