The path towards certification, expected for 2020, and the first flight, scheduled for next year, of the Advanced Turboprop (ATP) is proceeding swiftly. Meanwhile, at the Munich research centre, mid-October marked the start of the ACCV (Axial-Centrifugal Compressor Vehicle) test focused on the high pressure compressor, in particular to verify and fully understand its efficiency, performance and operational features. A very significant test along this path because, just like those carried out on the combustor at Avio Aero in Pomigliano, it focuses on an entire engine component and on its validation.
This quick pace is no surprise for a program on which GE Aviation has invested about 400 million dollars in development and over one billion for the industrial development of additive technology. The technology helped reduce a total of 855 conventional ATP components to 13 components in additive.
Among these components printed in 3D with machines that use lasers or electron beams to melt metallic powder and create an object starting from a CAD drawing, particular attention often goes to the combustor liners (donut-shaped parts) and various other casings and structural elements. However, among the 13 components made with additive technology, there is one, or rather a family of components with three variants, that fulfils a vital function for the engine and for the aircraft equipped with ATP. It’s the new Cessna Denali by Textron Aviation.
We are talking about the family of Heat Exchangers (HX), aircraft engine accessories with a high technical, functional and engineering complexity, which we will try to make as understandable as possible. Heat exchangers resemble small boxes or rectangles positioned outside the components, longitudinally at the beginning, middle and end of the ATP.
The first thing to know is that these HXs are an amazing innovation because they are made using additive technology with aluminum, cobalt or chromium-based alloy powders. It all depends on the variant, or rather the type of HX. GE Aviation has already developed them for the GE9X program, but also on GE90 and A700 engines. Heat exchangers are essentially liquid or gas cooling systems for the operation and life of an aircraft engine: air, oil, and fuel, suitably mixed according to the heat exchange that is performed. Each HX manages two elements, so the three HXs of the ATP are: the air-to-air HX (called High Pressure B Sump Heat Exchanger, HPBS HX), the Air-Cooled Oil Cooler (ACOC), and finally the Fuel Heater (FH).
Without going into technical detail, we can simply describe the HPBS exchanger as an air-to-air thermal exchanger. In the case of the ATP, it functions to cool the B Sump, the part that houses the turboshaft bearings, and the PT (Power Turbine) assembly using the air extracted partly from a compressor air intake and partly from the cold air circuit passing through the operating engine. With very simple words, it conveys the cold air coming from two different flows to cool the engine's hot parts, with the task of maintaining the air temperature at acceptable levels for a running engine.
The air cooled oil cooler (ACOC) exchanger is a different story, because this accessory is usually the airframer's responsibility. This time, the advantage of the additive technology and the main function of cooling the oil passing through the various lines of the entire engine, crossing the main mechanical components in order to lubricate them and guarantee their performance, have ensured its development like the other HXs. Necessarily maintaining the integration with the aircraft, the ACOC draws cold air from the nacelle (the outer casing of the engine, on the fuselage) through the air vents and also by means of special ejectors located on the HX terminal itself when the aircraft is stationary. The latter has wings and flat parts that are among the thinnest ever designed and manufactured at GE.
Finally, the Fuel Heater, the oil-to-fuel HX, could be one of the most popular engine accessories for high-altitude motor car touring enthusiasts, except that some technologies are not within the reach of automotive engines, which always have several heat exchangers.
Therefore, using the transmission gear oil, the FH does not cool but rather heats the fuel if its temperature drops below the 90°F (32°C) threshold. The oil that heats the fuel in order to optimize the fuel system performance and avoid freezing can reach higher temperatures.
All HXs are designed by CAD, of course, and made with additive manufacturing, and small parts such as valves, joints or inserts are then added to the HXs. Avio Aero's engineering (in particular the resident teams of Brindisi, Rivalta and Bari), working with the Aviation Technology Center in West Chester (Ohio), coordinate the design of these small thermomechanical masterpieces, with light weight and dimensions not to exceed 25cm. HXs are a concentrate of technology that is added to the responsibilities of Avio Aero, which are Engine Systems, Controls and Accessories, Combustion System and both power and accessories gearboxes, for this first engine fully developed in Europe.
“We have never designed entire HXs at Avio Aero in the past," says Antonio Caimano, Technical Leader for exchangers. "The HXs are assemblies, we start from built-in additive with DMLM technology (Direct Metal Laser Melting), then we treat it, machine it and add the necessary parts. Along with a team based in several countries we have created the product know-how with the complication of the new design in additive, but also with a valuable support from the Baker Huges GE engineering.”
For the HXs, a new interesting GE Store collaboration model between Aviation and BHGE took shape: Antonio Cardillo is Lead Engineer Mechanical Component for the ATP HXs and works at the former GE Oil&Gas plant in Vibo Valentia (Italy). “In Vibo, we deal with the thermal design of the air-to-air exchanger and we design the test benches for all three HXs”, says Antonio Cardillo. The exchangers will be one of additive products that we will begin printing in 3D at the Avio Aero plant in Brindisi, thanks to the Concept Laser machine that is arriving soon.
We are approaching the main milestones of this remarkable turboprop engine in record time, with little left to do before assembly begins at the GE Aviation facility in Prague, to be followed by the test of the first developed ATP engine, about 20 months in advance on the usual times for such engines. All this thanks to advanced technologies, but also, and especially, to the passionate and interconnected work of engineers and production technicians. You will soon hear about ATP again.