Cool is Key to Jet Efficiency
The headaches of holemaking
By Robert Aronson
The main design goal for turbine engines for military planes is performance, while with commercial planes it's cost. In a fighter, a fraction higher speed or acceleration can mean the difference between victory or defeat. With a commercial aircraft, just a small nudge in efficiency can really pay off during the life of an engine.
For all the turbine engine manufacturers, including the Big Three (GE, Pratt & Whitney, and Rolls-Royce), the goal is the same: Improve engine operating efficiency.
The principal way to do this is to have the engines run hotter. But engines now are running at temperatures that will melt components without cooling.
So the main way to cool is through effusion cooling, that is, circulating air through the hot sections, chiefly the combustor and the blades and vanes immediately behind it.
For some time this cooling has been done by making a labyrinth of channels within the rotor blades and vanes and forcing air through them. The air flow across the part surface keeps the blade and vanes from deteriorating while allowing higher combustion temperatures.
The challenge in engine design is to develop tighter air flow control for the film-cooling holes. This can be done by evaluating all aspects of engine design and manufacture are under review including the control of incoming material, both consumables and raw material.
The turbine section of the engine is the most expensive module on the aircraft because it is made up many costly parts that have to go through a number of operations. Each operation contributes some error, so error stack up is an issue.
Currently there are heavy losses in vane manufacture, beginning with the ceramic vane cores. These cores determine the size and positioning of the ducts within the blade that feed the cooling holes. The cores are costly and can have a high failure rate.
Another problem being studied is drilling through the protective ceramic coating on the blades. The edges of the coating layer may be damaged when the wall is drilled through.
One plus in this work is the availability of more computing power. This allows more computation and corrections to previous process.
Performance is the issue and the two main areas driving that development are materials, particularly coatings, and air-flow control. For example, the engine that powers the Joint Strike Fighter has well over a million holes per engine. In the past, performance of cooled components has not been well controlled, because manufacturing has had difficulty controlling hole size, and size equals flow.
More efficient combustion means higher temperatures in engines, requiring more efficient cooling. The cooling preferred by design engineers, called effusion cooling, is needed to maintain the integrity of the engine. It also lowers the pollution generated and reduces fuel consumption.
To achieve this new level of cooling requires significantly more cooling holes in the hot sections of the engine—the combustor and blade and vane systems behind it. That, in turn, requires greater precison in the placement and forming the holes.
"There is a cost problem because of the unacceptable level of scrap and rework associated with this type of manufacturing," explains Mark Barry, vice president of Laserdyne (Minneapolis) the company that currently boasts 90% of the market for turbine engine holemaking.
"With modern conventional milling or grinding operations scrap and rework rates generally at acceptable levels. However, for some manufacturers holemaking operations can have rates as high as 25%. One problem is some companies are still using 25–30-year old manufacturing systems and the control of hole size is still not well understood. Holemaking and therefore air flow with this level of technology is still a bit of a black art."
In the company's efforts to improve laser systems, a major change was the introduction of AutoFlow Compensation. This proprietary software has the capability of checking the flow results of a part against a data base, then automatically making changes in the system to correct a problem. The software in combination with the laser and system will in effect automatically control hole size.
"One particular problem with existing systems was we relied on operator control of beam focus position and power to the laser," explains Barry. "These two variables determine the size and quality of the hole. The change we have implemented was to remove the requirement for operator judgment and put the intelligence needed into the laser control, just as it had been for some time with conventional machine tools. The combination of software and hardware now allows the engineer to create a process that drills at the focus point of the laser, while change hole size with optical elements instead of power changes or by defocusing.
In addition to this change, some companies have added robotic loading which further reduces operator influence and produces consistent production rates.
"Cooling holes are normally drilled at angles to the surface as extreme as 20° or less to ensure the desired flow across the face of components, says Barry. "The laser we use for this process is the Convergent Laser P50L."
There is always a general wearing of the blade in service. The edges can be eroded from debris in the flowing gas. The rest of the blade wears just from the motion in the hot gas. A growing business for both OEM and contract manufacturers is refurbishing parts. Usually the blade/vane is sprayed to replace the lost metal. However this often plugs the holes and they have to be redrilled.
Positioning of the blade relative to the laser is critical. Before it was done with fixturing, which is difficult. Modern systems first grab the blade root or Christmas tree. Then the blade is scanned automatically and the program guides the motion system and laser beam to the correct hole location.
MC Machinery (Wood Dale, IL) supplies a lot of EDM equipment to the aerospace industry to improve production of turbine engine manufacturing. "Our most recent project has been six-axis CNC sinker EDM equipment that is being used specifically for cooling hole-making in the blades and vanes of turbine engines," explains Greg Langenhorst, technical manager. "This covers both commercial and military applications, but is extensively used on the 'super cruise' engine for the JSF."
"We use either the Mitsubishi EA series or the Ingersoll gantry series sinker machines with a servo controlled A and B axes added. The holes are made with either a single-point or comb-shaped copper electrode that produce a clean rectangular hole." In some cases the electrode is given a slight orbital path to make sure the hole size and finish meets spec and the corners come out sharp, very critical to the longevity of these high temperature components.
One advantage of the laser hole-makers is that the process doesn't need complex part location. The blade is positioned in a simple machine fixture and probed to determine its position. The probed location data is then used to automatically position the laser to produce the holes in the proper location.
"Currently, with the EDM system, part location depends on proper fixturing. So workpiece setup has to be done manually, on fixtures that can be interchanged between a setup station and the EDM machine. This adds cycle time and can introduce human error," he explains. "We expect that in the near future we will have that part of the cycle automated and have the probing and positioning capability built into the EDM's control," he concludes.
At Agie Chamilles, (Lincolnshire, IL ) they are working with a lot of exotic materials. The harder the metals behave like glass and are subject to fracture. "With EDM the material isn't cut, it's melted in a rapid heatcool cycle the trick, is to eliminate or minimizes fracture cracking," explains Eric Ostini product manager. AgieCharmilles fast spark cycle, measured in milliseconds does not allow heat to penetrate deep into the material, therefore eliminating fracturing.
"Our newer systems have five-to-seven axes and they were developed specifically for aerospace applications. The product to be worked on is positioned by the machine. The electrode does not have to be maneuvered to make a cut but moves only in the Z axis. The operation is easer to control through simplified adaptive-control software. For some aerospace applications the need to create hundreds of holes which are not round on 3D shapes, have turned to EDM for better accuracy and to eliminate micro fractures on aerospace exotic materials.
In another aerospace application, wire EDM is used to cut exotic material such as titanium, which is as thick as a human hair to exact dimensions without electrolysis bluing or deforming the 3D shape of the material.
Ebtec Corp. (Agawam, MA) is a manufacturing/service company that specializes in high-energy beam processing. It offers a wide array of manufacturing processes which include laser, electron beam, abrasive waterjet, EDM, as well as conventional machining and heat treating.
According to company president, John W. Leveille, "We service three key markets—aerospace, industrial and medical. Each of our processes can be utilized on a single project." To manufacture an industrial turbine diaphragm, EBTEC uses two and foveaxis abrasive waterjet systems for cutting thick metal sections and lasers to cut thin metal requirements. The electron beam is utilized for deep penetration welding and wire EDM for thick metal sectioning. All processes involved replaced slower, less accurate methods used only a few years ago.
The same is true for the manufacturing of aerospace components. Turbine engine rings, blades and vanes now have stringent requirements for cooling holes. "Tighter tolerances for hole size, placement and angle can only be accomplished by utilizing the latest technology, such as five-axis laser system equipped with auto-focus, breakthrough detection and a fiber laser designed with enhanced beam modes for precision welding," explains Leveille. Each of the high energy beam systems requires back-up equipment for inspection requirements. Air flow benches are utilized as a final inspection on all laser drilled holes. A helium mass spectrometer is required to test hermeticity of electron beam welded implantable pumps.
"Within the markets mentioned, quality is the most important factor," says Leveille. "There is a trend for more testing and stricter qualification requirements. What once took three days to qualify a given process within a given program, now it takes on average of three to six weeks."
Turbines aren't the only aircraft element that is inspiring process changes. A large number of structural airframe parts are manufactured from aluminum plate stock. "These monolithic parts have high volumes of material to be removed," explains Randy Von Moll, Aerospace Product Manager, Cincinnati Machine (Cincinnati, OH). "The complex parts have contoured angled features that are a perfect fit for our range of 5-Axis machining centers and profilers to produce at the lowest cost per piece for our customers."
The move to harder metals, chiefly titanium, has caused an increase in demand for machines capable of machining the tough material. First, titanium being much harder than aluminum, has to be machined more slowly. "Some of the hundreds of Cincinnati Machine's five-axis profilers world-wide originally purchased for aluminum machining are now being used exclusively for machining the harder alloys," says Von Moll. "Plus, the older, slower machines have the necessary dynamic stiffness to tackle titanium machining without chatter."
The spurt in aircraft construction, both military and commercial, has increased the demand for machine tools such as those of Cincinnati Machine, including retrofits. For example, their three-spindle five-axis gantries do a good job with aerospace assignments after replacement of the spindles and updating the CNC hardware and software.
"There is also greater emphasis on precision for two reasons," says Von Moll. "First the aircraft, the new airframe designs contain components that are much complex, integrating features historically found in multiple parts riveted together. Second, the aircraft industry is trying to produce aircraft on a automotive-like assembly basis.
How Rolls-Royce Does It
In gas-turbine engine design, the overall challenge is maximizing the gas temperature following combustion to improve the specific fuel consumption. To survive these operating temperatures, turbines exploit the lastest cast alloy materials, use of ceramic thermal barrier coatings which are a kind of insulation material, and utilize cooling air in the turbine components. This cooling air passes through sophisticated passages within the turbine components prior to exiting into the hot gas stream through film cooling holes to provide a cool film around the outside of the component.
These engines are divided into three principal areas, compressor, combustion and turbine. The high temperature challenges are concentrated in the combustion and turbine components. In those areas temperatures (in the order of 1600°C) are higher than the melting point of the turbines' nickel-based alloy materials.
Advances in manufacturing techniques have enabled complex internal shapes within the turbine components to enable efficient convective cooling. On the Trent 1000, the high-pressure turbine blade utilizes for the first time a soluble core casting process, that enables the production of re-entrant features using a single core die. This has resulted in a more advanced cooling solution that enables the blade to run at higher temperatures with lower levels of cooling air.
To make the film cooling holes, both laser and EDM drilling techniques have been developed. Rolls-Royce uses a combination depending on application. The present need is for more sophisticated hole shapes to optimize the flow over the blade and optimize cooling. The size, position, and shape of each hole are carefully controlled so that this external flow provides optimum cooling.
The cooling air (typical temperature around 700°C) is bled off the engine's compressor section. Clearly the source pressure of this flow must be greater than that of gases flowing around the turbine components to provide effective film cooling. In a typical Trent engine the compression section compresses the inlet air to 40 times atmospheric. Hence the cooling air that is bled off for cooling turbine components must be limited to avoid unnecessary performance penalties.
In terms of CFD, both technical capability and computer processing speed have moved the capability from the challenge of evaluating one component to the current situation where the turbine aerodynamicist can now evaluate the performance of an entire turbine system.
Turbine Systems Engineering Director
The Major Trends
The main pressures on the aircraft engine industry are to reduce its cost of production, meet the environment requirements for emission of noise and pollutants, and reduce engine replacement. Two kinds of aircraft are emerging. First the small jets with four to six seats and jumbo jets such as the A380.
For the first scenario, small jet, the engine for this kind of application has a cycle that includes take off, about one-hour flight and landing, repeated at least five times in a day. For this hard cycle, durability and reliability are critical.
For the other scenario, the mega jumbo jet, long flights mean long cycles, and one or two flights per day. For these engines the cycle is not long.
For both designs, engine maintenance schedules and MTBF are closely matched to each engine type. Therefore, cost of repair, overhaul, and spare parts are a big part of the profit the companies will make.
Senior Process Engineer
Pratt & Whitney Canada
This article was first published in the March 2007 edition of Manufacturing Engineering magazine.