By HANK HOGAN, Engine Air Magazine

This is a reprint of an article that appeared in the Summer 2012 issue of Engine Air Magazine.

The latest incarnation of the world’s best-selling plane, the Boeing 737 MAX, is slated to pass an important milestone this summer. At facilities in the United Kingdom and the United States, wind tunnel testing will confirm the plane’s low- and high-speed performance, respectively, according to Michael Teal.

Mr. Teal is Chief Project Engineer and Deputy Program Manager for the 737 MAX program at Chicago-based Boeing.  “Wind tunnel testing is on the critical design path of the program,” Teal stated in his announcement of the testing. “Based on previous work in the wind tunnel, we are confident this final phase of testing will substantiate our predictions of the aerodynamic performance of the airplane.”

That’s a good thing, as the airframe maker has received more than 1,000 orders and commitments from sixteen customers for the new plane, which is scheduled for first delivery in 2017 – 50 years after the first 737 flew. Boeing predicts the 737 MAX will offer a 10 to 12 percent fuel burn improvement over aircraft today and a 7 percent operating cost per seat advantage over the competition tomorrow.

Success of the 737 MAX in hitting such targets is important. Singleaisle jets like the 737 account for 70 percent of the total jet market on a unit basis. There is significant competition in the category for the billions of dollars the market segment generates.  Since the 737 airframe will remain essentially unchanged, the ability to achieve the stated goals will come largely from advances in the engine.

That might seem risky, but there are reasons why Boeing could consider this the best and safest approach.  “It’s the same engine OEM (original equipment manufacturer) as the prior 737, so that relationship is there, and it has worked incredibly well,” says Kevin Michaels, Vice President at ICF International. The Fairfax, Virginia-based company provides consulting services and technology solutions to government and commercial clients in aerospace and other areas.

ON THE HIGH-PRESSURE FRONT

The development of the engine, the CFM International LEAP-1B, is like CFM International itself: a partnership between GE (General Electric) Aviation of Evendale, Ohio, and SNECMA, of the Safran Group, based in Courcouronnes, France. GE Aviation is handling the high-pressure part of the engine, while SNECMA develops the low-pressure part, including the fan.  It is a collaborative process on many levels, says Dale Carlson, General Manager for LEAP Technology Strategy at GE Aviation.

As they develop the engine, GE Aviation and SNECMA also are working closely with the airframer. The LEAP-1B is larger than the engines previously used on the 737, and as part of a continuous improvement process, Boeing has modified the aft fuselage, struts, and nacelles. Those modifications, in turn, impact the engine, which features a slightly smaller fan than other LEAP applications in order to better meet Boeing’s needs. “As Boeing iterates on their design – the airframe design – we are right there iterating on the engine design,” Carlson says.

He also notes that the engine features lean burn combustion, an approach that yields low nitrogen oxide (NOx) emissions. That will allow it to exceed the latest NOx guidelines from the Committee on Aviation Environmental Protection standards (CAEP/6) by 50 percent. It also will reduce fuel burn, and carbon dioxide (CO2) emissions, by 15 percent below those of existing CFM International engines in the same thrust category.  In addition, the engine will feature higher thermal efficiency, courtesy of a higher pressure ratio and consequently higher temperatures.

By way of comparison, the bypass ratio of the new engine will be about 8:1 versus 5:1 for current CFM International designs.  The engine achieves this improved performance through a combination of innovative materials, advanced simulation and modeling during design, and sophisticated computer controls. The latter helps ensure that pilots have full thrust authority over the entire throttle range, Carlson says.  Material upgrades include using the latest in ceramic matrix composites in the hot gas path, an approach that offers several advantages.

For one thing, the composite is a third of the weight of metal, thereby cutting weight in the engine. What’s more, it requires less cooling flow. Reducing the need for cooling helps increase fuel economy and reduce CO2 emissions. As Carlson explains, “Cooling flow in a jet engine is considered detrimental to fuel burn.”  Carlson also points out that the metal in the engine, of which there will be plenty, will be used in locations where the higher core temperature of the LEAP-1B will not have an impact. In fact, metal temperatures in the new engine will run about the same as current CFM International engines. This will help the engine reliably go through many repeated cycles of taxiing, take-off, cruising, and landing. In advance of entering service, the new engine will go through 18,000 testing cycles, double the number of comparable CFM International engines that are in use now.

In preparation for a greater use of ceramic composites, GE Aviation and Safran have entered into a joint venture with the Tokyo-based company Nippon Carbon. The trio expects to launch a new company, with one of the motivations being to create a secure supply of the silicon carbide fiber that is a raw material for the ceramic composite. Manufacturing of the composite itself will be done at a GE subsidiary, Ceramic Composite Products of Newark, New Jersey.

As for the lean burn combustion regime that the engine will operate in, the analogy in everyday life for the issues that make this difficult to achieve is the pop heard when a gas flame goes out. That is caused by an acoustic instability, created when the gas flame transitions from fuel rich into lean burn.  GE Aviation has solved this problem.  Consequently, as Carlson explains, the LEAP-1B has a lean-burn combustor with superior operability characteristics and efficiency, in addition to industry leading low NOx emissions.

Part of this is due to active controls and fuel staging, but another reason is careful attention to design. Thanks to three-dimensional simulations of aerodynamic flow over surfaces, engineers know exactly what shape to build parts of the engine before construction actually takes place. In addition, thorough knowledge of the materials involved enables models of how they will age to be accurate, ensuring tight tolerances for parts and the clearance between them.  Of course, engines operate in an environment where they can ingest large and small particles. This debris can accumulate in areas within the engine core, such as the cooling holes in the turbine blades, and impact engine life and efficiency.

To handle that, LEAP engines incorporate a debris rejection system that keeps dirt and debris out of the engine core. A clean core should lead to a 1.4 percent better fuel retention, particularly as the 2020s and later decades roll around.  “As the engine gains cycles, the debris rejection system will keep us onwing longer,” Carlson says. He notes that GE Aviation has experience with using such a system on other engines. However, this is the first time it will be used in a CFM International engine.

MEANWHILE, ON THE LOW SIDE…

Another challenge facing the LEAP-1B team is fan blade size. At a reported 68 or 69 inch diameter, the 737 MAX fan will be larger than that in previous 737 engines. On the other hand, it is smaller than the 128-inch diameter composite fans currently in commercial use on the GE90 engine.  However, while the composite blades may be smaller, the birds that they may encounter are not. Thus, simply scaling the blades down in all dimensions from the larger version was not really an option.

At the same time, the weight of the engine had to be carefully controlled, so beefing up existing, smaller fan blades also was not an option.  The solution involves a new fan material and a new way to fashion that material into blades, says Olivier Longeville, Vice President for Product Strategy and Markets at SNECMA. Previously, composite fan blades were laminates. The stacking of layers led to constructs that were stronger in two directions than in the third, which is the thickness of the blade; however, new techniques have changed this. “With our technology, you have another carbon fiber that is going in that direction,” Longeville says.

The process involves weaving the carbon fiber into a preform, which is placed inside a mold. Resin is injected into the mold at a high temperature. When removed, the result is a fan blade in the final shape.  Because the carbon fibers form a three-dimensional matrix within the fan blades, they are both strong and light.  Such parts may make their first appearance in the 737 MAX, but the technology has been under development for more than a decade.

Around 2000, engineers and scientists first started seriously working on implementing these woven composites into something suitable for a commercial engine.  Before that could happen, though, numerous tests had to be passed and the equivalent of thousands of hours of flight time had to be accumulated.

BUILDING UP FOR PRODUCTION

In addition, a manufacturing supply chain had to be set up. SNECMA and some key partners are looking ahead to a time when there is large demand for the new fan blades and the materials that goes into them. “We have in mind several sites in the United States and in Europe that can support the production that we have to face,” Longeville says.  He added that no firm decisions as to plant location or timing of plant construction have been made, although there is an existing prototype plant in Rochester, New Hampshire. One plant likely will be in the United States, perhaps somewhere near the existing one; another probably will be located in France.

As for other materials, the engine also will include aluminate titanium at the last stages of the turbine. Along with a host of other innovations, that will boost the efficiency of the engine while reducing its weight. Many of these changes will be in the guts of the turbine and elsewhere, hidden away from the casual viewer. The fan, on the other hand, will be a different color and easily visible.  “It’s not a metallic color. It’s black, so you see it,” Longeville says. “For the turbine you will not see the changes and for the core you will not see them, but there are a lot of technology revolutions inside.”

Images Courtesy of The Boeing Company. Copyright 2013.