GE’s all-new Advanced Turboprop engine is built from proven technologies to deliver big performance benefits with low risk.
For nearly a decade, GE has made significant investments to build a Business & General Aviation program focused on pushing the boundaries of engine technology. And in November 2015, GE announced the all-new Advanced Turboprop engine, selected by Textron Aviation to power its new Cessna Denali aircraft. The engine is designed to deliver more power, greater efficiency and the first fully integrated digital engine and propeller control in its class.
The Bike Shop sat down with Gordie Follin, Engineering Leader for the Advanced Turboprop (ATP) program, to learn more about the materials and technologies inside this all-new engine.
Inside the Advanced Turboprop, Part One: Additive Manufacturing
Bike Shop: Let’s talk about additive manufacturing. How is it used in this engine?
Gordy Follin: We’ve used additive for a total of 12 parts, which replace more than 855 parts. Additive fundamentally changes the way we design. We’re not just printing a conventionally designed part, instead, we actually design it from the very beginning, knowing we’re going to additively manufacture it. This allows us to combine features and optimize the design to minimize weight and also infuse speed in the design, development and manufacturing process.
BS: And where are those 12 parts? Are they in different sections?
GF: They’re throughout the engine and include a number of big structural components –like the inlet frame and the exhaust case – but also more complicated parts like some of our bearings, sumps and housings, as well as our fuel nozzles.
BS: And you say that this eliminates more than 855 parts?
GF: That’s right. One of the fundamental benefits of additive is you can print assemblies as opposed to building parts and then assembling them. You can combine many parts into one big printed assembly that serves the same function. Typically, the weakest element of an assembly is where parts come together. It’s a place where you can have air leakage, or you can have wear between parts if there’s any sort of relative motion. And by using additive, you eliminate the leakage paths. You eliminate potential wear paths as well.
BS: So it eliminates leakages, what about weight?
GF: When you combine parts together, you must have some way of joining them. You have bolts or you have welds – all those interfaces also add weight. And so, one way that additive reduces weight is by eliminating those features that attach the parts. The other way is that, again, it allows you to optimize the design.
BS: Can you give me an example from the Advanced Turboprop?
GF: Sure. Take the exhaust case, which has two functions. It provides an aerodynamic flow pass to allow the air to exit the engine with minimal pressure loss, and it has a structural function. It must be able to withstand the pressure of the flow going through it.
In conventional manufacturing, you’d have to design the whole thing to a thickness dictated by the weakest point. But with additive, you can have much more complex shapes, and therefore you can print the aerodynamic shape, and then you can separately add features for structural stiffness. So, our exhaust case has a very, very thin liner, which is the aero shape, and then there’s external spars printed on the outside of it that provide the stiffness. Additive lets us put the strength where it’s needed, not across the whole structure.
BS: Additive is still a relatively new technology. Are there risks associated with that?
GF: We look at it as developing a set of new materials. When you print materials, they have different properties than if you forge them or cast them. So, as part of our development, we’re doing a lot of work to requalify these materials in printed form. We’re doing material-testing, we’re printing cutups in test specimens and so on, in order to ensure that we’re going to get the properties we expect.
BS: This is the first time additive’s been used in the turboprop space but not in Aviation, correct?
GF: That’s correct. The LEAP engine, which will be our largest selling volume engine in the aviation portfolio, has fuel nozzles that are additively manufactured, and those are the first certified production parts in the world.
BS: It seems like this is the way the industry is heading.
GF: That’s absolutely right. ATP is going to be the first program to use additive at this level and this extensively, but I expect the next new products we develop are going to go even further with additive.
This is the second in a three-part interview with Gordie Follin, engineering leader for the Advanced Turboprop engine program, discussing the materials and technologies inside GE’s Advanced Turboprop engine. Today we’re talking about the engine’s impressive efficiency gains and the innovations that make it possible.
BS: This engine is going to run significantly hotter than current engines in its class. What makes that possible?
GF: The fundamental technology here is advanced materials in the high-pressure turbine along with advanced cooling technology. So, the turbine blades run hotter than the melting point of the metals of which they’re made, and the way that we enable that is through cooling. Basically, cold air is passed through the inside of the blade and comes out through tiny holes to create a cool buffer zone around the blade so you can have hot gas path temperature and cooler metals because of this nice, thin film of cool buffer air.
BS: That’s a technology that GE has used in other engines in service, correct?
GF: That’s correct. We’ve been using cooled turbine blades for more than 40 years. But the Advanced Turboprop will be the first engine in this class that’s had cooled high-pressure turbine blades.
BS: What’s new about the aerodynamics inside the Advanced Turboprop?
GF: The modern 3D aerodynamic design of the compressor blades allows us to run a 16:1 overall pressure ratio. And that 3D aero design also allows us to optimize the aerodynamic efficiency of the compressor, which translates into fuel efficiency for the aircraft.
BS: Are 3D aerodynamics used in other places in the engine?
GF: We also used 3D aero to optimize the shapes in both the high-pressure and the low-pressure turbine to maximize efficiency of those components. And for the low-pressure turbine, we also have counter-rotating airfoils. They spin in the opposite direction of the high-pressure turbine airfoils, and this allows us to minimize swirl coming out of the back of the engine, which drops pressure and improves the overall efficiency of the engine.
BS: Tell me about the variable stator vanes. What benefits do they bring to the Advanced Turboprop?
GF: In the compressor, we have two stages of variable stator vanes, which is unique in this class. Most engines either have no variable geometry or one stage of variable geometry. Having two stages allows us to optimize the aerodynamics through the compressor around the envelope.
Typically, if you don’t have variable stator vanes, you can really only have optimum efficiency at one point in the envelope. But with variable stator vanes, we can help adjust for changes as you increase altitude or increase cruising speed, and that allows us to get the efficiency up at a higher level around the envelope. There’s a little more weight associated with the extra variable geometry, but when you look at the performance benefits, it’s definitely worth it.
BS: Tell me about how you reduced some of the air leakages in this engine.
GF: For engines of this size, clearances are really important because they establish how much leakage you get either around the end of your turbine blades or around the flow path. We put a lot of effort into minimizing the clearances in a way that would be sustainable.
One of the things that we did is go through and look at clearances and gaps throughout the gas path and used additive technology to combine parts in ways that would eliminate leakage paths.
BS: Can you give me an example of that?
GF: In areas around our structural frames, our bearings – traditionally where you have to put seals or you have to maintain tight clearances in order to reduce leakage – we don’t have a gap, so there is no leakage. And so, again, this is a place where additive really helps us enhance the performance of the engine.
BS: So how does all that translate into performance benefits?
GF: At the end of the day, you’ve got an engine that will produce at least 5% more power at altitude than the current products in the class, 15% better SFC and 35% better life. So, you get more power, better fuel efficiency, and longer time between overhauls.
BS: What about range and payload?
GF: The benefit of having more power is you can push a bigger aircraft through the air at the same speed. The fuel efficiency allows you keep range while you do that and also be able to carry more payload for a given distance.
BS: Fuel prices are low right now. Why does efficiency matter to turboprops?
GF: To take a bigger aircraft and push it through the air at the same speed takes more power. If you just have more power that’s fine, but if you don’t have the fuel efficiency, you have to put more fuel on the aircraft, which gives you less room for payload, or range comes down. So the only way you can have the same payload with the bigger aircraft, flying at the same speed, same range, is you’ve got to have the fuel efficiency. And in order to make this aircraft work, you’ve got to have a step-change difference in efficiency. You’re not going to get there with 5% or 8%. You need 15%.
BS: The Advanced Turboprop has been chosen to power the new Cessna Denali. What impact will it have on aircraft performance?
GF: What the Denali is bringing to the market is a jet-like pilot experience with a larger, more luxurious cockpit and cabin. And so, in order to have a bigger plane and fly just as fast and just as far, you’ve got to have more power, and you’ve got to have better fuel efficiency. In that sense, there’s a great alignment between what Denali is bringing to the market and what it needs, and what the ATP engine can provide.
This is the third in a three-part interview with Gordie Follin, engineering leader for the Advanced Turboprop engine program, discussing the materials and technologies inside GE’s Advanced Turboprop engine. Today we’re talking about the engine’s Integrated Propulsion Control and what it means for turboprop aircraft.
BS: Let’s talk about the Integrated Propulsion Control. This is something totally new to the turboprop space. So why did GE decide to make it part of this engine?
GF: I think one of the things that pilots don’t like about turboprops is they’re harder to fly than a jet aircraft because they have separate controls for the propeller and the engine. What we wanted to do was bring a “jet-like” experience to the cockpit. So, with this Integrated Proportion Control, the pilot will have a single lever and a FADEC, a Full Authority Digital Engine Control system, that allows us to optimize the control of the propeller with the engine itself to get the best performance we can and reduce pilot workload. Basically, it makes it easier for the pilot to fly the plane.
BS: And it comes with some other features that change that experience for the pilot, correct?
GF: That’s right. Creating the integrated control system allows us to optimize the experience. We can do more optimization in terms of how we do ground taxi to minimize damage to the propeller during ground taxi. We can use it to optimize rate of climb during takeoff, and we can use it to optimize efficiency during cruise. So, once we have the capability, we can use it to make the experience better around the whole flight envelope and on the ground.
BS: What does that mean for the pilot? Describe the difference in flying a current turboprop design versus a new aircraft with this IPC.
GF: Right now, when a pilot gets in a traditional turboprop aircraft, they have to manage the propeller and the engine separately, and so typically what they’re doing is they’re trying to maintain either propeller speed or torque. And they’re doing that every so often, by looking at a gauge or listening to how the propeller is running. The IPC basically takes that whole control loop out of the equation.
And by doing that, it’s not only easier for the pilot, but also lets us optimize it in a way that the pilot can’t, based on what they can see on a gauge or what they can hear or feel through the stick. Again, we can optimize the speed of the propeller versus the engine around the envelope to get better performance, to get better power during climb and takeoff, to get better efficiency at cruise, and to reduce noise. Things that a pilot couldn’t do, we’re able to do because of the automated control system.
BS: I understand the IPC collects performance data from the engine and the aircraft. What is that data used for?
GF: The performance data opens up a whole world of possibilities in terms of analytics and analysis of the performance of the engine. It allows us to better understand how the engines are operating in the field and look for ways that we can optimize the control system for how pilots are operating it. And in some cases, it can help us get early detection of potential problems that we may need to address in the fleet and be able to respond more quickly.
BS: GE is pioneering digital solutions across many different industries. Are there other ways you see this data being used to enhance the Advanced Turboprop experience?
GF: One thing we envision is condition-based maintenance. Instead of having to bring your engine in at a predetermined rate, regardless of how you use the engine, by being able to access the data through the integrated propulsion system, we would be able to optimize maintenance intervals based on your utilization of the engine.
BS: There are a lot of new technologies on this engine. How confident is GE that this engine will perform at service entry?
GF: We’re very confident. And the reason for that confidence is that while all of these technologies are new for this class, they’re not new technologies for GE. We’re leveraging experience from across our entire portfolio of commercial engines and other engines in our Business and General Aviation portfolio. So, while there’re new technologies here, they’re not new technologies altogether. That’s why we’re very confident we’re going to be able to bring these to market on time, and that we’re going to see the benefits we’re saying we’re going to have.