Turbine Tech

(April 2011) TAKE A CLOSE LOOK at the inner workings of a modern turbofan engine and you'll be glimpsing mankind's ultimate triumph over that most fundamental of untamed ancient natural elements — fire. The science behind basic turbine engine design, after all, represents simplicity at its minimalist best: A single moving part compresses the air and mixes it with fuel to create a very hot fire, which in turn produces the thrust that propels an airplane forward. Not since cavemen first understood how to make fire by rubbing two sticks together have humans achieved something so monumentally significant from such straightforward engineering principles.

Since the widespread commercial adoption of turbine engines, starting with the Boeing 707 narrow-body jetliner in 1958, designers have sought to make improvements in every facet of their manufacture. Today’s modern turbofan engine is lighter, quieter, more reliable, more fuel-efficient and cleaner-burning than anything Sir Frank Whittle could have envisioned when he filed his first patent for the turbojet in 1930. In fact, designers of the latest turbofans are so adept at extracting the maximum performance and efficiency from their engines’ computer-perfected cores that you might be tempted to think they’ve reached limits of what’s physically possible.

But you’d be very wrong.

Engine manufacturers are broadening the technological boundaries of what they can achieve by turning to exotic superalloys and new types of material coatings capable of withstanding ever-higher temperatures, which translates into improved efficiency and higher thrust. Engineering teams are also sculpting new 3-D geometry for fan blades, compressors, turbines and even the engine nacelles themselves as they seek to squeeze every last ounce of fuel efficiency and performance from their designs. Some manufacturers are even crafting entirely new types of turbine engines that rely on geared turbofans, spinning blisks, rear-facing swept rotor blades or multicoupled internal stages, painstakingly assembled to increase turbofan bypass ratios and extract extra power from smaller engine cores.

Suddenly, the engineering that goes into modern turbine engine design doesn’t seem so basic after all.

“It’s not quite rocket science, but it has become incredibly sophisticated in the last decade as we focus our attention on improving every last detail of our designs,” said Walter Di Bartolomeo, vice president of engineering for Pratt & Whitney Canada.

The details include everything from optimizing the basic physics of the engine — such as compressing the incoming air to ever-smaller volumes — to improving how internal components are cooled. For example, Pratt & Whitney Canada’s newest engine, the PW800 family developed for large-cabin business jets, has achieved a 10 percent increase in fuel economy compared with current engines in the same thrust class by improving the airflow and adjusting the overall cooling scheme, while also incorporating the fourth generation of Pratt & Whitney’s Talon (Technologies for Advanced Low NOx), which not only addresses how much fuel is burned during the combustion process, but also how thoroughly it is burned.

Air Force Driving Switch to Biofuels The U.S. Air Force recently approved a blend of jet fuel and plant-based biofuels for use in the Boeing C-17 Globemaster, representing a milestone in its quest to switch to a 50:50 jet/biofuel blend by 2016. The Air Force tested a blend of biofuel called “hydrotreated renewable jet fuel” and JP-8 jet fuel and found no degradation in the C-17’s engine performance.The Air Force last spring flew its first test flights in jets powered entirely by the biofuel blend. The flights took place at Eglin Air Force Base in Florida with an A-10 Thunderbolt II burning a combination fuel derived from camelina oil and conventional jet fuel.On the commercial side, biofuel company Solazyme recently announced a partnership to develop aviation fuels with Qantas. All the world’s major turbine engine producers have begun testing biofuel blends. Researchers say it would take about 65,000 square miles (roughly the size of Wisconsin) to continuously produce the plant matter needed to meet all of aviation’s biofuel needs.

War of the Global Jets
In the 1990s, Gulfstream and Bombardier became locked in a battle to introduce the world's first ultralong-range business jets, capable of flying more than 5,800 nautical miles nonstop. The Gulfstream V and Bombardier Global Express introduced in the late 1990s both derived their power from the Rolls-Royce BR710, a then all-new turbofan that burned less fuel per pound of thrust than any engine in its class.

Now the companies are back at it, scraping for every last mile of range in an escalating sales war.

“The practical goal is being able to fly 12,500 miles, plus some margin beyond that,” says Jim Kroeger, director of propulsion systems engineering and technology for Honeywell Aerospace. “That allows you to fly to any point on the globe nonstop. Once you can fly halfway around the world, range no longer becomes a limiting factor in where you can go.”

Converting from statute miles, the goal would be 10,800 nautical miles. Add in a couple hundred miles of safety margin and the magic number becomes about 11,000 nautical miles.

“There is this perpetual quest for more range, and as long as people want to fly farther, I don’t see us ever reaching a point where we’ll say we can’t go any farther,” Kroeger said. “How soon we get there is the big unknown.”

Gulfstream and Bombardier are taking an important step toward reaching that eventual goal with the new G650 and Global 7000 and 8000 business jets. Gulfstream's G650 will be capable of flying 7,000 nautical miles when throttled back to its long-range cruise speed. Power comes from Rolls-Royce BR725 turbofan engines, the next generation in the BR700 series. The BR725 engine benefits from some of the core advances made with Rolls-Royce's Advance2 technology initiative — a new family of turbofan engines Rolls-Royce is developing to power tomorrow's business and regional jets.

Competitor GE Aviation, meanwhile, is developing the TechX engine for large cabin-class business jets. Selected for the Bombardier Global 7000 and 8000 (which boast max range targets of 7,300 and 7,900 nautical miles, respectively), the new engine borrows some of its technologies from GE’s commercial and military engines and scales them to dimensions that make sense for business jets.

The hallmark of the TechX engine will be its improved efficiency. The engine will feature a high-pressure-ratio compressor incorporating optimized 3-D aerodynamics to help give it a fuel burn advantage over competing engines. The TechX engine’s 52-inch-diameter fan is a blisk — a one-piece design fashioned from “bladed disks.” You’ve probably heard the term. Some small turbofan engines, such as the Williams FJ44 on the Beech Premier I and Cessna CitationJet, use fan blisks, but their adoption in the TechX engine marks the first time the technology is being brought to large-cabin business jets. Besides easier maintenance, fan blisk technology offers improved aerodynamics and less vibration.

Return of the Propfan
Another way of extending range and improving fuel economy could center on the adoption of "propfan" technology, an idea that gained traction for a short time in the 1980s but is making a comeback as designers revisit the concept. Propjet engines use large-diameter unducted rotor blades to do the job traditionally performed by ducted fans. Also known as ultrahigh-bypass engines, propfans offer the speed and performance of a turbofan engine with the fuel economy of a turboprop.

The inherent problem of hanging a giant unducted rotor system outside the nacelle has always been noise — and lots of it. GE and NASA are performing tests to determine whether the propfan idea can indeed make it into production, using the same test rigs created for a prototype propfan engine called the GE36, which first ran in 1986. Rolls-Royce, with its recently announced Open Rotor technology demonstration program, predicts a propjet engine of its own design could be in service by 2025. Intended for the 100- to 200-seat airliner market, the engine would improve fuel economy by a whopping 30 percent over current-generation engines, Rolls-Royce predicts.

Pratt & Whitney, meanwhile, has publicly dropped the idea of a propfan propulsion concept in favor of its geared turbofan technology, which uses a sophisticated gearing system and newly designed core to achieve claimed fuel efficiency improvements of 10 to 15 percent over competing engines. Pratt & Whitney’s PurePower PW1000G engine uses gearing to separate the engine fan from the low-pressure compressor and turbine. The geared turbofan concept enables the fan to rotate more slowly while the low-pressure compressor and turbine spin faster, boosting overall efficiency and delivering lower fuel consumption, emissions and noise.

Honeywell's Flying Engine Lab
The sight of Honeywell's Boeing 757 experimental flying testbed has been making people stop and stare ever since the company started flying the airplane in 2008. The distinguishing feature of the 757, obviously, is its large test-engine pylon, mounted high on the right side of the fuselage between the cockpit and wing.

“Other pilots start checking us out as soon as we begin to taxi,” joked Honeywell flight-test chief pilot Joe Duval. “They must wonder what in the world we’re doing.”

Honeywell uses the 757 to gather data on the engines undergoing certification testing and technical evaluation. The goal is to ensure all the kinks in a particular engine design are uncovered and remedied long before an OEM customer ever fires it up.

On the day of my visit to Honeywell’s engine flight-test center at Phoenix Sky Harbor International Airport, the 757 carried an HTF7000-series engine undergoing testing in preparation for certification flight trials aboard the Embraer Legacy 450 and 500 business jets. The plan was to run the engine at various altitudes and airspeeds and under loads atypical of normal corporate jet operations. In other words, torture testing.

“We try to sniff out anything unusual in the engine so that the customer isn’t the first one to discover it,” said Ron Rich, vice president of propulsion systems for Honeywell.

That involves slamming the power full forward and then immediately pulling it all the way back (all controlled by computers) as well as doing fun things like inducing compressor stalls — yes, on purpose — at altitude. Modern turbofan engines are carefully designed to avoid full compressor stalls — also known as surging — when operating in their normal range. Surging was a common problem on early jet engines, but the phenomenon has been virtually eliminated by better design and the use of electronic control systems such as full authority digital engine controls (fadec).

Still, with the press of a few buttons, a flight-test engineer sitting at a workstation in the cabin can cause a test engine to surge.

For our trials we headed south from Phoenix and spent a few hours following a racetrack pattern high above the Arizona desert near the border with Mexico. Because of the scarcity of air traffic and perpetually good weather, this sliver of airspace is perfect for these types of evaluations. Our trip aloft followed a predetermined test regimen involving step climbs to 15,000 feet, 25,000 feet and, finally, 31,000 feet. At each new altitude, the engineers commanded the HTF7000 engine to transition instantaneously from max power to flight idle and back to max power — something you’d probably never do during a regular flight, but because you can, it has to be tested.

Next, the test engineers seated at various stations in the 757’s cabin prepared for the intentional surge. The sequence involved closing and locking two of the engines’ three bleed air valves and then slowly reducing N1 power. Soon, the engine let out a one-second belch. To me it sounded like the cannon burst of an AC-130’s side-facing 20 mm guns — not as loud, perhaps, but enough to get your attention.

All of the testing and minute attention to detail appear to be paying off for Honeywell. The HTF7000 engine — selected to power the Legacy 450 and 500, Bombardier Challenger 300 and Gulfstream G250 — boasts the highest dispatch reliability (99.96 percent) of any turbofan the company has built.

How does a turbine engine work?The easiest way to understand what makes a turbine engine work is to compare what’s happening with another type of internal combustion engine: the piston engine in your airplane.The four basic steps of any internal combustion engine are: 1. Intake of air 2. Compression of the air 3. Combustion (in which fuel is injected into the compressed air and burned) 4. Expansion and exhaust (in which the converted energy is put to use) In a piston engine, the intake, compression, combustion and exhaust steps occur in the cylinder head many times per minute as the piston goes up and down. In a turbine engine, these same four steps occur simultaneously in a continuous flow in different places.As a result of this fundamental difference, the turbine engine has sections called: 1. The inlet section 2. The compressor section 3. The combustion section 4. The exhaust section Air enters the inlet section at the front of the engine and passes more or less straight through, front to back. Along the way, a compressor squeezes the air in the compressor section to about one-10th or one-15th of its original volume. Fuel is added and burned in the combustion section, and finally the air is ejected through the exit nozzle to produce thrust. Just after the combustor but before the exhaust nozzle is a turbine (connected by a shaft to the compressor) that harnesses some of the energy in the discharging air to keep the compressor spinning.To achieve the 10:1 to 15:1 compression needed to develop adequate power, modern turbine engines are built with many stages of compressors stacked in a line and usually separated into segments. Most turbine engines built today are dual-rotor designs, meaning there are two distinct sets of these rotating components. The rear compressor, or high-pressure compressor, is connected by a shaft to a high-pressure turbine. This is the high rotor (the guts of the engine), often referred to as N2. The front compressor, or low-pressure compressor, sits in front of the high-pressure compressor and is connected to another rotor, called N1.Turboprops are turbine engines that drive a propeller, and turbo-shafts are turbine engines that drive a shaft connecting to a helicopter’s rotor blades.A turbofan engine is a type of turbine engine in which the N1-stage compressor rotor is much larger in diameter than the rest of the engine, and is called the fan. The air that passes through the fan near its inner diameter reaches the remaining compressor stages in the core of the engine and is further compressed. The air that passes through the outer diameter of the fan rotor does not pass through the core of the engine, but instead passes along the outside of the engine. This air is called bypass air, and the ratio of bypass air to core air is called the bypass ratio. High-bypass-ratio turbofan engines used in most business jets are quieter and more fuel-efficient than turbojet engines.

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