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By David Esler

June 24, 2008

 

While aviation's more extreme critics claim that the worldwide industry has allegedly been lax in reducing its carbon impact on the atmosphere, engine and airframe manufacturers have quietly proceeded with development of a new generation of aircraft a quantum leap more efficient than state-of-the-art equipment.

 

Innovative engineering, computational fluid dynamics (CFD) software, 3-D computer lofting for improved engine and airframe aerodynamics, use of lighter materials and advanced flight control systems to save weight, and adoption of other avant-garde methodologies and technology are yielding powerplant and airframe combinations promising what are touted as "step changes" in efficiency and, consequently, emissions contributing to global warming. In the second of this three-part series on the "greening" of business aviation, we'll take a look at some of the next-generation turbofan engines and business aircraft currently in development that will enter service early in the next decade.

 

We'll begin with engine technology, as it's obvious that the more fuel-efficient the engine, the less it will belch harmful emissions, such as carbon dioxide (CO2) and carbon monoxide (CO), and the more completely it combusts its fuel/air mixture, the less unburned hydrocarbons, or nitrous oxide (NOx), it will spew into the atmosphere. Or as Pratt & Whitney Canada (P&WC) Executive Vice President John Saabas put it, "When we look at emissions, we look at CO2, which is how much fuel you burn, and NOx and hydrocarbons, which is how well you burn the fuel."

 

Leveraging a Common Engine Core

 

P&WC is well along in testing its PW810 turbofan, about which we reported in "A New Engine Class Emerges: The '10K's'," in the August 2007 issue (page 38) when the powerplant was known by its engineering designation, the Advanced Turbofan, or ATF. Sized between 10,000 and 20,000 pounds thrust, the PW810 is unusual, among many other reasons, in that it shares its core with Pratt & Whitney's Geared Turbofan, or GTF, a larger variant rated up to 35,000 pounds thrust and oriented toward the next generation of single-aisle jetliners and, possibly, large business jets.

 

As such, the development of the common core represented a collaborative effort between P&WC in Longueuil, Quebec, and "Big Pratt" in Hartford, Conn. Considerable technology and expertise were traded in the definition of the respective powerplants, and the resultant engines probably represent the most intense cross-pollination to date between the two United Technologies divisions. Describing the PW810 as the outgrowth of "a joint-technology plan," Saabas went on to say that P&W and P&WC were "optimizing components and sharing them across the company."

 

Both engines have been successful in scoring launch applications, Cessna's super midsize Columbus business jet for the PW810, and the Bombardier's C Series and Mitsubishi MRJ 70-to-100 passenger regional airliners for the GTF. Described in the accompanying "P&W's Green GTF" sidebar, the GTF is distinguished from the PW810 by its signature geared fan. The engine's reduction gearbox, which enables the GTF to operate a large (80-inch diameter) fan at a relatively slow rotational speed, elevating its bypass ratio to an unprecedented (for a production engine) 12:1, is the key component allowing the common core to do double duty in engine variants with such widely divergent thrust outputs.

 

The PW810, with a more conventional direct-drive fan, also places P&WC, which hitherto specialized in relatively small gas turbines, into an entirely new class of 10,000-pound-plus-thrust business jet and regional airline engines. ("We have a product mandate of between 15,000 and 20,000 pounds of thrust," Saabas said.) The shared core consists of an eight-stage axial compressor -- the first employed in P&WC's product line not to rely on a centrifugal compressor -- driven by a two-stage high-pressure turbine. Separating the two components is a variant of P&W's "Talon" float-wall annular combustor, touted for its low emissions.

 

Featuring stratified fuel injection and a 2,000ýýF fore-to-aft temperature rise, the Talon II is credited for a claimed 50-percent reduction in NOx below ICAO's Committee on Aviation Environmental Protection (CAEP) 6 standards and a 35-percent drop in CO2, making the PW810 "the greenest in its class," according to Alain Bellemare. Indicative of the closer relationship between the United Technologies engine divisions, Bellemare wears two hats: P&WC president and P&W executive vice president for group strategy and development. "We are trying to understand what will be the next set of regulations [mandating possible emissions caps and/or trading schemes]," he said. "We have to be as aggressive as we can in terms of reducing fuel burn, emissions and noise." The last -- noise -- is claimed to be lowered "significantly below" Stage 4 limits.

 

But what about CO2, the primary constituent of greenhouse gases accelerating global warming? "In reducing emissions from the engine," Saabas said, "the big thing is specific fuel consumption [SFC], since every pound of fuel you burn produces three pounds of CO2." So lower the fuel consumption and you lower CO2 with the additional advantage of reducing operating cost, given the current price of fuel.

 

P&WC isn't revealing the PW810's SFC, but as Senior Editor Fred George reported in his March 2008 cover story on the Cessna Columbus (page 44), at cruise power settings it is expected to be better than 0.625 pounds of fuel per pound of thrust delivered. As a result, P&WC can claim correspondingly lowered CO2 emissions contributing to Bellemare's assertion that the engine leads its class in environmental friendliness.

 

The next generation of highly efficient engines is "all about attention to detail" in design, Saabas continued. "We look at overall pressure ratio, bypass ratios, mixing ratios, the whole cycle perspective, as well as component efficiency, or how you design your airfoils and the transitions within the engines [i.e., stators and diffusers]." Compared to earlier turbofans in this thrust class, the PW810's weight and size have been lowered via incorporation of lightweight materials, such as carbon fiber in the bypass duct and other applications, and advanced aerodynamics allowing a considerable reduction of stages and airfoils (see GTF sidebar).

 

Why Not Gear the PW810 Fan?

 

The PW810 power spool consists of a two-stage low-pressure turbine and a titanium fan fitted with wide-chord, severely twisted and curved blades. P&WC hasn't revealed the size of the fan, but it appears in artists' conceptions to be in the 45-inch range. Bypass ratio is in the neighborhood of 5:1, calculated for the business jet application. If the engine's larger airline sibling can more than double bypass flow and reflect an even lower SFC, why not also incorporate a geared fan here?

 

"The 810 doesn't need the geared fan for the application," P&W's Alan Epstein, vice president, technology and environment, said. "It would increase the weight and the thrust and so the commonalty would be less -- and that would take it out of the [business jet] application."

 

But if both the GTF and PW810 rely on a common core, why would commonalty be lost by gearing the smaller engine? "To use the geared fan on the 810 would require that the core be shrunk," Epstein answered. In other words, to gear the 810 in order to extract the GTF's bypass ratio and fuel burn advantages -- the primary reason why you'd want to do that -- the engine would have to use a larger fan, similarly sized to that of the GTF. Consequently, you'd wind up with the same engine, or nearly so.

 

To turn a smaller fan -- say, in the 50-inch-diameter range -- slower would require a smaller core. And for the purpose-built business jet application (not a converted airliner) like the Columbus, the engine's overall size has to be smaller and lighter, as big fans are difficult to mount on small airframes and generate a lot of drag for the application. To gear the fan, the core would then have to be shrunk or scaled down, reducing the development cost advantage of using a common core for both engines, which is the beauty of the PW810/GTF program. "Cessna selected [the PW810] because it offered more value and lower total cost to its customers," Epstein said.

 

Saabas put it another way: "Because business aircraft typically fly at higher altitudes and airspeeds and want to climb more quickly, that drives a different thrust-to-altitude curve than you want between a regional jetliner application and a business jet. Traditionally, that would keep the bypass ratio of the business jet below six. If you look at the GTF, you would want to have a bypass ratio larger than six to get the fan-speed benefit. It's a different thrust-to-altitude curve for that application." The larger-thrust engine provides a fuel-burn advantage, Saabas pointed out, "so the gearbox makes a lot of sense because you want a demarcation between the speed of the fan and the speed of the turbine. In the lower thrust class, you have too small a core to justify the gearbox, as there's a scaling effect [either for the fan or the core]."

 

On the other hand, the geared fan could eventually migrate to a larger business jet, Saabas claimed. "The environmental benefits of the GTF versus the weight you would carry in a business jet engine from the larger-diameter fan may offset each other, and the geared fan could buy its way onto the engine. You could get improved noise reduction, as well."

 

Added Epstein, "I think in the future you may see geared turbofans on business aircraft, and it'll start at the larger sizes and work down. The geared fan yields more than a 12-percent gain in efficiency, and it's based on three technologies: the lightweight, long-life gear system [a 20,000-hour TBO]; a super efficient fan with fewer blades; and the larger size and lighter weight of that fan [due to incorporation of hollow titanium blades]."

 

Using the common core for such widely divergent applications makes sense, Epstein observed. "It's a core with all the technology that you'd put into the narrow-body [airliner] market that could be applied directly to the business aviation market, which is a breakthrough. One of the advantages of the PW810 is that the costs are kept in check because of economies of scale, since you're bringing a large-run commercial product technology down to the smaller engine."

 

That product philosophy also leverages off of evolutionary upgrades to the engine from the commercial sector that can be passed across to the business jet application. "The other thing the commercial market brings to the business aviation sector is enormously reduced noise, so not only do you get the fuel burn advantage, you get noise technology as advanced as that in the latest commercial airplanes," Epstein pointed out.

 

And this is why the GTF is a "comprehensive solution" to fuel burn, emissions reduction and noise, Epstein claimed, "complementary to the environment. And this same technology will be shared with the PW810." P&WC is projecting FAA and Canadian DOT certification for the PW810 in 2011, two years before the GTF's anticipated FAA approval.

 

Delving Into a 'Big Toolbox' of Technology

 

In March, Rolls-Royce announced the BR725, the latest variant of its BR700 series that was launched in Germany in 1993 in a partnership with BMW. The first progeny of that alliance -- which has since been dissolved, although the engine series continues to be manufactured by Rolls-Royce in Dahlwitz, near Berlin -- was the BR710, which went on to power both the Gulfstream V and 550 and Bombardier Global Express and Global 5000. (A larger engine, the BR715, saw a short-lived application on the discontinued Boeing 717.)

 

The new variant, the BR725, is being developed to power the gestating Gulfstream 650 superjet, the subject of our April 2008 cover story (page 24). The obvious principal design goal in launching the variant was improved SFC in a larger thrust package to motivate the G650 all the way to 0.925 Mach.

 

According to Rainer Honig, Rolls' project manager for the BR725, "even if we apply the best technology, we will still have a gap against a sustainable aviation emissions level. To bridge this gap, we need to talk about emissions trading as a way to buy time, and to close the gap, we then need to introduce biofuels or those that lead to a sustainable emissions cycle. That's the framework. So our goal was to get us at least a 4-percent improvement in SFC relative to the BR710-C4-11, which powers the G550. The exact number is 0.657, based on our predictions from modeling and component testing."

 

The 725's nominal trust rating is 16,100 pounds, or 4.6 percent more than the BR710. "The more impressive number is climb thrust," Honig said, "which is a 12.6-percent enhancement. Gulfstream was very challenging on the runway performance, as they want to get out of small fields with a 100,000-pound airplane, airports like Teterboro and others. One of the unique features of the BR family is a big core that produces industry-leading cruise thrust." And that thrust translates to big-airplane performance: The G650 is projected to cruise between 0.85 and 0.9 Mach and climb directly to FL410; certified ceiling will be 51,000 feet.

 

In conceiving the new variant, Honig said Rolls relied on "a big toolbox of global technology. We went through the engine from front to back and studied every component . . . to get efficiencies up, and then we looked at the cycle and adjusted it."

 

The 625's bypass ratio was boosted with a new 50-inch-diameter fan -- two inches larger than the BR710's -- equipped with 24 swept solid titanium blades derived from Rolls-Royce's Trent 800. "Even though it's larger," Honig said, "we were successful in squeezing it into the same size nacelle to control drag rise. At 0.9 Mach, the high-speed Mach number for the G650, drag rise is very high and we dealt with this by keeping the nacelle the same size as its predecessor. However, it is an entirely new nacelle and thrust reverser." The nacelle supplier is Wichita's Spirit Aerosystems.

 

The BR725 compresses its air with 10 high-pressure axial stages, five of which are blisks. "We chose blisks because with them you save weight and gain some efficiency benefits by eliminating steps and gaps [presented by rotors with insertable blades]," Honig said. Fabricated from nickel alloy, the compressor's efficiency was enhanced with 3-D aerodynamics to create contoured end walls, or curvatures toward the casing.

 

A two-stage high-pressure turbine drives the compressor, shrouded to better control tip clearances and leakage across the stages. Not surprisingly, conventional single crystal alloys were chosen for the turbine rotors and blades to endure the high temperatures of the hot section. Rolls also borrowed from the Trent for an active-case cooling system. "In flight, we can close the tip clearance by cooling the casing actively with a series of valves linked to the FADEC that control the flow," Honig explained. "This cools the casing, which shrinks and closes the gap. This is a very powerful measure to increase turbine efficiency."

 

The LP turbine consists of three stages with high-loaded blading to reduce blade count, a substantial reduction over the BR710. Thanks to the lower parts count, weight of the three stages is claimed to be "not much more" than that of the two-stage LP turbine on the BR710 and 715.

 

Core and bypass flows are mixed in a 16-lobe scarfed exhaust claimed to improve efficiency and lower core noise. "It reduces low-pressure losses while simultaneously shielding noise from the core flow," Honig said.

 

Weight savings were a major initiative in defining the BR725 -- a lighter engine burns less fuel. "One way to save weight is use of composite media in both the nacelle and thrust reverser," Honig said. Thanks to its large doors, the reverser is claimed to be one of the most efficient in its class but relatively compact fore to aft. "The trick here is to control the flow with fewer losses and drag while developing significantly more [reverse] reverse thrust than the BR710 reverser," Honig said. The reverser meets "a very tough requirement" from Gulfstream for landing distance, which it is said to reduce through improved braking action. The engine's external accessory gearbox is also optimized for lighter weight in terms of gear lines and an integrated oil tank.

 

The payoff for all this, plus a new combustors design, is reduced emissions: a 43-percent improvement in the CAEP 6 margin for unburned hydrocarbons, 21-percent improvement in NOx, and complete elimination of visible smoke, a 72-percent improvement over CAEP. "Under no circumstances is there smoke," Honig affirmed.

 

Cost of ownership was also addressed in the BR725 through choice of materials that are claimed to eliminate the requirement for a midlife inspection, clearing the engine for 10,000 flight hours, a 43-percent improvement over the BR710. "With no midlife, you don't have to exchange parts and save materials," Honig pointed out.

 

Following "hundreds" of component tests, the full engine was run for the first time at the end of April. Throttled to 80 percent of its thrust rating, the test turbofan was claimed to be meeting its design performance targets. Five engines are being used in initial ground testing with another eight designated for flight work. Certification is expected in summer 2009 with first flight following immediately on the G650. "There will be no flying test bed," Honig promised. "We go right to the G650 prototype."

 

[As we reported a year ago, Rolls-Royce is also developing a new 10,000-pound-thrust turbofan that has been chosen by Dassault Aviation for a new twin-engine super midsize Falcon. Rolls-Royce executives were asked to provide information on the engine's status for this report but declined. -- Ed.]

 

Small Engines Are Getting Cleaner, Too

 

At the other end of the thrust spectrum, similar efforts are being made to lower emissions and fuel consumption, even though engines in the 1,500- to 3,500-pound-thrust band by virtue of their size pollute very little. At Williams International, Vice President for Business Development Matt Huff reported that the company is dedicated to lowering emissions "by continuously reducing fuel burn. The three engines Williams is certifying now -- the FJ44-4, FJ44-3AP and FJ33-19 -- will burn 15 to 20 percent less fuel than did early FJ44 models." As is Williams' custom, however, the company refuses to reveal SFC numbers on its products.

 

According to Huff, constant improvement in CFD software and other design tools enabled the Walled Lake, Mich., manufacturer to continuously improve the aerodynamics within its engines. "The more efficient the airfoils, the less fuel burned."

 

In addition to supplying engines to airframe OEMs for new aircraft, Williams is working with modifiers to replace early generation turbojets and fans on light business jets like the Learjet 25 and 35 and Cessna Citation II with consequent improvements in fuel consumption and emissions. "Several companies are offering retrofit kits that provide exhilarating climb and cruise performance while burning on the order of 25-percent less fuel when outfitted with FJ44 engines," Huff claimed.

 

Williams is now facing new competition in the small gas turbine market with Honda Motors' entry into the aviation realm. Partnered with General Electric as GE Honda Aero Engines, the Japanese transportation conglomerate is currently engaged in certification of its HF120 turbofan, nominally rated at 2,090 pounds of thrust. The engine has had nearly a 20-year development cycle and will be manufactured in the United States. In addition to powering the Honda Jet VLJ, also moving toward certification in Greensboro, N.C., the HF120 has been chosen by Linden Blue's Spectrum startup venture for its S-40 light jet.

 

"When Honda first began development of a small turbine, the HF118, the engine was a different configuration," recalled Jim Dougherty, GE Honda's marketing and sales manager. Following formation of the 50/50 alliance, the engine's architecture was modified to the HF120 configuration by adding second stages to both the core booster and the low-pressure turbine on the LP spool. "You want a high-pressure engine for good power density but a lower pressure, more efficient fan to balance out the light core," Dougherty pointed out. "Accordingly, we added 3-D swept blades to the fan and increased its diameter to 18.5 inches, a GE innovation borrowed from the GE90 and GENx engines. Swept blades draw more air into the engine and make for a quieter fan."

 

Bypass ratio is between 3.0 and 4.5. In an unusual measure, GE also introduced counter-rotating HP and LP spools, claimed to reduce weight and parts count. "With counter-rotation, the downstream turbine needs fewer airfoils, and we eliminated a full stage in the HF120," GE Honda President Bill Dwyer explained. "By turning the spools in different directions, you increase the turbine efficiency. Fewer parts means lower weight, less maintenance and reduced fuel burn. We also lower some of the temperatures and convert less fuel into energy. By eliminating the extra turbine stage, we can shorten the engine, and that also makes it stronger for less bending under the stresses of flight. Materials selection becomes important [in lowering overall weight], too.

 

"So the HF120 architecture is a single-stage centrifugal compressor, a two-stage axial booster on the LP spool, a one-stage HP turbine and a two-stage LP turbine," Dwyer continued. "We designed 3-D aero blades for the HP turbine and reversed blades to reduce shock downstream." Meanwhile, the combustor is a reverse flow arrangement for compactness and low weight, which Dwyer claimed also pays off in reducing unburned hydrocarbons and NOx. Advanced cooling features allow the use of less air for cooling.

 

Upsetting the Methane/Oxygen Balance

 

"We also designed it to minimize Stochiometric combustion, [a phenomenon] where the fuel/air ratio is balanced with just the right number of methane and oxygen atoms to produce NOx, which of course you don't want," he elaborated. "The technology incorporated into the combustor minimizes the time where Stochiometric combustion exists, the so-called rich/lean mixture, where the fuel/air mix is introduced rich and translated to a lean zone at the other end, minimizing the amount of time where NOx can be created. We again borrowed this technology from GE's larger engines. On the other hand, we attack CO2 emissions with reduced fuel burn."

 

Dwyer claimed the HF120 is "superior to the competition, the most efficient VLJ available" in terms of lower emissions and fuel burn, but refused to divulge the actual numbers for "competitive reasons." He went on to admit, however, that "our competition [Williams and P&WC] don't publicize their SFC or emissions numbers, so we don't either. All we can say is that the HF120 is designed to be 'advantaged' over the competition."

 

Part of that advantage Dwyer credited to technology trickling down to the partnership from GE. "We have to balance the cultural norm in our business with what our customers will value," he said, meaning the partnership had to choose just the right amount of expensive technology and features available from the high-utilization commercial engine product line to incorporate into the low-utilization and cost-conscious VLJ engine. "So we have tried to be smart about where we've taken advantage of the technology portfolio at our disposal."

 

One of those "smart" decisions was apparently in choosing materials for the engine that, as with Rolls-Royce's BR725, will allow it to obviate the hot-section inspection. "We will be able to go all the way to 5,000 hours with no HSI," Dougherty claimed, "so the turbine is designed to be 300 percent more durable than competing engines." Maintenance support will be shared by the partners through a combination of GE and Honda infrastructures ensuring worldwide service.

 

Reportedly, Honda Motors legendary founder Soichiro Honda -- reverently referred to as "Mr. Honda" in the halls of the company's Japan headquarters -- had right from the beginning wanted his company to ultimately have a presence in the aviation industry. "First two wheels [motorcycles], then four wheels [autos], then three wheels [tricycle geared airplanes]," he was quoted as saying. Now, after laying the groundwork for a turbofan engine and an airplane powered by it in the 1980s, the first product of that initiative, the HF120, is advancing toward certification. "We are testing demo engines and optimizing some of the structures," Dwyer said. "We've run eight builds of the core and six of the full engine and expect certification in the second half of 2009." The HF120 is to be flight tested on a Citation CJ1 late this year.

 

The engine will be manufactured completely in the United States, initially between two facilities, one Honda's and one owned by GE. The process will begin at GE's engine plant in Cincinnati and eventually migrate completely to Honda's new factory at Greensboro. Asked if the powerplant will be grown, Dwyer said, "We will look at other attractive opportunities," adding, "Our agreement with Honda covers the 1,000- to 3,500-pound-thrust bracket."

 

There are similar cultures in both partnership companies, Dwyer concluded, "as we are both oriented toward quality and reliability. "One of the prevailing thought processes here is anticipating the regulatory process [regarding greenhouse emissions] in both the United States and Europe. In effect, we're building a house on stilts anticipating the flood. We are designing in the ability to meet pending regulations; consequently, it will be one of the most fuel-efficient business jet engines available, as well as one of the cleanest burning, and that will establish the HF120 as a leader in environmental performance. Our intent is to have the cleanest engine in the class, consistent with our mutual cultural goal to deliver value."

 

The Airframe Connection: Designing for Aerodynamic Efficiency

 

But efficiency and reduced fuel burn and emissions aren't confined to the engine regime. Some 60 years into the evolution of the jet airplane, aeronautical engineers are still learning how to make airframes more efficient. And this translates to various drag-reduction strategies and the constant battle to lower weight. A more-efficient airframe will require less fuel to sustain a particular Mach number or satisfy a given mission -- and this means less emissions, especially CO2.

 

Cessna Aircraft wholly embraces that sentiment. "Our public position is that we have always been committed to reducing the environmental impact of our products, continually working with our engine suppliers to obtain optimum engine performance and low emissions, and so with each new generation of aircraft, Cessna business jets show improvements in emission levels," Robert Stangarone, Cessna's communications vice president, said. "For instance, our new Encore+, when compared to the previous generation Citation V, features the following emissions improvement: CO2, 16 percent; unburned hydrocarbons, more than 80 percent; NOx more than 80 percent; CO, more than 50 percent; and smoke, more than 84 percent. We remain committed to a strategy of continuous improvements. That's been our baseline over the past several years."

 

Michael Thacker, senior program manager, advanced design, picked it up from there. "Aircraft design -- the whole thing -- is always a great compromise, and Cessna has maintained for a number of years a balanced design approach, focusing on the efficiency of the airframe and the cost to us and the customer."

 

Efficiency lends itself to environmental friendliness, Thacker believes, because it allows a lower fuel burn, releasing less harmful emissions. "So in a sense, consideration of the environmental impact has been incorporated all along and today has a higher profile," he said. "The engine is the largest piece of that puzzle. We have pushed the engine OEMs hard on the efficiency of their products -- to meet or exceed our numbers -- as well as noise and emissions. The remainder is airframe and system efficiency, and we continue to work on that. This allows us to make a lighter and less expensive airplane to complete the same mission."

 

Wing design, naturally, is one of the key areas engineers and researchers turn to in efforts to improve efficiency.

 

Cessna's in-house aerodynamics research department is equipped with CFD capability to design wings matching the performance requirements of conceptual aircraft. "CFD allows you to model and accelerate your design iterations at a rate unprecedented from the old days when we had to wind tunnel-test each iteration," Thacker said. "Today you can do a number of interim iterations, test them in the computer with the modeling capability, and then verify them in the wind tunnel. You have a more optimized design as a result."

 

(Millions of grid cells are imbedded in CFD calculations, exponentially reducing engineering workload. CFD capability thus significantly shortens time necessary to develop an aircraft design. Manually calculating the iterations cited by Thacker in the pre-CAD/CFD era could take years.)

 

"The aerodynamic improvements are incremental," Thackery continued. "We learn over time, and we get better with each one. It's often about the integration of the major components -- the wing, the empennage, the pylon, how it all goes together. If you look at the cross section of the Columbus, it's slightly smaller than the competition, and we've compensated for that by putting the floor frames on the outside of the fuselage, then covered them with the wing fairing. This allows us to drop the floor and gives you the sensation of a much larger cabin. This slightly smaller cross section reduces forward drag, so there's a payoff there, too."

 

The Columbus eschews winglets, Thackery said, "because you can design a wing from the start to be the right wing for the airplane. "When you grow the airplane, then you might need winglets to stay with the same wing. But starting from scratch, you can design an efficient wing without the need for winglets by optimizing the wing for the application."

 

The quest for efficiency also extends to aircraft systems. "There is a continuing move to get more efficient systems in the airplane," Thacker said. "By efficiency in the systems, I mean the amount of energy you expend to get a given result. In an environmental system, for example, you have to either heat up or cool down air -- in other words, condition it -- and how much energy does it take to do that? So what we try to do is continually improve the efficiency of those systems to tap less energy from the engine."

 

The efficiency effort also targets a power distribution system for avionics, as well as other systems. "All have certain efficiency requirements," Thackery said. "In the past we have designed systems and worked with component vendors to supply the major pieces, but we have been the principal architect of those systems so we can have control over them end to end."

 

The French Connection: Lighter, Smaller and Wired

 

At Dassault Aviation, there has always been a "different shading" from competitors in how the French OEM has pursued efficiency in its airframe designs, according to Olivier Villa, senior vice president for civil aircraft. He maintained that Dassault's business jets have always been "lighter, more compact and basically more efficient. If you compare a Falcon 900 to a GIV, there is a significant difference in terms of weight, and if you compare missions, you will find a 30-percent difference in fuel consumption -- even as much as 40 percent, depending on the mission. Also between the Falcon 2000 and Challenger 604, you'll find the same order of magnitude."

 

Villa credits this to Dassault's heritage in designing and manufacturing tactical fighter aircraft. As a result, "it has always been very important to be at the outer edge of structural and aerodynamic design and developing the best tools for integrating these into a very efficient aircraft," he said. "The fact that we have these tools for fighter design made them available for business jets. . . . We are not the cheapest, but we are very advanced in terms of technology."

 

Green issues were of little concern when Dassault launched the development program for the Falcon 900 widebody business jet in the early 1980s, Villa said, but the manufacturer incorporated technology now accepted as de rigueur in efficient design into the aircraft "because that was our heritage." But now, he pointed out, "there is more attention given to it due to the cost impact and concerns about global warming. So we will continue with that trend, as it's becoming more and more important to the market."

 

High-lift wings, weight control and fly-by-wire control systems are all examples of technologies and methodology Villa says Dassault has and continues to exploit to build efficient airframes that culminate in a smaller carbon footprint. Here's a primer from Villa on how Dassault approaches the task of designing for high efficiency.

 

"When we design an aircraft, the customers want it to go fast and high and be able to use short fields," he said. "But going fast makes more drag and going high causes greater fuel consumption during the climb. When you raise your cruise altitude by 2,000 feet from, say FL 410 to 430, that means that your wing area needs to be larger by 10 percent, but that is significant in sizing an aircraft and a major design driver. And considering short-field performance, reducing BFL by 500 feet has a major effect on wing area, too, increasing it by 20 percent, which again is major."

 

But employing what Villa called "smart design," you can avoid this effect by using high-lift devices on the wing and add 50 percent lift compared to the clean wing. So what you want in designing a green aircraft is to use all this technology. By not using slats, you can get the performance for altitude, speed and short field with a bigger aircraft and more fuel consumption. But with high-lift devices, you can use a smaller wing and smaller engines and have a more efficient aircraft."

 

On the other hand, there's "no easy recipe" for controlling weight, Villa said. "Each aircraft needs a different solution. For example, on the Falcon 7X, we found that the best design for the empennage was to use composite [media] structure." The vertical fin, for example, is a single part fabricated of a carbon fiber process that Dassault terms RTM, for "resin transfer molding." According to Villa, RTM "gives us light weight and a single part."

 

For the wing, Dassault elected to stay with an aluminum structure "basically because the wing is a fuel tank, and that needs to be protected against lightning. This protection adds weight with composite structure [as a metallic mesh must be molded in with the composite media]. Also, when you have a fuel tank, you need to optimize the internal shape for maximum fuel quantity, and we found that if we were to design it with composites, we would have had lower internal volume for fuel." This is because higher skin thickness is necessary with composites, Villa claimed, and the central box structure needs to have continuity. "Also," he pointed out, "with metal you can use the central box structure for fuel."

 

Then, too, the carry-through structure must be continuous, "and that's easier to do with metal and you can put a lot more fuel into it," Villa continued. "There is not a one-size-fits-all solution -- each time you design a part, you need to compare technologies, and for each part, it will be a different solution for the type of aircraft you are building. You can do it [use composites] with the [Boeing] Dreamliner but not with a smaller aircraft, for example."

 

Designing in Instability for Enhanced Efficiency

 

But Dassault's piece de resistance is the digital fly-by-wire control system it incorporated into the Falcon 7X. "With traditional flight control systems, the aircraft has to be stable with a strong positive stability margin," Villa said. "That means you design an aircraft with a big and strong horizontal empennage, which during cruise provides negative lift. Unfortunately, this induces drag.

 

"What we can do with the digital flight control system, however, is reduce the stability margin," Villa continued. "We did this on the 7X. The empennage becomes smaller and this reduces negative lift and therefore the amount of drag. Sooner or later we will be authorized to certify aircraft less and less stable, and I'm sure we will see civil aircraft developed that are less stable like fighters, and we will have less drag."

 

One of the 7X's design points was high speed, requiring a thin wing with a high sweep angle. "With this kind of long and thin wing," Villa said, "typically if the gear struts are not long enough, you have a risk of touching the wingtips if it rolls [on takeoff or landing]. So to add safety margin, you can put longer gear struts or more dihedral on the wing. If you choose to have long struts, they will be heaver, requiring more space in the region of the wing where the fuel tanks are located, and the stairs will have to be longer. All this adds weight and reduces efficiency. "So if you are trying to optimize, you will choose more dihedral, but this will have an effect on stability for Dutch roll in that it will be more prone to that condition. With a conventional flight control system, you will need to move the fin farther aft or make it bigger to increase its stability effect. But if you use your digital flight control system, you can counter the Dutch roll and add artificial stability. This opens the bag to many new possibilities. The 7X aircraft was designed around the flight control system."

 

Another solution put forward to reduce emissions through more efficient operations is deployment of a truly modernized air traffic management system, e.g., the FAA's NextGen, which will facilitate more direct routings and allow less complex approaches and departures, all saving flight time. Terming these procedures "more demanding," Villa claimed future ATM will require more capable aircraft, "meaning that in terms of approaches you will need aircraft that can follow steeper descents . . . [or] more precise 3-D trajectories.

 

"If we have these capabilities, [flight crews] will be able to follow more efficient routes. If not, they will be put on slower routes or be required to hold -- more ATM constraints, which will negate their efficiency. It's all about aerodynamics, propulsion control and systems design. Bleed air for anti-icing requires engine power above a minimum level, and that means that your engine will give you more thrust than needed in the descent phase, so we need to be smart in designing [these systems]."

 

Consequently, he said, designers will need to focus on leading edge anti-icing systems to make them as efficient as possible. "If you approach them as a heat exchanger, you can design them more efficiently and thus use less bleed air from the engine. This takes a lot of care. In the future, we may even use electrical power for anti-icing -- maybe."

 

[We also asked Dassault to comment on its gestating new twin-engine Falcon, but like Rolls-Royce, company officials declined the opportunity. -- Ed.]

 

Starting With a Clean CAD Screen to Design 'Children of the 21st Century'

 

Being one of the newer entrants to the business aviation market, Embraer has had the luxury of starting with what Satoshi Yokota terms "clean-sheet design." Yokota is the Brazilian airframer's executive vice president for technology development and advanced design and is currently overseeing the definition of Embraer's proposed Midsize Jet (MSJ) and Mid Light Jet (MLJ) projects.

 

"Since we are newcomers in this market, we were not bound by limitations to take advantage of existing designs," he said.

 

Yokota said Embraer is working not only to expand its business aircraft offerings, but to introduce aircraft that will be more competitive and environmentally friendly than older designs of its competitors. "We have an active technology program within the company and are spending a significant amount of money on it," he said. "It addresses propulsion, structures and systems, and we are using the results of this research on our new designs and retrofitting them to older designs."

 

Efficiency, a balanced set of capabilities, and low operating costs were the objectives when Embraer commenced its definition of the MSJ and MLJ. "Since we are not bound by using an existing airframe, we started from a clean sheet and designed the optimum fuselage, wing and propulsion system for the aircraft," Yokota said. "Using fly-by-wire, we can reduce control surface sizing and deflection and therefore reduce weight and drag." He cited weight savings of "several hundred pounds" compared to conventional control systems. "Also, with a closed-loop FBW like we're using, you can have a more optimized control system and increase the safety envelope. It provides very powerful envelope protection."

 

Antonini Macedo, a product strategy environmental engineer at Embraer who is focusing on aircraft systems, added that another goal is optimizing the relationship between systems in terms of interfaces. "In the past, people would design a very good specific system, but perhaps it would not interface well with other systems on the aircraft." So to optimize all the MSJ/MLJ systems so they can talk to each other, Embraer is borrowing technology developed for its EMB170 and 190 airliners.

 

Embraer engineers are relying on a range of computational tools to maximize aerodynamic efficiency and achieve the most balanced design in terms of aircraft capability, including CFD and Multi Disciplinary Optimization software integrated with a CATIA design suite. Additionally, full digital mockups of the airplanes have been created for integration purposes.

 

"Using these techniques, we have a clean-sheet wing design," Macedo said, "in which we have fine-tuned the curvature of the wing and the winglets and the blending of them to provide a more fuel-efficient design in several flight regimes rather than optimizing for one performance point. We're modeling and verifying it in the wind tunnel and achieving several percent improvement in lift coefficients for takeoff, SFC and specific range. We are confident that we have achieved a very good design with very efficient fuel consumption."

 

Both the MSJ and MLJ will be "taller and wider" than their competitors, better optimized in more flight conditions and situations, Macedo claimed. "They will offer more efficiency and specific range [3,000 nm at 0.8 Mach and 2,300 nm at 0.78 Mach, respectively, both with NBAA IFR reserves] and should be lighter overall, as well, very balanced and efficient. In today's green paradigm, we need to seek more balanced designs, more efficient overall, not excelling in only one area like speed."

 

A single engine type has been chosen to power both variants, Honeywell's HTF7500, rated at 7,000 pounds of thrust for the larger MSJ and 6,000 pounds for the MLJ. Yokota said Embraer is also participating in programs for development and distribution of alternative fuels, including biofuels for which Brazil has carved out a leading role (see "Alternative Fuels for Jet Engines," September 2007, page 82). "We intend to make them viable and offer them to our customers," he promised. "Our role is essential to close the loop [on global warming]."

 

In concert with that initiative, Embraer has worked out a voluntary carbon trading program that Yokota claimed is "revolutionary." The manufacturer planned to announce the program in May at EBACE in Geneva; we will provide a description of it in Part III of this series in the July issue. The MSJ and MLJ were also expected to be formally named during EBACE.

 

The new business jets will enter service in 2012 and 2013. Because Yokota predicted that "the future generation of airplanes will be flying in a different ATC environment," NextGen in the United States and SESAR in Europe requiring advanced nav systems, the pair will be so equipped with advanced avionics capable of accommodating ADS-B and RNP. "These are truly 'children of the 21st Century,' so you won't have to retrofit them with the new avionics later on," he said.

 

Much of the new technology just described won't enter service for several years, so next month Part III of "The Greening of Business Aviation" will examine ways in which operators of contemporary and legacy aircraft can plan strategies for dealing with global warming concerns. There's much that can be done to help keep aviation's contribution to total CO2 among the lowest of any mode of transportation.