Fewer trips for the fuel truck

Sept. 13, 2007
Developments debuted at the Paris Air Show put airliners on a track toward flying more passengers while using less fuel.

Robert Rowe
Contributing Editor

"It really was way ahead of its time."

We're talking about the Piaggio Aero Industries P180, a perennial regular at the Paris Air Show. The Avanti is an ‘efficiency machine' conceived out of the oil crisis of the late ‘70s and hatched by the late ‘80s. Despite wise-cracks about its looks when it first appeared (like, "Is it flying back wards?"), events at this past summer's Paris Show foretell its re-emergence as in-vogue platform because of its high performance and fuel economy. It has props but cruises at 41,000 ft, runs 460 mph, and on a typical 345-mile flight arrives within five minutes of a jet. But that jet will burn 50% more fuel!

The Avanti, as well as several other inspirations aimed at conserving fuel and minimizing emissions, are now in the spot light. Features exemplified by the Avanti's configuration could be a reference for a future where reduced fuel consumption will be paramount.

The Show illustrated that the aviation industry, which has usually followed its own lead, is acknowledging environmental influences. Show-goers saw copious quantities of green grass and flowers proliferating through the stands. The idea that aircraft and engine manufacturers, operators, and authorities wanted to convey was that they are waking up to the ‘green challenge.'

The EU's subscription to the UN's February 2005 Kyoto Protocol on climate control has fostered an array of well-publicized clean air initiatives for aviation. At Paris, the U.S. nevertheless was also promoting its own stock of serious aviation-oriented environmental efforts.

It's not like aviation has stood still in operational efficiency or in reducing noise and pollution. Consider the often-cited productivity factor, passenger miles/gallon (pmpg), a measurement that accounts for the economics of the engine and airframe as a system. This is the fuel economy of a transport airplane averaged over its passenger capacity. The Airbus A380 and the Boeing 787, both now close to entering service, are expected to get over 80 pmpg. This is a whopping 3X improvement over early jets (i.e. 707-120). The air transportation fleet overall average today is about 47 pmpg. Modern 70-seat twin-engine short-haul turboprops get about 52 pmpg.

The industry has also been at work on noise reduction. The old 707-120 produced an ear-splitting equivalent of 200 hp in sound energy on takeoff. As witnessed in Paris, the newest jets such as the A380 have reduced perceived noise by about 40% (-10 dB) with respect to noisy fleets of the late 1960s. (See MACHINE DESIGN , 9/1/05, pg. 18) The A380 is also reported to be 50% quieter in noise nuisance than the classic 747.

These improvements have taken place primarily under the industry‘s self direction, a result of the incessant technical quest for better overall efficiency to cut operating costs. Noise abatement was a natural result of perfecting the combustion process and of high by-pass ratio (BPR) engines. On the airframe side, use of composite materials to structurally lighten the aircraft has also concurrently reduced flight energy needs.

The Green Challenge
Yes, something was in the air at Paris and it wasn't just air planes. Rather, it was the wind of change in aircraft design and operations. Paris is like a ten-ring circus of aeronautical events. But what might be considered a keynote of the Show was the formal announcement of a joint EUU.S. AIRE partnership (Atlantic Interoperability Initiative to Reduce Emissions). Major airlines and industry from the U.S. and EU are signed up as partners.

The objective of this agreement is to integrate just-started programs for transatlantic flights operations. Each side has an alphabet soup of efforts in this area. The most visible E.U. program is Sesar (Single European Sky Air traffic management Research) and for the U.S., NextGen (Next Generation Air Transportation System). The scope of this program includes more than just emissions issues. The umbrella goal is to figure out how to handle a three-fold increase in airline traffic by 2025. But its green issues may be boiled down to one aim: Reduce fuel consumption by any and all means.

The major talking point at the show was the greenhouse effect and controlling CO 2 emissions. Some countries are proposing carbon taxes as one means of addressing CO 2. The issue is basic: Combustion processes using petroleum-based fuels produce 3 lbs. of CO 2 for every pound of fuel burned. Research into new fuels is trying to reduce this figure. But the more obvious way to cut CO 2 and all other emissions is to just burn less fuel. Of course, this is nothing new. Fuel consumption has been on the front burner since the Wright flyer.

It's one thing to economize by driving the family car a little less. But it's much harder to reduce fuel burn and emissions by changing rigid procedures that govern air traffic management (ATM). Yet that is what NextGen, Sesar, and the combined AIRE, must do.

For instance, no more holding patterns. Computers will take a much bigger role in gate-to-gate operations control, integrating considerations such as evolving weather, desired time of arrival, emergency activity, and so forth. GPS satellites form the basis of this system and will continuously keep an eye on the situation.

We can thank the supersonic Concorde for one specific technique now being proposed to save fuel. The Concorde descended from cruise directly toward its landing runway. Now ATM planners envision conventional air traffic adopting the same idea. The technique, called CDA (Continuous Descent Approach), is already in test and eventually may be applied to the entire field of air traffic. With CDA all aircraft will theoretically descend smoothly from their cruise altitude. Their engines stay at flight idle, over a constant descending approach, at the highest altitude sustain able, to the landing point.

"Aviation contributes just 2 to 3% of all CO 2 emissions, but given growth predictions we must do our part," stated FAA administrator Marion Blakey. "We can save a ton of CO 2 per flight with CDA." Left unsaid at Paris, however, was how airports would cope with the ground congestion that would accompany the 3× airline traffic flow that NextGen and Sesar anticipate.

For the most part, the newer forthcoming green issues bear little impact on planes that are flying now. Even the latest big jets, the A380 and B787, reflect current legislation and application of composite materials to improve operating efficiency.

The latter plane benefits from advances in composite technology not considered for A380 simply because the A380 design is nearly four years older. This lets the 787 equal the A380's 80 pmpg though it is smaller and carries fewer passengers. Composites will make up nearly 100% of the 787 skin and 50% of all materials in the plane. By comparison, the A380, with a similar level of engine technology, rests on a structure that is just 25% composite.

Back to the future
Despite the green steam roller, new aircraft projects are all designs evolved from conventional jet planes. In that regard, turboprop power is get ting more attention, as it often does whenever fuel efficiency returns to vogue in aviation. Actually the first gas turbine went into commercial service nearly 60 years ago. It is nowadays called a turboshaft engine; it is basically a turbine driving a conventional propeller through a reduction gearbox.

In this regard, short-haul turboprop planes are selling well again. Examples include the ATR 72 twin-turboprop regional airliner built in Europe by ATR and used in the U.S. by American Eagle Airlines; and the 70-seat Bombardier Inc. Q400 (more commonly known as the Dash 8). Military heavy lifters are awaiting the Airbus A400M, a four-engine turboprop trans port/tanker whose maiden flight is scheduled for early next year. Its new 11,000 shp TP 400-D6 engine is billed as the most powerful Western turboprop ever. (See MACHINE DESIGN, 2/17/05, pg. 98)

Aircraft makers are taking an other look at engine designs of all kinds. The late ‘80s fuel crisis, for example, inspired a propeller derivative called the ‘propfan' promising good fuel efficiency. Baptized UDF (un-ducted fan) by GE and UHB (ultra high bypass) by PW-Allison, these were technically successful, demonstrating a 30% reduction in fuel burn over conventional high-BPR turbofans of that period. But the airline industry rejected them as being "unattractive."

Propfans resemble turbo props in that the prime mover is basically a high power turbo shaft engine. Their big technology breakthrough was the development of prop blades having highly swept-back blade tips. This overcame the conventional prop's breakdown in efficiency at airplane speeds exceeding 450 mph. Thus it enabled prop fan-driven airplanes to fly at jet speeds while also cutting their fuel consumption.

One difficulty with such engines has been noise. Nevertheless, several aircraft engine makers have built unducted fan prototypes. NASA also devised a concept in the 1980s that served as a take-off point for other studies.

Today, economic and regulatory strains have made some airlines clamor for the prop's return. One carrier, EasyJet Airlines, has even designed its own airplane! At Paris, EasyJet in the U.K. unveiled its ecoJet design, with dual propfans mounted in the rear in the style of the P180 Avanti. In fact, we may see a second reintroduction of the prop. At Paris, engine supplier CFM International, a joint venture of GE and Snecma, announced plans to study UDF-type prop fans. These propfans are candidates for the next generation of single-aisle transports about the size of a 737 or A320. But this time the propfans are being called open-rotor turbofans.

Still, these would be fairly customary airplanes operating from normal airports in a regular fashion. More efficient airliner operation will go for naught if it causes more congestion, energy use on the ground, and pollution. That's a real possibility if airports and metro areas see the kind of ground traffic that would go with air travel levels envisioned in Sesar/NextGen. One way to address this problem would be to bring the airplanes closer to the passengers. That might be possible with a network of small airports dispersed throughout broad population centers for both point-to-point travel and to serve as feeders for big jets.

In these regards, an important debut in Paris was the Bell\ Agusta Aerospace Co. nine-passenger convertiplane, the BA609 tiltrotor. It is a smaller civilian off shoot of the Bell-Boeing V22 Osprey. (See MACHINE DESIGN , 9/14/06, pg. 76) After coming through a long, thorough military development program, the tiltrotor now has proven its viability as an air vehicle system. Combining 315 mph speed with true VTOL (vertical take-off and landing), the BA609 and its relatives might be enablers for launching such a network of smallish airports.

A vision of the future:
The Open-Rotor Regional Airline

A main component of this concept, called the "ecoJet," is the open-rotor turbofan engine. A brainchild of European economy airline EasyJet, it incorporates open un-cowled multiple-swept-blade propulsion fans. These are powered through a reduction gear box by a modern high-performance turboshaft engine. EasyJet says its ecoJet design also reduces fuel burn by eliminating slats on the front of the wing to minimize drag and through use of a slightly forward-swept wing profile that allows the plane to maintain laminar flow over a significant proportion of the wing.

Coming to an airliner near you:
Open-rotor turbofan engines

The open-rotor turbofan engine consists of open un-cowled multiple-swept-bladed propulsion fans, powered through a reduction gearbox by a modern high performance turboshaft engine. A turboshaft engine consists of a gas-generating core-engine {compressor\combustor\turbine} whose energy output is extracted by a fan driving power turbine.

The open-rotor concept evolved from the contemporary turbofan, whose large propulsion fan bypasses about five times the amount of air needed for combustion around the core-engine (giving it a 5:1 BPR or By-Pass Ratio) directly to a propulsion nozzle. There, in combination with the hot core exhaust, it generates aircraft powering thrust. This arrangement produces a high takeoff thrust and great overall efficiency.

It has been demonstrated that high BPRs boost efficiency. At BPRs of 30:1 or more, weight and drag make it impossible to retain the fan's outer case and external cowl.

For the open-rotor fan to operate effectively in the free air stream at jet-plane speeds, specially built highly swept-tip fan blades (analogous to the plane's swept wings) become necessary to prevent a drop-off in efficiency.


Though it is perhaps best known for the bit part it played in the movie Miami Vice, the P180 Avanti's real claim to fame is its fuel efficiency in combo with high performance. Fuel-efficient features were a major theme at the recent Paris Air Show.

Tiltrotor convertiplanes like the Bell/Agusta BA609 could be harbingers of future efficient air travel from dispersed airports.


Holding patterns may be a thing of the past if satellite-guided flight plans and continuous descent approaches come into common use. CDA schemes save fuel by eliminating multiple applications of thrust during landing descent. Planes also generate less ground noise during part of the flight path.

In 1989, the Allison Engine Co. (now part of Rolls Royce) worked with Pratt & Whitney to flight-demonstrate the model 578-DX ultra-high bypass engine on an MD80 airplane. Note the curved double-rotation blades shaped for efficiency at high flight speeds. This type of engine is a forerunner of open-rotor turbofans.

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