Author: Michael Schirber Published on July 24, 2007

Ecologically conscious travelers eliminate their guilt for global warming by promoting carbon offsets from wind farms and reforestation. At the same time, aeronautical engineers are taking another route, designing a more environmentally friendly and sustainable aircraft, which may overturn the long-standing concept of flight engineering. "I want to get rid of the image of a winged cylinder," said Etnel Straatsma of Delft University of Technology in the Netherlands.

In Straatsma's vision, the future plane may be as wild as a flying saucer. She and other engineers are experimenting with lighter materials, and some are thinking about radical ideas, such as returning to propeller aircraft as an environmentally friendly alternative to passenger aircraft. 

Straatsma is responsible for the recently established CleanEra project, which aims to design an "ultra-environmentally friendly aircraft" that emits 50% less carbon dioxide per passenger mile than current passenger aircraft. According to the recommendations of the European Aerospace Commission ACARE, the project’s "greenliner"-depicted as a flying saucer in the design illustration-will also reduce other pollutants and noise. Alexander de Haan, also from Delft but not involved in CleanEra, said that the challenge is that modern aircraft are close to being optimized to the maximum. He checked various design modifications and found that they can reduce carbon emissions and noise levels by as much as 10% to 15%. "These ideas can't keep up with the [aviation] industry's 5% growth year after year," De Haan said. There is growing concern that the aviation industry accounts for about 2% of global carbon dioxide emissions, but many people worry about the industry’s growth rate. According to a 1999 report by the Intergovernmental Panel on Climate Change, air travel may account for 5% or more of the total warming impact by 2050. Although estimates vary, every mile and mile in an airplane emits about 1 pound of carbon dioxide emissions, which is roughly the same as driving the same distance in a car. According to Andreas Hardeman of the International Air Transport Association industry organization, continuous modification of aircraft can help reduce emissions per passenger by 2% to 2.5% per year. But he agrees that the current paradigm may have reached the "end". "Since the Boeing 707 first flew in 1957, the airliner... its basic appearance has hardly changed," Hardman said. "However, as it becomes more and more difficult to improve basic designs in the future, economic and environmental pressures mean that the reasons for radical changes are getting stronger." Fundamental changes may mean the introduction of new materials and shapes, or even the restoration of "old" The propulsion system. One idea of ​​retro propellers is to reuse propellers because they are more fuel-efficient than jet aircraft-even if the "jet" (gas turbine) engine is turning the propeller blades. The disadvantage is that propeller-driven airplanes are usually slower-which Straatsma considers unacceptable. "One of my requirements is to improve comfort, and we cannot do this by extending flight times," she said. Nevertheless, there are still some propeller designs—some with thin, specially curved blades—that can reach the current passenger jet speed of Mach 0.8 (approximately 530 mph). The problem is that the blade must rotate so fast that the tip will generate a very large shock wave. Straatsma said her team will look for ways to reduce this noise, perhaps by lining up two propellers with counter-rotating (opposite-rotating) blades. Scrap metal composites—plastics reinforced by woven fibers—are another approach. They can be as strong as metal and at the same time lighter, thereby reducing fuel consumption. Boeing recently announced its 787 Dreamliner, which is 50% "plastic" by weight. British low-cost airline easyJet recently revealed the design of a short-haul airliner that will reduce carbon dioxide emissions by 50% and may be completed in 2015. But De Haan believes that the composite material is simply replaced by aluminum. In order to take full advantage of new materials, engineers need to "consider composite materials" from the beginning.

If the fuselage is made entirely by winding fibers into the desired pattern instead of splicing metal plates together, "you can reduce the weight by 30%," De Haan said. In addition to being lighter, composite materials provide greater freedom in shaping aircraft, which may benefit aerodynamics. Straatsma said that an all-composite aircraft may not be realized in the short term because composite materials are not as heat-resistant as metals, and some of them absorb moisture-a problem especially at the leading edges of the wings. Other alternative biofuels may be a way to reduce the carbon footprint of air travel. Some aircraft now use 30% biofuel blended with kerosene, which is a common jet fuel. But there are concerns that higher biofuel mixtures may freeze at sub-zero temperatures at high altitudes. Hydrogen fuel may be another option, but it may have to wait for technology that can store this gas in smaller tanks than currently required. Another possibility is to place more passengers on an airplane. The so-called hybrid wing body, in which the seat extends into the wing, can carry 800 passengers. However, there are concerns that people sitting far from the central axis may ride more unstable. It is still unclear whether an aircraft with such a large capacity can be operated on many routes. Even if these fundamental changes prove to be feasible, their full implementation may take decades. This is why de Haan advocates researching policy and lifestyle changes as well. However, he does not necessarily believe that people should reduce flying. One solution might be to save fuel by allowing vacationers to fly in slower and slower planes, and then "start the party on the plane," De Haan suggested. It will not be a flying saucer, but a cruise ship in the sky.

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