Green skies: How engineers are developing the more efficient aircraft of tomorrow

6 min read

Aviation and air travel are riven by disagreement and difference, with various perspectives and interests driving diverse and often contradictory actions. While many want to bolster the proliferation of the commercial aviation industry to help drive economic growth, others level ever-greater criticism of its environmental impacts.

Despite the amazing technological feats of pedal-powered and solar powered aircraft being researched and flown with great success, this remains novelty engineering and it is unlikely to offer any kind of alternative to the industry's current predicament. It is much more likely that engine technology and aircraft structures will continue evolving, with airframes and engines not looking too dissimilar to many of today's aircraft, but with an environmental impact that is, perhaps, unrecognisable. An average transatlantic flight can consume 60,000 litres of fuel – more than the average motorist will use in 50 years. However, this should be qualified. For the moment large gas turbines are the most efficient machines we have for converting fuel into mechanical power, sometimes at over 50% efficiency. In addition, some aircraft carry over 500 people, meaning that per-person efficiency is actually very good (a car would need to do around 100-120 miles per gallon to be equivalent). Equally, despite the aviation industry continuing to lower fuel burn by about 2% per year over the last 50 years, the sector is the planet's fastest growing CO2 producer. This, the industry says, must change. Be they environmentalists, industrialists, businessmen or consumers, most people want to see the same thing; quieter, cleaner aircraft that ideally produce zero-net CO2. That goal may well be met in the future with biofuels created from organic matter. Drop-in, bio-based alternative fuels can create kerosene equivalent fuels and chemical scientists continue to try to make this process both consistent and cost effective. In the shorter term, however, industry is looking to engineers to offer solutions. One of the most anticipated breakthroughs comes from classic mechanical territory in the form of a highly efficient planetary reduction gear. US engine maker Pratt and Whitney has been developing the concept for some 25 years. Turbofan engines use a 'bypass ratio', the air flowing through the fan disks (which bypasses the engine) against the air drawn in to the engine core for combustion. At the moment the very best engines are up to 8:1 or 9: 1, but they are reaching a limit. One of the limiting factors is that the engine shaft transmits power directly to the fan blades, i.e. they rotate at exactly the same rate. "You can't fool Mother Nature," says Dr Alan Epstein, Vice President of technology and environment at Pratt and Whitney. "Mother Nature, aka thermodynamics, tells us what we have to do. It is clear that a gear is a good idea in terms of changing the speed ratio between the core of the engine that makes the shaft power, and the fan that propels the aeroplane. But, in practical terms what that means is, the gear cannot weigh very much or cost very much, and must be very efficient. So, for a medium-range narrow-body aircraft, the gear is about 0.5m in diameter, weighs about 100kg, but transmits about 25MW at over 99% efficiency." The gear added to a conventional engine improves fuel burn by 2%. The larger fan blades have a slower rotation and this reduces losses and also reduces the aircraft's noise footprint. The next technical challenge for Pratt was building the ultra-lightweight fan blades necessary for further efficiency gains. "We thought they would be composite, so developed composite fan blades for it," says Dr. Epstein. "But, it's not. It turns out it's a type of hybrid metallic that is lighter than composite and much less expensive." The structure of the engine means a higher bypass ratio can be achieved and therefore a bigger fan can be used, which needs to sit in a nacelle. The nacelle needs to be very lightweight and also have low drag so that it doesn't overwhelm any performance advantage gained from the increased size. The result is a thin, low-drag, lightweight design that incorporates a variable area nozzle. The fan blades and nacelles both offer a further 2% fuel burn saving each, but instead of the sum of parts offering a total of 6% improvement, optimisation of the entire system means the engines offer some 16% improvement of fuel burn over predecessors. Dr. Epstein adds: "The bad news is, if the engineers do not deliver – if one of these technologies doesn't actually work to the degree that we need – then we don't have 16-2 = 14% fuel burn improvement, we have 4%. And the difference between 16% and 4% is an utter disaster." Both the major large commercial airframe manufacturers, Airbus and Boeing, have offered their designs, which are likely to be flying for at least the next 20 years. The Airbus A350 XWB and Boeing 787 Dreamliner are both highly-optimised technical marvels that use the latest materials and technology to make them the most refined aeroplanes ever flown. That said, however, take off all the logos and to most novices they look the same, with slight differences in terms of wing sweeps, wing tips and the cockpit, but the essential layout is the same. Furthermore, take off the engines and, size apart, there is not a dramatic difference between them and a 747-400 or even the first Boeing commercial airliner, the 707. The aircraft industry is by nature conservative. It has to be sure of its next step, so radically different airframe designs, such as flying wings, boxed wing sections or even multiple fuselages, are probably a long way off – if they ever arrive at all. Airbus has recently come up with its 'Concept Plane' design which shows what aircraft could look like in the distant future, somewhere between 2030 and 2050. The Concept Plane is quite distinct. First, it has much longer, more slender wings than today's aircraft. These will allow the aircraft to glide more easily and reduce fuel burn significantly. The engines are also semi-embedded into the aircraft structure. These are placed toward the rear of the aircraft to reduce cabin noise and the integration of the engines is made possible because the increase in reliability will mean that engine access does not need to be as regular as it does at present. In addition, the exhaust is on the top of the tail section to reduce the noise footprint on the ground below. Boeing did have fairly radical plans for the future and carried out numerous studies in to a Blended Wing Body layout. It has even flown a prototype called the X-48B, which does without a conventional tail or rudder, and instead uses 20 control surfaces on the trailing edges of the wings and rudders on the winglets. However, Boeing found that the layout was not popular with passengers, so dropped the design for a passenger aircraft, but left open the possibility of using it as military refuelling tanker. Dr. Allen Adler, vice president of enterprise technology strategy at Boeing, says: "We see beyond the materials we are using right now and understand that they will enable us to take on new designs. If anything is going to lead to a revolutionary [airframe] it is going to come from a materials breakthrough. But things spend a long time in a laboratory before being incorporated into an aeroplane." In the nearer term, the company is undertaking a programme called the ecoDemonstrator, in which it aims to trial and commercialise various technologies that can be quickly rolled out to help reduce environmental impact. The chevrons that are commonly seen on engine nacelles were the result of a similar programme undertaken in 2001, and go a significant way to reducing noise. Jeanne Yu, director of environmental performance at Boeing, says: "The goal of the ecoDemonstrator programme is to accelerate integration of these technologies for more fuel efficient, quieter, cleaner, more advanced solutions for the future. This helps us incorporate these technologies more rapidly." It begins a 45-day test programme later this year to trial different fuel-saving technologies. Active engine vibration control systems are to be fitted that could potentially cancel out any vibrations created at low engine speeds. At the moment, many pilots have to throttle up to stop engines vibrating unduly on approach to a runway. This wastes a quantity of fuel that, although fairly insignificant on the individual flight, multiplied over many years in fleets throughout the world, the savings becomes significant. Mechanical engineers have also come up with an ingenious mechanism to adjust the trailing edge of the wing to give optimised aerodynamic characteristics at different phases of flight (which are at the moment fixed). However, perhaps the most interesting is the variable area fan nozzle which adjusts the size of the engine's exterior nozzle to again allow it be optimised for certain flight conditions. An onboard fuel cell will also be trialled as an onboard power system. It hopes all these technologies will soon be rolled out. With its parent company EADS, Airbus is also making small but technologically significant strides to improve its footprint with rapid manufacturing technology. Jean Botti, chief technology officer of EADS, says: "Additive Layer Manufacture (ALM) is a 3D rapid manufacturing technology for engineering not just for research. We can build parts that are very complex. This is not for high-volume, so is quite well suited to our industry which is about lower volumes. We can make very complex parts that optimise the weight. "The machines exist, we are keeping an eye on the alloys and materials themselves but we can save a lot of weight and have much less wasted material so we see this as a key development." It is crucial for the aviation industry to develop cleaner technologies so it can sustain its current activities but this also offers tremendous opportunity in wider engineering activities. The aviation sector is pushing at the moment for breakthroughs. And like many of its previous innovations, the trickle-down of technologies to other sectors and industries is almost guaranteed. It is likely the implementation of new materials, power transmission and even rapid prototyping technologies are likely to find other applications in many other sectors.