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Back to the Propeller?
How the "Propfan" can change air travel
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In the 20th Century, commercial jets made it possible for everyday people to travel the world like never before. With each new iteration of aircraft, engineers made them progressively safer while reducing fuel consumption and emissions. But we may be approaching the limits of traditional jet engines. The future of jet aircraft may require looking to the past.
How a Jet Engine Works
Put very simply, jet engines work like any other combustion engine. Air enters the front end, compressor blades compress that air and direct it into a combustion chamber where fuel is added and the mixture ignited. This mini-explosion forces the exhaust gasses out the back of the engine, driving turbines connected to the compressor, and generating forward thrust in accordance with Newton’s Third Law of Motion.
Since their invention in the 1930s, jet engine technology has steadily improved in the pursuit of higher thrust, better efficiency, and safety. Compressor and turbine blades, for example, are now made of exotic alloys, precisely shaped, often air-cooled, and coated with ceramics to make them lighter and ever more tolerant to heat and stress.
Commercial jet engines, which operate in the transonic realm, have also evolved increasingly large fans in front of the compressor. This is because there are two ways to generate thrust, one is to increase the velocity of engine exhaust, and the other is to increase the total mass flow of air forced out the back of the engine.
The latter is more efficient for commercial airliners that need to minimize fuel consumption. So modern engines use a large frontal fan to push air around the engine, what is known as a bypass. The fraction of air that bypasses the engine is the bypass ratio. In a high bypass engine, known as Turbofan engines, it is the fan, not the engine itself, providing most of the thrust.
The higher the bypass ratio, the more efficient the engine can be. The problem is, the larger these fans become, the larger and heavier the nacelle that encases the engine must also become. The future may require revisiting an old concept that, ironically, resembles a propeller; the Propfan.
The Propfan engine was a concept that nearly entered production in the 70s and 80s. The propfan moves the fan from inside the engine nacelle to the outside. This saves weight by reducing the size of the nacelle as it no longer needs to encase the fan. It also allows the fan to be built larger, improving the overall bypass ratio.
The concept works, but Propfans haven’t caught on for two reasons. One, to keep up with traditional jet engines, they typically use two contra-rotating fans, one behind the other, instead of a single fan. This maximizes efficiency but also requires a complex gearbox that poses safety and reliability concerns. Second, because the fans were no longer encased, they were much noisier than a Turbofan engine.
As a consequence, despite the fuel efficiency gains they offered, the Propfan was largely forgotten. In the last few years, however, Propfans have gained renewed interest. Engineers have figured out how to shape and space the blades so as to minimize the noise problem. They claim that new Propfans will meet all current and future anticipated noise regulations.
The CFM Rise engine concept (pictured above) aims to test a working prototype Propfan in the latter half of the 2020s. The hope is to demonstrate the viability of Propfan technology for use on a jetliner after 2030. The RISE concept, notably, eliminates the second fan entirely and replaces it with viable pitch stators. This is potentially transformative.
By replacing the second fan with stators, engineers can still increase the fan pressure ratio, but further curtail noise and also eliminate the need for a complex gearbox. It is hoped that the RISE concept will prove the safety and viability of Propfans, while reducing fuel consumption and emissions by up to 20 percent compared to the latest Turbofans.
Why it Matters
All human progress is the expansion of capability; the ability to do more with less. Propfans offer the potential to allow planes to fly further, conserve precious fossil fuels, and reduce environmentally damaging emissions. If combined with new wing designs, such as Boeing/NASA’s TTBW, the next generation of jets could use upwards of a third less fuel than their contemporaries. Opening the door to more affordable and sustainable travel.