The Future of Jet Engines May Be, Ironically, Propellers

A new generation of open fan turbine engines promises greater efficiency for airliners.

Propellers driven by internal combustion engines were the first airplane propulsion technologies and powered the Wright brothers’ first flight in 1903. While still used in smaller aircraft, most large modern commercial airplanes use jet engines to achieve higher speeds at greater altitudes than it is possible to achieve with internal combustion engines.

Turboprops—jet engines with propellers—are popular for smaller airplanes and large cargo aircraft, but a new generation of open rotor fan engines that combine some of the benefits of high bypass ratio turbofans and turboprops promise to revolutionize large aircraft design. CFM International and Airbus have announced a plan to test this new technology under the RISE program and aim to achieve a 20 percent reduction in fuel burn, with a similar lowering of CO2 emissions.

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When the Wright brothers took to the air in 1903, their flimsy craft was driven through the air by a primitive gasoline engine spinning two propellers. This method of gaining thrust made obvious sense to anyone who had seen a ship’s propeller, and the popular but erroneous notion that propellers drag an airplane through the air in the way that a wood screw penetrates a two-by-four was adequate for the fabric-covered biplanes of 110 years ago.  

Refinements of this basic technique still exist in smaller aircraft today, but by the end of the Second World War, turbojet engines promised unheard of speed and altitude capabilities for military airplanes, and civilian airliners soon followed.  

Propellers were eventually married to turbojet engines to create the turboprop, but for large airliners, the speed and altitude advantage of pure jets was insurmountable. In an age with few environmental concerns and two-dollar-a-barrel crude oil, other factors simply didn’t matter—but the propeller never really went away. For applications where range and payload were more important than speed and altitude, aircraft like the ubiquitous C-130 Hercules have been in production for 65 years.  

Today, a variation of the turboprop principle is under development as an alternative to turbofan engines for airliners, and it promises to deliver better fuel efficiency and lower emissions than current technology.  

CFM International, a joint venture between General Electric and Safran Aircraft Engines, has announced an agreement with Airbus to flight test a new open fan propulsion system as part of CFM’s Revolutionary Innovation for Sustainable Engine, or RISE program. The test engine will replace one inboard engine on an A380, mounted on a highly modified pylon. Rotor diameter will be 13 feet.  

While open rotor and unducted fan engines have been in development since the 1980s, the RISE system is substantially different. The core engine will be more compact and operate at higher pressures and will use a recuperator to recover waste energy from exhaust heat by preheating combustion air. Outside the core, the technology is also substantially different. Although it looks like a classic contra rotating assembly, the second stage blades are fixed, though the stators are variable pitch with active control. They act as flow recovery vanes and with active control, can increase effective fan pressure ratio and control rotor loading.  

Like modern very high bypass turbofan engines, the rotor will be connected to the turbine through a reduction gear set. Composite materials will be used extensively, including ceramic matrix composites in the hot section. But what will be gained?  

Engineers are aiming at a 20 percent reduction in fuel consumption and, of course, proportionate lowering of CO2 emissions, with thrust of up to 35,000 pounds that is competitive with current geared turbofans.  

It’s been about 60 years since passengers in large airliners looked out the window at spinning propellers, but a new generation of open rotors powered by gas turbines may be the answer to fuel burn and CO2 emissions issues, with turbofan speed.

Written by

James Anderton

Jim Anderton is the Director of Content for ENGINEERING.com. Mr. Anderton was formerly editor of Canadian Metalworking Magazine and has contributed to a wide range of print and on-line publications, including Design Engineering, Canadian Plastics, Service Station and Garage Management, Autovision, and the National Post. He also brings prior industry experience in quality and part design for a Tier One automotive supplier.