Linda Larsson recently defended her doctoral thesis at Chalmers University of Technology, where she has been studying two promising aircraft engine concepts. Of the two, the propeller or dual propeller open rotor can save up to 15% and the geared turbofan up to 4%.

Over the past 40 years the average annual increase in passenger miles traveled by air has been 5.8% and fuel sales have increased by 2.2% annually. New technological solutions are needed if aviation is to reduce its fuel costs.

Dual Propeller Counter Rotating Open Rotor Design Illustration.  Click image for the largest view. Image Credit: Linda Larsson, Chalmers University of Technology.

Dual Propeller Counter Rotating Open Rotor Design Illustration. Click image for the largest view. Image Credit: Linda Larsson, Chalmers University of Technology.

Larsson is revisiting the open rotor concept that was studied in the 1980s after the oil crisis and resulting fuel price hikes. It was apparent already then that it worked. However, fuel prices dropped and the technology lost its appeal.  It is now starting to get noticed again. Larsson’s thesis makes the modern case.

Larsson said, “The cost of fuel is one of the biggest costs faced by airlines, and there are also obvious environmental reasons and political incentives to reduce fuel consumption and emissions. A great deal of research is being conducted on the open rotor concept right now, and I believe the concept could be in place around 2030.”

“Both of the concepts demonstrated very good potential in terms of reducing fuel consumption: geared turbofan by up to 4% and open rotor by up to 15%. Naturally, though, the technology has to be developed and implemented,” she said.

An open rotor engine design generates most of the thrust from two counter-rotating propellers instead of a ducted fan. This enables a larger engine diameter and thus a higher propulsive efficiency, without resulting in excessively large and heavy engine nacelle.

What differentiates a geared turbofan from a regular turbofan is that the large fan at the front of the engine operates at a lower speed than the turbine that drives it. There is a gearbox between the turbine and fan that reduces the number of revolutions, which enables a lighter turbine and a higher turbine efficiency.

Larsson’s study of the open rotor concept was conducted on a small aircraft that is a possibility for the future; the small aircraft would operate short routes such as between Gothenburg, Sweden and Berlin, Germany.

A method to represent propeller performance was needed in order to do the calculations. Calculation models of this type already exist in the aircraft industry but only for specific designs, which means they are not generally accessible. Larsson has thus produced a method that can be used freely and widely and that can be re-scaled as more data becomes available.

That could make the dual counter-rotating propeller open rotor design concept possible a few years down the road, but geared turbofan applications are not that far away.

“I have studied how great the potential of the architecture itself is. I have not studied any specific models from manufacturers, but aircraft using the geared turbofan concept will be available in the market as early as next year,” said Larsson.

Air travel is enjoying a remarkable fuel and efficiency improvement trajectory into the future. Even though the passenger mile efficiency is already quite good and the nature of air flight vs. automobiles over the road requires higher capital costs with much longer life spans the business will continue to improve because the skill set needed for operation isn’t likely to ever allow everyone to have a pilot’s license.

Most planes will fly with enough people to be efficient on a fuel per passenger mile basis well into the future. If you’re in need of speed or to cover vast distances this is a very good thing.


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