Two aircraft engine concepts, geared turbofan (earlier post) and open rotor (earlier post), can enable a significant reduction to aircraft fuel consumption. With open rotor, the potential reduction is up to 15%, while the geared turbofan could provide reductions of up to 4%, according to Linda Larsson at Chalmers University of Technology in Sweden.
Larsson recently defended her doctoral thesis at Chalmers, where she has been studying the two promising aircraft engine concepts.
The two concepts have a propulsive efficiency; in other words, the energy generated by the core engine can be efficiently converted into thrust.—Linda Larsson
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.
An open rotor engine 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.
The open rotor concept was studied in the 1980s after the oil crisis and resulting fuel price hikes, and 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 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 and Berlin. A method to represent propeller performance was needed in order to do the calculations. Models of this type already exist in the industry but only for specific designs, which means they are not generally accessible. Larsson instead produced a method that can be used freely and widely and that can be re-scaled as more data becomes available.