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Analysis suggests open rotor engine could reduce aircraft fuel consumption by 15%

27 May 2014

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.

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Open rotor has huge noise problems, GTF doesn't.

GTF is in production now and will be on the A320Neo and Mitsubishi MRJ in 2015.

They used to say that Open rotor would be 30% better than current 1980's state of the art, but I suppose that has moved on a bit if it is now only 15% better.

If there is only an 11% difference between open rotor and GTF, it looks like curtains for Open rotor.

The advantage is already a long known fact. The issue is the demand side.

The open rotor ("propfan") engine's noise issues would be greatly reduced in some of the airframe designs kicking around which put the engines above the rear of a blended wing.  Eliminating most of the fan shroud and its drag would be an advantage that no ducted fan could beat.

@EP, you can see the designs with google(open rotor aircraft designs) but I feel we may be waiting a long time to see them as the airliner industry is so conservative.

They said that they wanted a 15% fuel economy increase, and the airframers offered it with winglets and latest generation engines (A320Neo and B737max). The airlines and lessors ordered about 5000 of them (2600 A320Neo and 2000 B737) in 2-3 years. That is about $4 trillion's worth of business - for a 15% improvement!

Then Airbus scrapes a further 3% improvement out of the A320 by adding an extra row of seats - they do this by fitting thinner seats ! (So simple, why did it take so long ?)

Also, the western airframers are maxed out in term of engineering resources, struggling to launch all the updated versions they want to (A320Neo, A330Neo? A380Neo ??) on top of the A350 completion and -1000 variant.
They havn't the engineers (or money) to build a 90 seat turboprop.

Maybe the Chinese will do it, but I imagine they will get their current generation designs (C919 + widebody to come) out first and life with them for a while before tying anything fancy.

But you never know what motivates people (like national pride) and the Chinese might give it a go and then you might see a propfan in 10-15 years.

Sorry - about $400 Billion, not $4 Trillion - still a lot of money.

China may soon start to build 1000+ Bombardier Q400E with up to 90 passengers, for the local market.

The Q400E is one of the most efficient commercial airplane in use and will be more so in about 2-3 years with new turboprop engines.

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