How open-rotor engines could change the shape of short-haul passenger airliners
Airbus
Airbus A30X could be the future replacement for the familiar A320
Take a short-haul flight anywhere in the world and the chances are you’ll fly on a twin-engined jet made by either Airbus or Boeing. Thousands of Airbus A320-family and Boeing 737 airliners are in service, providing reliable, safe and efficient inter-city travel.
But is the familiar layout – two turbofan engines mounted below the wings – the most efficient design for an airliner? Or is there a better way to build a short-range 100-to-200-seater? Many in the aerospace industry believe there is – and that the future involves not jets but a form of propellers.
We’re not talking about the clanking piston-engined propliners of the past, with evocative names like Lockheed Constellation and Boeing Stratocruiser. And we’re not even talking about modern commuter turboprops such as the Bombardier Q400 or ATR-72. But pressure on airlines to cut operating costs is sparking a revival of interest in a concept that first emerged in the 1980s – the unducted fan (UDF), or “open rotor” engine.
These engines are a close relative of the turbofans used on aircraft such as the A320 – but instead of the fan being shrouded inside the engine casing, it sits outside the main turbine. This allows a larger diameter of fan to be used, increasing the engine’s propulsive efficiency – more fan, less jet.
Rolls-Royce
Rolls-Royce RB-2011 unducted fan engine is in development
The idea returns
Why has the UDF engine come back on to the radar? Quite simply because it uses considerably less fuel than an equivalent turbofan. Research by UK aerospace trade body SBAC claims a UDF engine would offer a 25-30% reduction in specific fuel consumption compared to a typical turbofan of today. At current fuel prices, this equates to a saving of around $3 million worth of fuel per aircraft, per year, according to engine maker Rolls-Royce, which is working on a UDF engine called the RB2011.
The airlines like the concept. Fuel prices are the single biggest element of their operating cost base – 40% for major carriers and 60% for low-cost carriers – so savings of 25 to 30% would be welcome.
Indeed, one low-cost carrier, easyJet, is so keen on the concept that it recently devised its own concept of a future short-haul airliner based around the UDF engine. Called the easyJet ecoJet, it was first shown in 2007.
Mark Bursa
EasyJet’s EcoJet proposal could cut airliner CO2 emissions by 40%
A new way of working
It combines UDF engines with a radically different airframe. Engines are mounted at the rear of the plane rather than under the wings. The ecoJet also features swept-forward wings, designed to get the aircraft off the ground quickly, while allowing a cruising speed that’s almost as fast as a conventional jet.
As well as cutting fuel consumption, the ecoJet promises a 40% cut in CO2 emissions thanks to the UDF engines and the lighter weight of the aircraft, which would use carbon composites and other lightweight materials. The ecoJet would generate less than 47g of CO2 per passenger km, easyJet says. That’s about the same as two people travelling in a Toyota Prius hybrid car. And it doesn’t just cut carbon emissions. Harmful nitrous oxide (N2O) emissions would be 75% lower, easyJet claims.
OK, it’s easy enough for an airline to come up with a “dream airliner” like ecoJet. But the concept is based on a firm foundation. Both Boeing and Airbus have revealed concept aircraft that look remarkably like easyJet’s idea. As recently as June 2012, Airbus released images of a proposed A320 replacement, called A30X, which features the same rear-mounted open-rotor engines.
Airbus
Airbus is also considering regular turbofans to power the A30X
Testing is under way
Airbus plans to flight-test a UDF engine on an A340 flying testbed within the next couple of years as part of a €1.6bn European research project into green aviation technology called Clean Sky. Beyond that, a prototype A30X could be flying before the end of the decade. Airbus is hedging its bets – it also revealed images of the A30X with conventional turbofans, again mounted at the rear of the fuselage rather than under the wings.
This is sensible - clearly the engine makers aren’t going to abandon the turbofan – indeed, more efficient geared turbofan (GTF) engines will be available on the latest versions of both the A320 (A320 neo) and Boeing 737 (737 Max).
GTF engines add a gear that effectively decouples the fan and the turbine – so they don’t have to turn at the same speed. In fact the turbine can spin up to three times faster than the fan, so a simpler and more efficient turbine can be used. A fuel saving of 12-15% is promised as a result. The upside for the aircraft manufacturers is this can be achieved without major redesigns to their airframes – the A320 neo looks pretty much the same as a current A320.
The A320 neo and 737 Max are significant upgrades to the current models, but ultimately they are stop-gap designs that give the airlines an improvement in fuel efficiency, but not the sort of step-change that a UDF-powered airliner could offer. Both Airbus and Boeing say all-new next-generation short-haul airliners would have to be ready by 2030 at the latest, more likely around 2025.
AP
We've come a long way: A 1929 Ford Trimotor, one of the most successful early airliners
Different strokes
And that’s the sort of timeframe that could see UDF-powered aircraft become a reality. They require a different design to either the A320 or 737. You can’t hang the engines under the wings, as the diameter of the open-rotor fan is so much larger. That’s why both the A30X and the ecoJet have the UDF engines mounted above the rear of the fuselage instead. Positioning the engines at the rear of the plane also cuts cabin noise, which is a problem with current turboprops with wing-mounted engines.
Rear-mounted engines also help solve any safety concerns over the open rotor concept. If a fan blade breaks loose in a conventional turbofan engine, it will be contained within the engine casing. But if an open rotor fan blade were to become detached when the engine is running, it could fly off at high speed with disastrous consequences, puncturing the fuselage or damaging the aircraft’s control surfaces.
Mounting the fan at the very back of the airframe minimises this risk. When the aircraft is travelling at speed, any detached blade would be most likely to fly off behind the aircraft. The probability of a blade becoming detached is “extremely remote” – in the region of one failure per 100 million flying hours, according to SBAC.
The other obstacle to UDF engines is noise. Engine makers Pratt & Whitney and GE both tested UDF engines in the 1980s mounted on conventional Boeing 727 and McDonnell Douglas MD-80 airliners, but while they worked well, they were very loud.
Burkhard Domke
GE tested open-rotor engines in the 1990s – noise was a problem
However, advances in rotor design since then seem to have solved those issues. GE has tested open-rotor engines in wind tunnels and claims the noise performance was “outstanding”, saying the engines would be able to meet forthcoming Chapter 5 noise targets, which are due to be implemented around 2020, with enough headroom to meet any future rules beyond that timeframe.
Finally, a UDF-powered airliner would be a little slower than a conventional turbofan equivalent. Cruising speed would be around Mach 0.7, against between 0.75 and 0.8 for, say, an A320. This would add around 10 minutes to a two-hour flight, which could easily be made up in reduced time on the ground – such as less time needed to refuel.
The UDF concept has been in and out of fashion over the past three decades. But with renewed interest from aircraft makers, engine makers and airlines, and Airbus itself predicting a doubling of the number of airliners in service from 15,000 at present to more than 30,000 worldwide by 2030, its time could finally be here
http://innovation.uk.msn.com/design/back-to-the-future-new-planes-youll-be-travelling-on-soon
