Anyone think open rotors are for real this time round? By open rotor I mean propfans, unducted fans – engines with the fans on the outside. Engine manufacturers are again talking about open rotors, but as part of a long-term technology plan that begins with today’s turbofans, advances through ultra-high bypass engines (those with the fans on inside, like Pratt & Whitney’s Geared Turbofan) and finally gets to open rotors around the end of next decade.
But airlines like easyJet, with its “eco-friendly” aircraft design, are pushing for open-rotor engines by 2015. And NASA, which has begun developing technology for a 2015-timeframe next-generation narrowbody, says it has had requests to revisit open rotors.
Propfans came close to a launch in the late 1980s, but fuel prices were too low. This time round the driver is emissions and the environment.
Because of the work done in the 1980s, manufacturers could probably launch development of open-rotor engines today, with today’s technoplogy, and deliver substantial reductions in fuel burn – maybe as much as 30% – and therefore in carbon emissions. The problem is noise. Back in the late 1980s the best NASA thought was possible for a propfan was Stage 3, but new aircraft now have to meet Stage 4 – another 10dB reduction in allowable noise.
It is clear open rotors will not meet the noise limits without some help – aircraft configurations that provide noise shielding, or active noise-cancellation techniques – that would take time to develop and would probably push propfan-powered airliners beyond the 2015 horizon. That would likely mean waiting until the next next-generation narrowbody, which airlines like easyJet might not be too happy about.
But the current clamour over aviation’s contribution to global warming could abate, and ultra-high bypass engines (with the fans inside) could deliver substantial enough fuel and emissions savings, combined with lower noise, and mollify the airlines. It may be premature to fire up the propfan, but best to dust off the old reports and take another look.