Remember Blue Thunder, the high-tech police helicopter from the eponymous movie and TV series of the early 80s? The one with the 'Whisper Mode' for silent flight? Well according to an eminent rotorcraft academic, aural stealth at the push of a button might just be possible. He calls it 'acoustic cloaking'.
Video: via Youtube
According to Dr Fredric Smith, professor of aerospace engineering at the Univeristy of Maryland giving the Nikolsky lecture at AHS Forum 65 in late May, there are two main kinds of rotor noise. Blade-vortex interaction (BVI) noise is the annoying one, caused by'each blade slicing through the tip vortex shed by the preceding blade. BVI noise is directed downwards and best mitigated by modifying the approach to put the vortices above or below the rotor.
The other main source is thickness noise, 'which is caused by the blade displacing the air and is directed in the plane of the rotor. This is the one that makes helicopters detectable at a distance, and the one you need to get rid of if you want a stealthy rotorcraft.
Prof Smith reported on recent tests that showed the shape of the acoustic pulse from thickness noise could be modified by controlling the drag force on the blade as it rotates. Using trailing-edge devices on the blade to control drag it's possible, at the point where the pulse is directed towards the observer, to generate an anti-noise pulse to cancel out the sound from the rotor. 'It's basically a beam,' he said. 'You can kill the noise in a certain direction.'
It's still early days in the research, nothing has been flown and the impact of drag control on rotor performance has not been established, he said. Also the technique is likely to send noise in other directions. But with helicopter manufacturers experimenting with active rotor control using on-blade flaps and slats, 'acoustic cloaking' could yet emerge as one of the key benefits."