He mimics the sound of a 55-metre-long rotor whirling round a windmill 's mast .
他模拟的这种声音是由一种55米长的旋翼在围绕风车桅杆旋转时发出的。
From the surface it looks like nothing more exciting than a stationary grey barge ; but below are two spinning turbines , known as foils , which look like a whirling carbon-fibre double-helix .