I remember standing on a grandstand at the end of Silverstone's remaining runway a few years ago when a couple of GR7s departed. After taking off, the lead aircraft carried out a circuit followed by a low, fast run down the runway. The aircraft passed by about 75 feet away from where I was standing. About five seconds later a tremendous half second pulse of air sent my prized (free) McLaren cap on a 50 yard flight into new ownership.
This pressure wave was strong enough to knock a certain CFS(H) chap off his feet accompanied by much merriment from his colleagues (except me. I'd lost my bl**dy cap). The wind at the time was calm, so this short term gust was without doubt caused by the aircraft. The surprising thing, apart from the strength of the pressure wave, was the five second gap between the aircraft passing us and the arrival of the wave. Anyone explain?