Thanks for that link RRR, although much of it went over my head. By mentioning folio 24, I take it you are thinking 'slat noise'? You may be right and I note that the noise propagation is to the rear of the aircraft, which might chime well with my, admitedly very unscientific, observations. I don't ever recall
seeing the aeroplane I had just heard make the noise and surmise it had always just passed. With so many houses in close proximity round here, I don't get to see them very long.
We definitely must be talking about different sounds here. All the aircraft I have noticed making this brief groaning sound are flying the SID's and still below 6000ft.
It is definitely not the spoilers or a config change.. Not quite sure how some of you on the ground seem the know the flight crew are deploying the spoilers? X ray vision?
I'm still convinced it is some weird sound reflection of certain terrain, was hoping for a definitive answer from someone.
Brief groan is what I'm talking about Noland3 and
only one. It is loud enough and sharp enough to be the only thing that calls my attention to that particular aeroplane. It is only unusual noises that make me look up much. Concorde always did, of course.
I always thought SIDs are 'Standard Instrument Departures', but I'm pretty certain the noise is
only from aeroplanes descending and turning to intercept. I think my house is about 14 nautical miles from LHR and typically aircraft passing overhead come off the Biggin or Epsom stacks and are flying roughly north west. To give them 3000ft 10nm from touchdown, they need to be close to 4000ft overhead my house.
You guys will know exactly, but I can see from the ground that all sorts of config changes are going on above me, up to and including - rarely nowadays - wheels down or coming down. I can and have heard throttle up to arrest decent and throttle down to peg 240kts (is it 240 at that distance?)
My sense - as an engineer although not an aviation one - is the word resonance. Something about the noise says a resonant sound to me and since blowing across a bottle neck is exactly that, I can visualise slat or flap movements generating them. Just as they begin to move, new and temporary airsteams 'blow across' temporary cavities - bingo; a resonant oscillation that damps out almost immediately as the device reaches its normal or first operating position. It won't happen again because the cavity is no longer resonant.
I may be talking bo11ocks, but you have to admit it's good bo11ocks!
Roger.