Originally Posted by Trackmaster
Could it have been a V-22 Osprey.
Big blades whacking through the air?
Big blades do not turn that fast. Too keep the tips going supersonic this fan had to be small... blade length of no more than 20" to 24"... from the sound the blades were really spinning fast.
If the AD 150 was a single fan aircraft I could see it looking like.... also AD 150 (sorry they won't let me post the link, it's on a Wiki
but it's top speed is 300knots+
If this was in the working model era I would have thought that would be what I heard.
Bit more explanation: What I heard would have been single fan. My speed guesstimate is based on the unblocked (except for trees) distance I heard it in, V transit time. Transit time is a guess, absolutely no more than 15 seconds. Just long enough to hear something "different", and put all my attention to it to try identify it, before a ridge blocked the sound.
Very high speed fan, my guess is that was passing about 1000' below and a mile south of my cabin. (I live on a mountain). Most definitely a single sound source, or a PERFECTLY synchronized dual sound source. (single engine driving two identical fans?)
To try to show why I say a single sound source, think what you hear in the case of a Kiowa verse the sound of a Chinook. Only problem with it being a known prop aircraft, VTOL, or a helicopter, is the transit time, Way too fast for anything I know of that is prop/fan driven short of a WWII fighter or equivalent. A 421 Cessna isn't even that fast (and we have one of those that does fly around here.)