You asked me to proofread, so I'm taking notes and will re-read with care a few times. Only item to pop up is in ADV.HTM, last paragraph - "NOISE"
In there you state: "If anyone is thinking that helicopters make a hell of a row - that is mostly the noise of the engine rather than the blades."
This may be true of some helicopters, but in my experience the obtrusive noise is at the blade passage frequency "chop-chop-chop etc". It's noisy due to the higher pressures involved in most helos. They need to crank their tip speed up closer to the speed of sound than is best for silence since (short of entering the sonic region) they get better efficiency this way.
I think it's the same thinking as we applied to our propulsive ducted fans. There is a design choice between lots of blades running slowly at a coarse pitch and a few blades running swiftly at a fine pitch. Both can meet the coarse air movement criteria.
However, lots of blades at a coarse pitch has more lift component imparting torque to spin the airflow, and we used stationary downstream foils called flow straighteners to recover this torque as a thrust. Properly designed flow straighteners are thrust forward towards the impeller by the flow. In practice we used fixed flan blades of approximately the right section and then tweaked the pitch empirically.
Helos can't easily do this, so to maintain efficiency they have to spin the blades fast rather than slow. Hence the tip speed and the noise.
We had a lot of fun devising slow-running (300 ft/sec odd at the tip) fans that were quiet and adequately efficient.
Click here to see Bedford Scientific Homepage
Actually, there is practically *NO* blade noise on the Brabazon boat. I sailed round Cowes Harbour in it, and I could hardly hear anything - just a very gentle swishing.
This may be because the blades are solid (filled with foam) ?
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