Quote:
Originally Posted by freebeard
I see a lot of Prandtl influence, with lots of pitch at the hub with washout to zero at the tips.
More blades allow slower rotation? They really sell the sound signature.
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Slower rotation keeps the tips subsonic and quiet, but it does not chop up the column of air passing through the prop disk finely enough to push on all of it with only two or three blades. Using such a coarse pitch puts a lot of totally wasted spin on the propwash - this would be a good place for contra-rotating props, if they don't have new sound issues.
Fan jets are basically multi-bladed turboprops with tip shrouds. Every time they brag about better mileage, the size goes up, and the wasted momentum in the exhaust is reduced.
Attached is a picture of a prop showing the correct pitch all the way across. The rate of change is highest at the hub, and it never goes flat at the tips.