Aircar
Well-Known Member
This is probably one for Jan Carlsson -- the biggest defect in the normal propeller is the bloody noise -- the little data I have seems to indicate that it mostly originates from the tip vortex (and this seems to be the case with helicopters as well) natural enough. The thought occurs that if the tip vortex was eliminated then possibly so would be a lot of the noise . The Cl (and area) at the tip of the blade determines the strength of the tip vortex as much as anything else --the blade 'lift' in effect . So the thought is to see if something like the bell shaped Horten lift distribution could have an advantage for propellers --to untwist the last bit of the tip and likely also reduce the chord to not drag too much 'dead' blade at the high speed outer end . Sacrificing some efficiency would be acceptable in the name of humanity (less noise) I guess the first test of this basic idea is to build a propeller with NO pitch and see how much noise it still makes (maybe Jan has done this already?) If there was a relationship between the 'activity' of the tip and the noise created such that extending a blade tip at no pitch but small chord was not too bad for efficiency but was much better for noise then it might be done by reprofiling a given blade experimentally even . Like to hear some thoughtful comments or any experience that could bear on it.
