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Maximum propeller diameter

  1. Oct 16, 2011 #1
    Ok i'm trying to work out a few things with propellers but I am struggling to work out if I have got this right.

    If I have an 1800 shp engine running at about 2000 rpm and I want my aircraft to have a maximum speed of approximately 350 mph what is the maximum diameter I can have the propeller. I have read in a few books that it is best to keep propeller speed between 0.7 and 0.8 Mach.

    Working back from this I have calculated the diameter to be 78.5 inches however I know the spitfire used to run at 3500 rpm with at prop diameter of 10ft so I am assuming that I have done something wrong in my calculations.

    Any help would be appreciated.
     
  2. jcsd
  3. Oct 16, 2011 #2
    The Merlins or Allisons used in Spitfire or Mustangs had a reduction gearbox that I think was about 2.5-1 So while the engine might have seen say 3500 rpm the prop was only turning at 1400 or so.

    The rpm/mach number on your smaller prop actually sounds correct Might even be a bit on the high side. While you loose efficiency about all that happens is the prop "snarls". It gets loud as it breaks the mach. A Cessna 185 has a 310hp engine. When taking off it makes the usual noises until you have it full throttle and the prop tips bust the mach and then it gets a bit loud.
     
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