----- Original Message -----
    *From:* Russell Duffy <mailto:russell.duffy@gmail.com>
    *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
    *Sent:* Sunday, September 17, 2006 6:12 PM
    *Subject:* [FlyRotary] Re: ducted fan questions

    ing a ducted fan(not a shrouded propeller).           Now that's just way too cool Richard :-)           Was the fan running directly off the engine, with no redrive?  Did
    you feel that the performance was better than it would have
    been with a traditional redrive and prop?  If I get serious about
    trying this, I'll certainly take you up on your offer to share
    info.            Cheers,
    Rusty
              There are plenty of ducted fans, look at the bypass stage of every
    turbofan engine. The difference between a ducted fan and a
    shrouded propeller is the blade solidity. This is the ratio of the
    fan cross section to the total blade area. A turbofan is around
    "1" and a shrouded propeller, I have seen, may be as low as .1 or
    less.
    My fan with the KAWA engine was running reduced at 3400RPM, in
    order to allow for more solidity with the benefit of lower noise.
    When I changed to direct drive with adjusted solidity at the same
    HP, the fan efficiency went from 70% down to less then 50%, and
    the noise was almost unbearable.
    The fan I flew had six blades, and the test fan on direct drive
    had three blades, which were smaller too.
    BTW the diameter was 26".
    The conclusion is, if you aim for a solidity of 1, or close to 1,
    for a given hp, there is only one fan size and RPM. Now, what
    compromises can be made to arrive at a practical design? Your
    imagination, and expectations,  are the only limits.
    My 90+hp design has nine blades at 26", giving a solidity of .6 .
    This is not very good, however, I do not want to make a new shroud
    and I also have a given max RPM, which locks me in. Nevertheless,
    the data would be as good as any, knowing the deficiencies.
         So much for now.
         Should we discuss this on another E-Mail? I don't want to bore our
    rotor head friends.
         Richard Sohn
    N-2071U