Conference vmszoo::rc

    
    
        Instead of a 2 or 3 blade large diameter prop, Ducted fans use
    multi blade small diameter "props", therefore, even though they are
    spinning twice as fast, the tip speeds don't get excessive. I think the
    prop tip speeds of a RC model typically don't get much above the 400
    -500 mph range, so there is room avialable for the ducted fans to
    really spin up before going supersonic.
    
        Another example is a full size Heli. In forward flight ie; 200 mph,
    the tip speeds of the forward moving (large diameter ) rotor can approach
    supersonic ( and creates a barrier to making 500 mph helis ), where as
    there is usually not much danger of having the prop speeds of a piper
    Cherokee ( small diameter ) flying at 200 mph approach the same range.
    
                                                                DW2

    Re: -1
    
    Thanks Dan, that makes sense.
    
    How does the number of blades affect propellor/fan efficiency?
    
                                              
                                                              Thanks,
    
                                                              Jim

    Multi blade props are probably less efficient in most cases.  The main
    reason for multiple blades is to keep the diameter down for a given
    disk (wing) loading.  This is necessary for prop clearance, and for
    keeping tip speeds reasonable.
                Control Line Speed ships use single blade props!
    
    	One problem with multiple blades is that one blade can start
    running into the wake of the previous one.
    
    Charlie
    

    
    It's interesting to consider the Harvard trainer (full size!). I
    believe you call in the Texan in the States?
    
    One of these at takeoff power generates an incredible howl, which I
    have always understood to the the propeller tips going supersonic. It
    doesn't seem to affect the thrust too badly, since the ship gets
    airborne!
    
    Does anyone know if the "supersonic tip" theory is correct?
    
    Nigel
    

    Trying desparately to remember what I was taught loooong ago.
    
    The propellor tips of many aircraft do indeed exceed the speed of
    sound. But once you approach or exceed the speed of sound you get all
    sorts of nasty shockwaves doing all sorts of nasty things to the
    airflow and aircraft structure.
    
    The airflow behind a shockwave is peculiar. I have this hazy
    recollection that it acutally flows backward i.e. if the propellor is
    valiently pushing air from left-to-right, then the airflow behind a shock
    wave goes from right-to-left. This is bad news as far as a propellor is
    concerned.
    
    In a ducted fan, the airflow is different to a propellor hanging out in
    the breeze. Its restricted, there is a sort of 'ram' effect and its is
    pretty close to laminar. But perhaps the biggest difference is in what
    the tips are doing and what they are wallowing in. They should be
    spinning through a boundary layer i.e. the boundary layer formed on the
    walls of the duct. This your normal propellor just doesn't do. BOundary
    layers changes the rules somewhat; how, I don't know: I hated fluid    
    mechanics and the Naviar-Stokes equation in particular.
    
    If I remember correctly, one of the design considertions of ducted fan
    (and of the turbine/compressor blade in jet engines) is to take account
    of supersonic shock waves coming off fans blades. These shockwaves can  
    actually be used to enhance the efficiency of the engine. 
    
    Angus

    Tom Hunt wrote an interesting article in MAN earlier this year
    about the effects of wind on propellor performance.  Tom works
    as an engineer at Grumman Aerospace and used their lowspeed wind
    tunnel to perform the tests.
    
    Tom is an avid electric flyer and wnated to determine the best prop
    to use for the 7-cell limited run class.  
    
    The test base was an Astro 05 geared motor on 7 cells.  Tom tested
    three props: Aeronaut 12.5x6.5, Master Airscrew 12x8, and a non folding
    Top Flite 10x8. 
    
    The results of the tests were really interesting.  The prop that did
    the best in no wind conditions(Aeronaut), did the worst in a 15 mph
    wind.  The reverse was true for the Top Flight which performed
    miserably in calm conditions and the best of the 3 at 15 mph. The
    Master Airscrew split the difference.
    
    Thrust is what provides the motive force to move a plane through the
    air, not static read RPM.  So, the prop that gives the highest
    static rpm may not be the best prop to use on a windy day.  The sport
    minded flyer may not care about these differences, but the competition
    minded pilot can see performance improvements by better understanding
    propellors and how they function under different conditions.
    
    I highly recommend reading the MAN article, it is excellent!
    
    
                                                          Regards,
    
                                                          Jim 

    In Martin Simon's book I ran across an interesting formula for
    estimating the proper propeller for your model:
    
    D= 4th root of:      BHP
                    ___________________      x 10,000
                    
                    RPM(squared) x mph x 53.5     
    
    
    For example:  I want to build a quarter scale cub to tow my gliders. 
    It will be powered by a Quadra 42cc motor which puts out 2.4 BHP.  I
    want to spin the prop at 5000 RPM and fly at 30 mph.
    
    Using the above formula and consulting the pitch chart in Simons' book,
    I find I need a 28" x 6" prop.  Well I know from magazine tests that
    the Quadra 42 will not spin a 28" prop at 5000 rpm.  So I could add a
    gearbox or recalculate at 6000 rpm which gives a 25" x 6" prop.  Well
    magazine tests tell me that the Quadra will spin a 24 x 8 prop at 5400
    rpm, so this prop might work o.k.
    
    Using my original criteria and moving up to a Zenoah G62 which puts out
    4.7 bhp, I recalculate and find that a 33" x 6" prop is what is
    required.  Well such large props are hard to come by and I don't know
    if the G62 will spin a 33" prop, so I probably need to up the flying
    speed or engine rpm.  
    
    Anyway it is fun to work examples and it seems that the results are
    reasonably accurate.
    
    Regards,
    
    Jim