Conference vmszoo::rc

    I don't know of any rule (and would be interested if there were
    one) but when I tried one on my pattern bird I dropped one inch
    in pitch and diameter. I was running an 11/7 two blade and went
    to a 10/6 three blade. For my application I went back to the
    two blade.
    
    						Tom

Let us first accept that a giving engine under a given setup will provide a
given amount of power. We use that power to move air and hence give forward
motion to our models. Moving that air requires power, the more air you move the
more power required. 

Simply if we increase the diameter or increase the pitch we move more 
air, hence we absorb more power. I know more factors are involved but ignore 
them for now.

Let us now assume we have an engine that produces 1 bhp at 20,000, the power 
band will have the familiar bell shape so as we deviate from the center line of 
20,000 rpm + or - the available power will be diminished disproportionately to 
the deviation.

To answer your question what you will have to do is adjust the prop size 
(diameter and/or pitch) to maintain the rpm that best suites your engine. The 
diameter is the obvious choice to be reduced as prop disc size if generally the 
incentive to go multi blade.

Other notes have covered props; working and selection, multi blades are 
usually selected for the following reasons:

	1. Scale effect.
	2. To reduce noise by tip speed reduction.
	3. To increase ground clearance.
	4. Confined areas (eg. pusher configurations).

This is a simplistic explanation ignoring other factors around prop efficiency
so don't expect and immediate performance improvement or reduction by going
multi blade; you will have to experiment. 

john

    The rule-of-thumb I used to go by was: when going to three blade
    from two, drop diameter one (1) inch. (and use a commercial prop)
    I don't have a formula for 4 bladed ones ( anyway, spliced props
    are rather weak and dangerous; the only new thing in the
    wind is the recent article in, I believe, Model Airplane News that
    demonstrated spling with epoxy and carbon fiber tape)

    don

    Duke Fox ran an experiment on prop & engine noise reduction &
    discovered the two props at 90 deg of each other on the same shaft
    worked just fine but reduced propeller noise substantially. I have
    a copy of the report. Excerpts of the report were also published
    in a recent AMA mag issue..
    
    md

    To put two props on, you'd need a longer shaft.  I bet that the
    prop would be quieter if the blades were not at 90 degrees.  If
    you look at the blades on the fan in front of a car radiator, (not
    the new fangled electric ones, but the ones driven off of the water
    pump) you will notice that the blades are not spaced evenly.  This
    is done to reduce noise.  This works because the frequency and phase
    of the blade noise does not all add evenly.

    I recently saw a picture of a Russian "BEAR" class bomber, and it
    had two props per engine.  Do you suppose that they do this for
    more power, or less noise??????
    
    Ken

>    I recently saw a picture of a Russian "BEAR" class bomber, and it
>    had two props per engine.  Do you suppose that they do this for
>    more power, or less noise??????

Experts correct me if I am wrong.
I think you will find that they are counter rotating props - so they
do it to cancel the effects of torque.

I would assume that they are louder and slightly less powerful but
who are the neighbors going to complain to!

              _!_      
Bye        ----O----   
Kay R. Fisher / \     

================================================================================

    Ken/Kay,
    
    Realizing you [Ken] have toungue firmly buried in cheek, I'll bite
    too.
    
    The setup Kay refers to is called "contra" rotating props (one rotates
    clockwise, the other counter clockwise on a common shaft).  I believe
    some other reasons [besides torque cancellation] for usiong this
    setup is for max thrust/prop efficiency, minimum vibration and balanced
    shaft loads.  Also, I'd expect this scheme to be [perhaps] quieter due
    to the props cancelling each others beat frequency...just a guess.
    
    Adios,	Al

                              SIZE CHART
    
    
    There's an artical in a back issue of AMA mag. about three blade
    props. One that tells all about switching from a two blade to a
    three blade per engine size,revs,pitch and etc.
    
    Underline: If you are currently using a 10.6 two blade you can use
    a three blade 9.6 or "add a blade drop back one size".
    
    TWO BLADE              THREE BLADE
    
    12.6                      11.6
    12.5                      11.5
    11.6                      10.6
    11.5		      10.5
    10.6                      9.6
    10.5                      9.5
    
    
    Get the idea.
    
    
    bye for now ART