Ken,                       
KE>  I have a question that I have asked so many and everyone at the local
KE>flying field here has a different answer.  First, my question, is
KE>there guide lines that a person could go by when trying to decide what
KE>propeller to use with a particular size engine???  I have bought
KE>several used engines and I cannot figure out what is the right prop to
KE>use and which is wrong.  I have the following types:  OS .10, TIGER
KE>.40, K & B .61, and a .25 .......    If someone could tell me what the
KE>proper props are to use (and what makes one different from another) I
KE>would greatly apprecaite it....
    I'll try and help you out a little here.  The prop size varies greatly 
with 
the type of aircraft you have the engine mounted on, the type of fuel (amount 
of nitro) that you are using and whether the engine is using a tuned pipe or 
muffler.  
    The Tiger .40 will run well on a 10 x 6 or 10 x 7, the K&B .61 an 11 x 7 
or 11 x 8, the .25 should run well on a 8 x 6 (I don't have any experience 
with 
.10 size motors).  The Tower Hobbies catalog has a section in that lists 
serveral prop sizes for most motors.
    Using the above sizes as a starting point you can go up in diameter by an 
inch or so if you go down in pitch the same amount (you can also reduce the 
diameter if you increase the pitch).  For example an 11 x 5 could be used 
instead of a 10 x 6 if you need a larger diameter to pull a high drag model 
or 
want to slow one down.
    You will have to experiment a little to find just the right size for your 
particular engine/airplane combination.  If you tune the engine on the ground 
for peak RPM and then richen it a few clicks, it should hold this setting 
pretty well throughout the flight.  A motor that leans out during flight 
(particularly during a climb), is most likely over proped (too big in either 
pitch or diameter).  If it goes rich in level, high speed flight, then it is 
likely under proped.  
    Larger diameter props offer a greater flywheel effect for improved idle, 
but often sacrifice ground clearance.  The need to reduce the pitch to match 
the load on the engine properly may produce flight speeds that are less than 
optimum for the model.
    Using a tuned pipe can throw a whole other set of adjustment 
considerations 
into the mix.  If you're using one of them, seek out assistance from an 
experienced modeler that has good luck using one.  They can be a great source 
of power improvement, but can be tricky to get set if you're not familiar 
with 
the tuning procedure. 
    Sorry to get so long winded.  Hope this helps you out.
    
    Don Blewett
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