RE: Re: R/C AIRPLANE PROPS
BY: Ken Ellison to Roger Marshall on Sat Dec 09 1995 12:35 am
 > RM> Hello, Ken...
 > 
 > RM> KE> I have a question that I have asked so many and everyone at the
 > RM> KE> local flying field here has a different answer.  First, my 
question,
 > RM> KE> is there guide lines that a person could go by when trying to 
decide
 > RM> KE> what propeller to use with a particular size engine???  I have 
bough
 > 
 > RM> There are good guidelines for choosing the proper propeller for an 
engin
 > RM> I'm surprised that you found so many different answers.  These guides 
ca
 > 
 >   I have a copy of the current Tower Hobbies catalog and the guidelines
 >   as specified in there was what I was using.  The problem was in
 >   particular on my PT-40.  I have a GS-40 on it and was running a 10 X 6
 >   on it.  After a few flights I had some pilots ask what size prop I was
 >   using and they said I should use a 10 X 8.  I tried it and did not
 >   really notice much of a difference.  I noticed even in the Tower
 >   Hobbies catalog it gives two or three different sizes for each size
 >   motor.  Maybe this is a topic that can't never really be answered.
 >   Maybe if I can get a better price for a 10 X 6 I should use that or if
 >   they are having a sale on a 10 X 8 I should use that size.  Thanks for
 >   your reply though.
 > 
 >  Ken >
 > 
Prop Sizes can dramatically affect the performance and life of your engine.
the best way to prop a motor is to see what it says the practical RPM range 
is 
 and come close to that.. if you run too prop that is too big or has too much
 pitch you can overheat and burn up the engine.. same goes for a prop that is
too small.  you really need a tach to tune an engine for a prop. 
the first thing you should do is look at what the suggested prop sizes are. 
but
sometimes they can be completly off for the type of flying you want to do and
the perfoarmance your looking for. the next thing to look at is the practical
RMP range. for example.. the practical RPM for an OS FX .25 is 2,500 - 19,000
but you don't want your engine to turn up to 19,000.. if it could it would
most likely burn up. they suggest a 9x5,9x6,10x4-5 and a few in-between. it's
usually safe to say you will have the engine running to it's Safe potential
about 3k less than the top rpm. now I know from Pylon Racing that the prop
sizes they recommend will never get anywhere near 16,000.  I Always use an 
x6
prop and get about 17,000 peak, and it has just the right amount of pull to
really groove in the turns.. and when I tune it on the ground I tach it with
the throttle full open and lean the needle valve in until it reaches it's 
eak
then I richen it up until the rpm drops about 300-500. this is to keep the
engine from running up to lean and over heating, once in the air the prop 
ill
unload and the RPM will increase. if you just peak the engine out and fly it 
by
the second lap the engine will start to heat up and loose lots of RPM and
power. and possibly burn up. for bigger motors and ones with Pipes I keep 
hem
very rich to the point where they lag until about 100 yards into flight.. but
again this all depends on the plane, engine, and type of flying you wish to 

Neil S.
--- GEcho 1.00
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