KE>>   I have a question that I have asked so many and everyone at the lo
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 
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 pro
KE>>   use and which is wrong.  I have the following types:  OS .10, TIGE
KE>>   .40, K & B .61, and a .25 .......    If someone could tell me what
KE>>   proper props are to use (and what makes one different from another
KE>>   would greatly apprecaite it....
Hi Ken:
Prop selection for model airplanes is not an exact science. The engine
manufacturer should have included a recommended prop size in the
literature you got with the engines. If you bought them second hand,
this information is probably missing. In any case, the recommended prop
size is a starting point from which you can vary either the prop pitch
or prop diameter or both to get optimum performance for your engine.
It isn't surprising that you got many different answers to your
questions. Prop selection is a personal thing ... ie. what works best
for you is best for you. What works best for me might not work at all
for you.
(Smile...) Sorry, there isn't an short, easy answer.
Most folks simply use the recommended prop size and let it go at that,
but if you are serious about performance you need to get a tachometer
and learn to optimize your props.
Find a pylon racing enthusiast in your area who can work with you and
show you how to "clean up" your props.
This was a performance suggestion and did not deal directly with your
question, however. So, Lessee now ....
                OS .10  =  7 X 6
                ?? .25  =  9 X 6
             TIGER .40  = 10 X 6
               K&B .61  = 11 X 6  -  12 X 6
These are "starting points" not hard and fast nail-em-down sizes you
have to run. Lots of things determine which prop you need to run. Some
factors are ...
How high above sea level do you fly? (ie. how thin is the air?)
How much nitro do you use in your fuel?
What condition are the engines in? [new ... used but strong ... crappy]
What type of flying do you intend to do? (chug chug or unlimited aerobatics)
I'm sure if you think about it, you can come up with more questions to
make the problem seem more complicated. Prop selection is like gearing
on a car. The lower the diameter or pitch, the higher the RPM and vice
versa.
Points to keep in mind are (1) Engines develop optimum power within a
narrow band of RPM. (2) So do props.
The trick is to match the two. Before you can do that, you have to know
what those two number ranges are. That means you may have to contact the
manufacturers of the engines and get the data sheets for each one. Then
contact your favorite prop maker and get the recommended RPM ranges for
their props. Then make your selection based on the two sets of numbers,
keeping in mind the factors I mentioned above along with any others you
might come up with on your own.
I was going to add some information about the math involved but I'm sure
you get the idea that prop selection is more involved than pick a size
and go with it. There are lots of variables and no hard and fast rules
to follow. Just try different sizes to see what effect it has on your
model(s).
You are going to find that often, even the same prop will perform
differently on two identical engines (what fun!) When I was into Q-500
racing, I set up a prop box with my favorite racing props and tried to
build matched sets for different conditions. Then I only used them for
racing. For practice flying I kept the duds I didn't mind breaking on a
rough landing. But my good props would not work as well on other
people's engines.
I've noticed the same thing in tunnel hull boat racing. Props are not
necessarily the same even though they have the same numbers stamped on
them.
As a parting thought, are you aware of what the numbers in a prop size
represent? For example 10 X 6 ?  The first number (10) represents the
diameter of the prop from tip to tip. The second number (6) represents
the theoretical distance the propeller will travel in one revolution at
100% efficiency. It is equated with the average pitch of the propeller
blades. 
So, a 10 X 6 prop is 10 inches in diameter and will theoretically travel
6 inches each full turn. So, some math tells you that at 10,000 rpm it
would travel a total of 60,000 inches per minute or 5,000 feet per
minute or 0.95 miles per minute or about 57 mph.
Now, will it really do that? NOPE!  Why? Because propellers and engines
are not 100% efficient. I seem to remember from somewhere that 70% is
closer to the real world efficiency, so the above prop turning 10,000
rpm would yield a figure of somewhere around 40 mph. Looks more
realistic, doesn't it?  Good luck. If you find more information about
props in your research, please post it here. This is the kind of thing
that is helpful.
________ Eddie ________
... If anything is used to its full potential, it will break.
--- FLAME v1.1
(1:3628/5)
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