<< George Lednicer wrote:

To all,
Everyone talks of the speed of sound and how the prop tip must be kept below
the speed
of sound (which is correct )- to the novice/uninformed this is only half the
info in regard to
understanding WHY !
Although I don't really understand it all, the reasons or explanations given
to me are:-
1. The prop tip does some crazy things when it exceeds the sound barrier
(I found this one really informative)
2. The prop loses lift when it excceeds the sound barrier
3. Things go into reverse when the sound barrier is exceeded

Now that I have been told these things I feel totally and fully informed -I
don't think !!

However it does give me some understanding, if someone wants to expand upon
this
I would be forever grateful.
George(down under)

When I was in college studying physics my professors would tell me these
things
and I felt the same way. Here is how it works. Some guy tried it and that is
what
happened :-) So another guy tried it and it happened to him too. Several
others
also tried it with the same results and it became a law of physics. Nothing
more
nor less than this. Why it does what it does is another matter.
See the page below from the classic book "Theory of Wing Sections"
by Abbott & Von Doenhoff

The ratio of the diameter to the circumference of a circle is
pi. Why! Some Greek guy drew a bunch of different size circles in the sand
several thousand years ago  and measured the circumference and the diameters
and found that they all came out to the same ratio 3.1416xxxxxxxxxxxxxxxx

Paul Lamar

Picture the shock wave shape as it moves across the front and back of the
prop tip. The front of the blade with air moving a bit faster has a curved
shape. The back of the blade has a more nearly uniform shape. As the shock
wave grows across the blade tip, it acts like a giant clamp with the jaws
misaligned, trying to twist the blade tip first one way then the other. The
blade tip twists into more pitch then into less pitch at a very high
frequency. They will howel and carry on when this occurs. Metal props are
little affected but eventually will start dropping little pieces off the
tips. Wood props just split open. If you hear a loud buzzing sound at full
throttle, shut down and check the blade tips for a split. Write down the RPM
that will get your prop above 900 FPS and don't go there.

Lynn E. Hanover
lehanover@aol.com

 
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