If you look at a diagram of the streamlines around a wing, which is all a
prop is, you'll see that the velocity and direction of the air is changed a
surprising distance above and behind the wing. One of your prop blade tips
would be like a wing flying two feet above and behind another. The effect
at this distance would not be significant but would exist.

However, the same prop climbing out at 90 mph and 2800 RPM would put each
blade only 11.15 inches "above" the preceding. This is close enough that
each blade will encounter air that already has some component of motion to
the rear. This reduces the change in velocity (lift) that the blade can
impart.

The three blade prop will be less efficient per unit of area than the two
blade where it counts, near Vx with trees in the windshield. Given a
limitation on length however, the extra blade area of the three blader can
easily offset the efficiency loss by a substantial margin.

Another factor in the efficiency equation is the tips. The tip losses and
vortexes are a big factor in wings which is why there is such emphasis on
making tips small (high aspect ratio) and things like winglets. A three
blade prop has an extra tip which will effect the effeciency without any
help from the blades ahead.

--

Roger Long



"Corky Scott" wrote in message
...
Sorry, I got my information wrong when I stated that a three bladed
prop advanced 15 inches during each revolution at 200 mph.

I now have the article in front of me and the exact quote is as
follows: "At 200 mph and 2,800 rpm, the blades on my three-plade prop
follow three distinct helical paths through the air, and each blade is
25" ahead of the previous blade at the same point of rotation."

I repeat that I am not a prop engineer nor do I have any formal
training in aerodynamics but it appears to me that by advancing 25"
during it's revolution, the affect of one blade might have upon the
next one would seem to be pretty inconsequential.

Corky Scott