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The hp and thrust of a prop is based on the amount of air
which EACH blade sweeps out in one revolution. Since that is a function
of disc area, which is proportional to the square of the
diameter, if you change from a two-blade prop to a three-blade prop, you can
reduce the diameter to 82%, or to 71% with a four-blade, and still have the same
swept area. So if you have a 72" 2-blade, you should be able to get away with a
59" three-blade, or even a 51" four blade, and still have the same or better
efficiency IF the prop blade has a streamlined shape where it enters the
spinner. The reason that the myth about multi-blade props which give better
take-off and climb but are not as good in cruise, is that they had such
terribly klunky shapes in the root sections, which gives a lot of drag. Tom
Aberle's Phantom biplane racer had a 64" diameter two-blade prop in 2003
which gave him 221 mph. In 2004 he had my three-blade design of 59" diameter
which gave him 241 mph at 250 rpm less. This year he did 252 mph with my 59"
four-blade design with the same rpm he had with the two-blade.
The advantage of a shorter multi-blade prop, spinning at the same rpm, is
that it will have less noise from less tip loss which adds to its
efficiency. So if you want more ground clearance, reduced noise, more efficiency
for better cruise, and better takeoff and climb, go find a prop manufacturer who
understands this and buy a three or four-blade prop from them! And no,
scimitar tips are in the same league as the swept vertical stab on the Cessnas.
If your prop has any chord whatsoever at the tip, it just generates drag
proportional to the chord and absolutely no thrust. If it's a thin or
symmetrical airfoil, its parasite drag will be much greater than a 12% or even
15% thick airfoil. And the hp loss is greater the farther out on the blade you
go since hp is the product of torque and rpm, and the greater the radius where
you have drag, the greater the multiplier of that drag through the radius to get
torque. One pound of drag at 6" radius is 0.5 lb-ft, at 12" radius it's 1 lb-ft,
at 24" radius it's 2 lb-ft, and at 36" radius it's 3 lb-ft. Now that drag can be
either parasite or induced, so if your planform generates higher lift with the
squared dynamic pressure at the outboard radii, the rearward lift vector, the
so-called induced drag, uses much more power to generate thrust than do the
inboard sections.
p.s. Fan jet engines have multi-blade, fixed-pitch props
at the front end, which generates the majority of the thrust!
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