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Peter:
You say "As every inch counts for
efficiency,..." That's true for low
speed applications, but not necessarily true for high speed props. Consider:
thrust is a product of mass flow and velocity. At high speeds, each blade of the
propeller intercepts a volume of air that is a tube a little larger
that the prop diameter and as long as the forward speed. So by keeping the
diameter large, you intercept a greater volume of air with the square of the
increase in diameter. However, as you increase diameter, for a given engine rpm,
the tip Mach and resulting tip drag increase at a much higher rate. Going from
M0.65 to M0.85 can increase the parasite drag CD by six to
eight times. So to keep M low, you must decrease rpm. When you do that, you
decrease available power, which is basically linear to rpm over a wide range on
our engines. So for good efficiency, you want to keep m-dot high and v low. Most
pilots are not aware that v on our high speed planes is very low; it's not the
40 mph people think it is. On my little 235, at 200 mph TAS, 2800 rpm, 63"
three-blade prop, it's 4-6 fps! As I previously stated, each
blade of the prop intercepts that tube volume. So by going to more
blades, you can increase the mass flow while at the same time reducing the
diameter to keep the tip M low. The two biggest sources of efficiency loss on
props are the tip radius and chord, and the draggy, klunky shape of the roots. A
tip has no lift, only drag, and that drag is proportional to area which is a
function of chord. More chord, more drag. Then that drag force is multiplied by
the tip radius to give torque. The more tip chord, the more drag, and the more
radius, the more torque acting against the engine. Nothing but pure loss! A
three-blade, 60" diameter prop with the tip chord narrowing to a point and
streamlined blade roots would give the most efficiency. And wide, swept
tips are as functional as the swept v-stab of a 172!
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