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<< Lancair Builders' Mail List >>
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<<Regarding your question of static thrust, good luck. You can perform a
detailed series of calculations which use simplifications and
assumptions to arrive at an answer that is an estimation for a case that
almost never happens and is transient OR you can do what I do......Guess.
For variable pitch propellers in the diameter we are interested (74-80")
a rough guess of 3 pounds per horsepower will get you close to the
manufacturers published (and whispered) empirical figures. The larger the
diameter the more static thrust you get per horsepower>>
Thanks, Brent. since I am concerned about prop diameters and any loss of
static thrust I may suffer by choosing a smaller, higher rpm prop I wonder
if a reasonable approximation might be to assume that the static thrust is
also proportional to disk area (diameter squared). Except that would only
hold true if the rpm and pitch stayed the same. With a smaller prop the
pitch can be increased and therefore, the air velocity as well. Instead of
making it squared let's assume the first power. Let's say for a 2700 rpm,
300 hp engine with a 77-inch prop your 3X number is correct. Then:
Diameter Thrust
74 865
77 900
80 935
360 4,200 (helicopter)
That almost sounds right - a 300-hp helicopter with a 30-foot could maybe
weigh 4,000 pounds. The implication of all this is compromising the prop
diameter to add more blades, increase ground clearance or run at higher rpm
does reduce static thrust, but not all that much. Going from a 77 to
74-inch prop might reduce takeoff acceleration by 4% or so. Sound about
right?
Gary Casey
2800 rpm ES project
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