In Soaring VI #2151, Richard in Seattle asks how much weight a five HP gas
engine with a 24" prop can lift straight up from a standing start.
A useful formula for static thrust is:
T = 10.414 x F x(D x HP x sqrt(Sigma))^(2/3)
Where T = thrust in pounds.
F is a measure of merit (fudge factor) for the prop. 0.7 is good for
airplane props, 0.8 for helicopter rotors. Approximately.
D is the prop diameter in feet.
HP is power in horsepower.
sqrt(Sigma) is the square root of the density ratio. Sigma is 1.000 at sea
level on a standard day (15 deg C), less at altitude.
So for this example question, assuming sea level standard day, and F = 0.7,
static thrust is:
T = 10.414 x 0.7 x (2 x 5 x sqrt(1.000))^0.667
T = 33.8 pounds, approximately.
That's all for now,
Blaine Beron-Rawdon
Envision Design
San Pedro, California
http://members.home.net/evdesign/
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