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Static Thrust Calculator

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n3puppy

Well-Known Member
Joined
Jun 25, 2019
Messages
337
Can someone help me understand how to use the Static thrust calculator that is often quoted when discussing engine hp numbers?
Static Thrust Calculator - STRC

This is a common format I see when results are tabulated
3300 rpm with 72" Prop = 988.7 lbs of Static Thrust at 77 Degrees with a (3) blade prop at .9 CF
72" x 10 = 988.7 lbs Static Thrust. Needs 82.671 hp
72" x 5 = 988.7 lbs Static Thrust. Needs 41.335 hp
72" x 2= 988.7 lbs Static Thrust. Needs 16.534 hp
72" x 1 = 988.7 lbs Static Thrust. Needs. 8.267 hp
72" x 0.5 = 988.7 lbs Static Thrust. Needs 4.133 hp

Instinct tells me that the same 988lbs thrust for any pitch is not plausible.
Every formula (not calculator) I have seen always has pitch as a variable when calculating thrust. Change the pitch - thrust changes. Example formula below. (Set Vo to zero for static thrust)

If I am to believe the output of this calculator.....
I could build VTOL ultralight with a 500lb MTOW
Hang it from a 72in 3 blade prop with a virtually flat pitch (0.5inch)
Prop makes 988 lbs thrust according to calculator
Climb vertically at almost 2G (Thrust/Weight ratio 1.98 to 1)
All while using a $120 Harbor Freight 6.5hp motor (only need 4.133 hp)

Am I interpreting the results wrong?
Or is the calculator wrong?
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