Turbojet - Thrust-producer /// Piston-prop - Power-producer

[keith williams]

The formula

Thrust = BHP x 325 / TAS in knots

is just based on the following:

1 hosepower = a work rate of 33000 ft lbf / min

1 knots = 101.3 ft / min

Thrust = work rate / TAS

So thrust per horsepower = 33000 ft lbf / min / 101.3 ft / min = approximately 325 lbf

So at any given TAS we get Thrust = HP x 325 / TAS in knots


You are correct in saying that this would give infinite trust when TAS = zero.

The problem is in the assumption that propeller efficiency = 100%

Prop Power output is in Thrust Horsepower or THP, which is thrust x TAS

Prop power input = BHP

Prop efficiency = Power output / Power input.

When TAS = zero prop efficiency = zero, so the 100% efficiency assumption cannot be used.