Question

What is the formula for net thrust of the engine?

(a) F = -∯pdS

(b) F = ∯pdS

(c) F = -∯\(\frac {dS}{p}\)

(d) F = -∯p^2 dS

I got this question in exam.

This interesting question is from Jet Propulsion Engine Thrust in chapter Integral Form of Conservation Equation for Inviscid Flow of Aerodynamics

Answer 1

Correct option is (a) F = -∯pdS

To elaborate: Usually the pressure distribution is more dominant than the effect of shear stress thus for computing thrust, pressure force is only considered. The net thrust is given by the formula

F = -∯pdS

Where, p is the pressure distribution

dS is the outer surface over which it is integrated and thus vector points away from the surface

The negative sign is present because thrust acts in the opposite direction of the acceleration.