Question

Which of the following is the correct equation for net thrust?

(a) FN=ṁ(Vj+V)

(b) FN=(Vj-V)

(c) FN=ṁ(Vj-V)

(d) FN=ṁ(V-Vj)

This question was addressed to me in semester exam.

The query is from Thrust Producing Power Plant topic in section Aircraft Force Systems and the Equations of Motion of Aircraft Performance

Answer 1

Right answer is (c) FN=ṁ(Vj-V)

For explanation: The equation for net thrust is given by FN=ṁ(Vj-V) where FN is net thrust, ṁ is mass flow rate, Vj is exit velocity and V is inlet velocity. In other words the net propulsive force is the difference between the stream force entering and leaving the engine.