Question

The absolute jet exit velocity from a jet engine is 2800 m/s and the forward flight velocity is 1400 m/s. The propulsive efficiency is
1. 33.33%
2. 40%
3. 66.67%
4. 90%

Answer 1

Correct Answer - Option 3 : 66.67%

Concept:

The propulsion efficiency is defined as the ratio of Propulsive power (or) thrust power to the power - output of the engine.

It is given as

\(\eta_p = \frac {2u}{C_j + u}\)

Where Cj – Exhaust gases velocity, u – Flight speed;

Propulsive efficiency is the measure of the effectiveness with which kinetic energy imparted to the fluid is transferred into useful work.

Calculation:

Given u = 1400 m/s, C_j = 2800 m/s;

From the propulsive efficiency relation,

\({\eta _p} = \;\frac{{2{u}}}{{{c_j} + {u}}} = \frac{{2 \times 1400}}{{2800 + 1400}} = 66.67\;\% \)