@Bobregaz: That was quick for you ! Well done.
What do you mean by: "why jets dont have one (couldnt answer that)." ? Is it because the engine don't rotate like propellers and so the thrust force is at equal distance from the normal axis? Compare to propellers which if they are not counter rotating one will have is thrust force further than the other one from the normal axis (Due to descending propeller going faster) and so the other one is the critical engine.