So, we must have a limit to what efficiency we can have from the Propeller, and you can see from here that a is ratio of incremental velocity to Free Stream Velocity, which is always positive is less than 1, but it is positive; that means, the velocity actually increases across the disc. This V 1 is always positive; that means that is an increment of axial velocity. There is no decrease, because of this type of pressure distribution, there is always an increase in velocity. Therefore, this a that we have written is always greater than 1, greater than 0. What this means? Efficiency is always less than 1. It is not equal to 1 nor greater than 1 as per this simple axial momentum theory, understood?