Propeller Efficiency calculation different by two methods
Defining "useful" as moving the plane forwards, the rate at which useful work can be done is equal to whatever power which has been imparted to the fluid, i.e thrust x velocity at disc. This is propeller power. The rate at which useful work is done which is equal to whatever power which has gone into actually making the plane move forward, i.e thrust x freestream velocity (since freestream velocity and airspeed are interchangeable depending on the frame of reference). This is propulsive power 

Consequently there are two different measures of efficiency, the propulsive efficiency which is a measure of how efficiently the plane uses the propeller power and the propeller efficiency which is a measure of how efficiently the propeller uses shaft power. The equation on the MIT webpage is for propulsive efficiency but you want to know how efficiently your shaft power (2π x torque x RPS) was used, therefore you have to calculate propeller efficiency so you don't use that equation.