While winding the propeller, you exert a torque T_s = Kθ where K is a proportionality constant depending on the properties of the rubber band and θ = 2πN is the angle through which the propeller has been turned. Then the work done to turn the propeller through a small angle dθ is dW_s = Kθdθ. Integrating, we find the total work required to wind the spring as

The proportionality constant K can be measured by winding the propeller N turns and measuring the torque required to keep the propeller from turning. A simple way to measure torque is to tie a string a known distance from the propeller center and then determine how much weight must be hanging from the string to keep the propeller horizontal without moving. Then torque is the distance of the string from the propeller center multiplied by the weight on the string.

If K is measured, then overall efficiency can be computed directly as