How to decelerate a spinning object? I want to animate a helicopter land on a landing pad and the blades would slowly decelerate taking about 2 minutes until they stop.

I can only find tutorials on how to keep a wheel spinning forever but I want the spin to slowly over time reduce in RPM (rotation per minute) until it stops.

There must be a physics simulation for this. I have yet to use a mechanical simulations in Blender. I know one thing, calculating RPM to manually keyframe deceleration doesn't look right and is time consuming.

So how do I set up the blades to simulate the engine shutting down and decelerating gradually until it stops? 

This is for an animation rendered in cycles not for a game engine.
You only need 2 keyframes to control the way the rotor decelerates. Insert the keyframes, then go to the animation view. There you can adjust how the rotation shall behave over time. The default is an S curve:



But you can change that e.g. to an exponential curve: