Welcome to the forum, kevin.
One quick way is to create a circular selection the same diameter of the prop on a new layer below the prop layer and filled with 50 % gray. Lower the opacity to suit.
For added effect, copy the prop on a new layer, rotate it 45 degrees and lower the layer opacity.
It may not be the best way, but a quick one I can think of right now. Unless I go to the airport in the morning to see how a spinning prop really looks like and re-create it in PS

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Hope this helps you.
vee