"Disc'ing" drag
I was recently asked why the drag from a windmilling prop was greater than that from a stationary prop. No problem (I thought) and proceeded to give the standard vague explanation - the stude then asked for a bit more detail and I ended up falling on my sword and promising to have an answer by the next time we met.
So, are there any budding aerodynamicists out there who can give a rigorous explanation?