Try this source
http://icebox-esn.grc.nasa.gov/
It is exactly the frost which makes for unpredictable loss of cl (lift) and increase of cd (drag). To properly determine the impact a granularity must be defined and then tested in the wind-tunnel.
So, the best is stick to the "clean wing" principle and add that extra time to properly de-ice even the slightest touch of the winter fairy.
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