Little bit more info would help to get you some answers. Like which fan unit? Which actual F20? What esc?

Just for S&G's I tried the 4w with a Wemo mini 480 fan on 5 cells.
Worked out like this. 16.7 amps, 27.8 oz. of thrust, prop speed of 118.8 mph.

I had the HET F20. It ran the 2w20 motor with the Wemo mini 480. Lot more amps to get decent speed and thrust.