At 20 yards, an arrow with aggressive helical spins no more than 3 complete revolutions. Ya'll have to remember, that arrow strikes the target in 1/100th of a second. There just isn't much time for spin.

To answer the question about broadheads that spin, they don't do that for "power"....they do that for stabilization. The theory is, if the broadhead is spinning along with the arrow it won't windplane as badly, making it fly better.

You're thinking is right in that the spin does create more drag as the arrow enters strikes it's target, but it's minimal and the spin ceases almost immediately. Spin is all about stability, not penetration.