... This is one of the reasons I like this site so much, where else would you get such high standards of model-making and levels of discourse on something as mundane as oil staining?
One excellent isn't enough.....
Mate, this is looking sweet as a nut.
To allow for eddy-shedding. It's for the same reason that industrial flues have spiral flow-deflectors fitted to them, to stop them resonating in strong winds (well, basically. Due to Reynolds numbers and sh!t)..I stand corrected.
There's probably a computational fluid dynamics calculation that will explain it.
Airflow can do funny things. The Kiwis (from memory) had a problem with their P-3 Orions, for instance. They fitted a camera dome on the underside and it caused problems with ordnance release from the weapons bay. The solution was to paint it in very rough paint - and sort of grouting rough - in order to un-smooth the airflows it caused.
Joanna needs dusting, jus' sayin'