Ok,
Been thinking about it again,
The smaller dimension rule, must rule.
As Sven7 said above
Quote:
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Then the air is slowed down somewhere and it needs to "catch up", changing its speed and therefore pressure
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Ignoring the underside of the vehicle for now,
When the vehicle pushes through the air, there are two fixed boundary's, the ground and vertical centreline of vehicle and these primarily govern how air is distributed around vehicle, so the lesser dimension will allow easier airflow, in my case it is the sides as my vehicle is taller, and unless your vehicle is twice as wide as it is high from the ground this is the way for most.
As for overlays,
The calculation for taper over the top is governed by the height, but only until it meets the side taper, ie the flat roof area going down.
The side taper is calculated from the half width dimension and where it meets the top taper it rules.
If both top and sides use taper based on the longer top dimension, then the sides will not come in fast enough and result in air rushing in over the sides to top creating vortexes and turbulance.
If both are based on the shorter dimension, then the top will be too steep, resulting in seperation over the top, which will pull air in from the sides, again creating vortexes and turbulance.
So that means different calculations are required for top and sides and when viewed side on, the taper over the top will exceed the 22* rule as the tail will finish sooner as the side taper will reach centre point (0) well before the top taper is supposed to.