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Belly pans seem to get mixed results, some report expected improvements, others none or even negative results, so there is definately more devils in the detail.
Apart from getting the approach angles for the front undertray and the departure angles right on the diffuser one must also consider what is going on with the boundry layer.
Even though the underside may be a mess and is causing turbulance it may well be that the boundary layer is set by the lowest point, say the axles and these may be attached at their lower limits as the vehicle travels over and skims at these points.
If you go and hang a belly pan the full length of the vehicle, even if it is at lowest axle height, this will result in the boundary layer transferring to this surface and then adding to effective frontal area, which may be more than enough to offset the gains of reduced turbulance.
The difference is a flow through a rough walled larger pipe compared to that of a smooth walled smaller pipe, there is a point where the larger pipe, even if rough flows more air.
So my thinking is a balance between getting a smooth underside, but with a minimum reduction of existing ground clearance, the limits may be somewhere approaching the lowest point, (axles?), but not all the way and almost certainly not exceeding them.
I need to do some reading on ducts, anyone here have some ideas on how ducts operate and what primary constraints exist?
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