Quote:
Originally Posted by Frank Lee
I forget, is it 36"+ where the gap really starts to hurt aero?
It's a tough situation, making it as close as possible without risking hitting the trailer when backing up, going over bumps and dips, etc. Perhaps the best solution is some sort of flexy fairing that can close or at least partially close the gap without damaging anything... I seen some attempts at that on tractor/trailers but not at the consumer level.
I'd call Sven's proposal (c)- it looks quite good... now put radii on the leading and upper edges.
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In Hucho's 2nd Edition,p.180,Fig. 4.98,he shows a Morris Minor 1000 notchback,and Traveller squareback pulling a small caravan trailer.
For the notchback,the drag curve shows a maximum @ 36" gap.
For the squareback,the drag maximum is at about 39.3".
For SAE Paper 870714,with matched-height,parallel-face tug/trailer, a 21.6" gap produces only a 2.3% drag increase.
For stock cars in a 2-car draft,the trailing car,going from bumpers touching,to about a 36" gap would raise the Cd from 0.185,to 0.193,about 4.3%.
For two identical Mercedes-Benz buses,going from touching bumpers to around a 36" gap,would raise the Cd of the trailing bus from 0.167,to 0.21,about a 25% drag increase.
Since the buses don't have 'bumpers' per se,they might best reflect the significance of gap drag.
In Abbott and Von Doenhoff's Theory of Wing Sections,they depict a Clark-Y airfoil with varying degrees of gap(slots) along different positions of the cord.The Cd of the wing can vary from a minimum of 0.0152,to 0.0208,based on surface area skin friction drag.
NASA reported 19% drag for their C.O.E.semi-trailer with 31.2" gap.