Quote:
Originally Posted by Phase
is there a reason rooflines seem to be more important when tapering, verus tapering the sides? isnt closing the wake the same thing, whether horizontally or vertically?
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It's conditional.
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On a notchback, it cannot be predicted with the probability of fastbacks and squarebacks.
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On mass-produced fastbacks, the roofline is doing the lion's share of pressure recovery, while the sides cannot begin any meaningful section reduction until the rear wheels are passed.
Then, the same care as the roofline would have to be respected, to ensure that no super-deceleration and adverse pressure gradient were produced, otherwise, this 'boat-tailing' would provide no benefit.
Beyond the rear wheels, the added plan-view boat-tailing adds a synergetic benefit to pressure recovery.
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For an 'actual' squareback, where the top, sides, and bottom end at the same place on the body, the sides and diffuser shape would be of equal importance as the roofline. No surface would be allowed to be so 'steep', as to produce the adverse pressure gradient super-deceleration Hucho, and all others tell us to avoid like the plague.
Boat-tailing top and sides along a simple straight angle provides a 60% drag reduction benefit compared to just doing a straight, angled roofline. And if you don't radius the edges you lose 13% of the drag reduction potential.
You lose also if the diffuser isn't elongated ( let's wait until Jeff Howell conducts this research for these numbers, and let's wait until Jeff Howell conducts tests for tumblehome impact, and ditto for 'cambered' boat-tail surfaces all around ).
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For notchbacks, one would initially modify the aft-body into a 'fastback', then, afterwards, you would proceed as with a fastback.
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After another 3-generations of boat-tailing investigations, Jeff Howell et al. will have caught up to the aerodynamic state-of-the-art for boat-tails, as of 1986.
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The trick is to reconstruct the 'entire' longtail vehicle from it's blueprint ( photographs cannot provide the dimensional information we need ), top, sides, and bottom, all the way to it's farthest extremity; then, when elongating 'ANY' surface of the body, you stay along this imaginary contour suggested by the longtail blueprint you've created.
Hucho has used 'contour comparison' for this type of exercise.
It's why it's so important that everyone know how to draft and be able to perform scale measurements, or CAD, AUTOCAD, etc..
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Once you 'know' where 'the rest' of the vehicle would be, streamlining becomes 'modular'. 'Plug-and-play.
Just pick a 'length' you think you can live with, then simply elongate the body out to that length, along the imaginary contour you drew.
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Drag reduction will be a function of the reduction of wake area, compared to the OEM wake area.
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