Quote:
Originally Posted by Big Dave
It’s always a tradeoff between minimizing wake area and having an excessively steep slope. My bed cover does a nice job of minimizing wake area – a 47% wake area reduction – but does so at the price of having a seventeen degree slope.
It is time for me to revise my wake cover to make it handier to use. What I am thinking of is to reduce the slope to 11 degrees (1-in-5). This will mean my wake area reduction will be only 27%.
Going to a 1-in-4 slope (14 degrees) would result in a 37% reduction in wake area.
Does Hucho have a graph showing the effects of wake area reduction on drag?
I can’t do a full boattail. That would result in a thirteen foot long “tail” if one stuck to the one-in-five slope I’m planning. The truck is enough of a bear to park as is. The other extreme is simply a flush commercial cap. Zero degree angle but zero wake area reduction.
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Big Dave,I think Hucho shows Mair's boat tail chart with all the results for varying the length.Mair uses a cylinder with a stretched-out hemispherical nose.This body of revolution is out of ground effect,so you have to bear that in mind.-------------------------------------- with no aftbody,the form scores Cd0.165.Mair recommends stopping the tail at a length equal to 1.125 times the diameter of the body.Cd at this point has dropped to 0.03.Also,by this point,the body has been gently bent into a 22-degree angle.If you continue the tail all the way at the 22-degree angle,the Cd drops to 0.0225.I looked at Jaray,Klemperer,Lay,and Korff.They basically recommend that you end the tail at 50% frontal area so you don't bang stuff with the long tail.You still have half the drag leftover behind you,but to cancel that means all that additional length.Any 10% reduction in wake area cuts drag 10%.----------------------------- A note of caution.On bondo's original bedcover,he did not have enough gentle roof curvature behind the F-150's cab,and the flow separated immediately.His new cap has generous curvature ahead of the final slope angle.Kamm wouldn't go steeper than 10-degrees.The ideal teardrop ends up at about 20-degrees,but it takes a lot of length to get there.I think your 11-degrees is real good,and even if your wake is larger,it will be a a higher pressure and drag should be less.I may have Mair's graph in my photo album.I'll look.