Quote:
Originally Posted by BabyDiesel
I read redneck's boat tail thread and saw something interesting. He stated that 50% of the length will give 90% of the results. Was he correct in saying this? The above models of the truncated tails seem to point to yes.
Speaking of shark skin, would having trip strips laying perpendicular to airflow along the boat tail have any positive effect? Is a TBL wanted at this point, or are we teying to have the airflow go closer to laminar?
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I have scans ready for NASA's boat-tail research.They're full boat-tail netted Cd 0.238,while their truncated tail netted Cd 0.242,not much of a gain in drag.
This is getting into what Prof. Alberto Morelli referred to as 'phantom' tail phenomena.
Once the rear flow is cambered down,inwards,and upwards with the boat tail,there is a length at which you can truncate the tail with zero drag penalty.
The quasi-annular mass of boundary layer,when shed at the truncation stuffs the wake with a mass of air which the inviscid outer flow just skips over,as it would with a locked-vortex.
Dornier Aircraft Corporation was the first to discover and patent a wing based upon this principle in 1920.Technically,you don't want to remove more than 5% of the overall length to enjoy the zero penalty.
here's the formula for wing truncation drag
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With respect to trip strips,if the aft-body is to the 'template' contour,any roughening would only increase drag.The boundary layer is already fully turbulent which allows fully laminar flow in the inviscid layers 'above.'
If the contour is hypo-template,then some sort of boundary layer stimulation could help with separation.This has been researched in Europe for the U.S.Air Force with respect to the C-130 Hercules' fuselage radical up-sweep separation issues.Sub-boundary layer-thickness VGs,placed from empirical testing allowed some good effect.