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Old 11-29-2014, 01:04 PM   #1 (permalink)
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Virtual boat-tailing

The aerodynamicist Hucho once mentioned 'reenergizing the dead wake' in one of his books as a method to reduce rear end drag without the added size of full boattailing.

AFAIK he only mentioned that but he never elaborated on that.
Please correct me if I'm wrong.

I just found this technical paper
http://www.vortaflow.com/consulting/...0reduction.pdf

It mentions blown boundary laters, aero 'boat-tails', longitudinal traverse and swept grooves, porous surfaces and vortex generators as means of getting a virtual boat-tail.

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Old 11-29-2014, 01:43 PM   #2 (permalink)
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I came across a 1st gen prius on google that had vortex gens, they tuft tested it and the air stayed attached, and the rear angle of a 1st gen is not good! I think a compact boattail is possible this way.
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Old 11-29-2014, 02:47 PM   #3 (permalink)
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Vortex gens, tuft test, 1st gen prius

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Originally Posted by Hypermiler1995 View Post
I came across a 1st gen prius on google that had vortex gens, they tuft tested it and the air stayed attached, and the rear angle of a 1st gen is not good! I think a compact boattail is possible this way.
I think you mean this autospeed article:
AutoSpeed - Blowing the Vortex, Part 4

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Old 11-29-2014, 03:18 PM   #4 (permalink)
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Originally Posted by California98Civic View Post
I think you mean this autospeed article:
AutoSpeed - Blowing the Vortex, Part 4

James
Yes that is it.
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Old 11-29-2014, 09:52 PM   #5 (permalink)
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Thanks for posting this. It has a good summary of the work we've seen from NASA Dryden and Georgia Tech. I'm especially interested in the variety of Coanda nozzles shown. And the grooves are something new to me.

One of the hardest parts of deciding to go electric with my next vehicle is giving up the 1500 cubic feet per minute of hot, pumped air that air-cooled VWs produce right where it is needed. Here's an example where Coanda nozzles are used to virtually truncate a boattail (the one on the left):

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Old 12-01-2014, 05:10 PM   #6 (permalink)
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virtual boat tail

The concept has been demonstrated in aviation since the 1920s,with blown slots on wings,in Ludwig Prandtl's research.
There are STOL aircraft which use it today.
J.J.Cornish,III,chief aerodynamicist,Lockheed Aircraft,advocated its use in automobiles and a wind tunnel photo of a suction-slot sports car is in Paul Van Valkenburg's book,Race Car Engineering,you can see before and after smoke images of the technology in action.It IS remarkable!
So far the issue has been how to provide enough pump power for either blowing or suctioning the slots.Historically,the pump absorbs more energy than is saved with the slot.Not an issue with military aircraft.
Georgia Tech has had some limited success,has some patents,and continues research.
Knowing the thermodynamic limitations of this technology,Hucho suggested we take another look at the 1935 inflated boat tail.
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Old 12-02-2014, 09:00 AM   #7 (permalink)
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I've always wondered if an airfoil in the opposite of 'downforce' mode could provide a virtual boat-tail - not enough to cause much extra lift to speak of, mind you, just enough to encourage the wake to converge a little quicker - possibly at the sides as well. It would be a fairly small airfoil with very mild camber and a very gentle angle of attack, placed just behind and 'out' from the separation point on a station wagon or similar - just far enough out to catch a little bit of the free stream without impacting on the frontal area...

No idea if it would work or pay off in drag of course - just one of those random thoughts .
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Old 12-02-2014, 05:31 PM   #8 (permalink)
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I remember NASA in the 80s evaluating forced-air slots in a wing to help reduce drag and increase lift. The holes or slots used were very small, and were prone to getting obstructed, so they concluded that it wasn't feasible to put them on real-world aircraft.

-soD
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Old 12-02-2014, 06:02 PM   #9 (permalink)
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Madact -- Like this?



At the very least it will keep the glass clean.

some_other_dave -- More like the British in the 1930s. [citation needed]

Last edited by freebeard; 12-02-2014 at 06:09 PM..
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Old 12-02-2014, 06:43 PM   #10 (permalink)
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airfoil

Quote:
Originally Posted by Madact View Post
I've always wondered if an airfoil in the opposite of 'downforce' mode could provide a virtual boat-tail - not enough to cause much extra lift to speak of, mind you, just enough to encourage the wake to converge a little quicker - possibly at the sides as well. It would be a fairly small airfoil with very mild camber and a very gentle angle of attack, placed just behind and 'out' from the separation point on a station wagon or similar - just far enough out to catch a little bit of the free stream without impacting on the frontal area...

No idea if it would work or pay off in drag of course - just one of those random thoughts .
Hucho says nothing has worked except boat tails and box-cavities.

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