Thoughts on gain from boat tail on 94 eclipse

[Xist]

Without using a wind tunnel, you cannot know what the air is doing beyond the reach of wool tufts.

[aerohead]

Originally Posted by ennored

Thanks Phil. I have already started to consider my RV a vertical wing with respect to the tail. That is, it's 100" wide, and about 130" tall. The top 50" can get a tail designed for a 50" height, and the lower 80" gets it's tail solely based on the sides.

I'm still a bit lost with less than optimal tails though. How come the Dryden van tail looks to have attached flow? It was 80" long (and truncated to 40") on an 80" wide vehicle. The optimum tail would have been 142" long based on half the width (and much longer in side view). Does it really only have attached flow for part of it's length? Did the tufts lie? We KNOW it's better than a flat rear end. And we KNOW it's less than optimum. Is it just the filling of the turbulent wake that is improving the drag? Makes me think about the straight edge tail, like the trailer tail, or the flat 10° stuff you're talking about. Would they would better with a rounded shape?

Really makes me want to build something. Been investigating removing my rear cap. a 36" tail would fit in my garage. Another 36" or so could be folded out a la trailer tail. I only need 50" to replicate the half tail from the Dryden van. Suggests I could get as good as it or better than it!

*DOT length restrictions have limited most research to tails within a 48-inch overall length,or 60-inches for inflatables.
*None of these limited-length tails can provide full drag reduction potential.
*But at 48-inches,there are these 'optimized' architectures we can use.
*If you're going for all-out drag reduction,and a 'civilian',we don't have the same constraints as a commercial vehicle.
------------------------------------------------------------------------
Kamm and Koenig-Fachsenfeld both knew the potential of the 'Template.' It was the starting point for all research.
And the 'solution' they arrived at for 'practicality' was to build as if you were going for the entire tail,then just slice it off wherever you deem it practical.
-----------------------------------------------------------------------
Fachsenfeld made the mental leap,that a long tail could still be utilized for extra-urban situations by use of an inflated tail which adds the length Hucho and everyone demand for really low drag.
--------------------------------------------------------------------------
Walter Korff (Lockheed Aircraft Co.) depicted an 18-wheeler in 1963 which embodied some boat-tailing of the van itself,with clam-shell doors completing a full tail of a length equal to 1.88X body height.It was similar to the USAF 'Convert-a-plane' boat-tail concept of the 1950s.
-------------------------------------------------------------------------
The thing to bear in mind with the NASA Dryden tail,is that the Econoline only reached Cd 0.238.This is an 'optimized' Cd.
With a 'trailer-tail' constructed to W.A.Mair's specs,or the 'Template' it could have seen on the order of Cd 0.12.This would be an 'ideal' Cd.
It's all a matter of how far do we want to go.

[ennored]

Thanks once again Phil.

So, there's really no such thing as a "less than optimum" tail, other than a truncated ideal tail.

Not quite clear why the Dryden van works as well as it does, I'll have to accept "it just does" for now.

Looking at some percent drag reduction vs. percent of tail stuff, the Dryden van could have gotten to where it was (with 04" and 80" tails) with about an 18" (~15%) truncated template tail. A tail that short would only have drawn in 1". Could that really have worked? Wow...

[aerohead]

Originally Posted by ennored

Thanks once again Phil.

So, there's really no such thing as a "less than optimum" tail, other than a truncated ideal tail.

Not quite clear why the Dryden van works as well as it does, I'll have to accept "it just does" for now.

Looking at some percent drag reduction vs. percent of tail stuff, the Dryden van could have gotten to where it was (with 04" and 80" tails) with about an 18" (~15%) truncated template tail. A tail that short would only have drawn in 1". Could that really have worked? Wow...

All we have to go on is what made it into publication.There may be proprietary designs awaiting Patent Office Gazette publication which awaits access to our eyes.
Other than that,we can only infer or deduce potential design performance based upon data which exists in the public domain.
--------------------------------------------------------------------------
If you're doing an 18-wheeler,and we already know that rounded edges give 11% lower drag than square edges,then we can lean on HVAC technology,creating a boat tail as an inside-out square-to-round duct transition exo-plenum.
Throwing in Mair's careful curvilinear profile,we protect the boundary layer against the too-sudden pressure rise which would force the air down an up-escalator,or downhill skiing into an uphill avalanche,forcing the turbulence we must absolutely avoid if we're to ever reach low drag.
------------------------------------------------------------------------
Again,the Cd will be totally dependent upon the length.The aft-body is responsible for a 566% drag reduction on the body.If you're looking for the Lost Dutchman Mine,this is it!

[Rango318]

Thank you everyone for the ideas. I am sorry I have not been on to reply. Been without internet for a few weeks. In that time, I have done more driving, and have not been able to tell any improvement with the hatch up. Also, it was not possible, with only card board and duct tape, to make the hatch water proof, so i ended up with a very wet trunk a few times. For those reason, i have currently put the hatch back down. Maybe if i have the time this summer i will try again. Any more ideas are still much appreciated.