Class a motorhomes aerodynamics

[aerohead]

Originally Posted by sregord

Your patience is very much appreciated.


more from 8.5.4.5...The best geometry, with a cavity depth of 0.13d, reduced the drag coefficient by 5 per cent.

0.13 times 11'(tall) = 17.16"
0.13 times 8.5'(wide) = 13.26"



gluing, shaping, epoxing 2 or 3" thick pink foam to the perimeter of the rear edge, adding 13+" to the overall length of Moby ...like building a foam tail on an Insight... maybe?

I suppose, under an exact reading, Hucho's 'rear-end extension panels' qualify as a 'box-cavity', however, we can 'blow' by that performance, using Continuum Dynamics', aerodynamic backward's-facing step, captured-vortex technology.
It performs much like a tonneau cover on a pickup, except that, it travels down the rear sides too, and along the bottom, enclosing a four-sided 'step.'
And the 'length' and transverse 'inwards displacement of the 'step' coincides with the location a streamlined tail would intersect, as illustrated in Bilanin's patent.
Flow separates at the trailing edge of the body, but reattaches onto the end of the step, capturing a 'locked-vortex' in between.
Outer flow travels over this captured-vortex much as it would if the void were actually skinned over as a curved surface. It's a Lanchester/Prandtl surface of discontinuity in reverse.
As the outer flow moves rearwards, it also displaces 'inwards', along the 'curve' of the vortex, reducing the effective local cross-section, just like a boat-tail.
As it does this, it's decelerating, regaining pressure ( Bernoulli's constant total energy continuity ), and when it DOES separate at the 'new' trailing edge, it's at a higher static pressure, plus it's vectored towards the wake.
You end up with not only a 'smaller' wake, but a wake of higher base pressure, the goal of streamlining.
It's not as 'good' as a proper boat-tail, but it gets you 'towards' the performance, with less material, a lighter structure, and a few orders of magnitude easier to fabricate. All the reasons for Trailer Tail.

[sregord]

Originally Posted by aerohead

I suppose, under an exact reading, Hucho's 'rear-end extension panels' qualify as a 'box-cavity', however, we can 'blow' by that performance, using Continuum Dynamics', aerodynamic backward's-facing step, captured-vortex technology.... it's at a higher static pressure, plus it's vectored towards the wake.
You end up with not only a 'smaller' wake, but a wake of higher base pressure, the goal of streamlining.
It's not as 'good' as a proper boat-tail, but it gets you 'towards' the performance, with less material, a lighter structure, and a few orders of magnitude easier to fabricate. All the reasons for Trailer Tail.

So, you are suggesting this?
https://patentimages.storage.googlea...ngs-page-5.png

by "gluing, shaping, epoxing 2 or 3" thick pink foam to the perimeter of the rear edge, adding 13+" to the overall length of Moby ...like building a foam tail on an Insight"(ala 3-Wheeler), I meant modelling the adds to the very last iteration of the Betterflow semi ...with a bottom, not retractable...since I don't need to back up to a loading dock. That would be better ..yes?

[aerohead]

Originally Posted by sregord

So, you are suggesting this?
https://patentimages.storage.googlea...ngs-page-5.png

by "gluing, shaping, epoxing 2 or 3" thick pink foam to the perimeter of the rear edge, adding 13+" to the overall length of Moby ...like building a foam tail on an Insight"(ala 3-Wheeler), I meant modelling the adds to the very last iteration of the Betterflow semi ...with a bottom...since I don't need to back up to a loading dock. That would be better ..yes?

Understanding Bilanin's tail 'conceptually' is something worth knowing.
I'd throw a caution flag out with respect to his upper step as depicted. It may just be a conceptual drawing, and not meant to be taken literally as one would with a 'blueprint.'
The Continuum Dynamics tail was funded under an initial $50,000 SBA/ DARPA research grant, and subsequent $ 325,000 commercialization award. It's the 'real deal,' tested at NASA Ames', 'world's largest' wind tunnel ,at Palo Alto ,California.
Qualitatively, you'd project a tail like Mair's or NASA's onto the back of Moby, measure out 4-feet, then measure 'down', 'over', and 'up', vertically, sideways, and then vertically again, to define where the ends of the four top, sides, and bottom panels would terminate; creating the footprint of the 'steps' on Moby's rear.
If you were to be more aggressive, you'd run the risk that you'd fail to achieve flow reattachment. A disaster!
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If you place an epoxy compatible release barrier between Moby's surface and the blue Styro or pink Formular, you could fabricate the panels, knowing they wouldn't 'stick'.
Hard points could be glassed in for conventional fastener attachment.
I built Spindletop's boat-tail, fascia, and rear skirts directly on the HONDA using polyethylene 3M painter's film as a release. I used polyester resin.
EAA workshops at Oshkosh, Wisconsin showed us how to fabricate enormous sanding boards using belt-sander material 3M spray adhesive- adhered to straight or curved backer boards with handles.
We used blue foam exclusively, and all initial shaping was with a hot-wire.
West Systems makes a great line of resins for wet layups and bagging. Post-cure is just 'parking' the part in the summer sun. UV block under primer and top coat.
Definitely do the 'bottom.'

[sregord]

I'm not finding a "Continuum Dynamics tail"..but "... aerodynamic backward's-facing step" is plentiful. These only show the step(off) surface & the reattachment zone surface being parallel... except the step in a wing.
Following your graciously provided instructions "measure 'down', 'over', and 'up',"..etc. It appears that the length down stream the step could be longer than the reattachment zone..based on the Mair tail, just not shorter? Is there any concern regarding the 3-4" radius of Moby's corners..of the step?

The Ame's wind tunnel at Moffett Field is familiar. I called NASA (1998) requested a tour, "for my son's high school science project". We didn't get into the biggest, but had a great day touring a smaller one. One of those sure would be handy.

[freebeard]

Thanks for linking the pic of a 'Lanchester/Prandtl surface of discontinuity in reverse.'

I find this illustration particularly comprehnsive.



You find Wake boards in the upper left. I've never seen an example of the High momentum mud flap. The stepped box is especially helpful with the tail lights.

I think the egg-crate construction in the patent drawing could be reduced to triangles, but anything more would reduce rigidity, so why not?

[aerohead]

Originally Posted by sregord

I'm not finding a "Continuum Dynamics tail"..but "... aerodynamic backward's-facing step" is plentiful. These only show the step(off) surface & the reattachment zone surface being parallel... except the step in a wing.
Following your graciously provided instructions "measure 'down', 'over', and 'up',"..etc. It appears that the length down stream the step could be longer than the reattachment zone..based on the Mair tail, just not shorter? Is there any concern regarding the 3-4" radius of Moby's corners..of the step?

The Ame's wind tunnel at Moffett Field is familiar. I called NASA (1998) requested a tour, "for my son's high school science project". We didn't get into the biggest, but had a great day touring a smaller one. One of those sure would be handy.

Their research coverage may have been limited to Popular Science or one of the like periodicals. I'm sure I had images in my Photo Bucket account, but they were hijacked and I've lost the use of them.
As to step length, what we're trying to do is something Frederick Lanchester addressed in 1907. I published a link to his book, which illustrates a streamline body of revolution reduced to a very low-resolution bar-graph calculus.
Both the forebody and aft-body are presented as 'steps', of which the air makes its way across, even though reduced to 'chunks' of the original contour.
Think of the airship Hindenburg made up of tiny steps rather than a smooth ,continuous contour.
As long as the gross contour remains, you're still maintaining sectional density within enough resolution that, any pressure spike present, does not have enough adverse pressure gradient as to permanently 'trip' the boundary layer flow, with zero chance for reattachment.
NASA's tail, has an aggressive 'buttocks', and while it never slopes beyond 20-degrees, it's 'radical' compared to Mair.
Mair has a more subtle buttock, allowing a more gentle pressure rise, less adverse than NASA's.
Walter Korff, of Lockheed Aircraft Co., and designer of the Cd 0.1165 Summers Brothers' 409-mph' Goldenrod', has an even more gentle 10-wheel tractor-trailer van boat-tailing, Cd 0.20. It's reminiscent of the Fairchild C-119, 'Flying Boxcar' aft-body. That may be a 'lost' image as well.
Korff's tail uses the body width as the foundation for design. It would be a very 'safe' architecture, with high probability of success to a non-millionaire.
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The rear radii on Moby will separate at 5-'seconds' after 'noon' on their curvature, if you're observing from the 'sides', relative to flow. They've got to.
So you can just ignore them, and treat them basically as a 'straight' edge, if that eases fabrication and attachment, leaving all the actual streamlining to the new additions.
PS That's great that you guys got to see Palo Alto. It's on my bucket list.

[sregord]

Originally Posted by freebeard

Thanks for linking the pic of a 'Lanchester/Prandtl surface of discontinuity in reverse.'

surface of discontinuity in reverse = backward's-facing step
I like it.
Seems to be alot of pouching of graphics in aerodynamic.
I found this one in the: Alternative approaches to rear end drag reduction
Technical Report, Torbjörn Gustavsson, Stockholm 2006

[aerohead]

Originally Posted by sregord

surface of discontinuity in reverse = backward's-facing step
I like it.
Seems to be alot of pouching of graphics in aerodynamic.
I found this one in the: Alternative approaches to rear end drag reduction
Technical Report, Torbjörn Gustavsson, Stockholm 2006

I believe that it's one of his papers which goes into the shortcomings of NASA's boat-tail, and that of the entire tail structure of the Lockheed C-130 Hercules.
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And since the Continuum Dynamics project was tested at NASA Ames, that graphic you posted may be associated with that very research.
With a full belly pan, level with the bottom of the rear axles, a somewhat weaker 4th vortex could be added at the bottom.

[sregord]

So ..I'm headed to HomeDepot for 8 pieces of 1" pvc pipe.
strap them to the 4 corners of Moby with corner tees on the other end, wire then together & pull until I'm at 20*.

You guys are great. to put up with me.

[freebeard]

Seems to be alot of pouching of graphics in aerodynamic.

Is that like gunny-bagging?