Quote:
Originally Posted by ennored
OK, took care of the 5 posts with an intro over in the intro section. On to my questions!
I've read the Aero RV thread, and the "Dryden Van" paper. I wonder what a boattail would look like on my motorhome.
The aero template tool to the rescue:
Hmmm, looks like it would need to be long, VERY long. 1.78 times the height (132") is almost 20' long! That can't be, the Dryden van tail is only 3 1/2 feet. The Aero RV is like 4'. Hmmm, maybe the template is misleading. How much would a 4' tail actually taper in? Some math (from another thread) says that at the 20% point (4' of the 20' tail) it'd be at 7.5° and in about 5.5". Yup, template DOES look right.
Thought more about the Dryden Van, and templated it:
Well lookie there, it doesn't fit! Those dummies at NASA....
I didn't see a nice shot of the Aero RV to template, but by eye from the pictures that are here says it doesn't fit either.
More thinking. Find more examples.... I've known about the "TrailerTail" product for a while. Armed with the new knowledge on templates and such, looked again at it. Interesting. Flat sides (not curved), 4' long. Tapered in at 15° (yields about 12" "in").
That's where I'm at. In my head are the template, and a few actual large vehicles that have about a 12" taper (Dryden van is like 11" by my calculations).
Is there something to large vehicles that "confuses" the template? Maybe there's a length factor not captured? Or is it simply the boattails I've found are not optimum? TrailerTail and Aero RV, maybe, but the Dryden Van? Maybe it's the shape and 22° that are really important?
I welcome the experts' thoughts. Cool thing for my motorhome is I have a separate fiberglass rear cap that is a bit over a foot long now. Something that adds 2 or 3 feet is actually reasonable. A 3 sided, hollow centered tail could actually be done quite neatly.
(Also of note, I have a email in to a former aero engineer I know that is now at Navistar, the folks that did the Monaco Vesta. I hope to have some answers from him I can share here soon.) |
My suspicion is that since the DOT will allow only a 48- inch rigid boat tail extension,or a 60-inch inflatable boat tail,that folks are trying to arrive at an 'optimum' architecture for these arbitrary lengths, knowing that they'll never approach the drag minimum.
They're going to be looking at mass (lost cargo capacity),so a 'minimum' of material (weight) will be a driver in the ultimate design.
Since the vans are square-edged,there's already a body of research for these shapes (please see the full-boat-tail trailer thread for pictorial drag tables ).
I'm conservative when it comes to spending others time and money and it's my opinion that a truncated 'ideal' boat tail is better than a 'full' mediocre boat tail.
From the body of wind tunnel research it is apparent that all the really low- drag boat tail designs embody very gentle initial curvature to protect the boundary layer.
Streamline bodies,airships,submarines,bullets,fuselage,artill ery projectiles,turbojet tailpipe 'bullet' valves,hydoelectric Pelton wheel nozzle jets,birds,fish,marine mammals,etc.,all utilize these very subtle initiating aft-body contours.
Hucho states that the entire premise for road vehicle aerodynamic streamlining is the reduction,or elimination of flow separation in the aft-body.
Contours such as in the 'Template' or W.A.Mair's boat tail,or in an analog clock face from 12:00,to 4-seconds after 12:00,all possess the ability to foster attached flow within a diverging flow regime.Anything steeper and you've got separation.
The thing about the 'Tempate' is that it is intended as a contour for a complete boat tail of zero wake,not a compromised length.
For a 'short' optimum tail look to General Motor's 'Optimum' boat tail of the 1990s.Texas Tech University's Mechanical Engineering Department's Aero Lab was employing it in their 18-wheeler drag studies of 1995.
Baron Fachsenfeld achieved Cd 0.159 with an inflatable boat tail on a bus he designed for the Omnibus Corporation in Germany in 1936.It's contour is very close to the 'Template.'He worked with Professor Kamm at the FKFS.His tail probably represents the state-of-the-art in boat tail design.
Any large RV or commercial vehicle should be able to mimic the Cd 0.159 architecture of 1936.These shapes are at least a half-billion years old.
