Project, the Third - Now contains collabsible boat tails (1971 VW SuperBeetle)

[freebeard]

Cd -- Thanks. I need to address other issues before I move forward with this.

arcosine -- That's a pretty tall tire. Equivalent to 7.60-15? I've got stock tires (165R-15, equivalent to 6.50-15) on the back and 165/50R-15 on the front.

Was the variable main jet in a stock carb? Google shows them for Dellorto and Weber carbs. Did it run hot?

I've done the brakes front and rear, and rebuilt the front suspension. My problem must be in the alignment or bearing pre-load. Has anyone done a comparison of coast-down time across various vehicles? My son's SRT-8 takes a lot longer to shed 20 mph, and that's with much wider tires. I should probably raise the question in General Efficiency.

[arcosine]

It was the stock carb (solex) for the 1500cc single port engine and an aftermarket adjuster that fit into the plug in the float bowl, maybe search ebay or JC Whitney. Don't remember where I got it. I don't know if it ran hot, I did get a vapor lock once.

Adjusted front and rear toe in for zero toe using string. HR78 on rear, Gr78 tires in front.

[aerohead]

If you'll go over to the 'full-boat-tail-trailer thread,there are some pictorial drag tables I put together from wind tunnel test reports on boat tails.You should be able to get a sense of what would do what.
When I did the initial inflated tail on the truck it developed an uncontrollable vertical 'wag' oscillation.
I ended up placing a rigid bonded spar underneath it which killed the displacement.
Your fold down structure would provide for the support you'd need for a stable inflated tail.
When Carl Breer's team put a 'bad' boat tail on the Chrysler DeSoto Airstream test mule they were still able to pull off Cd 0.244.The Beetle is very similar to the 'beaver-tail' Airflow and I think you'd be laughing all the way to the bank with the tax-free pay raise the tail would give you.
The challenge is providing cooling air,but NACA submerged inlets could work for that.And the tail itself would allow the boxer engine to run cooler.
I got my VW bus as high as 35 mpg with the 1,500 cc,57-bhp engine.
You could use taller gearing for top gear.I still have to address that one.

[arcosine]

Arrowhead is referring to the taller bus 4th gear, which can be swapped in.

[freebeard]

arcosine -- I'm with you on the taller gearing. One of the parts I have is a 5-speed conversion transaxle. It basically has a low and a high 4th. I've seen the original receipt recently and it tells the ring and pinion ratio, but I don't recall. With swing axles and Type III drum brakes, it is intended for the panel van.

I found the variable main jet discussed on thesamba.com. It's out of production and it was for the 30 PICT. I've got a 34 PICT. Once I've got a mileage baseline, I'd like to try the Gene Berg Jet-A-Vator. It's a restrictor plate, plus.

aerohead -- So far I've found Full-boat-tail trailer with gap-fillers for Toyota T-100 pickup and 2011 Prius to get full boat tail w/ faired trailer that spun off from it, but I haven't found the earlier post referenced on the first page. But that's 38 pages anyway, so I've got some reading to do.

The challenge is providing cooling air,but NACA submerged inlets could work for that.

I've given some thought to taking engine air from a horn shaped scoop that runs along the drip rail as it departs from the template line. I tried to picture it in Photoshop but I wasn't satisfied. I need to get back to modelling in 3D. Until then just imagine sucking up to 600 cubic feet per minute of the air that's trying to spill over the edge. Do you think that could kill vortex formation?

[aerohead]

Originally Posted by freebeard

arcosine -- I'm with you on the taller gearing. One of the parts I have is a 5-speed conversion transaxle. It basically has a low and a high 4th. I've seen the original receipt recently and it tells the ring and pinion ratio, but I don't recall. With swing axles and Type III drum brakes, it is intended for the panel van.

I found the variable main jet discussed on thesamba.com. It's out of production and it was for the 30 PICT. I've got a 34 PICT. Once I've got a mileage baseline, I'd like to try the Gene Berg Jet-A-Vator. It's a restrictor plate, plus.

aerohead -- So far I've found Full-boat-tail trailer with gap-fillers for Toyota T-100 pickup and 2011 Prius to get full boat tail w/ faired trailer that spun off from it, but I haven't found the earlier post referenced on the first page. But that's 38 pages anyway, so I've got some reading to do.



I've given some thought to taking engine air from a horn shaped scoop that runs along the drip rail as it departs from the template line. I tried to picture it in Photoshop but I wasn't satisfied. I need to get back to modelling in 3D. Until then just imagine sucking up to 600 cubic feet per minute of the air that's trying to spill over the edge. Do you think that could kill vortex formation?

The vortex formation will be triggered by shearing forces of tranverse flow where slower,higher pressure air alongside the body attempts to bleed off into the faster,lower pressure air above the drip rail.
The VW Transporter,as a 'square-back' type body won't produce vortices,and the configuration of their cooling air inlet scoop allows for outer flow attachment downstream of the ram opening.
With the boat tail,it will provide an exo-duct diverging nozzle outside of which the streamlines can even better blend together as they gain pressure.
On my bus,I never modified the inlets and the boat tail was just integrated into the existing curvature,especially on the roof.I drilled through the rear drip edge and used it as a hard point for attachment of the tail's skeleton.

[freebeard]

Originally Posted by aerohead

...an exo-duct diverging nozzle...

Whenever I think I understand things, I just go read that again.

Like a lot of others, I started by comparing The Template to the side view of my car. I was dismayed to find that the boat-tail would be 98" long—about equal to the wheelbase of the car.

Oppan Tropfenwagen Style
Working only in plan, from a belt-line width at the back of the doors of 56", I find that a complete boat-tail would terminate ~36" from the present back bumper, making a 14' car 17' long.

I ran into a stumbling block on another project, so rather than be thwarted, I grabbed two 12' pieces of Redwood bender board, bungied them to my bike rack and clamped the tips together. The vertical stake holds the end at a height that is just out of sight in the rear-view mirror.

It appears that 10" lengths would have been better and I'm not sure about the acute angle at the very rear.

Then I made measurements and took them home to compare with <<template MRT1022.jpg>>. It has a fineness ratio of 2.5 and a max width 30% back from the tip.
I consider 3 stations on the shape. Setting the rear length to 7/4ths of max width (56") gives 98". At 41" behind that the very ends of the drip rails are 41" apart, slightly narrower than the template line. At 61" back we find the bike rack at 36" wide, slightly wider than the template line. 98" less 61" leaves only 37" from the inside of the bumper (it's curve and thickness add ~12" to the overall length) to an un-truncated tip.

I'm curious about the Template compared to the Dymaxion and Schlorwagen. The line in plan on the Dymaxion falls between the other two, but the Tropfenwagen looks to be pretty sharp.


I'm thinking that a backlight like a Studebaker Starlight coupe would be easy to fabricate with two sheets of lexan and a center divider; but I also like the idea of leaving the stock backlight exposed with a little triangular Redwood deck below it.

The next mock-up will have pieces cut to the angle of the drip rail, and have a properly angled wedge at the back. Aluminum flashing sandwiched behind the fender beading will offer an opportunity for mounting points below the drip rail, leaving the stock fenders and tail-lights un-modified.

I'm not seeing a lot of collapsible at this iteration. The trailer frame and wheel are starting to look like a lot of dead weight. I can make a curved triangle from 1" or 1 3/8" tubing that will plug right in like the bike rack does.

An update on gas mileage: Early in October I did 106 miles, mostly in convoy at 55mph, and saw 21.8mpg. Needless to say I was a little bummed. I filled up again on the 31st. Once again I traced my route in Google Maps and got a distance I can compare with my odometer. There is some uncertainty because I may have made one 22 mile round trip 4 times instead of three. Google says 259.9 or [282] miles. The odometer says 390 miles. So my actual mileage is either 66% or 75% of the odometer reading, and my gas mileage, driving normally (freeway mixed with intown and two-lane shortcuts through the hills, e.g. pretty average for me), is either 26.8 or 28.8 mpg. Or I could be stupid and say I'm getting 40.2 already!

I'm pretty OK with that. When I refilled it was with E85 regular. I've been been burning Ethanol-free Premium, which is down from $5.19 to $4.69 within this month. I'll probably alternate tanks to compare the differences.

[aerohead]

Since the Beetle is kinda tall with respect to it's width,then consider using a symmetrical wing section as your 'Template' in plan view.
*On the last page of the 'Aerodynamic Streamlining Template Part-C' thread is some data on sections.
*The second pictorial drag table has the lowest drag section.It has a chord-to-thickness ratio of about 3.9:1.You can measure it off the table.
*Anything 'thinner' or more 'bluff' will have higher drag.It is the drag MINIMUM.
If you'll match your roof curvature to the sides you should have zero longitudinal vortices.
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If you're going to do rear wheel skirts,then I recommend you leave their width alone,but add fairings in front of,and behind them to soften the pressure spike which they cause which will impact the boat tail.
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If you're going to have exposed wheels and run MOONs,then I recommend you reduce their frontal area by trimming them and extending them down close,and flush with the outside of the wheels.
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If your tail extends only as far as the rear edge of the rear bumper you should see Cd 0.244 territory as with the 1934-1/2 Chrysler DeSoto Airflow experimental.
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The inflatable tail will get you lower.
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*The Rumpler Tropfenwagen only achieved Cd 0.28.You can do better.
*The Dymaxion Car is estimated at Cd 0.25 by one source.You can do better.

[freebeard]

Thanks for your comments.

It has a chord-to-thickness ratio of about 3.9:1...
*Anything 'thinner' or more 'bluff' will have higher drag.It is the drag MINIMUM.

I'm looking for something in the 'wake-filling/attached flow' range. But maybe more aft length would compensate, is some general way, for the less than optimum fore-body. A fineness ratio of 4 suggests ~160" of aft body without truncation.

If you're going to do rear wheel skirts,then I recommend you leave their width alone,but add fairings in front of,and behind them to soften the pressure spike which they cause which will impact the boat tail.

This is exactly where I'm headed; treating the windshield and back as one case, and the rear fenders and back as another. Maybe on the back side of the front fenders I have an opportunity to use Coanda nozzles.

I'm headed out right now to take another pass at the mock-up, and also my aerodynamic garage.

[aerohead]

At GOOGLE IMAGES there is a photo of a Beetle in VW's modern tunnel with smoke being blown over it.I think it's in 'WINDKANAL' as a search title.
Anyway,the forebody looks okay,it's only at the top of the backlight that she 'loses it'.
As with the Airflow,I think the Bug would respond favorably to the tail even if you were to do nothing at the front.
I would work on the sides.All that you can stomach.
But (yeah my English teacher would spank me) it would be better to do a truncated tail of proper design,rather than a 'full tail' of improper design.
I just posted a thread with W.A.Mair's boat-tailing.You might want to give it a look.