Why does The Template have to be a half body of revolution?

[freebeard]

Tell you what. I'm sitting on some intellectual property that I refuse to disclose to the predatory capitalists (i.e., patent) but I have some responsibility to give back to Humanity. I'm curious about Opensource Architecture and Creative Commons Licensing.

Do you have any opinions on that?

[aerohead]

Originally Posted by freebeard

My understanding is that The Template is a half body of revolution to insure that air doesn't want to move laterally because of pressure differentials between the top and sides, which leads to vortex generation.

If that's the case, wouldn't a half-square cross section serve as well, given equal cross-sectional area? If air is not wrapping across the edge, would no vortexes be created?

Here are three aeroforms, a cylinder, a superellipse and a square. Would their Cd be equivalent?


(These are not dimensionally highly accurate; I just eyeballed the proportions against The Template)

The next question: If the height/width proportion changes, what happens to the respective tapers? If the half revolution form is 7' wide, then it is 3 1/2' high. If the height is proportionally greater, does the side taper increase or relax?

TIA

I've been chugging back through Aerodynamiks des Kraftfahrzeugs by Koenig-Fachsenfeld and looking at material he worked on under Kamm at the FKFS which is germane to the 'organic' vs 'cubist' teardrops.
It looks like they tested both images (left) and (right) as passenger car bodies,with wheels,at 'standard' ground clearance.
The (left) body produced Cd 0.12,the (right) body Cd 0.21.
I've contacted a local German tutor to help with translation,but from the tabulated data I think this is what they found.
FIAT,in a 1986 SAE paper investigated fastback bodies and demonstrated how the Cd is corrupted without both tumblehome and edge rounding,something which is a feature of the body of revolution.
Jaray cheated the form with his pumpkin seed of 1922 so we've got 'options.'
I'll try and have something for Saturday at the latest.

[aerohead]

Originally Posted by aerohead

I've been chugging back through Aerodynamiks des Kraftfahrzeugs by Koenig-Fachsenfeld and looking at material he worked on under Kamm at the FKFS which is germane to the 'organic' vs 'cubist' teardrops.
It looks like they tested both images (left) and (right) as passenger car bodies,with wheels,at 'standard' ground clearance.
The (left) body produced Cd 0.12,the (right) body Cd 0.21.
I've contacted a local German tutor to help with translation,but from the tabulated data I think this is what they found.
FIAT,in a 1986 SAE paper investigated fastback bodies and demonstrated how the Cd is corrupted without both tumblehome and edge rounding,something which is a feature of the body of revolution.
Jaray cheated the form with his pumpkin seed of 1922 so we've got 'options.'
I'll try and have something for Saturday at the latest.

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I'm still working through the German-English translation and I haven't heard yet from the tutor but have roughed-in some text and believe that the following is an accurate portrayal of what K-F and Kamm did at FKFS:
*Using a slice of airfoil twice as wide as the section height,with a thickness ratio of 15.38%,with a ground clearance equivalent to standards circa 1938 (around 9.25-inches (235mm)) they got Cd 0.19.
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*Adding wheels gave Cd 0.24.
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*Adding plan-view camber to the body produced Cd 0.1938.
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*Adding rounding to all the upper edges produced Cd 0.161
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*By restoring the plan-view radius to the nose produced Cd 0.12.
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*K-F/Kamm did 'remeasuremnts' (Nachmessungen) to a reproduction of Walter Lay's 1933 aerodynamic research model from the University of Michigan.They recorded an identical Cd 0.12.
*Lay's model# 10 with 'JEEP Wrangler' windshield couldn't better Cd 0.24,as with the FKFS compromised nose.
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FIAT's aeronautical laboratory,in their SAE Paper# 860212 reported that a 'fastback' car (Template) suffered a 32.6% drag increase with compromised forebody leading edges, which reinforces the conclusions of Lay,K-F,and Kamm.
*Fiat's research also showed that of fastback (Template),notchback,and squareback body styles,that the fastback was most sensitive to upper body rounding and suffered the greatest drag increase without it.
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*Jaray and Kamm were both airship 'Luftschiff' people.
*Jaray was designing Zeppelins at the same time Kamm was serving in military with observation balloons.
*Kamm's doctoral dissertation was on streamlined teardrop-shaped balloons of which he patented circa 1919 as the K-Balloon.
*All Allied barrage-balloons of WW-II were lineal descendants of the K-Balloon.
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*At FKFS,the half-airship 'halbeluftschiff',Half-streamline body of revolution 'halbestomlinienrotationsk'o'rper',was always the 'Alpha' form from which all the attempts to 'cheat' were done.
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*The half-body has a good enough forebody for subsonic flow.
*Most importantly,it has the sectional density in the aft-body necessary for the gradual pressure rise which will prevent the reverse-flow that triggers separation and the attendant pressure drag aerodynamicists demand that we avoid if we're to achieve really low drag.
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*We can push the shapes around in frontal elevation.Jaray did it.Lay did it.Kamm did it.Which means that production automobiles are game.It is a condition of boundary layer 'legislation.'
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*We can't mess around with it much in side elevation or plan-view.If anything,you'd go hyper-template,but never steeper.
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*With 'zero' streamlining you're looking at Cd 0.86.
*With ' complete' streamlining Hucho says we can approach the drag of the body sans wheels (0.07-0.09).
*Hucho also says that these lower values will be achieved only through airfoil and half-body forms.
* You can consider the 'practicality' of a 'wing-car.'
*And we've demonstrated above what happens when you 'template' a 'wing-car'.
You can fool most of the people most of the time.You can fool the atmosphere non of the time.

[freebeard]

Thanks for thinking about this.

The (left) body produced Cd 0.12,the (right) body Cd 0.21.

That's gold right there. I assert that even though the squircle is mathematically demonstrably half-way between, it is closer to the half-body aerodynamically ...but/and better suited to packaging passengers.

At FKFS,the half-airship 'halbeluftschiff',Half-streamline body of revolution 'halbestomlinienrotationsk'o'rper',was always the 'Alpha' form from which all the attempts to 'cheat' were done.

This would describe my approach.

I'll have more to say later but I'm mocking up a Breer's-style stinger on the '58 Beetle body. It will have a three-piece windshield similar to the old Shalako dune buggy (posted as the VLC conncept) and a boattail that extends the interior space straight back. It's an hexagon at the bulkhead 35" across at the top 43" at the center and 30 " deep, i.e., 39x30=1170sq in. This tapers to a line 16" tall 48" back, but truncated to an 8x16" base plate (768sq in). The front and rear wheels and the diffuser enclosing the engine are To Be Determined. I'm learning that the gutter and door cuts are splayed outward. A nice subtle change would be to pie-section the body vertically so the A- and b-pillars are the same width moving the widest point forward half the width of the door.

I notice it's my thread so I will go off-topic with my own permission:

Here's a screen grab from a few moments ago. I'm seeing a body kit for the Beetle that has front fenders that trade the iconic Beetle crown for a BMW-style shave, a rocker extension that replaces the running board, a panel that mounts on the lower door that swallows the lower door hinge and raises the swage line, and a slot on the inner edge of the fender that exposes the break to the inner fenderwell (opposite to the vent on the Corvette).

Possibly 4 crescent shaped box cavities on the fenders?

Edit: Well, I can't sleep. the whole body kit could be done in PolyMetal as simple curves. With curved scores in the flat sheet and basically a sector bent into a conic section. I made fender flares for a '62 13-window once that way.

The edge in the wheel arch would be wire-wrapped (tests soon) and the edge at the body mounting bolts would be crimped—probably heavily— between the mounting holes.

The rear fenders could start as a scoop at the door opening and terminate in a semicircular arc starting behind the rear wheel openings and canted back at about 30°.

Speed-whiskers off the tops of the front fenders to bury the top door hinges.

[aerohead]

The squircle would be real good.So many production vehicles already have a body section like that.
Jaray's was like it in the center of his pumpkin seed.
The FKFS K-cars were like it for the same reasons you mention.
As long as the contours are gentle the boundary layer will be protected.

[ennored]

Originally Posted by aerohead

The (left) body produced Cd 0.12,the (right) body Cd 0.21.

Guy I used to work with always said "If you were an air molecule..."

So, if you were an air molecule headed straight at the front of the three shapes, where would you like to go?

The left body is easy, any path around it is the same. The ones to the right are different. Superimposing a clock face, the 12, 3, 6, and 9 o'clock paths are the same simple paths from the left body. Anything but those paths is a longer path. And the war is lost...

[freebeard]

I should really redo that graphic with the shapes truncated to half-bodies. There is no 6 o'clock.

But, still... I get what your saying, but would not the individual molecules simply be less likely to take a path at 10:30 or 1:30.

aerohead nailed it. The battle is lost in cross-winds, the nominal condition.

[ennored]

Originally Posted by freebeard

I should really redo that graphic with the shapes truncated to half-bodies. There is no 6 o'clock.

But, still... I get what your saying, but would not the individual molecules simply be less likely to take a path at 10:30 or 1:30.

Yeah, half bodies, I didn't type 6 when I first typed a response.

Sure, the air molecules would LIKE to take the easy path. But the air molecule before you already did. Easy path isn't so easy. Air molecules bumping into one another. Energy wasted.



Another thought. A .21 Cd for the shape on the right makes things like the Dryden van (which is shaped like the right shape) seem pretty good at a .25 Cd (approx. - from memory) in the real world. Hmmm....

[freebeard]

Air molecules bumping into one another. Energy wasted.

Brownian movement. I think of them as little herd animals in a stampede, or little schools of fish or flocks of birds. Each, though inanimate, trying to reach its happy place. Energy is only wasted when that 'intent' is thwarted.

[aerohead]

Originally Posted by ennored

Guy I used to work with always said "If you were an air molecule..."

So, if you were an air molecule headed straight at the front of the three shapes, where would you like to go?

The left body is easy, any path around it is the same. The ones to the right are different. Superimposing a clock face, the 12, 3, 6, and 9 o'clock paths are the same simple paths from the left body. Anything but those paths is a longer path. And the war is lost...

Hucho 'said' that where we get into trouble with the square edges is that as the high pressure under the vehicle will try to seek equilibrium with the lowest pressure up on top.
As the two airstreams collide at differing velocities,they'll whip up into attached vortices on the corners,as rising moist super-cell Gulf air over Colorado would collide with the jetstream overhead,initiating the vorticity that will spawn tornadoes as the vortex rolls eastward eventually rotating into the vertical super-vortex over Kansas.
With any body section closer to a body of revolution,as you say,the pathways are more similar,and so to too with velocities.
With sharp lower edges,and even running boards on a pickup,it's harder for the high pressure underneath to even telegraph itself up onto the sides of the vehicle.