elongation and drag

[aerohead]

Hucho mentions in his books that,after detail optimization,lower drag can only be achieved by elongating the body to reduce or eliminate the turbulent wake.(This is the entire premise of ground vehicle streamlining)
Here is an illustration to show what he's talking about.


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The lowest drag for a 3D-flow automotive body is achieved when the body has a length which is 5X it's body height,or an effective fineness ratio of 2.5:1


For 2D-flow,as with a motorcycle,the minimum drag is achieved when the body is about 4X as long as the body is wide


As you can see,for either 3D or 2D flow,each body has a 'sweet spot'.If the body is shorter or longer,the drag rises.
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If the project vehicle has attached flow up to it's rear edge,then simply boat-tailing the body will provide the separation-free reduction,or elimination of the wake as you can see in Mair's boat tail research.

One only needs to decide what car length they're willing to live with.
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'Active' aerodynamics offers the option of a 'longer' car only on the highway,while a 'shorter' car for around town traffic and parking






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Here is the Daimler-Benz,Mercedes-Benz C-111 III of 1978 at three different lengths which illustrate what the boat-tailing did to drag coefficient


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[freebeard]

*The forebody of the Seven looks like it borrows from a 1956 Porsche 356A,of Cd 0.34,according to Hucho

Rather tenuous. The bigger, squarerer windshield kills it.

*If they had Kamm's (sense of surface development) we might be looking at Cd 0.155.

Did I mention it's hideous and I hate it? Volkhart-Sagitta fan.

Edit: Sorry for the negativity. So the logical way to extend a Beetle-shaped body would be with Tatra-style cooling intakes.

I'm thinking air scoops that are fender skirts with the whole front edge open, the inner face of the tire making one face of the duct. Then the rear edge following the curve of the back edge of the rear tire, like that '31 Bugatti Type 51 I posted in the Interesting Cars thread. Lay a sheet on a circle, pull the free edge down maybe 10°. The sheet will deform into s softly curved shape. Hammer-form that.

Then the upper body taper would be maximized (vertically) with ±3ft added length, with a 4-piece backlight like a '48 Studebaker Starlight coupe but with a 45° gathering angle.

[aerohead]

Here are some low-drag examples Hucho shared,from Rolf Buchheim et al.' wind tunnel research at the VW climatic wind tunnel.





[IMG]http://i1271.photobucket.com/albums/jj622
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A hand full of the cars are at Cd 0.16,to Cd 0.15.
"A limit of between 0.14 and 0.16 results for a basic body with the same dimensions,but this time with wheels."
Lengthening the body is required to get below Cd 0.15,as in the long-tail VW 'Flow' body

Walter Lay measured Cd 0.12 with his long-tail model of 1933


Kamm and Fachsenfeld evaluated Lays model at the FKFS a decade later and got the same number.
All of 'Kamm's' research was predicated upon the streamline half-body of 1922,or wing sections,experimenting with truncation position vs drag











Kamm strove for an optimized drag compromize between the ideal long-tail,

and a 'practical' length variant

Kamm's best effort realized Cd 0.23,a little batter than a 2016 Tesla S


Wolfgang Klemperer measured Cd 0.15 for this model in 1922

Klemperer measured Cd 0.15-0.13 for Jaray's 'pumpkin seed',depending on lower edge radii according to Hucho

Jaray's body incorporates the first-known diffuser.

[aerohead]

The Illuminati Motor Works 'Seven' entry for the X-Prize competition illustrates what Hucho's been talking about since the mid-1980s.
If you imagine starting with something like a 1950s Porsche 356,Cd 0.34,and stretch the body out to the rear,you end up with Cd 0.165 according to the Illuminati team.



Had the rear track been narrowed,and the pontoon fenders blended away,the uniform plan-taper would have allowed for even lower drag.

[freebeard]

aerohead -- What benefit do you reckon from leaving the tail as-is and going to a Talbot-Lago/Volkhart-Saggitta style cowl and windshield area?

[aerohead]

Originally Posted by freebeard

aerohead -- What benefit do you reckon from leaving the tail as-is and going to a Talbot-Lago/Volkhart-Saggitta style cowl and windshield area?

From wind tunnel experiments,the big dogs say that there's nothing magic about the fore-body of a car as far as drag goes.
Once the leading edge radii are optimized and you've got attached flow,your essentially 'done.'
For further drag reduction you've got to extend the rear in a way which allows the flow to de-accelerate,lose it's velocity,and regain it's former static pressure (minus what is inevitably lost due to the turbulent boundary layer); reducing or eliminating the turbulent wake.
This is the entire premise of automotive streamlining.It's all about the back of the car.
Both the Lago and Sagitta suffer from attached longitudinal vortices and lager-than-necessary wakes because of their steep rooflines.Hucho calls them 'pseudo-fastbacks.'
In this photo series,the center image is a 'pseudo-fastback' with large flow separation over the roof and attending longitudinal vortices.
The bottom image is a proper fastback,with only TBL flow disturbance basically,and zero vorticity.

These cars are all pseudo-fastbacks









They all have short aspect ratio greenhouses.'Kamm' was obsessed with this body relationship which was referred to as something like Ver'u'ngenverhaltnis.
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These cars are proper fastbacks (truncated)





Non-truncated

[freebeard]

I take your point about the back half. I'm just suggesting that if they had stuck with a Speedster windshield and less angular mid-body it would have more of a 356 feel and possibly better cross-wind performance.

[aerohead]

Originally Posted by freebeard

I take your point about the back half. I'm just suggesting that if they had stuck with a Speedster windshield and less angular mid-body it would have more of a 356 feel and possibly better cross-wind performance.

Yeah,I'm not sure where they were coming from.The original architecture of the 356 is hard to fault.

They were in a 4-seater class and may have felt impelled to soften the tumblehome to create more headroom in the cabin,then do the bubble roof to trim back on frontal area.Don't know.
I wonder why they did the pontoon fenders at all.There's no aerodynamic reason for them,unless pylon testing showed up a problem with body roll or something,and they thought they needed a wider track.
As far as the cross-section goes,it's perfectly okay to go from a semi-circle to a square section,then morph back to a semi-circle for interior packaging reasons.The air doesn't care.It 'IS' very important to get the aft-body correct though if you do it.
The fastback form has the lowest possible drag only if it has the 40-degree minimum tumblehome,and soft upper edge radii.

[freebeard]

I suspect they let the internal structure dictate the external form.

http://porsonly.com/matrix-143-porsche-troutman-barnes-911-4-door-green-1967/

The Troutman-Barnes 4-door 911 (in 1/43rd scale)

The fastback form has the lowest possible drag only if it has the 40-degree minimum tumblehome

Measured off the vertical or horizontal?

[aerohead]

Automotive fastbacks are the lowest drag form cars according to Hucho,and would always be measured from the vertical dimension.
'Narrow' vehicles,as with Semis,delivery trucks,and some land speed vehicles might favor the 2-D airfoil profile in plan-view.
The 'wide' 1957 MG EX-181 went ahead and used the airfoil profile for it's plan-view