The most aerodynamic shape possible
 

Is there anything that is more aerodynamic than the airfoil shape?

does something that perfectly flat that runs horizontally (something like the shape of a CD) more aero dynamic?

context
 

*'aerodynamic' has no real meaning until a context is applied.
* trees are aerodynamic,so are mountains,fences,lungs,vocal chords,trumpets,........................... you get the picture.
*If you mean 'lower drag' than something else,then probably not.
*Investigators have found that a thickness at the leading edge,and then gentle taper to a point produces the lowest drag.
*For 2-dimensional flow,a symmetrical wing section of chord/thickness ratio = 3.94:1 produces the lowest drag.
*For 3-dimensional flow in free flight,a streamline body of revolution of length/diameter = 2.1:1-to-approx. 3:1 has demonstrated the lowest drag.
*For 3-dimensional flow in ground proximity,a 'half-body' of length/height = 5:1 has demonstrated the lowest drag,given the peculiarities of bluff-body flow in ground-effect.
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If any of these structures are shortened,they experience a drag increase due to separation-induced pressure drag.
If any of these structures are lengthened,they undergo a drag increase due to increased skin-friction drag.
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There are some 'laminar' forms investigated for submarines and airships which as of the mid-1990s had not evolved beyond the CFD stage.Any spanwise flow on these forms triggers an immediate transition to turbulent boundary layer and all benefits are lost.
For automobiles these forms offer no gains,as the 'source flow' itself is turbulent once to 20 mph and again,the transition to TBL is immediate.

Nice post Aerohead, I got something new out of it, or just maybe it's starting to all sink in.

Taper41, if you have a lot of time to kill, start at page one of the thread below.
http://ecomodder.com/forum/showthrea...-c-9287-9.html

From page-9 the template I like to use:
http://ecomodder.com/forum/member-ba...dow-format.jpg

The idea car? (Page-7) Aerohead did this one
http://ecomodder.com/forum/attachmen...p;d=1296941870

Hey Aero,

if we take the car in the picture above, and start right above the back window.

then we angle down at 20 degrees. Will we end up with almost the same aero, but have a much shorter car?

No, as far as I understand it (but I'm not Aerohead).

I think thats the point of the 5:1 comment in close proximity to the ground, verses the 3:1 of "free air".

A car is three dimensional, so the 2D profile of an airfoil is only part of the picture. the sides should be tapered -- in fact, the sides can be tapered about as much as the top, and since the taper happens on both sides, this would shortened the length quite a bit, and end up forming a "fish tail".

Look at the Edison2 VLC (Very Light Car) for an example of this. The VLC was tested in the GM wind tunnel and got a Cd (using the older SAE formulas) of 0.145; so this is comparable with most of the Cd numbers we have used here on EM. The newer SAE formulas gave a Cd of 0.164. The VLC has a relatively "pointy" front end and outboard wheels, so if you apply the above profile to a chassis that has enclosed wheels (and a much narrower rear wheel track), then the Cd could likely be even lower.

Correct, here is one of the other useful images from page-1 of the C-template thread:

http://ecomodder.com/forum/showthrea...rt-c-9287.html
http://ecomodder.com/forum/attachmen...p;d=1247946422

20 degrees
 

I'd hoped to have Walter Lay's work posted by now.Hucho touches on it but kind of butchers what he did.
Lay did not follow the 'Template' per se,but he did pull the roofline down gently for the 1st 15% of the 'Template' to a 18-degree angle,and then held that constant slope out to 88% of the 'Template.'
He bent the sides in gently,starting at 10% of 'Template',into a 12-degree inward slope and held that constant 12-degrees all the way to 88%'Template',where the body ended in a point.
Lay published Cd 0.12 for the model.
Four of his models were reported at Cd 0.12.They all had 100% tails.All the tails were identical.The models only varied at the front.
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His models end at 88% of 'Template' with Cd 0.12 for a simple model with wheels.
I'm representing Cd 0.13 at the same length and I mean to be conservative in light of the wheel fairings.
Lay's tail would 'tin-can' and would require internal bracing across all spans,like Lindbergh's Ryan Flyer or Beech Bonanza.
The 'Template',being ovoid,is among the strongest structures known,would require no bracing and can be made the lightest of all,which is a non-aerodynamic driver for its choice.Of course,it's also the most complicated to fabricate as there isn't a straight line on it anywhere.
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20-degrees might work.It did okay on the VW and I think NASA stayed around 20-degrees with their Ford van.
Lay's design would be a no-brainer,it's a little more conservative,but he got pretty tasty numbers.With cooling system mods,skirts,MOONs,and wheel fairings,I don't see any reason why the gas pumps wouldn't be suicidal towards it.

2 other views
 

Neil,I did post a front elevation and plan-view of the 'Template' but I had to do it with a separate thread.I'd have to dig back to find it.
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'new images for 'template' ' is the other thread where the other views are.These were just drawings that were in the works.When you see Jaray's,Lay's,Koenig-Fachsenfeld/Kamm,and Korff's plan-tapers,you'll see where I was going.

think
 

Yes,if the 'Template' is placed over a mirror,and you look at it and its reflection,you see that its L/H=5,and L/D=2.5.
Without wheels,in ground effect it is Cd 0.08.
Without wheels,in free flight,it is Cd 0.04.
With wheels,and in ground effect,it would be on the order of Cd 0.12+,depending on the wheels/tires.
With comprehensive wheel fairings,as seen in record cars and solar racers,the Cd would go below 0.12.
The 2.5:1 L/D is about the shortest ratio which still respects Mair's 22-degree back-slope limit for attached boundary layer flow.

The "most aerodynamic" shape possible? A very long, thin PIN!

with the pointy end towards the back!

Could Vortex Generators (or similar technology) have potential use in this regard? Theoretically have a positive affect with a shortened body of a vehicle. If properly placed, assisting with continued attached airflow past the "ideal" distance/degree of tapper.

Of course, not to the extreme point of Reducing Drag, but to keep an identical/matching Cd value below the stated ratios, compared to its longer more ideal form?

But then when you shorten the cylinder portion of the pin to reduce the amount of skin drag, guess what you end up with? A very tiny teardrop.

...get the L/D-ratio huge enough and the teardrop shape goes away.

Does the template assume that the air flow will be in line with the vehicle direction. No?

Do they take in consideration the amount of ambient wind? It is common to experience a 20 mph wind. Sometimes cross.

A shorter vehicle might prove to have less drag in a cross wind.

If lowering a vehicle gets t close to that half blimp shape, what about raising it? It would be closer to the real blimp

It'd need to be too high off the ground to be practical - it'd still need wheels and the ability to turn without toppling.

I think at one point they had the Aptera high for a lower Cd number, but then lowered it for better handling. I cannot seem to find the photo with the two versions next to each other.

EDIT: Found it in one of Neil's old posts.

http://ecomodder.com/forum/showthrea...p-1h-5671.html

The bullet, could be the most aerodynamic shape.
With bullet shaped wheel covers a projectile shaped car could drive.

A bullet is much lower drag if it was moving backwards -- the blunt end in front and the taper to the rear.

I always wondered why a pointy nose was not good, after all you have to cut through the wind. Like a jet, if it had a pointy nose it could slip through the air easier. Must be an aerodynamic conspiracy with nature

Think Zen. The most aerodynamic shape is the one that isn't there..........

only when subsonic

Right, a pointy nose only helps above ~250MPH. Below that speed, a bullet is lower drag going backward.

"Cutting through" the air is not really how it works, below ~250MPH. It is far more important how cleanly the air is allowed to return to place as the vehicle moves through it.

bullet
 

*The bullet depicted has a sub-transonic drag coefficient of Cd 0.147.
*In ground proximity,and sectioned longways to create a 'template',the bullet would have Cd 0.294.
* Adding wheels would get Cd 0.334- 0.344.
* It would have a lot of wetted area for its frontal area,and like your VW,without a tail,only mediocre Cd.
* The 'Template' would produce Cd 0.12-0.13,depending on wheels/tires

backwards
 

Yes,but then we'd have to put nose fins on it to keep the nose behind the tail.:p

pointy
 

The sharp nose is critical for supersonic flight,which is ruled by shockwaves.
When you consider that much sporting ammunition 'flies' faster than a Lockheed SR-71,you realize the importance of the nose.
Below 250 mph,a convex hemispherical nose is the choice for low drag,especially in crosswind conditions.

OK, setting aside the crosswind aspect, how come no modern sailplane has a hemispherical nose? After all, back in the '30s most of them did, but now none do, so why did the designers consistently go with more pointed shapes?

sailplane
 

*I don't 'know' the answer to your question.
*I suspect,that since it's an aircraft,operating in 'flight' conditions,the designer may be attempting to delay the transition to turbulent boundary layer as far back on the fuselage as possible.
*With area-ruling/Whitcomb-waisting/Coke-bottling/sectional density/inverse pressure gradients......... we could locate the position of max cross-sectional area and first minimum pressure, and see if this is 'where' the nose actually ends on the fuselage.If so,then this is exactly what they intended.
*It would be a 'laminar' design,allowing a long region of favorable pressure gradient which could sustain a laminar boundary layer way back on the fuselage,and thereby significantly reduce skin friction,which is all an aircraft has basically.
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We can't do this on cars,as the air itself coming at the car is already turbulent due to viscous shearing forces present in ground proximity,something aircraft don't have to deal with.Abbott and von Doenhoff have very strong language when addressing this situation.