'Ideal' shape for low drag

[California98Civic]

Originally Posted by gumby79

... This studdy agrees with your sediment as I understand it.
Dream big chisel down to reality

Thanks for the find. That's how I read the lit, too. Including Hucho: "dream big, chisel down to reality."

[California98Civic]

Hucho's 4th edition from 1999 has a few things to say about ideal shapes and research into road vehicle aerodynamics. On pages 224-229, and a few other places, Hucho describes aerodynamics as a field in which not all relevant research has a specific car or specific part in focus. There are ideal shapes derived from basic research. In fact, Hucho's book can be read as a history of the rise of “detail optimization” and “shape optimization” as ways of compromising “ideal" shapes with market and other design requirements.

As gumby79 said: “dream big, chisel down to reality.” Julian’s throttle stop testing method is one of the good testing methods for chiseling, and a knowledge of some of this theory is also useful.

Hucho (4th Ed.), p.51:

[JulianEdgar]

I honestly cannot see what optimising a wing to provide maximum lift/drag ratio for an aircraft flying in ground effect has to do with the shapes of cars.

But I can see several reasons why it wouldn't be useful, and in fact could be quite deceptive.

From the paper:

...two optimization objectives are considered—maximization of the lift coefficient and the lift to drag ratio. - Neither is the aim with car aerodynamics.


The airfoil shape that results in lowest value of 100 x Cd/Cl... gives the shape of the optimized airfoil. - So, as you'd expect, they are chasing maximum lift and minimum drag. We don't want maximum lift in a car!

If the paper was about the best shape, in proximity to the ground, for lowest drag and lowest lift, I'd be all ears. (But even then, of course, they'd need to replicate the relatively rough surface of a car versus the dead smooth surface of an airfoil.)

I guess if you want your car to fly with the least power being consumed, this looks like an ideal paper to read!

[California98Civic]

Originally Posted by JulianEdgar

I honestly cannot see what optimising a wing to provide maximum lift/drag ratio for an aircraft flying in ground effect has to do with the shapes of cars.

... If the paper was about the best shape, in proximity to the ground, for lowest drag and lowest lift, I'd be all ears. (But even then, of course, they'd need to replicate the relatively rough surface of a car versus the dead smooth surface of an airfoil.) ...

There really is more for you to learn from foundational research about lift and drag, but you routinely just dismiss all of it. "Replicate the relatively rough surface of a car" is not not the path to lowest drag and lift. It is the path left available by an automotive industry, regulatory regime, and consumer market that makes more "ideal shapes" not practical on the road.

EDIT: But I have to admit no interest in driving a "template" shape as my DD, even though that whole discourse has been a pathway for teaching me valuable aspects of the aerodynamics of my car. I love the shape of the 90s Hondas. My goals are also a little like yours, Julian: optimization through testing mods, tweaking, retesting.

[Piotrsko]

If you increase lift WITHOUT increasing drag, you have generally increased overall efficiency, possibly going to all laminar flow which AFAIK, THE HOLY GRAIL. At that point you get to start attacking/approaching the zero lift angle which should reduce drag further and mutes the lift argument.

I notice that both of these ordinates are significantly undercambered. I am not sure why that is relevant. Could be a traction enhancement.

[aerohead]

* ' [T]here is not a simple relationship between overall lift and drag.' Hucho,page 123, 2nd-Edition,1987.
* ' [B]ecause the aspect ratio... is very small for vehicles a breakdown of the flow field into two-and three-dimensional components is not possible.The effect of the vortex field can therefore not be separated from that of the other flow field,as is possible for aircraft wings with high aspect ratio.' Hucho,page 122, 2nd-Edition, 1987.

[gumby79]

Edgarwit's
Are a prime example of a WOG( wing in ground effect.) Your front and rear fenders appears to lack the roughness you refer to being present preventing such research from applying to your specific or any use case with wheels. one of us has the wrong context
You challenge the community to find you a better airfoil profile the community responded, Your response to your challenge being answered,is the subject is not relevant do to roughness of automobiles?
I'll push you a very specific question. Witch one has the best chance of maintaining attached flow mounted 20-80% of cord off the sheet metal the GEO 222, NACA 4412, or the optimised NACA 4412



testing you proved your 222 was not the right design . at any angle of attack it cannot achieve attached flow Beyond 1/3-1/4 cord . I guess only functioning 25% is considered a great success to a "published professional". Imagine how much better your guide plains would function if you use the correct airfoil, preventing the added additional Dynamic frontal area caused by the seperation. ( I do acknowledge that they have already showen a reduction the factory Dynamic frontal area, your testing shows 66 to 75% has been left on the table. thank you for presenting us with the evidence, now follow through and make your changes necessary to get your best result unless you are satisfied with 25% )

Use case 2
Rear spoiler
This profile may give you much better results on your rear wing then a profile that is designed to be a multi element 360 degree angle of attack as used.
Unless you don't think the back of your Prius is smooth enough or there's no ground effect off your back glass before you wing.

Use case 3
Flat floor
Take me into account the pressure wave rebound and making the floor not quite so flat in a way to take advantage of this rebound energy that is demonstrated in the pressure and velocity graphs of this document that you say can never provide useful energy formation to wheeled vehicles.

Use case 4
Building a West coast-style mirror (old school tow mirror)that acts like your edger wits but at the A-piller.

Of course I can provide useful use cases for the presented paper cuz I presented it I saw useful I thought others obviously not you but others could benefit from the things I see

dump out a bucket of dots I'll draw you a picture, all one has to do is connect them

[JulianEdgar]

Originally Posted by California98Civic

There really is more for you to learn from foundational research about lift and drag, but you routinely just dismiss all of it. "Replicate the relatively rough surface of a car" is not not the path to lowest drag and lift. It is the path left available by an automotive industry, regulatory regime, and consumer market that makes more "ideal shapes" not practical on the road.

I am comparing the relatively rough surface of a car with the surgically smooth surface of an aerofoil.

No production car will ever have such a smooth surface, as the production costs would be too high for a mass-produced item.

As surface roughness increases, so does the thickness of the boundary layer. The thicker the boundary layer, the more likely flow separation will occur. When flow separation occurs, the airflow is no longer following the shape.

Therefore, the surface roughness of a car needs to be considered when assessing 'ideal' shapes.

An 'ideal' shape that implicitly assumes a very thin boundary layer (as aerofoils are modelled to) has grave shortcomings when applied to a car.

[JulianEdgar]

Originally Posted by Piotrsko

If you increase lift WITHOUT increasing drag, you have generally increased overall efficiency, possibly going to all laminar flow which AFAIK, THE HOLY GRAIL. At that point you get to start attacking/approaching the zero lift angle which should reduce drag further and mutes the lift argument.

Yes I did consider this. So in that case we'd:

1. Start with an aerofoil that has been optimised for maximum lift/drag ratio in ground effect for an aircraft

2. Change its angle of attack until we'd achieved zero lift (ie stop the aerofoil doing the very thing we've just optimised its shape for)

3. Claim we've developed a low drag shape for cars.

I don't see why such a design would have lower drag than a shape developed to have low lift and drag near the ground - as the primary criteria for its development.

It seems rather like developing a ladder for maximum height, laying it on the ground - and then saying we'd developed the best possible chair.

[Piotrsko]

Originally Posted by JulianEdgar

I don't see why such a design would have lower drag than a shape developed to have low lift and drag near the ground - as the primary criteria for its development.

Because it would be optimized for the conditions experienced not just a calculated set of non infinite parameters. Or as aerohead calls it rolling floor effects. If you take your calculated profile and then modify it for local conditions, either the ordinates would be copies or very similar depending on how the aspect ratio is dealt with. Otoh, I have seen 1/1 aspect ratio low drag profiles and they are........ odd.

By the way, look out the window of a commercial airline in flight and notice how lumpy the upper surface is. Definitely not surgical smooth. Ditto for one piece composite or military although those are smoother.