Let's talk about Fineness Ratios ( pointy front ends )

[RobertSmalls]

Skin friction contributes about 0.04 to the Cd of a compact car. If you were to increase the wetted area by 10%, which is a lot, you'd increase the drag by about 0.004, which is not a lot.

Take a look at the flow across the front of this body:



The airflow's behaviour would be pretty much the same whether the front end were flattened, rounded, or pointed. The center streamline comes to the stagnation point and stops. Any air above that streamline flows above the object, any air below goes below. It turns out round has the lowest drag, but the difference is small.

If you want to move your stagnation point upward, grille blocks and airdams do this. A wedge-shaped air splitter would do it to, and of all the ones we've seen, the one that Mr. Kim brought to the AMEC FE Run looked like the most effective, but it also reduced his approach angle.

Will a splitter that does not interfere with approach angles be too high to be effective? I dunno, but it's probably close. So, a splitter is not on my to-do list.

[Cd]

Originally Posted by Piwoslaw

My guess is (I don't know if this came up in the other thread, as I didn't get through it) that the 'wedge nose' is to lower the stagnation point and reduce the amount of airflow under the car. The slight increase in skin friction is outweighed by the decrease in undercar drag. Whether this works (and how well) with an almost perfect bellypan - I don't know.

That was my guess too, but as my tuft test showed me, the majority of the air goes around the sides and over the car.









Note how that a simple flat plate grille block ( in this case my license plate ) directs the air up and over the hood when compared to the brown tuft to the right of the plate .

[winkosmosis]

I think a ground effect is acting as an air dam. But just because the air is going upward, doesn't mean it doesn't take more energy than if there was a wedge shaped nose directing it upward gradually. I think the focus on attachment and smooth vs dirty flow causes people to ignore that it takes energy to move air out of the way.

Yes, of course air will flow around a completely flat nose. It HAS to. But that doesn't mean it's a good idea to design a vehicle that way.

[NeilBlanchard]

Yes, if the bottom of the car is smooth and streamlined, then a pointy nose will either not help, or it might even hurt a bit if it redirects air the "wrong" way. So, to make a well designed pointy nose, you would have to determine where the natural stagnant point is -- and put the pointy nose there.

If the bottom is smooth and the ground clearance is properly designed, then a blunt front end lets the air "decide" which way to flow i.e. it is not forced to go where it doesn't "want" to go -- which means it will be lower drag.

BTW, one of the main things that was tweaked on the Schlörwagen was the ride height: if it was too low, then the drag increased, and if the ride height was too high, this also increased the drag. The right height was between 6 and 8 inches, if my memory serves...

[winkosmosis]

Originally Posted by NeilBlanchard

Yes, if the bottom of the car is smooth and streamlined, then a pointy nose will either not help, or it might even hurt a bit if it redirects air the "wrong" way. So, to make a well designed pointy nose, you would have to determine where the natural stagnant point is -- and put the pointy nose there.

If the bottom is smooth and the ground clearance is properly designed, then a blunt front end lets the air "decide" which way to flow i.e. it is not forced to go where it doesn't "want" to go -- which means it will be lower drag.

BTW, one of the main things that was tweaked on the Schlörwagen was the ride height: if it was too low, then the drag increased, and if the ride height was too high, this also increased the drag. The right height was between 6 and 8 inches, if my memory serves...

But even a perfectly smooth bottom, if the car is low (IE not an Aptera or offroader), saps energy because the air has to be compressed. So you'd want to direct as much flow as possible upward.

[Frank Lee]

http://ecomodder.com/forum/showthrea...etro-5623.html

[Piwoslaw]

Originally Posted by winkosmosis

But even a perfectly smooth bottom, if the car is low (IE not an Aptera or offroader), saps energy because the air has to be compressed. So you'd want to direct as much flow as possible upward.

I think Neil means that with a round front the air will not compress under the car, but will pick to go over.

Originally Posted by ChrstphrR

In addition to that, not having a huge schnoz on a car that makes it efficient for straight ahead travel, but a tad inconvenient for turning corners, or parking in a standard parking lot space.

So maybe an automatic nose that extends only above a certain speed? To go with the speed-actuated extending boattails and lowering suspension that have been discussed here.

[ChrstphrR]

Air compression wouldn't be an issue except at high mach numbers; thus the whole concept of that is irrelevant, off-topic scatter being raised up.

I think RobertSmalls' made a much more photogenic backing of what I mentioned about how a pointy nose doesn't matter for well below Mach one. That's just part and parcel of the reason why it hasn't been widely persued.

I've two more drawbacks to dredge up, one's aerodynamically related, one isn't.

A long supersonic nose for a car would cause more separation, turbulence, and thus, more drag with anything but a headwind or tailwind. Yes, that means boattails also have this drawback.

However, we shouldn't just consider pure aerodynamics for the sake of meagre gains versus... vehicle dynamics and stability.

A Boattail does have the same drawback as a huge nosecone I just mentioned; the boattail with a side wind will increase the tendency to understeer, handling/dynamics wise, which is generally easier for drivers to counteract and anticipate.

A large pointy schnoz, that makes your car look like a MiG 29 or a Tomcat from the front, will contribute to oversteer with sidewinds, which is generally NOT stable, and harder for most drivers to anticipate, or correct for.

The other angle on vehicle dynamics: Corvette Stingrays were infamous for the uplift their high, pointy noses caused.

The "ideal" pointy nose would be almost a wedge that veers to the ground, but that isn't practical, since most people have to have at least ONE end of the car, if not two, that can clear the transitions* from steet to driveway.

Thus, a practical pointy nose, would have it's furthest point foward up from the ground. The generally rounded shapes that have been cited as ideal push air overtop the car, and to the sides, mostly -- with a small amount going underneath, comparatively.

Too high a pointy nose, too much air is directed underneath, and you get lift on the front axle, which gives the driver a lightened steering response at highway speeds, or above. This is simply not conducive to SAFE driving.

Walter H. Korff detailed that effect in his tome, Designing Tomrrow's Cars.

And from Julian Edgar's practical test on just little tiny front tire deflectors, from an oft-cited set of aero articles from Autospeed:

Firstly, fuel economy slightly improved, indicative of the drag again being decreased. The gain wasn’t nearly as great as achieved by the new undertray but the best-ever 100 km/h freeway 5-minute fuel consumption was achieved – 24 km/l (4.2 litres/100 km)... and that was with the air con on! Secondly, 5-minute fuel consumptions of 22 km/litre were also more frequently achieved with the deflectors in place.

However, the aero stability of the car was poorer than standard. While I stated above that the new undertray may have resulted in a stability improvement, with the deflectors in place, stability was without a doubt inferior to standard. The reason that stability had declined can be sheeted home to an aerodynamic pressure build-up on the undertray ahead of the new deflectors. With the deflectors in place, steering corrections were more frequently needed and the car was more susceptible to the bow waves and wakes of cars in adjoining lanes.

I’d already bought the high density foam rubber with which I’d intended to make the final versions of the deflectors but after testing the quickie foam-and-duct-tape prototypes, I decided to not go ahead. Here was a clear case of deciding to either further reduce drag while trading-off stability – or to have lower drag and lower stability.

I chose to maintain the stability!

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...darn! where's the MythBusters when you really need them? A drop of falling water is round, not teardrop shaped.

Teardrops slinking down a cheek do have something closed to a bobbed "teardrop" shape.

Until my commute approximates free-falling for miles on end, I'd rather approximate a teardrop which hugs an approximately flat** roadway instead.

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* I live in Canada, and some civil engineers and city planners think that an 8" curb at 45 degrees is a reasonable transition from road to driveway. I think these people just hate mufflers, tailpipes, and oilpans.

** I live in Canada, we have these collections of potholes and patches they call "roads" up here; so, yes I did mean approximately.

[Cd]

Lots of good information guys ! Keep this discussion alive - I'm learning new things !


Here is how I lowered my stagnation point without installing a nose cone :



I simply made my car into a 'bottom breather' by cutting the bottom section of the bumper and adding a spoiler that directs the air upwards into the fan.

[moonmonkey]

cd what was the result of your bottom feeder corvette/firebird style cooling system? is it aero superior to a small hole in a full grillblock? looking at basjoos aerocivic front end his train style front scope is there to direct air and deer up and over his car. its not pointy though it goes accross the whole front of his car (mostly), and lowers his stagnation point, i dont know how much that particular part lowers his cd though.