New bug, .38 CD why?

[ShadeTreeMech]

Originally Posted by ChazInMT

Shadetree, I don't know whether to thank you or not.....what do you mean by the last paragraph? "If you want to read it from the experts..."

What I meant is I'm not one, only that I too have been doing a lot of reading and know where to look for info, so here's some useful links......not certain what may have been implied, but I'm no good at implicationing, if that's even a word

to be quite honest, I wouldn't have thought a roof spoiler would do much good, but considering aerohead has been doing this sort of thing longer than I've been alive, I'd go with it.

[bnmorgan]

Well, found some on Ebay (item # 280524854009) , but not sure what their ratio of "correct" versus "decorative" is. I'd guess if they're even close it's worth a shot, even for $72. what do y'all think?

[ChazInMT]

The thing I don't like about this is it is going to create a fair amount of drag by kicking the air up. So the strip itself is going to be trying to hold yer V Dub back.

What is more ideal is something that follows the roof line then just ends abruptly, this way the added "Spoiler" does nothing to create its own dynamic drag. I have looked hard for a commercially produced alternative but can't find one, certainly not one painted to match for under $100.

I like this for cheap and easy, for your car this may be the trick because I believe the trade off between the serious lift drag and strip induced drag is worth it.

[bnmorgan]

well, i'm ok attempting to fabricate something i suppose. I'll see if i can get hold of some 1/8" abs sheet somewhere along the way. I ain't going to get in a hurry for the time being, i'm beating the EPA est by almost 22%, still, i'd love to get better than 40.

[Patrick]

Originally Posted by ChazInMT

Morgan, You DO NOT want to keep airflow attached. VG’s are used to try and keep flow attached when a rear shape tries to change too quickly for it to remain attached but not quickly enough for it to be a clean break. VG's would be useless here, the slope of the Bugs back is such that flow is remaining attached way too long already.

Understand that when I say you are generating too much lift, it is not all in the straight up direction, a very good portion of the lift is in the backwards direction, which is a huge drag load on your car. This is why you want it to separate at the top of the windshield. If you fear that you are creating a huge area of dead air that is starting at the top of your windshield, you should not worry about it, because this “Dead Air” drag will be there no matter what you do to your car....I hope that makes sense.

Below is a basic illustration of what I’m trying to say. By keeping the air from moving down the rear window, you eliminate the large lift, (Black Arrow) and it’s associated Drag Vector (Red Arrow).



The "Dead Air" drag is about the same as "Aerodynamically Shaped Perfect" drag. As soon as the air on your bug starts going down past the top of the rear window, it is costing you. So VG’s would do no good since the flow is already attached.

Go with a roof spoiler as I suggested earlier, it will yield the best results.

The dead air that forms will be less drag than the lift.

Where do you get this theory from? Lift is always in a vertical direction and drag is always horizontal.

If you detach the flow at the top of the backlight you have a very large base area at the back of the car in a turbulent airflow. This would be similar to a "stalled" wing, which has very high drag. I think you would be better off trying to keep the airflow attached as long as possible to minimize the base area, which is what the large spoiler treatment is trying to do.

[autoteach]

http://www.dragtimes.com/images/1097...gen-Beetle.jpg

I don't know where to find it, but a Grandfather/grandson team did a cross country tour with a new beetle using a wing like this. They saw large gains. I wish that I still could find the pictures of it.

[autoteach]

http://www.samcar.be/caracter-carros...newbeetle1.jpg

[ChazInMT]

Patrick, it's aerodynamics basic stuff. It is a lift vector which I have broken down into its components in the vertical and horizontal directions. Your statement above shows you have a few misconceptions about aerodynamics.

I am responding to you in case some lurker shows up and thinks that your somehow right, and I'm probably wrong.

Your “Stalled Wing” is not a fair comparison, and, your statement that it has very high drag is not quite correct. The wing, in fact, has its highest drag just before it stalls, that's what is keeping the airplane aloft. When the flow detaches from the wing in its stalled state, the low pressure area drops dramatically, the lift goes away, The Dynamically Created Low Pressure Area No Longer Exists. Think long and hard about that, if you don't understand wing theory, read up on it, cause it directly pertains to the situation at hand with this Beetle.

Of course the wing is a high drag shape when it is being shoved through the air bottom first, just like plywood, or anything large and flat.

The real stretch for you to understand is that the low pressure created behind a moving vehicle CAN NOT BE ELIMINATED! It is a 2 sided coin we're dealt here, the high pressure created in the front of your car has a corresponding low pressure behind it. There is no getting around it. We can only try to minimize it as much as possible by striving to shape our cars as close to the aerodynamic ideal as possible.

The flat area behind a car is not always a turbulent, drag producing thing. You have that all confused. If the air detaches cleanly, that's the best we can hope for, short of building full boat tails on all our cars.

What is happening on the VW Beetle back side is that flow IS remaining attached all the way down its backside. This is because the shape is totally smooth with no breaks in the curve from the bottom of the front windshield all the way to the bottom of the trunk. The website where the flow diagrams in Post 1 were retrieved from show the “Raw data” in the form of tuft tested VW’s, the air flow is doing what's depicted in that diagram. Now once the air goes past the top of the rear window, it is headed down really fast, this is creating a substantial amount of dynamic low pressure, that is what I depicted in my drawing. Since the area of low pressure is at an angle going down the back, the force being felt on the car is perpendicular to it. That is why I broke it down into it's vertical and horizontal components to make it easier to understand.



It is not always good to maintain attached airflow once our cars rear curve gets beyond the ideal arrow shape, you are better off letting it detach and creating what is essentially a dead air space behind our cars. Just cause it's detached and still above the back of the car does not make it turbulent. I have a pickup truck with big ol side rearview mirrors on it, when I put my hand close to the mirror in the air bubble formed behind there, my hand is not buffeting about wildly from the turbulent air, same thing on the back of a car, the air just puffs about a bit, no huge drag creation.

Do you think this car is creating a huge amount of drag behind it??





The problem with detached airflow is when it happens in an uncontrolled fashion over a large area of the back of a car, this creates all sorts of shredding, turbulence, and vortex's which create the drag. This is in fact happening at the trunk area on the Beetle which is probably another reason for the drag.

And lastly. Here is a VW Factory Option for the turbo Beetle.



Hmmmmmm.

[COcyclist]

Originally Posted by autoteach

I don't know where to find it, but a Grandfather/grandson team did a cross country tour with a new beetle using a wing like this. They saw large gains. I wish that I still could find the pictures of it.

I think this is the link to Ernie's wing you are speaking of and here is another that may be useful. It has charts from the Hutcho book.

http://www.max-mpg.com/html/tech/main.htm

http://ecomodder.com/forum/showthrea...info-2072.html

[Patrick]

Originally Posted by ChazInMT

Your “Stalled Wing” is not a fair comparison, and, your statement that it has very high drag is not quite correct. The wing, in fact, has its highest drag just before it stalls, that's what is keeping the airplane aloft. When the flow detaches from the wing in its stalled state, the low pressure area drops dramatically, the lift goes away, The Dynamically Created Low Pressure Area No Longer Exists.

I have to say that you have a few misconceptions about aerodynamics.

The stalled wing is a fair comparison and the wing does NOT have its highest drag "just before it stalls." Quite the contrary. See the graph below. The Clark Y stalls at about 18 degrees. After that the lift drops and the drag increases dramatically due to flow separation. The drag does not "keep the airplane aloft," lift does. And the lift does not "go away" after the wing stalls, it just decreases.

Originally Posted by ChazInMT

The flat area behind a car is not always a turbulent, drag producing thing.

Yes it is, and we should try to minimize it as much as possible. The closer we get to the "ideal" shape, the lower the Cd is, due to the elimination of the flat area.

Originally Posted by ChazInMT

The website where the flow diagrams in Post 1 were retrieved from show the “Raw data” in the form of tuft tested VW’s, the air flow is doing what's depicted in that diagram.

And what Cds did they get with the stock car vs. the "detached" airflow? Or did they not test that? Are you just assuming that the car would have a lower Cd with detached flow? You know what they say about the word assume.

Originally Posted by ChazInMT

I have a pickup truck with big ol side rearview mirrors on it, when I put my hand close to the mirror in the air bubble formed behind there, my hand is not buffeting about wildly from the turbulent air, same thing on the back of a car, the air just puffs about a bit, no huge drag creation.

Your hand is in the low pressure area, and the drag is there, even if you cannot "feel" it. There is at least one graph on this website showing the huge amount of drag a flat plate creates.

Originally Posted by ChazInMT

Do you think this car is creating a huge amount of drag behind it??

No, not a huge amount of drag, but some nonetheless, and it is due to the separated airflow. The car is not the optimum shape for low drag, but Toyota did the best they could within the other constraints for the car (styling, overall length, utility, etc.). Would it have lower drag with a full boattail and fully attached flow? Certainly.

Under your theory, the Prius would have lower drag if we deliberately cause airflow separation up by the B or C pillar. I disagree.