New bug, .38 CD why?

[fud2468]

I usually lurk here because I know little about aerodynamics. However I'm old, have a good memory and need to challenge the idea that there was pressure into the cooling air intakes of old Beetles.
Though it was corrected later on, 36 HP Beetles run at high speeds in hot weather had a cavitation problem which restricted cooling air flow. Overheating then caused the soldered joints in their oil coolers to melt, resulting in oil loss.
At least one VW repair guy set up shop near the freeway where drivers from the Bay Area hit the Sacramento Valley heat-- he had a good business offering quickie engine replacements.
Ray Mac

[Frank Lee]

Originally Posted by bgd73

power as in hp, is not a boxer. the 1938 would rip a civic a new ... err hum. Don;t chain the miconception of power together.

the new beetle is sideways drivetrain for one, crammed in aplace it should never go..that is instant brick wall in the wind...underhood physics of transverse is the reason I run from them. aero benefits for thenew beetle would be diffuser holes down low in the rear...maybe even an inch of rake positive.

No I don't think so.

[donee]

Hi Dave,

Yes, that is right. The oblique view from above gives a clear view of the swirling air flow at a seated head position.

As it is a model, they may not have matched the Reynold's number to that of of a full sized vehicle. But what is shown in the model wind flow seems to me to be similar to what the air pattern would be based on what I was hearing when I was in the new beetle.

Is this a case where the automotive speed range makes a big difference in the air flow? Is it the unique shape of the beetle that causes this? Does the Cd change with speed dramatically in the beetle?

[Dave's Civic Duty]

Hi donee,

Thanks! Makes sense to me! Nice clarification.

Dave

[aerohead]

Originally Posted by donee

Hi Aero....,

How about this one: http://inter.action.free.fr/images/a...t-aerodyne.jpg

The main link to this sight gives a very interesting road vehicle aerodyamics section. Thank you Google for the autotranslation feature!

Check out http://inter.action.free.fr/ Then hit the "Aero Lab" link, and follow by hitting the "Aerodyamics of Road Vehicles" link.

donee,I skipped this image,as it is a small scale model in a model wind-tunnel,and from the look of the smoke trace I'll bet a rubber donut that this was a very low Reynolds number environment and what we're seeing is not what it would be like in the real world.
If you reverse the image of the powder blue bug above it,you'll see that this photo comes from VW's full-scale tunnel and is above critical Reynolds number.
That site has some great images.
They stole my paste-up job of the Honda Insight range-extender trailer I created.My French is lousy but it looks like they're giving photo credit to Honda.What a laugh!

[aerohead]

Originally Posted by donee

Hi Dave,

Yes, that is right. The oblique view from above gives a clear view of the swirling air flow at a seated head position.

As it is a model, they may not have matched the Reynold's number to that of of a full sized vehicle. But what is shown in the model wind flow seems to me to be similar to what the air pattern would be based on what I was hearing when I was in the new beetle.

Is this a case where the automotive speed range makes a big difference in the air flow? Is it the unique shape of the beetle that causes this? Does the Cd change with speed dramatically in the beetle?

For the size and velocities of automobiles,critical Reynolds number is achieved by about 20-mph,and from there,up to 250-mph,where compressibility effects begin to rear their ugly head,the drag coefficient is a constant along with air patterns.
Of course,that's all predicated on zero wind,or zero-yaw condition when wind exists.

[bnmorgan]

ok, typical new guy question here: would keeping the airflow attached until further down or around the car be better or worse? for instance, in the picture of the NB showing separation and turbulence behind the front fender, and again at the crest of the rear fender, and at the base of the rear glass in that large "box" area that pretty much IS the rear of the car. Would keeping that air attached be better or worse for fuel economy. Or instead would filling that area somehow to relieve the low pressure and "bubble" being dragged along help more?

I think i can figure out how to help keep it attached if i just know if it's beneficial to do so. I don't need downforce, i need slipstreams.

also wonder what the reynolds number for this is. I was trying to use the flow illustrator just to see what there is to see.

[ChazInMT]

In my thought on this, the flow remains attached on this vehicle as it is purported in the initial drawings. These drawings were created from string tests that were done on the car. The main drag I see being created is from the huge amount of "Lift" being induced on the top of and along down the rear window, and a lot of this lift is being created such that it is acting rearward dragging the car back, this is where things go horribly wrong with this shape. So if you have an ideal aero shape, and cut off it's tail, you are better off even if you have a huge flat area on the back of the vehicle, because you are not creating this rearward vector in the lift component that the New Bug is famous for.

So to answer your question, you want to detach flow cleanly from your car when it starts to deviate from the ideal aero shape by too much, this will prevent drag created by lift acting in a rearward vector.

In the Beetles case, find a roof spoiler to attach near the top of the window that follows the curve of the roof remaining lift/downforce neutral.

Something like this maybe:

[bnmorgan]

I wonder if the small "trip lip" or whatever they're actually called that are used on the top edge of many motorcycle windshields, esp sport bikes, would help. They're gentle but abrupt at the same time, helping throw a lot of wind up over the riders head. They kinda create a blade effect that shoves air up as a deflector instead of trying to move all the air....or at least that's how they're explained.

[XJguy]

My dad who spent many years in Brasil (the country with the largest number of original Beetles in the world) told me years ago that they were notoriously horrible at high speeds and were known for taking flight and people losing control and crashing.