beetle and aero

[FJCamper]

Hi Bren,

The rear wing is drag up to about 60 MPH, at which point it begins to exert an infulence in breaking up the low pressure area behind the car by "spoiling" the airflow.

At about 80 to 90 MPH, when (with no wing) drag is maximizing and instabilty increasing because of lift and buffeting, a winglet or spoiler above the rear window helps break the car body partially loose from the backwards pull of the drag, and both speed and stability increase.

As you know, VW provided an electric spoiler on the New Beetle above the rear window to deploy at about 70 MPH. I'm not sure about the actual deployment speed -- maybe someone who knows exactly could tell us.

FJC

[brenmobile]

What do you think about this wing?
http://www.aircooled.net/new-bin/viewpr ... 0386673010
Would it help or hurt a drag beetle that might reach 75mph?
Bren

[FJCamper]

Above: A Herrod's Helper mounted high.

Hi Bren,

75mph is just on the edge of aero devices making any big difference.

My honest opinion is you get less function out of a wing just below the rear window, much less than a wing or spoiler above the window.

Think for a moment. VW put the engine air intake ducting just below the window because that spot is a high pressure area, and air would be forced into the low pressure body cavity.

The high pressure is created by the air that races along the roofline, then tries to conform to body shape and travel down the window toward the rear bumper. But the air cannot make that full trip because the bottom of the rear window is the separation point, and buffeting/swirling pressure is highest there.

As the air separates from the body, a partial vacuum is created just behind the lower edge of the rear window with a width almost the same as the car body. There is our drag.

A wing or spoiler at the lower edge of the window is an easy place to mount, and looks right (as it is generally consistent with what we see on other cars) but it is not in the most advantageous place. If you break the airflow higher up, it takes less wing or spoiler, and is more efficient.

The Ron Lumus wing will work, and certainly looks good, but is a performance tradeoff. The bottom line is high spoiler or low wing, either one is better than nothing.

FJC

[Dubfreak1]

What advice can be offered comparing a drag car sitting level Vs one that has a slight nose down stance? It would seem that having the back higher would be a plus on the top surface of the car, but a negative on the bottom. True? Which is more of a factor?

[ProctorSilex]

What advice can be offered comparing a drag car sitting level Vs one that has a slight nose down stance? It would seem that having the back higher would be a plus on the top surface of the car, but a negative on the bottom. True? Which is more of a factor?

The drag car is going to squat so the rake would make the front lift less relative to the rear, which would be important at high speed to help keep all four wheels down (front especially) and, in extreme cases, the car from flipping.
I do not know why a higher rear would be an aerodynamic problem on the bottom (aside from unnecessary losses). It should cause a lower pressure region thereby pulling the rear down, but under car turbulence might factor in ways I do not understand.

[Dubfreak1]

I should have been more specific. This is a 1600 Drag Car, approx 90hp max. I'm talking about the car's stance in 4th gear going down the track (1/4 mi). Hi speed stability probably won't come into play, but the low pressure underneath probably causes some aero resistance. True? If so, is it overcome by the reduced drag from having the rear sit higher. I want to maintain a relatively "stock" appearence, so any wing is not in the cards for me.

[Bruce2]

Don't worry about the wing. You won't be going fast enough to cause one to have an effect.

[Dubfreak1]

Don't worry about the wing. You won't be going fast enough to cause one to have an effect.

Exactly, but what about the way the car cuts through the air. "Nose Down" or "Flat"? I'm sure "nose up" is just plain wrong.

[Piledriver]

I'm sure "nose up" is just plain wrong.

VW disagreed.

[ProctorSilex]

I should have been more specific. This is a 1600 Drag Car, approx 90hp max. I'm talking about the car's stance in 4th gear going down the track (1/4 mi). Hi speed stability probably won't come into play, but the low pressure underneath probably causes some aero resistance. True? If so, is it overcome by the reduced drag from having the rear sit higher. I want to maintain a relatively "stock" appearence, so any wing is not in the cards for me.

I am not an expert. My somewhat educated guess is that you are correct that rake will slow your particular car down. In fourth with that engine, the car probably will not be squatting much at all from the acceleration at which point the rake might just be slowing you down and not returning commensurate benefits in other areas.

VW disagreed.

Please explain. Are you saying that nose up is beneficial?

[Piledriver]

VW disagreed.

Please explain. Are you saying that nose up is beneficial?

Yes. The later T1s sat slightly nose high intentionally, it greatly improved the stability at speed.
VW spent a LOT of time in wind tunnels trying to improve that, although the general appearance of the car didn't change much.
Yes, it looks funny. It was not an error.

Nose down/ass up is for looks and (perhaps) rear tire clearance.

Also, If you are drag racing and not going FAST, nose high can improve weight transfer on launch, and that's where drag racing is won or lost.
If it's an otherwise ~stock class and that's all you can mod, that's the mod to do.

[r@lf]

What advice can be offered comparing a drag car sitting level Vs one that has a slight nose down stance? It would seem that having the back higher would be a plus on the top surface of the car, but a negative on the bottom. True? Which is more of a factor?

wat u can do is... "arrange" the flow below the car...

more so like influence it.. or aid it..

if the nose is lower.. there is a bigger gap under the rear.. suffering uneven airspeed etc

you can put a pan/floor/roof whichever way u look at it.. and implement diffusers to aid flow into arranging them in ways that it exits the rear ideally

[ProctorSilex]

Yes. The later T1s sat slightly nose high intentionally, it greatly improved the stability at speed.

Do you understand how that works?

[Piledriver]

Yes. The later T1s sat slightly nose high intentionally, it greatly improved the stability at speed.

Do you understand how that works?

It had something to do with air pressure distribution, bt it's been many years since I googled for it.

[ProctorSilex]

It had something to do with air pressure distribution, bt it's been many years since I googled for it.

I took a look on Google, but did not turn up anything.
My guess is that it was a free method of increasing stability (rather than adding a front air dam) under the T1's modest intended driving conditions rather than an ideal method. I have a guess at how it would help, but it is pure speculation.

Thanks for the info. I will have to keep an eye out for the details.