Air Tabs

[ecomodded]

Do you mean the Flow Simulator
here's the N Beetle with a roof spoiler, it looks to me like the air stream is already in a good flow with out the VG's
The VG's may as they are designed, bring the air back down, in my case onto the hatch glass.
I cannot find the video of the beetle without the roof spoiler but it was around at one time.

A benefit could be found for other vehicles but for mine I fear it will cause lift-drag, I can now deduce, after revisiting this video.

http://www.youtube.com/watch?v=fw2qkR3bFoM

or this next one, it looks more realistic

http://www.youtube.com/watch?v=i16zo...eature=related

[ChazInMT]

Keep in mind that the air affected by both Aircraft and Cars is a Quantum Leap greater in volume than the air within 1-2 inches of the skin. I have seen pictures of aircraft flying over water and creating a disturbance on the surface 150 feet below....as in Directly Under the aircraft, not a wake. Even experienced pilots and people who design aircraft don't really understand how lift is generated, it is Vodoo stuff, anyone who tells you they know exactly how it works is full of their own hot air. The classic "Faster across the top low pressure theory" is crap. Every time a plane flies upside down in an air show (or anywhere else really) without immediately slamming into the ground dispels that myth. Current theory holds that a large volume of air is being directed downward by the wing, the opposite reaction to this downward thrusted air is upward thrusted airplane.

But don't take my word for it, read about it here. Nasa saying "Traditional" explanation of how a wing works is Horse Hockey

Look around the NASA sight to help yourself understand what we don't really know better. This page, for example, says what I said lift is...maybe. Nasa Tries to Explain Lift

I reason that Car aerodynamics are very similar in complexity and misconceptions. There are huge volumes of air involved in both, and the "Air Tab" gimmicks may affect a small part of this air, and may show "Attached Flow" at the surface, but, you still need to think of the MUCH bigger picture and ask how it is affecting the air at 1 foot, 3 feet, and 20 feet away.......chances are, it ain't, and if it is, it is not likely to be helping you out. This is why "Form Drag" is what we try to minimize on a car, and "Skin Drag" is what we try to minimize on an airplane. There is no question that VG's help aircraft in some ways. The Form Drag of an aircraft better be minimized as much as possible.

Skin drag vs. Form drag along with ground effect are the reasons that you Can Not take much away from Aircraft Aero and apply it to Car Aero, you're wrestling 2 different demons in 2 different environments.

Skin drag on a car is fairly insignificant because of the speeds involved, and the surface area of a crappy shape is going to be equal to an ideal shape meaning the skin drag will be the same on both....so why even bother to talk about it, besides the air is by definition never in laminar flow on a car, it is already turbulent. Bottom line Skin drag is a non-issue.....for now. Maybe when we're all driving VW XL1 type cars we can revisit this.

Attached flow IS NOT the holy grail for achieving better Cd, so just because you can get some air blowing down your back window does not mean you have lowered your cars drag. It is much more complicated than that. Great aerodynamic shapes with low drag have attached flow, shapes with attached flow are not always low drag. You must understand this distinction to move past this blind fixation towards "Attached Flow".

As for VG's, they are silly 99% of the time on a car. In my opinion, they are pure.......

[baldlobo]

Originally Posted by ChazInMT

"Faster across the top low pressure theory"

nasa disagrees with you on that, or did you not comprehend what your links say.

{The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, this part of the theory is correct. In fact, this theory is very appealing because many parts of the theory are correct. In our discussions on pressure-area integration to determine the force on a body immersed in a fluid, we mentioned that if we know the velocity, we can obtain the pressure and determine the force. The problem with the "Equal Transit" theory is that it attempts to provide us with the velocity based on a non-physical assumption as discussed above.

ps. it's site; not blind sight

additional reading for people; yes it's wiki
http://en.wikipedia.org/wiki/Lift_%28force%29

i got my airtabs, but then again my jeep is about as sheer as semi-trailers rear(62x68 inches of vacuum)

[euromodder]

Originally Posted by skyking

flight does not equal or represent flow on a car on the ground.

True, but that doesn't mean we must throw everything we see overboard.
This video shows the effect of VGs on airflow rather well - preventing reversal of the flow over the aft part of the wing.

These air speeds are comparable to car speeds.

These VGs do what we'd also want them to do : bend the airflow to make it conform to the surface we want it to flow over - wether it's a wing or a rear window.

Placement and size will be critical to determine wether they reduce overall drag, as they add drag by themselves and by the vortexes they produce.

[ksa8907]

I think im confused? Are people trying to keep attached flow over the window or reduce the low pressure zone at the rear. I think the later is where the real gain is to be made.

[aerohead]

Originally Posted by ksa8907

I think im confused? Are people trying to keep attached flow over the window or reduce the low pressure zone at the rear. I think the later is where the real gain is to be made.

They go hand in hand.
*The base pressure behind the VW is dependent on where the boundary layer separates,as the entire wake will be at this pressure.
*If you can move the separation point further back the 'new' separation point will typically be in a region of higher static pressure,which will raise the base pressure a little closer to the forward stagnation pressure;which reduces the delta-P across the car,lowering the pressure drag which is the largest component of profile drag.
*VGs are supposed to create small eddies which transfer kinetic energy from the 'invisid' outer flow,down into the boundary layer,helping to maintain attachment in the presence of an adverse pressure gradient (downstream of the point of minimum pressure).
*VGs are typically for flow reattachment onto the boot of a notch back car.
*Since the Beetle has no 'boot',there's really nothing to reattach to.
*Without the 'reattachment',any pressure gain you might get from the VGs are eaten away at by the constant increase in interference drag and induced drag of the vorticity they produce.Gains could be 'negative.'
--------------------------------------------------------------------------
*The Beetle needs a rear end.
*The New New Beetle has a better rear.
*The AUDI TT rear is better.
*The PORSCHE would be even better
*The 'Template' is reported to be the lowest drag.
--------------------------------------------------------------------------
If you can find the thread for the aero-modded Chrysler Airflow,you can see what Carl Breer would have done to your car.It was actually the world's first K-form car.

[ecomodded]

Quote: euromodder

Placement and size will be critical to determine wether they reduce overall drag, as they add drag by themselves and by the vortexes they produce.

I found these small VG's on a website for small single and twin engine planes that are less obtrusive.

text:
Micro Vortex Generators are small aluminum blades placed in a spanwise line aft of the leading edge of the wing and tail surfaces. They control airflow over the upper surface of the wing by creating vortices that energize the boundary layer. This results in improved performance and control authority at low airspeeds and high angles of attack.

Micro Vortex Generators - Over 500 Models of Aircraft Certified

[ecomodded]

Quote:aerohead

*Since the Beetle has no 'boot',there's really nothing to reattach to.
*Without the 'reattachment',any pressure gain you might get from the VGs are eaten away at by the constant increase in interference drag and induced drag of the vorticity they produce.Gains could be 'negative.'
------------------ reply:

With the Beetle i have thought that the best place for a rear spoiler is not at the bottom of the glass where it is in stock configuration when mounted,
but at the bottom of the hatch itself, on the top of the bumper/ license plate indent about 24" below the rear glass.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Quote:
If you can find the thread for the aero-modded Chrysler Airflow,you can see what Carl Breer would have done to your car.It was actually the world's first K-form car.
~~~~~~~~~~~~~ reply

I bet he would boat tail it and but good.

[redorchestra]

i thinkI might have proof of airtabs working for me. I drive about89km each direction to work everyday. Last week my tank to tank was 36.28mpg that pretty stock as I removed my airdam and grill block. This weeki added 6 airtabs that I made and mounted on magnetic sheets. My mileage for this week was 37.69mpg. I will remove them again for next week and see what happens. My testing its not strict but I set the cruise control for the same speed every day andI take the exact same route to and from work. Most variables should even out in the course of the week anyway.

[aerohead]

Originally Posted by ecomodded

Quote:aerohead

*Since the Beetle has no 'boot',there's really nothing to reattach to.
*Without the 'reattachment',any pressure gain you might get from the VGs are eaten away at by the constant increase in interference drag and induced drag of the vorticity they produce.Gains could be 'negative.'
------------------ reply:

With the Beetle i have thought that the best place for a rear spoiler is not at the bottom of the glass where it is in stock configuration when mounted,
but at the bottom of the hatch itself, on the top of the bumper/ license plate indent about 24" below the rear glass.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Quote:
If you can find the thread for the aero-modded Chrysler Airflow,you can see what Carl Breer would have done to your car.It was actually the world's first K-form car.
~~~~~~~~~~~~~ reply

I bet he would boat tail it and but good.

Professor Hucho refers to your car as a Pseudo-Jaray form.The rear is too fast for attached flow.Even if Flow Illustrator was accurately depicting the Beetles flow(and it is not),it would only be at the centerline and would not indicate the longitudinal vortices attached on each side which is killing your MPG.
Hucho would go on to tell you that so far,nothing which works on aircraft has demonstrated like efficacy in ground proximity on an automobile.There is no well of free-flowing air below the car.
Propellers and struts are 2-dimensional flow devices and their dynamics are not germane to automotive aerodynamics investigations.
The Beetle violates the 115-degree rule of boundary layer flow attachment.Unless you relax that curvature,more like VW's latest 1-liter car,you have no chance for meaningful drag reduction.