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Old 05-31-2020, 09:22 PM   #1 (permalink)
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passively variable angle VG; fixes crosswinds

Taken the idea from modern airliners' nacelle strakes which is basically a huge VG on the side of the engine parallel to the airflow in level flight. Let's say the angle between the airflow and lengthwise centerline of VG, θ is 0 here. At level flight, the θ is 0 so no vortex is generated and marginal amount of drag introduced.

Now imagine the aircraft still moving level, but pitched up ~20 degrees; the airflow remains the same but the plane is turned up, resulting in θ = 20.

NOTE : In case you didn't know, it took me years until I was able to finally grasp the reality that airplanes rarely fly in line with the centerline of fuselage outside cruise. They pitch up and still slide forward (and downward a bit when landing) to create more lift at less airspeed but less efficiently. Which is why the VG is "under" the bottom of the wing but the air through the VG flows over the wing.



Now how is this relevant to a car? I can't actively pitch my car up or roll it to help turning, but the wind changes and I have to yaw (steer) the car to counteract the sideways forces which is never constant. And VGs on the trailing edge of a body seem to help "pressure recovery" as seen in the airtab wind tunnel civic data.

I've played around with several dozens of airtabs for past 5 years and even had 6 airtabs along the top of the rear window nearly eliminating any gusts when the weather app said up to 30mph.
Here's a little side note. For every airtab placed anywhere on the top or the side of 2007 Acura TL, it decreased my highway MPG pretty linearly by approximately ~0.5%. I tested from 2 to 12 airtabs using OBD2 fuel consumption data on a roundtrip of 7 miles (2 rural exits on the interstate little over 3 miles away) averaging around 21MPG.

Anyway, I concluded that airtabs having 1" height is the thickness of laminar flow at the end of 80ft box, not on a 15ft car. I needed something with less height as these are going to be placed across the hood, along the transverse frontal edges of stagnation bubble in the transition from hood to windshield. Airtabs could work in a pinch, but they're not as efficient (ECOmodder right?) being too tall for the airflow.

So what fits my bill? DIY VGs for general aviation planes.
These are 3/8" high and rigid.
https://www.blackmaxbrakes.com/airwa...tex-generators
These look about the same but flexible.
https://www.stolspeed.com/
Both are less than $1 / piece, less than $1.375(2.75/2) / piece for Airtab.

Since I don't want people tripping and falling on my car's hood and suing me for the "dangerous barbed plastic pieces" on my car, I decided to go with the soft one.

Some advantages
- Designed to work around 60mph (40-60kts in testimonies)
- Clear color (most won't bother to notice)
- Semi-flexible (no injury)
- Removable, semi-permanent, paint safe adhesive unlike airtabs' the "Ultimate Epoxy in Sponge form" which never comes off clean.
- (PROBABLY) not going to induce much drag even if I fail to install it at exactly θ = 0 since they're meant to be on an airplane without inducing too much drag at an angle at all times.


After finding out the direction of airflow with tuft test, they will be placed in a obtuse V-shaped arrangement with the pointy side forward, aligned θ = 0 in no crosswind airflow.
MS Paint drawing to help visualize...



Some visualization of airflow around a sedan in crosswind. From this link
https://hal.archives-ouvertes.fr/hal-01449542/document on page 8 by Rossitto et. al.


Whatcha think??

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Old 05-31-2020, 10:23 PM   #2 (permalink)
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Lockheed Georgia airtab study

Because people will ask for it, I went and found the cited airtab study (I think this is the one):

[Anon,] "LOCKHEED GEORGIA LOWSPEED WIND TUNNEL HONDA CIVIC HATCHBACK AIRTAB(R) MODIFICATION RESULTS" http://www.airtab.com/test-results.htm

"Executive Summary:
This report shows conclusively that the Airtab® product reduced aerodynamic drag
forces at the base area (the rear facing surface) of the test vehicle. The test showed a 4%
reduction in horse power required to maintain a steady speed of 55 mph. Only the sides
of the vehicle were fitted to assure the most aerodynamically symmetrical run data. By
adding Airtabs™ to the rear roof line as well, a conservative performance benefit
extrapolation of these findings would be in the 50% range resulting in a further HP
required reduction to approximately 6%. The test also shows that the vehicle drag
coefficient (CD) is reduced at every yaw angle from zero to thirty degrees angle from the
airflow and that this CD reduction increases at greater yaw angles."









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Old 05-31-2020, 10:34 PM   #3 (permalink)
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Quote:
Originally Posted by California98Civic View Post
Because people will ask for it, I went and found the cited airtab study (I think this is the one):

[Anon,] "LOCKHEED GEORGIA LOWSPEED WIND TUNNEL HONDA CIVIC HATCHBACK AIRTAB(R) MODIFICATION RESULTS" Airtab | Aerodynamic Fuel Savers | Test Results

"Executive Summary:
This report shows conclusively that the Airtab® product reduced aerodynamic drag
forces at the base area (the rear facing surface) of the test vehicle. The test showed a 4%
reduction in horse power required to maintain a steady speed of 55 mph. Only the sides
of the vehicle were fitted to assure the most aerodynamically symmetrical run data. By
adding Airtabs™ to the rear roof line as well, a conservative performance benefit
extrapolation of these findings would be in the 50% range resulting in a further HP
required reduction to approximately 6%. The test also shows that the vehicle drag
coefficient (CD) is reduced at every yaw angle from zero to thirty degrees angle from the
airflow and that this CD reduction increases at greater yaw angles."
The above test was done on a 1982 model Honda Civic with a Cd of 0.45. I very much doubt the relevance of that testing to any more modern car shape.

Ford tested Airtabs and found increased drag.

I tested AirTabs over a 150km daily round trip and found increased drag.

In my book I cite a paper using VG (not Airtabs) at the upper rear of a hatch-shaped car and they increased drag and lift.

I love using AirTabs to promote flow attachment but I don't think they work in reducing wake drag.

To the OP: good find on the flexible small VG, and I think the basic idea of VG behaviour changing with crosswinds is sound.
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Old 06-01-2020, 12:19 AM   #4 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
The above test was done on a 1982 model Honda Civic with a Cd of 0.45. I very much doubt the relevance of that testing to any more modern car shape.
I was not vouching for the study. The OP mentioned it. I was curious. I am not willing to assume it means they work, either. Anyway, my understanding it that the Evo design's airtab placement took a lot of windtunnel time. I imagine micro adjustments matter a lot.
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Old 06-01-2020, 02:53 AM   #5 (permalink)
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Interesting, not sure how much concrete benefit you'll get for the effort though.

My thinking is that generating vortices in a crosswind could possibly make the lateral forces on the car worse.
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Old 06-01-2020, 05:11 AM   #6 (permalink)
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I now have 40 of the Stolspeed VGs on their way. Very cheap including postage to the other side of the world - AUD$40. Good guy to communicate with too.

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Last edited by JulianEdgar; 06-01-2020 at 05:20 AM..
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Old 06-01-2020, 09:49 AM   #7 (permalink)
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Quote:
Originally Posted by serialk11r View Post
Interesting, not sure how much concrete benefit you'll get for the effort though.

My thinking is that generating vortices in a crosswind could possibly make the lateral forces on the car worse.
Quote:
Originally Posted by JulianEdgar View Post
I now have 40 of the Stolspeed VGs on their way. Very cheap including postage to the other side of the world - AUD$40. Good guy to communicate with too.
Those look good. 1" height?

I don't think the OP's plans will work, if I understand correctly what he is attempting. He bought 3/8" hieght instead of 1" height and plans to place them on the hood, positioned with yaw in mind. The idea being to increase pressure on the low pressure side under yaw. I agree with serialki11r. It'll create more drag, yaw or no yaw & make crosswinds worse.
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Old 06-02-2020, 03:11 AM   #8 (permalink)
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I appreciate criticism but only when provided with a reasonable thought process. If one can say it won't work I can also say it will and it is not contributing to anything other than a blobby mess of opinions. All I got is "It ain't gonna work" without any hows. (That's how online forums get multiple threads claiming they get better mpg with hotter coolant but that's another story)

Let me give you an example. I think these will work in similar ways that roof extension spoilers work to reduce lift and drag in hatchback vehicles. Crosswind is never constant, which also promotes more abrupt separation on the trailing edge (in this case, A pillar and front roofline). The VGs when activated by crosswind, "...will prevent abrupt air separation at the trailing edges and therefore, contributes to lesser Wake creation, which in turn reduces the drag of the vehicle", quoted from explanation of hatchback rear spoiler.

This is why I also had an idea to just attach something to promote cleaner separation on A pillar but it already creates giant vortex as is which is out of my thinking capacity at this moment. End example.


I like to look at Toyota stuff since they seem to be going crazy with VGs for the past decade, which is my go-to item for when I can't modify the actual shape. The idea of letting the wind vary VG angle is probably not my original. If you look at any 15+ Toyota/Lexus vehicle from behind you can see the plastic cover under the trunk floor has four 1" x 6"(approx.) 2.5cm x 15cm plastic protrusions that are in line with the length of the vehicle. And I'm not going to bother posting visualizations of airflow under the car in varying crosswind because you should know it before you reply in this thread.

Toyota got the variable angle part down, and yes I'm still not certain if it wll help reduce the pull caused by crosswind and how to measure it if I could at all. Especially having seen that Airtabs do not "fill the wake" or "increase pressure on the trailing side", if I had a Camry or RX400 I'd already be testing the car with the strakes covered vs uncovered on windy interstate.


If those little pieces in line with airflow "creates more drag", I strongly believe it is smaller than margin of error in real world testing which is the only thing I could do so I won't be able to see it. But I will keep an eye out just in case I made wrong assumptions.
And, sure, airliners only have 2 of them but if they affected FE in any significant way, they wouldn't be on LH commercial airliners.

In crosswind, I actually do not know if they will help fight crosswind or not for the Airtabs did not reduce drag anywhere on my car. But like I explained in the OP I'm crossing fingers they would be efficient considering they're designed for vehicles much more sensitive to aero drag than cars.


And no those are 3/8". If one would care to read several posts above, we have been just talking about how 1" airtabs hurt MPG even on the trailing edge. It can be visualized how thin the laminar flow is on the front part of the car, put tufts on a straw perpendicular to the surface and give it some diagonal supports and go fir a drive.
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Old 06-02-2020, 03:27 AM   #9 (permalink)
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You tend to write a lot which makes it hard to follow what you intend doing and why you intend doing it. So, is this right?

Fit fin-type vortex generators on the hood in front of the windscreen to change A-pillar separation and so reduce susceptibility to crosswinds.
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Old 06-02-2020, 11:42 AM   #10 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
You tend to write a lot which makes it hard to follow what you intend doing and why you intend doing it. So, is this right?

Fit fin-type vortex generators on the hood in front of the windscreen to change A-pillar separation and so reduce susceptibility to crosswinds.
I tried quoting but it won't accept more than 3 so I tried to just lay them out. I don't think any of them were replying to you tho... not too sure what I was exactly thinking but yeah.

And yes. Gosh I'm going to suck at writing thesis..es...

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