brianrheffron

Link for Andy-GTS.
http://www.airtab.com/main.html

andy-gts

thanks brian.

IcemanG17

I talked to these guys a while back.....VERY HELPFUL..... I was interested in how their product could improve the "bad" aerodynamics of the Estate.....they actually sent pictures of my car to their head aerodynamics engineer who sent me an email....my 1st thought was to remove the S spoiller off the roof and just add airtabs....he said a better solution was to make a new spoiler at the same "gain" with airtabs on the end of it....so I don't loose downforce, only drag....but he also said the shape of the S spoiler I have in nearly laminar flow will yield a similar result to their product....

mark kibort

thats why i cut large holes in the rear bumper where the license plate is.

mark kibort

Carl, your on the right track with much of the stuff you are experiementing with, but these two tests are complete BS. 8mpg savings?? carl, i once tailed (drafed) a semi on hyway 5 right after I filled up . we went 60mph for 30miles . as was seen on myth busters, this technique of being near 20ft behind a semi gave me near 5mpg, when i filled up at the final destination) YOU GOT NO WHERE NEAR this benift with little "tabs"

Now, road america.. with lap times of 2:40 a lap, and a prodigous amount of HP, it is NO stretch that you could have come off the curve with more speed, or that you were that much better that or those laps , or track conditions were changed to get 5mph on the main straight . I would bet and give you $1000 if you got 5 mph more speed, and came in, took off the tabs and gave me you car! I would match that speed Guranteed! (and so would you! )
Now, a little science . if you would get near 5mph down a main straight, its something very close to near 30-40hp to do that. and at the speeds you are travelng at, thats near 75-100lbs of drag saved.
To put it in perspective, a 250lb downforce wing at 120mph costs about 25lbs of drag, which takes about 8ft-lbs of torque out of the engine. thats the entire wing cost!! you think your little tabs creat the same effect a pulling off the drag of a 1000lb downforce wing?? doubtful.

Also, another data point, is if you could save 5mph up to 150mph on that straight due to aero savings, it would almost equal ALL your modifications to date for bonneville! work the calculations. you will see. 5mph down the main straight of road america increase will have to be something greater than little "tabs". Much greater!

This ranks right up there with the guy that thought when he lost 5lbs off each wheel he gained 1 second a lap and 5mph down the main straight of Cal speedway. Scientifically, when we looked at the DATA, he was coming off the turns better and applying the gas earlier consistantly! it was obvious.
with telemetry, you would see the same thing on your 5mph run, and it WASNT due to aero changes. if you got 1mph for a major aero change, that would be HUGE!! ever watch racing. if someone that has 1-2mph better speed, he litterially , walks away from you . little aero changes dont even do this, but they all add up , I agree.

Ive seen plenty of races where someone losses a wing, or has a bumper flaying in the wind and are able to still match their competitors lap times and speeds. I still remember kleinubing when he got hit in his speed touring car, and his rear hatch was flapping up and down with the wing attached, it didnt hurt him at all! many of these racing mods are subtle at best on a straight light measurement.

an other proof point. I've been to T-hill a bit, right? so, my debut of my car with NO aero on it. no wing, no splitter, i was still able to muster a scary lap at 2:02 vs my best of 1:59.8 at the time, with crappy toyos . same straight line speed of 130mph down the main straight. witih the wing, splitter and better tires, 1:59.xx was achieved, and the same 130mph down the main straight. no surprise, because the wing is 275lbs of downforce , or 27lbs of drag or 8ft-lbs of torque off the engine at near terminal speed , BUT, better exit speed off the prior turn more than make up for any drag at the high speeds, (110mph plus) which only account for less than 50% of the time down the straight anyway.

mark kibort

one more thing on vortex generators. they dont re-attach flow, they create turbulent flow, so that disturbed flow stays close to the surface. attached flow is simething entirely different. again, aircraft manufacturers use them to get air flow to the control surfaces (although as dirty as the flow is , and as high drag as it is) so that control is maintained during near stall conditions. a great example of how they are used on cars, are ones with boxey rear ends. (i.e. subaru or lancer eVO). it forces the air to head dowward , disturbed slightly, so it can hit the wing and create downforce. for gas efficiency, you would lose them. (or if you had a really slated rear hatch like we do, and the wing is in the right position to recieve the air flow)

mark kibort

Brian, you dont need to worry about drag with the estate at the speeds we race at. put a bit wing on the back of the estate, and lenthen your splitter up front. it looks great and is doing a good job i imagine at turn 8 at thunderhill!
wait until you put the bigger tires on ! now, that will be fun!

andy-gts

dang, man!!

IcemanG17

http://www.grc.nasa.gov/WWW/k-12/airplane/foil3.html

Here is a cool simulator I found....bummer is I don't know enough to get anything useful out of it

Glenn Evans

From http://www.airtab.com/how.html:

Air flows into the "V" of the Airtab, is deflected away from the surface of the body and forms a pair of contra-rotating vortices. Mark is correct in that they could not re-attach the airflow to a sloping surface. They work on a tractor or trailer by breaking up the large vortices which are shed from their rear edges. At the rear of the trailer, in particular, these vortices shed from each side alternately, and can be powerful enough to make the trailer sway, as the lower pressure associated with them moves from side to side.

The pressure in the disrupted airflow would not be as low as it would within a vortex the height or width of the trailer, so drag would be reduced significantly. Similarly, no large vortices would form behind the tractor cab to suck large volumes of air into the gap; the pressure in the gap will not be as low and there won't be any large, energy-absorbing vortices.

Although the Aerotabs are located well within the boundary layer at the rear of a 40ft trailer, the airflow would be enough to generate the vortices at highway speeds. At 88 ft/sec (60mph), the velocity one inch (the height of the Aerotab) away from the surface would be about 68 ft/sec; at 0.5 in: 62 ft/sec and 0.25 in 56 ft/sec.

Glenn Evans

It models a wing in the free stream (airflow unaffected by anything in front of it). Even wings mounted as high as those on Carl's and Mark's cars would be affected by the airflow over and around the car.

What is interesting is that you can create aerofoil sections which would not be used on aircraft, so good data for which is not available. For instance, a highly-cambered wing, like that on Carl's car, can be modelled.

If you want some explanation so that you can play around with it, start a new thread rather than go off topic here. I'd say PM me, but I suspect that more people may be interested.

mark kibort

actually, the effect of our wings, (normal race wings, not carls kick tail spoiler) that are as high as they are, is really not much off free air. you can see from carls streaming video, the air is just deflected downward at a 8 degree angle. not much turbulence if any. Yes, you can make a high cambered wing to get the same data as carls wing. my data from my wing match matched the simulator. i measured about 275lbs of downforce at 120mph. the books said i was looking at a L/D ratio of about 10:1 at the angles I was running

yep, i believe they would work on trucks and blunt rear end objects. (as the site even points out. even the representive said that the shape of the estate spoiler does what they said the little gysmos woud do.
They certainly wouldnt give HUGE results on a race car , and certainly couldnt give a 5mph straight speed at road america. again, to get that kind of gain, by any measure , you would need about 35hp, and there is no way the tabs are capable of doing this .



Its a great simulator. Ive used it and others like it for all sorts of pet projects.

Mother

Just an idea and probably not in regs.

Glenn Evans

I wasn't suggesting that wings mounted as high as yours and Carl's would be in turbulent air, rather that the incidence of the flow onto the centre section of the wing will be affected, as you have said. Carl's wing looks to be set at about 7 degrees nose down so, if the air flowing onto the centre section is flowing downward at 8 degrees, the centre section is actually at an incidence of -15 degrees.

IMHO, the vortices generated by the Aerotabs are likely to disrupt that generated by the step at the top of the hatch glass, and cause the flow to detach. Presumably, they would still reduce the drag of such a separated flow, so the nett effect would be to straighten the flow onto the centre section of the wing for little drag.

I cannot see how the sort of vortices the Aerotabs generate would cause a separated flow to attach to the surface of the hatch.

Using FoilSim, if you use "Size" to set the wing span at 7ft and chord at 1ft (about the size of Carl's wing); "Flight" to set the velocity at 130mph and "Shape" to select "Ellipse", set Thickness/chord at 12.5% (1.5 inch maximum thickness:chord of 12 inches) and camber to 15%, you get a wing section resembling Carl's.

The difference in drag between the wing at -15 degrees incidence over the centre 3.6ft and -7 degrees over the rest, and -7 degrees across the whole span, is 36lb (77lb for the 10% thickness/chord). Assuming a Cd of 0.40 and cross-sectional area of 22ft^2, the no-lift drag at 130mph would be 380lb and at 135mph 410lb.

Using the above assumptions, the car with Aerotabs at 135mph would have 26lb less drag than without at 130mph so, IMHO, Carl's 5 mph increase due to drag reduction alone is plausible.

YMMV. :-D

Carl Fausett

That's an excellent answer, Glenn. Spot on.

I'm also glad you pointed out that that the vortex generators do not re-attach the boundry layer.