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Old 06-27-2007, 10:13 AM   #1531
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Quote:
Originally Posted by DLS II View Post
John, your idea about a collapsing rear wing rang a bell. Did you ever run slot cars? Remember their wing/air dam system? They had a stiff piece of lexan stapled to the front end to keep air from getting under the body, side dams that ran the length of the body. They also had a rear dam that was bent into something like a scoop. At speed the side dams would collapse and the rear dam would straighten out. Group 7 (open class) cars could get around a Blue King 155' track in less than 4 seconds! Don
Thats exactly what i was thinking, but im guessing John would be looking for a compromise, as those slotcars were looking for nothing but more downforce. If you went for this setup, you would get way too much drag. very interesting topic, actually, look into the air shock supported wings on the IMSA super modifieds, they have some spring kind of wing, which lowers on acceleration and raises through the corner..... hmmmm
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Old 06-27-2007, 11:27 AM   #1532
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Thanks for the posts. I did run slot cars and have used the lexan stapled to the front on a slot car dragster. I have one taped to my high speed body right now. I may be able to put my collapsing rear spoiler back on if I use the front piece. It will collapse as well. I have some problems with the new wind machine that may be solved by raising the plate up on little pedestals. I think it needs wind flow below the plate or the plate lifts all by itself with no car. I did not seem to have this problem with the previous setup.

I did some more experiments behind the car. Over the tires the air flow is stagnant at the back. The impeller goes forward slow then backwards slow. In the center top there is up to 1.8 mph into the car body. The airflow out is indeed there but it is down low. I get a good jet down there with no front soft dam.
John

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Old 06-27-2007, 01:32 PM   #1533
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john, a quick and easy alteration to try. take some masking tape and cover the various chassis slots that would normally allow air in.

John you really need to get some sort of separator (air straightener) between the fan and the car. while you can compare test to test, i would wager that much of the data is a bit skewed from the fan creating vortices. air at ground level, moving or not is more uniform than what the box stock fan will create, whether you're on the positive or negative side of the air flow.

search "build a wind tunnel" and the links on the first page alone cover grade school to workhorse style wind tunnels. I want the magic smoke machine to show air turbulence also!

keep up the enthusiasm and efforts.
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Old 06-27-2007, 01:39 PM   #1534
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just go to uhaul and but a bod specifically designed for packing glasses. they are corrugates intelocking seperators..
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Old 06-27-2007, 05:46 PM   #1535
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Mason, TallyRC-Thanks for the posts. I will try the tape tricks. If my Lexan had been bigger I would have a bigger belly pan. After I completed the tapered box I found the buffeting to be reduced and changes in wind speed to be less. The square section of the tapered box helps a lot to straighten the air flow. It would be better if the small section was longer. My meter detects very little variation in velocity over the front face of the box. Even though I am reporting numbers realize this would be better done on Nicks school wind tunnel. My fan is better than a box fan. It is round and powerful. If I come accross some separators, I might try that, but work in the tunnel nears an end. Note that I did take a look at Nicks research and his schools's research grade wind tunnel.

Track test 97F, Damp as hell. Not fit for human or beast.

I tried the high speed body out at the track. I needed to pick up some race reports from the regionals there to post. I stood at the end of the straight to gain practice from lower elevations.

The body had no soft dam but a partial belly pan. The front end was planted over the bumps at normal track speeds. Usually I need the soft dam for this. Good.

I had sufficient forward traction to reach full throttle in about 180 feet. Our parking lot has a much greater distance. So forward traction is good.

I had sufficient straight line stability once I got up to about 20 mph. This is good. So the body with collapsible spoiler, belly pan, and no soft dam is a go. I'm ready for a buddy to hold the radar gun. Nick has some important personal buisness to attend to for a couple of weeks. I note that the side dams have some torsional flutter about their long axis. I expected this from feeling them. This is probably OK. They are not bending side to side. That car looks really fast when it passes you only a couple of feet away. Nick almost took me out when we were video taping his last session. I was looking at the little screen but I really could not see the car in the sunlight. Lucky the car was in the pic at all. He does not trust me to drive the car.

I did drive a couple of laps on the partially damp track. Front steering on slow corners was better than before. Rear grip on the sweeper was absent as you would expect from the very low wing.

Wind Tunnel Improvements

My tunnel has three sides now. There is good wind speed behind the car. Elevating the setup board on pedestals allowed me to set up the front at 0 downforce with 0 variance with, wind speed increase, without the car on it. There is a little variance but it wobbles back and forth between plus and minus and so averages to 0. I had trouble with this previously. The rear seems to have a .5 ounce preload with the wind for some reason. Possibly the tunnel needs to have the fourth side. This is included in the data as the start point.

Here is some extrapolated data. My front downforce at 50 mph should be in the neighborhood .15 lbs. This is great. My rear downforce at 50 mph is .25 lbs, I corrected the zero error see the posts below. My weight distribution is exactly 50-50 with this body on. That is probably enough.
The lower rear downforce will reduce drag. It seemed stable at about 50 mph on the track. Not near as much braking potential as before, but it is sufficient.
John



Note the bottom of the box is tapered up on the inside. Picture in the middle shows the original setup and the fan.
Attached Thumbnails
Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-wind-tunnel-complete-resized.jpg   Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-wind-tunnel.jpg   Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-graph-spoiler-downforce-c-resized.jpg  

Last edited by John Stranahan; 06-28-2007 at 05:59 PM. Reason: shrank the graph
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Old 06-27-2007, 06:41 PM   #1536
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John

Can't you tare the scales with windtunnel blowing with no car on the scales to remove the preload?
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Old 06-27-2007, 06:51 PM   #1537
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Sydewinder-I can but the rear changes a little with speed. The scales however need to be zeroed with car on to make the data easy to read, so that makes it not practical also. It is not off much, so I just included it. At the front which is my important measure it is very good and steady near zero with incresing wind speed. I am trying to solve the blowover problem. I think the solution to the rear zero is to provide a shield near the bottom of the pedestals that they would stick through that would prevent leakage of wind into the scale area from the sides. The lead scale is shielded by a stack of metal bars.
John

I found a baffle. I cut out a balance shield. I'll test the wind tunnel again tonight.

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Old 06-27-2007, 08:27 PM   #1538
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Here is the latest incorporating all input. You can see a horizontal baffle. You can see the finished Balance Shroud with viewing windows. The 0 is now very stable. There is less than .1 ounce of variance off zero at all speeds. Great. I'll restest and adjust the numbers on the graph. I'll probably get some numbers on the standard body.
John
Attached Thumbnails
Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-wind-tunnel-last-mod-resized.jpg  

Last edited by John Stranahan; 06-27-2007 at 09:58 PM.
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Old 06-27-2007, 09:31 PM   #1539
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More Wind Tunnel Results
The baffle cut my wind speed by 2 mph. This is all that the box improved it, so I removed the baffle. I was able to put about a 2 inch gap between the outlet of the box and my balance table and a 1 inch iron bar to shield the balance pedestals. I got good zeroes now front and rear that did not increase or change with wind. I redid my experiment with the high speed body. Collapsible rear spoiler, Front body pan, no soft dam. The downforce at the front at 50 mph is predicted to be .3 lbs. This is similar to before, but I had the previous graph mislabeled. (I have corrected that graph now). The rear numbers changed because they contained the zero error. The prediction is now .4 lbs of rear downforce at 50. I am happy with both of these numbers. It should produce a stable run with no blowovers and reasonable downforces at 80 mph. Too soon to tell but it was fun doing these experiments during a couple of rainy day.

Technical stuff
Now can I really predict what is going to happen at 50mph from data at 16mph and below. Well the answer is yes and no. If the car maintains it shape it is more yes. If there is a theoretical basis for using the quadratic equation to fit my data then the answer is more yes (The answer to this is in one of Nicks Books). This formula fit the data quite well.

If the car wads all up like Nicks plexiglass wing at 80 mph, then the answer is no. I have on purpose put some flex in my rear spoiler on the high speed body, so the number I quoted above would be the maximum and would be reduced by any flex. At the front I don't expect much deformation. It probably is a reasonably good prediction. This will hold the front end down over the bumps and undulations.

Less technical stuff

Graph on the left is the High Speed Body it tells me several things. The red line is above the blue line, this means rear down force is higher than front downforce. The lines curve upward. This tells me the downforce increases at good rate with increasing wind. The lines on the graph beyond the data markers can be used to make a rough estimate of the downforce at higher speeds than available with the wind tunnel.

The graph on the right is the Standard type Toyota GT1 body with a 2 inch spoiler chord which is the maximum allowed by the rules. I had to change the scale on the left side of the graph because the huge rear spoiler is much more effective at producing downforce. If I plotted the rear downforce of the GT1 body on the graph at left the Red Rear Downfoce line would look much steeper. This indicates much greater downforce. At 50 mph it develops about 2.2 lbs. This is backed up with track data that shows the rear suspension fully loaded at this speed (all the way down, 2 lbs on a scale). At 80 mph there would be an excessive 6.5 lbs requiring hugely stiff springs and diminishing any suspension action. This wing position would also create a lot of extra drag and slow top speed down.

Note that my front downforce is improved by a factor of 2!! with the addition of that big spoiler. I ran the 1/2 belly pan on this test.
Attached Thumbnails
Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-graph-high-speed-body-resized.jpg   Pantoura, 1/10 Pan Car, 2S LiPo, Brushless, Tips and Tricks.-graph-toyota-gt1-resized.jpg  

Last edited by John Stranahan; 06-28-2007 at 10:18 AM.
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Old 06-28-2007, 09:10 AM   #1540
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wow!
that just shows what can be done with some card board and duct tape!!
You have done what I have been thinking about, and probably never would have built it. You wind tunnel is very cool.

I may have to purchase a couple digital scales and a strong fan. the card board and tape is already there. this would really help me set up the aereo package for my velo car. although I do know that things change above 50 mph. down force that worked below 50, failed at over 50 on the track.

can you get the wind to over a true 50 mph with the venturi tunnel?

Wayne
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Old 06-28-2007, 09:31 AM   #1541
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jon, after reviewing your wind tunnel post.

I think I know the problems above 50.

although I use a smaller rear wing, it is either bending down and/or vibrating causing no effective down force. I will install a support strut or two to stabilize the wing.

I hope it works, it is not fun to have the rear end let go above 50 coming out to a turn!!

thank you for your scientific approach to speed and effeciancy in a hobby that has a hot rod approach to speed.

yours.
Wayne Nagata
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Old 06-28-2007, 10:07 AM   #1542
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Wayne-Thanks.
I agree that a wing that collapses too far is going to mess you up on the oval. Maybe a little collapse would help on the straights. I think to get over 50 mph with the tunnel would require two things. First the tunnel would have a tendency to expand, it would need to be made out of stronger stuff. Second you would need a hell of a fan. I notice that Nicks school's wind tunnel has a multiblade fan and no interior baffles to straighten the flow. There is a shot of the interior posted above. It relies on a long stretch of square section to straighten the flow. I also notice it is outside or in an adjacent room probably because of the roar. The fan required would almost have to blow 50 mph without the tapered tunnel. The square tapered tunnel is used to straighten the flow, reduce turbulence, and to slightly increase the wind speed.

Thanks for the posts.

I thought I would list some of the dramatic things that I learned playing with the tunnel.

Wind Tunnel Conclusions

Spoiler Cutouts
The cutouts when made on the Toyota Spoiler reduce rear downforce by 33 %. (they don't increase it by moving the flow downward). This is probably the same on the Peugeot.

Partial Belly Pan
Adding a forward Belly Pan significantly increases front downforce. It almost doubles. I don't know how crash worthy it is. I have a plastic bumper coming to reinforce the front. Then I can use thinner Lexan.

Rear Downforce at 80 mph
The stock 1/10 pan body with a really stiff rear spoiler can produce 7 lbs of rear downforce at 80 mph, if it is not collapsing from the force. I have trackdata to back up the 50 mph number so I have some confidence in the 80 mph prediction.

Rear Down force with nose up from a bump
The belly pan significantly improves downforce if the front end rises from a bump.

High cutouts of the back of the body
The flow of air at center back top of the body but under the spoiler is reverse. It is flowing air under the body. Maybe such a high cutout is not warranted back there. Certainly from the top of a bumper down there is a flow outward. I measured both with the wind meter.

John

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Old 06-28-2007, 10:47 AM   #1543
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I've always had a hard time with statistics that extrapolate so far from so little.

I don't mean this in any way to denigrate your work!

It's just that how can we really look at 15-20mph and get real results at 3-4 times faster? The number of variables increases dramatically - the dynamics of the air change. Where you won't get vortices at 15-20mph (since the air can flow around), you may very well get a lot of them at 45 or more.

Point to illustrate - the area near the rear, just before the spoiler ramps up. . . at low speeds, I can see the air actually getting down there. However, at higher speeds I would expect this area to build its own vortex, allowing the main stream of air to completely bypass it - thus one reason for those little cutouts: they disturb this pocket and "pull" the airstream lower. . .



btw - have you looked at how the Mythbusters built their windtunnel? They used straws to "straighten" the airflow so they could use a little smoke to see where the air was going. . .
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Old 06-28-2007, 10:53 AM   #1544
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I agree that you cannot get a rock solid estimate at 80 from data at 16. However, I have supplementary downforce data at 50mph from the track at the back. It agrees closely with my curve. In other words I have real data out to 50 mph. That gives me more confidence in the 80 mph number at the back. I have trouble with the I would expect part of RC (of course I do this myself). Thats why a lot of stuff is backwards. I personally think you are going to get laminar flow up to 50 mph and maybe those vortices at full size GTP speeds. That body is really sleek and smooth.

If you take a turbo dyno and measure power of a stock motor at 10 - 30 amps in four steps, you can predict the whole curve, if you fit a quadratic, up to stall at 75 amps. This is because, in theory, the motor Power vs Amp curve is a quadratic. The numbers are backed up by the Robitronic dyno which can measure at higher amperage. It is possible to extrapolate sometimes. It is not statistics it is Science.

If you look at Nicks data up to 80 mph you will find that some of the curves have a similar shape to mine at the higher wind speeds (they almost look straight). His data is different in that the base body had a lot of lift and possibly a lot more flex in supporting the wing. (My peugeot is a thick body. That .040 Lexan spoiler is stiff as hell.) This flexing masked the upward curvature that the device itself might produce on a stiffer non lifting body.

No smoke is forthcoming. It is not laying around the house and thus violates my speed run rules.
John

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Old 06-28-2007, 12:36 PM   #1545
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Boomer- OK. Here is the theoretical basis for the quadratic. With this, the confidence in my predictions are improved. Not super just improved. I have put plenty of qualifyers on this extrapolation.

I can add statistics to my curves like 90% confidence limits. It is trouble, and I would then need to do more experiements. Some replicates would be nice. The data was smooth, so I did not do replicates. I only do enough experiements to please myself, not the statistician. I expect that you would see the confidence band lines diverge apart at the higher velocities indicating more uncertainty, but not huge due to the smoothness and repeatability of the data.



D= 1/2 x (WS x H x A0A) X F x p(greek letter Rho) x V^2

Where:V is velocity in m/s

Note that the downforce D increases as the square of velocity V. Thats the quadratic I was looking for.
That's probably enough math for the month.

John

Last edited by John Stranahan; 06-28-2007 at 05:43 PM.
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