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Author Topic: Drag coefficient fundamentals  (Read 6600 times)
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Rex Schimmer
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« Reply #15 on: November 17, 2016, 05:39:21 PM »

Jack Gifford said "When you mentioned area being reduced by lowering a car, I assume not an open-wheel car? I don't see that lowering a lakester could change its area." One of the things that you need to consider when lowering a tank (or any round bottom car) is that as the bottom of the tank gets close to the ground it begins to choke off air flow and can and will cause the attached air in the rear lower region of the tank to become attached which is the highest drag condition. Being low looks neat but it may not be fast. As I always reference: Look at the Contrivance modified roadster as a great example of providing sufficient ground clearance to allow the air to flow underneath it. (If you don't happen to know, the Contrivance modified roadster holds all of the gas records from E through AA plus some blown records also!)

Rex 
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« Reply #16 on: November 17, 2016, 07:00:27 PM »

I don't know aero from arrow.

But years ago, a friend of mine built a bike liner using what he believed were the aero theories used for the wing and belly tanks.  He'd found out that they were placed one radius below the effected body structure of the wings -- to be neutral.  So when they were jetisoned, the plane did not react to them.

The bike was underfunded and was designed for two Kawasaki triple 2-strokes and only ran with one Harley, so who knows what it ever proved.

(Disclaimer -- most or perhaps all of this is repeated with a failing memory.)

Stan
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Rex Schimmer
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« Reply #17 on: November 18, 2016, 02:05:48 PM »

I said "tank to become attached which is the highest drag condition" I meant to say "unattached"! Attached flow is low drag unattached flow is high drag. As usual I will blame it on age! at my age everything has something to do with age!

Rex
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« Reply #18 on: August 22, 2017, 09:30:02 AM »

A2 uses small, compact straps on the front tires with a deflector to "simulate" the tire rolling. When you change tire sizes or wheel discs you find out how much HP you gain or lose real quick!  cheers
Regarding various components - it's a 3-dimensional interactive problem - leading to the two most common wind tunnel comments: "Wow, look at that!"  shocked and "I have no idea!"  huh
TEST - DON'T GUESS!  cheers

Woody I suspect that tire "addition" is an attempt to get wheel lift to match rolling road conditions. I wouldn't put much stock into drag values off it.

It's amazing how many of the community have been steered towards using this style of tunnel. We need to have a serious discussion before someone gets hurt!
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Stainless1
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« Reply #19 on: August 22, 2017, 10:55:14 PM »

Well... Start having it right here if there is one to be had  undecided
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Stainless
Red Hat 228.039, 2001, 65ci, MSA Bockscar Lakester with a little N20 
MSA Bockscar Lakester #1000 my fastest mile 245 and change, 84 ci turbobusa motor... but Corey's 233 MPH H/BFL record is still 3MPH faster than mine.
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« Reply #20 on: August 23, 2017, 10:21:20 AM »

Well... Start having it right here if there is one to be had  undecided

As with any tool, if used incorrectly you can take away the wrong answers.

My concern is a seeming lack of transparency on some key factors with this design of tunnel:

Blockage Ratio
Reynolds Number
Static Pressure Gradient
Boundary Layer Displacement Thickness

They may be doing everything they can to advise customers about the differences they might see between the tunnel and the track, I just don't know.

And that's what makes me nervous.
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Peter Jack
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« Reply #21 on: August 23, 2017, 10:46:12 AM »

Well... Start having it right here if there is one to be had  undecided

As with any tool, if used incorrectly you can take away the wrong answers.

My concern is a seeming lack of transparency on some key factors with this design of tunnel:

Blockage Ratio
Reynolds Number
Static Pressure Gradient
Boundary Layer Displacement Thickness

They may be doing everything they can to advise customers about the differences they might see between the tunnel and the track, I just don't know.

And that's what makes me nervous.

It might be more constructive if you based your insinuations on fact instead of pure speculation. Do a little research first if you're going to make spurious remarks.

Pete
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Bratfink
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« Reply #22 on: August 24, 2017, 12:13:33 PM »


It might be more constructive if you based your insinuations on fact instead of pure speculation. Do a little research first if you're going to make spurious remarks.

Pete

That's my exact point. What are the facts? Lets see a tunnel commissioning report, lets see the data reduction methods. Q control etc.

I have done 10 years of aerodynamic research in many different tunnels, scale and full size, rolling road and static ground. That's why I am concerned.

It could be I have no reason to be concerned, it could be that full disclosure of the testing limitations is fully explained to all customers. But I all I see is pictures of 2 and 300 mph (and some 400 mph) cars in a tunnel only capable of running 130mph with a blockage ratio of over 20% and the tail section of the car way into the collector of the tunnel in some cases, with what looks like a distributed suction system in the floor on a static ground. With comments like "some good downforce gains found".

Perhaps it's not me who should be doing a little more research..... 
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Stainless1
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« Reply #23 on: August 24, 2017, 11:30:52 PM »

So go to either of the available tunnels and evaluate their methods, procedures and data, we will expect a full report from your expert evaluation...  rolleyes
 cheers
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Stainless
Red Hat 228.039, 2001, 65ci, MSA Bockscar Lakester with a little N20 
MSA Bockscar Lakester #1000 my fastest mile 245 and change, 84 ci turbobusa motor... but Corey's 233 MPH H/BFL record is still 3MPH faster than mine.
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« Reply #24 on: August 25, 2017, 08:44:21 AM »

So go to either of the available tunnels and evaluate their methods, procedures and data, we will expect a full report from your expert evaluation...  rolleyes

Someone invite me to one of their tests as an observer. Or someone send me their data for review.

I won't publish any data in public.
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Rex Schimmer
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« Reply #25 on: August 26, 2017, 01:30:33 PM »

I think you have brought up some very valid questions regarding many of the wind tunnels now being used by landracers. Standing in the tunnel with a smoke wand and the air speed probably around 50-60 mph really doesn't show much from a data point of view. I have also questioned the "blockage ratio" of many of the tunnels we see on various videos, only being a "side line kibitzer" and not a seasoned wind tunnel user, as Bratfink appears to be I have read enough to see that some of the present tunnels are questionable with full size cars. With the always improving CFD "world" I think that I would sooner give Woody some money to crank my little lakester through his computer if I need aero data.

Rex
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« Reply #26 on: August 26, 2017, 01:52:41 PM »

That's about my line of thought too Rex! I do all my aero through experience & logic gained over the years & there's a few here that have taken my advice on their cars & picked up MPH.
  Sid.
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Ron Gibson
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« Reply #27 on: August 26, 2017, 03:59:18 PM »

Wonder what it would cost for my street roadster?  Ha,Ha,Ha, Ha,Ha, Dead Horse Dead Horse Dead Horse
Kidding aside I see some of Bratfiks points, but I think smoke could tell of serious problems. Other than that I agree completely with Rex and let Woody do it.

Ron
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« Reply #28 on: August 27, 2017, 10:25:56 AM »

I'm Just here to learn.

But didn't redbull formula 1 team just experience this issue at the beginning of the season? The data collected from their wind tunnel did not manifest the same in real life? To the point that it was a significant detriment in races. And then they finally figured out how to adjust what they saw on the tunnel data or change something with the tunnel... and now their cars are finally picking up pace.

Different type of racing but it's still 4 wheels and a soap box punching through the air.
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Seldom Seen Slim
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« Reply #29 on: August 27, 2017, 10:42:20 AM »

Ron, you've asked a good question - What does it cost to spend a session in a wind tunnel?  How many bucks do you need?

I dunno.  Back when the A2 tunnel (guy) was active on this Forum I seem to remember a cost in the $500/half day area.  That's only a hazy memory, though, and fresh information would be welcome.  I have no idea what Layne and Tom are charging at the Darko Tunnel - only that a lot of racers (and others) are using it - but they've got open times available.
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