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Old 01-03-2008, 09:58 PM   #31 (permalink)
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Clarifying some terms. Cd, A, CdA and such.

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Originally Posted by AndrewJ View Post
Sure, this will increase the CdA of my car, but it will also take care of two (or three) nasty areas of aerodynamic drag, thereby decreasing my Cd.
What I *should* have said was:
"Sure, this will increase the frontal area (A) of my car, but it will also take care of two (or three) nasty areas of aerodynamic drag (Cd), thereby decreasing my coefficient of drag (CdA).

Basically the coefficent of drag is CdA which is comprised of the Cd multiplied by A. See wikipedia for further detail.

So the big idea behind airdams is that they will improve your aerodynamics IF you can improve your Cd without adding to much A. If you tip the balance the other direction, by adding a lot of frontal area without streamlining the shape much then you will actually be hurting your aerodynamics.

This is why an airdam like mine would actually be a bad idea for basjoos's CX. He has a full belly pan, so adding an airdam wouldn't improve his Cd any, and it would add a bit to his A, thereby raising his CdA.

As for the original post, "You have to hear what I'm thinking."

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Old 01-04-2008, 06:32 PM   #32 (permalink)
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The testing drags on....

I went out to do some more testing today and while I didn't get to do all of the testing I had planned to do, the intrigue continues.

I did 2 of 6 runs of coastdown testing in "stock" form to verify that the Excel spreadsheet was giving me decent numbers. By comparing the Cd that the spreadsheet gave me with Honda's Cd of 0.33 for a stock 5th generation Civic hatchback, I can see how far off (if any) the numbers in the spreadsheet are.

Now, I only got to do 2 runs, one in each direction, before I had to quit due to the sudden appearance of a fierce crosswind. I didn't want my data corrupted by any wind, so I just used those two sets of data to plug into Excel.

I know, I know... two points of data isn't much, in fact, it's probably safe to consider it flawed.

But then again...

Guess what Excel came up with based on my 2 data points?

Cd = 0.33

Grrr....more testing....damn wind.


Given the windyness situation I headed on over to my "calibrated" hill for a little session of A-B-A. The hill is usually windy, but today it was on the leeward side of the wind, so it was unusually calm on the downhill side.

I brought the car to 55mph at the top of the hill, and at the same point in all 3 runs, cut the engine reset the trip odo. I then kept track of speed and at what distance on the trip odometer those speeds were reached. The runs ended when I coasted back down to 55mph at the bottom of the hill.

---------------------------A(stock)----B(aero)---A(stock)
Initial speed-------------55mph-----55mph-----55mph
Slowed to----------------52mph-----55mph-----52mph
Miles@lowest speed-----0.375------0-0.4------0.375
Top speed---------------66mph-----71mph-----65mph
Miles@top speed--------1.1---------1.05-------1.05
Miles@55mph (end)-----1.35-------1.45-------1.25

Also, on the "B" run, 66mph was reached at 0.75 miles.

Looks like this aerodynamics stuff might actually work
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Old 01-04-2008, 07:00 PM   #33 (permalink)
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Haha, interesting results,

Stick with it though, I'm glad you have the patience to be so methodical about it,
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Old 01-04-2008, 07:19 PM   #34 (permalink)
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Quote:
and while I didn't get to do all of the testing I had planned to do,
Welcome to the club!

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Originally Posted by AndrewJ View Post
Looks like this aerodynamics stuff might actually work
Nice work. I envy you people and your winter time testing weather.
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Old 01-04-2008, 08:48 PM   #35 (permalink)
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In my opinion, an airdam is just a band-aid to use if you have a aerodynamically dirty underside you want to divert air away from. In a max FE application, we're not overly concerned about generating added downforce, which is the other main purpose of an air dam. On my car, I have a totally smooth underside and the only "airdams" on my car are the two air diverters (wheel spoilers) that sit ahead of my front wheels to split the airflow around the wheels. This split airflow is maintained via the double side skirts to the rear wheels and then ideally (future project) should have a small boattail fin behind each rear wheel to reduce its eddy. If you look at the Loremo and the Daihatsu UFEIII, they have airflow splitters in front of each of their front wheels and the UFEIII has a small boattail behind each rear wheel. If you insist on going with an airdam, you can make it a variable height airdam that is deepest in front of each wheel and highest in the section between the wheels. Some airdam-equipped Audis and BMW's go this route.
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Old 01-04-2008, 09:22 PM   #36 (permalink)
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I agree that the airdam isn't optimal FE-wise. However, I'm finding that it is a really nice thing to have as part of a work-in-progress car.
I will be modifying the airdam further as I continue to work on the car, eventually resulting in it being just a grille block/wheel spoiler type thing.
But for now, while I won't be able to do a full bellypan anytime soon, it's helping to staunch the aero losses.

I like the mini-boattail idea behind the wheels, I may have to work that into my car somehow. I've been thinking of trying sideskirts next, maybe work them into the mix there?

And after re-reading IWillTrys post I think I'll have to look for a new place to do my next round of coastdown testing. It looks like the tests might be more accurate if I start out with a higher top speed, where the areo-drag is more pronounced.
My current testing circuit is a 50mph speed limit, and right next to the Highway patrol headquarters. Probably not wise to speed

basjoos: How did you get your Cd numbers? Any tips?
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Old 01-04-2008, 11:21 PM   #37 (permalink)
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Quote:
Originally Posted by AndrewJ View Post
I agree that the airdam isn't optimal FE-wise.
The full belly pan/undertray thing is not that hard to do. You need a couple 8*4 sheets of 0.6mm aluminum, probably some sort of heat resistant padding to stick in the middle somewhere. Jack stands, your regular jack, a friend, a drill, screws with big heads, some metal shears and about 5 hours is all you will need.
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Old 01-07-2008, 07:00 PM   #38 (permalink)
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Airdam or Airdamned?

One thing that I feel hasn't been discussed too much yet about airdams vs. bellypans is the point of diminishing returns.
So far we know (at least in theory) that a bellypan is better because of the lower frontal area, and also the probable decrease in Cd from all the hanging bits on the underside.

But what we don't know is how much better a belly pan is. Has anyone seen any data on this so far? The most relevant thing I've found so far has been this refrence (PDF file) to a 1958 study
"Hoerner 1958 discusses the underbody as the biggest single avoidable
aerodynamic drag component and gives an old example where drag goes
from 0.3 for a smooth underbody to 0.6 for an open and rough as
customary car!"

Still no word on how much an airdam would knock off that 0.6

So, I hope eventually when I do get myself a jack, some jackstands and a new (working) drill I can get a belly pan together to test it out and get some numbers.

This list of things to do is getting quite long.
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Old 01-07-2008, 09:33 PM   #39 (permalink)
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Quote:
Originally Posted by AndrewJ View Post
One thing that I feel hasn't been discussed too much yet about airdams vs. bellypans is the point of diminishing returns.
Well, one thing Phil Knox has said in terms of the lowest extension of an air dam is that it shouldn't go lower than the lowest hanging suspension/underbody bit, or else you're just adding frontal area at the expense of Cd.

That said, your tests so far, and Coyote X's, seem to suggest that more can in fact be better.

Quote:
But what we don't know is how much better a belly pan is. Has anyone seen any data on this so far? The most relevant thing I've found so far has been this refrence (PDF file) to a 1958 study
I'd be wary of relying on 1958 data. Cars back then would have had MUCH worse undersides (basically being body-on-frame truck like construction) than today.

Here's a couple more modern references:
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Old 01-08-2008, 12:33 AM   #40 (permalink)
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Hoerner Dr.-Ing. S. F., Fluid-Dynamic Drag, 1965

Quote:
Originally Posted by AndrewJ View Post
One thing that I feel hasn't been discussed too much yet about airdams vs. bellypans is the point of diminishing returns.
So far we know (at least in theory) that a bellypan is better because of the lower frontal area, and also the probable decrease in Cd from all the hanging bits on the underside.

But what we don't know is how much better a belly pan is. Has anyone seen any data on this so far? The most relevant thing I've found so far has been this refrence (PDF file) to a 1958 study
"Hoerner 1958 discusses the underbody as the biggest single avoidable
aerodynamic drag component and gives an old example where drag goes
from 0.3 for a smooth underbody to 0.6 for an open and rough as
customary car!"

Still no word on how much an airdam would knock off that 0.6

So, I hope eventually when I do get myself a jack, some jackstands and a new (working) drill I can get a belly pan together to test it out and get some numbers.

This list of things to do is getting quite long.
Technically, Hoerner is referencing "Sawatzki-Weiss, Autom.tech Zts. 1941; (2,d)" in Figure 9 (page 12-6) that is paraphrased in your qoute (assuming I'm reading the bibliography correctly). . The 0.3 is for a "completely smooth model" while the "1/2 faired (Tatra or Volkswagen)" is Cd = 0.5, and "Open and rough as customary" is Cd = 0.6. However, I don't think these are meant as overall coefficients. Instead he (seems) to reference the "completely smooth" (with no cut outs for turning of the wheels) is worth a DeltaC_d_o of 0.3 over the open/rough bottom.

Hoerner doesn't describe airdams in particular, but the paragraph on "Cooling Air (also page 12-6)" he seems to blame most of the cooling drag on the turbulence of the air exiting the bottom of the engine bay. There is a factor of C_D_box = 0.4 where "box" represents the "circumscribed area" of the open radiator with that number coming from Kamm's "Automobiltech Zeitscrift 1939, page 447". He also mentions achieving thrust from the cooling air via the proper "exit nozzle" and "energy transferrred to the cooling air by the engine fan (and some heating.)" Which may be harder for us to realize with fans no longer being attached to the crankshaft.

I heard this recently went out of print, so I'd go find a used copy if you can find one.


Last edited by Fuzzy; 01-08-2008 at 12:58 AM.. Reason: mis-spelled "airdam'
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