Landracing Forum
Tech Information => Aerodynamics => Topic started by: QikNip on November 26, 2014, 11:57:03 AM
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I've searched on line everywhere (and every way) I can think of and can't find a calculator (or formula) to determine the reduction in air resistance at altitude. I'd assuming that at Bonneville's 4,400 altitude, air resistance will be reduced by X% verus sea level. There are lots of calculators that can be used to predict HP loss at altitude, but I've been stymied on this aspect of my research. I'm betting this forum has the answer!
Rick
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It's directly proportional . The formula for drag is air density x Cd x speed squared x sq ft x .5 . On a hot Bonneville day density is about 82% of sea level air .
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The air density part of the drag calculation changes (rho) go here to find air density htpp://www.denysschen.com/catalogue/density.asp
Down under we are penalised an extra 12-13% drag wise compared to Bonneville due to our low altitude air density
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It's directly proportional . The formula for drag is air density x Cd x speed squared x sq ft x .5 . On a hot Bonneville day density is about 82% of sea level air .
John:
Thanks, but I must be doing something wrong in applying this formula, because I get an odd number.
The Cd of the car is .33
The target speed is 155 MPH (24,026 when squared)
The frontal area is 22 square feet
When I use that data in the formula I get: .82 X .33 X 24,026 X 22 X 0.5=71,515. If that's the correct product, what is that number? Or if I'm just making a math mistake, please straighten me out. As you can see, I need a bit of help! :?
Rick
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The air density part of the drag calculation changes (rho) go here to find air density htpp://www.denysschen.com/catalogue/density.asp
Down under we are penalised an extra 12-13% drag wise compared to Bonneville due to our low altitude air density
But you have a potential 12%-13% more HP :-D,
Sumner
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It's 82% of the air density. Density is missing.
Regards, Neil Tucson, AZ
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Qik
The Speedweek air density to put in the calculation is .00190 . Use ft/sec , mph x 1.47 .
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QikNip,
Just a thought... Frontal area is a WHOLE number... air thickness won't matter for what you are doing.. size, shape, and weight will.
also... your target speed is... pretty low, 300 hp will get you there if weight is under 3000 lbs
so lighten the car up as much as reasonable..
bob
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.... I'd assuming that at Bonneville's 4,400 ...
Don't forget that the density altitude can be a lot higher than the elevation.
(http://purplesagetradingpost.com/sumner/hooley/2008-9.jpg)
Above almost 6,000 feet before 8 in the morning and....
(http://purplesagetradingpost.com/sumner/hooley/2008-7.jpg)
... and 6,500 at 10 am and in the afternoon it can get to over 7,000.
What you gain in less aero resistance you loose in less HP and the other way around. You will go the fastest when you have the perfect tune HP wise for the DA at the time.
Personally for a car that is going to run over 250 or more I'd like to run in the morning if at all possible since the traction just seems to be better then. If there is moisture there the heat in the afternoon can bring it to the surface and if you are in a traction limited car (you have the HP to run over 250) all the traction you can get will be helpful. If you have the traction and can accelerate faster and have a higher speed at the 2 1/4 you are going to have a higher 5th mile speed.
Good luck,
Sumner
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Sumner ...that only applies to N/A vehicles ....when you bring the extra O2 to the party ( boosted N2O) you get a very healthy benefit from the thinner air aerodynamically ....that's why we down under will never match the boosted speeds of Bonneville :cry:........Traction now there's a problem we both have.
cheers
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Sumner ...that only applies to N/A vehicles ....when you bring the extra O2 to the party ( boosted N2O) you get a very healthy benefit from the thinner air aerodynamically .....
That's true, when are we all going to go to Bolivia :-D ....
(http://upload.wikimedia.org/wikipedia/commons/5/53/Uyuni_landsat.JPG)
http://en.wikipedia.org/wiki/Salar_de_Uyuni
... with our boosted vehicles?
Sumner
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You'd better charter a couple of C-130s to get you in there, Sum. :-D
Regards, Neil Tucson, AZ
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QikNip,
Just a thought... Frontal area is a WHOLE number... air thickness won't matter for what you are doing.. size, shape, and weight will.
also... your target speed is... pretty low, 300 hp will get you there if weight is under 3000 lbs
so lighten the car up as much as reasonable..
bob
Bob:
I'm (as usual) a bit confused, so bear with me. First I did use a whole number for frontal area (it was the 22) in the formula above. That aside, are you saying that lightening the car is the way to go? I was under the impression that LSR cars are all about being beefy to aid in traction. Being a road racer, I'm very used to getting things as light as possible. So can you elaborate a bit on your lightening advice?
Rick
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If you're mostly or entirely traction limited or have a lift problem heavy can be good . If you're mostly power limited and handling isn't a problem light can be good . If you peak before the end of the run weight doesn't hurt .
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Thanks John. That makes perfect sense.
Rick