Author Topic: Air resistance altitude calculator?  (Read 5601 times)

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Offline QikNip

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Air resistance altitude calculator?
« on: November 26, 2014, 11:57:03 AM »
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
138.0 G/CPRO 4/18 Arkansas Mile Challenge Record
145.632 G/CPRO 8/17 Bonneville Record
149.825 G/CPRO 8/18 Bonneville Record
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Offline John Burk

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Re: Air resistance altitude calculator?
« Reply #1 on: November 26, 2014, 01:23:25 PM »
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 .

Offline hawkwind

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Re: Air resistance altitude calculator?
« Reply #2 on: November 26, 2014, 02:21:17 PM »
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
slower than most

Offline QikNip

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Re: Air resistance altitude calculator?
« Reply #3 on: November 26, 2014, 02:55:14 PM »
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
138.0 G/CPRO 4/18 Arkansas Mile Challenge Record
145.632 G/CPRO 8/17 Bonneville Record
149.825 G/CPRO 8/18 Bonneville Record
142.443 G/CPRO 9/20 Loring 1.0 Mile Record
156.343 G/CPRO 9/20 Loring 1.5 Mile Record

Offline Sumner

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Re: Air resistance altitude calculator?
« Reply #4 on: November 26, 2014, 04:30:47 PM »
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

Offline manta22

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Re: Air resistance altitude calculator?
« Reply #5 on: November 26, 2014, 04:31:27 PM »
It's 82% of the air density. Density is missing.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline John Burk

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Re: Air resistance altitude calculator?
« Reply #6 on: November 26, 2014, 04:42:20 PM »
Qik
The Speedweek air density to put in the calculation is .00190 . Use ft/sec , mph x 1.47 .

Offline 7800ebs

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Re: Air resistance altitude calculator?
« Reply #7 on: November 26, 2014, 05:22:36 PM »
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


Offline Sumner

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Re: Air resistance altitude calculator?
« Reply #8 on: November 27, 2014, 10:45:30 AM »
.... I'd assuming that at Bonneville's 4,400 ...

Don't forget that the density altitude can be a lot higher than the elevation.
 


Above almost 6,000 feet before 8 in the morning and....



... 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

Offline hawkwind

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Re: Air resistance altitude calculator?
« Reply #9 on: November 27, 2014, 02:19:12 PM »
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 
slower than most

Offline Sumner

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Re: Air resistance altitude calculator?
« Reply #10 on: November 27, 2014, 02:52:55 PM »
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://en.wikipedia.org/wiki/Salar_de_Uyuni

... with our boosted vehicles? 

Sumner

Offline manta22

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Re: Air resistance altitude calculator?
« Reply #11 on: November 27, 2014, 04:19:10 PM »
You'd better charter a couple of C-130s to get you in there, Sum.  :-D

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline QikNip

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Re: Air resistance altitude calculator?
« Reply #12 on: December 02, 2014, 05:58:19 PM »
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


138.0 G/CPRO 4/18 Arkansas Mile Challenge Record
145.632 G/CPRO 8/17 Bonneville Record
149.825 G/CPRO 8/18 Bonneville Record
142.443 G/CPRO 9/20 Loring 1.0 Mile Record
156.343 G/CPRO 9/20 Loring 1.5 Mile Record

Offline John Burk

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Re: Air resistance altitude calculator?
« Reply #13 on: December 02, 2014, 06:29:22 PM »
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 .

Offline QikNip

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Re: Air resistance altitude calculator?
« Reply #14 on: December 08, 2014, 04:06:43 PM »
Thanks John. That makes perfect sense.
Rick
138.0 G/CPRO 4/18 Arkansas Mile Challenge Record
145.632 G/CPRO 8/17 Bonneville Record
149.825 G/CPRO 8/18 Bonneville Record
142.443 G/CPRO 9/20 Loring 1.0 Mile Record
156.343 G/CPRO 9/20 Loring 1.5 Mile Record