TEMP vs DRAG Estimator (winter / summer)

[3-Wheeler]

Hi Aerohead,

Thanks for posting your chart about air drag versus temperature!

I'm not questioning the presented data, but simply pointing out that there must be other factors in the ISO table besides absolute temperature ratios.

For example, I work allot with air flow at work, and have to compensate for temperatures and barometric effects of the air, and the resultant mass flow rates of various flow meters we have.

Fortunately, barometric pressure is already in absolute numbers, i.e. inHg, kPa, or psi, which ever units you prefer. As the barometric pressure goes up on a given day, the air drag on our cars goes up by the same amount, just based on air density due to barometric pressure.

When working with baro readings from radio stations or TV, make sure to convert their reported number to the true barometric pressure at your specific elevation, otherwise you can be easily 2% off in your calculated numbers.

And air temperature has the same affect, except as the temperature goes up, and air density goes down. And we have to deal with air temperature in absolute units, rather than relative. Absolute units are typically Rankin or Kelvin, whereas relative units are Centigrade or Fahrenheit.

To get the absolute outside temperature in Rankin, just add 460°R + the relative temperature in °F, so room temperature is 460°R + 70°F = 530°R

Then if you want to know how a change in temperature affects air drag, just calculate the ratio between the temperatures to find the effect on air drag.

Such as other things in life, the ISO paper most likely has other factors that affect air drag that I don't know about or haven't considered. There is the z-factor compressibility of air, that is used when dealing with the true density of the air at pressures higher than 100 psig or so, but that factor is typically so small for our purposes here, that it does not need to be considered.

Again Aerohead, thanks for posting your info. It's a useful tool for those who are interested.

Jim.

[aerohead]

Originally Posted by 3-Wheeler

Hi Aerohead,

Thanks for posting your chart about air drag versus temperature!

I'm not questioning the presented data, but simply pointing out that there must be other factors in the ISO table besides absolute temperature ratios.

For example, I work allot with air flow at work, and have to compensate for temperatures and barometric effects of the air, and the resultant mass flow rates of various flow meters we have.

Fortunately, barometric pressure is already in absolute numbers, i.e. inHg, kPa, or psi, which ever units you prefer. As the barometric pressure goes up on a given day, the air drag on our cars goes up by the same amount, just based on air density due to barometric pressure.

When working with baro readings from radio stations or TV, make sure to convert their reported number to the true barometric pressure at your specific elevation, otherwise you can be easily 2% off in your calculated numbers.

And air temperature has the same affect, except as the temperature goes up, and air density goes down. And we have to deal with air temperature in absolute units, rather than relative. Absolute units are typically Rankin or Kelvin, whereas relative units are Centigrade or Fahrenheit.

To get the absolute outside temperature in Rankin, just add 460°R + the relative temperature in °F, so room temperature is 460°R + 70°F = 530°R

Then if you want to know how a change in temperature affects air drag, just calculate the ratio between the temperatures to find the effect on air drag.

Such as other things in life, the ISO paper most likely has other factors that affect air drag that I don't know about or haven't considered. There is the z-factor compressibility of air, that is used when dealing with the true density of the air at pressures higher than 100 psig or so, but that factor is typically so small for our purposes here, that it does not need to be considered.

Again Aerohead, thanks for posting your info. It's a useful tool for those who are interested.

Jim.

Jim,I do not have any of ISO material.The last SAE material I bought was at a time before the international standards came to be adopted.
I did dig into my rats nest and found something which I believe dovetails into what you've brought up which generates the most accurate reflection of ambient conditions at a particular location,whereas my table is for only standard atmosphere and elevation.
My SAE J1263 which deals with coastdown testing has an air density correction which use absolute ambient temperature,either Kelvin or Rankine,and standard along with ambient baro pressure:
( Pstd )X( T ambient )
___________________
( P observed) X ( T std )

My data came from Psychrometric tables which are for 29.92" Hg and 59-Degree F
Since I'm working with US standards,standard pressure only,and using the same example at -20 F:

( 29.92 ) X ( 5/9 -20+ 459.67 )
____________________________

( 29.92 ) X ( 5/9 59 + 459.67 )

= 0.84768,then taking the reciprocal =
1.179685,or 17.9685%

For the more accurate measure you'd factor in the actual baro pressure.
So the ISO method is clearly the superior.I see another page to print up!

[3-Wheeler]

Originally Posted by aerohead

... My data came from Psychrometric tables which are for 29.92" Hg and 59-Degree F...

Hi Aerohead,

Ahh, now we know where part of the difference in our numbers came from...

Our STP (Standard Temperature and Pressure) at work is considered 29.92 inHg and 70°F. This is what most air flow calibration labs in the U.S. use nowadays.

I remember some 30 years ago or so, 60°F used to come up when dealing with the 'official' height/density of water columns when measuring inches of water pressure, which is about 1/13th of one inch of mercury.

If one were to use 60°F instead of 70°F then the equation looks like:

= (460°R + 60°F) / (460°R + (-20°F))
= (520°R) / (440°R)
= 18.2%

So now we can see that your psychrometric table gives essentially the same response as simply considering the absolute temperature ratio.

Jim.

[Frank Lee]

STP - Standard Temperature and Pressure & NTP - Normal Temperature and Pressure

Appears there are several "standard" temps and pressures to choose from!

I looked that up because I was unaware of anything else than the 59degF/29.92Hg aviation standard, which has not changed.

[gone-ot]

...that's the beauty of STANDARDS...you have such a w-i-d-e range to pick and choose from...ala' carte'!

...if you don't like one, simply pick another one to suite.

[3-Wheeler]

Originally Posted by Frank Lee

...Appears there are several "standard" temps and pressures to choose from!...

Just to make it more interesting!!

Notice the STP values on this portion of an air flow calibration certificate.



Jim.

[Domman56]

I have a question about this

Doesn't humidity have to do with it a little too? Because i see where the temp would make a difference but say somewhere like florida


SUUUPer humid in the summer therefore the air is stickier and more dense so driving though that would be less efficient than dry desert air like where i live correct?

[aerohead]

Originally Posted by Domman56

I have a question about this

Doesn't humidity have to do with it a little too? Because i see where the temp would make a difference but say somewhere like florida


SUUUPer humid in the summer therefore the air is stickier and more dense so driving though that would be less efficient than dry desert air like where i live correct?

The SAE says specifically to ignore humidity for drag-related testing.Air density is the overiding concern.
If you were doing engine dyno work it would be a different story.