Author Topic: Air tempeture effect on air flow  (Read 3389 times)

0 Members and 1 Guest are viewing this topic.

Offline jdolecki

  • New folks
  • Posts: 22
Air tempeture effect on air flow
« on: December 10, 2018, 09:21:40 AM »
Does the temperature of the air flowing across a body have different effects on the aerodynamics?

Basically are they better on a cooler day verses a hot day?

What about engine exhaust, exiting a car mid body and flowing over the body verses coming straight out the back?

I know is kind of a general question but i don't know the answer.

thanks, john





Offline TheBaron

  • Full Member
  • ***
  • Posts: 122
Re: Air tempeture effect on air flow
« Reply #1 on: December 10, 2018, 10:13:10 AM »
Air drag, lift, and downforce all go up and down with the air temperature.....

The "rule of thumb" rate is 1% per 11 degrees F of change...

Exhaust thrust is very real with larger supercharged power plants....

Exhaust should be directed as aft as possible and into a "low pressure"
region if possible (such as the wake of the vehicle)....

The WWII P-51's forward thrust (at extreme altitudes where the propeller became more and more inefficient) it was estimated that 20% was contributed by the exhaust and the radiator ducting.....

Robert "Smitty" Smith
Red Baron Race Team

Offline mtiberio

  • Full Member
  • ***
  • Posts: 248
  • ElMo MPG1K 141, LTA MPF1K 137, ECTA MPG1K 118
Re: Air tempeture effect on air flow
« Reply #2 on: December 10, 2018, 10:13:18 AM »
since hot air is less dense than cool air, I'd say yes.

Offline Bratfink

  • Full Member
  • ***
  • Posts: 81
Re: Air tempeture effect on air flow
« Reply #3 on: March 08, 2019, 04:05:33 PM »
Humidity, temperature and ambient air pressure all play a part.

Drag Force = 1/2 * Density * Velocity^2 * Cd * Frontal Area.

Density = pressure(absolute) / Specific Gas Constant * Temp(absolute).

Temp goes up, density goes down and so does drag.

However if pressure goes up it increases density and drag.


Offline Stan Back

  • Hero Member
  • *****
  • Posts: 5879
Re: Air tempeture effect on air flow
« Reply #4 on: March 08, 2019, 05:13:57 PM »
. . . and then there's Bolivia – which, I'm told, fooled everyone.
Past (Only) Member of the San Berdoo Roadsters -- "California's Most-Exclusive Roadster Club" -- 19 Years of Bonneville and/or El Mirage Street Roadster Records

Offline SPARKY

  • Global Moderator
  • Hero Member
  • ***
  • Posts: 6908
Re: Air tempeture effect on air flow
« Reply #5 on: March 09, 2019, 11:49:46 AM »
SB did the HP go up as well as the drag ?  :-o
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!

Offline Stan Back

  • Hero Member
  • *****
  • Posts: 5879
Re: Air tempeture effect on air flow
« Reply #6 on: March 09, 2019, 04:29:44 PM »
I seem to remember accounts of unblown bikes going faster there – apparently the drag was reduced by such an extent that the horsepower loss was less a consequence than anticipated.
« Last Edit: March 09, 2019, 08:53:08 PM by Stan Back »
Past (Only) Member of the San Berdoo Roadsters -- "California's Most-Exclusive Roadster Club" -- 19 Years of Bonneville and/or El Mirage Street Roadster Records

Offline SPARKY

  • Global Moderator
  • Hero Member
  • ***
  • Posts: 6908
Re: Air tempeture effect on air flow
« Reply #7 on: March 09, 2019, 08:34:31 PM »
isn't that the same as B'ville cleaner cars go faster in the AM  dirtier  cars in the afternoon?
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!

Offline Stan Back

  • Hero Member
  • *****
  • Posts: 5879
Re: Air tempeture effect on air flow
« Reply #8 on: March 09, 2019, 08:53:45 PM »
Huh?
Past (Only) Member of the San Berdoo Roadsters -- "California's Most-Exclusive Roadster Club" -- 19 Years of Bonneville and/or El Mirage Street Roadster Records

Offline DallasV

  • Sr. Member
  • ****
  • Posts: 578
Re: Air tempeture effect on air flow
« Reply #9 on: March 09, 2019, 11:14:48 PM »
Having a naturally aspirated roadster (brick) I will gladly take more drag from dense air to better feed the noisemaker. Those slick little skinny rolling pencils, especially with forced air, probably are on the other side of the fence.
Records or parts, I didn't come all this way not to break something.