Air tempeture effect on air flow

[jdolecki]

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

[TheBaron]

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

[mtiberio]

since hot air is less dense than cool air, I'd say yes.

[Bratfink]

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.

[Stan Back]

. . . and then there's Bolivia – which, I'm told, fooled everyone.

[SPARKY]

SB did the HP go up as well as the drag ? 

[Stan Back]

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.

[SPARKY]

isn't that the same as B'ville cleaner cars go faster in the AM  dirtier  cars in the afternoon?

[Stan Back]

Huh?

[DallasV]

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.