Cd constant... or speed dependant?

[freebeard]

Reynolds Number is related to scale.

1/4th to 1/5th scale is the edge of similarity. At 1/10th scale and down, water fudges the Reynolds Number in the other direction. Upside down, with hydrogen bubble streamlines.

The other problem with wind tunnels is blockage, there needs to be free air 4-5x the area of the model/vehicle. Else the effect of the body is compressed (interference drag) and the attached turbulence of the tunnel wall intrudes into and skews the data collected.

edit: Lest it sounds like I'm pontificating, I've been in one at Darko with aerohead and Gumby79.

[NeilBlanchard]

Right, that is where the Reynolds number enters in - the scale of the model has to be compensated for with either pressure or velocity. I think?

Quarter scale = 2X pressure (or?) 2X velocity, if I am not mistaken.

[Vman455]

Reynolds number is a function of test length and fluid velocity. Decrease one by a factor of 10, and the other must increase by the same factor to get the same Re in fluid of the same density.

Re = [(density)(velocity)(length)]/(dynamic viscosity) = [(velocity)(length)]/(kinematic viscosity)

[Hersbird]

Ok here is another variable. What if the drag is more a result of designed downforce vs say a brick with just wasted drag?

[freebeard]

Absorbed and re-released through the [never-inactive] coil-overs?

[aerohead]

Originally Posted by Hersbird

Ok here is another variable. What if the drag is more a result of designed downforce vs say a brick with just wasted drag?

It won't make a difference.Coefficient of drag is solely a function of profile drag,regardless of 'source.' As long as dynamic similarity is respected (Reynolds number),model-for-model,the Cd will always be the same, as long as the critical Reynolds number is maintained and you're beyond Rn= 860,000.