Poorman's wind tunnel: car model placed over speeding car roof?

[Big time]

I wonder if it has ever been tried.

Mounting the model over the roof of a car and measure its drag. You can even place smoke generator oulets in front of it.

Of course you will need very tall mounting brackets to separate the car model from the speeding car by several feet, so the flow distortion from the car won't disturb the model.

Maybe you would also need a extremely streamlined car (kind of solar racer) to minimize flow distortion?
Maybe you'll need to place the car model over a board parallel to the ground to simulate ground effect.

I know it sounds weird but has it been tried or proposed before?

[MGB=MPG]

Build Your Own Wind Tunnel

i think i have seen air bodies on top of automobiles being tested .. its the observation while its up there is the problem i see.
http://www.grc.nasa.gov/WWW/k-12/airplane/shortt.html
how about this one



or this

[Sven7]

You could surely extend the "tunnel" out in front of the car, provided the cantilever doesn't make it bounce too much and it's out of the high pressure area.

However, while decreasing the model's scale the speed of the air has to increase exponentially to get useful results. If I recall correctly it's something like this.

1/1- 20mph
1/2- 40mph
1/4- 80mph
1/8- 160mph
etc.

Essentially unless you have a quarter scale or larger model you won't get useful results. Phil can correct me the numbers are off

[some_other_dave]

NASA did this with aircraft models in the late 40s at the very least. I remember seeing pictures of models that were mounted on the wing of a Mustang. The 'Stang was, as I recall, very qualified for the job, not only because it was one of the fastest aircraft of the time, but also because it used a laminar-flow wing that led to less-disturbed flow across the model.

-soD

[aerohead]

Originally Posted by Big time

I wonder if it has ever been tried.

Mounting the model over the roof of a car and measure its drag. You can even place smoke generator oulets in front of it.

Of course you will need very tall mounting brackets to separate the car model from the speeding car by several feet, so the flow distortion from the car won't disturb the model.

Maybe you would also need a extremely streamlined car (kind of solar racer) to minimize flow distortion?
Maybe you'll need to place the car model over a board parallel to the ground to simulate ground effect.

I know it sounds weird but has it been tried or proposed before?

The Rutan Brother's Scaled Composites employs a rig which extends ahead of a pickup truck so as not to affect the source flow to the model.They have access to the taxi ways,if not the runways at Mojave Airport,California,and can do safe testing away from traffic.
I think you'd want a 'ground board' under the model and you might have to look into boundary layer thickness there,as some tunnels vacuum off some of this layer to more accurately portray 'real life' conditions.Alan Pope has a fine book on low-speed wind tunnels.Could be helpful.
Force measurements might be a real challenge if the 'laboratory' itself is displacing around in space.
Tuft testing would reveal flow separation and that would be valuable,but you could do that at full-scale on any vehicle.