Ground Clearance vs Aerodynamic Drag

[aerohead]

So far,in the literature,minimizing all 'internal ' flow is emphasized.
We want as much air as we can get on the outside body surface.
It minimizes wetted area and surface friction.
It minimizes super-velocities which increase surface friction and encourage vortex formation.
The only benefit a tunnel would provide is downforce,which in itself is induced drag which we'd like to eliminate altogether.
Since streamlined bodies don't produce lift there's no good reason to design in downforce unless you're going to race the vehicle.
If the body's right,the air will get around it without separation,delivering almost all of its kinetic energy to the wake area.
With all due respect to Bucky,the idea of lifting the rear off the ground could spell disaster in a split second.

[freebeard]

Ya think?

I guess what I'm thinking about is vs a flat transom. Maybe curved branches like veins in the flat bottom (something like what you see under race cars) that would wrangle the turbulent air as it exits. The plate becomes an edge.

If it generates down-force to boot, no problem. The interesting question is: What would it really do?

What's really going on under there? Sound waves bouncing back and forth between the road and the car?

[serialk11r]

Originally Posted by aerohead

Since streamlined bodies don't produce lift there's no good reason to design in downforce unless you're going to race the vehicle.

But tires cause lift, so maybe a little downforce to counteract that might be a good idea Just a little bit done via a flat underbody, since a car is already relatively close to the ground you might as well.

My car definitely "drifts" a little at higher speed on the freeway. Headwind makes that happen too. Less of that would be nice to have.

[aerohead]

Originally Posted by freebeard

Ya think?

I guess what I'm thinking about is vs a flat transom. Maybe curved branches like veins in the flat bottom (something like what you see under race cars) that would wrangle the turbulent air as it exits. The plate becomes an edge.

If it generates down-force to boot, no problem. The interesting question is: What would it really do?

What's really going on under there? Sound waves bouncing back and forth between the road and the car?

The only turbulence will be due to the tires and that can be fixed with fairings.
A flat bottom and gentle diffuser is all that's required for low drag.
If you see chines down there,then the diffuser is too steep and they're attempting to control transverse contamination of the downforce (low base pressure telegraphed forward under the tail).

[aerohead]

Originally Posted by serialk11r

But tires cause lift, so maybe a little downforce to counteract that might be a good idea Just a little bit done via a flat underbody, since a car is already relatively close to the ground you might as well.

My car definitely "drifts" a little at higher speed on the freeway. Headwind makes that happen too. Less of that would be nice to have.

If the tires are shielded within the body,then all they do is add some windage drag,and drag associated with pressure spikes and separation-induced turbulence as they erupt from the underbody.
I don't think lift has been an issue with production cars since the mid-70s,when Hucho and others published their research and shared it with all the automotive engineers at SAE Congresses.
Mediocre designs are already 'fixed' with airdams and rear spoilers.
The weight of the car is all the downforce you need unless your at extralegal speeds.
If you do track days you'll want downforce for cornering.It'll kill your mpg coming and going from the track though.
If you drive a supercar on the unlimited sections of Autostrada or Autobahn,then the manufacturer has already covered your behind.
108-mph is uneventful in my truck,even in a blowing crosswind.It has -30 lbs downforce at the nose at 130-mph,and +22-lbs lift at the tail at 130 mph,in a truck with a 4,200 lb travel weight.The 'lift' generated is meaningless compared to the weight of the truck.

[aerohead]

The July 2013 MOTOR TREND had an article about Volkswagen's Bugatti Veyron Grand Vitesse which included some aero tidbits:
*in top-speed mode,the car configures itself for less downforce (lower drag ) to protect the tires from self-destructing beyond 233-mph.
*flaps at the front of the tunneled diffuser close,forcing the air to take a flat pathway under the diffuser's belly.
*the rear spoiler tucks in tight into the body.
*DROPS THE CAR LOWER TO-AND LEVEL WITH-THE ROAD.
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In 'Handling-Mode',the rear spoiler creates as much as 770-pounds of downforce (also acts as an airbrake).
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So,in light of the PROBE's inclined belly,Volkswagen (current King of production car fuel economy) takes a different tack.
PS,the Veyron,at top speed,consumes it's entire 26.4-gallon tank of fuel in 8-minutes,covering 35.73 miles until fuel starvation.Range anxiety?

[freebeard]

Come on—surely 36 miles in 8 minutes is 4.8mph?

Interesting that there are active aerodynamic aids underneath.

[BamZipPow]

Originally Posted by freebeard

Come on—surely 36 miles in 8 minutes is 4.8mph?

Interesting that there are active aerodynamic aids underneath.

Um...if yer gonna divide miles by minutes wouldn't the rate be mpm vs mph?

So the actual rate is 4.8mpm (miles per minute) or converted to mph it would be multiplied by a factor of 60 (minutes in an hour) which would be around 288mph.

Rate of fuel consumption is 3.3 gallons per minute!

[freebeard]

I can't even claim I was up too late.

In the cold light of dawn, it's pretty obvious isn't it.

Fuel consumption is faster than I could fill may car out of a 20-liter gas can.