Drag Racing Aero Help

[Bennett_Racing]

Originally Posted by aerohead

If the windscreen/A-pillar weren't so generous in radius I'd say go ahead and push the blister out to the edges,but from the photos she looks really slick and don't think you'd see improvement at the trap.
The blower and bug-catcher can be extremely dirty and a lot of guys at Bonneville are finding more speed with the pro-comp type aero scoop/fairing covering the whole shebang.
The headers are also a source for interference drag,both from the structure itself and from the exhaust blast.You might look at some of the P-51 Mustangs which run the Reno,Nevada air races and look at how they configure their exhaust.If the epoxy finish can't handle the Btus,you could skin the affected area with stainless.
Safety/directional stability is your biggest concern at the speeds you run.
I was looking at photos of the Crash of the Chicken Hawk.They lost a freeze plug,squirted the tires,and got sideways;went over and 'inside' the guardrails for a total wreck.Driver went to the hospital.That was at only 111 mph!
I'd be tempted to maintain the wide boot ( no tapering ) just to have the drogue effect of the wider transom back there,helping to keep the car straight in the lane.
One day they may have a rate-gyro to automatically fire the chutes if a car gets sideways.I suspect the drivers are already plenty absorbed,concentrating on going 'forwards.'
Race safe!

Thanks for reply, sorry but what is a drogue effect?

[DonR]

I was referring to the side of the car as it goes rear past the tail lights. Which would be just a few inches outside of what I think you called a spill plate. I don't know how they would work being so close to the spill plate.

[AeroModder]

Originally Posted by Bennett_Racing

When you say clean up the rear underbody? Do you mean for air to exit from under the car cleanly?

Exactly. If the air is turbulent as it leaves the car, it creates drag. The ideal situation is to disturb the air as little as possible, keeping the flow as clean as you can.

[Bennett_Racing]

Originally Posted by DonR

I was referring to the side of the car as it goes rear past the tail lights. Which would be just a few inches outside of what I think you called a spill plate. I don't know how they would work being so close to the spill plate.

Would it help if the spill plates go past the rear of the body? How much overhang? Will it help?

[Bennett_Racing]

Originally Posted by AeroModder

Exactly. If the air is turbulent as it leaves the car, it creates drag. The ideal situation is to disturb the air as little as possible, keeping the flow as clean as you can.

Ok that makes alot of sense.

So at the back of the car, do we leave the surface flat or add a gurney flap to detatch the air?

[aerohead]

Originally Posted by Bennett_Racing

Thanks for reply, sorry but what is a drogue effect?

If the car had a full tapered tail it would be prone to behave more like a lake canoe or racing kayak,requiring constant inputs of the oars to maintain direction.
With the tail chopped off,the base pressure created behind the chop acts in a 'drogue'( braking ) effect directly opposed to the direction of travel,as the drogue-chutes do at the end of the run.
This drag is a trade-off for directional stability,keeping the boot going same direction as bonnet.

[AeroModder]

Originally Posted by Bennett_Racing

So at the back of the car, do we leave the surface flat or add a gurney flap to detatch the air?

On the underside, leave it flat. A lip spoiler on the boot is the equivalent of a gurney flap, just turned upside-down.

If the rules allow, a diffuser at the end of a smooth undertray would be ideal, like this:

[autoteach]

http://www.kwaninternational.com/ima...car%20race.jpg

This has two ideas that are good. the hood hump goes out to the edge of the windshield, and the air intake is almost embedded in the hood of the vehicle (think LARGE NACA duct). The red and black car also has canards in front of the exhaust (barely visible).


I am not sure what the rules are for the underbody/undertail. I would say that the number one thing that you can do is to create as much down force using the shape of the vehicle so that you don't have to have a wing create that force (which is also providing aero braking). Making air take the long way underneath and the short route above (low and high pressure, respectively) Sounds like a fun project.

[XJguy]

I would test that rear "wind shield" area to make sure the air stays attached. The angle looks a bit steep, you might have some turbulence that can be resolved via vortex generators.

[Bennett_Racing]

Originally Posted by aerohead

If the car had a full tapered tail it would be prone to behave more like a lake canoe or racing kayak,requiring constant inputs of the oars to maintain direction.
With the tail chopped off,the base pressure created behind the chop acts in a 'drogue'( braking ) effect directly opposed to the direction of travel,as the drogue-chutes do at the end of the run.
This drag is a trade-off for directional stability,keeping the boot going same direction as bonnet.

That makes alot of sense, thanks for that.

Also we researched this body shape, (its a 2003 Pontiac Firebird)

And the photos we found, they actually dont use spill plates....

Is this because the body is effient enough?