Increasing downforce while reducing drag?

[freebeard]

This picture had what may turn out to be a life-altering effect on me. I'd stopped thinking about fastening composite materials, but it reminded me. Basically you bore through all three layers and counterbore through one skin and the substrate. A stepped washer shape grips one skin on both sides and the full material on both sides.

I went to a local fastener supplier to see what they had. It looks like I will have to take a high-grade washer and machine the step myself.

In case you missed —my 'World's fastest motorhome' project has morphed* into a Lakester to run in the 36HP Challenge.

*competion is too stiff--> 138mph.

[Smokeduv]

I think that the better way to obtain downforce without adding too much drag (or actually reducing it) would be ground effects with the use of a rear diffuser (Like the Lotus 79 posted above)

[aerogreen]

Most downforce increasing methods also increase drag.
Consider that F1 car have a Cd of about 1 while most passenger cars have Cds of less than 0.4

Most production cars have lift so to get to have downforce you must eliminate all lift
Things that really work to improve downforce while also reducing drag

- Smooth undertray or a front air dam. While an air dam increases drag, if you got a rough underbody the dam drag is compensated by the reduced underbody drag.
- Diffuser. Up to 4 degrees reduces drag. Anything beyond 4 increases both drag and downforce.
- Boundary layer suction
Look it up in wikipedia
Even better if you could run some solar panels to power the suction engine. By doing so you wouldn't use any fuel.
- Placing exhaust to fill vacuum behind car.
- Redirecting air heated by the radiator either over the hood or behind the front wheels.
- Variable opening grille.
- I think smooth wheel covers will reduce both drag and wheel induced lift.
- AFAIK all rear wings will increase drag.
- Fender holes to reduce lift caused by rotating wheels. Be aware fender louvers will increase drag.

[kach22i]

Originally Posted by aerogreen

Most downforce increasing methods also increase drag.

The word here is "most".

If we to stray the conversation for a moment to include some of Luigi Colani's 300 mph Corvette designs we would perhaps find some exceptions to the rule.

Luigi Colani Corvette



The rear spoiler on the MIT comparative examples might be more an indication of getting power down to the road via torque than anything to do with a lower Cd.

For example I know that I can launch my pick-up truck faster off the line with two sand bag tubes over the rear axle. I would not argue that I got faster 0-60 times because these sand bags lowered my Cd.

[freebeard]

I was thinking Colani, but I'll go with this (I'm blanking on the name at the moment):

[cfd_guy]

Originally Posted by aerogreen

Most downforce increasing methods also increase drag.
Consider that F1 car have a Cd of about 1 while most passenger cars have Cds of less than 0.4

Most production cars have lift so to get to have downforce you must eliminate all lift
Things that really work to improve downforce while also reducing drag

- Smooth undertray or a front air dam. While an air dam increases drag, if you got a rough underbody the dam drag is compensated by the reduced underbody drag.
- Diffuser. Up to 4 degrees reduces drag. Anything beyond 4 increases both drag and downforce.
- Boundary layer suction
Look it up in wikipedia
Even better if you could run some solar panels to power the suction engine. By doing so you wouldn't use any fuel.
- Placing exhaust to fill vacuum behind car.
- Redirecting air heated by the radiator either over the hood or behind the front wheels.
- Variable opening grille.
- I think smooth wheel covers will reduce both drag and wheel induced lift.
- AFAIK all rear wings will increase drag.
- Fender holes to reduce lift caused by rotating wheels. Be aware fender louvers will increase drag.

A big reason F1 cars have a high Cd is because they are an open wheel car. Yes the wings also increase this, but the open wheels are why they are as high as they are. Their "low downforce low drag" setup for tracks like Monza still create lots of drag.

How does a diffuser over 4 degrees magically make drag, but 3 degrees don't??? This is such a broad statement and not true. Any diffuser itself creates drag because it creates induced drag. However in a system that utilizes a diffuser well should reduce drag because it "fills in" the low pressure region behind the car.

How does fender louvers automatically cause drag?

[kach22i]

Originally Posted by freebeard

I was thinking Colani, but I'll go with this (I'm blanking on the name at the moment):

This website (see link below) has that same graphic labeled as "Colani C-Form",

Similar project actually built:
Hail Luigi Colani



EDIT: Google image search using the term "Colani C-Form".

1968 Colani C-Form
1968 Colani C-Form - Studios

[kach22i]

Originally Posted by cfd_guy

How does a diffuser over 4 degrees magically make drag, but 3 degrees don't???

Going off memory, rear underbelly angle of inclination should be limited to 10-degrees.

This is similar to the over the body angle being limited to 22-degrees.

I think with rear diffusers with vertical strakes or fins contain the air and keeps it from wandering or rolling off to the sides.

This containment zone between the fins I believe allows for angles greater than 10-degrees to exist and function as intended for ground effects.

I would think the 3-4 degree thing may apply to the initial curvature upward and not a mandatory angle to be maintained the entire length.

[cfd_guy]

Originally Posted by kach22i

Going off memory, rear underbelly angle of inclination should be limited to 10-degrees.

This is similar to the over the body angle being limited to 22-degrees.

I think with rear diffusers with vertical strakes or fins contain the air and keeps it from wandering or rolling off to the sides.

This containment zone between the fins I believe allows for angles greater than 10-degrees to exist and function as intended for ground effects.

I would think the 3-4 degree thing may apply to the initial curvature upward and not a mandatory angle to be maintained the entire length.

That is true USUALLY of 10 degrees to keep the flow from detaching. I have seen much steeper angles than 10 degrees with flow attached however. There are some nifty tricks that can be done This does make the car more sensitive to ride height and yaw changes though (racecar application only).

He was talking about drag though which doesn't matter. The diffuser will make drag (as an isolated part) at any angle because of induced drag. In a system, it will usually decrease drag even at lets say 7 degrees (especially on street cars).

[kach22i]

Originally Posted by cfd_guy

..........7 degrees (especially on street cars).

You meant to type "7 percent" perhaps?