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Old 04-03-2009, 06:55 PM   #1 (permalink)
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Aero design question

In the effort of designing a more slippery vehicle while still maintaining decent looks, is it possible to see a lower CD by offering a "channel" that would allow air to flow thru/under the vehicle?
I noticed the Loremo design is like this, the floor is higher up except at the tires. I'd be interested in having this channel be narrower but far taller, forming a "center console" between driver and passenger that is for example 12" wide and 18" tall..
Air would enter the grille area say 12" tall by 36" wide, and instead of just running into an engine, be routed thru the channel back under a rear mounted engine and out, pulling exhaust with it.
The squeezing of the channel would accelerate the air under the car and help provide a more balanced shape, as a generally slippery car shape tends to resemble a wing and can get dangerous at higher speeds.
I keep looking at all these sleek cars heading for the x-prize competition wondering if there is more to be gained by using such a method and getting rid of more frontal area....

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Old 04-04-2009, 01:37 AM   #2 (permalink)
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Some racecars already have channels in the underbody panels. What you are talking about though would take up a lot of interior space.
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Old 04-04-2009, 12:41 PM   #3 (permalink)
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Doesn't the channel on race cars fan out at the back, creating suction, to increase down force? That would be counter productive for MPG's. I could see a strait line back may improve things.

What your talking about sounds a lot like a ram-jet engine. Squeeze the air really tight and warm it up with the exhaust, see if you can get some push from it. I wanna say they did that with the P-51 mustang. Got a few extra MPH's out of it.
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Old 04-04-2009, 01:57 PM   #4 (permalink)
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Quote:
Originally Posted by FastPlastic View Post
Doesn't the channel on race cars fan out at the back, creating suction, to increase down force? That would be counter productive for MPG's. I could see a strait line back may improve things.

What your talking about sounds a lot like a ram-jet engine. Squeeze the air really tight and warm it up with the exhaust, see if you can get some push from it. I wanna say they did that with the P-51 mustang. Got a few extra MPH's out of it.
Sloping the back upward creates downforce just like a fastback creates lift but a fastback is more aerodynamic than a wagon back.
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Old 04-04-2009, 03:26 PM   #5 (permalink)
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channel

Quote:
Originally Posted by rkcarguy View Post
In the effort of designing a more slippery vehicle while still maintaining decent looks, is it possible to see a lower CD by offering a "channel" that would allow air to flow thru/under the vehicle?
I noticed the Loremo design is like this, the floor is higher up except at the tires. I'd be interested in having this channel be narrower but far taller, forming a "center console" between driver and passenger that is for example 12" wide and 18" tall..
Air would enter the grille area say 12" tall by 36" wide, and instead of just running into an engine, be routed thru the channel back under a rear mounted engine and out, pulling exhaust with it.
The squeezing of the channel would accelerate the air under the car and help provide a more balanced shape, as a generally slippery car shape tends to resemble a wing and can get dangerous at higher speeds.
I keep looking at all these sleek cars heading for the x-prize competition wondering if there is more to be gained by using such a method and getting rid of more frontal area....
So far,it looks like all the really low drag designs minimize internal flow as best they can.When duct work is necessary,it's held it to a minimum,and the air is always vented to a place on the body which maximizes drag reduction,not necessarily at the back of the car.----------------------- Your point about high speed stability is well taken,however,safe,zero-lift low drag designs have been hammered out now since the late 1980s.Pininfarina's CNR,Ford's Probe-IV,and GM's PNGV would be some examples you could look at.
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Old 04-04-2009, 11:47 PM   #6 (permalink)
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The probe in one of my favorite shapes and if I was going to start with a rolling shell it would be one of those...but then theres plain old steel and heavy everything....nah.
Take a 1st generation Mazda Rx7 for example. The bullet shape of the front end was pretty decent, but the rear fastback of the car got loose at high speed...I raced one for several years both autocross and road racing and found that I had to change setups because the higher speeds got the rear end loose and low speeds understeer'd...
My goal I am trying to achieve is to reduce frontal area and therefore gain a few % on the designs already out there. Take the Avion for instance, look at it's front view and then take a big bite out of the bottom center of it....
Slowly tapering this channel smaller and shallower towards the back(and so it doesn't totally kill the passenger space), should yield a lower CD and help balance the effect of lift on the back of the car due to it's shape(fastback type shape). In the beginning I was hesitant about even posting because I am unsure if this can be answered without wind tunnel testing or computer testing of the shape itself.
FYI in high school I based my co2 dragster design on a similar concept and made basically a bullet shape at the cylinder with a pair of vertical triangles that picked up the axles. Frontal area was as little as it could be, and despite that I had doubled the side surface area by hogging out the center, it was the fastest by over 5mph of all the ones tested.
A couple of wood 1/12 scale models and a visit to my friend at the VRI program may be in my future..
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Old 04-04-2009, 11:59 PM   #7 (permalink)
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Hi,

The crux of the matter is, that the whole body has to be aerodynamic, in order for it to work best. You can't solve the overall drag by channeling the air under the car; in order to make the outside look "better". The air will always drag on the parts of the car that are styled instead of engineered.

I think we need to change our ideas of what looks good?
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Old 04-05-2009, 12:22 AM   #8 (permalink)
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I totally agree, but at the same time lets look at some alternatives. You configure the car to be like a jet cockpit with pass behind driver and narrow it up. Then you have a car that is prone to sidewinds and tipping over in a windstorm...
Crap my car is on it's side in the neighbors yard again lol...

There is going to have to be compromises made that will yield a practical car with acceptable mileage. Sure I could take a empty torpedo shell and mount 4 aero wheels on it and probably get 200mpg laying on my stomach looking out a clear nosecone, but hows that gonna work for old people, fat people, or spoiled people?
Wheres the cruise control and DVD player?

Flat and wide is stable, run air thru the parts of the car you can instead of around it...
I will post some scans of my drawings up once autocad gets here next week and I can get them transferred in..
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Old 04-05-2009, 02:48 PM   #9 (permalink)
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Quote:
Originally Posted by rkcarguy View Post
I will post some scans of my drawings up once autocad gets here next week and I can get them transferred in..
oooohhhh i can't wait! I love this stuff! Haven't a clue about the mathematics of it though!

ollie
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Old 04-06-2009, 01:59 PM   #10 (permalink)
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rkcarguy

To help us out for your aero concept is a narrowed design criteria of what you want to accomplish. This way your we could have an engine, powertrain, drive, passenger/cargo configuration & car size (i.e., city, subcompact, compact, mid, full, etc) classification defined. And what is probably the most difficult thing to settle is what do you define as 'decent looking'. Pure aero forms would either look biomorphic (fish-like) or mathematically-defined forms, it would look odd to most and people usually are accustomed to well defined proportions set by the car design industry. Besides the Loremo & RX 7, what other examples do you consider a 'decent' inspiration for your project. A 'clean' paper project is going to be a great project. Even a heavily modded existing platform chassis with a new 'kitted' aero shape would still be a great challenge and would be closely followed by this group, if you don't mind a design-by-'enthusiast'-committee.

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