bbundy

Mister Bernoulli thinks you don’t have a clear understanding.

High velocity laminar flow relative to an object generates a lower pressure than lower velocity and turbulent flow.

Keeping the air sliding smoothly under the car at a high relative velocity to the car generates a much lower pressure on the car than having a rough underbody catch and slow the air and haul along a chunk of air traveling with the car caught up on underbody geometry of the car.

Unlike an airplane wing a car has ground interaction effects. You can actually create a venturi effect and actually speed up the flow relative the car thus generating even lower pressure under the car even though it doesn’t look like the traditional wing shape from the side.

Lift is simply the difference between the pressures on upward facing surfaces and downward facing surfaces.

Bob

Miater

^Very well put. I figured there was a reason engineers, professionals, and hobbiest alike have been covering their bottoms for about 100 years.

Seefo

iTrader: (5)

edit: NM, I am going to do some reading

bbundy

I also think a lot of people misunderstand how and what a diffuser does. Essentially it connects a path for the air passing under the car to feed into the low pressure wake zone behind the car. It kind of like sucks the air out from under the car and thus increases the velocity of the air passing under the car in so doing. It is way more effective with a flat bottom though.

Think Brawn double diffuser. The upper portion of that thing is using the low pressure and drag producing wake region behind the car to suck air out from under the middle of the car both reducing lift and drag.

Now that has me thinking of trying to figure out how to make a duct from the aft upper transmission tunnel to the license plate area through the trunk. Seems like you could suck a lot of drive train heat out and reduce both lift and drag. 

Bob

greeenteeee

I referenced scarbsf1's blog plenty of times in this thread:
http://clubroadster.net/vb_forum/sho...iffuser&page=2

This is exactly what I've tried to accomplish.

Splitter only, made the rear end less stable at speed. Rear diffuser helped stabilize that, but I did not like the flow patterns I was seeing. Despite appearing like turbulent flow at the diffuser, there is a noticeable difference at 70-90mph in rear end behavior, and the undertray made it much more efficient. That is what the string videos show.

Do you know what kind of temp measurements I can take at the diff to see if I need to be concerned with diff temps? I've measured 170* after a 30 minute street drive. I understand what is okay on the street may not be okay on the track.

Also, if my corrugated plastic is supposed to fail, when will it? It has held up to 100mph speeds and the most this car will ever see is probably 120-130mph around the roval of AutoClub.

ZX-Tex

iTrader: (15)

What Bob said above. Word. Saved me the trouble of typing out my long winded engineering type response.

Savington

iTrader: (31)

+1, Bob nailed it.

Mobius

iTrader: (2)

Bob r smart.

Greentee I don't think there's much chance of the underbody material getting ripped off unless it's poorly attached. As long as the leading edges of it can't flap at all, and can't begin to scoop air in any way, (ie as long as your front air dam and splitter are doing their jobs) the underbody stuff should be fine. I would check that your zipties aren't fraying through their mounting holes periodicallly. You could probably find some sort of snap grommets to put on those holes to prevent that.

Bernoulli gets vastly too much credit for making airplanes fly. Paper airplanes fly with no airfoil at all. Aircraft with semi-symmetric airfoils can fly upside down (although not as efficiently). The majority of lift comes from the mass of air being deflected downwards by the "bottom" side of the wing, as referenced by the angle of attack.

Since you have a cutout for the diff, can't you extend a channel from that cutout to the diffuser? Essentially like a naca duct, or a diffuser just for the cooling fins on the diff, so it sees some airflow.

sixshooter

iTrader: (12)

Negative. Lift is generated on the top surface as well. It is a lower pressure (think of it as suction) area attracting the higher pressure area. The top of the wing is pulled upon and lifted into that vacuum.

This is why an airfoil is not just a flat surface with its leading edge angled upward relative to the angle of attack. And it is also why it is possible to achieve stall in an aircraft with plenty of airspeed at a high angle of attack. Air still pushes on the underside of the wing but lift (which actually gives us flight) goes to zero.

ZX-Tex

iTrader: (15)

+1

And regardless, we are not talking about a wing in the case of an undertray. With an undertray, there is airflow between two roughly parallel surfaces. This airflow creates a pressure drop, and pulls the two surfaces together, creating downforce. There can indeed be a significant load on the undertray if it is smooth and flat, and the air flow velocity is high enough. The pressure differential may be small, but the surface area is large. Splitters work on the same principle.

It is the same flow behavior that can cause two large ships sailing in parallel in the same direction to be pulled into each other if they are not careful. It has to be compensated for at the helm.

It's Bernoulli. Basic closed fluid flow behavior.

TorqueZombie

iTrader: (6)

So if I semi flat bottom the '99 should the undertray go to the bottom of the front bumper or to bottom of a Garage Vary lip? In my head if I mount my new to be made belly pan (soon to go flat ish bottom) to the bottom of the bumper to make it smooth now. When I add the lip it will make a kinda sideways L shape. bad?

Edit: It's a street car with dreams of a few track days.

Twibs415

iTrader: (8)

you want it from the bottom of the lip for it to work effectively.

Savington

iTrader: (31)

Or a bit further, if you can live with it on the street. There are diminishing gains after a certain point, but if you can get the "undertray" out beyond the lip a bit, it becomes a splitter and you get to utilize the high-pressure zone at the nose to generate a little downforce as well.

Laur3ns

How far would you say is optimal with a GTC200 on the back and 250rwhp? Two inches out?

ApexAddict

Sorry to resurrect this thread, but what material is best? I heard 5051 is the easiest to shape?

ThePass

iTrader: (3)

Are you trying to shape it? Operative word... Flat bottom

ApexAddict

True, but there are a spots where I'm going to be bending it, such as bend it from the frame rails to the pinch welds, and possible over the exhaust. I've seen people have just a flat panel under their car and I've seen others cut out a channel down the middle for the exhaust. Ideally I'd like to have the panel cover everything including the exhaust but I don't see how people do that, so i'm assuming I'd have to do a little bit of bending. If anyone could chime in and give me some advice I'd appreciate it, clearly I'm pretty clueless.

ApexAddict

Also, if I go thinner than 1.5mm will I have issues? I don't see the problem with going as thing as possible yet I see most people getting 1.5mm-2mm thickness sheets, why is this?

On another note, where are you guys getting your sheets of aluminum? Please don't tell me you're ordering them online...

shlammed

iTrader: (8)

with ultra thin sheets, your going to need bracing if you cantilever it out at any point past where its supported.

go to a metal shop and shake a sheet of 1/16" (or less thick) aluminum and it will move. Part of the benefit of aero is in its rigidity. so if your going to try to save weight, your going to need more complex bracing to do it well. or go up in thickness. something like the true middle of the car between thr frame rails would be easier to go thinner based on how its supported. something like past the rockers would suffer from being thin unless your adding some bracing.

open a local directory and look for a metal supplier. you shouldn't need a boutique "metal supermarket" type supplier if your buying full sheets and prices will be marginally more for a half sheet as a full sheet anyways.

ApexAddict

Ahhh that makes sense, so 2mm would probably be my best bet? And thanks for the advice by the way, I appreciate it.