911 rake front/rear

[BogieMan]

Hmmm. I think the low pressure area would be right underneath it, and then the pressure would increase as the area increased.

Let's try an analogy. This has limitations and shouldn't be taken to far, but at least its something everyone can directly relate to. If you are on a 4 lane highway with bumper-bumper traffic that reduces to a 2 lane highway, everyone must go twice as fast to avoid a traffic back-up. When the highway opens up to 4 lanes again, everyone slows down again (assuming they remained bumper-bumper the whole way through).

The velocity of the air going through a venturi is like this. By venturi, I mean a classic hourglass shape. The initial flow must speed up to go through the narrow section, and then slow down as the area increases after the throat. Remember that the faster the air goes, the lower the pressure and vice-versa (unless you add energy). The pressure at the begining and the end are identical (if the cross-sectional areas are equal). At the narrowest point, the airflow increases and the pressure drops. This is how the carburetor draws in fuel. The increase in area downstream of the throat slows the flow down and the pressure returns to the initial value.

The underside of the car is like that, with the narrowest point at the front. Thus the front has the lowest pressure. When you talk about tilting the back up or the front down, the height of the rear increases. Thus the velocity will relatively decrease, and the pressure will rise. Think of it like the bottom of the hourglass.

I'm perplexed by your comments from last night. You seem to be talking in circles:
________________________
TYSON: I disagree. Yes it trades kinetic energy for potential energy, but that doesn't necessarily have to mean pressure. It can also mean vacuum. (Well, low pressure actually, it's all relative.)
________________________

A vacuum just means that the local pressure is less than everything else around it (ambient). Air can never have a negative pressure -- it just doesn't make sense -- unless you are talking about gage pressure specifically which is relative to ambient. So when you say the air doesn't trade movement energy for pressure energy but instead trades it for "vacuum" energy it sounds like gibberish.

I'm also perplexed by your other statement:
_______
TYSON: If the air flowing under the car slows down as it enters a larger volume space, then there is even less of it to fill the space. The body of the car attempts to fill that space, and that provides downforce.
_____________
How is an upwards sloping underbody "attempting to fill that space"? That just doesn't make any sense.

Some of your comments actually make sense if you are exceeding the speed of sound (Mach > 1). Just how fast do you drive?

Hope this clears things up.
-BogieMan

[Tyson Schmidt]

If the lowest pressure in the carburetor is at the narrowest point of the venturi, then why are all the fuel inlets quite a bit down stream of the venturi, and the small venturi portion of the pre-atomizers?

Also, if the air going over the car is sped up as it travels over the hood and roof, then does it speed up, slow down, or stay the same as it travels downward over the sloping rear window and decklid?

I would assume that it either stays the same speed, or accelerates as it passes down over the rear, otherwise you'd have down force back there, correct? I always assumed it slowed down, just like you're saying the air under the car would, as it traveled rearward into the higher volume of the underbody of a raked car.

[BogieMan]

I'd have to see the carburetor to respond fully, but I suspect the vents are moved slightly downstream to account for friction (viscous) effects. There is a mass of slow moving air near the surface of the venturi tube (i.e. boundary layer), and it takes a small amount of time to accelerate it up to speed. You can observe a boundary layer by rinsing a dirty car with water. It doesn't realy get anything off because the water touching the surface doesn't move. Take another look at your venturi and you'll see the fuel vents are much closer to the narrowest point than the exit.

The air over the top of a 911 both increases and decreases speed relative to the car's velocity. There is a general increase in speed though and this causes lift. The sloping rear deck does tend to decelerate the flow back to freestream, but it is actually a little too aggressive and separates off (changes with speed). The deceleration on the rear deck does increase the pressure relative to the suction peak on the roofline, but it is still at lower pressure than the underbody.

Bottom line: Air speeds up over the top and causes lift. Same effects top and bottom, although venturi analogy is more appropriate between the underbody and the ground.

-BogieMan

[Tyson Schmidt]

Originally posted by BogieMan
Hmmm. I think the low pressure area would be right underneath it, and then the pressure would increase as the area increased.

Let's try an analogy. This has limitations and shouldn't be taken to far, but at least its something everyone can directly relate to. If you are on a 4 lane highway with bumper-bumper traffic that reduces to a 2 lane highway, everyone must go twice as fast to avoid a traffic back-up. When the highway opens up to 4 lanes again, everyone slows down again (assuming they remained bumper-bumper the whole way through).

The velocity of the air going through a venturi is like this. By venturi, I mean a classic hourglass shape. The initial flow must speed up to go through the narrow section, and then slow down as the area increases after the throat. Remember that the faster the air goes, the lower the pressure and vice-versa (unless you add energy). The pressure at the begining and the end are identical (if the cross-sectional areas are equal). At the narrowest point, the airflow increases and the pressure drops. This is how the carburetor draws in fuel. The increase in area downstream of the throat slows the flow down and the pressure returns to the initial value.

The underside of the car is like that, with the narrowest point at the front. Thus the front has the lowest pressure. When you talk about tilting the back up or the front down, the height of the rear increases. Thus the velocity will relatively decrease, and the pressure will rise. Think of it like the bottom of the hourglass.

I'm perplexed by your comments from last night. You seem to be talking in circles:
________________________
TYSON: I disagree. Yes it trades kinetic energy for potential energy, but that doesn't necessarily have to mean pressure. It can also mean vacuum. (Well, low pressure actually, it's all relative.)
________________________

A vacuum just means that the local pressure is less than everything else around it (ambient). Air can never have a negative pressure -- it just doesn't make sense -- unless you are talking about gage pressure specifically which is relative to ambient. So when you say the air doesn't trade movement energy for pressure energy but instead trades it for "vacuum" energy it sounds like gibberish.

Didn't you see the part where I wrote, "Well, lower pressure actually, it's all relative." So, I agree with you.


I'm also perplexed by your other statement:
_______
TYSON: If the air flowing under the car slows down as it enters a larger volume space, then there is even less of it to fill the space. The body of the car attempts to fill that space, and that provides downforce.
_____________
How is an upwards sloping underbody "attempting to fill that space"? That just doesn't make any sense.

I guess what I was trying to say is the the bottom of the car wants to level out, (become less sloped at the rear) so that the pressure would remain constant front-to-back.

Some of your comments actually make sense if you are exceeding the speed of sound (Mach > 1). Just how fast do you drive?

Hope this clears things up.
-BogieMan [/B][/QUOTE]

[island911]

“behaviours that are usually difficult for the average layman to grasp. “
Yeah!? . .. that’s what all the lousey engineers say.

I say kudos to Tyson. While his “hand out the window” analogy wasn’t the most accurate, he brings the point quickly to where people can grasp it. ( a good thing to strive for)

While I give bogie credit for trying to explain some aerodynamic priciples, I don’t believe too many are following.

BTW;
"worth roughly 0.7% of the total number.
. . .for anything below Mach 1. You may not like the answer, but that's how it is.
. . .. if you are exceeding the speed of sound (Mach > 1). Just how fast do you drive?"

Well that is great news. I like it, I like it.
Then all the that talk of "loosing volumetric efficiency," as an engine revs, is pure BS.
Wooh-whoo I'm getting new cams tomorrow

.. . yeah, Im flip'n ya sheit.

Im not saying that the compressable nature of air is a huge infulence on street cars. I am saying, some here will not want to just dismiss it and consider the simple model of " I always figured a Porsche was a low flying vehicle that just happens to be touching the ground."

Understand, Tyson deals with setting up track cars. This means dealing in the subtle aspects of ground effects. (among other subtleties)

So bogie, get your head out of the clouds, and keep reading-up on ground effects. It’s interesting stuff.

THEN, you can come back and "school" us all for your buddy.

[BogieMan]

OK.
Sounds like we're in agreement now.

Cool.

-BogieMan

[}{arlequin]

Whew! That was some intense reading/debating. Very educational, though. Thanks to all, engineers and laymen.

[Tyson Schmidt]

I just had an epiphany.

I think that the air is sped up after the venturi (bottleneck, restriction, whatever.)

Like the nozzle on a garden hose. The pressure is in the hose, and discharged out through a restriction. The smaller the nozzle, the faster the water is discharged for a given amount of pressure.

So, lowering the nose of the car would restrict the pressurized air in front of it. The air that gets through the restricion is then accelerated as it discharges under the car into a larger volume, and continues to accelerate as the volume under the raked underbody increases. By raising the underside of the rear of the car, it would accelerate the air even further, effectively lowering the pressure even more.

This would hold true with the top of the car as well. The air is accelerated as it ramps both up, and then down as it travels over the car. If the back of the car slopes at a steeper angle than the front, then it would actually speed up even more as it passed toward the rear. This would increase the lift in the rear. If you were to raise the rear decklid, it would slow down the airflow and increase in pressure.

It all makes sense to me now. The increase in volume under the car in the rear would actually accelerate, not decelerate the airflow, causing a decrease in pressure under the car.

That's why the fuel inlets in a carburetor are much further downstream of the venturis and pre-atomizers. Because the air is sped up directly after the bottleneck.

[BogieMan]

Tyson: I think that the air is sped up after the venturi (bottleneck, restriction, whatever.)
_____________________

In the words of the Car Talk guys: Bogus!

Go ahead and believe what you want. What you are proposing would mean that mass or energy would need to be created as the air flows under the car. Doesn't sound very reasonable to me. Wouldn't that violate most of the laws of physics?

I'm not even going to bother responding to "Island911". He's to far gone, and seems to be working on soundbites instead of logic. I think he's part of some Tyson fan club. This isn't about personalities or ego. It's just reality. That's just how things are. You can complain or disbelieve it or insult me, but it's still there and that won't change anything.

-BogieMan

[350HP930]

Poor BogieMan. This is turning into a good example of why I usually try to avoid teaching physics on a bullitin board.

I think the thread about how turbos work on TampaRacing.com ended up being over 15 pages and most of that was people refusing to accept the valid explanations from scientists, engineers and knowledgeable technicians such as myself because it was incongruent with how they BELIEVED things works.

Just like with politics people start taking it personally when you are 'attacking' their beliefs, even though you are just explaining the laws of physics to them.

And Tyson, the air does not speed up after the restriction, it speeds up at the restriction because in low speed flows (which is pretty much anything less than the speed of sound) the air does not compress (at least not more than 1% which is negligible as all ready stated by BogieMan).

The only reason that their might be additional acceleration AFTER the restriction is because disturbed air at the surface of the restriction (the boundry layer) slows down and stalls, effectively becoming part of the restriction, increasing the area of the restriction beyond the solid object, whether that restriction is a nozzle or a car.

[masraum]

Hopefully this will help
(don't grade the art, and understand that I intended a rake like we are discussing to exist on the car, but it doesn't really look like it.)

[Tyson Schmidt]

There's really no need to get pissy. I'm just trying to get my mind around something here, and if I think out loud, I can figure everything out.

I appreciate you guys staying with this thread, since I think a lot of people are reading this and learning a lot. I totally understand about 95% of what you guys are saying, it's the last 5% that's really bugging me, and I'm trying to make that click. There must be 1 key ingredient that's missing for this all to make sense to me. I'll find it. That's how people learn, by not being menatally lazy and just giving up.

Island is definitely not in my fanclub, and enjoys arguing with just about anybody. He enjoys a good argument, and is just giving me props for hanging in here against some engineering heavyweights, and not backing down, despite being outgunned.

I know this next question will probably give you fits, but, isn't the position of aileron on a wing similar to the tail of the car being shaped either in the sloping upward or downward? How do the two relate?

Isn't the aileron there to change the attitude of the wing in the airstream? Wouldn't the the upward sloping underbelly combined with the upward sloping rear decklid (If it were raised enough) combine to mimic the aileron on a wing being raised to make the front of the wing lift and the back of it drop? (relative to one another.)

I know we're dealing with close proximity to the ground here, so I'm sure that changes things, and is probably the answer. The venturi effect winning out over the wing effect?

[MotoSook]

Wait! I thought the area over the rear lid of our cars and behind it were low pressure regions?

Bogie, can you run a 2-D model using the profile of a 911?

I always thought that flow rushing to fill the region behind the car helped to draw air out from under the car.

P.S. I don't know Tyson, but guys like he and I use to piss the hell our of our Calc teachers

Tyson, I've had my share of fluid dynamics classes/labs, and even tested a model of an open wheel race car in a wind tunnel complete with a drag scale and smoke. Yet, I still have a lot to learn about aerodynamics of vehicle. Keep probing.

[masraum]

Tyson, I think the aileron question is covered by the profile of the 917lh, but I don't think you could really apply it to a regular 911 without giving it a treatment more like the tail on the 935/78. I think (but could very easily be wrong) that most of the downforce from this setup comes from the top of the wing, not negative pressure from under the wing. It's probably exactly what the tail on a 911 is doing. Also, this would cause the front of the car to become light if you didn't have the right frontend.

[island911]

Originally posted by BogieMan
. . .
I'm not even going to bother responding to "Island911".

You just did.

He's to far gone, and seems to be working on soundbites instead of logic.

What a "logical" explaination, you have there.

I think he's part of some Tyson fan club. This isn't about personalities or ego.

Trust me, Tyson and I have had a few good arguments. . . it's just that he's not a pansy about it like SOME.

Look, you want to keep the argument in the realm of what you know. Fine. I'm not saying the majority what you are telling us is wrong; just misdirected. . . I'm telling you racers do want to know about that 1%.

Example:
If this were about tires, and I did the knee-jerk mechanical engineering reaction of saying 'the coeffient of static friction is constant . .. .end of story. . . like it or not. . .thats just the way it is!' I tell you people like Tyson (with emperical evidence / hands on experience) would be all over my shiet.

Bogie, you obviously came here to "school" us all . .. but your model of reallity is in saying 'it's all wing theory.'
. . ..Bring the subject down to earth.

[69911e]

Souk: I believe the deck area pressure changes dramaticaly depending on which deck is used, no tail, duck tail, or whale tail. Also I believe (I could be wrong, as I have no aero background) the dominant problem in the top rear is flow separation/turbulance not the venturi effect which has been the above discussion.

[MotoSook]

Well, I should have been more clear. The general shape of the 911 sans ducktail or others is what I was refering to.

I know the top side of a tail-less 911 has nothing to do with the a venturi effect, but what I was trying to point out is that in the real world, it may be that air flowing to fill the region behind the car may give Tysons reasoning some basis, i.e. the mass flow issue.

Lets not start the discussion on the flow over ducktails and whaletails yet...that is another one I would like to see a simple 2-D model on Bogie (separation delay and wake management??).

[MotoSook]

Steve it has always been my assumption that the taller, larger wings of race cars like the 917, F1 cars, etc. all DO behave like a "wing". They are generally in clean airflow and provide downforce via "lift" (truely inverted "wing") -> downforce.

[350HP930]

The area over the tail of the car, and behind the car too since the car is dragging along a lot of turbulent air behind it which you can in a way concider part of the car, is low pressure relative to the undisturbed air stream.

The only purpose of the factory tail on 911 is to help create a localized high pressure zone (though the pressure is still lower than the undisturbed air) over the grill to help push air into the engine compartment and act as a spoiler. It does not act as a wing or an aileron.

As I mentioned above the car creates a zone behind itself where the air does not come together smoothly and creates a wad of turbulent air which the car drags along behind it.

The larger this area the greater the drag it creates pulling on the back of the car.

A spoiler is just a simple device that creates a votex that helps to stabilize and minimize the size of this turbulent air behind the car.

[350HP930]

As an additional aid, here is a pic of my car that I added some things to to help clear this up a little bit.

The black lines represent the slip streams, giving you an idea of how the air is going around the car. Its not exact, but gives you the right idea.

The swirls behind the car represent the turbulent air pulled along by the car. This area grows as the speed of the car is increased.

The radial lines from the car indicate the deviation from atmospheric pressure. Red indicates elevated pressures, and the blue lines indicate lower pressures.

The length of the blue lines give you an idea of the magnitude of these pressures and how they change over the length of the car and the turbulent area.

This is meant to be a very intuitive diagram of what is going on over your typical porsche with a tail.

I will let you all soak it in, and if you have any questions or comments about it I will do my best to respond.