Belly pans for aerodynamics

[Guest Night_rider_383]

I'm with mike on this. I'm 100% sure he knows what the risk is and knows what hes doing. When some one has there heart, mind and soul set on doing something then its gonna get done or at least go out in a blaze of glory. If you think about it we take our lifes in our own hands every time we walk in to a store, drive to work, drive down the road. etc. I know how having a goal is thats for sure. My goal for the last 4 years is been to hit the 6's in the 1/4 and still be able to drive on the streets. I came close a few times with 10's but in a chevy body to get any faster the engine would have to be un street able. Thats why i'm going with a z car less weight, better aero. So mike i say good luck to you bud. Keep digging you be there soon

[pparaska]

Mike, Michael Ol might have some ideas on wind tunnels or water channels. He likes to use water channels to get easier flow-vis.

 

I wonder if you could get an aerospace engineering student to do some scale or full scale stuff on this at a university. Univ. of MD has at least one wind tunnel - it might even be large enough for a Z?

 

Anyway, the thought is that if the University sees it as a good senior or non-thesis masters degree project for a student, they might buck up and give some time in the tunnel. Just a thought.

 

There are a few Z bodies around you could play with in a tunnel without using yours .

[Guest Anonymous]

Just a quick thought here, if you look at the design of the belly pans on a ferrari (courtesy of the ferrari website) you'll notice they have somewhat of an hourglass like appearing set of channels in the bottom. This is on the 550 Maranello. The point of this is that it creates a venturi effect and drops the pressure in the air that is under the car. Why couldn't this sort of design be adopted onto the Z? There's a nice picture of the bottom of the 360 modena on there, apparently they don't have the bottom pic of the 550 anymore (now I wish I'd have saved it somewhere). There's wind tunnel testing involved I'm sure, but I suspect if you follow their lead on the design you'd probably be ok. Plus it is pretty sexy looking which is always a plus. Saw a guy driving a 360 Challenge which is like the club racing version of the Modena a while back and it was quite beautiful to look at that sucker even if the guy driving it was a bit too into the whole gold chains thing.

[Mikelly]

Thurem, I assure you this will not take place on a public highway. I'm planning runs on the Salt Flats, and also at the various open road races across the country.

 

As for pulling high speeds on the highway, my little brother just got handed his final sentance on Tuesday for being convicted of reckless endangerment in Virginia - excess speed - 104mph in a 55mph zone. His sentance is 30 days in jail, 6 months restricted license (To and from work or school only), $1250 fine and mandetory one day drivers improvement course. He originally was sentanced 10 days in jail, $750 fine, 6 months restricted license and the one day drivers improvement course. He didn't want to sit in jail, and he listened to his attorney. I'm currently working with the local Detention center Captain to get my little brother released to me under house arrest and fitted with a tracking device on his leg. Fortunately the Captain of the facility knows me from my years in law enforcement and is willing to work with me and knows that I'll make the boy tow the line while he's with me... Lots of chores around the house that will be completed daily. If all goes well I'll have him at my house tonight where I can start the process of un-doing years of bad example. The kid is 20 years old and looks to me as being his hero, and I haven't set the proper example. We'll work to correct that with time.

 

Bottom line is I personally will not be setting any jail house records in my little beast. I'll be doing it in safe restricted areas.

 

Mike Kelly

 

[ June 15, 2001: Message edited by: Mikelly ]

[Guest Jason]

Instead of full size wind tunnel testing, I would go out and buy The Franklin Mint's extremely detailed and accurate 240z replica (saw it at the mall once), and use it for miniature wind tunnel testing (you could even build your own tunnel). You can correlate the model to the full sized car with the help of someone who knows their fluids calcs. When you're done, you'll have a miniature Z for the coffee table and a ton of money left over.

[pparaska]

Jason, I was hoping the university would pick up the wind tunel bill Good idea though on the Franklin Mint model. I have a BRE Z model that'd be good enough and cheaper.

 

Guys, I know Mike and believe me the kid will NOT get off easy if Mike is the warden!

[Mikelly]

Pete,

That is a true statement. I'm actually having to make sure I'm not "Too" hard on him, as the folks at the detention center actually had concern over that issue... Imagine that. Oh well, I tend to see things very black and white and tend to lean to the hard edge rather than being Politlcally correct/ warm and fuzzy... I'll have to mkae sure I don't leave any visible marks on the boy...

On the plus side, I'll get my deck sealed, privacy fence sealed, walkways powerwashed, siding wahsed, lawn mowed, weed eated, tons of fill dirt moved, interior of the house painted, and the garage cleaned up and garage floor repainted... Hmmm wonder if he would have rather stayed in jail??? We made him sign a contract to do these things...

Mike

[Guest Frank Rich]

I've been following this thread for a while, and I'd just like to offer some comments based on something Z3bra mentioned - the refined shapes used in the underbody panels of Ferraris and other high performance, high speed cars.

 

I know that several (maybe most) of you understand the basic principles of how an asymmetrical airfoil shape generates lift, but for those who may not, just bear with me for a minute - and I apologize for insulting anyone's intelligence with my wanderings.

 

In lay terms, as a lifting (asymmetrical) airfoil (basic wing) shape moves through the air, the air flow is divided by the leading edge of the airfoil into two separate streams - one which flows along the curved upper surface of the airfoil(wing), and one which flows along the (relatively) flat under surface of the airfoil. The air moving over the curved upper surface has to travel a greater distance than the air moving over the flat lower surface in the same amount of time (shortest distance between two points is a straight line), so it must attain a higher velocity than the air moving over the flat bottom of the airfoil. This results in an area of Low Pressure on the curved upper surface of the airfoil, which is opposed by an area of High(er) Pressure created by the slow(er) air moving over the flat bottom of the airfoil. The "technical" term for this combination of effects is "Lift". Lift is joined by "Thrust", "Drag" and "Gravity" in the equation for flight, which is: Thrust + Lift > Drag + Gravity = flight.

 

Now here is the problem as it applies to our Z-cars: If you create too smooth an underbody panel for your Z, you are actually IMPROVING the Lift coefficient of the Z body shape! Think about it - if you create an even straighter line (shorter distance) for the air flowing under your car to travel, then it will move even more slowly than the air traveling over the curved, aerodynamically "friendly" lines of the Z upper body - remember this air mass started out in one "piece" (stream) before it hit the nose of your Z-car, and once it was separated by your Z's body work, the forces of Thrust, Lift, Drag and Gravity all came into play. Add Thrust in the form of 300 - 400 h.p. to the extra Lift generated by a smooth underbody panel and you will easily overcome Drag and Gravity - and you have just achieved...Flight. Oh Sh*t!

 

That's why lots of high performance cars - and many open-wheel racing formulas - add "venturi" or ground effects channels to the underbody panels - the idea being that you make the airflow follow a curved, or longer course, or that you accelerate the airflow through a partial venturi effect tube - anything to try and increase the velocity of the air moving under the car, so that the area of Low(er) Pressure is generated beneath the car, not above it. You are literally trying to "fly" the car into the ground (negative lift) for better grip.

 

Okay - Mike K., and all you other guys looking at this area of performance improvement, I know what I've said so far sounds kinda negative, but here's the good news - underbody panels, when properly shaped, can contribute as much downforce as a good external body kit - airdam, side sills, rear spoiler, wing, etc. - but by cleaning up the airflow and directing it over smoother surfaces YOU REDUCE THE DRAG COEFFICIENT OF YOUR CAR'S SHAPE! Less Drag = more speed at the same Thrust (horsepower)! So the trick here is to design and create a (relatively) smooth underbody panel that will have enough venturi effect to accelerate the airflow under the car to at least match and ideally exceed the velocity of the airflow moving over the car - all the while taking into consideration minor things like cooling and suspension travel. Yeah, right!

 

I'd really be interested in getting much further into this with somebody - and I have my own ideas about bargain basement wind tunnel testing - I used to fine-tune sailboat sails using big industrial warehouse exhaust fans, lots of taped-on yarn tufts and colored smoke, and a sail is just another form of airfoil!

 

"Lead me not into Temptation - I'm getting ther just fine on my own"

 

Frank

[Guest Anonymous]

I guess it would depend on the effeciency of the wing (in our case a 'Z') but I wonder what the stall speed is on a airborn Z?

 

 

Regards,

 

Lone

 

Ps: Frank that was a excellent explaination of airfoils and lift BTW.

[DavyZ]

Frank, as Lone just mentioned, that was an excellent piece of work. I actually understand how this whole thing works. I think for my own purposes I'll not have a bellypan other than maybe something relatively small to keep the road debris and water from the front part of the motor. My idea is to eventually make a GT-40 style hood like Terry Oxandale's so the air gets directed in, up, and out the top of the hood for increased downforce--but that would be adding to the low pressure on top now wouldn't it? Hmmm, this is an intellectual feast now isn't it?

 

David

[Guest Anonymous]

Thanks Frank, great explanation.

 

I think given the bottom of the Z doesn't have much room for adding huge channels at best we'd probably be looking at enough to equalize the low pressure between the top and bottom and then make up for the top via spoilers etc.

 

Of course if someone wanted to take it really hi-tech, you could look at implementing some sensors on the bottom of the car that either measure the amount of weight being transmitted to the wheels and if it exceeds a certain threshold activates some sort of lift spoiling device (like the spoilers on the top of airliner wings that pop up after touchdown).

 

Alternately you could probably rig something that measures the air pressure of the underbody and works the same way. Rather pie in the sky really but that's basically how the flight control systems on the helicopters I used to work on in the Army worked. Can't imagine you'd need anything that hi-tech to accomplish a similar effect on the Z since you're only worried about movement in 1 plane of motion instead of all 3.

 

Just a few "out there" thoughts for everyone to toss around in their noggins. (Then again sometimes I even worry myself with the wierd crap I think of)

 

[ June 15, 2001: Message edited by: Z3bra ]

[pparaska]

Frank that was great!

 

Hey, I made a foot room sacrifice to put in 2x3" subframe connectors that are between my feet and the passengers feet also. How about making a tunnel (top and sides anyway)in the same area? This channel would aid in the structure as well. I'm thinking out loud about how to put a venturi path under the legs/seats so as to not impact the interior too much.

 

DavyZ's point about letting the radiator air exit the hood like on Terry (blueovalz's) car could reduce the underbody pressure couldn't it? If you had no grill, but ducted all radiator intake air from below the chin splitter, you'd be transporting some of the flow mass stream originating below the splitter to the flow stream above. I'd think that would help. This would require a forward canted radiator, like Terry's done. Terry, how much radiator intake air comes through the grill on your car and how much from below the spoiler?

 

Mike Kelly also mentioned to me an idea about cooling the diff with a duct. This would create another longer path for underbody air. Have it exit the rear panel maybe, between the tail lights?

 

Just some crazy stream of consciousness stuff - sorry if it's nutso.

 

[ June 15, 2001: Message edited by: pparaska ]

[BLKMGK]

Hrm, anyone notice the bottom of that RX7? Belly pans all along it - very nice. Looked liek he caught a gust that put him a little sideways and disrupted the wind, over he went when he presented a good neough edge. He also looked to be sitting a little high - I could swear my street driven RX7 sits lower than that one!

 

Heh, did you know TT RX7 have some belly pans from the factory? Some of mine are apparently missing but stock they're there. Well built cars but that darned motor is so fragile!

[Guest Anonymous]

To premise, what I know of aerodynamics you could blow out the window with a breath, but are there 'virtual' wind tunnel softwares out these days? Something you'd check a wireframe (or whatever) model against? Just curious..

 

Regards,

 

Lone

[Guest Anonymous]

Gosh, now nobodys going to believe my 205mph story.....tell you how I did it, instead of a belly pan i used one of those fringy things like you see on the back of motorhomes on the front..

 

Sorry, couldn't resist

Â¥

[pparaska]

Thanks for chiming in Michael. I see all those business trips around the country are beginning to pay off for us at least .

 

Lone, you're probably referring to CFD - Computational Fluid Dynamics. Cools stuff, but the problems with it are various and germane to all types of computer simulations of physics:

garbage in = garbage out,

To do it right takes a skilled and experienced analyst,

To get to good answers means doing it over again several times to converge on the (correct?) answer.

 

Add to that, that the physics is not always modelled in enough detail (full Navier-Stoke's equation of fluid dynamics) to give the answers that get to the problem at hand.

 

Not to say this can't and isn't being done, but it's no small undertaking either.

 

Michael, what's the chance that a small model in a water tunnel would give any kind of results for various underbody designs that would even point to the correct design?

 

How about putting a design on an actual car and putting a bunch of pitot-tube ports on it, drive it around and have someone record data for all the ports? I still think a better way to see if a design is good would be to build it on a Z, test carefully, and instrument the strut attachments to the body (load cells). At least you could see if you were helping or hurting the downforce/lift problem.

[Michael]

Folks,

 

Regarding the very start of this discussion – I would opine that going 200 mph in ANY car is risking your life – that is not a risk specific to the Z.

 

The main problem with car aerodynamics is that the car’s shape is not “smooth”, so it’s very hard to predict what the air flow will do. Most of our methods, from the hardcore theory down to handbook-style approximations, assume that whatever the vehicles does to the air, those changes actually differ at most modestly from the undisturbed flow. Airplanes are, as a rule, “smooth” – and so, much easier to analyze.

 

A good example is the air dam vs. smooth floor pan dilemma. The air dam is supposed to block flow, while the underbelly enhances fast, low turbulence flow. Contradictory engineering objectives, but both are ultimately after the same thing: low drag, good downforce. In most cases, an air dam can be slapped onto a generic car without any analysis, and things will improve. Not so for the underbelly – that can only work if the whole car is taken into account. The underbody venturi effect, for example, won’t work if the entrance conditions at the tunnel are bad (flow angularity, pressure loss, etc.), or if the pressure recovery at the aft end is unfavorable (flow separation).

 

To improve stability at 130 mph at the end of the 1/4 mile, the air dam is a much more practical choice. But at 200 mph – that almost automatically requires redesigning the whole car. No advice on that one…..

As Pete Paraska mentioned, there are universities that might be interested in a student project on Z aerodynamics. Keep in mind that as the aerospace industry declines and facilities are idled, stuff like car aero is a great way to keep the facilities going. For example, NASA Langley essentially “gave” their famous 30’x60’ wind tunnel to Old Dominion U, who makes good money selling wind tunnel time to NASCAR and the like. If you are in the mid-atlantic area, also check with Virginia Tech, U of Maryland (who has an excellent wind tunnel – see their web site), and Penn State. All of these places have a “public relations”-type guy associated with their aero facilities, who would be glad to chat about car aerodynamics and retail customers – or even in-house cooperation with the students.

 

Unfortunately, a bad wind tunnel is worse than a half-assed road test. It gives bad data, but makes you think that it’s good data because it’s “scientific”. Water tunnel flow visualization of a 6” wood model of a Z will give a simple picture that sort-of matches reality when gross flow features like massive separation over the rear hatch are concerned, but it is NOT a design tool. Neither is a cardboard wind tunnel driven by cooling fans.

[Drax240z]

From what I've read on CFD software, its really best used to confirm and tweak wind tunnel results, as opposed to standing alone.

 

A company called Advantage CFD is getting really close to having a setup that will work on its own though. Advantage is a Reynard offshoot. Doesn't do us a whole lot of good though, considering the computer that they test on has 16Gb of RAM. Not much chance of getting one of those to take home.

[johnc]

And the recent discussion about the Advantage CFD software in the June issue of Racecar Engineering confirms that, without a rolling ground plane, wind tunnels are inaccurate by some significant numbers when testing underbody aero.

 

More Advantage CFD testing trivia: Running your race car close to "the wall" generates drag and creates a yaw force turning the front of the car away from the wall and pulling the rear of the car towards it.

[pparaska]

That's a fortunate effect - good thing it doesn't pull the front end to the wall!

 

Thanks for posting about that mag - I should add that to my subscriptions. I've heard some good things about it. Maybe I'll replace Road and Track with it .