Poor Man's Ground Effects -- I need some input
 

The thread on ground effects, and Yellowbird RS's link to this document has got me thinking.

As some of you know, Tyson and I put a sheet of ABS plastic on the underside of my car running from the front AC condenser to the rear swaybar. When I was talking to Tyson at the GAF, he mentioned continuing the flat surface back in the way the under-the-engine piece did on 964's and 993's.

A stock 964 undertray won't fit to my 964 3.6, because I run a different tranny and exhaust. But it's a pretty straight shot, back there, and I could easily run a 3x5 sheet of aluminum from the rear swaybar back to my funny rear curved license panel, adding vertical guides on the rear as it curves up.

Obviously, I'd need to add some holes to allow air that's passing over the cylinders to continue out from the engine. But what else would I need to consider on this? I worry about trapping to much air around the headers and exhaust, although I guess they lose most of their heat out the tailpipe rather than through radiating.

Removeable side skirts might make sense, if they can be done with aluminum as well. And I'm probably going to try a bolt-on extension for the front valence to bring the splitter lower.

Is this all worth trying? For the record: I've got no access to a wind tunnel. This is strictly at the DIY level.

Here's a look at the back of my car. If you look closely, you can see the flat black panel ahead of the rear axle line.

http://forums.pelicanparts.com/uploa...1084558491.jpg

Jack,

I want to be first to say that ideas like this are traditionally run in the "Dream Tunnel", the quasi experts place of choice for ponitifications about aerodynamics.

I reckon you try it, back to back, on the track and see if it's better. In my experience, in both the wind tunnel and the dream tunnel, virtually all air management under the car will be an improvement.

Regards

Hayden

OK...just one admittedly smart-assed question.......why? While all the 964 and 993 guys are ditching their engine under-trays, you're thinking of adding one. Do a search on the 993/964 board on Rennlist for a myriad of reasons not to do it including:

-No perceptible difference in aerodynamics (my experience also with my old 964).
-Potential for excessive heat leading to premature exhaust valve roasting.
-More weight.
-Less flat 6 music.
-and on, and on, and on...

Honestly, without the benefit of a wind tunnel, I'm guessing there's as good of a chance that you will increase lift rather than reduce it. Those engineers at Ferrari, Porsche, etc., spend a lot of time in the tunnel making the tiniest tweaks and adjustments to get it to work...and they understand this stuff! Just slapping some sheeting on the underbelly and hoping for the best doesn't seem in keeping with the planning and thought processes that went into BB2 to begin with.

OK...there you go. One party-pooper has spoken.

Yes, I think you can tell if a large effect is present via track testing. You know how to do wool strips?
You'll be doing things the way the pros did them before big wind tunnels were available - inclunding PAG with the 356 and maybe the early 901s. Instead of having friends standing on freeway overpasses, you can use video cameras mounted on the car.

You could even do some smoke injection studies while on a track (I'd tell them about it first...). You'll need a way to generate smoke - TiO2 is the most common smoke used. I've set off smoke grenade fireworks in a lab before, but we needn't get into that...

I suggest using NACA duct shapes for air inlets - they will work on the bottom or sides -- no need for them to be on ext. surfaces.

The 'dream tunnel' is all they'll give me the keys for. ;)

I wish there were a way to measure it more precisely than lap times, which -- even on the same day -- can involve so many variables.

It would probably also be a good idea to test (in addition to aerodynamic changes) the differences in engine temps (cylinder head temp and oil) between both setups on a track.

Except that the forward sheet of plastic lowered my lap times at Willow Springs by about a second and a half. That's a huge improvement (especially considering the $29 price tag).

From where, and directed toward where?

Certainly can't argue with that. Are you confident that the plastic sheet is responsible? Maybe you're just getting better! ;)

Seriously, be careful to allow plenty of airflow underneath the engine to keep things from getting too hot. Another way to measure the effect might be to do some accurately timed 40-100 mph runs with and without (maybe with a G-tech)...in a safe environment...of course.

if it ain't broke............:D
jack what about making the current undertray more efficiant, lexus has been using thousands of dimples (like a reversed golf ball) to stabilize turbulence, maybe improve on what you already know works.

An easy to fab potential solution is to dam the front and sides of the car. By making the air flow around the car instead of under it, you will create lower pressure under the car adding to anti-lift. The next step would be to allow some air under the car, but manage it. That would be harder on a homebrewed test mule.

Another potential advantage of the above, might be less eddies and turbulance that will create drag, but then eddies and turbulence may have some benefit too...

The dams will increase your frontal area slightly but if you allow for some streamlining the positives may out weigh the negatives.

Just my time in the Dream Tunnel :) , but I have operated a wind tunnel with smoke and drag measuring devices...

Jack, I guess I didn't make it very clear when we talked. My plan was to put the stock 964 undertray back on, then connect the underbody tray to the engine undertray with a custom piece, or modify a 964 piece.

It won't be a difficult task. You just need to bring me the car some night.



Oh, and about the 964 undertray issues regarding heat. The real culprit there is city driving. They are fine at speed where there are NACA ducts for the trans, and carefully managed airflow openings so that the engine cools just fine at high speed. It's around town stop-and-go driving that kills the valve guides.

y guess is that Jack would simply remove it for the hotter short-tracks, and put it on at tracks like Willow Springs and Fontana.

Just looking at the silhouette of the 911, one finds the airfoil shape. What I believe is needed is something to either reverse the airfoil shape or create a partial vacuum under the car.

The partial vacuum could be helped by a very deep air dam and side skirts to keep the air from entering beneath the car as it moves forward.

Well this advice is what it's worth. Cheap.

David Duffield

Bravo, Jack!

PCA class rules do not allow us this kind of aero enhancement, but WE WISH THEY DID!!

Unfortunately, I can not imagine whether or not your front configuration blocks all air possible from under the front splitter, or encourages some flow in the middle.

Either way, to be most effective, it appears from your photo there is considerable "daylight" coming in from each side of the bottom, between the front and rear wheels. I would attempt to limit that exchange of air from beneath the sides of the car.

Your sheet aluminum idea is a good one for the diffusser. Perhaps attach it to continue your front understay. As it "progresses" rearward, only the center section continues to (or slightly past) your rear "bumper". On each side, the aluminum stops just forward of the header, leaving heads and headers to breathe.

For maximum effectiveness, the center section which continues rearward should be bounded by short "walls" which extend as low as possible to the track surface. Once the center section is rearward of the motor, it should gently angle up from 8 to 12 degrees, and terminate a couple inches behind the "bumper". The "walls", however, should grow longer, to continue parallel to the track surface.

With the car's side rails properly blocked, the net effect will be a single flat column of "dead" air under the front of the car, spreading into three "tunnels" as it evacuates from the rear. The tunnel on each side will unfortunately have lots of turbulance due to the wheel rotation and the header shape. (But not nearly as turbulant as UNMANAGED air under our cars!)

Jack, I am certain that you already know this, but for those others who may want to try, a couple of caveats are in order:

* When you add downforce, you usually sacrifice top speed. For this configuration to work properly, the car must have plenty of power available.

* Someone suggested using high-speed acceleration runs. These will only work to evaluate DRAG REDUCTION measures. In fact, if your DOWNFORCE efforts are successful, high-speed (straight line) runs will be less dramatic!

* A flat bottom or diffusser configuration is much more sensitive to ride height and rake. Once the diffuser is dialed in, one must be VERY CAREFUL when increasing ride height, like for rain, or for bumpy tracks. The front end can become very "light"!

Keep us posted on your findings.
Ed LoPresti
RacePro Engineering

Jack - you were taling about heat issues - you would use NACA ducts to provide more air if you run into that. They might also help in getting air out from under the vehicle.

How far do you want to go with this intellectually speaking? I think a lot of the air flow mgmt issues are susceptible to "conceptual analysis" and some crude measurements. You might for example, want to read Steve Vogel's book which is very accessible (little math). I posted the cite a week or 2 ago if you wnat to search.

You could make some simple Bourdon tube manometers and use those to measure the pressure in various spots. There are lots of things an inventive mind can do for cheap in this area. ... You could even write a book about your experiences...

What are you trying to do? Reduce drag, or increase downforce?

I’m going to guess that the scale of lap time reduction you noted by adding the ABS was not due primarily to reducing drag or increasing downforce, but instead was due to changing the location of the center of pressure. Moving the center of pressure forwards of backwards is going to affect understeer or oversteer at high speeds.

I’ve only played with practical aero a little bit, but I did quickly realize that moving the center of pressure has dramatic effects- move it significantly forwards of the CG and the car will be nearly un-drivable at speed (as me how I know).

I think if you’re going to try and find a bunch more speed from the underbody you won’t find it by reducing drag- you’ll need to create downforce. When you do, I’d try to balance the underbody downforce independent of the upper body downforce, ie wings and splitter.

What you don’t want to have is the underbody downforce moving the total center of pressure around dramatically as it’s affected by the ride height (bumps, speed). You basically want both the center of pressure from the wings and the underbody a little ways behind the CG.

So in my “dream tunnel” I’d give the car some rake, but keep most of the floor flat or even give it a small reverse rake. This flat floor would end foot or so behind the center of gravity, were it would connect to a diffuser (aluminum or whatever) angled up below the motor at the steepest rake allowable given all the stuff in the way. I’d keep the side skirts small- you don’t want them getting too close to the ground when the ride height changes, as this will make the car all the more sensitive to height changes (though it would increase downforce if you got it right). Basically the center of pressure of this setup is going to be just in front of where the diffuser starts. Since the car is sensitive to this I think I’d start out by trying to make this location (ie the length of the diffuser) adjustable so that I could tune the downforce front to rear. Like I said I think this will make more of a difference that the total downforce. In theory (and no I have not tried this) this setup could give you both reduced drag and a decent amount of correctly placed downforce at speed. Making it all work wouldn’t be too easy, though...

I would not try to make too much downforce. If you did the rest of your suspension setup (springs, etc) would need to change to match.

I love these kinds of posts. I've been thinking about this since the last one a couple of weeks ago.

I was trying ot install a new muffler with R type wide spread outlets Fri. night before the GAF. (It didn't fit). So, the car sits with only the HE's looking out from under there.

Now, I'm thinking, w/o the HE's and no licence panel, THERE'S A HUGE FREAKING TUNNEL (well, two, one on each side) UNDER THERE!!

It didn't help one bit to see some Carol Shelby recent Cobra stuff on TV the other day with the little cosmetic diffuser on the back.

But think about it. You just have to completely rework the standard notion of exhaust routing and ESPECIALLY the muffler.

As a little side note, I ran a piece around behind my seat on my kart that was connected to and on plane with my side pods. I did it just because I do silly things when I have time if I think it looks cool.

Well, I got about 4 laps into practice when my backside got awfully warm. I pulled in, drilled out some rivets, popped that thing off and all was well again. This dodad wasn't more than 4 " of extension on an aerodynamic pig. It just exacerbated the already and only calm place on the whole rig.

EDIT: I guess I ought to note that on a kart, the expansion chamber swings right behind your head and across the back at the rear.

Don't worry, Jack is going to remove the heat exchangers right after he tosses out the A/C...

OK....sanity check.....

For grounds effect to work ( think about this)....you need to get a Bernouli effect by changing the cross-section of the tunnel as the air flows underneath the car. For *that* to work ( and as found out by the pioneering race cars), you need a combination of :
- stable platform....I'm talking serious spring rates that don't allow any roll and pitch.
- side skirts and undertray venturi tunnels that hug the ground closely. If you got 3-4" ground clearance...I can't see how all this can work.

The successful racing and formula cars that use this principle ended up having essenttially "no" ground clearance....and spring rates measured in the thousands pounds per inch.....

How do we make this system work for a more "normal" car....is the core question....IMHO......

Wil

Wouldn't the panel under the car need to be shaped like an inverted wing, for it to actually suck the car down. Allowing air both under and over it but at different speeds of flow?

I just read the article that Jack reff'd to... Good reading (specially for a simpleton like me :D)

Basically, it states that any smoothing of the bottom of the car will reduce drag and will provide some downforce (Hayden also already mentioned this)
So if it works for cars ranging from a street Audi A4 to a track NSX, why wouldn't it be an improvement for ours?

If you change the diagonally positioned muffler for 2 seperate units that mount lenghtwise, you would have a good clearing to add a venturi/diffusor

My guess is that Ed is right. that is, Ed has information that you're on a path to learn by experimentation. The tricks you are considering are known tricks. And the smooth underbelly idea is good, but the air dam idea is better, with skirting on the sides to maximize the negative pressure under the car. The good news is that we know what maximizes a track car's aerodynamicity (did I just invent a new word?). The bad news is that you must make your car look like a brick to achieve it. Your car is much more beautiful with the "fuselage" sides. The brick shape is different and ugly. In my humble opinion.

Board: A diffuser allows the air traveling underneath the car a place to expand and decelerate back to road speed as well as providing wake infill. As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this "high velocity" air back to normal velocity and also helps fill in the area behind the race car making the whole underbody a more efficient downforce producing device by reducing drag and increasing downforce.

Understand that a true flat bottomed car (one without a diffuser) will produce downforce in and of itself when run in rake. Essentially the entire flat bottom becomes one large diffuser. It too has two suction peaks, one upon entrance, the second at the trailing edge of the flat undertray. A diffuser acts to enhance this underside suction, it acts like a pump, encouraging better flow under the car.

One thing to note is that the rear wing interacts with the diffuser "driving" it. The proximity of the low pressure side of the rear wing encourages better flow through for the underbody
http://forums.pelicanparts.com/uploa...1095175090.jpg

Yellowbird:
OK....so the platform has to be incredibly stable for this to work ( 2000 lb/in springs or more !) ....no pitch and no roll allowed....and certainly no 4" ground clearance...more like 1/2".

All things you said are true...but I take exception that a grounds effect car reduces drag...if the car is sucked-down onto the road...."drag" will increase....

--Wil

To motor racing engineers, wing might be a good solution to lift, but still far away from what they really want. A typical formula one racing car corners at around 4g lateral acceleration, that requires substantial downforce to keep the tyres firmly on track. Install a huge wing with high angle can satisfy this requirement, but also deteriorates the drag coefficient.
In the 70s, Collin Chapman (again) invented a completely new concept to provide downforce without altering drag - Ground Effect. He incorporated an air channel into the bottom of his Lotus 72 racer. The channel is relatively narrow in front and expand towards the tail. Since the bottom is nearly touching the ground, the combination of channel and ground forms virtually a closed tunnel. When the car is running, air enters the tunnel in the nose and then expands linearly towards the tail. Apparently, air pressure is reducing towards the tail so that downforce will be generated.
Ground Effect is so superior than wing that it was soon banned in Formual One. In 1978, Brabham's Gordon Murray tried again with different means - instead of expansion channel, he used a powerful fan to create low pressure near the tail. Of course FIA banned it again.

Ground effect is not too suitable for road cars. It requires the bottom to be very close to the ground to form a closed tunnel. For racing car, this is no problem. But road cars should have much higher ground clearance to suit different rough roads, up hill and down hill etc. This greatly reduce the effectiveness of Ground Effect. McLaren F1 road car followed Brabham's trick by using 2 electric fans to create ground effect, but honestly speaking, no tester had ever praised its down force. Dauer 962, a so-called "road car" but it is actually a road-legal Porsche 962 endurance racing car, use conventional air-channel ground effect as the race car. Adjustable ride height allow it to run in rough road (slowly) and make good use of Ground Effect in Germany's Autobahn. Nevertheless, it can barely generate 40% downforce of the racing car
http://forums.pelicanparts.com/uploa...1095176286.jpg

undertray
 

We can also reduce the influence of underside airflow by covering the car's bottom by a smooth undertray, as shown in this Ferrari F355. This avoid turbulence and lift.
http://forums.pelicanparts.com/uploa...1095176420.jpg

Jack,

Good thread, bringing all sorts of opinions forward.

I am sure you and Tyson are sufficiently independently minded to run the idea to conclusion, however you choose to do it.

Even complex solution begin with a single thought - here is an example of one of mine, from another lifetime, when I had significant resources at my disposal, but it did grow from a single thought and a bunch of unanswered questions.

http://www.mulsannescorner.com/allardj2x.html

One other test procedure that you might consider - a car like yours is rpm limited at Willow Springs, perhaps try the experiment on a car with less h.p., one where Vmax will be aero driven, drag-v-hp.

The day we were there in Jan, my car would pull the limiter into T8, and about 800 rpm less into T1, despite being in 5th since well before the pit in. A reduction in drag will be far more obvious on the top speed of a car that is drag limited. The influence in that case was the wind - a variable I know, but with about 150 - 170 hp the test should show some results.

You could then have some expectation of the same parts installed on your own car, plus end up with discrete 2 data points, a luxury many experiments never enjoy!

Alternatively reduce your rpm limit (electronically) until your Vmax is drag limited and perform a back to back with the parts.

However you do it, perservere and enjoy the process, you will for sure learn something, either to use or avoid.

Regards

Hayden

I say try it. What have you got to lose? Some cool fab time? You already have a lipstick cam, right? Put it under the rear and observe how the wool strips flutter w/ and w/o the contraption. Smooth *upturned* strips will be your friend. Keep us posted and good luck.

Counter-intuitive, perhaps...in light of what I'm saying here...but look at the Ford GT web page or similar data...and notice that this car uses added-on diffusers at the very rear of the car.

If you can overcome the blocking effect these may have on the exit cooling air from the fan blowing air over the fins....you may have something here......

---Wil

i've posted this before on this subject:

here is a well respected book on race car aerodynamics.

<A HREF="http://www.amazon.com/exec/obidos/ASIN/0837601428/piombocom-20"><IMG SRC="http://images.amazon.com/images/P/0837601428.01.MZZZZZZZ.jpg" border="0" alt="cover" hspace="3" vspace="3"></A>

i believe ground effects were banned in f1 due to the danger of the sliding side skirts losing their seal with the ground. this would cause an immediate loss of downforce from ground effect!

The more posts, the more mis-information.... I don't know where to start, but one thing I can say is that any smoothing of the underbody will help certain figures of merit at least somewhat.

Jack -

Please post these pics of the front of your car:

Head on
Head on and low
Underneath

---------------------------------------------------------------------


Having spent time at a wind tunnel (Boeing, 747, 777) and working in the aerospace field (airplanes) as an engineer, I can weigh in on this for you --

I can virtually guarantee you will accomplish but one thing by using sheeting - reduction of turbulence in the front and middle portions of the underbody. This is not a bad thing, just don't expect measurable results.

*Your main goal must be this - reduce the amount of air going UNDER the body.* This is critical.

Reducing turbulence in the rear will be a hard trick; creating downforce will be nearly impossible. You just do not have room for tunnels which you would need -- these increase in height towards the rear to promote that all desirable 'suction' gives you the mechanical grip.

On a 911 we have the additional 'problem' of air cooling - air is coming DOWN off the engine, effectively trashing many good ideas at reducing lift and turbulence in the rear.

So, I'd like to see those 3 pictures:

Head on
Head on and low
Underneath


and we can go from there.
We can come up with something that truly works!:p

What doubly stinks is that a rear diffuser would be nearly impossible due to the exhaust location - bummer!

your solution sir...
http://forums.pelicanparts.com/uploa...1095182910.jpg
;)

I know from the 964 that I had - the bottom was virtually smooth all the way back. I would try and modify the underbody panels to try and fit as much as possible. And the engine tray that I had on mine had two small NACA ducts. My car had no problem keeping cool at speed - but stop and go was a whole different issue!

Great information, here. Thanks.

To reiterate, this is a no-resources project, pretty much. I'm looking to see what a couple of trips to the hardware store could do to improve my laptimes.

For our baseline, here's what I've got now.

Head-on, 16 inches up:

http://forums.pelicanparts.com/uploa...1095186469.jpg

Head-on, ground level:

http://forums.pelicanparts.com/uploa...1095186525.jpg

Here's as good a picture as I can manage of the belly-pan, right now. I added approximate measurements of ground clearance at the beginning of the sheet, the front axle line, and the rear axle line. (Keep in mind, my belly pan has an AC condenser that lowers -- and flattens, somewhat -- a normal 911's profile.)

http://forums.pelicanparts.com/uploa...1095186595.jpg

In terms of blocking air in front, I have one of these, that I could modify to fit my bumper. Removeable aluminum sideskirts are also a possibility.

http://forums.pelicanparts.com/uploa...1095187268.jpg

From what I'm reading here, it seems that I could go in two directions, initially. One would be to continue my flattening of the underbelly with a little (maybe-helpful-maybe-not) diffuser in the back. The other would be to drop down my front splittler and do some skirts to control air coming in (and going out) on the sides.

My guess is that the splitter-and-skirts solution would do more for me, but would increase my frontal area and slow me down somewhat in a straight line?

More specifics. Here's a look at the rear of the car, with the pieces made more visible.

http://forums.pelicanparts.com/uploa...1095189841.jpg

And here are some measurments, in a (not-to-scale) drawing.

http://forums.pelicanparts.com/uploa...1095189904.jpg

Jack, I think this is a great idea. Maybe use your DL-90 with RPM input to record RPM's/GPS speed at a known point on one of the tracks you run. There's a new Beta version of Trackvision out, you could couple the output of the DL-90 with the video feed from a low-light lipstick cam mounted under the car with some yarn tufts. This would allow you to visualize the flow as you move around the track.

By the way, before you fellas cut a NACA duct anywhere on your car, there's an interesting article by Peter Garrison in the recent issue of FLYING magazine that basically says that "submerged ducts" were NOT recommended by the original guys at NACA for oil coolers, carburetor air intakes, or anything requiring a high velocity airstream.

These are the original references: Charles W. Frick, Wallace F. Davis, Lauros M. Randall and Emmet A. Mossman, An Experimental Investigation of NACA Submerged-Duct Entrances, NACA ACR-5I20, November, 1945;

Emmet A. Mossman and Lauros M. Randall, An experimental investigation of the design variables for NACA submerged duct entrances, NACA RM-A7I30, January, 1948;

Alvin H. Sacks and John R. Spreiter, Theoretical investigation of submerged inlets at low speeds, NACA TN-2323, August, 1951, pp. 49. http://naca.larc.nasa.gov/reports/1951/naca-tn-2323/

Excellent information provided, Jack - here is my answer.

#1 priority is to *reduce* the ground clearance of the front end, ie
the splitter you have needs to be 'lower to the ground' by about at least 1 inch or ideally (but not realistically for street use) 2 inches.

You must reduce the air getting under the car - I see some turbulence inducing elements that are not taken care of - ie the A/C nerf bar with torsion attach points.

http://forums.pelicanparts.com/uploads3/Underbelly1095186595.jpg

Fantastic job of smoothing the undertray! Well done.
I propose that nothing else should or could be done in the rear - you are left with opposing forces in this area, ie:

1) Air flow over & through engine for cooling vs. airflow mgmt under engine. These are at odds w/each other - what to do?

2) No room for a diffuser - and a diffuser just cannot work in this above mentioned (1) environment.

Bring that front lip/splitter down in such a way so that it is the lowest point of the front 1/3 of the vehicle. Keep the air from getting under the car - then you have optimized your 911 and will have one hell of a great setup - the best you can do. You are already 80% of the way to this point -

Nice to have those dims to play with. So if I were going to try something I would not try the blocking all air route- the downforce you get from this is very sensitive to ride height, and I believe you would need stiffer springs to make this work. It is very effective at making downforce, but I've found it can be too effective- in my case the car would suck the front down and take the travel out of the suspension (increasing downforce as it did and moving the center of pressure forwards- the car was very scary). Thus I think you’d want to make a number of other changes to make this work right.

Looking at your car I would change the height of the belly pan about a foot behind your jack point to about 3” from the current 4” or 4.25”, probably with an aluminum plate. I’d then connect a second aluminum plate to this (the diffuser) going up at the max angle you can get away with- you’ll probably basically touch both the heat exchanger and the heat pipe, and end up with about 5 degrees. I’d put about 1.5” side skirts, bugger in the diffuser.

I believe this setup would create real downforce right where you want it (just behind the CG) without making the car un-drivable as the ride height changes. It would be banned under most modern rules, as they often don’t let the diffuser start forwards of the rear axle. Maybe that’s an indication that it’s probably fast?

Perhaps Hayden or someone else that’s tried this could weigh in with an opinion on this, but like I said I’ve tried the basic low splitter/ diffuser approach others are advocating, and I think that would require you to stiffen the suspension up in order to insure the car doesn’t get scary as the ride height changes. I’d clean up the entrance to the underbody at the front while I was at it, btw.