Window Slats

[Stratos]

This thread got lost when the database crashed last night.

I believe Steve Ramm had justed posted some information about the aerodynamics, or the lack of aerodynamics, of the rear louvres.

David May and Scott McGill had posted about the aerodynamics of the rear louvres.

Guy Mayers had posted some useful information about fitting the louvres.

It might be helpful if these members could repost their original information.

[chris.richard]

I'm sure I've read about mounting a fan above the engine under the louvres to increase the extraction. (Make it bigger and get ground effect?):D

[Steve_R]

It must have been my long winded reply that crashed the system.

Scott had it right, the pressure difference between the louvres and the underside of the car causes the air to flow upwards. There's also a bit of convection as well to help out although that would be more apparent when the car is at rest. One thing that screws up the venting would be the rear spoiler. The spoiler generates a large high pressure region in front of it and would reduce the pressure differential and therefore reduce the flow. Not much that you can do about that though. A belly pan under the engine compartment with some forward facing scoops would certainly help and reduce drag at the same time. Sizing the scoops, etc. would be trial and error unless you could find out the local air speed under the car as well as the velocity of the air coming out of the louvres. It could be measured with a pitot rake, but that's getting into science experiment territory. If anyone is willing, I'd be glad to help out. I'd volunteer, but I moved across the country and my kit didn't.

On the other hand, I have access to a 16 cpu Linux cluster with CFD software, so if anyone has access to decent CAD data for the stratos, I'd be glad to build a model and run it. One thing that I'd be interested in is the aerodynamic downforce split (front/rear) and the effectiveness of front spoilers.

-Steve

[Stratos]

Steve,

If I didn't have the slots in the rear louvres, do you think I'd loose much cooling effect?

Would having round holes in the rear boot section provide as much, or more, airflow as the holes in the rear louvres?

[Steve_R]

Yeah, I think you'd probably have some cooling problems without the slots. Are you thinking of cutting out the boot all together? If you cut holes in the rear and open up the boot to the engine compartment the cooling will certainly be better than the louvres. A little bonus is a bit of drag reduction.

The Diablo and Murcielago have their radiator exhaust ducts exit in the rear into the car's wake. I'm not sure how optomised those are (probably not very), but on race cars (especially prototypes) the exit duct is placed just above the diffuser and sized so the velocity of the air exiting energizes the air exiting the diffuser allowing a more radical diffuser angle without risking flow separation ... and more downforce.

I always thought it was interesting that the Stratos' rear is something like a diffuser, except that the muffler and frame get in the way of making it work.

-Steve

[Stratos]

Originally posted by Steve_R
Yeah, I think you'd probably have some cooling problems without the slots. Are you thinking of cutting out the boot all together? If you cut holes in the rear and open up the boot to the engine compartment the cooling will certainly be better than the louvres. A little bonus is a bit of drag reduction.

-Steve

So, for a competition car, which would not store anything in the boot area (trunk space ;) ), it would be beneficial to:-

Have the louvres without holes

Remove the rear panel between the boot space and the engine

Cut holes in the rear bodywork, as shown in this picture of the Marlboro Stratos.


Yes?

[Steve_R]

Have the louvres without holes
I would get rid of the louvres and replace them with a perspex panel. The smooth panel will cut down on the turbulence.

Remove the rear panel between the boot space and the engine
Yes. If this is a tarmac rally or track car you could benefit from moving the silencer up a bit and constructing a diffuser in the rear. Diffuser's work much better at low ride heights so won't be as effective at gravel stage ride heights.

Cut holes in the rear bodywork, as shown in this picture of the Marlboro Stratos.
Most definitely. Removing the spoiler and replacing it with a wing would help performance as well. Spoilers aren't very efficient. But I personally think the Stratos just looks wrong without the spoiler.

Hope this helps.

-Steve

[mogul_x]

Originally posted by chris.richard
I'm sure I've read about mounting a fan above the engine under the louvres to increase the extraction. (Make it bigger and get ground effect?

Technically speaking, I'm not sure if this qualifies as "ground effect" - sounds a bit more like "sucker car". Besides, you'd need a really big fan!

As an aside, I heard that the Chaparral 2J is supposed to be at Goodwood this year. That car used a 55 horsepower auxiliary fan to draw air from beneath the car, which was sealed to the ground via a fairly complex system of lexan skirts. On the plus side, it developed something like 2000 pounds of downforce standing still.

[guy mayers]

Just to reiterate the previous (lost) posting! And thanks for the reminder!
Don't force the louvre panel up into its' aperture on the rear body panel. The return edge still gets bolted/screwed to the underside of the top section but there should be a gap at the top under the roof spoiler. It can be forced right in but as soon as the louvres get hot the will warp really badly. The trick is to make sure that they are put in so that the securing bolts pull the sides of the louvres into contact with the rear body panel, putting them under a little tension. No matter how hot they get they won't warp.
Guy

[Stratos]

Originally posted by Steve_R

If this is a tarmac rally or track car you could benefit from moving the silencer up a bit and constructing a diffuser in the rear. Diffuser's work much better at low ride heights so won't be as effective at gravel stage ride heights.


-Steve

Do you mean a diffuser on the silencer?

AND, sorry but I'm going to show my ignorance here, what is a diffuser?

[Steve_R]

Here's a little background on underbody aerodynamics (ground effects). A simple underbody resembles an upside-down airplane wing, or basically one half of a venturi. So, the front of the car becomes a nozzle that's used to accelerate the air, a throat and a diffuser. The throat is the flat (or near-flat) area under the car and the angled part at the rear is the diffuser. The diffuser returns the air to near free-stream. The idea is to accelerate the air which causes a drop in pressure. The pressure drop usually isn't very large, but consider that a decrease in pressure of 1 psi acting over the entire throat (say 24 sq. ft.) would equal 3456 lbs. of downforce. Now, you won't get 1 psi of pressure drop unless you run side skirts, etc. but you can get significant downforce without skirts.

One of the keys to efficient underbody aerodynamics is a smooth underbody, which is why I suggested that the silencer could be moved upwards to allow room for the diffuser.

I hope all of this makes sense ... here's a website that has diffusers for Honda/Acura NSX's. (http://www.scienceofspeed.com/products/exterior_performance_products/NSX/TAITEC/JGTC_diffuser/) Of course, there are a bunch of little tricks that you can do to make diffusers work better, and the longer and flatter a throat is the more pitch sensitive the car will be. By that, I mean that the car will gain or lose downforce as the car pitches and the center of pressure will move fore and aft. This isn't a good thing and a lot of wind tunnel testing goes into trying to reduce pitch sensitivity.

If you want to see what happens when aerodynamicists go wild with fairly relaxed rules, check out the DTM cars. To keep things in perspective, even the current WRC cars have smooth undersides and diffusers.

-Steve

[Stratos]

Steve,

Many thanks for that info and the link. I understand your previous posts much more now.

[chris.richard]

Diffusers and the like sound as if they need proper aerodynamic design: not the sort of thing that I could knock up with a sheet of ally and a hammer? Or is sheeting the underside potentially useful? The Hawk's cabin are is flat sheet already, is there useful gain in sheeting the front area?

[Stratos]

I used a sheet of Kaylan as a guard at the front underside of my car.

This means I have a flat undretray all the way from the front of the car to the start of the engine compartment.

[CorseChris]

There was some work done in a wind tunnel a fews years back with a Marcos Mantula. The upshot was that due to the relatively flat bottom and the wedge-shaped seat recesses in the floor, a small change in front/rear ride heights (leading to a change in the angle of attack of the car) changed the downforce (well, lack of lift really) quite considerably. So much so that if one got it wrong (front high), the car went VERY light at the front at speed.

I too have panelled the front of my car (to keep the crud out mainly) so will be careful to make sure that it runs a bit low fornt/high rear to stop it going even lighter at the front.

All empirical of course.......


...and I'll get me coat.

;)

[chris.richard]

What's Kaylan?

[Stratos]

It's a form of plastic. Very Strong and very light. The WRC works teams use it it protect their suspensions and underfloor.

I've run a section of it, the width of the front chassis from the front of the car all the way along the underside, until just before the engine compartment. This means that the whole of my front compartment, plus most of the underside of the car, including the pipes running through the tunnel, are all well protected from bumps, stones, rocks, etc.

Sounds from some of the above comments that I also made the car more aerodynamic in the process.

It cost about £35 for a 3mm sheet of about 4m x 2m. I've still got loads left over from my sheet.

Their website is www.kay-dee.co.uk

[Steve_R]

Glad I was of some help. Just about every late model Ferrari and Lamborghini that I've seen has had a flat underbody and the F355 are actually designed to produce reasonable downforce. The key as Chris pointed out is to make sure the front doesn't get higher than the rear. Something that Mercedes found out the hard way at LeMans. Even with tons of cash, wind tunnel testing and CFD you can still screw up really badly.

-Steve

[David May]

I hate to spoil things, but I'm afraid that a big flat floor pan has to be just that. The Startos with its raised nose and no splitters just does not lend itself to the correct front attack. Its got to have the aerodynamics of a boat, will all the lift that comes with it!
I have a feeling that mine is even worse at high speeds with the front underbody guard fitted.

Dave May

[Steve_R]

I always wondered why they always have such a nose-up attitude. Are there any downsides to running it nose-down? I figured that it was to avoid bottoming out on non-tarmac events, but I don't really have to worry about that.

-Steve

[David May]

I think thats just it. The tarmac spec cars were quite low and even used splitters, but rally cars could not afford to scrape thier radiators.
I set my front suspension low for tarmac events, but the whole shape of the nose profile means that it can never be really low-lift aerodynamic.

Dave May

[Steve_R]

Yep, it's the '60s-70's Gandini front end. The miura, et al had the same deal. I read an account by Bob Wallace about driving the Miura at speed. He said that he got it up around 180mph and there was so much front end lift that you could turn the steering wheel and still go straight. Time for a front spoiler.

-Steve

[Jerry B]

Can anyone supply aluminium window louvers, as I`m going to need some and they might not burn as well as GRP

[John]

What effect would opening up the rear spoiler have? I mean the area under the profile of the spoiler above the rear hood surface- making this a little like the roof spoiler. Not a good explanation this but I've just had a bootle of Valpolicella!
Any ideas?

John

[guy mayers]

Hi, I know Geoff Turton has made at least one set in aluminium but they were expensive! I am hoping to see him tomorrow to get my ss exhaust repaired so I will ask him for a rough price.........
think of a number and quadruple it!
Guy

[guy mayers]

Ok, I saw Geoff today, a rough price for a one off would be £500 plus VAT................ You may be able to negotiate a discount for cash or volume and it does save all that fiddly cutting, shaping, fitting and sanding! Call Geoff direct on 01514203420.
Guy

[Jeff Davison]

If we can do it, I'm in for one!
Me so like ally, me no so like fibre.


Jeff Davison

[Steve_R]

John - I'm not sure I know what you mean by "opening up the area under the profile of the rear spoiler". Replacing the spoiler with a wing would be much more effective, but IMHO the spoiler looks right one the Stratos.

-Steve

[chris.richard]

Originally posted by Steve_R

I would get rid of the louvres and replace them with a perspex panel. The smooth panel will cut down on the turbulence.


Mine was originally built without the louvres. It looks quite good that way - this is how a perspex panel would look.

[chris.richard]

Here's one solution

[JohnB_SPY8808053]

I think that they raised the louvers on the vs Olio Fiat car to get around the big rally air-box fitted to that car. At least that was my inpression from the other pictures of the car. You can see in this picture that the front edge of the lowest louver is notched to fit around the air box. You can also see that the 2nd louver is being pushed up in the middle by it. If they went all the way down, the air box would cut the second louver in half.

It's not a bad idea though, might help ventilate the engine better.

John B.

[rutthenut]

This same trick of raising the louvre panels is also apparent on the Rothmans-livered car of Ken Tomblin.

[chris.richard]

http://www.carclassic.com/html/DG67.htm

[chris.richard]

What Guy was warning about, I presume.

[guy mayers]

That's exactly what happens when they are compressed and heated. Not pretty. I see Jeff is the only taker for a set of ally louvres so far.
Jeff, do you want me to approach Geoff on your behalf, it may be an idea to get a shipping quote first! If you want to call him direct try 00441514203420 from there. It's his workshop direct line so if he don't answer he ain't there! And you won't be waking him!
Guy