Over and Under or around the sides

[greenpower dude reloaded]

Right, I did a couple of runs over the weekend with a slightly different profile (pics to follow) and I got the lift down to around 0.05 great! but the Cd shot up to 0.45 darn - the car seems to have a suprising amount of blue on it.

Although you'll see this from the pics later, I'll describe the changes. In plan view it has a slightly wider nose (as per this years car) slightly narrower body overall much more gradual tail at the rear and a small "diffuser". I'll try and run Breezy's suggestion over the next few days

I'm going to see if I can round the edges off over the next couple of days, I suspect that'll make a big difference. Not sure on whether it'll make a positive difference or not but it will certainly make one. testing earlier models that took us down from about Cd 0.28 to Cd 0.17 so I'm hopeful. It was also a very similar design but we're suffering from separation at the rear of the vehicle. This more gradual tail should cure that.

[flynfrog]

http://windtunnel.brawngp.com/gallery.asp

check out the green blob

[greenpower dude reloaded]

Wow!!! I was going to email them earlier but decided it best to get on with some (paid) work I had a quick browse of that site but han't looked at the gallery heres the 2nd run

this looks a lot worse than i realised. The front really looks flat. But I can assure you it's not THAT bad.

The side profile is actually vastly improved. The plan is to develop this carefully over the next month or so. The corners are all due to be rounded off which i'm sure will make a difference but we'll see...

I'm basically trying to make a Low, 4 wheel, open cockpit of this...

PAC CARII the car that that green blob is based on.

[flynfrog]

looking at your wheel base you might want to check you Center of pressure vs center of mass.

also I am guessing to get as curvy as the pac car you will need a new chassis.

http://inventors.about.com/library/inve ... ontrol.htm

article on stability

[flynfrog]

Curving the bottom a little more might help with the lift/drag problem

[dumrick]

Have you tried that basic configuration but, behind the driver, with flat top and low surfaces on it, ending on a fin?
You basically want to generate as little wake as possible and, in the tested configuration there may be some interference from flow coming from top and bottom and the lateral flow. Also, some of the lift might be originating on the drooping down of the tail, being a low pressure area (generating wake).
Concerning aero stability, you must evaluate if the racing speeds involved really make it an issue.

[greenpower dude reloaded]

Yes, I have had quite a detailed look in to possible design options and having seen enough high cars roll over I think it'll be best to create this half way house.

There are currently 3 main options used by teams to making front of the grid cars.

1 Wheels out. Tiny frontal area but more draggy shape. Wheels out has never attracted me. Doesn't seem as realiable as wheels in in an endurance race. Option of banging wheels an bending critical parts, less protection for the driver. etc etc. Hasn't won a major competition for almost as long as I can remember, interestingly most of the field are wheels out.

2 Wheels In and low to the ground, Massive FA, no escaping that. Great for development, lots of space to try new things etc. Can get an excellent shape which balances the FA out.

3 High up, wheels in. anything up to 200mm gound clearance. At the end of the day you are putting kids in these cars, and there is no two ways about it rolling over is one of the most dangerous things that could happen. So we will not be persuing this route. there are many other actual advatanges to this design I'm just not prepared to persue it. If I were driving it'd be a different matter.

What I'm proposing is sort of a 2.5 half the ground clearance of most high cars (75mm) but a more natural shape.

I think I may have cracked the problem with the car! (or at least what caused this step backwards between runs. I bet it's that the vertical sides are now dead flat. They used to have a bit of a curvature to them, and now they don't so much lower pressure on the sides! it all makes sense now, plus the nose doesn't work with flat sides in this rendering.

Geuss what i'll be running tonight! I suspect that the shape in plan view is a lot more important than i've been thinking recently. I suspect this side profile is actually a much better shape. I'll have to lay some proper NACA profiles down and trace them tonight to try and get a nice plan shape.

Maybe this topic should have really been Over and under and around the sides?

To get something truly in the vein of PAC CAR yes it would be a new chassis and i'm not ruling that out but i am trying to avoid that. Once the edges are rounded off and this shape is refined, It'll look 100 times better. It's just so tough to
imagine it ever looking like a fast shape with the shape of it at the moment.

[BreezyRacer]

It looks to me like you've sacrificed the front (nose) to be able to taper the back completely. Keep in mind that your drag numbers (all aero behavior actually) starts at the nose. So I would shorten the rear overhang some so I could taper the nose more completely. Refine the front and then work backwards.

Then we get into a philosophical direction of raising the car to allow and manage airflow under the car to reduce drag (such as the PAC CAR). This is an ambitious goal from a fabrication standpoint (BTW did you know you purchase the CAD for PAC CAR body for 100 euros) and I wouldn't go in that direction without some consideration of the venues you race and the cornering demands for your car, along the personal resources you have to work with. Raising the center of gravity substantially (required to get enough airflow under the car) could seriously affect the cornering ability, to say nothing of the increasing complications of fabrication. While I couldn't find the top speed for PAC CAR I would bet that it's well below the speeds you attain. I believe that the amount you've raised this design has contributed to drag rather than reduced it. If you have the time it would be interesting to see the drag numbers if you dropped the car to 20 mm or so ride height and ran this test again, just to see how that affects drag numbers and airflow under the car. If my hunch is right I imagine the drag will drop. IMO, you would have to raise it quite a bit and sculpt the floor quite a bit to get the drag to go down by letting airflow under the car.

Note that your drag is primarily under the nose of the current design, not above the nose. That is a lift creator the way it is, no matter how the numbers turn out for the overall design.

As with the PAC CAR, you should make every effort to minimize frontal area. For instance they cambered the front wheels so they could reduce the frontal area .. a smart idea that you can easily borrow IMO, providing that track is measured from the axle line, or better yet the tire contact patch.

Also your roll hoop looks much higher than it needs to be. Is there a min height requirement or something?

Just some random thoughts for the moment. But deciding the raise the car to let airflow under the car and manage the underflow to reduce drag is something you need to analyze carefully. Once you make that decision there is no going back, either way.

Edit: I had not seen your last post before throwing this one up. Good to hear that you've gone away from the high design idea.

[greenpower dude reloaded]

wow, lots to consider there breezy, thanks

Your right, the nose has been sacrificied and it's honestly not quite as bad as it looks Paraview shows this simulation in perspective view rather than how a normal cad package shows it up.

I will take note, however and re-evaluate it this eve. I had noticed the high pressure on the underside. That's something I already know I want to try and get away from.

I'll try lowering but, it does shave off 0.028 off the frontal area of 0.349m2. But it'll be a good trade off to see whether it'll benefit us or not. There is no space really above the tops of the wheels so it's not as though the whole thing can just shift. I've got the book on Pac CarII, incredibly informative.

It features a graph that shows the benefit of every cm you raise the car. There are two ways to tackle it all, either stop air travelling underneath OR allow it to flow freely. There is a minimum clearance of 40mm which dictates you can't discourage it from travelling underneath with skirts or w/e.

They don't typically go any higher than 30mph, we don't typically go below 30mph which is my reasoning really for the half way design.

I've thought about cambering the wheels but there isn't much benefit as the rear wheels can't be cambered as well. So you can't really knock off much/any FA. It may enable a greater curve on the top edge.

Roll hoop could (according to the scrutineers) do with being a touch higher but is a really integral part of the chassis, so it's unlikely to change. A shame really be cause I would certainly make it a different shape if I were making a new chassis.

EDIT: just read your edit, it's not going to be skimming along the ground but it will be no higher than 75mm it's currently 60 in 2009 race trim

[BreezyRacer]

Are you saying that the rules stipulate 40mm minimum ground clearance? If so that's where I would be. But to know that that for sure, just drop this identical model down to that height and see how/if it affects your numbers. That would be a good thing to know and wouldn't require much effort to do. I'm lazy and gravitate towards low effort experiments.

If 40 mm is the min, can you go lower to shield around the tires or is that part of the same restriction?

[greenpower dude reloaded]

"Ground clearance must not be less than 40mm on all post‐2006 vehicles."

I think there is potential there for some cleverly designed wheel coverings but, they would be difficult to execute I think.

I'll have a go at dropping it down but it will need remoddling because otherwise the wheels will stick out the top ever so slightly.

It's all worth a try! what I really need is some sort of magic software that takes everything you have to cover and just works out the most efficient shape!

[Ciro Pabón]

I don't know, ground effect wings have less drag when they fly at the height of one half of its wingspan.

I deduce, using my awesome mathematical abilities, that you need 50 cm of ground clearance for a 1 m wide car, so, no wonder stability becomes a problem when you try to minimize drag through floor height.

If there is a relationship between height and drag, I wonder if length-wise channels grooved on the underbody would minimize drag, even if they're not too deep. You know, like corrugated aluminium roofing.

This would also help also to have a more rigid structure: deeper channels would give you a stronger floor, helping with your suspension behaviour (and maybe with your "chain derailed" problems).

About the sides, would any "extra-plates" attached to the sides help something? Like winglets, maybe? Can you use them per regulations? I think they were used in F1 to minimize induced drag, so, what about two, one at each side?

I believe winglets work only if they reduce vortexes in the "plane" direction you mention, altough your simulation seem "completely laminar", to my untrained eye. Is that because of the low speed or what?

Winglets have the additional benefit you already mentioned: they can be used also as weapons against those vulnerable open-wheelers.

[BreezyRacer]

"Ground clearance must not be less than 40mm on all post‐2006 vehicles."

I think there is potential there for some cleverly designed wheel coverings but, they would be difficult to execute I think.

I'll have a go at dropping it down but it will need remoddling because otherwise the wheels will stick out the top ever so slightly.

It's all worth a try! what I really need is some sort of magic software that takes everything you have to cover and just works out the most efficient shape!

I was saying that just to drop the model you already have to 40 mm just to see what happens to your drag numbers. Don't change anything else otherwise the results will be tainted. Don't worry about wheel repositioning right now, etc. Just adjust the height and then compare numbers and flows. You'll maybe find a different pattern on the underside of the nose too (surprise!). This is just to check to see if adding more ride height is a good thing or a bad thing in relation to drag. I was just asking about the wheel coverings because I wasn't sure if there was an exception for wheel covers. I guess they are governed by the same 40 mm rules. BTW, 40 mm is pretty low and if lowering is better we could do more in that direction such as adding a little splitter to the middle of the front, then feeding into a vortex generator on each side of the nose to reduce under body/tire drag some more. But first we must know .. when talking about ride heights of less than 300 mm is it better to go as low as possible or is higher, in fact, producing lower drag numbers.

[greenpower dude reloaded]

Ok, I will try it like that. I had wondered about possible tainting but then I thought if it wasn't workable lower (i.e.) wheels out top, then it's not a realistic result, because you'd never be able to run that shape

But we shall see.

You may be interested to know that Pac Car II actually runs at half of the clearance you suggested at 150mm their tests showed the effect started to plateau after that height.

[compo]

greenpower dude can you post the rules for me so i can see what is and isnt allowed

goony