has anyone ever seen a tunnel structure underneath a car. as in a cylinder (however flattend).
i dunno how to explain what im talking about well.
but the air moving through the pipe (underbody tunnel) would have a resultant force for this to have an effect.

if youve seen one, has anyone got any pics?

you help is much appreciated.

are you talking about the diffuser? The recent UTA cars should have it ... check out their websites. I think UTA has been having underbody works since early 2000

yes we run an undertray with two tunnels. i think i have pictures i can post up when i get home, but for now i would get an automotive aerodynamics book and read up on it.

here this is the best picture i have on my computer. it is of out 04 car. you can see the undertray has the tunnel running along it and near the end it flares up.

There are a few decent papers available on diffusers as well, by Kevin Cooper out of NRC in Canada and by Zhang at University of Southampton (I think.) Zhang's are written by his grad students but are more in depth and provide more insight into the flow physics; the Cooper papers are a much broader survey type.

Look at

www.mulsannescorner.com

The Audi R8 and the Nissan prototypes have a lot of good undertray pictures of real cars and windtunnel models. I haven't read the aero portion of the FSAE rules recently enough to suggest how applicable some of the methods would be. Most FSAE courses though are made to keep the speeds relatively low compared to what you'd see on a full-scale road course.

Downforce will increase with the square of forward velocity, so unless you keep the vehicle moving quick, you won't get much downforce. Below 50 mph, I wouldn't bother at all. 60 to 70 mph is the point where most full-size cars start to feel the benefit.

See Hucho et al "Aerodynamics of road vehicles"
Available from SAE publications.

Good luck,

quote:

Originally posted by NetKev92:
Below 50 mph, I wouldn't bother at all. 60 to 70 mph is the point where most full-size cars start to feel the benefit.

There's a ton of money being spent making road bicycles produce less aero drag- and they spend most of their time in the 15-30 mph range. Aerodynamics are not neglible below 50mph. Debatable on an FSAE car? Sure, but not negligible.

Aerodynamics is one of the most hand-wavy parts of Formula SAE. Especially in our program. Hopefully that will stop now.

Most full scale cars feel the benefit at 60 to 70mph.. and we're far from a full scale car.

Agreed, I think the real question of aero in FSAE is whether you have the means to truly develop any sort of aero package... ie design tools, manpower, fabrication tools and techniques... if any of those are lacking, the package likely won't do well and the overall car will suffer from lack of development in other areas of the car. Aero definitely works, but only when the rest of your car has its act together. Who cares about having more downforce into a corner if your suspension can't handle it?

Downforce is not something that "happens" at 60 to 70mph, we all know its a v^2 relationship.

There are numbers published for the aerodynamic performance of FSAE cars, they are not hard to find. With them you will also find wing profiles, CFD, wind tunnel and on-track measurements.

If anyone thinks that 60 kg of downforce is of no benefit at 60 km/h (yes kilometers per hour) then I suggest they leave aero to the hand wavers out there.

Scott

I also recommend mulsannescorner.com and Competition Car Downforce by McBeath. I heard Race Car Aerodynamics by Katz is good, and a bit more technical.

At the speeds we're running I think the aero package to have is high downforce comparable to A-mod autocross cars. Drag is negligable because there are no straightaways or large radius turns (never achieve anywhere near drag limited top speed). However weight of the package is a huge issue, and it's tough to make large wings weigh nothing.

As far as tunnels go, they'll probably stall when using aggresive diffuser angles unless you employ vortex generators. You'd want to use 3D CFD to design vortex generators though.

quote:

Downforce is not something that "happens" at 60 to 70mph, we all know its a v^2 relationship.

only true if the airfoil is not stalled.

quote:

Drag is negligable because there are no straightaways or large radius turns (never achieve anywhere near drag limited top speed).

You'd be suprised how much drag can be produced at our speeds. We have had configurations of our car drag limited to <60mph. To say that amount of drag has no effect on typical FSAE speeds is misleading.

quote:

Who cares about having more downforce into a corner if your suspension can't handle it?

Downforce makes pretty much any suspsension work better. Although i agree that cutting team resources will bite you in the ass. But not because your suspension can't handle it.

quote:

Originally posted by Marshall Grice:
You'd be suprised how much drag can be produced at our speeds. We have had configurations of our car drag limited to <60mph. To say that amount of drag has no effect on typical FSAE speeds is misleading.

What kind of engine are you using? Let's assume the car has the same amount of power as the Honda 600 F4i which has a top speed of about 160 mph. The car would have to be 19 times as draggy in order to have a top speed of 60 mph. I realize that the car with wings has at least twice the frontal area and a higher drag coefficient, but I still don't believe it.

quote:

Originally posted by CappyUMD:
What kind of engine are you using? Let's assume the car has the same amount of power as the Honda 600 F4i which has a top speed of about 160 mph. The car would have to be 19 times as draggy in order to have a top speed of 60 mph. I realize that the car with wings has at least twice the frontal area and a higher drag coefficient, but I still don't believe it.

Marshall's right, I've tested the drag limited top speed on a couple aero FSAE cars and you can set them up to be drag limited at pretty low speeds. There was also full scale wind tunnel data to back that up.

Also keep in mind mechanical drag. Motorcycles have much less mech. drag than an FSAE car with 4 larger tires and lots of toe angle. Then there's the power loss in the drivetrain, which is more complicated on an FSAE car (I don't remember seeing any CV joints on a motorbike). Then there's that pesky restrictor...

Your design goal is to develope a car to market as an autocross car. You would never be able to sell cars without aero. You can not win on an autocross without aero. All of the fast/quick autocross cars now running in A-Mod have aero packages.

AW

Im really starting to enjoy reading the responses here, most are well thought out. In regards to the suspension, what happens if you choose crappy dampers that bottom out from the downforce generated into a corner... woops. There was an A-Mod aero car a few years back I think it was called the phantom that had so much drag that it was limited to ~80 mph, but it could almost sustain speed through corners due to the immense amount of downforce gained. If you saw the thing, all it looked like was an A-mod car but with ginourmous aero! Supposedly it won almost every race it competed in.

see http://www.napylon.com/Phantom.htm

To look at the car would not strike you as anything revolutionary. Everything is very well done. As I remember it was around 300 hp. Most of the top autocross cars top out around 100 HPH and I cant think of any that are shifted more than once wile on course. Most are CVT. Most if not all of the top cars run tunnels and close to the full size on wings.

AW

quote:

Originally posted by awhittle:
Your design goal is to develope a car to market as an autocross car. You would never be able to sell cars without aero. You can not win on an autocross without aero. All of the fast/quick autocross cars now running in A-Mod have aero packages.

AW

Hmm, this is something we've pointed out, but apparently section 1.2 of the rules (or whichever one states of purpose of building an FSAE car) is being ignored these days.

As stated before, you have to have everything done right on the aero package to consider running it. You have to have multiple forms of testing to show you know what you did and you have to be able to build it properly (light, strong, accurate). Pretty much like anything else on a car that you want to be competitive. They're definately not something you want to be slapping on last minute, it has to be considered as an integral part of the package from the word go.

Well said Andyman, The thing that most impresses the judges, as it should, is when aero (or any aspect of the cars design) fits well in the grand scheme of things. The problem with aero on FSAE cars is that there are quite a lot of downsides. For high downforce, wings are ideal (preferably in conjunction with a diffuser set up) however, wings create a great deal of drag, add weight, increase MoI, increase CoG height, are rideheight/pitch sensitive etc. etc. and then there’s the practical issues with hitting cones. Some teams opt for a diffuser set up which has nearly none of the afore mentioned downsides other than rideheight sensitivity. If you are clever about it you can even decrease drag with the use of diffusers. However it is not likely that a large amount of downforce can be generated with diffusers alone. So in the end a team has to decide between effectiveness and efficiency, but the most important thing is whether it fits in with the teams design philosophy.
Multiple forms of testing is absolutely imperative. Each form of testing carries with it some difficult challenges but because aerodynamic devices can be so sensitive to minor changes in geometry or flow field, verification needs to be comprehensive.
Katz is a good starting point but is not particularly in-depth. SAE Papers are good if u can find the right ones. Unfortunately F1 teams don’t tend to divulge their secrets either so good practical information can be hard to come by.
Playing around with 2D CFD is the best way to find a good starting point for your geometry because its quick and easy (efficient use of resources can impress the judges too) but for refining the design it needs to be 3D. Wind tunnel and on track testing are also important when some prototypes have been made. Each method is inaccurate in some way which is why it is good to have multiple methods of verification.
Anyway, I think j_types question has been answered now.
Cheers