Frs vs s2000

[fatoni]

Originally Posted by Boxer486

Yeah Cd has nothing to do with size. Behold one of the best Cd found in the animal kingdom 0.06.

http://www.the-scientist.com/?articl...d-the-boxfish/

i went from despising you to loving you in an instant. haha

[Dave-ROR]

Originally Posted by gramicci101

Motortrend was saying 2750 for the FR-S and 2830 for a 2005 S2K.

Edit- ok, I'm a dumbass who can't math. One too many zeros there.

lol yeah 80lbs makes more sense

[chrisl]

Originally Posted by CSG Mike

It absolutely does.

Using just the Cd to say a car is aerodynamic, is like judging an engine by it's torque output.

I disagree - a good Cd means an object is aerodynamic in my opinion. Sure, a 24 inch diameter parachute has less drag than a 787, but the 787 is more aerodynamic, in my opinion. CdA is important for figuring out drag force, but Cd tells you whether the shape of the object itself is good for aerodynamics or not.

(Interestingly, from the numbers I can find, the CdA of a 787 is about 84 square feet, so at the same speed, the 787 has less drag than a fleet of 10 Veyrons...)

[CSG Mike]

Originally Posted by chrisl

I disagree - a good Cd means an object is aerodynamic in my opinion. Sure, a 24 inch diameter parachute has less drag than a 787, but the 787 is more aerodynamic, in my opinion. CdA is important for figuring out drag force, but Cd tells you whether the shape of the object itself is good for aerodynamics or not.

(Interestingly, from the numbers I can find, the CdA of a 787 is about 84 square feet, so at the same speed, the 787 has less drag than a fleet of 10 Veyrons...)

You're agreeing with me.

The Parachute (high Cd, low A) has less CdA than a 787 (ultra low Cd, high A), and the 787 is more aerodynamic (Cd).

[chrisl]

Originally Posted by fatoni

if you changed the scale of a car would it change the cd? like if you had a 4/5 scale frs would it have the same cd as our cars?

Well, kind of. It's complicated. To have a truly representative model (that would have the same drag coefficient), the reynolds number has to match between the model and the full size object. The mach number has to match too, though for mach numbers below about 0.3 (around 200mph), it doesn't really matter much. The reynolds number is more important however. For objects the size of cars traveling at normal car speeds, it won't matter very much as long as the two are fairly close in size (so your 4/5 scale car and full scale car will probably have very nearly the same drag coefficient). However, if your car was 1/10 scale (or smaller), you would have to pay more attention to the reynolds number or you could have significant variation in the results.

To calculate the reynolds number, you have to know the viscosity (μ) and density (ρ) of the air, a reference dimension of the object (L) (which one you choose doesn't matter much, as long as you're consistent - for cars, I would probably pick length though), and the velocity (V) of the airflow. The reynolds number is calculated as Re = ρVL/μ. If the reynolds number for one object is substantially different (I.E. more than an order of magnitude, in most cases) than for another, then the drag coefficient could be very different even though they're the same shape.

This is especially true if the reynolds number is very low - for example if you have a very small object, a very low velocity, or a very high viscosity. In this case, the fluid dynamics (and the drag) will be dominated by viscous forces, and the drag coefficient pretty much loses any relevance (since the drag force no longer even scales as V^2 in the viscous dominated flow - it scales with V instead). This is part of the reason you can't really do wind tunnel testing with a very small model of a car - the results will be completely wrong unless you do a lot of very careful (and potentially nonintuitive) things to ensure that you have a reasonable reynolds number for your model.

In high reynolds number flow, including pretty much any full size car, the drag coefficient will be approximately constant across a fairly wide range of velocities (up to about mach 0.3, as I mentioned earlier). You can get some significant changes in boundary layer behavior, which can impact intakes on the side of the car or hood scoops, as well as the behavior of spoilers and vortex generators, but for the most part, the drag coefficient won't dramatically change with reynolds number for a full-size car (it'll probably drop a bit as reynolds number increases, due to the changes in the boundary layer, but this is a small effect).

So, the short answer to your question is yes, a 4/5 scale car will have the same Cd (for all intents and purposes) as a full scale, but you can't just assume that drag and lift coefficients are completely scale-independent or velocity-independent, unfortunately, since there are some complications there.

[chrisl]

Originally Posted by CSG Mike

You're agreeing with me.

The Parachute (high Cd, low A) has less CdA than a 787 (ultra low Cd, high A), and the 787 is more aerodynamic (Cd).

Maybe I misread your post then - I interpreted your statement as saying that the Cd alone doesn't determine whether an object is aerodynamic, which is what I was disagreeing with. I do agree fully with your statement here (that the small parachute has a smaller CdA than the 787, but is less aerodynamic).

[CSG Mike]

I should have been more clear >.<

I'm still sleep deprived from 86 CUP.

[BRZ21]

Even though there has been basically no reference to an s2000 or an frs/brz in the last 3 pages, this has been an incredibly interesting read.

[Boxer486]

Originally Posted by BRZ21

Even though there has been basically no reference to an s2000 or an frs/brz in the last 3 pages, this has been an incredibly interesting read.

Can't fault a guy for trying.


http://www.ft86club.com/forums/showp...postcount=1200

[CSG Mike]

Originally Posted by BRZ21

Even though there has been basically no reference to an s2000 or an frs/brz in the last 3 pages, this has been an incredibly interesting read.

Dat new Vette though..

[OrbitalEllipses]

Originally Posted by CSG Mike

Dat new Vette though..

Ass looks better in person, but not sure if I love it yet.

[King Tut]

I love dis fread, and I still want an S2000.

[gramicci101]

Originally Posted by King Tut

I love dis fread, and I still want an S2000.

Sigh. I guess an M3 will have to do...

[King Tut]

Originally Posted by gramicci101

Sigh. I guess an M3 will have to do...

Nope. It is for sale, just waiting for one of the interested parties to come get it with the cash.