Car & Driver Aerodynamics Test

[oldtamiyaphile]

Originally Posted by Old Tele man

ANALOGY - imagine this airplane WING airfoil as a car (lefthand pix):

...or, think about this being UPSIDE down, and think what the inverse of LIFT-INDUCED drag would be (DOWN-FORCE drag!):

Your analogy is false because the GT-R uses it's floor/diffuser to create most of it's downforce which is basically 'drag free', and why underbody downforce is always so heavily policed in racing.

A diffuser's typical lift to drag is 10:1 and can even reduce drag while still generating modest downforce

Compared to a rear wing's typical 3:1. Front wings on open wheel cars can be as low as 10:1, but that's because they're basically front mounted diffusers.

[oldtamiyaphile]

Originally Posted by UltArc

I find that .26 very hard to believe.

VW quote 0.33 for the T5 Transporter:



I coast down tested my old MB100 and after running the numbers it came out to 0.36, and given the VW's vastly better coasting (basically identical frontal areas), I have no problem believing VW's 0.33 claim. Now given that a brick can be aerodynamic, I see no issue with 0.26 for a GT-R.

[aerohead]

I see a scale-model tunnel at Nissan.They cannot measure full-scale detail.
The Mercedes tunnel looks familiar.The FKFS is next door to them and they've been using the tunnel there,which Kamm used in the 1930s.
Anyone with Hucho's more recent books might have something on newer,climatic/accoustic tunnels.
Cd 0.26 is not quite 'low-drag,' which 'begins' at Cd 0.25.MIRA and Pininfarina said that any 1978 car could have been made with Cd 0.25 without dramatically altering the styling.
Cd 0.26 is not Cd 0.10,so there's still some fruit on the tree.
Cd 0.26 IS pretty good for a sports car.Sports cars are to stick 'fast' to the roadway.Low drag isn't necessarily driving their design.

[gone-ot]

Originally Posted by oldtamiyaphile

Your analogy is false because the GT-R uses it's floor/diffuser to create most of it's downforce which is basically 'drag free', and why underbody downforce is always so heavily policed in racing.

A diffuser's typical lift to drag is 10:1 and can even reduce drag while still generating modest downforce

Compared to a rear wing's typical 3:1. Front wings on open wheel cars can be as low as 10:1, but that's because they're basically front mounted diffusers.

Sorry, wrong. All down force is derived from forward motion, and thus (just as the illustraction geometrically illustrates) vectorally adds to the total forward drag...ie: the LONG side of a 3-4-5 right triangle.

[aerohead]

Originally Posted by oldtamiyaphile

Let's stick to road cars, not sure what F1 has to do with anything.

A McLaren P1 produces 1323 pounds of downforce at 161mph. Is the GT-R's 186 at 186mph still immense? If you build a car with low drag, it won't have much lift, add a good under tray and diffuser and you should have modest downforce with no drag penalty (compared to an unclad underside at least).

Yes,Goro Tamai says that zero lift is easy for low drag cars,and a little downforce can be had with little effort as well.

[Vman455]

Originally Posted by aerohead

IThe Mercedes tunnel looks familiar.The FKFS is next door to them and they've been using the tunnel there,which Kamm used in the 1930s.
Anyone with Hucho's more recent books might have something on newer,climatic/accoustic tunnels.

Looks like Mercedes opened its own aeroacoustic tunnel last year; buried in the press release is a reference to a climate tunnel opening two years before that.

https://www.daimler.com/dccom/0-5-71...0-0-0-0-0.html

[oldtamiyaphile]

Originally Posted by Old Tele man

Sorry, wrong. All down force is derived from forward motion, and thus (just as the illustraction geometrically illustrates) vectorally adds to the total forward drag...ie: the LONG side of a 3-4-5 right triangle.

If I didn't know that then why did I list typical lift to drag ratios? The point is a diffuser adds significant downforce with much less drag than associated with wings.

[UltArc]

It was clear, now it is not clear lol. Does anyone have resources I could review to expand my knowledge on this? I understand how a wing is going to hit the air and become an issue, but if it's creating less force than the underbody (which has less drag) wouldn't it noticeably increase the rolling resistance? I suppose RR should not be included in Cd lol

[gone-ot]

Ain't nothing for free...and since all LIFT and DOWN forces derive their respective results from the forward air motion, anything that creates either one, manifests itself as an increase in total drag.

[oldtamiyaphile]

Originally Posted by UltArc

It was clear, now it is not clear lol. Does anyone have resources I could review to expand my knowledge on this? I understand how a wing is going to hit the air and become an issue, but if it's creating less force than the underbody (which has less drag) wouldn't it noticeably increase the rolling resistance?

I would recommend Race Car Aerodynaimcs - Designing for speed by Joseph Katz.

If you add downforce you'll add Crr, but it's not going to be an issue at road speeds. At 93MPH, that GT-R will be making 44lbs of downforce, nothing to loose sleep about. At legal speeds, you might see 20lbs.

Ain't nothing for free...and since all LIFT and DOWN forces derive their respective results from the forward air motion, anything that creates either one, manifests itself as an increase in total drag.

Sure, my point is it's possible to take a standard car, add an undertray and diffuser, and improve both Cd and Cl. That makes it 'free' downforce. Yes it could have even less drag if designed purely for Cd. But:

To use the GT-R example, let's just assume that the rear wing is enough to create a lift neutral car, and the splitter/ underbody create the rest at the 10:1 ratio.

At 186MPH, 176lbs of downforce is costing 17.6lbs of drag. About the same as sticking your palm out the window. At road speed that 20lbs of downforce is costing 2lbs of drag, barely measurable. This is a great cheat that lets designers create a low drag street car, that can still demolish the odd race track.