# Drag Coefficient/ frontal area



## IamIan (Mar 29, 2009)

How accurate are you trying to get?


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## Kelmark (Oct 26, 2009)

I would like to get as close as possible, but a consensus from some more experienced members would be enough to start doing some math.


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## galeson (Oct 25, 2009)

Look on the Mayfield Co homepage. They have lots of Ferraris there( but no Dinos)


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## Kelmark (Oct 26, 2009)

galeson said:


> Look on the Mayfield Co homepage. They have lots of Ferraris there( but no Dinos)


My guess for drag coefficient .35 looking at other Ferrari's in 1986. Does that sound about right? Still not sure on frontal area 1.82m^2? I am guessing at this point.


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## IamIan (Mar 29, 2009)

Kelmark said:


> My guess for drag coefficient .35 looking at other Ferrari's in 1986. Does that sound about right? Still not sure on frontal area 1.82m^2? I am guessing at this point.


Seems like a reasonable guessing point.

Maybe add ~10% to the final combined CdA number just to be conservative.

but that should be good to get started on calculations and such ... you can always roll up your sleeves to get more specific if you want to later.

one basic / easy method to get a basic idea of the frontal / displacement area is to take a string or something and wrap it around the car ... find the largest part ... that is the circumference ... divide my pi... ~3.14 to get diameter ... diameter times diameter times ~0.7854 will give you a ball park frontal area.

( String / 3.14 )^2*0.7854

usually best to avoid protrusions like side mirrors and antenna.

That is an easy way to get in the ball park of your frontal area.


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## tomofreno (Mar 3, 2009)

One way to estimate frontal area is to take a photo from the front of the vehicle and draw a grid to scale on it. Add up the areas of the squares to estimate the area. Smaller the grid, the more accurate the estimate.

This site gives drag coeff of a number of vehicles. It gives 0.34 for a Ferrari F40. No idea how that compares to your dino.
http://www.madabout-kitcars.com/kitcar/kb.php?aid=327


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## Kelmark (Oct 26, 2009)

tomofreno said:


> One way to estimate frontal area is to take a photo from the front of the vehicle and draw a grid to scale on it. Add up the areas of the squares to estimate the area. Smaller the grid, the more accurate the estimate.
> 
> This site gives drag coeff of a number of vehicles. It gives 0.34 for a Ferrari F40. No idea how that compares to your dino.
> http://www.madabout-kitcars.com/kitcar/kb.php?aid=327


 
Do I just print an outline of the front view of the car on graph paper figure out the apropriate scaling per square add them all up. If I am understanding you correct, thanks that's a great idea!


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## tomofreno (Mar 3, 2009)

> Do I just print an outline of the front view of the car on graph paper figure out the apropriate scaling per square add them all up.


 Yes, that would work. You just need to ensure you have the scale correct. One way is to set something of known length on the car, like a piece of cardboard with large (3" tall) markings on it at say every 2 inches. Another way is to measure between two points on the car (like distance between inner edges of headlights) and use this to determine the scale of the printed photo.


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## Kelmark (Oct 26, 2009)

tomofreno said:


> Yes, that would work. You just need to ensure you have the scale correct. One way is to set something of known length on the car, like a piece of cardboard with large (3" tall) markings on it at say every 2 inches. Another way is to measure between two points on the car (like distance between inner edges of headlights) and use this to determine the scale of the printed photo.


My eyes are crossed from counting squares. But what I ended up with was approximately 1,124 squares with a scaling of 1.45" per square. I determined scaling by physically measuring the width 69 ¾” and height 37” of the car as it sits on the floor. (Just the body) That gives me 1,629 sq inches or 135.8 sq ft. That doesn’t sound right? Go figure I messed it up, I wasn’t very good at math better with a wrench! Can you help me out?


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## IamIan (Mar 29, 2009)

Kelmark said:


> That gives me 1,629 sq inches or 135.8 sq ft. That doesn’t sound right?


Square inches to Square feet... as in 12^2 = 12 * 12

1,629 Sq Inches = ~11.3125 Sq Ft... that is a pretty small frontal area.


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## MJ Monterey (Aug 20, 2009)

If I understand your measurements and counts:

Each grid is*: *1.45 * 1.45 = 2.10 sq inches

2.10 sq inches * 1124 grids = 2360 sq inches of frontal area

2360/ 144 sq in per sq foot = 16.4 sq feet of frontal area.



Side Note: If I remember correctly. It is standard practice to include the ground clearance cross section in the frontal area for drag calculations. The air is typically so turbulent under the car that it is considered a solid block.

MJ


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## Kelmark (Oct 26, 2009)

IamIan said:


> Square inches to Square feet... as in 12^2 = 12 * 12
> 
> 1,629 Sq Inches = ~11.3125 Sq Ft... that is a pretty small frontal area.


I did not include the air gap under the car just the frontal area of the tires body windshield. Am I supposed to include that?


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## IamIan (Mar 29, 2009)

Kelmark said:


> I did not include the air gap under the car just the frontal area of the tires body windshield. Am I supposed to include that?


Sense you are just getting an estimate ... I would recommend just adding +10% to the 0.34 Cd and the ~17 Sq Ft you have already estimated ... That should get you pretty close... you are after all just estimating the Cd and estimating the frontal area... a ~10% buffer should give you a conservative measurement without getting overly complicated in refining the Cd and A estimates you already have.

But generally speaking the turbulence under the car is reflected in the cars Cd number... Frontal area only refers to the frontal area of the car , wheels , mirrors , under-body hoses , etc... of the car itself.

Remember the Cd number is actually a combination number of lots and lots of other small contributing effects to the aerodynamic shape of the vehicle... this is why a vehicle with a smooth under belly has a better Cd than one with a smooth under belly.


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## DavidDymaxion (Dec 1, 2008)

Does the car run? You could do coast down measurements to actually measure the car's rolling resistance and air resistance.


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## Kelmark (Oct 26, 2009)

DavidDymaxion said:


> Does the car run? You could do coast down measurements to actually measure the car's rolling resistance and air resistance.


Unfortunately I did not have the car legal to drive when I bought it. Then I started tearing it down. Still re-assembling the chassis now so I will have to wait a while before I can do coast down measurements. I am just trying to verify my pack size by using as accurate data as I can. Because I am trying to keep the weight down I don't want to oversize the pack more than my goal range. This is listed on my build “Kelmark Solar Series Hybrid”.


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## Kelmark (Oct 26, 2009)

I found data on the Ferrari 360 Modena which is similar in shap and size. Its 2.03 sq m/21.9 sq ft Frontal Area and .34 for drag so I will use those numbers for best case and add 10% for a conservative.


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## Duncan (Dec 8, 2008)

Hi Kelmark

For my sins I once worked on a solar racer (we won!) any way one of the things I took away from that is that once you get a reasonable shape the devil is in the details,

All of the transition zones and any holes become critical, 
Two vehicles that looked similar could have a two to one variation in drag just because of the way the panels met up or the way the wheel arches were faired


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