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Old 04-12-2022, 01:28 PM   #111 (permalink)
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Quote:
Originally Posted by Piotrsko View Post
Total drag should increase as a result of speed increases. If it didn't, everybody here would drive like everyone else@100 mph and there would be no reason to add aero
Exactly. Hence trying to find a place that has a faster speed limit. I know a 55 or 60 mph long flat highway that I may be able to test on late at night. Itís over an hour away though

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Old 04-14-2022, 01:26 AM   #112 (permalink)
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whats a good material to extend the spoilers with?
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Old 04-14-2022, 04:41 AM   #113 (permalink)
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Quote:
Originally Posted by Piotrsko View Post
Total drag should increase as a result of speed increases. If it didn't, everybody here would drive like everyone else@100 mph and there would be no reason to add aero
Of course. I was being a bit pedantic in pointing out that the coefficient of drag, which we usually assume is one static number, it isn't really static. It varies with the air's speed, density, & viscosity. A rear diffuser shape/angle steep enough to be optimized for 40mph would begin to have some flow separation at 80mph.


Again, the differences are minute so you can just ignore it unless you're super OCD or trying to win a very close competition
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Old 04-14-2022, 11:40 AM   #114 (permalink)
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Quote:
whats a good material to extend the spoilers with?
Polymetal/Alumapanel/MaxMetal. All are a 3mm plastic sheet with aluminum skins and an enameled finish.



These samples were sheared, braked and rolled on a bench tool with an 18" lever arm, by hand. The brake was least successful as it burst the ouside skin. Perhaps not if the inside is scored beforehand.

I think a big advantage is that it's pre-finished.
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Old 04-14-2022, 12:20 PM   #115 (permalink)
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math

Quote:
Originally Posted by Phase View Post
ill for sure work on the boat tailing with the mid length. only difficult thing is going to be the side with the tail lights!

also since youre good with math, can you tell me what my highway mpg would increase to if i lowered the cd from 0.24 down to say just 0.20 without changing the engine or weight or anything system wise?
On purpose, I gave a math breakdown so everyone could learn to 'fish'.
I'd like you to see if you can figure the new numbers.
If not, I jump in.
Clues:
1) The aero power varies as the Cd at any given speed.
2) If you already know the Cd, and the aero power at the target speed, multiplying the original aero power, by the fraction created when the new Cd is divided by the 'original' Cd, will spit out the new aero power.
3) Rolling-resistance power never changed, so that's just added back for the total Road Load horsepower.
4) Total efficiency ( 95% ) didn't change, so dividing by that gives you the ' brake-horsepower equivalency ( Bhp-e ).'
5) Bhp-e multiplied by it's conversion constant ( 2546-Btu/hp ) yields your Btus per hour.
6) I allowed a thermal efficiency for the IONIQ's engine @ 40% ( 60% of the heat energy lost to cooling and exhaust ). So the Bhp-e divided by 40% (0.40 ) gives gross Btus available from the gasoline.
7) The gross Btus/hour divided by the specific heat content of 111, 836- Btu/gallon gives your new gallons/hour.
8) You're still going 80-mph.
9) 80 mph divided by your new gallons per hour = your new mpg.
This is an 'energy-balance' method of estimating, same as Chrysler Corporation used to calculate the aerodynamics of their 1934 DeSoto Airflow mule's modifications.
--------------------------------------------------------------------------------------
It's actually more simple to do than to explain.
I use a cheap CASIO, scientific pocket calculator from the thrift store, with ten decimal point accuracy and a single memory.
Once a value is obtained, it's used for the next operation, all the way through.
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Old 04-14-2022, 12:25 PM   #116 (permalink)
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material

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Originally Posted by Phase View Post
whats a good material to extend the spoilers with?
For your mockup, I'd recommend cardboard and duct tape, exactly as MetroMPG demonstrated with his Firefly and Insight etc..
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Old 04-14-2022, 12:36 PM   #117 (permalink)
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40-mph vs 80-mph

Quote:
Originally Posted by Drifter View Post
Of course. I was being a bit pedantic in pointing out that the coefficient of drag, which we usually assume is one static number, it isn't really static. It varies with the air's speed, density, & viscosity. A rear diffuser shape/angle steep enough to be optimized for 40mph would begin to have some flow separation at 80mph.


Again, the differences are minute so you can just ignore it unless you're super OCD or trying to win a very close competition
I'm gonna get anal-retentive and submit that, at sub-sonic ( below 250-mph ), incompressible flow, and above critical Reynolds number ( about 20-mph for automobiles ), that the Cd will be 'fixed' at all 20-250mph velocities, and separation points will also be fixed.
The context is:
1) testing occurs within a short enough window that air density ( hence viscosity ) doesn't alter in any significant manner. Or any change is monitored, recorded, and calculated for in the data reduction.
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Old 04-14-2022, 01:52 PM   #118 (permalink)
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MetroMPG's 'cardboard and duct tape ( CAD ) design

Here's a link to Darin's fabrication technique.
https://ecomodder.com/forum/showthre...uft-25223.html
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Old 04-14-2022, 02:15 PM   #119 (permalink)
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Quote:
Originally Posted by aerohead View Post
For your mockup, I'd recommend cardboard and duct tape, exactly as MetroMPG demonstrated with his Firefly and Insight etc..
Iím def going to use some cardboard or firmer poster board for the mock ups. Just gotta get some type of Skeleton frame material to hold the shape so the cardboard isnít flapping around
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Old 04-14-2022, 02:17 PM   #120 (permalink)
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Quote:
Originally Posted by aerohead View Post
On purpose, I gave a math breakdown so everyone could learn to 'fish'.
I'd like you to see if you can figure the new numbers.
If not, I jump in.
Clues:
1) The aero power varies as the Cd at any given speed.
2) If you already know the Cd, and the aero power at the target speed, multiplying the original aero power, by the fraction created when the new Cd is divided by the 'original' Cd, will spit out the new aero power.
3) Rolling-resistance power never changed, so that's just added back for the total Road Load horsepower.
4) Total efficiency ( 95% ) didn't change, so dividing by that gives you the ' brake-horsepower equivalency ( Bhp-e ).'
5) Bhp-e multiplied by it's conversion constant ( 2546-Btu/hp ) yields your Btus per hour.
6) I allowed a thermal efficiency for the IONIQ's engine @ 40% ( 60% of the heat energy lost to cooling and exhaust ). So the Bhp-e divided by 40% (0.40 ) gives gross Btus available from the gasoline.
7) The gross Btus/hour divided by the specific heat content of 111, 836- Btu/gallon gives your new gallons/hour.
8) You're still going 80-mph.
9) 80 mph divided by your new gallons per hour = your new mpg.
This is an 'energy-balance' method of estimating, same as Chrysler Corporation used to calculate the aerodynamics of their 1934 DeSoto Airflow mule's modifications.
--------------------------------------------------------------------------------------
It's actually more simple to do than to explain.
I use a cheap CASIO, scientific pocket calculator from the thrift store, with ten decimal point accuracy and a single memory.
Once a value is obtained, it's used for the next operation, all the way through.
How do I know my aero power?

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