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aero power

Quote:
 Originally Posted by Phase How do I know my aero power?
1) you'll find it in the original calculations that I posted earlier. 20.3183-hp.

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Quote:
 Originally Posted by aerohead 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.
I was taught low speed Cd was "fixed" in my undergrad fluid dynamics course 20 years ago, but I've subsequently learned it is more complicated than that. For time trial cyclists, it is not uncommon to see Cd change 0.0015 between 30mph and 20mph. An elite time trialist might have a CdA of 0.180 so we are talking about less than a 1% difference, but in high level competition that can make the difference between 1st and 2nd.

And that is just for 0-yaw in the wind tunnel. Out in the real world there are crosswinds and the slower you go, the higher the yaw angles you will encounter:

The ideal shape of your vehicle/object changes depending on the expected yaw angles. About 15 years ago Freightliner and International each released totally redesigned semi's. Freightliner boasted that its Cascadia was ~8% more aerodynamic than International's Prostar, which it apparently was in a 0 degree yaw wind tunnel. Out on actual highway loops in Colorado & New Mexico cross winds, the Prostar was ~5% more aerodynamic.

Freightliner Cascadia (8% better aerodynamics in a straight line wind tunnel):

International Prostar (~5% better aerodynamics in real world cross winds):

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Quote:
 Originally Posted by aerohead 1) you'll find it in the original calculations that I posted earlier. 20.3183-hp.
i tried throwing the numbers around in a calculator and just couldnt figure it out. way too complicated for me

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Quote:
 Originally Posted by freebeard 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.
where did you get those?

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Quote:
 Originally Posted by Drifter 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
ill probably keep my rear diffuser at around a 10 degree angle but have it extended a bit longer. dont want to cause early separation

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delta-Cd

Quote:
 Originally Posted by Drifter I was taught low speed Cd was "fixed" in my undergrad fluid dynamics course 20 years ago, but I've subsequently learned it is more complicated than that. For time trial cyclists, it is not uncommon to see Cd change 0.0015 between 30mph and 20mph. An elite time trialist might have a CdA of 0.180 so we are talking about less than a 1% difference, but in high level competition that can make the difference between 1st and 2nd. And that is just for 0-yaw in the wind tunnel. Out in the real world there are crosswinds and the slower you go, the higher the yaw angles you will encounter: The ideal shape of your vehicle/object changes depending on the expected yaw angles. About 15 years ago Freightliner and International each released totally redesigned semi's. Freightliner boasted that its Cascadia was ~8% more aerodynamic than International's Prostar, which it apparently was a 0 degree yaw wind tunnel. Out on actual highway loops in Colorado & New Mexico cross winds, the Prostar was ~5% more aerodynamic. Freightliner Cascadia (8% better aerodynamics in a straight line wind tunnel): International Prostar (~5% better aerodynamics in real world cross winds):
Thanks.
As a production automobile, for it's length, reaches critical Rn around 20-mph, you might imagine that, with the length of a bicycle, it will be at sub-critical Rn until a higher velocity is achieved.
So it's no surprise to see Cd variability with the bike.
As amateurs, with no 'means', we may just have to resign ourselves to the fact that driving weather will be whatever it is, with no control over crosswind or gusts.
Current automotive Cds are already 'crosswind-averaged', figuring a mean annual averaged 7-mph crosswind vector.
We'd be looking at \$4,000 /hour to validate a Cd in a full-scale tunnel, zero-yaw, or crosswind.
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Last edited by aerohead; 04-14-2022 at 05:08 PM.. Reason: typo

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10-degrees

Quote:
 Originally Posted by Phase ill probably keep my rear diffuser at around a 10 degree angle but have it extended a bit longer. dont want to cause early separation
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Quote:
 Originally Posted by Phase 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?
Are you using the Ecomodder calculator? Here it is with the assumptions I used (3500 lb with driver, .24 Cd, 23.5 Frontal area, .007 rolling resistance, 1000Watts parasitic load, 36% thermodynamic efficiency, 85% drivetrain efficiency, summer gas): https://ecomodder.com/forum/tool-aer...ToStep=5-200-5

That spits out ~20 horsepower aero drag and 42mpg at 80mph. Changing the CD to .20 results in ~16.5 horsepower aero drag and ~48mpg:
https://ecomodder.com/forum/tool-aer...ToStep=5-200-5

Plug in a Cd of .145 and you can get within a rounding error of 60mpg at 80mph.

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numbers

Quote:
 Originally Posted by Phase i tried throwing the numbers around in a calculator and just couldnt figure it out. way too complicated for me
1)[ dividing 0.20 by 0.24] X 20.3183-hp= 16.9319-hp @ 80-mph @ Cd -.20.
3) divided by 0.95 mechanical efficiency = 29.964-hp
4) allowing 0.978 accessory efficiency= 30.6380-bhp
5) X 2546-Btu/hp= 78.004.4417-Btu/hour
6) divided by 0.40 thermal efficiency = 195,011.104-Btu
7) divided by 111,836 Btu/gallon = 1.7437-gallons/ hour
8) into 80-miles/hour = 45.878-mpg, @ 80-mph, @ Cd 0.20.
I'm using Af = approx. 24.223-sq-ft ( 85.5% of gross area )
2.2% accessory losses ( Argonne National Lab.)
95% dual-clutch A6 transaxle ( Argonne National Lab.)
3,031-lb EPA dynamometer test weight
Gasoline Direct Injection, Atkinson-cycle @ approx. 40% thermal efficiency
REGULAR Unleaded E10 gasoline @ 111,836-Btu/gallon
I let the rolling resistance just fall out of the math, @ 0.004101/tire
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Last edited by aerohead; 04-14-2022 at 06:02 PM.. Reason: forgot accessories

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Quote:
 Originally Posted by Phase where did you get those?
That was remnants from MultiCraft Plastics in West Eugene. One popular usage is in sign shops.

There are three price levels, retail wholesale and distributor, generally about \$100 for a 4x8 sheet.

https://nudo.com/p_polymetal.php
https://www.maxmetal.com/
https://www.copeplastics.com/materials/alupanel/

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