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Old 11-02-2012, 03:15 PM   #11 (permalink)
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Quote:
Originally Posted by JRMichler View Post
The difference in aerodynamic drag is proportional to the square of the airspeed, or 40%.
Say that again?

Airspeed squared is the aero difference?

So, if I were going 10 mph faster, it would be 10*10...oh, I give up

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Old 11-02-2012, 03:28 PM   #12 (permalink)
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Quote:
Originally Posted by JRMichler View Post
But, at 60 MPH, that is the difference between 55 MPH and 65 MPH airspeed depending whether it's a tailwind or a headwind. The difference in aerodynamic drag is proportional to the square of the airspeed, or 40%.
Correct. In this case, the two airspeeds are 55 mph and 65 mph.

65^2 = 4225
55^2 = 3025
4225/3025 = 1.397, or about 40% higher.

Or to put it another way, 10 mph over 55 is an 18% difference. 1.18*1.18 = 1.39, or again almost 40%.
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Old 11-02-2012, 04:13 PM   #13 (permalink)
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Quote:
Originally Posted by JRMichler View Post
And once upon a time I got 41 MPG out of a 1995 Dodge Neon at 80 MPH. But it was a warm summer day, I was moving with traffic in the fast lane, and a tail wind.

Wind has a huge effect. Most people cannot even feel a five MPH wind. But, at 60 MPH, that is the difference between 55 MPH and 65 MPH airspeed depending whether it's a tailwind or a headwind. The difference in aerodynamic drag is proportional to the square of the airspeed, or 40%.
Reminded me of several of my crosscountry trips from CA to texas when I would be on the high plains on hwy 40.
Once I could literally feel the car being pushed along!!!!!
Another time the cross winds had all the cb anteneas leaning at 45degree angles!!!
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Old 11-03-2012, 08:59 AM   #14 (permalink)
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Something that wasn't touched on here is the possibility of the engine running more efficiently at the higher speed (higher rpm) than the lower rpm.
It could be the engine "coming on the cam", getting the best scavenging from the exhaust, both the tuning of the intake/exhaust plumbing, thermal dynamics inside the combustion chamber...etc.

Makes sense to me to have a target rpm range where a typical engine runs the most efficient...especially if this engine is connected to a CVT transmission...which isn't the case here, but just saying...
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Old 11-03-2012, 02:42 PM   #15 (permalink)
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wungun - It's possible, of course. Diesel Dave touched on that idea in post #8.

But it's pretty unlikely. Actually it would be the first modern, conventional 4-cyl car anyone's seen around here that behaves that way.
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Old 11-03-2012, 02:54 PM   #16 (permalink)
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Guess i speak a foreign language
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Old 11-03-2012, 03:01 PM   #17 (permalink)
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I have driven my car on the same road. At the same speed. At the same time ( reseting the trip mpg every 2 minutes ) and got different numbers every time because its almost impossible to have the same exact conditions even if they are only 2 minutes apart
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Old 11-04-2012, 04:17 AM   #18 (permalink)
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Wow, I knew that air resistance grew exponentially, but I didn't realize that a difference in 10 mph could make such a difference.

I wonder: What if trees were planted along all the highways? Once the trees grow, wouldn't that dramatically reduce wind resistance?

Although that might also reduce the benefits of a tailwind.
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Old 11-04-2012, 10:01 AM   #19 (permalink)
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Old 11-04-2012, 10:24 AM   #20 (permalink)
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Quote:
Originally Posted by wobombat View Post
Wow, I knew that air resistance grew exponentially, but I didn't realize that a difference in 10 mph could make such a difference.

I wonder: What if trees were planted along all the highways? Once the trees grow, wouldn't that dramatically reduce wind resistance?

Although that might also reduce the benefits of a tailwind.
Exponentially means just that...
And easy way to look at it is, to double your speed requires 4times the power.
So if your increasing your speed from 10mph to 20mph, it'll still require 4 times the power...except at those speeds, you might only be using 3hp to maintain 10mph (or 12hp for 20mph.)
But if you're doing 50mph, and you want to do 100mph, where it might be 50hp to do 50, you'll need 200hp to do 100mph...and exponential jump!

Someone correct me if I'm wrong....

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