The speed that aerodynamics are important

[sgtlethargic]

I have an old memory of someone posting a graph of when aerodynamic forces start taking a lot of energy. The conclusion was something to the effect of: aerodynamic forces are significant at 35 mph. The person posting was surprised. This was possibly ten years ago. I'd like to find that post and thread. I'm wondering if that person is still active here, will remember, and find the thread. Or if someone else can help find it.

[aerohead]

It appears to be completely 'relative', as some don't consider aerodynamic drag as something which would inform their thinking or behavior at all.
--------------------------------------------------------------------------------------
As CAFE standards have increased over the decades, along with frontal projected areas, there's been 'some' interest in aerodynamic drag, associated with regulatory compliance by automakers, and some nascent awakening as to the relationship between the climate and road vehicle aero drag ( global warming was mentioned in my 1974 college text for internal combustion engines ).
--------------------------------------------------------------------------------------
As to 'numbers', they've always lurked within the mathematics of the 'force' and 'power' equations as related to 'velocity'.
--------------------------------------------------------------------------------------
It has been accepted for some time now, that, one might consider aerodynamic drag at as low as 35-mph ( 56-km/h ).
At 70-mph, the drag force would be 4X what it is at 35-mph ( velocity-squared law )
At 70-mph, the horsepower / Kilowatts necessary to overcome the drag would be 8X what it is at 35-mph ( the velocity-cubed-law ) [ you're hitting twice as much air, twice as often, and twice as hard ].
-----------------------------------------------------------------------------------Aerodynamically, you could manufacture an automobile today, which would get the same mpg at 100-mph ( 161-km/h ), as it presently gets at 55-mph ( 88.5-km/h ).
If you drove 'that' car at 55-mph, it would resemble driving on the Moon.

[Phase]

35 mpg is about the speed my rainx starts to work and move on my windshield too

[Piwoslaw]

Anyone ever tried pedalling their bicycle to 35mph and keeping that speed?
Broke a sweat? That's because aero drag doesn't start above 35mph, it's already there at lower speeds.

When I made my Kammback, the first time I noticed a substantial increase in coasting distance was at city speeds of 40-50 km/h (25-30mph).

[freebeard]

The speed that aerodynamics are important

The speed at which aerodynamics are important.

[Isaac Zachary]

Whether they are "significant" at 35mph depends on what we are talking about exactly.

Every time you double your speed you use 4 times the amount of energy per mile to overcome aerodynamic drag. (At the same time rolling resistance doesn't change much and engine efficiency usually goes up, at least to a point.)

If going 35mph your car uses 2hp to overcome air drag and you travel 35 miles then you'd have used 2hp-hours of energy, or about a 2/3 cup of gasoline if the engine were 100% efficient.

If you travel twice as fast at 70mph you'd need 16hp to overcome air drag but at the same time you'd get there twice as fast meaning you'd use 8hp-hours of energy, or about 2 2/3 cups of gasoline to travel the same distance.

If you drove half as fast at 17.5mph then you'd use approximately 0.5hp-hours, or about 4 teaspoons of gasoline to overcome air drag. You'd also take twice as long (2hrs) to drive those 35 miles.

Of course in those three examples you still need to add rolling resistance, energy loss due to acceleration and braking, as well as inefficiency losses, which is why driving twice as fast isn't actually going to quadruple the amount of fuel used (it only quadruples the amount of energy needed to overcome air drag over the same distance).

[freebeard]

Thus the hot rodder's aphorism, "Speed cost money, how fast do you want to go?"

[Phase]

Originally Posted by Piwoslaw

Anyone ever tried pedalling their bicycle to 35mph and keeping that speed?
Broke a sweat? That's because aero drag doesn't start above 35mph, it's already there at lower speeds.

When I made my Kammback, the first time I noticed a substantial increase in coasting distance was at city speeds of 40-50 km/h (25-30mph).

when i snowboard, i typically dont feel the wind resistance until i get to about 40 mph. once i get above 45 i can start to feel the pressure and my jacket flapping and then crouching helps speed up a little more

[freebeard]

The upright human figure is a different use case to a torpedo.

[Piotrsko]

Originally Posted by freebeard

Thus the hot rodder's aphorism, "Speed cost money, how fast do you want to go?"

I heard it as :how fast can you spend it?


Carry on, We now return tou to the regularly scheduled debate.