Originally Posted by
MetroMPG
My understanding: "stall speed" isn't an appropriate analogy for road vehicles because the road vehicle doesn't have to generate lift to stay in the sky. So flow characteristics on a road vehicle's surfaces are similar throughout the full range of normal vehicle speeds because the angle of attack remains constant. To say otherwise implies that a car's Cd changes with speed, and it doesn't (in the range of normal speeds).
For a much more eloquent explanation, post on EcoModder -- Aerohead will probably explain it much more clearly than I can!!
That's not necessarily true about flow characteristics being the same at all speeds...
In fact, at around 65 MPH, I would expect flow separation from our spoiler to become almost an inherent issue. As the speed increases, the relative time that exists between the airflow and the vacuum produced by a downward slope in a cambered airfoil decreases, giving the air less time to "be sucked onto the surface" which causes issues of lift in aircraft, and flow characterics for vehicles. Whether or not our puny spoiler does have this issue, I'm not sure.
However, if since Mitsubishi reduced the angle of the spoiler on the new Mirage, I would be willing to say that there was separation at higher speed, and therefore, they reduced the angle to prevent that separation.
This is why slow flying aircraft can tolerate a much larger range of angles of attack. The stall angle of attack can be much higher at lower speeds, contradictory to at higher speeds. There is however, a point of no return even at slow speeds.
An airfoil moving at 10 MPH might be able to retain the airflow at increased angles far better than the same airfoil at 65 MPH.
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View my fuel log 2015 Mirage ES 1.2 automatic: 51.7 mpg (US) ... 22.0 km/L ... 4.6 L/100 km ... 62.0 mpg (Imp)