Aerodynamics of the new Mirage (0.27-0.31 drag coefficient varies with options)

[MetroMPG]

Stall speed? Not an issue for road vehicle aerodynamics, from what I've read.

Flow (thus drag coefficient) is consistent through the range of normal operating speeds. If the spoiler has attached flow at 10 mph, it will still have attached flow at 65.

[cyclopathic]

I am not sure flow will be attached to spoiler at 10mph. Small aeroplane wings have stall speeds in 40-60mph range, but this is probably not 1:1 carry over to hatchback rear window spoiler. Driving C I have observed falling leaves got suck up it trailing wake and stay there for many seconds at speeds 40-50mph, rotating vertically up and down. Unfortunately it is too late now to drive and see if VGs changed that, leaves're gone until next year.

You are right if flow is attached at 40 it is probably attached at 65. Question is if VG is beneficial at 30 in helping with flow attachment, will they be still benefitial at 65, were flow would be attached regardless? Or they will be just generating extra drag?

[Mitz]

@cyclopathic Could you post pictures how & where you placed the VG's on your C?

[Cani Lupine]

Flow detaches from the spoilers on our current cars, starting at the gap between the roof and the hatch. Morning dew doesn't move at all on my way to work, but the roof will be mostly dry.

[cyclopathic]

Flow detaches from the spoilers on our current cars, starting at the gap between the roof and the hatch. Morning dew doesn't move at all on my way to work, but the roof will be mostly dry.

Thanks! that is what I thought could be the case. So it looks the best location for VGs will be where the 2015's antenna located.

[cyclopathic]

@cyclopathic Could you post pictures how & where you placed the VG's on your C?

They angled on purpose should be ~22.5deg, set of EVO-style VGs cost me ~$12 shipped

here is the pic:

[Mitz]

Flow detaches from the spoilers on our current cars, starting at the gap between the roof and the hatch. Morning dew doesn't move at all on my way to work, but the roof will be mostly dry.

So, VG's would be better placed on the roof than at the spoiler as cyclopathic did?
I was thinking to place Airtabs on the spoilers, at the body side rear, and substitute them as rear tire air dams.

[inuvik]

VG's should be place on the leading edge of the surface you're trying to create turbulence (ie the front of the spoiler, not the rear).

[MetroMPG]

I am not sure flow will be attached to spoiler at 10mph. Small aeroplane wings have stall speeds in 40-60mph range,

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!!

[BRagland]

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.