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Old 08-28-2020, 01:04 PM   #1 (permalink)
aerohead
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Contour-corrupted rooflines and lift

Since this subject has come up I wanted to share one tid-bit of data, actually quite rare in the popular press.
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In 2014, CAR and DRIVER tested a 2014 Cadillac CTS VSPORT at the A2 Wind Tunnel.
When compared to the 'template,' the roof contour of the Caddy strays a bit, going hypo-contour for a smidgeon, then recovers at the decklid trailing edge.
Any flow instability due to the 'off the reservation' flow would be sequestered atop the backlight and boot, isolated to that region.
At 100- mph ( 161 km/h ) the VSPORT developed 30.6-pounds of rear lift, and 40.8-pounds front lift.
The curb weight is 3,998-pounds. EPA test weight would be 4298-pounds.
A gross, plan-view, 'wing area' is 98.02 square-feet. Gravitational wing loading = 43.1 pounds/ square-foot.
Aerodynamic loading is negative 0.72-pounds / square-foot at 100-mph.
At 100-mph, the VSPORT experiences 98.3% of it's static wheel loading.
As a 'wing', and with a plan-view Coefficient of Lift = 0.00007284227709, 'take-off' velocity for the car, at Le Mans, on the Mulsanne Straight, would be 776 - mph. Supersonic.
As someone who was born yesterday, and just fell off the turnip truck, I'm inclined not to concern myself with the 'lift' of production vehicles, as was the habit of Hucho when he published his second- edition.
I'm especially inclined not to concern myself with vehicles who's rooflines resemble the streamline profile.
Spirit was essentially zero-lift at DARKO. Hoerner has published pressure profile data for streamlined cars which clearly demonstrate positive loading at nose and tail, cancelling any lift in between.
I'm compiling a list of contour-corrupted fastback cars which would suffer separation- induced lift, some of which are in production today. I'll add that as a near-future addendum.
I anticipate push-back, from a certain gentleman in Australia, who, no doubt will completely discount the validity of the DARKO and A2 data. All I can offer is, an invitation for him to take every vehicle previously tested, to a moving-road wind tunnel and publish the contrasting data. There exists no other way to defend criticism of extant data.

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Last edited by aerohead; 08-28-2020 at 01:08 PM.. Reason: add text
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