Quote:
Originally Posted by JulianEdgar
Incidentally, for anyone who is actually interested in facts rather than supposition and erroneous theories, take the Porsche Taycan as an example of a modern fastback - and let's look at the topic of aero lift.
In profile:
CLr (coefficient of lift, rear) : 0.16
(That figure is from a presentation Dr Thomas Wolf of Porsche was kind enough to send me - rear spoiler not raised.)
Frontal area 2.33m^2.
Calculated rear lift at 100 km/h - 17kg
At 200 km/h - 70kg
At 260 km/h - 119kg (that's 262lb!)
No wonder they fitted an automatically deploying rear spoiler....
All fastback shapes with attached flow develop lots of low pressure over the upper curves. That can only be offset by:
1) rear spoiler (usually only a partial offset of lift), and/or
2) very effective underfloor aero (can completely cancel lift)
But as I keep saying, there's absolutely no need to guess or use only factory coefficients. Just measure the actual, real, aerodynamic panel pressures on your car on the road.
My Insight's measured pressures (my car runs very effective undertrays) - Pa at 80 km/h:
Note the low pressures all the way across those upper curves. It's just what happens with attached flow and these shapes.
So the next time that 'Aerohead' states his completely wrong notion about fastback shapes having no lift, can we actually ask for some evidence - real evidence - and not just pretend the emperor has clothes... and that we should all be politely agreeing with claptrap? |
The Taycan, while certainly a 'fastback,' is not a streamline body. It's a composite, with the greenhouse independent of, and glommed onto the main body.
The roofline of the Taycan is a bit too aggressive, there's too much pressure regain for the length of the roof, compromising the boundary layer. The spoiler helps reach out to the streamline contour. It's not as good as simply lofting the end of the body.
Upper surfaces of the body, flanking the greenhouse represent very little deceleration, with high velocity, low pressure. Nothing like a streamline body, in which the body would boat-tail from the same point as the roof apex, building pressure the entire length of the aft-body.
I believe that a close reading of Doctor Wolf supports everything I mention. It's at home, not at hand.