Quote:
Originally Posted by aerohead
1) had the flow actually 'wrapped' the 911's aft-body, it would actually be at its highest-possible pressure, zero rear lift, and lowest drag.( This won't mean anything to you until you study boundary layer theory and Bernoulli ).
2) the flow separated when the inclination exceeded 23-degrees.That's the law. It's in Hucho, just where I said it was.
2) the high drag, Cd 0.40, is produced by low base pressure and attached, counter-rotating longitudinal vortices, which 'induce' a 'downwash' (this is not the same as attached flow) which is responsible for the alignment of the tufts. Hucho might have clarified this better. Seeing shouldn't be believing!
3) the tufts indicate a small wake when, in reality, there's an enormous turbulent wake and vortical flow, following for hundreds of feet behind the 911. Only smoke will reveal this.
4) The spoiler(s) reach upwards, through the separated flow, attempting to harvest kinetic energy from the overlying inviscid flow by reattachment (Hucho page 281). You've got the physics completely inverted.
-----------------------------------------------------------------------------------
A) drag comes from the aft-body.
B) the drag is pressure drag.
C) pressure drag is a function of flow separation
D) the 911's high drag cannot be explained by ' air wrapping over the long curve.' Bernoulli's Theorem will explain this.
E) Spoilers do not trip flow, they're there to capture,or over-capture flow and deflect it.
----------------------------------------------------------------------------------
* In 1968, while the 2,360-pound, Cd 0.40, 911 was plying the Autobahn at 132-mph, the 1968, 1,240-pound, Cd 0.27, Porsche 907, long-tail was plying the world's race courses at 180-mph,without spoilers or wings. A look at the difference between the two car's aft-bodies gives a clue about rear design, drag, and lift. And ironically, the 911 could have been Cd 0.27 in 1968, but it would have impacted the car's silhouette. Law of the Paris Dressmaker.
|
You know, if what can actually be measured on real cars matched your theory, you could well be right.
But there's a major problem.
On a range of cars, here's what doesn't match your theory:
- surface body pressure measurement
- tuft testing showing separated/attached flow
- manufacturer's stated coefficients of lift.
I think if I had theories that were so far from reality, I might look pretty carefully at what I was saying...
The simplest way of you realising the size of the errors you are spreading would be to make some on-road measurements for yourself.