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Old 05-27-2020, 09:56 PM   #15 (permalink)
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Yes I did miss this.

Originally Posted by California98Civic View Post
When I read this post the first time I appreciated the references, but I got stuck on a few things. Your example from Hucho (2cd Edition, p.281) seems to be an image of three differently streamlined bodies. Yet you seem to regard only the last as streamlined. Why? And isn't the difuser angle also relevant, not just the sweep of the top of the body?
No, of course they're all streamlined. Not sure how I suggested otherwise? Aerohead has argued that streamlined bodies do not create lift. Here is an example, where the more streamlined the body (lower Cd), the higher the lift. And yes, the diffuser angle is relevant, so it's interesting how the more heavily cambered body has more lift, even with the diffuser.

Originally Posted by California98Civic View Post
A second point you made is contradicted by the evidence you show from Katz’s Race Car Aerodynamics. Katz’s images on p.48 show increases in drag and lift between body shapes 4, 5, and 6. The “low drag body of revolution” has zero lift and the lowest drag. The half-body has the second lowest lift and drag. And the generic contemporary production shape has higher lift and drag than either 4 or 5. Yet you claim that “low drag shapes that have upper curves coming down to a small wake area have higher lift.” But body shape #6 has the higher lift.
There is no point in looking at a streamlined body of revolution (Cl = 0) and then applying that to a road car. When such a streamlined body is cut in half and placed near a road, it develops lift.

I am not saying that in every case as drag goes down, lift goes up. You can have low drag bodies with low lift, or high drag bodies with high lift - as in #6 (and that's fairly common). You can have almost any combination you want, but in a road car chasing low drag with a small wake, you need to be very careful that you don't develop a body with lots of lift due to the high degree of camber. Many of the shapes suggested on this site will have very high lift. (In fact, just refer to the thread on a wing in ground effect, a profile that was optimised to produce maximum lift. A car shaped like that, with the suggested ground clearance and angle of attack, would probably fly).

There are solutions, and the easiest is a proper undertray. But to ignore lift/downforce is to ignore literally half the aerodynamic forces acting on a car.

Originally Posted by California98Civic View Post
Lastly, you gloss aerohead's text this way: “Aerohead's theory is that lift is caused by separation, rather than attached flow.” And you conclude: “The primary cause of lift on road cars is the attached airflow on upper curved surfaces. To argue otherwise is to show a grave misunderstanding of the mechanism of lift on road cars.” But aerohead actually argued almost the same thing as you: “Lift is a function of low pressure acting 'over' a portion of the body.All separation should be at the back of the car. If it's not,it's in a lower pressure regime,as it's at a higher velocity.The closer you get to the windshield the lower the pressure and the higher the lift.”
Yes lift is a function of the pressures acting on the body. The debate is not about that, it is about the causes of that low pressure. Aerohead has again and again argued that low pressure is caused by separation. In fact, as the references I have given show, it is caused by attached flow over the cambered surface of the car - just like an aircraft wing. The greater the curve, the lower the pressures over it. In fact, premature separation will reduce lift - the opposite of what Aerohead argues. That this is the case can be clearly seen on the tuft pictures of the Porsche, where earlier separation was produced in order to reduce lift. (But according to Aerohead, those tufts don't show real flows!)

Originally Posted by California98Civic View Post

I'll grant that aerohead sometimes writes haikus, or else with a lot of jargon, but I think his point about lift and pressure was clear enough.

Unfortunately, I think that around half of what Aerohead writes is misleading or simply wrong. I do agree with him on some things: coastdown testing on the road is pointless, and testing of small models in wind tunnels is very likely to be misleading. But of all I have seen him write, they're about the only things I can think of that are helpful guidance (with good evidence underpinning them) for people modifying their cars.
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