Aerodynamic lift- A real problem

[JackMcCornack]

Honest to gosh, is there something about this topic header that lights people up?

And speaking of lighting people up, one way I discourage tailgaters is I took the back body panel off MAX--it has a clear fuel tank, so anybody behind me can see the fuel sloshing around. My fuel is cooking oil, but it looks as scary as gasoline.

Originally Posted by MadisonMPG

... lift causes drag. Drag is our enemy. If we start producing a lot of lift...

I'm in full agreement re 'a lot of lift', but insufficient lift causes drag too. As long as our cars stay on the top side of the asphalt, zero lift will have more aerodynamic drag than some lift. That's my opinion but it seems a common opinion in the trade, and at extreme speeds (which I never drive) car designers are willing to accept the disadvantages of some lift, in exchange for the benefits of low drag. Case in Point: the Veyron, which won't even hit 250 mph unless you take it out of Handling Mode and put it in Speed Mode--the difference is the spoiler retracts in speed mode. The spoiler goes up in Handling Mode to reduce lift, but it sure doesn't reduce drag.

[Frank Lee]

The thread needs this:

http://www.youtube.com/watch?v=0Bmhjf0rKe8

[Nerys]

Ok that is one of the cutest damned videos I have ever seen in my life! Very cool :-)

I love driving my Cherokee. it has a rear wiper with washer. the nipple points STRAIGHT BACK. all I have to do is disconnect the rubber hose (oops) and I have a rear facing water gun.

INSTANT tail gate eliminator :-) works 100% of the time without fail in 400,000 miles :-)

even made a nj trooper back off. He was REALLY on my ass (I could not see half his hood in my mirror)

he backed off. you want to read my plate do it from a safe distance moron :-)

also to solve road rage? just carry a camera (not a cell phone camera not the same thing)

so far (about a dozen incidents including some pretty nasty incidents) I have a 100% success ratio. When I pull out the camera and say SMILE asshole "something" happens in the human mind. I guess the camera does something that "snaps" them out of it and very harshly brings them back to reality IE what the hell am I doing here?

every time when I whip out the camera and that flash goes off the rage CEASES instantly. and I mean like a twig snapping BANG rage gone. They turn tail and go away.

Word of caution folks. I don't care how big you are or how strong or fast you think you are GETTING OUT OF YOUR CAR to confront someone with road rage is pure unfettered stupidity.

YOUR SITTING INSIDE A 2000-5000 POUND ARMORED TANK.

are you NUTS getting out? Just rev it up and turn the wheel toward them. They will "get the hint" really really quickly. Get back in your car or I am going to mother fraking crush your ass. :-)


as for lift. I am no expert but I can not agree with some lift being good (unless it has a specific need)

if any lift causes drag THEN ALL LIFT causes drag.

the only way lift could reduce drag is if you were extract over unity out of it. You just can't do that to the best of our knowledge.

what is actually happening most likely is you have OTHER DRAG somewhere else and adding that little lift ADDS DRAG from adding the lift but REDUCES the other drag and the reduction in the other drag is larger than the INCREASE in lift drag so you get a net benefit.

but that lift is STILL adding drag.

[winkosmosis]

A lift reduces drag from stuff on the ground

[winkosmosis]

Seriously though, you're wrong about lift not reducing drag.

What Compare a pickup with a tonneau to one with an aero cap. The aero cap is creating lift that a tonneau doesn't, because of the angle, but of course it's also decreasing drag. Similarly, a rear "diffuser" reduces drag by adding taper, but also creates downforce.

[MadisonMPG]

I still think that lift hurts MPGs.

[Frank Lee]

The very act of creating lift, creates drag.

[winkosmosis]

Originally Posted by Frank Lee

The very act of creating lift, creates drag.

The "perfect" teardrop shape creates lift at the top rear edge. It also creates downforce on the bottom rear edge, lift on the bottom front edge, and so on, so it balances. The point is that a shape that is meant to reduce drag can also create lift. A brick has no lift, no downforce, and a lot of drag. Would you rather have the teardrop or the brick?

The design of a car is constrained by the fact that it moves along on the ground. So most of the rear is always going to be slanted downward to minimize drag, and that surface is always going to create lift.

You can counteract that force with a "diffuser" which also reduces drag, and also creates downforce because of its angle.

Draw yourself a vector diagram.

[winkosmosis]

To put it another way, EVERY angled surface of a vehicle creates force in some direction, either up, down, left, or right. The question is whether it produces less rearward force (drag) than the alternative.

So what is the alternative to the downward slope of a "fastback" or pickup aero-shell? The alternative is a box shaped rear like my XJ's, a nice lift/downforce free surface, which is exactly what everyone here wants to get rid of because it instead creates a huge vacuum.

[JackMcCornack]

Originally Posted by Nerys

every time when I whip out the camera and that flash goes off the rage CEASES instantly. and I mean like a twig snapping BANG rage gone. They turn tail and go away.

Maybe they thought it was a muzzle flash. Maybe they turn tail thinking, "Whoa, I'm not going up against a professional road rager, he's got a silencer and everything!"

Anyway, back to our subject: Aerodynamic lift - A real problem

Originally Posted by Nerys

...but that lift is STILL adding drag.

This thread has metamorphed in an interesting direction. It started as a discussion of lift as a safety issue (the consensus was it's not a major factor at ecomodding speeds), now it's about lift as a component of aerodynamic drag.

Nerys:
> if any lift causes drag THEN ALL LIFT causes drag.

MadisonMPG:
> I still think that lift hurts MPGs.

Frank Lee:
> The very act of creating lift, creates drag.

If we put it to a vote, it looks like we'd conclude that all lift causes drag and we should strive to get lift down to zero. Here's my take on it; please consider me the Loyal Opposition rather than a troll.

I think there's an optimal Cl for minimum Cd for every solid body, but it's different from body to body and it's different from environment to environment, and in most cases, optimal Cl is other than zero.

In free space the optimum shape is a symmetrical three dimensional teardrop at zero angle of attack and a Cl of zero, but cars don't drive in free space. I think in the ground effect environment, a teardrop is no longer the lowest drag body, and as the body gets closer to the ground, the optimum shape gets flatter on the bottom. It is no longer symmetrical and its lowest drag angle of attack is no longer at Cl 0.0

Asymmetrical airfoils which have Cl > 0.0 at Cd min are the rule, not the exception, as has been reliably demonstrated in wind tunnels since NASA was NACA. From the aerodynamic drag standpoint, too much lift is worse than too little lift, but some lift is better than no lift.

(note: while I was typing, winkosmosis was posting--I guess this post is a wordy version of: +1 what winkosmosis said.)