The Underbody (flat belly pan is suboptimal - explanation & solutions offered)

[Xist]

Originally Posted by jimepting

I'm not sure how these two picture snuck in but they are wonderful examples of what Hucho concluded, from wind tunnel testing, to be the very best way to exit cooling air. The disadvantage is that the warm air is now on top of the hood, with easy access to the climate control intake at the base of the windshield. Hucho concluded that this arrangement worked well for race cars, but not so well for other applications. Second best option was exit through the front wheelwells.

Wow. I was just about to unsubscribe and suggest that others did also, but then you went and contributed!

[jime57]

Originally Posted by 2000mc

I make that final diffuser angle to be 8-10 deg. This is steeper by far than Hucho would recommend. I do note that the diffuser appears to be curved, assuming the final angle over some distance. It is just another example where details sometimes diverge from the text. It would be nice to see some GM testing, but I suspect it isn't available.

The Honda Insight ends it's diffusion area, which is about 4 deg, with an upward curve molded into the bumper cover. There is also a "fancy" little undercut slot whose purpose appears to be the introduction of turbulence.

[UltArc]

What is the climate control thing? I mean, why would the hood vents not work as well on a basic production car?

[MetroMPG]

UltArc - what he means is the cabin fresh air intake is generally at the base of the windshield. If you vent very hot air from the cooling system through the hood just upstream, you can see the potential problem.

[freebeard]

For the record, in cold light of dawn, I agree with UltArc and jimepting. And I should say 'sorry' to MetroMPG.

I'm willing to ignore a little 'attitude' to get to what you have to say; others, not so much.

[Xist]

I do not agree with any of you! Arnold Schwarzenegger is the best singer ever!

[jime57]

Originally Posted by xist

i do not agree with any of you! Arnold schwarzenegger is the best singer ever!

lol.

[UltArc]

Originally Posted by MetroMPG

UltArc - what he means is the cabin fresh air intake is generally at the base of the windshield. If you vent very hot air from the cooling system through the hood just upstream, you can see the potential problem.

I would think it could be okay. I do see your point, but in my head I imagine engine air exhuasting on the sides of the top of the hood, and induction in the center. So it would dissipate while in motion, the only time warm air is really coming out. That is a great way to not do a grill block, and lose drag, it seems.

I have seen these on the new Mustang, but I cannot recall if they are real or not. I think they are real openings, because I aleays throw a fit and complain when manufacturers make fake ones lol -but I don't remember my findings when I explored them.

[MTrenk]

I knew some people would be upset when I suggested that flat underbodies don't really work that well (if at all), but considering people were killed and exiled for figuring out the world was round, I think I can take some unwarranted hate myself. =P

As far as the hood vents, if you wanted to make them effective and not ruin the cooling intake, you could design them (point them) in a vector/line toward the sides of the car so the warm air could roll over a minimal area of the intake at the windshield.

I don't intend to anger people, just put what I know out there (teaching, sharing info... same thing or not? I don't know).

Sorry for seeming 'arrogant' or 'claiming to be smarter than everyone' or 'not taking credible sources seriously' or whatever. Hucho got a bunch of stuff right, but he isn't the only guy out there with some good knowledge on how this stuff works.

As I've said many times before, I'm just letting you guys know some stuff I know. Believe it or not, that's up to you, but you probably aren't going to change my mind on what I said in the first post. mcrews, would you like to contribute something useful to this forum? Don't bother us if you have nothing useful to say.

There's a reason Formula 1 requires a flat section under the car: it lowers top speeds because of the huge amount of drag it creates. In no way shape or form would the F1 engineers choose to put that flat surface there on purpose.

[jime57]

Originally Posted by MTrenk

I knew some people would be upset when I suggested that flat underbodies don't really work that well (if at all), but considering people were killed and exiled for figuring out the world was round, I think I can take some unwarranted hate myself. =P

I'd love to see the information you have on this issue. I personally believe that underbodies do help based on information and studies that I have seen posted here on this site. I also believe diffusers work, based on Hucho's text.

As far as the hood vents, if you wanted to make them effective and not ruin the cooling intake, you could design them (point them) in a vector/line toward the sides of the car so the warm air could roll over a minimal area of the intake at the windshield.[

That might avoid the heat problem, don't know. It sounds rational. Hucho only devoted a small amount of discussion to the question and did not consider diverting the hot air to the side. It would be worth a CFD study, or some wind tunnel testing, but we are not likely to get either.

I don't intend to anger people, just put what I know out there (teaching, sharing info... same thing or not? I don't know).

Sorry for seeming 'arrogant' or 'claiming to be smarter than everyone' or 'not taking credible sources seriously' or whatever. Hucho got a bunch of stuff right, but he isn't the only guy out there with some good knowledge on how this stuff works.

I don't think anyone ever said that Hucho was the be-all, end-all source. He is just the convenient source that many folks have at hand. He has compiled much of his data/information from the work of others and properly credits them. It isn't even his own work entirely. Get your hands on a copy of the book and check it out.

BTW, Hucho discusses your original "thoretical" flat plate transition issue in section 2.3.3.1 of the 4th edition. There are lots of "qualifications" which probably make the flat plate analysis inappropriate to the underside of an automobile. The section is long winded and difficult. I'll leave it for others to decide if it actually applies to undertrays, which don't even satisfy the most basic of the qualifications. Wind tunnel testing would be a better approach IMO and that has been done and documented here on this site.

As I've said many times before, I'm just letting you guys know some stuff I know. Believe it or not, that's up to you, but you probably aren't going to change my mind on what I said in the first post.......snip...........

Please don't hold back on information. Please provide your personal sources, tests, documentation so we can all benefit from it