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

[jime57]

Originally Posted by UltArc

I am not trying to disagree, but I can NOT believe everyone who has done a belly pan is just imagining the results. I will trust that the theory on improving the belly pan is accurate, or plausible, but to say that a belly pan does nothing? I can't believe that one bit. Too many people have done too good of testing for me to believe that a belly pan is worthless unless it has dimples or whatever.

I hate to keep harping on Hucho, but in section 4.4.7 of the 4th edition, he restates some information developed by Buchheim who worked on the Audi 100. According to Buchheim's measurements of underbelly sections, the front section(to just behind the front wheels) was worth Delta Cd of -.015, the middle of the car was worth Delta Cd of -.015, and the diffuser section (starting front of rear wheels was worth delta Cd of -.015. The total is Delta Cd of -.045, which is pretty large in aero terms. Hucho has just a short summary of the results. He references the original paper which would be nice to see, but it is in German.

My own Insight underbelly can be seen here:

http://ecomodder.com/forum/showthrea...s-22691-2.html

post #17.

There are a number of aero improvements on the car, but I recently did 112MPG over a 90 mile segment with rolling hills, so something is working on the car

[freebeard]

freebeard, most cars aren't designed to be aerodynamic in the side direction

Some of the happiest times in my life were spent sideways on gravel roads; but I don't do it anymore, too hard on the paint.

There used to be a band that played in Bluegene, OR, called Driving Sideways

[aardvarcus]

I have a couple questions. Accepting the premise that a flat belly pan is suboptimal due to the flow detaching as compared to an airfoil shape, what is the minimum curvature of the airfoil shape required to negate this effect? For example, if we take a vehicle 240” long, what minimum width of airfoil cross section would be required to prevent this effect? Are we talking neighborhood of 1”, 5”, 10”, or 20”?

Second question- when you say “airfoil” are you referring to symmetric or non-symmetric camber? (the teardrop shape or more like the top/bottom of an airplane wing shape) Would this lower airfoil generate lift/down force depending on the camber or lack thereof as compared to a flat shape, and would that introduce drag?

Thank you.

[Otto]

Aardvarcus, I have essentially the same question: Exactly what "airfoil" did the poser of that concept have in mind? Because if it's curved like virtually all airfoils are, that means it curves up and away from (or down and toward) the ground, creating constrictions to flow.

So, will the "airfoil" guy who does not like Hucho please chime in here and explain?

Also, what sources and studies can he cite regarding optimum under car airflow?

Inquiring minds want to know.

[freebeard]

When the OP appeared yesterday, he seemed more concerned about hood vents than the underbody. It's like he whacked the hornet's nest and ran.

I'm still curious about the diffuser (and the exhaust 'above' it), so get in line. I just make an inane comment once in a while to bump the thread.

For my take on underbody camber, I will repost this:

[TheEnemy]

Originally Posted by freebeard

Some of the happiest times in my life were spent sideways on gravel roads; but I don't do it anymore, too hard on the paint.

There used to be a band that played in Bluegene, OR, called Driving Sideways

Meh, paint is over rated. However my wife gets a bit upset when I'm using her windows to see where I'm going.

More seriously, crosswinds of 40-50mph are regular here so having some consideration for how that will affect aero, or more importantly stability would be somewhat important.

[meanjoe75fan]

In re: the OP (perhaps many of these points have been raised, but I didn't see 'em all at once).

What you have to understand about laminar flow is that it's VERY unnatural--air wants to go turbulent at speeds and sizes relavant to motor vehicle travel. To sort of "cheat the air," aero-engineers have created the (NACA, NLF) natural laminar flow airfoils. These have pressure distributions whereby the top-side pressure gradient increases to a maximum fairly far back on the airfoil (and are thus typically characterized by max thickness fairly far back on the wing chord). These allow up to around 70% (as of the printing edition of my Theory of Wing Sections) **under ideal testing conditions, and across a narrow range of coefficient of lift**.

What this means is laminar flow is very hard to achieve, to the point that a riveted aircraft skin--even flush rivets--will most likely "trip" laminar flow and you'll not realize the benefits thereof. That's why most of the achieved laminar flow is from composite wings (creating the term "fast glass.") Grumman Co. went as far as to make it's AA5A/AG5A light aircraft with BONDED aluminum wing skins--but manufacturing variances and wing flex in flight were too significant to realize much laminar flow.

Also, "surface roughness" (akin to frost or just an unwashed, unwaxed surface) will kill NLF, too.

What that means for the home aero-modder is twofold:

1) "Fuggedabout" laminar flow, unless you have a machine shop capable of meeting or exceeding Boeing/Airbus standards. Consider Basjoos' AeroCivic: he did all the right things, got great economy...but "fit and finish" wasn't really up to Boeing standards...or even Chevy standards. Surface roughness out the wazoo!

A perusal of the site shows that rough fit+finish is the rule, not the exception here, thus chasing after NLF is waste of energies that could better applied to maintaining attached turbulent flow across as much of the vehicle as possible.

Not only will you lose NLF over surface roughness, airflow through the unsealed gaps in the hood will, too (how the OP intends to maintain NLF aft of the spinning wheels is anybody's guess), as will smashed bugs on the leading edge, etc, etc.

Furthermore, if you really want NLF (and have the CNC machinery to prove it), you can't just make your car's top+bottom "kinda look like a wing": no, you have to get a NACA NLF-series airfoil and match your car's profile EXACTLY to that specific airfoil (note that most airfoils are turbulent across the majority of the chord, yet those planes fly just fine).

2. You don't really need laminar flow, anyways. Note...all those planes flying just fine with turbulent-flow airfoils, as well as Basjoos mentioned previously. You can probably get north of 100MPG with optimized, turbulent, attached flow and a decent engine...and if that's not good enough, well, THEN start looking into CNC lathes and such.

Actually, turbulent flow is good in that it adheres to curves better without detaching...if one DID actually make a NLF-auto, it would likely be impossibly long!

[Frank Lee]

Furthermore, if you really want NLF (and have the CNC machinery to prove it), you can't just make your car's top+bottom "kinda look like a wing": no, you have to get a NACA NLF-series airfoil and match your car's profile EXACTLY to that specific airfoil (note that most airfoils are turbulent across the majority of the chord, yet those planes fly just fine).

Pretty tough to do in ground effect- just ask the Aptera boys.

[02Eclipse]

First time poster but long time lurker, quick question... Does the chart you supply in the first post apply to an flat plane in the ground plane? Like in the 10 inches or so from the ground that most bellypans are? One other question, since most cars employ a rake (front of vehicle is lower to the ground than the rear) is this closer to an airfoil shape that you refer to or is it closer to a flat plane?

[Otto]

Originally Posted by 02Eclipse

First time poster but long time lurker, quick question... Does the chart you supply in the first post apply to an flat plane in the ground plane? Like in the 10 inches or so from the ground that most bellypans are? One other question, since most cars employ a rake (front of vehicle is lower to the ground than the rear) is this closer to an airfoil shape that you refer to or is it closer to a flat plane?

The OP seems to have disappeared.

I'd like him to define "airfoil shaped" as regards a belly pan. Exactly what does that mean? Got pics?


I'd also like him to define "optimal:" If a belly pan is "suboptimal," then what's optimal? Exactly how does an irregular or rough shape become less draggy than a smooth one? Where else in aerodynamics is this true? After all, the fact that a surface is upside down should not effect how slippery it is or isn't.