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

[oil pan 4]

Originally Posted by MTrenk

This will create a low pressure under the car.

But I want to create slightly lower pressure under my suburban. One of the problems with these trucks is cooling. On 2 wheel drives its worse, if you get at bumper level and look under there you have a cross member that catchs about 6 inchs of air and forces it up into the engine bay.
Air cant really go around the bottem of this cross member becaus its got a 1.5 inch lip around the bottem. like an "L" cross section that scoops up the air, so air has to go up into the engine bay.
I want to block the air flow over this massive low hanging cross member and pull more air through the radiator.
And also by cutting away some of the body that covers the radiator, I can gain about an additional 3 inch by 15 inch strip of radiator fins exposed to flowing air, I will have much better cooling.
And yes I know tons of unneeded air flow over the radiator is bad for Cd that is where the active grill shutters come in.

[MetroMPG]

Max, thanks for starting this thread. Hope you don't mind I fleshed out your thread title to make it more descriptive & attract more eyeballs.

I think you will find that the members who have made & tested undertrays with flat materials have found improvements for the reason you stated:

it may improve a little bit over the pumpkin and other bits that hang from the bottom of your car

So I think it's important not to throw out the easy-to-construct baby with the optimally-designed bathwater.

For that reason, I'm glad to see you tempered your language from the earlier assertion in other threads that a "flat belly pan is no good" to a flat belly pan isn't going to do as much good as one with these optimal design properties (paraphrasing).

[slowmover]

As with the post by oilpan4 above, the already low independently-suspended cars have it easy . . pickup trucks (and derived SUV's) are a bit harder to figure (cooling needs, suspension articulation, etc; the things that an "underpan" might prevent from allowing full work ability from the vehicle) for those of us contemplating this. My truck is not an off-road vehicle -- that might be the line not crossed for this -- and the problems of keeping that kind of limit from declining is fundamental.

Thanks from me for the "idea" of seeing this vehicle type from top and bottom as being complementary for best execution, I had not seen this clearly. May well make an underpan more realistic. I'll have to look at FF ground clearance (IFS) as I have not looked at it before (Post #9).

The truck I have is -- like others -- ultimately limited by cooling capacity (climate & terrain against HP demand) where gearing might be optimized (raised) for better FE when solo. But problematic when towing (for a gasser, just when loaded heavily solo). Grill shutters are one approach, yet egress of heat exchanger[s] cooling air is just as important to include when thinking on a pan as an aero aid.

.

[freebeard]

I will discuss why:
...
•Any other questions that need to be addressed.

Well since you stated that, and it's on-topic, would you care to comment on this:


Also, on the stock VW Beetle floorpan there are lateral raised ribs, I take it that these are counter-productive? There are open cavities ahead of and behind the wheelbase. Which would benefit more from an added partial pan?

Someone here, I think basjoos, has double sidewalls aligned with the inner and outer faces of the tires. Do you see any benefit there?

Thanks for this post.

[Xist]

I keep trying to get my thinker going for school, but all that I have gotten so far is "That EV is cool!"

MTrenk, will you kindly show us the pictures of your car?

[I am mean! ]

[oil pan 4]

If you feel your cooling system is holding you back and you just so happen to have a diesel then try water injection.
You can put an up to an additional 100,000 BTUs per hour (maybe even more than that) out through the exhaust rather than the cooling system with water injection.

[Nevyn]

What if the pan isn't 100% flat, and it creates a kind of venturi chamber under the car? Is that worth anything more than a complete flat panel?

[freebeard]

Nevyn -- As shown above, that was my thinking. I didn't mention the fans forcing air like the Chapparal race-cars:


Although if little air gets underneath in the first place, the air flowing over the open back window might suck (like an airbrush) enough to work passively.

In the 1920's R. B. Fuller proposed the opposite with his Omin-directional Transport:


(Hey, 'pneumatic guard rail'——airbags in the 1920s) But of course he wanted to fly the rear wheel with a horizontal elevator. The only other person I heard of doing that was 'Jocko' Johnson:


A passing mention after some interested discussion of transonic/supersonic cars here:
July 2009 Update

And of course Bucky dropped it in his Dymaxion car.

I'm hoping MTrenk drops into the thread.

[aerohead]

*The flat underside limits under body turbulence and in so doing,allows the flow field under the car to deliver useable kinetic energy to the wake area,and at a higher static pressure.
*Diffusers provide for non-separated flow 'up' the deceleration ramp,with simultaneous pressure regain,raising the base pressure behind the car,reducing pressure drag.
*The airflow volume is a constant,fixed by velocity.
*The golf ball effect is in effect,all over the car,above 20-mph.
*Dimpling the underside could act as turbulators although I wouldn't know the benefit.
Many modern underbody sheetmetal deformations have more to do with noise/vibration/harshness,altering a harmonic or resonant frequency of the belly so as not to telegraph certain frequencies into the passenger compartment.The sheetmetal is a sandwich construction.

[MTrenk]

aerohead:
--> Does the flat side really limit turbulence? I think there's enough potential from such a large flat plate to end up creating MORE drag than pumpkins and exhaust bits. I'm completely serious. Flat underpans are required in Formula One to reduce top speeds. :P
--> Diffusers are good.
--> If airflow volume is constant at fixed velocity, then what good does it do to analyze it when cars have changing velocity, not only due to acceleration, but changes is wind speed?
--> Is the golf ball effect really all over the car ever? Because at 20 mph I'd bet money that the hood has laminar attached flow, and a 20 mph golf ball has turbulent attached flow. Perhaps I don't know what you are trying to say.
--> Dimpling the underside of a bellypan would in fact allow you to make a flat one, which is obviously more convenient than turning the underside of your car into an airfoil profile. Let's say that a smooth sphere and a smooth flat underbody are traveling at 40 mph along side a golf ball, and a dimpled flat underbody. Suddenly they encounter wind resistance, nothing is pushing them. We all know what happens to the sphere vs. golf ball (equal diameter). Can we imagine what would happen to the underbodies (same dimensions)?
--> Yes, creasing metal panels, dimpling, or whatever sort of bending manufacturers do is related to many things: panel strength, good looks, airflow, damping properties, etc.

Nevyn:
--> Ask Formula One, they know all about dealing with flat panels and Venturi tunnels under and behind the car, and they use them to their advantage in certain way, while doing their best to eliminate them in other ways. This deals with aerodynamics that I won't know much about until I go to graduate school in approximately 3 years. Some of this stuff is trade secret ya know?

Xist:
--> As for the drawings, I just have to find someone to let me borrow their scanner.
--> I haven't owned a car in about 8 months. I'm paying for my degree by working and playing in the university orchestra for scholarship money. I have just enough for rent and food. Cars are expensiveeeeeeee to keep legal.

freebeard:
--> I'm not really sure what's going on in that diagram... Raised ribs just increase panel strength, but also increase frontal area and probably don't do anything other than just push the air around. I'll try to talk more about sidewalls later, in conjunction with underbodies and front air dams/splitters.

slowmover:
--> It's my opinion that many cooling techniques used on the tops of cars can do well on the bottoms of cars.




Or you could even just use metal mesh type holes. If you cover the bottom of your truck with a belly pan, I think you will be able to achieve the kind of cooling you need. You'll need to go with something heat resistant/something that doesn't get destroyed by heat over time. I've had lots of ideas about trucks and SUV's over the years, so maybe soon I can sit down and draw them out.

oil pan 4:
--> Not really sure I understood anything you meant.

Tried to answer posts bottom to top!