Belly Pan slowing me down
 

I installed a belly pan on my Hyundai Elantra last week out of coroplast. Ever since I did my mpg numbers have gone down. It feels like I have a lot more drag with it on. I do not have a front grill or bumper block. I am wondering if the air that goes into the engine bay gets stuck in there with now where to escape because the underside of the engine bay is completely covered? and the only place for it to go is back out the front of the car.

Any one have this problem before?

I am going to try a bumper block with some left over coroplast.

wildfire8 -

That's the first I have heard of a belly pan "gone bad". Do you have pictures of your configuration? Without seeing the details, I am guessing that you are experiencing a "parachute effect" where the air is getting stuck in the pan somewhere. I would shoot pictures from the POV of the airstream. Find and close whatever hole is causing the problem, and the belly pan will start working as intended.

CarloSW2

Hi,

I think you need to do an upper grill block, and probably a partial lower grill block, too. I also had this happen, when I taped up the opening in the chin pan -- it was dragging me down, and now that I have a smaller grill opening, it may be helpful to block the under pan again.

here are some pictures.

1 front of car
2 front of belly pan
3 front hooked to the bumper
4 showing all the way back (cut a hole for the tow hook) (the hole in the back is for the CAT)
5 showing the cut away for my tire from the front
the next pics are in next post.
6 the hole in the back is for the CAT so it wont melt
7 pic from side of car showing the back of the belly pan.
8 showing the cut away for my tire from the back

pics 6 7 8

make sure that there is an opening under the engine compartment that is at least as big as all the frontal openings from where the air can penetrate into the compartment. also make sure the opening is placed as far back as possible.
but i would consider reducing the frontal openings aswell.

now it seems your pan traps the air or slows it down, turning your car into a parashute. at best it slows you down, at worst it could blow off your pan at high speed so beware.

Hi,

Add an upper grill block! And I'd cover ~ half of the lower opening too -- the outside part, and experiment with leaving some open near the lower center. On my car, I only have about 1/5 of the original grill opening left open, and the cooling is fine.

even if i do a bumper block i will still need to have an opening?

i don't have a belly pan yet myself, but common sense tells me it's not a bad idea to have some opening underneath. make the opening on a horizontal or rear upward sloping part, so that the air going under the car doesn't enter through there. i would still leave a partial opening in the front grille so that there is a minimal airflow wich should avoid air from coming in from underneath and messing up your cooling and aerodynamics.

Can't believe people are telling him to block the grill, with the Utah summer a week away...

Why not just make some holes in it?

Hi,

The edges of the belly pan do not appear to be sealed, right? The problem is too much air is getting in -- and there is almost always too much air "trying" to get through the radiator. I think on many cars that you get better cooling by blocking off some of the grill(s). Some of the openings on the grills on this car seem to be well away from the radiator, and the upper grill is there for style.

I think that ~half of the lower grill should be left open and it will easily cool the car.

I did make a bumper block with the middle open. I am going to try cutting a hole in the back to get better flow threw the engine bay.

What do you mean by the edges appear not to be sealed? How would you go about that?

Is there a way to make exit holes for any air that is being trapped.. without causing more of a parachute effect?

You can put the holes in a spot where they'd be facing slightly toward the rear

That is my next thing i am going to try is make hole at the back of the engine bay in the belly pan to to let air out. I am just busy today. I really want to try it out but that will have to wait till tomorrow.

Not sure if I should make 1 in the middle or 2 off to the sides of the belly pan?
I was thinking maybe 1 foot long by 5 inch wide going the width of the car.

Nice belly pan work!!

Looking at the big cutouts for the front wheels, I think there's probably plenty room for air to exit the engine bay.

HOWEVER it might have to go all over the place to get out that way - so a nice opening in a location that's 'convenient' to where the air flow wants to go, might be a good idea.

Closing off unneeded air flow through the grill is a good idea - the question is, which air flow is not needed? See notes below.

If you have (or get) a ScanGauge you can monitor engine temps digitally which is WAY better than watching a gauge. That will tell you if your grill block is causing higher than normal temperatures. Otherwise you have a good chance of choking off the air flow so that your radiator fan comes on more often to keep the engine from overheating.

It wouldn't overheat (hopefully) and your gauge will likely stay at center - but you'll be running the fan a lot more than the designers intended. I know because I've been modifying my grill block and monitoring engine temps exactly that way.

Exit pathway is as critical as entry. Exit pathway should be smooth so the spent cooling air rejoins the slipstream as parallel to ambient flow as possible.

Think: Fish gill, with entry at the nose stagnation point (highest pressure) and exit at the point of lowest dynamic pressure along the side or bottom, with flow virtually parallel to the slipstream. That way, the slipstream sucks air through the gill (radiator).

NACA did a lot of studies of engine cowlings in the WWII era, much of which was forgotten in the dawning jet age. Unfortunately.

Exactly. Look at the cooling duct underneath the P51 mustang. It is said that the hot air exiting the rear of the radiator actually provides sufficent thrust to overcome the drag caused by radiator itself.

http://www.ipmsstockholm.org/magazin..._p51_b_156.jpg note the radiator situated underneath

http://www.ipmsstockholm.org/magazin..._p51_f_002.jpg

http://www.ipmsstockholm.org/magazin..._p51_f_004.jpg

Actually after reading closer, it apparently added a few extra mph to the mustang's top speed.
http://www.ipmsstockholm.org/magazin...il_p51d_01.htm

And, when you get the exit geometry sorted out, you'll find that you can then decrease the size of the inlet, with subsequent improvement in overall streamlining. This because with Bernoulli effect sucking the spent cooling air out the well-designed outlet, it takes a lot less ram air (and consequent drag) to stuff cooling air into the inlet in the first place. Less air will be doing more cooling, more efficiently.

BTW, make sure the cooling air has a nice, smooth path(s) between inlet and outlet.

Google for pics of WWII aircraft, and study the engine cowlings on, say, the B-29. Or, if you prefer a recent example, Google for Arnold AR-5 and have a gander at a remarkably efficient little plane, which despite fixed gear, tractor propeller, and cruciform tail, does amazing things on little power. Study the cooling vent on the side of the cowling, which works great but has no moving parts. Design your cooling outlet on the bottom of your Coroplasted car accordingly.

If you need more air hitting the oil pan (finned aluminum on both my BMWs and my Porsche, for enhanced cooling), then take a carpet knife to that Coroplast and make a nice NACA duct with ~7 degree ramp slope. Bits of Coroplast and some duct tape for duct sidewalls will finish the job nicely.

PS: You did a damn nice job installing that Coroplast belly pan, and once you get the exit done, I'll bet your fuel economy has significant improvement

I don't understand how the heat could result in thrust from the exit port. The heating and expansion of air wouldn't push air out the back any more than it would push it out the front. The way I see it, it should have no net effect on flow.

Here are the pics of my bumper block.
The hole cut out of the left front is for my ram air kit for my intake.
It is not attached to the bottom of the bumper.

Looks good (though you may find the cooling and aero might be better with the upper grill blocked and then open up the lower one a bit). Any initial impressions of whether this addresses your issue?

My number seem to me normalizing out with the bumper block. But its hard to really tell cuz my scan gauge calibration changed a few %'s with the last fill up. I did make an opening in the belly pan that should exhaust the heat and the incoming air from the engine bay, and rejoin the air stream. I have not yet tested. Made the opening and didnt leave the house after. I will post some pics tomorrow.

My temps did rise a little.
Normal was 182f 184f.
With the belly pan it stayed at 184f (+/-)(didn't check to much).
Then with the bumper block 193f.
I hope this opening helps with some of the heat.

seems to me like this might be more of an intake air temp issue than an aerodynamic one, in wich case, fining where the engine picks up it's air migth be a good thing.

i've had lower FE with grillblocks that extended my coastdown, so they must have helped aero, but they blocked the area where my engine picks up air. so it removed the ram effect and made the engine injest warmer air.

decreased engine efficiency might overrule aero improvemens as these are only very effective at higher speeds. it might be the wrong conlusion that the undertray is ineffective by itself. perhaps rerouting the engine air picup migth help things.

slowing
 

On my CRX,I have used stamped-steel 45-degree louvers from HVAC application to form the bellypan section under the firewall area of the car.Air passing from the nose shingles over the louvers with little disruption,and the louvers offer positive engine bay ventilation for the cooling system.I have as much free area of louvered outlet opening as I do grille inlet opening.The louvers are available at building supply or HVAC supplies outlets.

Rad thrust
 

The theory is that the assembly is like a mild ramjet. Instead of adding fuel and burning it, it just adds heat from a heat exchanger. However, the pressure inside is higher than the air around it, and the back is bigger than the front. Hence, a tiny thrust. As to Net thrust, there are skeptics. . .

The folks who flew Voyager around the world on one tank of gas had to re-do all the standard tables on heat exchange at low speed, and wound up with 20% cooling drag overall, even with careful adjustment of flow by the co-pilot.

Count me in as a skeptic on the heat-from-thrust question. Could be, other aspects come into play, such as drag reduction because the rear flap on the P51 radiator nacelle may act to prevent backwash from the rapidly constricted shape, and/or the hot exiting air attenuates the drag from that shape. Maybe "pressure thrust" per Goldschmied/Bushnell at NASA is at play here. One way or another, apparently the thing gets less drag, which is tantamount to an increase in thrust.

For a good, recent application of careful thought and design of a heat vent into the slipstream, Google for the Arnold AR-5 and note the configuration of the forward right fuselage wall where engine heat is vented. Also, have a look at the various aspects of the Arnold AR-6 racing plane, especially the cooling inlet at the front of the cowling, which is designed to let max ram air in during climb (when the engine gets hottest) but more streamlined in cruise (when the angle of incidence is less, for less air entering).

For purposes of efficient venting of engine compartment air on a ground vehicle, I suggest starting with a horizontal application of the side vent shown on the Arnold AR-5. Perhaps you could have one sheet of Coroplast from the front bumper on back to near the firewall, then an exit gap where a second sheet curves down to and meets the same horizontal plane as the front sheet. Or, perhaps the rear sheet curves down to a parallel horizontal plane perhaps an inch or so higher than the horizontal plane of the front sheet. This way, the slipstream acts to suck air out of the engine bay per Bernouli and/or Venturi effect(s).

Been busy.
I made an exhaust port on the belly pan. Check the pics.

I did try a grill block but 5 miles of driving and I saw my temp hit 220f so I pulled off the free way and took it off. Then just last night with the bumper block temps where hitting 200f it was fine when I was in motion but when I stopped at a light is when the temps started going up. I noticed that when my intake air temp went down my engine/water temp went down.

I took that off bumper block on just drove home on the streets 43mph and saw my MPG numbers go quite high and the load go lower then usual. And that was with no bumper/grill block and just the exhaust port. I don't feel the parachute effect any more. Temps where at normal 182-184f.

If you notice the middle dipping down in the exhaust port I fixed that. I tied it up so it stays up. Pic later.

Going to try with just a grill block and leave the bumper open and see how that works out.

I'd say this illustrates why the concept of a rear grill block makes no sense. By letting the air flow through, you decrease drag.

How has your belly pan mod affected cabin noise levels, esp. at highway speeds?

Going threw the engine bay the air has to flow around all the stuff in there. So that makes turbulent air. If you stop the air from going it then it will reduce drag.

Its the same. I have always had tire noise on the highway cuz they use a different kind of road. When I am on streets I don't have tire noise. Even if it did affect cabin noise I didn't notice.

That's what it was like when it was completely blocked off with no exit. No exit means no air going in. The grill is just acting like a parachute with air going in a certain distance and spilling back out


frank lee and I were arguing about this months ago.


The grill opening is small, the outlet is small. That means air slows down quite a bit going through the BIG engine compartment. Let's say the open cross section of the engine bay is 4x that of the inlet. That means the air would slow down to 1/4, and have 1/16 the drag as if it were full speed.

That would explain why the outlet vent on that plane reduces drag so much... Because of the small cowl opening, engine bay drag is low, and it acts as a tube to let air through.

Internal ducting is a black art, but an important one for minimum drag. NACA did a lot of research on this during WWII. Once the cooling air gets in, it needs to slow down and increase pressure to reduce its transit drag, but importantly to have time for its higher pressure air to absorb engine heat. Once having done so, it needs to re-enter the slipstream in as efficient a pathway as possible.

The inlet should be at stagnation point on the nose, where pressure is greatest for ram-air effectiveness, then exit at the point of max. speed and therefore lowest pressure along the sides or bottom. Nature has perfected this, as seen with fast-swimming fishes.

It looks to my non-aeronautical engineering mind the the shape of the P51D radiator housing is a classic air-foil and would form a lifting surface and that lift would oppose the lift generated by the wings. It's position slightly behind the wings would put its "lift" behind the aircrafts presumed center of gravity, pitching the nose upward. The enlarged exit duct looks like it would be a spoiler to that opposing lift and therefore reduce drag. In a seat of the pants measurement, reduced drag is increased thrust.

The Bernoulli effect is insignificant in the flight of a plane. I've read that the effect on the wings is something like 1% of total lift, and that's with BIG wings, rather than the little radiator cowl. I'm sticking with my theory that the bigger rear facing outlet vent allows the cooling system to act as a passthrough.

Edit: Usually the tail is used to push the rear down, and lift the front. So if there was an effect like that from the cowl, it would mean less of that force has to come from the tail. Adding the outlet thing, which would act like a spoiler, would then require more input from the tail.

just a question, you say that the front blocks made your temps go up. i suppose thats from reading your scangauge? but im interested to know if you also notice that your standard car temp needle or whatever is higher than usual? i would be interested to know that, as i have blocked my entire front and my temp needle stays at the verry same position as where it used to be before. but i don't have a scangauge to doublecheck.

I read somewhere that a properly designed NACA cowl over a radial engine could supposedly generate forward thrust. If that is true, I wish I could think of a way to harness that effect on a road car (and correctly model it).

i've never heared of a naca cowling generating thrust. it did however cut down drag greatly when it was introduced, because it allowed for both better cooling flow and reduced drag. previously it was believed that the best way to cool a radial engine was to have all the cylinders exposed to the airlow... most cowling that did exist before that where more wide and there to protect the pilot in his open cockpit from oil splatters (especially in rotary engines wich where radial engines where the entire engine turned with the prop for better cooling!)

because drag was reduced so much planes saw an increase in top speed, and because improve cooling meant leaner mixtures more HP was on tap from the same ammount of fuel.

However the p51 uses an inline engine, with water cooling and the radiator is indeed mounted in a spacial radiator housing, that was developed by North American, although the idea had been tried unsuccessful by Curtis. the shape of the radiator housing is such that the hot air that exits does actually provides thrust, even so much that it will cancel out the drag induced by the radiator. This was called the "Meredith Effect".

All of this happens at rather high speeds compared to cars, and what's more a plane is always moving, even when flying "slow", these planes still go faster than the top speed limit for cars... a car needs proper cooling when it's crawling along at a snails phase, to a completely sealed cooling duct might call for a fan to assist airflow, canceling the benefits.

that said there is possibly quite some room for improvements in the cooling departement