Belly Pans on Semi Trucks

[aerohead]

*NASA's airdam showed a 7% wind-averaged drag reduction
*Their belly pan gave 5%
*Full gap seal 17%
*Side skirts 19%
*Rooftop fairing 9%
*Tail 10%
*This truck's Cd,with all mods dropped from around 0.705,to 0.232.
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*Freightliner Corp.achieved similar results with their research
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Renault's V.I.R.A.G.E. showed :
*COE leading edge rounding to r=0.2H gave Cd 0.28,down from 0.92
*Full gap seal = delta 0.072
*Belly pan= 17.1% reduction
*Skirts = 27.2%
*Boat tail 394mm (15.5") = 0.025 reduction
--------- 750mm (29.5") = 0.035
--------- 875mm (34.4") = 0.041
*Their semi,with full gap seal and longest tail yielded Cd 0.222,@ 8.473 mpg (27.431 L/100 km) @ 38-tonnes (83,600 lbs),@ 100 km/hr (62 mph)
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FIAT's bus research yielded Cd 0.232 without any boat-tailing
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Fachsenfeld's 1936 Omnibus,with full belly pan and 80% 'Template' inflated boat tail showed Cd 0.159
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Walter Korff's semi concept of 1963 was predicted at Cd 0.20
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As of 1980,Freightliner Corp. did not consider the belly pan to be practical and ruled it out
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Laws and rules can be changed

[Diesel_Dave]

Originally Posted by aerohead

*NASA's airdam showed a 7% wind-averaged drag reduction
*Their belly pan gave 5%

Are these numbers independent (air dam by itself = 7%, belly pan by itself = 5%) or are the combined, (air dam + belly pan = 5% better than just air dam)?

Phil, I'd value your input on the benefit of air dam vs. belly pan. I've been wondering about it for my truck. As I understand it (and I may be wrong), the priciple benefit of an air dam is that it directs more air away from the underside of the vehicle (which typically has poor aero characteristics). Thus, even though the frontal area has been enlarged, there is a net benefit (Cd*A is smaller, even though A is larger).

My thinking has been that if one uses a belly pan to improve the aero characteristics of the underside of the vehicle, then an airdam no longer makes much sense. This is because there's no longer as much benefit from directing the air away from the underside.

Do I have this right? Or even with a belly pan is the underside aero still poor enough that an air dam has benefits?

[slowmover]

I'd bet that the most effective (work vs. results) for a pickup will be a partial bellypan (body inwards to frame, and, full coverage behind rear axle) with an air dam 8-10" from ground height. Were a pickup low to the ground it wouldn't matter much, but crosswinds have significant action on these high COG vehicles.

[aerohead]

Originally Posted by Diesel_Dave

Are these numbers independent (air dam by itself = 7%, belly pan by itself = 5%) or are the combined, (air dam + belly pan = 5% better than just air dam)?

Phil, I'd value your input on the benefit of air dam vs. belly pan. I've been wondering about it for my truck. As I understand it (and I may be wrong), the priciple benefit of an air dam is that it directs more air away from the underside of the vehicle (which typically has poor aero characteristics). Thus, even though the frontal area has been enlarged, there is a net benefit (Cd*A is smaller, even though A is larger).

My thinking has been that if one uses a belly pan to improve the aero characteristics of the underside of the vehicle, then an airdam no longer makes much sense. This is because there's no longer as much benefit from directing the air away from the underside.

Do I have this right? Or even with a belly pan is the underside aero still poor enough that an air dam has benefits?

The values are presented for each specific mod.
I don't have a one-size-fits-all answer with respect to the airdam/pan question.
On my T-100,the aluminum/rubber airdam pushed the mpg into new territory with a compromised belly.It was sized so as not to aggravate frontal area at all.6" ground clearance,no lower.
I just got my 2013 emissions testing/safety inspection which required a new muffler/tailpipe; and removal of the boat tail and most of the pan to facilitate that.
I'll be camping at Monahans State Park with the Gilkisons,early April and will leave the belly and tail off 'n see what shows up at the pump.It's about a 750 mile roundtrip (normally quite easy on a single tank).Hope to extract some data,It's wait 'n see.
I raced Bonneville with a number of panels missing underneath and my opinion is that the airdam helped compensate for the missing parts.
All the really low drag concepts use every trick in the book,including active suspension.Airdams and pans are always part of the recipe.
I see many contemporary cars,SUVs,and Crossovers which direct air underneath.They're doing it for approach clearance,but aerodynamically,it's not the best move.
Rubber turned out to be my friend on this one.