A lunch buddy yesterday was telling a story about a car drafting his winnebago and complaining about it.

I said who cares, he cannot hurt you?

He said it reduced his Mpg in his vehicle.

I called BS.

anybody have any science to support the arguement one way or the other?

If two race cars can go faster together they should also get better mpg together so this car should not affect the motor home except if to slightly increase fuel economy....in my mind.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Tom,
No science, but scientific intuition---needing massive federal funding to turn it into science.

The Winnie pushes the air out of the way. The amount of energy required to do this is directly related to the Winnie's speed, and shape ( and prevailing wind speed and direction). I see no way that a vehicle in the draft space behind the Winnie has any effect on the Winne--unless he extends a tow bar that latches onto the rear bumper of the Winnie......

Perhaps the Winnie driver believes all in life is a zero-sum game, so if the drafting vehicle benefits, then it must necessarily be costing the Winnie.

__________________
1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

OTOH the geese switch off the lead position....But that might only be because all the other positions are a little less strenuous.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Geese trade off the lead position because drafting is easier than breaking the wind, so to speak. (That reminds me of the movie "Spinal Tap." They were getting ready to come out with an album called "Break Like the Wind," as I recall.)

As I recall from engineering school (over 30 years ago!), the equations describing the aerodynamic drag on the Winnebego are parabolic equations, which means that the only conditions you need to know are the upstream conditions. What ever is happening behind the Winnebago has no bearing on the drag it sees. As I recall, the story changes as you cross the speed of sound. Then you get into hyperbolic equations that require you to know the downstream conditions as well.

Or something like that.

Your friend raises an interesting puzzler, though. On the surface, it seems like the guy dragging off the Winnebago is getting a free lunch, which generally doesn't happen in nature.

With the Nascar racers, two are less wind resistance than each separately and added together, so the second car makes the first car move through the air easier, and himself as well.

So the car behind the winnebago may help him along a bit.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

That's a good point concerning the Nascar racers. My guess is that they draft so close to one another that they basically form one body moving through the air. As such, they only leave one wake and it only pulls back on the rear car. If so, then each car helps the other. The Winnebago is a completely different size and shape than the car dragging on it, but there still might be a similar effect if the rear car is right up tight on the Winnebago's bumper.

A very small bit, but yeah, that's the idea. Having something fill in the space behind any vehicle with a sharp cut-off at the end reduces it's pressure drag.

HowStuffWorks "Drafting Strategy"

That said, maybe the Prius driver is stealing the Winnebago driver's brainwaves. I think it's time to triple-bag some tinfoil hats.

Thermodynamics.

It's the vacuum or low pressure behind the vehicle that causes suction. Put two cars together and it's like making one long car, only the one in front has no vacuum sucking it backwards. That's dealt with by the rear car. So they are sharing the aerodynamic load.

__________________
You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman

So if its a low pressure area, does the engine have higher pumping losses that lower its efficiency?
(Yeah, over- thinking)

__________________
1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

Theoretically, that might be true, but if the rear car is a normally-aspirated gas engine car, all it needs to do to overcome the reduced pressure is to open the throttle a bit more.

The real answer, I think, is that the vacuum created by the wake of the lead car is much, much milder than the suction created by the engine, so it probably doesn't make much difference.

That's my story and I'm sticking to it, at least until someone who knows what they are talking about chimes in.

If the tailgater is close enough, he could increase the MPG on the draftee (and the speed on the NASCAR example) by transferring the pressure drag from the draftee onto the drafter.

Slides from Stanford here, 8 and 9 are probably best examples: http://gcep.stanford.edu/pdfs/ChEHeXOTnf3dHH5qjYRXMA/10_Browand_10_11_trans.pdf