Aerodynamics acording to Myth Busters!

"Peter Dohm" wrote in message
...
|
| "Denny" wrote in message
| ups.com...
| On Jul 24, 12:20 am, "Richard Isakson" wrote:
| "Matt Whiting" wrote ...
|
|
|
|
|
| cavelamb himself wrote:
|
| It is more fuel efficient to drive a pick-up truck with its
tailgate
| down, rather than up.
|
| Busted
|
| Driving with the tailgate down actually increased drag on the
| pick-up
| and caused it to consume fuel faster than the identical truck
driven
| with the tailgate up. It was later revealed that the closed
tailgate
| creates a locked vortex flow that created a smoother flow of
air
| over
| the truck. With the tailgate down, the trapped vortex was
dissipated
| and
| the drag increased.
|
|
http://en.wikipedia.org/wiki/MythBus...%29#Blown_Away
|
| I remember seeing a wind tunnel test on this many years ago (I
think
| in
| the late 70s when I was an aerospace engineering student) long
before
| Mythbusters existed. However, there is no way you can convince
some
| people that this is a myth. They will argue to the death that
they
| gain
| mileage with the tailgate down, however, the wind tunnel tests
were
| very
| conclusive. A rotating vortex is established in the truck once
you
| get
| above a fairly slow speed (I think 30 MPH or so) and this acts
almost
| like a large balloon in the bed of the truck and directs the
airflow
| over the tailgate. You can see this pretty easily from the
smoke in
| the
| wind tunnel and the drag change was noticeable also.
|
| Not only do you not get better mileage with the tailgate down
.... you
| actually get worse mileage! Then again, most people can't even
check
| their MPG correctly...
|
| Matt
|
| YMMV!
|
| Rich- Hide quoted text -
|
| - Show quoted text -
|
| Paper material in the bed of my truck at 55mph rotates once or
trwice
| in that standing vortex then vanishes over the tailgate - never to
be
| seen again...
| Any empty, plastic gas can left against the tailgate slides up
against
| the back of the cab - again that standing vortex...
| Now, that the gate up has less drag overal still amazes me, in spite
| of knowing what is theoretically happening...
|
| denny
|
| I, for one, am not convinced that a p/u has less drag with the gate
| closed--although I concede that many trucks may gain a trivial benefit
at
| some particular speed.
|
| Their are just too many variables; including the shape of the cab,
shape of
| the nose, slope of the windshield, and length of the bed; to draw any
| meaningful concludions from just one or two tests. In addition, the
vortex
| grows in size with increasing speed--so that the forward moving
portion will
| be nearer the back of the cab at lower speeds and may be aft of the
tailgate
| at very high speeds. However, a radically sloped nose and windshield
may
| cause the vortex to be much longer front to back at all speeds; the
the
| lengths of the cab and bed will have an effect.
|
| If I had to wager on the outcome of a *real* test of this issue, I
would
| guess that there would be a slight drag reduction (on average) with
the
| tailgate down or removed--especailly if there is any cross wind--and
that a
| bed cover (a/k/a tonneau) would do at least as well under *all*
conditions.
|
| In any case, if you choose to run with the gate open or off, be sure
to
| secure your load! About 20 years ago, I personally watched a driver
lose a
| P&W Wasp Jr when he forgot it was there and mashed the accelerator.
That
| was a major "ah, darn"!
|
| Just my $0.02
| Peter


This issue came up on the Toyota PU site some time back. One of the
references was a university study that clearly showed that the mileage
was better with the tail gate up. Here is a link showing no meaningful
difference- http://www.scangauge.com/support/tailgate.shtml

The university study also tested bed covers and bed caps. The best
mileage was obtained with a cap that was cab high and had a rounded rear
end that blended into the tailgate.

While searching I found that in some states running with the tailgate
down is technically against the law.

--
OldPhart



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