Aerodynamics acording to Myth Busters!

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

"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

"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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"John Clear" wrote in message
...
In article .net,
cavelamb himself wrote:

I personaly can't argue either way.

Both answers seem to have merit - and then not...

But those webbed tailgate thingies have GOT to increase drag.

Don't they???

They re-visited the tailgate issue in a later episode, and re-busted
tail gate down being more efficient, but found that the mesh thingie
increased mileage. Still questionable since they were doing the
tests on public roads with traffic.


It's hard to measure small differences in fuel economy even when you are
running a vehicle over the same cycle on a chassis dyno with a million
dollars worth of analyzers- trust me on that one - I've looked at the
results of thousands of emission tests over the years.

Trying to see the difference on public roads in traffic? You might as well
throw some dice for anything less than 10 or 20 percent.

Doing coast-downs is a much better method - it gives a reasonably reliable
results for finding changes in aero drag.


--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

Peter Dohm wrote:

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.


Why is it so hard to believe. You have seperated flow, and seperated
flow equals high drag. Put the tailgate down and you have one huge drag
chute in the form of the cab. Put the gate up, and you get an air dam
in front of the gate that sort of creates an airfoil from the top of the
cab to the top of the gate. You've still got a big drag chute, just not
as big as before.

Someone smarter than me said that how you meet the air isn't as
important as how you leave it.

"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com wrote in message
It's hard to measure small differences in fuel economy even when you are
running a vehicle over the same cycle on a chassis dyno with a million
dollars worth of analyzers- trust me on that one - I've looked at the
results of thousands of emission tests over the years.

Trying to see the difference on public roads in traffic? You might as well
throw some dice for anything less than 10 or 20 percent.

Doing coast-downs is a much better method - it gives a reasonably reliable
results for finding changes in aero drag.


--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.


Exdellent point.

Peter

"Ernest Christley" wrote in message
...
Peter Dohm wrote:

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.


Why is it so hard to believe. You have seperated flow, and seperated
flow equals high drag. Put the tailgate down and you have one huge drag
chute in the form of the cab. Put the gate up, and you get an air dam
in front of the gate that sort of creates an airfoil from the top of the
cab to the top of the gate. You've still got a big drag chute, just not
as big as before.

Someone smarter than me said that how you meet the air isn't as
important as how you leave it.

It is not hard to believe at all--except for the proposition that either
trick (gate open or gate closed) works on all trucks at all speeds.

"Peter Dohm" wrote

It is not hard to believe at all--except for the proposition that either
trick (gate open or gate closed) works on all trucks at all speeds.

Trucks are not that different. Except for something like the old El Camino, thy
are mostly boxes on wheels, to the wind.

Sure, they have a small styling curve here and there, but most of them are
square chopped off behind the cab, and have tailgates about the same height.
--
Jim in NC

"Denny" wrote in message ups.com...

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 forget which brand truck we were looking at (Ford?), but it had a sculpted shape to the top of the tailgate about 8"
wide. The salesman said it helped to improve gas mileage...

"Blueskies" wrote

I forget which brand truck we were looking at (Ford?), but it had a sculpted
shape to the top of the tailgate about 8" wide. The salesman said it helped to
improve gas mileage...

He was wrong. It is to provide extra clearance for people towing 5th wheel or
gooseneck trailers.
--
Jim in NC