Aerodynamics acording to Myth Busters!

"cavelamb himself" wrote in message
link.net...
...
But those webbed tailgate thingies have GOT to increase drag.

Don't they???

Richard

OK, Following up on on old thread.

I spent the day working in an automotive wind tunnel today, and I thought to
ask the operator if he had tried tailgate up / down in the tunnel.

He said they did a bunch of tests on an F150 and found that a tonnau cover
was best, a cap was nearly as good, tailgate up was better than down (I
forget if there was a difference between down and removed) and the web
things were the worst.

With the tailgate up, when they used smoke, the smoke would go over the roof
and then come down and touch the top of the tailgate.

--
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.

On Oct 31, 6:17 pm, "Capt. Geoffrey Thorpe" The Sea Hawk at wow way
d0t com wrote:
"cavelamb himself" wrote in message

link.net...
...

But those webbed tailgate thingies have GOT to increase drag.

Don't they???

Richard

OK, Following up on on old thread.

I spent the day working in an automotive wind tunnel today, and I thought to
ask the operator if he had tried tailgate up / down in the tunnel.

He said they did a bunch of tests on an F150 and found that a tonnau cover
was best, a cap was nearly as good, tailgate up was better than down (I
forget if there was a difference between down and removed) and the web
things were the worst.

With the tailgate up, when they used smoke, the smoke would go over the roof
and then come down and touch the top of the tailgate.

--
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.

Jeez, there goes my excuse for not fixing the broken latch on the
tailgate... I hate it when that happens...

denny

"Denny" wrote

Jeez, there goes my excuse for not fixing the broken latch on the
tailgate... I hate it when that happens...

Ahh, don't sweat it too much.

A couple bungee chords will do just fine. g
--
Jim in NC

Capt. Geoffrey Thorpe wrote:
"cavelamb himself" wrote in message
link.net...
...
But those webbed tailgate thingies have GOT to increase drag.

Don't they???

Richard

OK, Following up on on old thread.

I spent the day working in an automotive wind tunnel today, and I thought to
ask the operator if he had tried tailgate up / down in the tunnel.

He said they did a bunch of tests on an F150 and found that a tonnau cover
was best, a cap was nearly as good, tailgate up was better than down (I
forget if there was a difference between down and removed) and the web
things were the worst.

With the tailgate up, when they used smoke, the smoke would go over the roof
and then come down and touch the top of the tailgate.

--
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.


I work on the RJ program and it was once explained to me, in a
discussion of devices to slow airflow in ducts, that a wire mesh
structure has far more drag and will create a much larger and more
permanent (longer lasting effect downstream) drop in velocity than a
single obstruction, even if the single obstruction is much larger. In
our case, a nozzle type restrictor used on the cockpit air duct had a
fraction of the effect on velocity downstream, that the mesh screen at
the outlet had, and was basically useless. So it's easy to see how
those tailgate screens are the worst thing to do for drag.

John

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

"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

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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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.