Aerodynamics acording to Myth Busters!

"Harry K" wrote in message
ups.com...
On Jul 29, 6:33 pm, (Scott) wrote:
On Sun, 29 Jul 2007 10:04:39 -0400, in rec.aviation.homebuilt, "Peter
Dohm"

wrote:
Have you 1) verified you odometer against at least 10 miles of highway
mile
markers, 2) verified your speedometer reading at 60MPH as a result of
the
same test, and 3) checked the cumulative fuel mileage, as shown on the
computer, against your fuel mileage calculated in the usual way?

Seconded. I've never seen an automotive trip computer that was worth
more
than the recyclable metals in it.

My Grand Prix has a trip computer, and what it tells me is scarcely
better
than a wild guess. It reports better fuel mileage than I really get,
and
underreports the amount of fuel I've used. When the tank's full it
tells me
I have a range of 430 miles (best I've ever gotten was ~350), and it
raises
a fuel alarm when I've still got 80 miles in the tank. Now and then, it
will raise a low fuel alarm when the tank is completely full.

It probably kicks puppies and steals candy from babies, too.

-Scott

Probably quite true but...

It is a useable instrument to detect what mode of driving is most
efficient even it it is not accurate. That is if it reports, for
example, 19 mpg over a stretch of road with tailgate down and then you
repeat the run in the same direction, same speed, same conditions,
etc. the reported mpg is then a useable bit of data. Yes, it will be
inaccurate but the comparison is useable and meaningful.

Harry K

Probably true. However, the coastdown test mentioned elsewhere in this
tread is probably the most consistently reliable method--when properly
controlled. The most obvious requirements a state the coast down from
the same place and speed each time, record the remaining speed at the
seconde marker, continue testing in the same direction, warm up the tires
before the first test, and choose a day (or days) with constant temperature
and wind conditions. There are certainly more, but those are enough to give
a far more reliable result than any test in traffic that I can think of--and
even then, since we are discussing the drag of detached flow, the test may
only be valid for the speed(s) at which the test was run.

Peter

"Harry K" wrote in message
ups.com...

It is a useable instrument to detect what mode of driving is most
efficient even it it is not accurate. That is if it reports, for
example, 19 mpg over a stretch of road with tailgate down and then you
repeat the run in the same direction, same speed, same conditions,
etc. the reported mpg is then a useable bit of data. Yes, it will be
inaccurate but the comparison is useable and meaningful.

I would not assume that a faulty instrument is going to give me readings
that are inaccurate in a consistent way across a given range.

-Scott

"Scott" wrote in message
news:46ae8308.426924174@localhost...
"Harry K" wrote in message
ups.com...

It is a useable instrument to detect what mode of driving is most
efficient even it it is not accurate. That is if it reports, for
example, 19 mpg over a stretch of road with tailgate down and then you
repeat the run in the same direction, same speed, same conditions,
etc. the reported mpg is then a useable bit of data. Yes, it will be
inaccurate but the comparison is useable and meaningful.

I would not assume that a faulty instrument is going to give me readings
that are inaccurate in a consistent way across a given range.

-Scott

Wait a minute Scott,

You got a little too ambitious with the clipping and the part you left in
was from a previous contributor. I only agreed that the errors in the fuel
computer would probably be consistent--electronics usually are--and then
recommended a proceedure to conduct coast down testing.

My contribution was the following:

"Probably true. However, the coastdown test mentioned elsewhere in this
tread is probably the most consistently reliable method--when properly
controlled. The most obvious requirements a state the coast down from
the same place and speed each time, record the remaining speed at the
seconde marker, continue testing in the same direction, warm up the tires
before the first test, and choose a day (or days) with constant temperature
and wind conditions. There are certainly more, but those are enough to give
a far more reliable result than any test in traffic that I can think of--and
even then, since we are discussing the drag of detached flow, the test may
only be valid for the speed(s) at which the test was run.

Peter"

Peter

On Tue, 31 Jul 2007 08:23:18 -0400, in rec.aviation.homebuilt, "Peter Dohm"
wrote:

"Scott" wrote in message
news:46ae8308.426924174@localhost...
"Harry K" wrote in message
ups.com...

It is a useable instrument to detect what mode of driving is most
efficient even it it is not accurate. That is if it reports, for
example, 19 mpg over a stretch of road with tailgate down and then you
repeat the run in the same direction, same speed, same conditions,
etc. the reported mpg is then a useable bit of data. Yes, it will be
inaccurate but the comparison is useable and meaningful.

I would not assume that a faulty instrument is going to give me readings
that are inaccurate in a consistent way across a given range.

-Scott

Wait a minute Scott,

You got a little too ambitious with the clipping and the part you left in
was from a previous contributor. I only agreed that the errors in the fuel
computer would probably be consistent--electronics usually are--and then
recommended a proceedure to conduct coast down testing.

Sorry about the piggyback post, but I was replying to HarryK, as the
(edited) attribution shows. I responded via your post because his was
already expired, and I was not ambitious enough to download it again.

I agree with you about the coastdown test. I disagree that a faulty fuel
computer will necessarily be reliable in any useful way. In my GP I have an
example of a computer that not only produces incorrect results, but produces
results that are wildly inconsistent when compared to results from the usual
method of measuring fuel mileage. Perhaps others have more reliable errors,
but you can't know without testing them first.

(To be fair, my GP's computer is probably very accurate based on the data it
gets. The problem is most likely with the analog data sources for fuel flow
rate and/or fuel level.)

-Scott

(Scott) wrote:

On Tue, 31 Jul 2007 08:23:18 -0400, in rec.aviation.homebuilt, "Peter Dohm"
wrote:

You got a little too ambitious with the clipping and the part you left in
was from a previous contributor. I only agreed that the errors in the fuel
computer would probably be consistent--electronics usually are--and then
recommended a proceedure to conduct coast down testing.

Sorry about the piggyback post, but I was replying to HarryK, as the
(edited) attribution shows. I responded via your post because his was
already expired, and I was not ambitious enough to download it again.

I agree with you about the coastdown test. I disagree that a faulty fuel
computer will necessarily be reliable in any useful way. In my GP I have an
example of a computer that not only produces incorrect results, but produces
results that are wildly inconsistent when compared to results from the usual
method of measuring fuel mileage. Perhaps others have more reliable errors,
but you can't know without testing them first.

(To be fair, my GP's computer is probably very accurate based on the data it
gets. The problem is most likely with the analog data sources for fuel flow
rate and/or fuel level.)

I think Peter's point is still valid - while the mileage computers
might not be entirely accurate, the inaccuracies should be relatively
consistent. That is, if you're doing a test where all other variables
are the same, and the computer tells you you're getting an extra 1mpg,
you can't be sure that the delta is precisely 1mpg, but you can bet
that there IS a difference, and that it's probably not TOO far from
that value.

For example, my Jeep Grand Cherokee has a computer, and I drive a lot
on fhe Arizona roads that are absolutely flat and have very little
traffic. I can get the cruise control set, let everything stabilize,
and see a 1mpg difference from clicking the overdrive on and off
(which is about what I'd expect the actual delta to be). Can I say
with any certainty that the difference is very close to 1mpg? Nope.
But can I surmise there IS an advantage to running with the overdrive
engaged, based on the computer? Absolutely.

Mark "now if I could only get it to go up 10mpg..." Hickey

"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

I admit, I didn't follow the original thread, but I've driven
a pickup for the past 30 years and I can tell you that each
time I've installed a cap I lost 2mpg.
Lou

On Thu, 01 Nov 2007 13:46:34 -0700, Lou wrote:

I admit, I didn't follow the original thread, but I've driven
a pickup for the past 30 years and I can tell you that each
time I've installed a cap I lost 2mpg.
Lou
And on my 1957 Fargo, putting the gate down gave me an extra couple of
MPH and MPG. Mind you that whole truck had no aerodynamics at all.
Stepside with sidemount.

--
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