Does glide ratio change with altitude?

During a recent discussion about calculating glide ratio, I began to wonder
what the effects of pressure altitude were on the glide ratio of an
aircraft. Since air is 50% as dense at FL180 as at sea level, would the
glide ratio increase (glide further) as altitude decreases? AND if there is
a difference in glide ratio as altitude changes, then what values do most
manufacturers use when they publish their numbers (if they do)?

Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the
wags can skip that part of the reply...

--
Jim Carter
Rogers, Arkansas

Jim Carter wrote:
During a recent discussion about calculating glide ratio, I began to wonder
what the effects of pressure altitude were on the glide ratio of an
aircraft. Since air is 50% as dense at FL180 as at sea level, would the
glide ratio increase (glide further) as altitude decreases? AND if there is
a difference in glide ratio as altitude changes, then what values do most
manufacturers use when they publish their numbers (if they do)?

Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so the
wags can skip that part of the reply...


Ratio math is good only if terrain is perfectly flat in a
perfect world. You got to figure in the geography to get the
actual ratio. Or...... splat

A little calculus anyone???

"Jim Carter" wrote in message
t...
During a recent discussion about calculating glide ratio, I began to
wonder what the effects of pressure altitude were on the glide ratio of an
aircraft. Since air is 50% as dense at FL180 as at sea level, would the
glide ratio increase (glide further) as altitude decreases? AND if there
is a difference in glide ratio as altitude changes, then what values do
most manufacturers use when they publish their numbers (if they do)?

As you descend, drag increases.

Remember that the purpose if having an engine (thrust) is to overcome
weight, but also DRAG.


Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so
the wags can skip that part of the reply...

Okay...I'll skip that point! :~)

--
Matt Barrow
Performance Homes, LLC.
Cheyenne, WY

"FredGarvinMaleProstitute" wrote in message
...
Jim Carter wrote:
During a recent discussion about calculating glide ratio, I began to
wonder what the effects of pressure altitude were on the glide ratio of
an aircraft. Since air is 50% as dense at FL180 as at sea level, would
the glide ratio increase (glide further) as altitude decreases? AND if
there is a difference in glide ratio as altitude changes, then what
values do most manufacturers use when they publish their numbers (if they
do)?

Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so
the wags can skip that part of the reply...


Ratio math is good only if terrain is perfectly flat in a perfect world.
You got to figure in the geography to get the actual ratio. Or...... splat

A little calculus anyone???

Ok Fred - that's a new one. What does terrain have to do with glide ratio? I
already noted that impact has an impact on the ratio, but only at point of
impact. Other than that one point on the continuum, what effect could
terrain have on glide ratio? And don't get started on turbulence, or wind
shear, or any number of other weather anomalies, because I'm asking about
glide ratio, not glide distance.


--
Jim Carter
Rogers, Arkansas

"Matt Barrow" wrote in message
...

"Jim Carter" wrote in message
t...
During a recent discussion about calculating glide ratio, I began to
wonder what the effects of pressure altitude were on the glide ratio of
an aircraft. Since air is 50% as dense at FL180 as at sea level, would
the glide ratio increase (glide further) as altitude decreases? AND if
there is a difference in glide ratio as altitude changes, then what
values do most manufacturers use when they publish their numbers (if they
do)?

As you descend, drag increases.

Remember that the purpose if having an engine (thrust) is to overcome
weight, but also DRAG.


Yes, (induced) drag increases as a factor of lift caused by the higher air
density, and parasitic drag increases as a factor of speed which isn't
changing. So back to my original question - sort of, does the lift created
by denser air at lower altitudes vary the glide ratio for any significance?

Or is the airfoil performance at a given speed static across the entire
flight profile?


Oh yeah, I do understand that glide ratio changes to 0:0 upon impact, so
the wags can skip that part of the reply...

Okay...I'll skip that point! :~)

--
Matt Barrow
Performance Homes, LLC.
Cheyenne, WY



--
Jim Carter
Rogers, Arkansas

"Marc J. Zeitlin" wrote in message
...
Jim Carter wrote:

During a recent discussion about calculating glide ratio, I began
to wonder what the effects of pressure altitude were on the glide
ratio of an aircraft. Since air is 50% as dense at FL180 as at sea
level, would the glide ratio increase (glide further) as altitude
decreases?

To a first order approximation, the maximum glide ratio (L/D) does not
change with altitude. Since what's changing is the density of the
air, and both the lift and the drag are linearly related to the air
density, the effect cancels out.

Since the max. L/D occurs at a given IAS for any particular weight and
CG position, the TAS will change as the altitude changes, but the L/D
will not (again, to a first order approximation).

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007

I was not sure the relationship between lift and drag was linear, so thanks
for clearing that point up. You've also answered the question very well.
Thanks.

--
Jim Carter
Rogers, Arkansas

"FredGarvinMaleProstitute" wrote:


Ratio math is good only if terrain is perfectly flat in a perfect world.
You got to figure in the geography to get the actual ratio. Or...... splat

LOL. Still got a way to go to catch Mxsmanic, though.


--
Dan

"Did you just have a stroke and not tell me?"
- Jiminy Glick

Jim Carter wrote:

I was not sure the relationship between lift and drag was linear,

*nitpick* That's not what he said. He said that both lift and drag are
linearly related to air density, so that if the air density changes, the
L/D stays constant (ceteris paribus, of course).

Ad.

--
The mail address works, but please notify me via usenet of any mail you send
to it, as it has a retention period of just a few hours.

Adhominem writes:
Jim Carter wrote:

I was not sure the relationship between lift and drag was linear,

*nitpick* That's not what he said. He said that both lift and drag are
linearly related to air density, so that if the air density changes, the
L/D stays constant (ceteris paribus, of course).

What does Airbus certification have to do with it?

Ad.

"Adhominem" wrote in message
...
Jim Carter wrote:

I was not sure the relationship between lift and drag was linear,

*nitpick* That's not what he said. He said that both lift and drag are
linearly related to air density, so that if the air density changes, the
L/D stays constant (ceteris paribus, of course).

Ad.

--
The mail address works, but please notify me via usenet of any mail you
send
to it, as it has a retention period of just a few hours.

*nitpick with finer granularity*

....and if lift is linearly related to air density and drag is linearly
related to air density, then aren't lift and drag in a linear relationship
to each other?

--
Jim Carter
Rogers, Arkansas