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

On Sep 19, 10:43 am, "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...
>
> --
> Jim Carter
> Rogers, Arkansas


This is a very good question. As others have mentioned, max (L/D) and
best glide IAS remain the same regardless of altitude. But that does
not immediately lead to the conclusion that glide ratio also remains
constant. L/D and best glide speed is about indicated airspeeds. At
higher altitudes, the best glide speed (IAS) wll result in a higher
true airspeed. So one might be tempted to conclude that the airplane
may be able to glide father, which is why I suspect you asked the
question in the first place.

It is not sufficient to argue that L/D remains constant, but you also
have to show that L/D is equal to the glide ratio. Glide ratio is the
physical horizontal distance traveled over still air for a unit
vertical altitude lost. Therefore, it is the ratio between the true
airspeed and the true vertical speed (TVS). I am sure there are many
ways to do this, but I find the energy argument the most intuitive.
Consider the power lost to drag, which is (D*TAS). This should be
equal to the rate of energy given up by the airplane due to its
descent. The rate of energy lost due to the descent is W*TVS, where W
is the weight of the airplane. We also know that W=L under
unaccelarated flight. Therefore, D*TAS = L*TVS and we can TAS/TVS =
glide ratio = L/D.

This might be obvious to some, but I had to do this step by step in
order to convince myself that L/D is indeed equal to the glide ratio.
Pehaps you might find it useful.

Jim Carter wrote:

> *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?

Well, yes. Anything else wouldn't make much sense, would it? Argh. Must've
forgotten to take my mediaction or s/th.

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.