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  #1  
Old November 4th 03, 06:02 PM
Bill Gribble
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Default Newbie question

Completely daft newbie question that I'm hoping will have a simple
answer.

Mention is frequently made of the term L/D, which I know is an
abbreviation of Lift / Drag. For example, the L/D cited in an advert for
a Ka6 recently was "L/d 33"

If L/D is the Lift/Drag ratio, why are the figures accompanying it
frequently not ratios? For instance, in the above, what is the "33"?

Is L/D used as the best Lift/Drag ratio the glider in question can
achieve? How does this translate in real terms? Does it describe the
best glide speed, or relate to the glide ratio in any way?

The glide ratio (eg. An ASH25 has a glide ratio of 60:1) seems a great
way of describing certain aspects of the performance of a glider. Yet
most the specifications I read describing gliders don't give this figure
in quite such a straight forward way. Is this information somehow
derived from other information given? Or is it too variable to generally
provide as a generic statistic (eg. One ASH25 might have a 60:1 ratio,
another less at 45:1)?

Am I managing to make any sense, or am I completely confusing myself (in
which case, no worries - more time, exposure and experience will
eventually rattle all this out for me, I'm sure)?

--
Bill Gribble

/----------------------------------\
| http://www.cotswoldgliding.co.uk |
| http://members.aol.com/annsweb |
| http://www.shatteredkingdoms.org |
\----------------------------------/
  #2  
Old November 4th 03, 08:12 PM
Waduino
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Default

People just tend to leave off the :1, e.g. 33 means 33:1. That's all.
Generally speaking the ratio applies to the model of sailplane and shouldn't
be too hard to find, with a little digging.
---


"Bill Gribble" wrote in
message .. .
Completely daft newbie question that I'm hoping will have a simple
answer.

Mention is frequently made of the term L/D, which I know is an
abbreviation of Lift / Drag. For example, the L/D cited in an advert for
a Ka6 recently was "L/d 33"

If L/D is the Lift/Drag ratio, why are the figures accompanying it
frequently not ratios? For instance, in the above, what is the "33"?

Is L/D used as the best Lift/Drag ratio the glider in question can
achieve? How does this translate in real terms? Does it describe the
best glide speed, or relate to the glide ratio in any way?

The glide ratio (eg. An ASH25 has a glide ratio of 60:1) seems a great
way of describing certain aspects of the performance of a glider. Yet
most the specifications I read describing gliders don't give this figure
in quite such a straight forward way. Is this information somehow
derived from other information given? Or is it too variable to generally
provide as a generic statistic (eg. One ASH25 might have a 60:1 ratio,
another less at 45:1)?

Am I managing to make any sense, or am I completely confusing myself (in
which case, no worries - more time, exposure and experience will
eventually rattle all this out for me, I'm sure)?

--
Bill Gribble

/----------------------------------\
| http://www.cotswoldgliding.co.uk |
| http://members.aol.com/annsweb |
| http://www.shatteredkingdoms.org |
\----------------------------------/



  #3  
Old November 4th 03, 08:36 PM
David Starer
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Posts: n/a
Default

Bill,

Your question is not daft, it just sounds as though you've been listening to
"experts" who tend to abbreviate the terminology in a way that might be
confusing to outsiders.

L/D is indeed the lift/drag ratio. It's possible to show by drawing the
diagram of forces acting on the glider that this ratio is equal to the ratio
of airspeed to sink rate. This is true for any speed since the drag changes
as the airspeed changes.

When you hear people talking about a glider's L/D or glide ratio, what they
usually mean is the best or highest value of this ratio that can be achieved
by a particular glider. An L/D or glide ratio of 33 is shorthand for 33:1,
i.e. the glider will travel forwards 33 times as far as it descends in a
given time. It's important to appreciate that this will occur at one
specific speed, known as the best glide speed or Max Glide. What this speed
actually is depends on the wing loading for any particular type, but is
typically about 50-55 kts for most modern gliders and increases as the wing
loading increases. The maximum achievable glide ratio is basically an
inherent property of any particular type though you can achieve small
alterations by moving the centre of gravity and other subtle modifications.
The important thing to understand is that a particular L/D is only achieved
at one single speed. In your example you are correct that the ASH25 achieves
60:1, but only at, say, 55 knots. At 100 knots its L/D might be, for
example, 40:1 and will continue to decrease as the speed increases.

If you consult the polar curve for any glider you can see how the glide
ratio changes with speed. Lay a ruler from the origin to any point on the
polar and its slope will represent the glide angle at that speed. Divide the
IAS by the sink rate for that point on the polar to get the numerical value
of glide ratio at that particular speed. It will be pretty obvious that as
you move the ruler's point of contact along the polar, there will be a
single position where it just makes a tangent to the polar, where the
ruler's slope is at its most shallow. This represents the best achievable
glide angle and you can read off the speed where this occurs. If the graph
contain several polars at different wing loadings, you can see how the glide
angle is affected as the wing loading changes. You will find that the actual
best glide ratio is more or less the same for each wing loading, but will be
achieved at higher speeds for the heavier glider.

David Starer

"Bill Gribble" wrote in
message .. .
Completely daft newbie question that I'm hoping will have a simple
answer.

Mention is frequently made of the term L/D, which I know is an
abbreviation of Lift / Drag. For example, the L/D cited in an advert for
a Ka6 recently was "L/d 33"

If L/D is the Lift/Drag ratio, why are the figures accompanying it
frequently not ratios? For instance, in the above, what is the "33"?

Is L/D used as the best Lift/Drag ratio the glider in question can
achieve? How does this translate in real terms? Does it describe the
best glide speed, or relate to the glide ratio in any way?

The glide ratio (eg. An ASH25 has a glide ratio of 60:1) seems a great
way of describing certain aspects of the performance of a glider. Yet
most the specifications I read describing gliders don't give this figure
in quite such a straight forward way. Is this information somehow
derived from other information given? Or is it too variable to generally
provide as a generic statistic (eg. One ASH25 might have a 60:1 ratio,
another less at 45:1)?

Am I managing to make any sense, or am I completely confusing myself (in
which case, no worries - more time, exposure and experience will
eventually rattle all this out for me, I'm sure)?

--
Bill Gribble

/----------------------------------\
| http://www.cotswoldgliding.co.uk |
| http://members.aol.com/annsweb |
| http://www.shatteredkingdoms.org |
\----------------------------------/



  #4  
Old November 4th 03, 08:48 PM
tango4
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Posts: n/a
Default

Bill

Generally they are treated interchangeably so your LD 33 ( K6e on
Gliderpilot ) is the same as 1:33 glide angle.

A Nimbus 4 has an L/D of 1:60 or a glide angle of 1:60 ( 60 bits of lift
for one bit of drag 60' forward for 1' of altitude lost )

Generally the figure quoted refers to the best glide angle that the aircraft
can acheive, there is no standard speed at which this is quoted so its a bit
misleading.

What everyone works from is the gliders 'polar diagram'. A plot of speeds
against sinkrate.

A few examples can be found at the SSA site
http://www.ssa.org/Magazines/Johnson.asp

These days the manufacturers still aim to develop good L/D ratios ( around
1:50 seems to be the mark for todays 18m ships ) but the game seems to be to
get the polar as flat as possible and to try to maintain good glide ratios
into the 150 kph or 80 kts range and higher.

Given two gliders with the same LD, say 1:42 ( 2nd generation glass ships )
one has best L/D at 90 kph the other at say 95 kph the theory is that the
second must win all the tasks ( higher average speed )

Reichmans 'cross country soaring' is great for filling your head with the
theory of all this. Well worth a copy even at the newbie end of your flying
career!

Ian



"Bill Gribble" wrote in
message .. .
Completely daft newbie question that I'm hoping will have a simple
answer.

Mention is frequently made of the term L/D, which I know is an
abbreviation of Lift / Drag. For example, the L/D cited in an advert for
a Ka6 recently was "L/d 33"

If L/D is the Lift/Drag ratio, why are the figures accompanying it
frequently not ratios? For instance, in the above, what is the "33"?

Is L/D used as the best Lift/Drag ratio the glider in question can
achieve? How does this translate in real terms? Does it describe the
best glide speed, or relate to the glide ratio in any way?

The glide ratio (eg. An ASH25 has a glide ratio of 60:1) seems a great
way of describing certain aspects of the performance of a glider. Yet
most the specifications I read describing gliders don't give this figure
in quite such a straight forward way. Is this information somehow
derived from other information given? Or is it too variable to generally
provide as a generic statistic (eg. One ASH25 might have a 60:1 ratio,
another less at 45:1)?

Am I managing to make any sense, or am I completely confusing myself (in
which case, no worries - more time, exposure and experience will
eventually rattle all this out for me, I'm sure)?

--
Bill Gribble

/----------------------------------\
| http://www.cotswoldgliding.co.uk |
| http://members.aol.com/annsweb |
| http://www.shatteredkingdoms.org |
\----------------------------------/



  #5  
Old November 5th 03, 12:56 AM
F.L. Whiteley
external usenet poster
 
Posts: n/a
Default

Peripheral, but pertinent to your understanding Bill. As part of his TLAR
exercise, Tom Knauff suggests this.

You are approximately two (of your) paces tall. Mark a spot and take ten
paces. Look back at the angle. That's 1/5 from your eyes to the spot.
Take 30 more paces and you have 1/20. Now try 30, 40, and even 60/1 if you
have the room. Gives a bit of meaning to the ratios.

TLAR stands for "That Looks About Right".

Frank Whiteley


  #6  
Old November 5th 03, 10:43 AM
Bill Gribble
external usenet poster
 
Posts: n/a
Default

Thanks all. Much, much clearer now!


--
Bill Gribble

/----------------------------------\
| http://www.cotswoldgliding.co.uk |
| http://members.aol.com/annsweb |
| http://www.shatteredkingdoms.org |
\----------------------------------/
  #7  
Old November 6th 03, 08:57 PM
Mark James Boyd
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Posts: n/a
Default

at one single speed. In your example you are correct that the ASH25 achieves
60:1, but only at, say, 55 knots. At 100 knots its L/D might be, for
example, 40:1 and will continue to decrease as the speed increases.

If you consult the polar curve for any glider you can see how the glide
ratio changes with speed. Lay a ruler from the origin to any point on the
polar and its slope will represent the glide angle at that speed. Divide the
IAS by the sink rate for that point on the polar to get the numerical value
of glide ratio at that particular speed. It will be pretty obvious that as
you move the ruler's point of contact along the polar, there will be a
single position where it just makes a tangent to the polar, where the
ruler's slope is at its most shallow. This represents the best achievable
glide angle and you can read off the speed where this occurs. If the graph
contain several polars at different wing loadings, you can see how the glide
angle is affected as the wing loading changes. You will find that the actual
best glide ratio is more or less the same for each wing loading, but will be
achieved at higher speeds for the heavier glider.

David Starer



Perhaps it's good to know why this is important, too.
In training and gliding, often in smooth air, you'll fly
the min sink speed or the best L/D speed and think that is
great.

In soaring, there may be lift in places, and hellacious
sink in places (caused by the ridge lee, rotor, divergence,
a cruel God, etc...). When in sink, you'll want to
get out of it in a shorter amount of time.

To answer how well you can escape the sinking air, look at
the L/D at the higher speeds (let's say at double the
best L/D speed). A really bad L/D at high speeds means
you'll be in bad sink, push the nose over to accelerate, and
notice the airspeed indicator hardly increase, while the
vario shows you going down a lot more.

This is called "lack of penetration." Fancy gliders
have negative flaps to reduce the sink rate at high speeds,
and ballast to make the best L/D speed a higher speed.

I noticed this most when flying a 2-33 with a heavy
passenger, vs. flying a 1-26 solo. Boy, the 1-26
doesn't speed up much, but just goes down if I push the
nose over. Fiberglass ships, especially with
ballast and negative flaps, accelerate very quickly
with little forward stick.

The 1-26, the PW-2, the Quicksilver MXIII Sprint
ultralight, and the Piper J-3 Cub are
"floaters", meaning they require very little lift
to stay aloft. On the other hand they "penetrate"
poorly, meaning drag and sink rate build up quickly
at higher airspeeds.

The other place this is important is in winds.
I got blown downwind from an airfield once at about
2000 ft, and didn't realize there was a 20-25 knot wind.
I really had to push the nose over a lot to get back to
the field and almost didn't make it back. Adding 15
knots over best L/D speed really increased the sink
rate a lot.

 




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