How to simply determine the L/D of your glider

On Jan 13, 2:40*pm, Gary Evans wrote:
I think something may have gotten lost in the translation. The
discussion I thought was the L/D value used as part of the required
flight computer data required to establish the correct polar for the
glider. The fact that it will change based on a number of variables
doesn't mean it is a meaningless value. Ideally the other variables
are also taken into consideration by the flight computer either by
manual input or sensors.

You are right something has become lost in this discussion. The OP
stated "Like you all I have questioned what the best L/D of my 3 (now
1)
gliders...were in reality suspecting that real L/Ds would be lower
than the manufacturers published values". He goes on to discuss how
measurements of glide performance during cross country flights showed
that his measured L/D did not match the claimed best L/D for his
gliders.

Several people have pointed out that the achieved glide angle on a
typical cross country flight has little relationship to best L/D since
XC flights are typically not made at best L/D speed and cross country
flights are typically not made in a motionless air mass.

It should be obvious that the ratio of distance flown to altitude lost
on a complete XC flight, or on any segment of an XC flight, is not a
measure of glider performance but a measure of the combination of
glider performance, air mass characteristics, and pilot skill. It is
my opinion that using the term L/D to describe this ratio is
misleading. Also, comparing this number with a manufacturer's claimed
best L/D tells you absolutely nothing about the validity of the
claimed best L/D value unless the primary objective of the flight, or
flight segment, was to fly at best L/D speed.

The title of the thread is "How to simply determine the L/D of your
glider". Several experienced XC pilots have pointed out that using
reported L/D data from typical XC flights is not a valid way to
determine the L/D of a glider.

None of the above denies that useful performance data can be derived
from making constant airspeed speed runs in a still air mass. None of
the above denies that a flight computer or post flight analysis
software can be useful in processing the data. None of the above
denies that the value of the ratio (distance flown)/(altitude lost)
achieved in a properly conducted flight test or by computer modeling
is properly described as "L/D". None of the above denies that the
maximum value of that ratio is properly described as "Best L/D".

Andy

On Jan 15, 2:00*am, "Paul Remde" wrote:
:The "Current L/D" window in SeeYou Mobile is very powerful because
you can
:compare it with the required L/D to see how you are doing on the way
to a
:goal. *I remember being impressed with it one day while flying in the
:Minnesota Soaring Club's SZD Junior. *The Junior is a wonderful
glider, but
:it has thick wings and doesn't penetrate wind very well. *I was
fighting a
:20+ knot headwind to get to my destination and could see the
destination
:clearly. *I noticed that my required L/D was onlly 20. *The Junior
can
:theoretically perform at a 35:1 glide ratio, but with the strong
headwind my
:measured "Current L/D" was 12. *I liked that the number was an actual
:measured performance number, not an estimate based on a previously
:measured wind and the entered approximate polar data. *I knew for a
fact that I
:needed another thermal - and I found one. *Cool feature!

It's very interesting that you used the term "L/D" three times, and
every single time it was an incorrect usage. What you were referring
to was in fact the distance traveled in losing a certain amount of
height, or in other words the achieved glide ratio.

And yet the ONE time that you could have correctly said "L/D" you said
"glide ratio" instead!!

A given aircraft at a given weight and in similarly clean condition
will always perform at a fixed L/D (lift/drag) at any particular speed
and G loading. It is an aerodynamic property.

The actual glide ratio achieved will depend on head or tail wind, lift
or sink encountered. As well at the air mass movement it is dependent
on the glider's L/D ratio, but it IS NOT an L/D ratio. It is a glide
ratio.

On Jan 14, 9:02*am, Andy wrote:
On Jan 13, 2:40*pm, Gary Evans wrote:

I think something may have gotten lost in the translation. The
discussion I thought was the L/D value used as part of the required
flight computer data required to establish the correct polar for the
glider. The fact that it will change based on a number of variables
doesn't mean it is a meaningless value. Ideally the other variables
are also taken into consideration by the flight computer either by
manual input or sensors.

You are right something has become lost in this discussion. *The OP
stated "Like you all I have questioned what the best L/D of my 3 (now
1)
gliders...were in reality suspecting that real L/Ds would be lower
than the manufacturers published values". *He goes on to discuss how
measurements of glide performance during cross country flights showed
that his measured L/D did not match the claimed best L/D for his
gliders.

Several people have pointed out that the achieved glide angle on a
typical cross country flight has little relationship to best L/D since
XC flights are typically not made at best L/D speed and cross country
flights are typically not made in a motionless air mass.

It should be obvious that the ratio of distance flown to altitude lost
on a complete XC flight, or on any segment of an XC flight, is not a
measure of glider performance but a measure of the combination of
glider performance, air mass characteristics, and pilot skill. *It is
my opinion *that using the term L/D to describe this ratio is
misleading. *Also, comparing this number with a manufacturer's claimed
best L/D tells you absolutely nothing about the validity of the
claimed best L/D value unless the primary objective of the flight, or
flight segment, was to fly at best L/D speed.

The title of the thread is "How to simply determine the L/D of your
glider". *Several experienced XC pilots have pointed out that using
reported L/D data from typical XC flights is not a valid way to
determine the L/D of a glider.

None of the above denies that useful performance data can be derived
from making constant airspeed speed runs in a still air mass. *None of
the above denies that a flight computer or post flight analysis
software can be useful in processing the data. *None of the above
denies that the value of the ratio (distance flown)/(altitude lost)
achieved in a properly conducted flight test or by computer modeling
is properly described as "L/D". *None of the above denies that the
maximum value of that ratio is properly described as "Best L/D".

Andy

Just look at some ridge flights on the OLC in SeeYou......"mean L/D"
is often 100 or 200 are even 9999........!!!

Meaningless....


Cookie

Doesn't air density affect it as well (altitude, temperature, humidity)?
Are you talking TAS, IAS or CAS?


At 03:07 15 January 2011, Bruce Hoult wrote:
A given aircraft at a given weight and in similarly clean condition
will always perform at a fixed L/D (lift/drag) at any particular speed
and G loading. It is an aerodynamic property.

It paaers we may be close to the root of much confusion. Most
everyone, myself included, uses L/D and glide ratio (path)
interchangeably. Even worse we often do not use the word "best" in
front of it when that's what we mean. This should be easily
correctable by a suitable penalty for the misuse of terms. Perhaps
washing out ones mouth with soap or just ostracizing the culprits.
When this is complete we can move on to the other winter topics of
contest finishes, best wax and sealing tape.

It appears we may be close to the root of the problem. Most everyone,
myself included, use L/D and glide ratio (path) interchageably. Even
worse we often don't use the word "best" in front of it when that's
what we mean. This should be easily corrected by a suitable pentalty
for the misuse of terms. Perhaps washing out ones mounth with soap or
just ostrsizing the culpreit. When this is complete we can move on to
the other standard winter topics like contest finishes, best wax and
sealing tape.

It appears we may be close to the root of much confusion. Most
everyone, myself included, uses L/D and glide ratio (path)
interchangeably. Even worse we often do not use the word "best" in
front of it when that's what we mean. This should be easily
correctable by a suitable penalty for the misuse of terms. Perhaps
washing out ones mouth with soap or just ostracizing the culprits.
When this is complete we can move on to the other winter topics of
contest finishes, best wax and sealing tape.

At 12:24 15 January 2011, Big Wings wrote:
Doesn't air density affect it as well (altitude, temperature,
humidity)?
Are you talking TAS, IAS or CAS?


At 03:07 15 January 2011, Bruce Hoult wrote:
A given aircraft at a given weight and in similarly clean condition
will always perform at a fixed L/D (lift/drag) at any particular speed
and G loading. It is an aerodynamic property.




In principle, no ... (L/D)max is a non-dimensional ratio, so air density
effects and IAS/CAS/TAS effects cancel out*. It's a constant and is
determined by the product of the zero-lift (profile) drag coefficient and
the induced drag factor.

Changes to the profile drag coefficient (dirty wing, flaps, airbrakes,
undercarriage etc) or the induced drag coefficient (flaps, winglets etc)
will affect the max L/D, but as Bruce says, for a given aircraft at a
given weight in a similarly clean condition at a given speed and g-loading
it will be a constant - as long as by 'speed' you mean IAS.

"Gary Evans" wrote in message
:

It paaers we may be close to the root of much confusion. Most
everyone, myself included, uses L/D and glide ratio (path)
interchangeably. Even worse we often do not use the word "best" in
front of it when that's what we mean. This should be easily
correctable by a suitable penalty for the misuse of terms. Perhaps
washing out ones mouth with soap or just ostracizing the culprits.
When this is complete we can move on to the other winter topics of
contest finishes, best wax and sealing tape.

I am sort of partial to the annual battery powered leaf blower
discussion, myself...

Zero One

On Jan 16, 1:24*am, Big Wings wrote:
Doesn't air density affect it as well (altitude, temperature, humidity)?
Are you talking TAS, IAS or CAS?

Sure, lots of things affect it, but it's fairly close if you're
talking IAS which takes altitude, temperature and humidity into
account fairly well in that they affect the ASI in about the same was
as they affect the wings.