How to simply determine the L/D of your glider

On Jan 11, 8:43*am, Eric Greenwell wrote:
On 1/11/2011 3:13 AM, BruceGreeff wrote:

Then ask yourself whether it is significant.

Best L/D is just one number that has dominated marketing for gliders.
Like most things marketing it is subject to a lot of creativity....

Actual performance, how well a wing uses energy from vertical gusts, how
it climbs, how sensitive it is to contamination, whether it gets
distorted over time. All these will affect how far and fast you fly -
Best L/D is a useful "summary" but it is a generalisation and subject to
a deplorable level of hype and exaggeration.

snip

* you would soon discover the vast difference in

achievable XC performance between the two.

This posting gets my vote for "best overall view of the situation".

I routinely exceed Schleicher's 50:1 claim for my ASH 26 E by 10% to
40%, flying 15 to 20 knots higher than best L/D. That's "Mean L/D" from
SeeYou statistics. It's easy in good conditions with plenty of lift,
cloud streets, or ridge lift. If the Mean L/D drops under 50:1, it's
almost always been a bad day with lift hard to find. So, I really doubt
this L/D statistic has any value for determining a point on your
glider's polar.

It is instructive to compare your statistics for the day to another
pilot flying a comparable glider. I've been surprised at how different
they can be, particularly the number of thermals taken, how fast they
cruise on average, and the percentage of circling times.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

My mean L/D is always much better than that. I routinely make flights
of 150-250 miles with a net loss of altitude of 2000 ft or less.
That's an achieved L/D (if we want to persist in using that term out
of context) of about 450/1. Pretty impressive for a standard class
glider. The manufacturer only claims 44/1.

Why do we want to continue using the terms L/D, and best L/D, out of
context?

Andy

On Mon, 10 Jan 2011 10:07:54 -0800 (PST), jan
wrote:


If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will find the measurements become
quite consistent which suggests that they might represent something
close to reality.

Hi Dave,

as others already pointed out, you are measuring an L/D that is
typical for your flying style, but not the performance of your glider.

In my AS 22-2 I usually get an L/D of 75 (!) on long flights, although
in reality its maximum L/D is only about 55.


Andreas

The purpose to me in knowing the real L/D is not for bragging rights
but to have the correct value in the flight computer.

On Tue, 11 Jan 2011 08:55:45 -0800 (PST), Gary Evans
wrote:

The purpose to me in knowing the real L/D is not for bragging rights
but to have the correct value in the flight computer.

The "correct value" at which speed?
I suspect that you don't want the correct value, but the correct polar
in your flight computer - which, obviously, cannot be measured in the
way described.


Andreas

On Jan 10, 11:07*am, jan wrote:
Hi Gang
* Like you all I have questioned what the best L/D of my 3 (now 1)
gliders - DB800B, Stemme S10-VT and SparrowHawk were in reality
suspecting that real L/Ds would be lower than the manufacturers
published values. I have used a PDA for many years switching it
between gliders. Software is GN11. After each flight I usually
download the log and review on a PC using SeeYou. Now GL11 *calculates
an average L/D which can be viewed with the stats for each flight.
Also one can straight edge any glide from a flight and calculate that
effective glide ratio. Especially on a non thermic day one can get a
feel for a gliders' performance. So what I have I found after crudely
averaging in my brain 10 years of flying these 3 glider. Fairly
consistently the DG800B came out around 43 (manufacturer claimed 51.5.
How the hell could DG claim a half percentage point in 51???? That
represents a 1% accuracy! What nonsense!) - the Stemme around 42
(claim approaching 50) almost comparable with the DG and the
SparrowHawk around 29 (claim 35)
* If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will *find the measurements become
quite consistent which suggests that they might represent something
close to reality.
Dave

L/D numbers are the things dreams are made of. Max L/D is much like
breast size, it gets a lot of attentions but the important thing is
what you do with it.


Ron

On Jan 11, 10:46*am, Andreas Maurer wrote:
On Tue, 11 Jan 2011 08:55:45 -0800 (PST), Gary Evans

wrote:
The purpose to me in knowing the real L/D is not for bragging rights
but to have the correct value in the flight computer.

The "correct value" at which speed?
I suspect that you don't want the correct value, but the correct polar
in your flight computer - which, obviously, cannot be measured in the
way described.

Andreas

This flight computer I used required 3 pieces of speed/sink rate data
to represent the polar. Best L/D, speed at best L/d and V2.

On Jan 11, 8:43*am, Eric Greenwell wrote:
On 1/11/2011 3:13 AM, BruceGreeff wrote:

Then ask yourself whether it is significant.

Best L/D is just one number that has dominated marketing for gliders.
Like most things marketing it is subject to a lot of creativity....

Actual performance, how well a wing uses energy from vertical gusts, how
it climbs, how sensitive it is to contamination, whether it gets
distorted over time. All these will affect how far and fast you fly -
Best L/D is a useful "summary" but it is a generalisation and subject to
a deplorable level of hype and exaggeration.

snip

* you would soon discover the vast difference in

achievable XC performance between the two.

This posting gets my vote for "best overall view of the situation".

I routinely exceed Schleicher's 50:1 claim for my ASH 26 E by 10% to
40%, flying 15 to 20 knots higher than best L/D. That's "Mean L/D" from
SeeYou statistics. It's easy in good conditions with plenty of lift,
cloud streets, or ridge lift. If the Mean L/D drops under 50:1, it's
almost always been a bad day with lift hard to find. So, I really doubt
this L/D statistic has any value for determining a point on your
glider's polar.

It is instructive to compare your statistics for the day to another
pilot flying a comparable glider. I've been surprised at how different
they can be, particularly the number of thermals taken, how fast they
cruise on average, and the percentage of circling times.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

Sorry to be pedantic but we need to be careful how we throw these
terms around. L/D is Lift divided by Drag which has little to do with
distance covered for altitude lost in cross country soaring. D/H has
far more to do with pilot skills selecting lift, avoiding sink and
decision making than with aerodynamics. It's unfortunate some glider
software misuses the term L/D when they mean D/H

L/Dmax is a one-dimensional metric used as a shorthand to evaluate the
aerodynamics of a glider design. I think that is what the OP was
asking about. While it's true L/Dmax is not commonly used in cross
country soaring, it is nonetheless a useful metric for aerodynamicists.

On Tue, 11 Jan 2011 13:56:32 -0800 (PST), Gary Evans
wrote:

This flight computer I used required 3 pieces of speed/sink rate data
to represent the polar. Best L/D, speed at best L/d and V2.

This what all flight computers do - they convert these three points
into an approximated polar curve.

But how do you get these three points by the method described by Dave?


Andreas

On Jan 11, 9:12*pm, Andreas Maurer wrote:
On Tue, 11 Jan 2011 13:56:32 -0800 (PST), Gary Evans

wrote:
This flight computer I used required 3 pieces of speed/sink rate data
to represent the polar. Best L/D, speed at best L/d and V2.

This what all flight computers do - they convert these three points
into an approximated polar curve.

But how do you get these three points by the method described by Dave?

Andreas

Well, I'll bite. Here's what I do for the planes I fly:

First, I've been fortunate enough to fly planes that have Johnson
reports
available. I know there are probably Akaflieg reports as well, but I
haven't
seen those. I carefully extract the data points from Dick's polar
charts and
correct them for my flying weight (unfortunately always considerably
higher
than Dick!). I input the adjusted values into my PDA (which just
wants
the sink rate at 3 airspeeds, rather than the numbers listed above).
Finally,
I set the Polar Potential via experiment. Typically I'll set it to
90% and then
see how well my final glides work out. If I have a bunch of altitude
left over
on a glide then I'm doing better, and I'll increase the potential. If
I tend to fall
below glideslope a lot then I'll decrease the potential. For the most
part I've
wound up with values around 90% or 92% (which probably means I need to
work harder at tuning up the planes I fly).

Essentially this is a refinement of the beginner approach to glide
slopes:
take the published value and divide by 2 as a safety factor. I divide
by something
closer to 1.1 and usually make it home just fine. The times I've had
to
break off have been because I was below glideslope to begin with.

-- Matt

On Jan 12, 4:56*pm, mattm wrote:
Essentially this is a refinement of the beginner approach to glide
slopes: take the published value and divide by 2 as a safety factor.
I divide by something closer to 1.1 and usually make it home just fine.

2 is a bit harsh!!

I use 32.8:1 plus circuit height no matter what I'm in :-)

It's about right for a PW5 (or K6), about your factor of 1.1 for a
Libelle or Grob Twin, and maybe 25%-30% pessimistic for a Janus or
DG1000.

Those are also in order (and I think reasonable proportion to) the
cost and embarrassment and inconvenience of breaking each of those
gliders!

It also happens to be 100 ft per km, which doesn't require a computer
to calculate.