Flown L/D vs. Required L/D to complete the task.

If your software has it how much are you using this feature? Do you
tend to use it more than traditional MC/ballast/bugs settings for
final glide calculation? I am suspicious of usefulness of this feature
in fast changing conditions. What is your experience?

AK

On Dec 24, 7:26 pm, AK wrote:
If your software has it how much are you using this feature? Do you
tend to use it more than traditional MC/ballast/bugs settings for
final glide calculation? I am suspicious of usefulness of this feature
in fast changing conditions. What is your experience?

AK

I use SeeYou Mobile and rely on L/D achieved a lot. But it is not
either or, you use both and just because there is a display of L/D
achieved in a nav box nothing else on the display has changed so I'm
not sure what the concern would be - go try it out for yourself. If
you think the achieved L/D is temporary and want to ignore it just do
so. See a consistent bad L/D and think you are in a big hole of bad
stuff or the wind is being unkind and want to assume it keeps going
that way then do that instead. Want to split the difference do that.
Want to ignore the stupid computer then do that. And achieved L/D has
the beauty of being simply derived from raw GPS data and is unlikely
to be wrong - and if nothing else when conditions are suitable you
want to make sure that your polar calculated numbers and achieved L/D
converge.

Within reason having more sources of data is a good thing. What you do
with that data to turn it into actionable information is what matters.

Darryl

At 03:30 25 December 2007, Ak wrote:

If your software has it how much are you using this
feature? Do you
tend to use it more than traditional MC/ballast/bugs
settings for
final glide calculation? I am suspicious of usefulness
of this feature
in fast changing conditions. What is your experience?

AK

The SeeYou mobile the manual recommends flying final
glides by comparing the required L/D ('Req L/D') with
the flown L/D ('Cur. L/D'). There are 3 things to
be aware of:

1) L/D is a misnomer in this context - the figures
are glide angle with respect to the ground.

2) You need to use a separate measurement to indicate
when you are within range before you start the glide
- ie to determine the starting required glide angle.

3) The 'Cur L/D' varies dramatically above and below
the 'Req L/D' as you fly through lift and sink and
so it is almost impossible to compare Crrent and Required
L/Ds in a short time frame.

However you can use these features very usefully -
albeit not quite as the manual says:

1) When approaching final glide height monitor your
arrival height with whatever buffer you choose and
when you start the glide take note of the Req L/D -
this is the glide angle from the start of the glide
to the finish point. NB this *is* dependent on the
accuracy of your polar, bugs, wind calculation but
it gives you a best guesstimate of when to start the
glide.

2) During the glide monitor the Req L/D and if that
number is getting larger you are falling below glide
- if so you can choose to back off the McReady setting
until you have your buffer height achievable again
and monitor the new Req L/D. The same in reverse if
your Req L/D number is getting smaller and you are
gaining on glide.

The advantage of flying glides by angle rather than
by the usual way is that it is entirely pragmatic and
gives good information irrespective of errors in bugs,
polar and wind. You are now finding out what you can
actually achieve on the glide. Also if you fly final
glides in the conventional way with an arrival height
buffer then you are flying parallel to the best achievable
glide angle towards your height buffer - i.e any extra
height above the buffer gets proportionately larger
as you get closer to home. If you fly by angle then
the Req L/D at you current McReady setting converges
vertically on the best achievable glide angle towards
your height buffer. So I think the glide angle method
is both more confidence inspiring and less wasteful.

You can still compare Cur and Req L/Ds but that is
only meaningful in areas free of vertical air movement.

John Galloway

Thank you both for your reply. I like John's approach which is
"validation" of traditional calculation which can be as much science
as it is art.

Note that some Garmin GPSes (e.g. my GPSmap 60CS, which also has a
barometer) can display "Glide Ratio" and and "Glide Ratio to
Destination". I find the latter most useful, monitoring whether it is
increasing or decreasing during a glide to an airport as well as
keeping track of whether an airport is within glide range. The "Glide
Ratio" value is not as useful, since the averaging time is rather
short so the display varies considerably and "mental averaging" must
be done. (BTW, to aid in using the "Glide Ratio to Destination" I
have added 1000 feet to the airport elevations in my database.) We
don't need no stinkin' PDAs! :-)

Jack

On Dec 24, 8:26*pm, AK wrote:
If your software has it how much are you using this feature? Do you
tend to use it more than traditional MC/ballast/bugs settings for
final glide calculation? I am suspicious of usefulness of this feature
in fast changing conditions. What is your experience?

AK

I don't use Flown L/D very much, but use Required L/D a lot. After
awhile, you get a good feel for what your glider can do in various
conditions. For example, in my LS6 I'm happy when I have 25/1 to the
place I want to land at (and I use L/D to the field elevation to
prevent weird numbers showing up at close ranges). 30/1 is good for
starting a long final glide. 35/1 is iffy but close. Makes keeping
track of landable fields easy at a glance, and makes a nice cross
check during climb to final glide and during the glider.

Kirk
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