required LD versus required MC to make it home ??

Hi All,

I'm aware of and use the math formula to get my rental Cirrus back to
home base, but I like to back it up with SeeYou mobile. I know it's
recommended to use required LD to target making sure you have entered
a correct polar and safety altitude. But this doesn't account for
winds does it? If you are flying away from your target wondering how
far you can safely fly, you can't depend on required LD because big
headwinds can make this number useless. As an example, I notice I've
got 25LD required to my home base. I turn around and because of the
headwinds, I can only make 18LC. Outlanding anyone.

I'm curious about MC required to target. Wouldn't that be better to
use if you make sure all data is correct such as polar, winds, safety
altitude and make sure the correct target is activated. This way, I
can wander away from my home field and I know if my MC doesn't fall
below about say 7 (which plays out to about 20 LD in no wind) I am
fairly assured of making it and that this MC will be wind aware. Of
course it can't know about hitting lots of sink, but it seems a better
way for my type of non task, local soaring.

Before I finish, I would like to note that the MC to target NavBox in
SeeYouM doesn't always update very quickly if you change the winds
aloft manually. For this problem, I scroll the MC value untill the
little glide slope type indicater on the left side of SeeYou centers,
then compare that MC to the required MC NavBox.

akiley

On Aug 22, 7:10*pm, akiley wrote:
Hi All,

I'm aware of and use the math formula to get my rental Cirrus back to
home base, but I like to back it up with SeeYou mobile. *I know it's
recommended to use required LD to target making sure you have entered
a correct polar and safety altitude. *But this doesn't account for
winds does it? *If you are flying away from your target wondering how
far you can safely fly, you can't depend on required LD because big
headwinds can make this number useless. *As an example, I notice I've
got 25LD required to my home base. *I turn around and because of the
headwinds, I can only make 18LC. *Outlanding anyone.

I'm curious about MC required to target. *Wouldn't that be better to
use if you make sure all data is correct such as polar, winds, safety
altitude and make sure the correct target is activated. *This way, I
can wander away from my home field and I know if my MC doesn't fall
below about say 7 (which plays out to about 20 LD in no wind) I am
fairly assured of making it and that this MC will be wind aware. *Of
course it can't know about hitting lots of sink, but it seems a better
way for my type of non task, local soaring.

Before I finish, I would like to note that the MC to target NavBox in
SeeYouM doesn't always update very quickly if you change the winds
aloft manually. *For this problem, I scroll the MC value untill the
little glide slope type indicater on the left side of SeeYou centers,
then compare that MC to the required MC NavBox.

akiley

What is "the math formula". I am aware of many different math
formulas, including many for calculating/estimating glider performance/
navigation. But what are you using?

Required Mc is a kind of noisy number, especially if you think the
difference between two large numbers helps you much. It is sensitive
to high speed polar data and if you tried to fly it in a rental glider
with an unknown actual polar without a lot of experience at pushing it
is likely meaningless.

For recreational flying, unless you are racing with lots of
experience, I would focus less on twiddling Mc (or virtually twiddling
with SeeYou Mobile telling you its Mc estimate to goal) and more on L/
D achieved and L/D required as one data pair and on arrival height as
another. Arrival height factors in wind, uses the polar, bugs, Mc. Set
some sane low Mc near what you actually fly at. Pad the polar with
%bugs (start with max of 30% if new to XC) and have an arrival safety
height (at least your usual pattern height, more when starting). You
can try adjusting it at times and see what it does to your arrival
height but if you are at the stage it sounds like mostly leave it set
and don't go chasing large Mc numbers. Hide the navbox, there are
better things to look at. In fact hide almost everything, except the
two L/D boxes and arrival height and use the wind indicator on the
main map to check it looks sane. And forget the rest, including the
silly glideslope display, I cannot think of anybody who really uses
that thing (oops now we'll hear from them...).

Required L/D to target tells you what you need to achieve. It makes no
sense to fold wind into that, its just the distance divided by the
difference in height. Achieved L/D tells you what you are getting
obviously with wind affects as well, all without any assumptions about
polars, mass, bugs, or wind. That is the beauty of working with L/D
required and achieved.

But even better than asking on r.a.s. can you find a local
accomplished XC pilot(s) who can mentor you on all this stuff?

Darryl

Required L/D to target tells you what you need to achieve. It makes no
sense to fold wind into that, its just the distance divided by the
difference in height. Achieved L/D tells you what you are getting
obviously with wind affects as well, all without any assumptions about
polars, mass, bugs, or wind. That is the beauty of working with L/D
required and achieved.

But even better than asking on r.a.s. can you find a local
accomplished XC pilot(s) who can mentor you on all this stuff?

Darryl-

Darry is spot on. I would go a little further and dispense with the
Achieved L/D - I just use L/D required and watch for the trend: if it
is getting better (lower L/D required) then you are gaining on the
glide and can either speed up or relax more. If it's getting worse,
or not changing and looks a bit high (say more than half your
published L/D), then you need to stop and get some altitude. That
takes care of the wind, bugs, etc.

Totally agree with getting rid of all the navboxes that are "info
only" - unless your PDA is hooked up to a 302 and getting air data,
using GPS for fancy speed to fly info is a distraction. Use it as a
digital sectional, with your task, airspace, and landable fields (with
L/D required) on it, and in most cases turn off the terrain (exception
is in ridge country where the terrain can be really useful). Less is
more!

Cheers,

Kirk
66

Well, just for a contrary opinion, I disagree with Darryl and Kirk. Mc
setting is the right set of units for everything in soaring.

If you must think about glide angles, the right units are D/L not L/
D. L/D goes through infinity when you run in to lift. D/L (feet per
mile, meters per kilometer) does not. If you gain 200 feet in lift vs.
lose 200 feet in lift, L/D shows radically different changes, D/L does
not.

The "safety profile" for making it to a goal with constant (say 99%)
probability follows a roughly square root function of distance.
(Square root follows if lift/sink are independent over distance) Most
of us approximate this with a relatively high Mc setting (3-4) plus a
reserve altitude.

Smoother conditions -- less lift or sink -- means less uncertainty
about your glide. So, paradoxically, you can use more aggressive
safety settings if there is no lift around, because then there is no
sink around. Strong lift mans strong sink; half chance of escaping in
10 knots, half chance of hitting the dirt in 10 knot sink. Therefore,
use a higher Mc setting and higher reserve altitude with stronger lift/
sink or general uncertainty.

To fly a safety glide you want to have the glide computer at a high Mc
setting, but fly slowly and accept weaker lift. Many pilots disconnect
the glide computer from the vario for this reason. Well, I do.
Instrument makers should recognize this difference and make it easier
to have a different Mc for glide than vario.

Wind is irrelevant here, with one exception. As you lower the Mc
setting heading upwind, you will discover a point at which lower Mc
setttings seem to make it worse. This is a featuer not a bug. The best
glide in wind occurs at a higher Mc setting. don't fly slower than
that, don't take weaker thermals than that, or you wont get home

John Cochrane

On Aug 23, 10:00*am, John Cochrane
wrote:
Well, just for a contrary opinion, I disagree with Darryl and Kirk. Mc
setting is the right set of units for everything in soaring.

If you must think about glide angles, the right units are D/L not L/
D. *L/D goes through infinity when you run in to lift. D/L (feet per
mile, meters per kilometer) does not. If you gain 200 feet in lift vs.
lose 200 feet in lift, L/D shows radically different changes, D/L does
not.

The "safety profile" for making it to a goal with constant (say 99%)
probability follows a roughly square root function of distance.
(Square root follows if lift/sink are independent over distance) *Most
of us approximate this with a relatively high Mc setting (3-4) plus a
reserve altitude.

Smoother conditions -- less lift or sink -- means less uncertainty
about your glide. So, paradoxically, you can use more aggressive
safety settings if there is no lift around, because then there is no
sink around. Strong lift mans strong sink; half chance of escaping in
10 knots, half chance of hitting the dirt in 10 knot sink. Therefore,
use a higher Mc setting and higher reserve altitude with stronger lift/
sink or general uncertainty.

To fly a safety glide you want to have the glide computer at a high Mc
setting, but fly slowly and accept weaker lift. Many pilots disconnect
the glide computer from the vario for this reason. Well, I do.
Instrument makers should recognize this difference and make it easier
to have a different Mc for glide than vario.

Wind is irrelevant here, with one exception. As you lower the Mc
setting heading upwind, you will discover a point at which lower Mc
setttings seem to make it worse. This is a featuer not a bug. The best
glide in wind occurs at a higher Mc setting. don't fly slower than
that, don't take weaker thermals than that, or you wont get home

John Cochrane

I did a little calculation for a standard Cirrus with no wind. I did
this by using SeeYou mobile in simulator mode. I did manual math for
angle and feet per NM. Here are a few numbers. They might not format
correctly.

Required L/D 38, 28, 20, 15, 10
Required MC 0, 3.5, 7.4, 12, 22
glide angle deg 1.7, 2.2, 2.9, 3.8, 5.7
feet per NM 158, 215, 300, 400, 600

Then I added a big headwind. Required L/D stays the same, but MC
corrects for winds. ... akiley

On Aug 23, 8:40*am, akiley wrote:
On Aug 23, 10:00*am, John Cochrane
wrote:





Well, just for a contrary opinion, I disagree with Darryl and Kirk. Mc
setting is the right set of units for everything in soaring.

If you must think about glide angles, the right units are D/L not L/
D. *L/D goes through infinity when you run in to lift. D/L (feet per
mile, meters per kilometer) does not. If you gain 200 feet in lift vs.
lose 200 feet in lift, L/D shows radically different changes, D/L does
not.

The "safety profile" for making it to a goal with constant (say 99%)
probability follows a roughly square root function of distance.
(Square root follows if lift/sink are independent over distance) *Most
of us approximate this with a relatively high Mc setting (3-4) plus a
reserve altitude.

Smoother conditions -- less lift or sink -- means less uncertainty
about your glide. So, paradoxically, you can use more aggressive
safety settings if there is no lift around, because then there is no
sink around. Strong lift mans strong sink; half chance of escaping in
10 knots, half chance of hitting the dirt in 10 knot sink. Therefore,
use a higher Mc setting and higher reserve altitude with stronger lift/
sink or general uncertainty.

To fly a safety glide you want to have the glide computer at a high Mc
setting, but fly slowly and accept weaker lift. Many pilots disconnect
the glide computer from the vario for this reason. Well, I do.
Instrument makers should recognize this difference and make it easier
to have a different Mc for glide than vario.

Wind is irrelevant here, with one exception. As you lower the Mc
setting heading upwind, you will discover a point at which lower Mc
setttings seem to make it worse. This is a featuer not a bug. The best
glide in wind occurs at a higher Mc setting. don't fly slower than
that, don't take weaker thermals than that, or you wont get home

John Cochrane

I did a little calculation for a standard Cirrus with no wind. *I did
this by using SeeYou mobile in simulator mode. *I did manual math for
angle and feet per NM. *Here are a few numbers. *They might not format
correctly.

Required L/D * * 38, * *28, * 20, * *15, * 10
Required MC * * * 0, * *3.5, *7.4, * 12, * 22
glide angle deg *1.7, * 2.2, *2.9, * *3.8, *5.7
feet per NM * * * *158, *215, 300, *400, *600

Then I added a big headwind. *Required L/D stays the same, but MC
corrects for winds. *... akiley

Okay, for eyeball calculations this makes sense. I normally use miles
per thousand plus 1000' arrival (plus field elevation) to estimate if
I'm high or low. I printed a table that showed how many miles I could
get per 1,000' as a function of Mc (0, 2, 4, 6, 8, 10) and wind (-25
to +25 mph).

If you are just starting to venture out from home and you are in the
US, consider getting a copy of GlidePlan software. It will allow you
to print minimum altitude contours directly onto a sectional chart
(you can even print alternate maps for different wind conditions).
Then all you need to do is figure out where you are on the chart and
it will tell you how high you need to be. Pretty cool. www.glideplan.com

Andy
9B

On Aug 23, 7:00*am, John Cochrane
wrote:

If you must think about glide angles, the right units are D/L not L/
D. *L/D goes through infinity when you run in to lift. D/L (feet per
mile, meters per kilometer) does not.

No expression using the terms lift and drag is appropriate for
defining a ground referenced flight path angle.

The proper term is, surprise, flight path angle. It has been used in
the aerospace industry for many, many, years. It has a range +/- 90
degrees. Required FPA describes the required geometry to reach the
goal and instantaneous FPA will descibe the current glider flight path
accurately whether climbing or descending.

Required and instantaneous FPA are just as easily calculated as the
incorrect term L/D or the misleadingly named term "efficiency".

Where did the use of L/D (the ratio of lift to drag) to describe a
ground referenced flight path angle originate? I know it has been
perpetuated by SeeYou, but did they start it?

Andy (GY)

On Aug 23, 12:20*pm, Andy wrote:

Where did the use of L/D (the ratio of lift to drag) to describe a
ground referenced flight path angle originate? *I know it has been
perpetuated by SeeYou, but did they start it?

That goes back to the 50s, very likely. Certainly, it was used on
circular slide rule calculators of the 60s (referenced in Sunship Game
and articles of the era). L/D *is* a little easier to say than "the
inverse of the tangent of the flight path angle", even if it *is*
extremely sloppy shorthand.

-T8

On Aug 23, 9:00*am, John Cochrane
wrote:
Well, just for a contrary opinion, I disagree with Darryl and Kirk. Mc
setting is the right set of units for everything in soaring.

If you must think about glide angles, the right units are D/L not L/
D. *L/D goes through infinity when you run in to lift. D/L (feet per
mile, meters per kilometer) does not. If you gain 200 feet in lift vs.
lose 200 feet in lift, L/D shows radically different changes, D/L does
not.

The "safety profile" for making it to a goal with constant (say 99%)
probability follows a roughly square root function of distance.
(Square root follows if lift/sink are independent over distance) *Most
of us approximate this with a relatively high Mc setting (3-4) plus a
reserve altitude.

Smoother conditions -- less lift or sink -- means less uncertainty
about your glide. So, paradoxically, you can use more aggressive
safety settings if there is no lift around, because then there is no
sink around. Strong lift mans strong sink; half chance of escaping in
10 knots, half chance of hitting the dirt in 10 knot sink. Therefore,
use a higher Mc setting and higher reserve altitude with stronger lift/
sink or general uncertainty.

To fly a safety glide you want to have the glide computer at a high Mc
setting, but fly slowly and accept weaker lift. Many pilots disconnect
the glide computer from the vario for this reason. Well, I do.
Instrument makers should recognize this difference and make it easier
to have a different Mc for glide than vario.

Wind is irrelevant here, with one exception. As you lower the Mc
setting heading upwind, you will discover a point at which lower Mc
setttings seem to make it worse. This is a featuer not a bug. The best
glide in wind occurs at a higher Mc setting. don't fly slower than
that, don't take weaker thermals than that, or you wont get home

John Cochrane

John, as usual, is correct, but in this case it's a bit of apples and
oranges. Darryl and I use "L/D" as a shorthand for a quick analysis
of the flight path angle required to make a destination. It's totally
Mc independent, and is easy to interpret at a glance on a moving map.
For accurate final glides in competition mode, where more accuracy is
desired, using Mc based on the last climb plus winds is the way to go
(and why I use my SN10 and not SeeYouM for final glides), but I also
crosscheck the two for a sanity check.

Always nice to have two opinions in the cockpit - the trick is to
decide which one is right!

Cheers,

Kirk

On Aug 23, 6:40*am, "kirk.stant" wrote:
Required L/D to target tells you what you need to achieve. It makes no
sense to fold wind into that, its just the distance divided by the
difference in height. Achieved L/D tells you what you are getting
obviously with wind affects as well, all without any assumptions about
polars, mass, bugs, or wind. That is the beauty of working with L/D
required and achieved.

But even better than asking on r.a.s. can you find a local
accomplished XC pilot(s) who can mentor you on all this stuff?

Darryl-

Darry is spot on. *I would go a little further and dispense with the
Achieved L/D - I just use L/D required and watch for the trend: if it
is getting better (lower L/D required) then you are gaining on the
glide and can either speed up or relax more. *If it's getting worse,
or not changing and looks a bit high (say more than half your
published L/D), then *you need to stop and get some altitude. *That
takes care of the wind, bugs, etc.

Totally agree with getting rid of all the navboxes that are "info
only" - unless your PDA is hooked up to a 302 and getting air data,
using GPS for fancy speed to fly info is a distraction. *Use it as a
digital sectional, with your task, airspace, and landable fields (with
L/D required) on it, and in most cases turn off the terrain (exception
is in ridge country where the terrain can be really useful). *Less is
more!

Cheers,

Kirk
66

I generally use arrival altitude for everything, especially final
glide. That way I know how much I need to climb to get to my goal and
wind is accounted for automatically in the computer. I typically
program in 1,000' for arrival altitude and speed up/slow down
depending on whether the arrival height is building or declining.
Typically I dial in 4 knots for the computation because it corresponds
to a typical cruise speed. I try not to set below 3 knots unless it's
a last resort. Except on very long glides low Mc settings just don't
yield enough glide angle margin - a little sink and you're at best L/D
or can't make it at all.

9B