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required LD versus required MC to make it home ??



 
 
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  #1  
Old August 23rd 10, 03:10 AM posted to rec.aviation.soaring
akiley
external usenet poster
 
Posts: 114
Default 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
Ads
  #2  
Old August 23rd 10, 05:32 AM posted to rec.aviation.soaring
Darryl Ramm
external usenet poster
 
Posts: 2,396
Default required LD versus required MC to make it home ??

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



  #3  
Old August 23rd 10, 02:40 PM posted to rec.aviation.soaring
kirk.stant
external usenet poster
 
Posts: 1,257
Default required LD versus required MC to make it home ??


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
  #4  
Old August 23rd 10, 03:00 PM posted to rec.aviation.soaring
John Cochrane
external usenet poster
 
Posts: 90
Default required LD versus required MC to make it home ??

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
  #5  
Old August 23rd 10, 03:02 PM posted to rec.aviation.soaring
Andy[_10_]
external usenet poster
 
Posts: 261
Default required LD versus required MC to make it home ??

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
  #6  
Old August 23rd 10, 04:09 PM posted to rec.aviation.soaring
Mike the Strike
external usenet poster
 
Posts: 951
Default required LD versus required MC to make it home ??

On Aug 23, 7:02*am, Andy wrote:
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


I do the same with arrival altitude, only am more conservative than
Andy. I increase or decrease MacCready accordingly. It seems the
only rational piece of data you need - arrival altitudes of less than
zero are likely to be less than useful.

Mike
  #7  
Old August 23rd 10, 04:17 PM posted to rec.aviation.soaring
akiley
external usenet poster
 
Posts: 114
Default required LD versus required MC to make it home ??

On Aug 23, 12:32*am, Darryl Ramm wrote:
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?


Thanks for the good advice Darryl,

My personal math formula is that, without winds, I want to be 15:1 L/D
from my home airport, (not a cross country pilot yet) with a 1000
foot safety buffer. So this is 400 feet per NM. So for 5 miles, it's
just 400x5=2000 feet AGL plus 1000 safety = 3000AGL. If there is a 5
knot headwind on the return home, I would divide the Cirrus 46kt best
LD speed by 41 to get 0.9. That .9 can be divided by my 400 feet per
NM to get 445 feet per NM for my new calculated L/D to target. I
guess there are other factors, but this is ballpark for me. When I
start to get 20 L/D from home I start to get nervous. But my formula
is roughly based on one half my best LD plus a bit. I can't remember
which book I got my math from, but it's airspeed plus or minus winds
divided by airspeed which gives you a number to modify your L/D. Then
divide 6000 by the modified L/D to get feet per NM required to reach
your target, then add any safety altitude you desire to that agl
altitude.

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...).


Well, I've flown long enough to know not to trust electronics. I have
600 power hours using all sorts of navigators. Funny, when you use a
Garmin 396 on a computer, it sets magnetic variation to user set
instead of auto. SeeYou has quite a few bugs and gochas too. So my
primary is look at the down angle back to the airport.

I do monitor arrival height, but it's really just another way of
looking at MC as far as I can tell. The little glide slope indicator
is also sort of another way of looking at your MC to target. I figure
if I keep an eye on all of those, I'm less likely to trip over a bug
in SeeYou which I've found several of.

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.


This is a good point, but since wind isn't factored into required L/D,
you don't know what your achieved L/D is unless you turn around and
head back to the airport. But it does seem to be safer and more
straight forward. And I suppose since you are always aware of the
winds, you can make a fairly accurate guess as to what you achieved L/
D is likely to be. If it's a straight headwind home at 5 knots, I
could just mentally modify what I expect to achieve.

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?


Yes, my club has several and I'm talking to them too. It's also funny
about gadgets in aircraft. My feeling is learn to use the autopilot
and whenever you can, learn navigators using simulators. Half
learning electronics is the most dangerous in my opinion. I enjoy
navigators, but I'm strict as to when and how to use them.

Darryl


  #8  
Old August 23rd 10, 04:37 PM posted to rec.aviation.soaring
T8
external usenet poster
 
Posts: 429
Default required LD versus required MC to make it home ??

On Aug 22, 10:10*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


It's probably also worth noting here that the Std Cirrus had some of
the least accurate static sources (under the wing) ever put on a
glider, so the accuracy of the ASI and any computer attempting to
calculate wind info using ASI will be compromised. Symptoms would be
significant changes in reported wind when you change heading or ground
track. At cruise speeds, Std Cirrus system reads higher than
calibrated airspeed, this should show as a headwind component that's
always there. There's a technical solution (tailcone statics), but it
will be at odds with your certification paperwork. I'll leave the
teaching of final glides via r.a.s. to the others :-).

-T8
  #9  
Old August 23rd 10, 04:40 PM posted to rec.aviation.soaring
akiley
external usenet poster
 
Posts: 114
Default required LD versus required MC to make it home ??

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
  #10  
Old August 23rd 10, 05:20 PM posted to rec.aviation.soaring
Andy[_1_]
external usenet poster
 
Posts: 1,565
Default required LD versus required MC to make it home ??

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)
 




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