Help calculating Speed To Fly for headwind and tailwind

On May 28, 8:20*pm, Tim Taylor wrote:
On May 28, 11:26*am, Andy wrote:

On May 28, 6:02*am, Andy wrote:

*The wind does not change the speed to fly. *It only
impacts best glide speed to a landing.

To be more clear I should have said the wind only impacts speed for
best glide range to a landing. *There are of course also cases where
the next thermal cannot be reached unless wind is taken into account
but that too is a best range solution not a best speed solution.

Andy

Thanks for all the help and suggestions. *I was mixing equations too
late at night. *Here is the data from the spreadsheet for a Standard
Class glider in MPH.

My understanding from reviewing the theory is these are applicable for
all legs and not just the last. *Looks like using about half the wind
speed would be a good rough approximation for most normal speeds.

MC * * *Headwind * * * * * * * *Zero * * * *Tailwind
* * * * 30 * * *20 * * *10 * * *0 * * * -10 * * -20 * * -30
0 * * * 71 * * *65 * * *61 * * *58 * * *55 * * *54 * * *52
1 * * * 84 * * *77 * * *72 * * *67 * * *64 * * *62 * * *60
2 * * * 94 * * *87 * * *81 * * *76 * * *72 * * *69 * * *66
3 * * * 103 * * 95 * * *89 * * *84 * * *79 * * *75 * * *72
4 * * * 111 * * 103 * * 96 * * *91 * * *86 * * *82 * * *78
5 * * * 118 * * 110 * * 103 * * 97 * * *92 * * *88 * * *84
6 * * * 125 * * 117 * * 110 * * 103 * * 98 * * *93 * * *89
7 * * * 131 * * 123 * * 116 * * 109 * * 103 * * 98 * * *94
8 * * * 137 * * 129 * * 121 * * 115 * * 109 * * 103 * * 99
9 * * * 143 * * 134 * * 127 * * 120 * * 114 * * 108 * * 104
10 * * *148 * * 139 * * 132 * * 125 * * 119 * * 113 * * 108

I'm not sure this is right Tim, unless you are thinking it is for a
special case like upwind/downwind turnpoints - and even then I'm not
sure.

John Cochrane's paper is a bit ambiguous on the point of speed to fly
versus how strong a thermal to take in the up/downwind turnpoint
scenario and I'm not totally clear on to what extent (or whether)
McCready theory accounts for wind drift while thermalling - even after
reading John's paper. If you are flying into a downwind turnpoint the
idea is you should be willing to take relatively weaker thermals to
get high so you don't have to do as much climbing into the wind after
making the turn.

Where I get into trouble thinking about this is that I can easily
glide 40 or 50 miles into a downwind turnpoint and I don't think I
should PLAN on taking a relatively weaker thermal - therefore my STF
should set to whatever my EXPECTED next climb will be heading into the
turn. As I get closer to the turn I may start dialing back my
expectations for the climb I'm going to find in the remaining
distance, depending on how things look ahead, how many miles I have to
the turn, etc. As I do that I suppose I would also slow down to
optimize the overall cruise/climb combination. I stop dialing back
McCready at the point that my expectations for post-turn (into the
wind) thermal give me a better overall time than my expectations for a
pre-turn (downwind) thermal.

Example: I'm heading into the (20 mph) downwind turn at 5,000 AGL and
am 5 miles out. It's a day with 5 knot typical climbs and occasional
10-knotters. So let's say I'm flying McCraedy 5. Looking at John's
chart I should be willing to take anything stronger than about 2.5
knots while heading downwind into the turn so presumably I am
progressively slowing down as my expectations for the lift I'm going
to find in the shortening distance go from 5 knots down to 2.5 knots -
my minimum. At some point I may realize I'm not going to get another
climb before the turn. In that case what do I do? Since I have plenty
of altitude, my climb expectations go from 2.5 knots (pre-turn) back
up to 5 knots (post-turn). Do I speed up or do I fly based on a ground-
fixed polar until I make the turn?

I'm pretty sure once I make the turn I am back to flying Mc=5 to
optimize my speed versus the airmass.

The logic is analogous but different for an upwind turnpoint. Here I
assume I am flying Mc=5 until I can reach the turnpoint with some
reasonable altitude left to find a decent thermal. Using John's chart,
I don't want to take any thermals weaker that 9 knots or so. Does that
mean I should fly Mc=5 until I think I can make the turnpoint flying
Mc=9 and then speed up to the STF for Mc=9? Presumably I will be low
at the turn, but high enough that I plan on running into at least a 5-
knot thermal. After the turn should I slow down to Mc = 5?

That seems like a lot of gear-shifting. The alternative possibility is
that the optimal thing to do is fly the same McCready speed (Mc=5),
but be pickier about what lift you take into the wind and less picky
downwind. It seems like once you make the turn you are back to
ignoring the wind since all cruise and climb will be subjectt to the
same wind vector and the STF that yields the fastest speed through the
airmass will also yield the fastest speed over the ground.

Thoughts?

9B

Thoughts?

9B

Here's my 2 cents.

If you're racing, not maximizing glide over the ground, and if you're
far from a turnpoint -- meaning you will certainly have to thermal
before you get to the turnpoint -- then as everybody notes, the wind
speed is irrelevant.

That assumes that thermals drift with the wind, and are as easy to
core going upwind as downwind. Thermals actually drift a bit slower
than the wind, and are anchored to ground sources. That means that
going upwind is harder; you're effectively in a lower-performing
glider, so in fact you have to fly more cautiously. I seem to have an
easier time centering when going downwind as well; that may be because
I hit the obvious core first rather than be seduced by the driblets
off downwind of the core. I also seem to stay in contact with streets
better going downwind. (In general, better performing gliders use
slightly higher Mc settings, because they are less likely to get in
trouble)

But back to theory which ignores all this stuff. The calculations in
"upwind/downwind" assume you're near the turnpoint. Here you're
making the decision "do I climb at x before the turnpoint or do I
wait, round the turnpoint and climb at y?" It's only valid if the
latter is an option before hitting the ground!

In any decent wind, it's surprising how much difference there is
between x and y. On the other hand, the graph quantifies common sense:
if you are in an 8 knot thermal and all the other thermals are 3
knots, take it even if it's upwind! The rule of thumb about turning
upwind low isn't always right.

I bug the clearnav team to put these numbers in about once a week.
When you're above glideslope to the next turnpoint, it could show the
equivalent Mc "after the turn" to your current Mc. So far no luck, but
they may correctly perceive that there are about 3 of us who
understand and care about this number.

Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!

If that isn't enough, you need a thermal, and the thermal has to be
stronger than this minimum Mc setting. If you're going downwind, a
slightly negative Mc setting will result in a better glide. I also
encourage my favorite insturment makers to not allow the Mc setting to
go below the value that gives the best glide over the ground, and
allow it to go slightly negative downwind. Again, I think they rightly
perceive this as unnecessary nerdiness.

In both cases, there really is no valid reason at all for cruising at
a lower Mc setting than the weakest (smooth, bottom to top average,
including all centering etc) thermal you'd take. Equivalently, if
you're cruising at Mc 2 and the weather gods grant you a smooth,
guaranteed 3 knot thermal, you're better off taking it and then
cruising at Mc 3 for a while. This is very hard to swallow, but it's
true.

John Cochrane

On May 29, 12:22*pm, John Cochrane
wrote:
Thoughts?

9B

Here's my 2 cents.

If you're racing, not maximizing glide over the ground, and if you're
far from a turnpoint -- meaning you will certainly have to thermal
before you get to the turnpoint -- then as everybody notes, the wind
speed is irrelevant.

That assumes that thermals drift with the wind, and are as easy to
core going upwind as downwind. Thermals actually drift a bit slower
than the wind, and are anchored to ground sources. That means that
going upwind is harder; you're effectively in a lower-performing
glider, so in fact you have to fly more cautiously. *I seem to have an
easier time centering when going downwind as well; that may be because
I hit the obvious core first rather than be seduced by the driblets
off downwind of the core. I also seem to stay in contact with streets
better going downwind. (In general, better performing gliders use
slightly higher Mc settings, because they are less likely to get in
trouble)

But back to theory which ignores all this stuff. The calculations in
"upwind/downwind" assume you're *near the turnpoint. Here you're
making the decision "do I climb at x before the turnpoint or do I
wait, round the turnpoint and climb at y?" *It's only valid if the
latter is an option before hitting the ground!

In any decent wind, it's surprising how much difference there is
between x and y. On the other hand, the graph quantifies common sense:
if you are in an 8 knot thermal and all the other thermals are 3
knots, take it even if it's upwind! *The rule of thumb about turning
upwind low isn't always right.

I bug the clearnav team to put these numbers in about once a week.
When you're above glideslope to the next turnpoint, it could show the
equivalent Mc "after the turn" to your current Mc. So far no luck, but
they may correctly perceive that there are about 3 of us who
understand and care about this number.

Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!

If that isn't enough, you need a thermal, and the thermal has to be
stronger than this minimum Mc setting. If you're going downwind, a
slightly negative Mc setting will result in a better glide. *I also
encourage my favorite insturment makers to not allow the Mc setting to
go below the value that gives the best glide over the ground, and
allow it to go slightly negative downwind. Again, I think they rightly
perceive this as unnecessary nerdiness.

In both cases, there really is no valid reason at all for cruising at
a lower Mc setting than the weakest (smooth, bottom to top average,
including all centering etc) thermal you'd take. Equivalently, if
you're cruising at Mc 2 and the weather gods grant you a smooth,
guaranteed 3 knot thermal, you're better off taking it and then
cruising at Mc 3 for a while. This is very hard to swallow, but it's
true.

John Cochrane

A common thread in this discussion is the need to know the
ACTUAL average climb over an entire thermal. Otherwise
you're typically plugging too high a number into McCready
theory, which doesn't work.

For you SN10 pilots, this is why the instruments I've
designed prominently display TAv - Thermal Average...
Use it ! Don't use the 20 second averager peak...

For John, perhaps time to switch back ;-)
And if you don't, you can still use the SN10 in my
plane to plan your task on the ramp ;-)

Hope this helps,
Best Regards, Dave "YO electric"

On May 29, 6:41*pm, Dave Nadler wrote:
On May 29, 12:22*pm, John Cochrane
wrote:



Thoughts?

9B

Here's my 2 cents.

If you're racing, not maximizing glide over the ground, and if you're
far from a turnpoint -- meaning you will certainly have to thermal
before you get to the turnpoint -- then as everybody notes, the wind
speed is irrelevant.

That assumes that thermals drift with the wind, and are as easy to
core going upwind as downwind. Thermals actually drift a bit slower
than the wind, and are anchored to ground sources. That means that
going upwind is harder; you're effectively in a lower-performing
glider, so in fact you have to fly more cautiously. *I seem to have an
easier time centering when going downwind as well; that may be because
I hit the obvious core first rather than be seduced by the driblets
off downwind of the core. I also seem to stay in contact with streets
better going downwind. (In general, better performing gliders use
slightly higher Mc settings, because they are less likely to get in
trouble)

But back to theory which ignores all this stuff. The calculations in
"upwind/downwind" assume you're *near the turnpoint. Here you're
making the decision "do I climb at x before the turnpoint or do I
wait, round the turnpoint and climb at y?" *It's only valid if the
latter is an option before hitting the ground!

In any decent wind, it's surprising how much difference there is
between x and y. On the other hand, the graph quantifies common sense:
if you are in an 8 knot thermal and all the other thermals are 3
knots, take it even if it's upwind! *The rule of thumb about turning
upwind low isn't always right.

I bug the clearnav team to put these numbers in about once a week.
When you're above glideslope to the next turnpoint, it could show the
equivalent Mc "after the turn" to your current Mc. So far no luck, but
they may correctly perceive that there are about 3 of us who
understand and care about this number.

Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!

If that isn't enough, you need a thermal, and the thermal has to be
stronger than this minimum Mc setting. If you're going downwind, a
slightly negative Mc setting will result in a better glide. *I also
encourage my favorite insturment makers to not allow the Mc setting to
go below the value that gives the best glide over the ground, and
allow it to go slightly negative downwind. Again, I think they rightly
perceive this as unnecessary nerdiness.

In both cases, there really is no valid reason at all for cruising at
a lower Mc setting than the weakest (smooth, bottom to top average,
including all centering etc) thermal you'd take. Equivalently, if
you're cruising at Mc 2 and the weather gods grant you a smooth,
guaranteed 3 knot thermal, you're better off taking it and then
cruising at Mc 3 for a while. This is very hard to swallow, but it's
true.

John Cochrane

A common thread in this discussion is the need to know the
ACTUAL average climb over an entire thermal. Otherwise
you're typically plugging too high a number into McCready
theory, which doesn't work.

For you SN10 pilots, this is why the instruments I've
designed prominently display TAv - Thermal Average...
Use it ! Don't use the 20 second averager peak...

For John, perhaps time to switch back ;-)
And if you don't, you can still use the SN10 in my
plane to plan your task on the ramp ;-)

Hope this helps,
Best Regards, Dave "YO electric"

And SeeYou has the VarA navbox (20 sec average) and the more useful
VarT (average since circling) navbox and on the statistics page shows
last four thermal averages, as well as rough graphical guide of the
entry and exit heights of those last four thermals (sometimes handy to
remind you what the climb was) and the average vario for all four last
thermals.

The stats pages also show global flight average vario, on task
average vario, and last 60 minute average vario. Sorry but I think
that beats the SN10 vario stats. But the stats pages don't show how
long pilots spend starting at all this stuff trying to work out what
is what :-)

I fly with a VarT navbox.

Darryl

On May 29, 9:22*am, John Cochrane
wrote:
Thoughts?

9B

Here's my 2 cents.

If you're racing, not maximizing glide over the ground, and if you're
far from a turnpoint -- meaning you will certainly have to thermal
before you get to the turnpoint -- then as everybody notes, the wind
speed is irrelevant.

That assumes that thermals drift with the wind, and are as easy to
core going upwind as downwind. Thermals actually drift a bit slower
than the wind, and are anchored to ground sources. That means that
going upwind is harder; you're effectively in a lower-performing
glider, so in fact you have to fly more cautiously. *I seem to have an
easier time centering when going downwind as well; that may be because
I hit the obvious core first rather than be seduced by the driblets
off downwind of the core. I also seem to stay in contact with streets
better going downwind. (In general, better performing gliders use
slightly higher Mc settings, because they are less likely to get in
trouble)

But back to theory which ignores all this stuff. The calculations in
"upwind/downwind" assume you're *near the turnpoint. Here you're
making the decision "do I climb at x before the turnpoint or do I
wait, round the turnpoint and climb at y?" *It's only valid if the
latter is an option before hitting the ground!

In any decent wind, it's surprising how much difference there is
between x and y. On the other hand, the graph quantifies common sense:
if you are in an 8 knot thermal and all the other thermals are 3
knots, take it even if it's upwind! *The rule of thumb about turning
upwind low isn't always right.

I bug the clearnav team to put these numbers in about once a week.
When you're above glideslope to the next turnpoint, it could show the
equivalent Mc "after the turn" to your current Mc. So far no luck, but
they may correctly perceive that there are about 3 of us who
understand and care about this number.

Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!

If that isn't enough, you need a thermal, and the thermal has to be
stronger than this minimum Mc setting. If you're going downwind, a
slightly negative Mc setting will result in a better glide. *I also
encourage my favorite insturment makers to not allow the Mc setting to
go below the value that gives the best glide over the ground, and
allow it to go slightly negative downwind. Again, I think they rightly
perceive this as unnecessary nerdiness.

In both cases, there really is no valid reason at all for cruising at
a lower Mc setting than the weakest (smooth, bottom to top average,
including all centering etc) thermal you'd take. Equivalently, if
you're cruising at Mc 2 and the weather gods grant you a smooth,
guaranteed 3 knot thermal, you're better off taking it and then
cruising at Mc 3 for a while. This is very hard to swallow, but it's
true.

John Cochrane

Okay I modelled this out just to see what the speed to fly
implications are - that is, what's the optimal STF going into an
upwind/downwind turnpoint depending on whether you thermal before or
after the turn and what kind of penalty do you pay flying off the
optimal speed.

First, it confirms John's theory on how picky you should be about
thermals and the consequences of going into a turnpoint low versus
high depending on the wind. If the there is strong wind it matters A
LOT how you do this. Just by way of a real world example - a couple of
years ago I stretched a glide into an upwind turnpoint on a contest
day when the winds were 30 knots on the nose, then caught a strong
thermal on the following downwind leg. The math shows that this cut my
total task time by more than 12 minutes! If you assume 20 miles of
glide into the turn, the time savings vary from 3 minutes for 15 knots
of wind up to 12 minutes for the aforementioned 30 knots of wind.

The other surprising thing is how much the optimal speed to fly varies
depending on whether you are planning to take one more thermal before
the turn or wait until after - the speed differences can be more than
20 knots, though the penalties for flying a bit too slow are only a
couple of mph.

Note to BB: when you did the theory for optimal MAT strategies did you
include the high/low turnpoint strategy for the up/downwind versus
crosswind courses?. If you assume even a few thousand feet of high/low
strategy I think the up/downwind approach may dominate the crosswind
one. I think it may also pay to up the number of legs to maximize the
number of times you can play the high/low turnpoint game per mile of
task. Try programming THAT into a ClearNav. :-)

9B

Note to BB: when you did the theory for optimal MAT strategies did you
include the high/low turnpoint strategy for the up/downwind versus
crosswind courses?. If you assume even a few thousand feet of high/low
strategy I think the up/downwind approach may dominate the crosswind
one. I think it may also pay to up the number of legs to maximize the
number of times you can play the high/low turnpoint game per mile of
task. Try programming THAT into a ClearNav. *:-)

9B

Yeah, the theory that says crosswind is better ignores streeting (or
sink streeting!) and the option to turn upwind low and downwind high.

The best MAT is probably to find three turnpoints in an up/downwind
line, and do all your thermaling going downwind. Also final glide into
the wind.

Another good reason to hope CDs don't call unrestricted MATs. That's
not a lot of fun.

John Cochrane

On May 29, 9:22*am, John Cochrane
wrote:
Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!


I was involved in an animated discussion about this just last weekend
when a group of us was dining out after a fine day of soaring. I was
assured that at least one popular glide computer does take wind into
account for final glide and that a setting MC=0 would always give the
best range glide solution for the known headwind or tailwind. In
other words the glide computer is finding the MC setting that gives
max range (more than zero for a headwind and less that zero for a tail
wind) and displaying that as the zero MC setting. I maintained that I
never had a glide computer that did that, but admitted it was possible
that someone had implemented it that way.

So designers, or users, of popular glider computers - Does your
instrument give max range at MC=0 regardless of wind or do you have to
adjust MC for the minimum vaue of required altitude?

On Jun 1, 11:44*am, Andy wrote:
On May 29, 9:22*am, John Cochrane
wrote:

Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!

I was involved in an animated discussion about this just last weekend
when a group of us was dining out after a fine day of soaring. *I was
assured that at least one popular glide computer does take wind into
account for final glide and that a setting MC=0 would always give the
best range glide solution for the known headwind or tailwind. *In
other words the glide computer is finding the MC setting that gives
max range (more than zero for a headwind and less that zero for a tail
wind) and displaying that as the zero MC setting. *I maintained that I
never had a glide computer that did that, but admitted it was possible
that someone had implemented it that way.

So designers, or users, of popular glider computers - Does your
instrument give max range at MC=0 regardless of wind or do you have to
adjust MC for the minimum vaue of required altitude?

That's a poor solution. A sticky knob that won't let you go below max
range would be better. If it shows Mc 0 but it's really set at (say)
Mc 2 because you're in a howling headwind, you need to know to cruise
at Mc 2 and not to take any 1.9 kt thermals. If the indicator shows Mc
0 you don't know that

John Cochrane

On Jun 1, 9:52*am, John Cochrane
wrote:
On Jun 1, 11:44*am, Andy wrote:





On May 29, 9:22*am, John Cochrane
wrote:

Many people make the mistake of thinking wind affects final glide. It
does not (except for the above meteorological considerations). There
does come a point, gliding in to the wind, that lowering your
MacCready setting actually results in a worse glide. You'll see that
-- you get low, turn down the Mc, and all of a sudden you're even
lower! ouch!

I was involved in an animated discussion about this just last weekend
when a group of us was dining out after a fine day of soaring. *I was
assured that at least one popular glide computer does take wind into
account for final glide and that a setting MC=0 would always give the
best range glide solution for the known headwind or tailwind. *In
other words the glide computer is finding the MC setting that gives
max range (more than zero for a headwind and less that zero for a tail
wind) and displaying that as the zero MC setting. *I maintained that I
never had a glide computer that did that, but admitted it was possible
that someone had implemented it that way.

So designers, or users, of popular glider computers - Does your
instrument give max range at MC=0 regardless of wind or do you have to
adjust MC for the minimum vaue of required altitude?

That's a poor solution. A sticky knob that won't let you go below max
range would be better. If it shows Mc 0 but it's really set at (say)
Mc 2 because you're in a howling headwind, you need to know to cruise
at Mc 2 and not to take any 1.9 kt thermals. If the indicator shows Mc
0 you don't know that

John Cochrane- Hide quoted text -

- Show quoted text -

On Jun 1, 9:52*am, John Cochrane
wrote:

That's a poor solution. A sticky knob that won't let you go below max
range would be better. If it shows Mc 0 but it's really set at (say)
Mc 2 because you're in a howling headwind, you need to know to cruise
at Mc 2 and not to take any 1.9 kt thermals. If the indicator shows Mc
0 you don't know that

Agree, but he seemed very sure that he had been told by his instrument
designer that MC zero would alway yield max range glide since the
computer knew the wind and would take account of it.

Maybe I should ask Dave directly since the SN10 was the instrument in
question.

Andy