Help calculating Speed To Fly for headwind and tailwind

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 2010/05/28 03:02 PM, Andy wrote:
On May 27, 10:49 pm, Tim wrote:

Anyone have a good set of equations or example of how to do this
simply?


The fastest speed through the air mass will give the fastest speed
over the ground. The wind does not change the speed to fly. It only
impacts best glide speed to a landing.

So add 0xW for a headwind and subtract 0xW for a tailwind.

Andy
I am nnot very good at this kind of arithmetic, so - Let's do a little
mental flying.
Your mount is a first generation standard class. (assume 1:37, L/D max
at about 85km/h, minimum sink at around 70km/h)

You are thermal flying in a fairly strong wind - say 30kt at 5000 AGL.
Thermals are consequently quite broken at lower height and quite
strongly angled downwind once they get organised at 3000AGL.
The turnpoint (an assigned area with a nn km radius) is upwind of you -
around 20km. You are at cloudbase with minimum VFR clearance.
There are cloud streets - producing an average 1kt climb. The best
embedded thermals are around 4kt.

If you fly Mc 1 into a 30kt head component you are flying ~95km/h into a
headwind of ~55km/h - so you are covering ground at 40km/h while
descending at around 125ft/m.

To cover the 20km is going to take 30 minutes in this situation. With a
height loss of 30*125 = 3750feet. This will get you into the turn area
at 1250feet AGL.

Maybe this is not such a good situation to be in - low down you are out
of the working band on the thermals, and lift is broken and difficult to
work. So you decide to thermal at the bottom of the working band - and
end up going backwards at ~40km/hour every 2000/125feet = 16min. In
those 16 minutes you have covered approximately 5km. Assuming you
connect one of those 4kt thermals it is going to take you about 6
minutes to centre and climb back up. In those 6 minutes you have
traversed back at least 4km. So at best you have a nett gain of 10km.
Turn in a 2kt thermal and you have made 5km, turn in 1tk and you are
back at start.

This is an example of there being a cross over on the MC/wind speed. At
some point your ground covering performance deteriorates to the point
that you have to go faster than theoretically economical to make headway.

Bad math?

Cheers
Bruce

--- news://freenews.netfront.net/ - complaints: ---

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

On Jun 1, 2:01*pm, Andy wrote:
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

Hmm.... I wonder what my PDA program does in this case? Anyway,
final glide is in table form for me (Soarpilot), which shows me glide
slope
numbers compared with different airspeeds. When there's a non-zero
MC value for best glide it's apparent on that table. Example screen
shown he
http://www.soaringpilot.org/dokuwiki...l_glide_screen

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

I pity our poor instrument designers. If they do nothing, mad pilots
will write in "something's wrong with your instrument, as I lower Mc
setting it shows me a worse glide angle!" If they make Mc=0 always the
glide extending setting, mad pilots write in with "something's wrong,
I select Mc 0 and it's telling me to fly 70 knots." If they set a
sticky knob that won't go below the glide-maximizing Mc value, mad
pilots will write with "something's wrong, I can't lower the Mc
setting."

John Cochrane