Help calculating Speed To Fly for headwind and tailwind

[Tim Taylor]

I am looking for the equations to determine the speed to fly for a
headwind or tailwind. I can calculate the the speed to fly for no
wind based on the polar from Equation V (Page 106, 1988 US Version) in
Reichmann. He gives a graphical method to determine the speeds for a
headwind or tailwind but I would like to translate this into an
equation.

I can do it by shifting the polar to the "true" ground speed, but then
I have to correct the predicted STF from the equation by adding or
subtracting the wind again. I'm not sure if this is the best way to do
it.

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

Thanks,

Tim

[Andy[_1_]]

On May 27, 10:49*pm, Tim Taylor > 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

[Nine Bravo Ground]

On May 28, 6:02*am, Andy > wrote:
> On May 27, 10:49*pm, Tim Taylor > 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 think Tim means STF for final glide where you are flying in
reference to the ground, not the airmass. John Cochrane's analysis
shows that you have different lift strength targets for upwind/
downwind turnpoints as well, though I don't know if this extends to
STF. John?

I have the final glide formulae in a spreadsheet, including effects of
wind and wing loading, if you are interested.

9B

[Nine Bravo Ground]

On May 28, 7:10*am, Nine Bravo Ground > wrote:
> On May 28, 6:02*am, Andy > wrote:
>
> > On May 27, 10:49*pm, Tim Taylor > 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 think Tim means STF for final glide where you are flying in
> reference to the ground, not the airmass. John Cochrane's analysis
> shows that you have different lift strength targets for upwind/
> downwind turnpoints as well, though I don't know if this extends to
> STF. John?
>
> I have the final glide formulae in a spreadsheet, including effects of
> wind and wing loading, if you are interested.
>
> 9B

To clarify, the ground reference STF is reserved for trying to
maximize distance, not speed. This means that your 4-knot final glide
is at the same speed irrespective of wind up until the best glide STF
accounting for wind exceeds the McCready STF - at that point you won't
make it home into the wind unless you speed up. That would only apply
in situations where you make a downwind turnpoint under weak
conditions with enough altitude to get home but without strong enough
lift to make sustained headway - that's only happened to me once - 1.5
knot thermals and a 40 mph headwind (I landed).

I think you could use a version of this logic in making an upwind
turnpoint - though again I think the situation would be rare. In this
case you are calculating your angle over the ground to see if you can
make the turnpoint before you need to take a thermal. I suppose it is
possible that the optimal solution is that you need to fly faster than
McCready speed to make the turnpoint, but I think that means that
you'd be unable to make sustained headway given the thermal strength.
Maybe if you were trying to duck into a turnpoint at the edge of a big
downburst or something.

9B

[mattm[_2_]]

On May 28, 10:10*am, Nine Bravo Ground > wrote:
> On May 28, 6:02*am, Andy > wrote:
>
> > On May 27, 10:49*pm, Tim Taylor > 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 think Tim means STF for final glide where you are flying in
> reference to the ground, not the airmass. John Cochrane's analysis
> shows that you have different lift strength targets for upwind/
> downwind turnpoints as well, though I don't know if this extends to
> STF. John?
>
> I have the final glide formulae in a spreadsheet, including effects of
> wind and wing loading, if you are interested.
>
> 9B

If you want to derive the formula you need a little bit of 1st year
calculus plus some algebra. Derive a line passing through
the point (-headwind, -MC) that is tangent to your polar. The slope
will be equal to the 1st derivative of the polar at the speed to fly.
I use it often enough when analyzing the performance of gliders
I fly (I've made more than a few prayer wheels in my day).

As far as speed to fly, Andy is correct. Fly through the airmass at
your MC speed. John's paper says you should nudge your MC
a bit up or down when you're flying into our out of an upwind
turnpoint (read the paper for details).

For final glide, THEN you can take the headwind into account.

-- Matt

[Andy[_1_]]

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

[Tim Taylor]

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

[Nine Bravo Ground]

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

[John Cochrane]

>
> 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

[Dave Nadler]

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"

[Darryl Ramm]

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

[Bruce]

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: ---

[Nine Bravo Ground]

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

[John Cochrane]

> 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

[Andy[_1_]]

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?

[John Cochrane]

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

[Andy[_1_]]

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 -

[Andy[_1_]]

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

[mattm[_2_]]

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 here:
http://www.soaringpilot.org/dokuwiki/doku.php/soarpilot/final_glide_screen

[John Cochrane]

>
> 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

[Dave Nadler]

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

Say what ???

No. No. No. No. (Did you hear me ?) NO.

When flying into a headwind:
Set MC 0 a note altitude surplus or deficit.
As you increase MC, the deficit will decrease,
reach a minimum, then increase again.

The SN10 takes into account the effect of
wind on each leg in future. That affects the
average speed per leg, the altitude required
per leg, and the surplus or deficit. All this is
calculated at whatever MC setting you input.

OK ?

See ya, Dave "YO electric"

[Ramy]

On Jun 1, 2:01*pm, John Cochrane >
wrote:
> > 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

While I am not arguing against the theory behind MC settings I would
like to add couple of points which I am sure most of you are aware
anyway:
1 - Using lower than calculated MC setting will hurt your speed much
less than taking a wrong turn or wrong line, so don't get too anal
about it.
2 - The theory does not take into account the terrain ahead, ground
clearance, the probability to find another good thermal, the risk of
getting out of glide of landable strips, the significant increased
risk of landout, and your comfort level when getting low which will
often translate (for most of us) in more time spending circling in
weak lift and even unecessary deviations.

For what it worth, my MC setting is usually between 0-3, regardless of
wind.

Ramy

[Nine Bravo Ground]

On Jun 1, 3:23*pm, Dave Nadler > wrote:
> 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
>
> Say what ???
>
> No. No. No. No. (Did you hear me ?) NO.
>
> When flying into a headwind:
> Set MC 0 a note altitude surplus or deficit.
> As you increase MC, the deficit will decrease,
> reach a minimum, then increase again.
>
> The SN10 takes into account the effect of
> wind on each leg in future. That affects the
> average speed per leg, the altitude required
> per leg, and the surplus or deficit. All this is
> calculated at whatever MC setting you input.
>
> OK ?
>
> See ya, Dave "YO electric"

Dave - you made me curious. Does that mean if you are abeam of the
home airport with a decent distance to run to a downwind turnpoint and
back to the finish that the SN-10 will calculate altitude margin based
on a single McCready setting for the entire way home or separate
McCready solutions for each leg - and if so how does the pilot input
separate values for each leg? I think the finish height maximizing
solution to get you home could easily be, say, Mc = -2 for the
downwind leg and Mc = +2 for the upwind leg. If the computer looks for
a single McCready value as the solution from wherever you are it might
not find a solution that gets you home. If the SN-10 does calculate
separate solutions for each leg, does the pilot need to remember to
reset the McCready at the turn, because if he doesn't as soon as he
makes the turn the computer could suddenly go from indicating that he
can make it to indicating that he can't. Also, does the SN-10 have the
ability to set a negative McCready value for the downwind leg?

I'm not sure any computer does this today as it is a complex problem
to manage in such a way that the pilot can understand what's really
going on. But on those long, dicey, late-in-the-day final glides
around a turnpoint when the wind is howling and the thermals are all
torn up it sure could help get you home. It may not happen all that
often, but when you need it you really need it.

9B

[Andy[_1_]]

On Jun 2, 5:45*am, Nine Bravo Ground > wrote:
> Dave - you made me curious. Does that mean if you are abeam of the
> home airport with a decent distance to run to a downwind turnpoint and
> back to the finish that the SN-10 will calculate altitude margin based
> on a single McCready setting for the entire way home or separate
> McCready solutions for each leg - and if so how does the pilot input
> separate values for each leg?

Last Monday's club contest task put us all in exactly that position.
The last turn of the area task was downwind of the finish and the
winds were quite strong. It was blue in the last turn area so most of
us made final glide altitude before entering the last area and went in
just deep enough to make the best use of the altitude. For me the
most difficult part of the puzzle is knowing what the winds will be on
the downwind leg and whether it will be the same on the up wind leg to
the finish. I have never flown with a glide computer that does a good
job of knowing the winds at all altitudes and uses that wind profile,
rather than a fixed value, for the final glide solution.

As I was already over min time and not wishing to landout because the
head wind on the last leg was stronger than expected I elected to turn
early and fly home fast. I have no doubt I could have earned a few
more points by going a bit further and flying slower on the last leg.

Andy