When are thermals not circular and do thermal helpers assume thatthey are?

Flew with updated SeeYou Mobile Thermal Assistant today - definitely better than the old one - reserving judgement on usefulness!

Mike

On Tuesday, July 30, 2013 1:54:13 AM UTC+1, jfitch wrote:
Thermals may not be circular, but your glider flies in a pretty good approximation of a circle. So the cylindrical graph depicted in Winpilot is all the information you can really use. XCSoar's polar graph might fool you into thinking that is the shape of the lift - but it is a polar graph of strength around a fixed radius circle, not a map of the thermal. Oh yeah they are different!

'fixed radius circle' implies to me that XCSoar's assistant is trying to estimate the lift over a fixed area - eg a 250m wide circle. Just to clarify: XCSoar's thermal assistant is simply a 36-point plot of the lift recorded vs heading over the last turn. While your heading lies within a single 10 degree block, one of these points is continuously updated with your instantaneous vario value. When your heading crosses into the next 10 degree block, the next point is updated, and so on.

In practical terms this means that any sections where you hold a constant heading are ignored on the plot. As an extreme example, imagine you're flying a racetrack pattern between two cores. You've turned the first 180 degrees in the first core and have just completed the remaining 180 degrees in the second core. The top half of the thermal assistant would show the lift encountered during your 180-degree turn in the first core, and the bottom half would show the result of your 180-degree turn in the second core. Any lift or sink you encountered on the straight leg between the first and second cores would not be represented on the plot.

The line in the middle of the plot just points to the centre of the polygon..

On Saturday, August 10, 2013 6:57:04 AM UTC-7, wrote:
On Tuesday, July 30, 2013 1:54:13 AM UTC+1, jfitch wrote:

Thermals may not be circular, but your glider flies in a pretty good approximation of a circle. So the cylindrical graph depicted in Winpilot is all the information you can really use. XCSoar's polar graph might fool you into thinking that is the shape of the lift - but it is a polar graph of strength around a fixed radius circle, not a map of the thermal. Oh yeah they are different!



'fixed radius circle' implies to me that XCSoar's assistant is trying to estimate the lift over a fixed area - eg a 250m wide circle. Just to clarify: XCSoar's thermal assistant is simply a 36-point plot of the lift recorded vs heading over the last turn. While your heading lies within a single 10 degree block, one of these points is continuously updated with your instantaneous vario value. When your heading crosses into the next 10 degree block, the next point is updated, and so on.



In practical terms this means that any sections where you hold a constant heading are ignored on the plot. As an extreme example, imagine you're flying a racetrack pattern between two cores. You've turned the first 180 degrees in the first core and have just completed the remaining 180 degrees in the second core. The top half of the thermal assistant would show the lift encountered during your 180-degree turn in the first core, and the bottom half would show the result of your 180-degree turn in the second core. Any lift or sink you encountered on the straight leg between the first and second cores would not be represented on the plot.



The line in the middle of the plot just points to the centre of the polygon.

Thanks for clarifying my explanation, that is precisely how I thought it (and Winpilot) works. A graph of lift as a function of angular (heading) displacement.

I do not understand the value of plotting lift strength vs. geographical position as is done by some programs. In the thermals I fly, the thermal is at least drifting with the local wind, and often not in an orderly way. The last circle's track has little predictive value for the next (they may be displaced by half a diameter or more) unless the last circle's track is drifted with the predicted wind - and that is not reliable near a rough western thermal. This does have some value in ridge or wave flying, but not in thermals.

Your glider is drifting with the wind, too...


"jfitch" wrote in message
...
On Saturday, August 10, 2013 6:57:04 AM UTC-7, wrote:
On Tuesday, July 30, 2013 1:54:13 AM UTC+1, jfitch wrote:

Thermals may not be circular, but your glider flies in a pretty good
approximation of a circle. So the cylindrical graph depicted in Winpilot
is all the information you can really use. XCSoar's polar graph might
fool you into thinking that is the shape of the lift - but it is a polar
graph of strength around a fixed radius circle, not a map of the
thermal. Oh yeah they are different!



'fixed radius circle' implies to me that XCSoar's assistant is trying to
estimate the lift over a fixed area - eg a 250m wide circle. Just to
clarify: XCSoar's thermal assistant is simply a 36-point plot of the lift
recorded vs heading over the last turn. While your heading lies within a
single 10 degree block, one of these points is continuously updated with
your instantaneous vario value. When your heading crosses into the next 10
degree block, the next point is updated, and so on.



In practical terms this means that any sections where you hold a constant
heading are ignored on the plot. As an extreme example, imagine you're
flying a racetrack pattern between two cores. You've turned the first 180
degrees in the first core and have just completed the remaining 180
degrees in the second core. The top half of the thermal assistant would
show the lift encountered during your 180-degree turn in the first core,
and the bottom half would show the result of your 180-degree turn in the
second core. Any lift or sink you encountered on the straight leg between
the first and second cores would not be represented on the plot.



The line in the middle of the plot just points to the centre of the
polygon.

Thanks for clarifying my explanation, that is precisely how I thought it
(and Winpilot) works. A graph of lift as a function of angular (heading)
displacement.

I do not understand the value of plotting lift strength vs. geographical
position as is done by some programs. In the thermals I fly, the thermal is
at least drifting with the local wind, and often not in an orderly way. The
last circle's track has little predictive value for the next (they may be
displaced by half a diameter or more) unless the last circle's track is
drifted with the predicted wind - and that is not reliable near a rough
western thermal. This does have some value in ridge or wave flying, but not
in thermals.

I have been using XCSoar thermalling assistant in 8 club gliders.
As pointed out earlier, it works better when connected with a
variometer. But it gives you some idea what is happening even
with GPS only. My experience is, that the most important thing
is to adjust timing of you corrections for each glider. It's easier
now, when we have 3 or our gliders with same variometer/flarm
systems.

I was flying few days ago using the latest XCSoar test version.
There was an interesting new feature - while circling, it is
drawing an arc in front of the glider, predicting where your
current bank and speed will bring you. It worked surprisingly well
when I was using it on backseat of our Janus, with LG phone and
without flight computer connection. The glider track was set to
color the lifting areas yellowish, sinks blue(s), and I was using
this new graphic tool to aim my course to the area of strongest
lift.

I also had the thermal assistant as an infobox, as well as the
thermal gain graph.

Since these systems tell you what has happened past 30 secs
and where, they might give you some information you can use to
make your corrections. Or to leave the thermal. Nothing more.

XCSoar seems to have a feature that calculates the central
point of the lift, but I have not found that useful. Maybe I have
not got the right idea off how to utilize it.


At 13:57 10 August 2013, wrote:
On Tuesday, July 30, 2013 1:54:13 AM UTC+1, jfitch wrote:
Thermals may not be circular, but your glider flies in a pretty
good
appr=
oximation of a circle. So the cylindrical graph depicted in
Winpilot is
all=
the information you can really use. XCSoar's polar graph might
fool you
in=
to thinking that is the shape of the lift - but it is a polar graph
of
stre=
ngth around a fixed radius circle, not a map of the thermal. Oh
yeah they
a=
re different!

'fixed radius circle' implies to me that XCSoar's assistant is
trying to
es=
timate the lift over a fixed area - eg a 250m wide circle. Just to
clarify:=
XCSoar's thermal assistant is simply a 36-point plot of the lift
recorded
=
vs heading over the last turn. While your heading lies within a
single 10
d=
egree block, one of these points is continuously updated with
your
instanta=
neous vario value. When your heading crosses into the next 10
degree
block,=
the next point is updated, and so on.

In practical terms this means that any sections where you hold
a constant
h=
eading are ignored on the plot. As an extreme example, imagine
you're
flyin=
g a racetrack pattern between two cores. You've turned the
first 180
degree=
s in the first core and have just completed the remaining 180
degrees in
th=
e second core. The top half of the thermal assistant would
show the lift
en=
countered during your 180-degree turn in the first core, and
the bottom
hal=
f would show the result of your 180-degree turn in the second
core. Any
lif=
t or sink you encountered on the straight leg between the first
and second
=
cores would not be represented on the plot.

The line in the middle of the plot just points to the centre of
the
polygon=
..

I have been using XCSoar thermalling assistant in 8 club gliders.
As pointed out earlier, it works better when connected with a
variometer. But it gives you some idea what is happening even
with GPS only. My experience is, that the most important thing
is to adjust timing of you corrections for each glider. It's easier
now, when we have 3 or our gliders with same variometer/flarm
systems.

I was flying few days ago using the latest XCSoar test version.
There was an interesting new feature - while circling, it is
drawing an arc in front of the glider, predicting where your
current bank and speed will bring you. It worked surprisingly well
when I was using it on backseat of our Janus, with LG phone and
without flight computer connection. The glider track was set to
color the lifting areas yellowish, sinks blue(s), and I was using
this new graphic tool to aim my course to the area of strongest
lift.

I also had the thermal assistant as an infobox, as well as the
thermal gain graph.

Since these systems tell you what has happened past 30 secs
and where, they might give you some information you can use to
make your corrections. Or to leave the thermal. Nothing more.

XCSoar seems to have a feature that calculates the central
point of the lift, but I have not found that useful. Maybe I have
not got the right idea off how to utilize it.


At 13:57 10 August 2013, wrote:
On Tuesday, July 30, 2013 1:54:13 AM UTC+1, jfitch wrote:
Thermals may not be circular, but your glider flies in a pretty
good
appr=
oximation of a circle. So the cylindrical graph depicted in
Winpilot is
all=
the information you can really use. XCSoar's polar graph might
fool you
in=
to thinking that is the shape of the lift - but it is a polar graph
of
stre=
ngth around a fixed radius circle, not a map of the thermal. Oh
yeah they
a=
re different!

'fixed radius circle' implies to me that XCSoar's assistant is
trying to
es=
timate the lift over a fixed area - eg a 250m wide circle. Just to
clarify:=
XCSoar's thermal assistant is simply a 36-point plot of the lift
recorded
=
vs heading over the last turn. While your heading lies within a
single 10
d=
egree block, one of these points is continuously updated with
your
instanta=
neous vario value. When your heading crosses into the next 10
degree
block,=
the next point is updated, and so on.

In practical terms this means that any sections where you hold
a constant
h=
eading are ignored on the plot. As an extreme example, imagine
you're
flyin=
g a racetrack pattern between two cores. You've turned the
first 180
degree=
s in the first core and have just completed the remaining 180
degrees in
th=
e second core. The top half of the thermal assistant would
show the lift
en=
countered during your 180-degree turn in the first core, and
the bottom
hal=
f would show the result of your 180-degree turn in the second
core. Any
lif=
t or sink you encountered on the straight leg between the first
and second
=
cores would not be represented on the plot.

The line in the middle of the plot just points to the centre of
the
polygon=
..

On Saturday, August 10, 2013 8:43:05 AM UTC-7, Dan Marotta wrote:
Your glider is drifting with the wind, too...




Unfortunately, the earth, and the GPS system that tracks with it, is not.....

What are you trying to say. Seems to me that the airmass is carrying the
thermal along and the glider with it. With that premise, circling in calm
air or in a wind makes no difference. I've thermalled straight up and with
a 20+ kt drift down wind. I never had to hold heading into a wind to
maintain center in a thermal.

Or do I misunderstand what you're saying? If that's the case, please
elaborate.


"jfitch" wrote in message
...
On Saturday, August 10, 2013 8:43:05 AM UTC-7, Dan Marotta wrote:
Your glider is drifting with the wind, too...




Unfortunately, the earth, and the GPS system that tracks with it, is
not.....

On Sunday, August 11, 2013 1:19:40 PM UTC-4, Dan Marotta wrote:
I never had to hold heading into a wind to
maintain center in a thermal.

Does it work like this east of Ohio?

On Sunday, August 11, 2013 10:19:40 AM UTC-7, Dan Marotta wrote:
What are you trying to say. Seems to me that the airmass is carrying the

thermal along and the glider with it. With that premise, circling in calm

air or in a wind makes no difference. I've thermalled straight up and with

a 20+ kt drift down wind. I never had to hold heading into a wind to

maintain center in a thermal.



Or do I misunderstand what you're saying? If that's the case, please

elaborate.







...

On Saturday, August 10, 2013 8:43:05 AM UTC-7, Dan Marotta wrote:

Your glider is drifting with the wind, too...









Unfortunately, the earth, and the GPS system that tracks with it, is

not.....

It makes a difference if you are trying to relate your thermaling track to a track projected to the ground. Some thermal assistants (like SYM) draw a track on the ground color coded by climb rate. If you are drifting with the wind, this track quickly drifts upwind relative to you and the airmass (which are drifting downwind). Therefore the next time around your circle, the track will not have much relevance. The only thing that matters (as you have said) is the glider relative to the airmass, which the GPS cannot accurately track and plot. That relationship has to be inertially derived, or simple inferred to be static. The GPS plot is relative to the earth, which can be thought of as moving underneath you at the speed of the wind. Such a plot is of little value when thermaling though they can be useful for ridge and wave, which are effectively anchored to the earth.

There have been some attempts to guess the drift of the thermal from an assumed or calculated wind speed, but most thermals where I fly are not so well behaved.