Why don't flight computers adjust STF for wind?

On Tuesday, 14 January 2014 04:46:19 UTC+2, Eric Greenwell wrote:
Steve Leonard wrote, On 1/13/2014 8:17 AM:

On Monday, January 13, 2014 2:34:40 AM UTC-6, krasw wrote:

Yes, theoretically *if* thermals would drift perfectly with

wind...



Uh oh. Next problem. If thermals don't drift at the same speed as

the wind, how accurate is that wind speed your computer derrives from

drift while you are thermalling?



My experience with a 302 and SeeYou Mobile was the differences were

small compared to other variations due to time, location, and altitude,

and small enough that I usually didn't notice any difference. But, there

may be places that do have bigger differences, and I just didn't fly there.



--

Eric Greenwell - Washington State, USA (change ".netto" to ".us" to

email me)

I think many computers (or software) counts for wind variations due altitude. For flying in convective conditions this is just plain wrong and misleading. Wind doesn't change much (or at all) between altitude just over surface friction layer (few hundred meters) and cloudbase. This is because convection effectively mixes airmass momentum in convective layer. (Wave and mountains are of course different matter). Most important variations are due location and time.

If you are gliding from one thermal to another, optimum speed is the same
whether you are going into wind, downwind or no wind, it just depends on
the rate of climb achieved in the next thermal. If you are gliding to a
point on the ground, final glide or round a turn-point, then optimum speed
will be higher into wind than downwind.

At 19:10 10 January 2014, Kevin Christner wrote:
My understanding is that modern flight computers (which seem to have 10X
mo=
re features than anyone could possibly use) don't make adjustment to STF
ca=
lculations for the wind. From my reading of recent "authoritative"
sources=
(Brigliodori, Kawa) it is optimal to increase speed into the wind and
decr=
ease speed downwind, relative to non-adjusted McCready. Also of critical
i=
mportance to me would be a final glide optimizer. Currently when making
a
=
final glide I "move the dial" up and down in McCready speeds as I get
close=
to final glide. Often changing the McCready value up into the wind and
do=
wn with the wind leads to lower required altitude.

Am I missing anything?

2C

Doesn't it depend on what you are trying to optimize?

* if it's maximizing your arrival height at a point, then you do use a
different STF for upwind and downwind

* if it's minimizing the time for a final glide, then you use the same
STF, based only on your current rate of climb

* if you are approaching a turnpoint, it's not about classic McCready
STF, but about risk management: fly the MC STF and you might have to
take thermal while going upwind; fly more slowly so you can use a
thermal on the downwind side, but risk a lower course speed because you
are flying slower than the MC optimum.

Or putting it in "classic" terms, for the "rounding the turnpoint"
situation: Always fly the MC STF, but leave your thermal as soon as you
can glide around the turnpoint to your next thermal. A practical example
is a big dust devil or gaggle short of the turnpoint: go to the
gaggle/dust devil after rounding the turnpoint, not before.

Chris Rollings wrote, On 1/10/2014 11:35 PM:
If you are gliding from one thermal to another, optimum speed is the same
whether you are going into wind, downwind or no wind, it just depends on
the rate of climb achieved in the next thermal. If you are gliding to a
point on the ground, final glide or round a turn-point, then optimum speed
will be higher into wind than downwind.

At 19:10 10 January 2014, Kevin Christner wrote:
My understanding is that modern flight computers (which seem to have 10X
mo=
re features than anyone could possibly use) don't make adjustment to STF
ca=
lculations for the wind. From my reading of recent "authoritative"
sources=
(Brigliodori, Kawa) it is optimal to increase speed into the wind and
decr=
ease speed downwind, relative to non-adjusted McCready. Also of critical
i=
mportance to me would be a final glide optimizer. Currently when making
a
=
final glide I "move the dial" up and down in McCready speeds as I get
close=
to final glide. Often changing the McCready value up into the wind and
do=
wn with the wind leads to lower required altitude.

Am I missing anything?

2C




--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

On Saturday, January 11, 2014 2:35:45 AM UTC-5, Chris Rollings wrote:
If you are gliding from one thermal to another, optimum speed is the same
whether you are going into wind, downwind or no wind, it just depends on
the rate of climb achieved in the next thermal. If you are gliding to a
point on the ground, final glide or round a turn-point, then optimum speed
will be higher into wind than downwind.

This is true from an optimal speed perspective. Kawa and Brigliodori's point (I think) has to do with increasing search range. Flying faster upwind and slower downwind increases your search range. Also, if you've ever looked at a graph for flying slightly faster or slower than optimum MC bumping up or down one know has very little effect on total speed. Finding a better thermal (or a thermal at all) would make you faster or keep you from going Aux Vauche...

Thoughts?

2C

On Saturday, January 11, 2014 2:35:45 AM UTC-5, Chris Rollings wrote:
If you are gliding from one thermal to another, optimum speed is the same
whether you are going into wind, downwind or no wind, it just depends on
the rate of climb achieved in the next thermal. If you are gliding to a
point on the ground, final glide or round a turn-point, then optimum speed
will be higher into wind than downwind.

Then, on Saturday, January 11, 2014 4:18:33 PM UTC-6, Kevin Christner wrote:
This is true from an optimal speed perspective. Kawa and Brigliodori's point
(I think) has to do with increasing search range. Flying faster upwind and
slower downwind increases your search range. Also, if you've ever looked at a
graph for flying slightly faster or slower than optimum MC bumping up or down
one know has very little effect on total speed. Finding a better thermal (or
a thermal at all) would make you faster or keep you from going Aux Vauche....
Thoughts? 2C

Take your polar, see what your speed is for MC3. Let's say it is 75 knots. Calculate your L/D from the polar. Now, fly into a headwind, say 5 knots, and determine your L/D over the ground. Now, check your L/D flying at 80 knots into that same 5 knot headwind. Did your L/D go up or down? Try adding another 5 knots to the cruise speed and see what happens with your L/D.. Try subtracting 5 knots and calculate your L/D Now, bump the headwind to 10 knots, then 15, and 20. Figure out how strong the headwind has to be for your L/D to actually go UP by flying at 80 or 85 knots indicated versus 75 knots indicated. Think you will find that flying faster into the wind will only increases your search range when the MC is low and the wind speed is high.

As for increasing your search range when flying down wind, why slow down since you have the tailwind increasing your search range?

But, those are increasing glide distance over the ground, which is not the same as maximizing cross country speed.

And, as BB said, MC for final glide is climb rate for your last thermal. Not more if the final glide will be into the wind. Altitude required increases for a headwind, but speed to be flown does not change because of wind.

Steve Leonard

My earlier post was referring to the theoretical considerations. The
essential point is that the thermals drift with the wind, applying simple
McReady theory gives maximum achieved average speed through the air, any
variation gives a lower average airspeed and therefore also a lower average
ground speed.

In the real World, for the last 40 years, my teaching and practice, in
UK/NW Europe conditions, has been: default setting 1 knot, if you're
getting low or it doesn't look good ahead set 0 knots, if it's a really
good day and you're high and going well set 2 knots, if it's a fantastic
day, the best you've ever flown on and you look like setting a record, set
3 knots. If you are tempted to set 4 knots, land, take 2 asperin and lie
down, you're feverish. For strong thermal areas like the American South
West, South Africa and Australia, add one knot to all the above, except
perhaps the 0 knot case.

You go faster flying between 6 knot thermals with 2 knots set than you do
flying between 4 knot thermals with 4 knots set. The lower setting you
use, the greater your range and the greater your chance of finding the
really good thermals.

This doesn't apply on final glide, when you do need to allow for head or
tail wind and you should use the instantaneous rate of climb when you leave
the thermal, not the average.

At 04:11 12 January 2014, Steve Leonard wrote:
On Saturday, January 11, 2014 2:35:45 AM UTC-5, Chris Rollings wrote:=20
If you are gliding from one thermal to another, optimum speed is the
same=
=20
whether you are going into wind, downwind or no wind, it just depends
on=
=20
the rate of climb achieved in the next thermal. If you are gliding to
a=
=20
point on the ground, final glide or round a turn-point, then optimum
spee=
d=20
will be higher into wind than downwind.=20

Then, on Saturday, January 11, 2014 4:18:33 PM UTC-6, Kevin Christner
wrote=
:
This is true from an optimal speed perspective. Kawa and Brigliodori's
po=
int
(I think) has to do with increasing search range. Flying faster upwind
an=
d
slower downwind increases your search range. Also, if you've ever
looked
=
at a
graph for flying slightly faster or slower than optimum MC bumping up
or
=
down
one know has very little effect on total speed. Finding a better
thermal
=
(or
a thermal at all) would make you faster or keep you from going Aux
Vauche=
....
Thoughts? 2C

Take your polar, see what your speed is for MC3. Let's say it is 75
knots.=
Calculate your L/D from the polar. Now, fly into a headwind, say 5
knots=
, and determine your L/D over the ground. Now, check your L/D flying at
80=
knots into that same 5 knot headwind. Did your L/D go up or down? Try
ad=
ding another 5 knots to the cruise speed and see what happens with your
L/D=
.. Try subtracting 5 knots and calculate your L/D Now, bump the
headwind
t=
o 10 knots, then 15, and 20. Figure out how strong the headwind has to
be
=
for your L/D to actually go UP by flying at 80 or 85 knots indicated
versus=
75 knots indicated. Think you will find that flying faster into the
wind
=
will only increases your search range when the MC is low and the wind
speed=
is high.

As for increasing your search range when flying down wind, why slow down
si=
nce you have the tailwind increasing your search range?

But, those are increasing glide distance over the ground, which is not
the
=
same as maximizing cross country speed.

And, as BB said, MC for final glide is climb rate for your last thermal.
N=
ot more if the final glide will be into the wind. Altitude required
incre=
ases for a headwind, but speed to be flown does not change because of
wind.

Steve Leonard

Not sure it's true ... but I recall Bill Bartell(US comp pilot) initially misunderstood MC ... and would set his MC value to altitude/1000 ... i.e. 5000' MC = 5 .... 2000' MC = 2.

He had some pretty good comp results during this period - as I recall.

:-)

kk

In the real World, for the last 40 years, my teaching and practice, in
UK/NW Europe conditions, has been: default setting 1 knot, if you're
getting low or it doesn't look good ahead set 0 knots, if it's a really
good day and you're high and going well set 2 knots, if it's a fantastic
day, the best you've ever flown on and you look like setting a record, set
3 knots. If you are tempted to set 4 knots, land, take 2 asperin and lie
down, you're feverish. For strong thermal areas like the American South
West, South Africa and Australia, add one knot to all the above, except
perhaps the 0 knot case.



You go faster flying between 6 knot thermals with 2 knots set than you do
flying between 4 knot thermals with 4 knots set. The lower setting you
use, the greater your range and the greater your chance of finding the
really good thermals.


This reflects current practice, and it's really interesting. "Range" doesn't really describe the reason, I think. In the southwest USA at 10,000' we have tons of range, we don't stop for under 6 knots, yet flying at 100 knots does better than 120 (go look up mc 6 speed!), and 95 won't kill you.

I think the reason is more that flying at warp speed, you (or at least I) lose the feel of the air; I can't adjust slightly to benefit from gliding through rising air. And, I might fly right through that great thermal and not feel it.

John Cochrane

On Saturday, January 11, 2014 1:35:45 AM UTC-6, Chris Rollings wrote:
If you are gliding from one thermal to another, optimum speed is the same
whether you are going into wind, downwind or no wind, it just depends on
the rate of climb achieved in the next thermal. If you are gliding to a
point on the ground, final glide or round a turn-point, then optimum speed
will be higher into wind than downwind.


Not quite true on final glide. If you're in a 3 knot thermal, you climb until you hit the Mc 3 glide height, and then set off at the Mc 3 speed to fly, and the wind makes no difference to this calculation. It does not make sense to sit in a Mc 3 thermal, drifting downwind, to get higher and then bash home at the Mc 4 speed just because it's in to the wind.

This presumes that the goal is speed, not stretching a glide.

This also ignores the fact that it's often easier to bump thermals into the wind, but much harder to scratch low going into the wind than downwind. Those features argue for a bit more aggressive approach early on an into-wind final glide, and a much more conservative approach later on an into-wind final glide. All my final glide disasters have been going in to stiff winds! When down to 1000 feet, it's awfully nice to be drifting downwind while you hunt around in those half know thermals.

John Cochrane

This was exactly my understanding until I read Brigliadori. On pages 359
through 364 they make a pretty good case for flying a bit faster into wind
on task due to the slope of thermals and vice versa, all with worked
examples.

Jim

At 07:35 11 January 2014, Chris Rollings wrote:
If you are gliding from one thermal to another, optimum speed is the same
whether you are going into wind, downwind or no wind, it just depends on
the rate of climb achieved in the next thermal. If you are gliding to a
point on the ground, final glide or round a turn-point, then optimum
speed
will be higher into wind than downwind.