Why don't flight computers adjust STF for wind?

My understanding is that modern flight computers (which seem to have 10X more features than anyone could possibly use) don't make adjustment to STF calculations for the wind. From my reading of recent "authoritative" sources (Brigliodori, Kawa) it is optimal to increase speed into the wind and decrease speed downwind, relative to non-adjusted McCready. Also of critical importance 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 down with the wind leads to lower required altitude.

Am I missing anything?

2C

On Friday, January 10, 2014 1:10:45 PM UTC-6, Kevin Christner wrote:
My understanding is that modern flight computers (which seem to have 10X more features than anyone could possibly use) don't make adjustment to STF calculations for the wind.

They do make very good adjustments for the wind in the sense that if you dial in MacCready 3 into the wind they will tell you you need a lot more altitude than if you dial in MacCready 3 going downwind.

The speed to fly calculation is not affected by wind, as long as thermals drift with the wind. You still fly MacCready 3 airspeed in MacCready 3 lift. The final glide calculation is also not affected by wind. You still leave a 3knot thermal when you hit the MacCready 3 altitude, corrected for wind.

There are second order effects -- thermals don't drift exactly with the wind, you may be able to bump more or less effectively depending on wind, etc. But it would take a whole new generation of theory and software to begin to quantify these effects.

The optimal MacCready setting is affected by wind, when you're going in to a turnpoint. You want to both fly faster and be choosier about thermals going to an upwind turnpoint, and fly slower and take weaker lift going to a downwind turnpoint. This is currently handled by just increasing or decreasing the MacCready setting, which is what Brigliadori and Kawa are advocating..

The setting choice can be done quantitatively, see "upwind and downwind" here

http://faculty.chicagobooth.edu/john...ring/index.htm

The only thing a flight computer can productively add is this calculation. I've been bugging the CN folks do implement this for a while. All it takes is to present the equivalent after-turnpoint mac cready setting for the current value. But they answer (correctly, from a business standpoint) "we're not putting in a number that only you care about!"

John Cochrane

The only thing a flight computer can productively add is this calculation.. I've been bugging the CN folks do implement this for a while. All it takes is to present the equivalent after-turnpoint mac cready setting for the current value. But they answer (correctly, from a business standpoint) "we're not putting in a number that only you care about!"

If Brigliadori and Kawa care, it might be worth adding!

2C

I think I would modify John's statements just slightly. He says "You want to both fly faster and be choosier about thermals going to an upwind turnpoint, and fly slower and take weaker lift going to a downwind turnpoint."

I would say, you want to fly faster 'after the last climb going into an upwind turnpoint' and be choosier about thermals (particularly the last one before the turnpoint) when going to an upwind turnpoint and 'be more willing' to fly a little slower, or take a weaker climb (particularly the last one before the turn) when going to a downwind turnpoint.

Key is that theory is different when trying to maximize altitude over the ground at a fixed point some distance away then it is to maximize achieved cross country speed through the air.

Sound about right, John?

George Moffat:
"Always go into an upwind turnpoint low"

Anonymous:
"Yep, Ol' George sure landed out a lot of his competitors with that one!"

See ya, Dave "YO electric"

On Friday, January 10, 2014 3:10:57 PM UTC-6, Dave Nadler wrote:
George Moffat:

"Always go into an upwind turnpoint low"



Anonymous:

"Yep, Ol' George sure landed out a lot of his competitors with that one!"



See ya, Dave "YO electric"

no matter downwind or upwind ... use the horseshoe theory cheers S9

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 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