We know flight computers are taking into account the polar, MC setting
and the current sink/lift to determine the speed to fly, but AFAIK
they don't (or at least my 302 doesn't) take into account the head/
tail wind component. As such, they will often command too slow speed
when flying in headwind, unless the pilots manually increases the MC
setting, which is not easy to determine. This is especially critical
in final glide against strong head wind. I would think it would be
straight forward to incorportae head/tail wind into the STF
calculation, or at least make it optional. Any thoughts why not?

Ramy

Standard STF theory says that the head/tail wind does not matter for
cruise flight. Does wind influence best speed for final glide ?

Todd Smith
3S

toad wrote:
> Standard STF theory says that the head/tail wind does not matter for
> cruise flight. Does wind influence best speed for final glide ?
>
> Todd Smith
> 3S


It's two separate issues. Speed to fly AND altitude required. Speed to
fly is NOT dependent on HW/TW. HW/TW will affect Altitude required.

toad wrote:
> Standard STF theory says that the head/tail wind does not matter for
> cruise flight. Does wind influence best speed for final glide ?


If you're flying at 50 knots into a 50 knot headwind you'll make how
much progress over the ground?

Tony V

Ramy wrote:
> We know flight computers are taking into account the polar, MC setting
> and the current sink/lift to determine the speed to fly, but AFAIK
> they don't (or at least my 302 doesn't) take into account the head/
> tail wind component.

Are you sure about that? Certainly the computer "knows" the wind
direction and velocity. It's then rather trivial to factor that in to
other calculations.



> As such, they will often command too slow speed
> when flying in headwind, unless the pilots manually increases the MC
> setting, which is not easy to determine. This is especially critical
> in final glide against strong head wind. I would think it would be
> straight forward to incorportae head/tail wind into the STF
> calculation, or at least make it optional. Any thoughts why not?

It *is* straightforward. SoarPilot does it. I would double-check your
assumptions.

Regards,

-Doug

On Mar 14, 6:51*pm, Gary Emerson > wrote:
> toad wrote:
> > Standard STF theory says that the head/tail wind does not matter for
> > cruise flight. Does wind influence best speed for final glide ?
>
> > Todd Smith
> > 3S
>
> It's two separate issues. *Speed to fly AND altitude required. *Speed to
> fly is NOT dependent on HW/TW. *HW/TW will affect Altitude required.

HW/TW also effect speed to fly to achive best glide over the ground.
The rule of thumb is to add half the wind speed to the best glide
speed. I am also not sure it doesn't matter for cruise flight. If you
have a 50 knots head wind you will not make any progress unless you
fly very fast. From what I understand from cambridge manuals they do
not take the wind into account for speed to fly.

Ramy

Ramy wrote:
> On Mar 14, 6:51 pm, Gary Emerson > wrote:
>> toad wrote:
>>> Standard STF theory says that the head/tail wind does not matter for
>>> cruise flight. Does wind influence best speed for final glide ?
>>> Todd Smith
>>> 3S
>> It's two separate issues. Speed to fly AND altitude required. Speed to
>> fly is NOT dependent on HW/TW. HW/TW will affect Altitude required.
>
> HW/TW also effect speed to fly to achive best glide over the ground.
> The rule of thumb is to add half the wind speed to the best glide
> speed. I am also not sure it doesn't matter for cruise flight. If you
> have a 50 knots head wind you will not make any progress unless you
> fly very fast. From what I understand from cambridge manuals they do
> not take the wind into account for speed to fly.

I have a 302, and I'm sure you are right, because "speed to fly" does
not depend on the wind. If you want to know the "speed to fly to achieve
the greatest distance over the ground into a wind", then you can (1) use
your rule of thumb, (2) use the MC setting that gives a theoretical
cross-country speed equal to the wind (another rule of thumb), (3) let
Winpilot (or perhaps one of the others) figure it out for you.

If you have a flight computer, you might be able to find the best
speed/MC setting by watching the arrival height for a point in front of
you while you adjust the MC from 0 upwards. I haven't tried that, but it
sounds good.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

The "airspeed to fly to produce the fastest time over a given path"
(for a given thermal strength) _does_ depend upon the wind. Simple
MacCready theory does not include wind effect but can be modified to
include it. Whether the 302 includes that effect I do not know. I do
know that wind effect is included in the STF calculations (including
that of airspeed) used for my BLIPMAP viewer "Track Average" popup.

Jack

On Mar 15, 3:29*am, Ramy > wrote:
> On Mar 14, 6:51*pm, Gary Emerson > wrote:
>
> > toad wrote:
> > > Standard STF theory says that the head/tail wind does not matter for
> > > cruise flight. Does wind influence best speed for final glide ?
>
> > > Todd Smith
> > > 3S
>
> > It's two separate issues. *Speed to fly AND altitude required. *Speed to
> > fly is NOT dependent on HW/TW. *HW/TW will affect Altitude required.
>
> HW/TW also effect speed to fly to achive best glide over the ground.
> The rule of thumb is to add half the wind speed to the best glide
> speed.

I thought it was add half the windspeed for a headwind, but take off
half for a tailwind? Or is it had half whichever direction you're
heading?!


Dan

Below is an illustration of the wind speed effect on a flight leg,
comparing a BLIPMAP Track Average "optimal flight" calculation for a
headwind of 20 kt to one with no head/tail wind (for a LS-3 polar). The
former requires a larger airspeed between thermals and a larger amount
of time spent thermalling to make progress. And of course the ground
speed is lower, but note that it is _higher_ than one would get by
simply subtracting the headwind from the "no headwind" groundspeed -
this is what is gained by using a higher airspeed when a headwind
exists. For this example the "add half the headwind" rule of thumb
would give an airspeed higher than optimal. (I suspect the column
formatting will be lost in this posting, so the numbers in order are the
leg distance (km), tailwind (kt), climb rate(m/s), leg time (min),
average groundspeed (kt), between-thermal airspeed (kt), and
percent time spent thermalling.)

---- Optimal-Flight-Avg ----
-- Tail Clmb Gnd Air Thm
-- Dist Wind Rate Time Spd Spd Pct
-- km kt m/s min kt kt %
-- 45 -20 1.4 43 34 85 60
-- *** WITH NO HEADWIND ***
-- 45 0 1.4 32 45 80 44

The 302 like all current instruments does calculate altitude required
correctly. If you put in Mc2, and fly at the Mc2 speed, and with
steady wind and no lift/sink, you'll get to the airport.

The 302 does not adjust the Mc setting for headwind / tailwind, nor
does it adjust it for strength of the next thermal for that matter.
That's up to the pilot.

The adjustment to Mc setting for headwind/tailwind for glide to a
point is rather minor, typically less than one knot for most winds we
encounter in thermic cross country gliding (20 kts or less). A
headwind is like shifting the whole polar to the left, a tailwind is
like shifting it to the right. Thus, you can visualize the effect of
headwind/tailwind on the usual polar diagram by drawing a tangent to
the polar from a point that is shifted up from the origin by expected
lift, and horizontally from the origin by the wind speed. You can then
see the correct Mc setting to put in the instrument by where this new
line crosses the vertical axis.

Modern glider polars are so flat, that this tangency line is also very
flat, so a large horizontal displacement corresponds to a very small
vertical displacement. If your glide angle is 30:1, then each vertical
knot of displacement correponds to 30 horizontal knots of displacement
-- a 30 knot headwind only changes your Mc setting by one knot!

What about the "half the wind velocity" rule we parrot for the FAA? In
the same modern glider, small chages in Mc setting give rise to rather
large changes in airspeed. One knot more Mc setting is about 10 knots
more speed, so the ballpark is right. (Try giving the above answer to
your FAA examiner on your next rating instead!)

The adjustment to Mc setting for rounding a turnpoint in headwind or
tailwind is considerably larger, because now you're not just gliding
to a point and thermaling with a headwind, you are thinking about
thermaling with a headwind vs. a tailwind. I wrote a "contest corner"
article in soaring magazine, together with a little chart you can
bring along in the cockpit in Oct 2006, it's on my webpage

http://faculty.chicagogsb.edu/john.cochrane/research/Papers/upwind_and_downwind.mht

End of theory class. When's the lab session already??

John Cochrane

Dan G wrote:
> On Mar 15, 3:29 am, Ramy > wrote:
>> On Mar 14, 6:51 pm, Gary Emerson > wrote:
>>
>>> toad wrote:
>>>> Standard STF theory says that the head/tail wind does not matter for
>>>> cruise flight. Does wind influence best speed for final glide ?
>>>> Todd Smith
>>>> 3S
>>> It's two separate issues. Speed to fly AND altitude required. Speed to
>>> fly is NOT dependent on HW/TW. HW/TW will affect Altitude required.
>> HW/TW also effect speed to fly to achive best glide over the ground.
>> The rule of thumb is to add half the wind speed to the best glide
>> speed.
>
> I thought it was add half the windspeed for a headwind, but take off
> half for a tailwind? Or is it had half whichever direction you're
> heading?!

The approximate answer is "Add some fraction of the headwind". Someone
suggested one half; I generally increase the MC about 1 knot, which has
a similar effect.

For a tailwind, the best distance will be achieved by flying somewhere
between the max L/D speed and the minimum sink speed, so I set MC to
zero (best L/D). If it's a really strong tailwind, I'll fly slower than
it indicates (closer to minimum sink). In the olden days, I could rotate
the MC ring to a negative number, but my 302 doesn't allow that!

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

On Mar 14, 10:14 pm, Tony Verhulst > wrote:
> toad wrote:
> > Standard STF theory says that the head/tail wind does not matter for
> > cruise flight. Does wind influence best speed for final glide ?
>
> If you're flying at 50 knots into a 50 knot headwind you'll make how
> much progress over the ground?
>
> Tony V

Come on guys, the classic "cruise" speed to fly theory is designed to
maximize average XC speed does not include headwind or tailwind,
because the assumption is that the thermal drifts with the wind. This
is the STF calculation programmed into all of our flight computers.

If you want to achieve best L/D, then yes you have to include head/
tail wind.

But if you are trying to achieve max XC speed, including final glide,
then you don't include wind.

This changes if you modify the assumption that a thernal drifts with
the same speed as the wind.

Todd Smith
3S

To be fully precise, though at the risk of getting too arcane, I'll
point out that the "shift the polar" treatment for the effect of wind on
STF only gives the true airspeed
_component_in_the_direction_of_the_track. The true "airspeed to fly"
also depends upon the cross-wind component of the wind, since one must
crab to fly the desired track. The effect of this additional factor is
typically small, less than 3 kts. In my previous example calculation I
used a cross-wind component of zero to make things simpler.

Jack

The fact that the best speed to fly interthermal is independent of the
headwind-tailwind component confused me at first too, and for the same
reason as one of the other posters. If your STF is 50 kts and you have
a 50 kt HW, you'll make no distance. The problem is, flying faster
than 50 kts (assuming it is the value predicted by MacCready's
optimization) will actually blow you backward. Why? You make headway
while flying, but when you stop to tank up before hitting the ground
(an assumption in the MacCready model that is not often explicitly
stated: you can find a thermal with the lift you've dialed in whenever
you want it -- not very realistic!), you are blown backward at 50 kts
due to the tilt of the thermal. Because you flew faster than optimal,
you must tank up longer and are blown back more.

When on final glide, the optimal speed to fly is different and clearly
depends on the wind. It no longer depends on the speed of the "next"
thermal, since there isn't one. It seems like a totally different
optimization to me.

Hope this helps.

Martin

On Mar 14, 8:29*pm, Ramy > wrote:
> On Mar 14, 6:51*pm, Gary Emerson > wrote:
>
> > toad wrote:
> > > Standard STF theory says that the head/tail wind does not matter for
> > > cruise flight. Does wind influence best speed for final glide ?
>
> > > Todd Smith
> > > 3S
>
> > It's two separate issues. *Speed to fly AND altitude required. *Speed to
> > fly is NOT dependent on HW/TW. *HW/TW will affect Altitude required.
>
> HW/TW also effect speed to fly to achive best glide over the ground.
> The rule of thumb is to add half the wind speed to the best glide
> speed. I am also not sure it doesn't matter for cruise flight. If you
> have a 50 knots head wind you will not make any progress unless you
> fly very fast. From what I understand from cambridge manuals they do
> not take the wind into account for speed to fly.
>
> Ramy

On Mar 14, 8:29*pm, Ramy > wrote:
> On Mar 14, 6:51*pm, Gary Emerson > wrote:
>
> > toad wrote:
> > > Standard STF theory says that the head/tail wind does not matter for
> > > cruise flight. Does wind influence best speed for final glide ?
>
> > > Todd Smith
> > > 3S
>
> > It's two separate issues. *Speed to fly AND altitude required. *Speed to
> > fly is NOT dependent on HW/TW. *HW/TW will affect Altitude required.
>
> HW/TW also effect speed to fly to achive best glide over the ground.
> The rule of thumb is to add half the wind speed to the best glide
> speed. I am also not sure it doesn't matter for cruise flight. If you
> have a 50 knots head wind you will not make any progress unless you
> fly very fast. From what I understand from cambridge manuals they do
> not take the wind into account for speed to fly.
>
> Ramy


To get the MC setting for best glide distance into a head wind just
rotate the MC knob to the setting that gives the minimum required
altitude. Unless the wind is constant as you descend you will have
to keep adjusting the MC as the low point shifts.

Andy

On Mar 15, 7:09*pm, wrote:
> When on final glide, the optimal speed to fly is different and clearly
> depends on the wind. It no longer depends on the speed of the "next"
> thermal, since there isn't one. It seems like a totally different
> optimization to me.

To determine the "optimum" speed to fly you need to state the
objective. The speed or MC setting to achive max glide distance may
not be the speed or MC setting that gives the shortest time to the
destination.

For best speed there is an interaction between thermal strength and MC
setting even though the final glide does not end with a thermal
climb. The MC setting is based on the strength of the thermal you are
currently using to reach final glide altitude. The combination of
wind, thermal strength, and distance to go, will determine not only
the final glide MC setting but also when to leave the last thermal.

Of course you can forget all that if you practice the start at zero MC
and bump up final glide technique.

Andy

I must retract my previous statements regarding wind effect on on the
"cruise" STF, i.e the true airspeed to fly to achieve the fastest
speed over the ground between two ground points. Further thought, and
working through a specific example to convince myself, tells me that
airspeed is _independent_ of a head/tailwind. The way I now think
of it is that the "optimal" interthermal-glide airspeed produces the
fastest obtainable "cruise" airspeed, i.e. with no altitude loss over
a thermalling/gliding cycle (for the given thermal strength
and glider polar) - a wind affects the resultant groundspeed but not
that optimal value through the air. This means it is possible to have
a wind for which no forward progress can be made without a net loss of
altitude.

I now believe I've made an incorrect assumption somewhere in script I
wrote to predict "optimal" speeds over a track (most likely neglecting
the thermal drift with the wind) and will need to correct that.

Jack

So to be clear, when not on final glide, to achieve the best speed-
over-ground the head/tail/cross wind should be ignored when
determining inter-thermal cruise speed?


Dan

On Mar 16, 11:53*pm, Dan G > wrote:
> So to be clear, when not on final glide, to achieve the best speed-
> over-ground the head/tail/cross wind should be ignored when
> determining inter-thermal cruise speed?
>
> Dan

I mean, when I think about it, I come to the conclusion that you
always have to (and can only) optimise your speed through the airmass
by flying MC (putting aside what that actually means in reality). If
the resultant XC speed given by MC theory is less than the windspeed,
well, you're stuffed.


Dan

> I mean, when I think about it, I come to the conclusion that you
> always have to (and can only) optimise your speed through the airmass
> by flying MC (putting aside what that actually means in reality). If
> the resultant XC speed given by MC theory is less than the windspeed,
> well, you're stuffed.
>
> Dan

That's correct. The assumption is that you're flying in thermals, and
thermals drift with the airmass. Actually thermals usually drift a bit
less than airmass, and if you've got ridge or wave, those are
stationary with respect to the ground. But otherwise, by definition
there is no way to fly faster than the optimum speed through the
airmasss -- the fact that the ground is moving around below you is
irrelevant.

John Cochrane

"BB" > wrote in message
...
>> I mean, when I think about it, I come to the conclusion that you
>> always have to (and can only) optimise your speed through the airmass
>> by flying MC (putting aside what that actually means in reality). If
>> the resultant XC speed given by MC theory is less than the windspeed,
>> well, you're stuffed.
>>
>> Dan
>
> That's correct. The assumption is that you're flying in thermals, and
> thermals drift with the airmass. Actually thermals usually drift a bit
> less than airmass, and if you've got ridge or wave, those are
> stationary with respect to the ground. But otherwise, by definition
> there is no way to fly faster than the optimum speed through the
> airmasss -- the fact that the ground is moving around below you is
> irrelevant.
>
> John Cochrane

For the ultimate competitor, going fast is their reason for being so the
MacCready formula is very important for them.

For the rest of us, staying in the air and getting home is also on the
priority list. Answering the question, "How far can I go from here?" is
important. That's why I really like the "Glide Amoeba " on the NK glide
computer and the "Glide Footprint" on GPS_LOG WinCE. These simple graphics
take wind and terrain into account as well as the current M setting.

Bill D

On Mar 16, 10:24 pm, "Bill Daniels" <bildan@comcast-dot-net> wrote:
> "BB" > wrote in message
>
> ...
>
>
>
> >> I mean, when I think about it, I come to the conclusion that you
> >> always have to (and can only) optimise your speed through the airmass
> >> by flying MC (putting aside what that actually means in reality). If
> >> the resultant XC speed given by MC theory is less than the windspeed,
> >> well, you're stuffed.
>
> >> Dan
>
> > That's correct. The assumption is that you're flying in thermals, and
> > thermals drift with the airmass. Actually thermals usually drift a bit
> > less than airmass, and if you've got ridge or wave, those are
> > stationary with respect to the ground. But otherwise, by definition
> > there is no way to fly faster than the optimum speed through the
> > airmasss -- the fact that the ground is moving around below you is
> > irrelevant.
>
> > John Cochrane
>
> For the ultimate competitor, going fast is their reason for being so the
> MacCready formula is very important for them.
>
> For the rest of us, staying in the air and getting home is also on the
> priority list. Answering the question, "How far can I go from here?" is
> important. That's why I really like the "Glide Amoeba " on the NK glide
> computer and the "Glide Footprint" on GPS_LOG WinCE. These simple graphics
> take wind and terrain into account as well as the current M setting.
>
> Bill D

Yes, and all of the glide computers out there take headwind or
tailwind into account when computing the altitude required.

Todd Smith
3S

On Mar 16, 7:07*pm, BB > wrote:
> > I mean, when I think about it, I come to the conclusion that you
> > always have to (and can only) optimise your speed through the airmass
> > by flying MC (putting aside what that actually means in reality). If
> > the resultant XC speed given by MC theory is less than the windspeed,
> > well, you're stuffed.
>
> > Dan
>
> That's correct. The assumption is that you're flying in thermals, and
> thermals drift with the airmass. Actually thermals usually drift a bit
> less than airmass, and if you've got ridge or wave, those are
> stationary with respect to the ground. But otherwise, by definition
> there is no way to fly faster than the optimum speed through the
> airmasss -- the fact that the ground is moving around below you is
> irrelevant.
>
> John Cochrane

So the conclusion is that the wind has effect only on the final glide,
and requires adjusting the MC setting manually (and keep adjusting as
the wind changes) to optimize the glide over the ground, which in some
cases (long glides into significant head wind) will make the
difference between making it back or landing short. However, while
normally MC values can not be determined by the flight computer since
it doesn't know if,when and how strong the next climb will be, the MC
value for best final glide into the wind CAN be determined by the
flight computer. Wouldn't this be a useful feature, to be able to
select an auto MC for final glides?

Ramy

Bill Daniels wrote:
> "BB" > wrote in message
> ...
>>> I mean, when I think about it, I come to the conclusion that you
>>> always have to (and can only) optimise your speed through the airmass
>>> by flying MC (putting aside what that actually means in reality). If
>>> the resultant XC speed given by MC theory is less than the windspeed,
>>> well, you're stuffed.
>>>
>>> Dan
>> That's correct. The assumption is that you're flying in thermals, and
>> thermals drift with the airmass. Actually thermals usually drift a bit
>> less than airmass, and if you've got ridge or wave, those are
>> stationary with respect to the ground. But otherwise, by definition
>> there is no way to fly faster than the optimum speed through the
>> airmasss -- the fact that the ground is moving around below you is
>> irrelevant.
>>
>> John Cochrane
>
> For the ultimate competitor, going fast is their reason for being so the
> MacCready formula is very important for them.

My experience with talking to "ultimate competitors" is that the
MacCready formula isn't even on their radar. It's mainly a topic the
rest of us obsess over, because we think it we just understood it well
enough, we'd go as fast as those other guys. The "ultimate" pilots know
the MC speed isn't important because we rarely fly in pure MC condtions,
and in any case, "risk management" means flying slower to optimize the
chances of finishing the task. John Cochrane's article is a wonderful
exposition of this.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

On Mar 16, 11:19 pm, Ramy > wrote:
> On Mar 16, 7:07 pm, BB > wrote:
>
>
>
> > > I mean, when I think about it, I come to the conclusion that you
> > > always have to (and can only) optimise your speed through the airmass
> > > by flying MC (putting aside what that actually means in reality). If
> > > the resultant XC speed given by MC theory is less than the windspeed,
> > > well, you're stuffed.
>
> > > Dan
>
> > That's correct. The assumption is that you're flying in thermals, and
> > thermals drift with the airmass. Actually thermals usually drift a bit
> > less than airmass, and if you've got ridge or wave, those are
> > stationary with respect to the ground. But otherwise, by definition
> > there is no way to fly faster than the optimum speed through the
> > airmasss -- the fact that the ground is moving around below you is
> > irrelevant.
>
> > John Cochrane
>
> So the conclusion is that the wind has effect only on the final glide,
> and requires adjusting the MC setting manually (and keep adjusting as
> the wind changes) to optimize the glide over the ground, which in some
> cases (long glides into significant head wind) will make the
> difference between making it back or landing short. However, while
> normally MC values can not be determined by the flight computer since
> it doesn't know if,when and how strong the next climb will be, the MC
> value for best final glide into the wind CAN be determined by the
> flight computer. Wouldn't this be a useful feature, to be able to
> select an auto MC for final glides?
>
> Ramy

I think the conclusion is that the wind has effect on the altitude
required for the final glide, but not the speed that you would fly it
at. The speed is determined by the climb rate at the top of the last
thermal.

Some flight computers do an automatic MC setting for a final glide. I
think both Winpilot and Seeyou Mobile do this.

Todd

Ramy > wrote:

>So the conclusion is that the wind has effect only on the final glide,
>and requires adjusting the MC setting manually (and keep adjusting as
>the wind changes) to optimize the glide over the ground, which in some
>cases (long glides into significant head wind) will make the
>difference between making it back or landing short. However, while
>normally MC values can not be determined by the flight computer since
>it doesn't know if,when and how strong the next climb will be, the MC
>value for best final glide into the wind CAN be determined by the
>flight computer. Wouldn't this be a useful feature, to be able to
>select an auto MC for final glides?
>

Try GPS_LOG WinCE. It has this feature. At least if I understand you
correctly. One of its displays shows MC and speed to reach destination
from the altitude you are at. all you need is to click it (and
confirm) to transfer MC value. One of "ultimate" :) competition pilots
asked for it years back.

Cheers,
Henryk Birecki

What we're really talking about here is; How high should I climb in
the last thermal? I think you should take it to the altitude where
your computer shows you have final glide at your last MC setting. Lets
say its 3, but your last thermal is pumbing out a solid 4 knots. Do
you stay longer (climb higher) or leave when you have final glide into
the current wind? The rule I use is; Climb higher, if the average
thermal strength is above 3 knots and then adjust my MC setting to
come home faster. If my final thermal is only 2 knots, I leave as soon
as I have final glide + pattern altitude ( I use 500 feet).
Other thoughts?
JJ

Ramy wrote:
> On Mar 16, 7:07�pm, BB > wrote:
> > > I mean, when I think about it, I come to the conclusion that you
> > > always have to (and can only) optimise your speed through the airmass
> > > by flying MC (putting aside what that actually means in reality). If
> > > the resultant XC speed given by MC theory is less than the windspeed,
> > > well, you're stuffed.
> >
> > > Dan
> >
> > That's correct. The assumption is that you're flying in thermals, and
> > thermals drift with the airmass. Actually thermals usually drift a bit
> > less than airmass, and if you've got ridge or wave, those are
> > stationary with respect to the ground. But otherwise, by definition
> > there is no way to fly faster than the optimum speed through the
> > airmasss -- the fact that the ground is moving around below you is
> > irrelevant.
> >
> > John Cochrane
>
> So the conclusion is that the wind has effect only on the final glide,
> and requires adjusting the MC setting manually (and keep adjusting as
> the wind changes) to optimize the glide over the ground, which in some
> cases (long glides into significant head wind) will make the
> difference between making it back or landing short. However, while
> normally MC values can not be determined by the flight computer since
> it doesn't know if,when and how strong the next climb will be, the MC
> value for best final glide into the wind CAN be determined by the
> flight computer. Wouldn't this be a useful feature, to be able to
> select an auto MC for final glides?
>
> Ramy

On Mar 17, 6:03Â*am, JJ Sinclair > wrote:
> What we're really talking about here is; How high should I climb in
> the last thermal? I think you should take it to the altitude where
> your computer shows you have final glide at your last MC setting. Lets
> say its 3, but your last thermal is pumbing out a solid 4 knots. Do
> you stay longer (climb higher) or leave when you have final glide into
> the current wind? The rule I use is; Climb higher, if the average
> thermal strength is above 3 knots and then adjust my MC setting to
> come home faster. If my final thermal is only 2 knots, I leave as soon
> as I have final glide + pattern altitude ( I use 500 feet).
> Other thoughts?
> JJ
>
>
>
> Ramy wrote:
> > On Mar 16, 7:07�pm, BB > wrote:
> > > > I mean, when I think about it, I come to the conclusion that you
> > > > always have to (and can only) optimise your speed through the airmass
> > > > by flying MC (putting aside what that actually means in reality). If
> > > > the resultant XC speed given by MC theory is less than the windspeed,
> > > > well, you're stuffed.
>
> > > > Dan
>
> > > That's correct. The assumption is that you're flying in thermals, and
> > > thermals drift with the airmass. Actually thermals usually drift a bit
> > > less than airmass, and if you've got ridge or wave, those are
> > > stationary with respect to the ground. But otherwise, by definition
> > > there is no way to fly faster than the optimum speed through the
> > > airmasss -- the fact that the ground is moving around below you is
> > > irrelevant.
>
> > > John Cochrane
>
> > So the conclusion is that the wind has effect only on the final glide,
> > and requires adjusting the MC setting manually (and keep adjusting as
> > the wind changes) to optimize the glide over the ground, which in some
> > cases (long glides into significant head wind) Â*will make the
> > difference between making it back or landing short. However, while
> > normally MC values can not be determined by the flight computer since
> > it doesn't know if,when and how strong the next climb will be, the MC
> > value for best final glide into the wind CAN be determined by the
> > flight computer. Wouldn't this be a useful feature, to be able to
> > select an auto MC for final glides?
>
> > Ramy- Hide quoted text -
>
> - Show quoted text -

This is all correct when you have enough altitude that all you need to
decide is how much higher you should climb to fly faster, a typical
contets scenario. I was thinking more of final glide at the end of the
day when you trying to stretch a marginal glide to try to make it home
against significant head wind (think final glide to Truckee from the
north end of the Pine Nuts or Airsailing). Normally you would put MC=0
when you want to maximize your glide, but if you have 20 knots head
wind a setting MC=1 will be better. Sounds like GPS-LOG is doing this.

Ramy

Ramy > wrote:

>This is all correct when you have enough altitude that all you need to
>decide is how much higher you should climb to fly faster, a typical
>contets scenario. I was thinking more of final glide at the end of the
>day when you trying to stretch a marginal glide to try to make it home
>against significant head wind (think final glide to Truckee from the
>north end of the Pine Nuts or Airsailing). Normally you would put MC=0
>when you want to maximize your glide, but if you have 20 knots head
>wind a setting MC=1 will be better. Sounds like GPS-LOG is doing this.


Well... not exactly. You should always put MC=0 (setting it higher
would give you safety margin as you can slow down) Flight computer
should give you altitude and speed corrected for wind for the MC
setting you request. If you have the altitude, then it can calculate
maximum speed to reach destination and from speed MC. GPS_LOG
recalculates speed (and MC) in final glide continuously based on
altitude and you can display it if you wish, or you can display other
types of calculated speeds.

Just remember that all instruments lie, and the ones that predict
something lie the most.

Cheers,
Henryk

On Mar 17, 10:27*pm, Henryk Birecki >
> Well... not exactly. You should always put MC=0 (setting it higher
> would give you safety margin as you can slow down)

Sorry, I don't acept that.

As I said in an earlier post - with a head wind present, increasing
the MC from zero will show reducing altitude required until it reaches
a minimum and then increases again.

Try it in simulator mode on any PDA based glide computer. If it
doesn't do that there something wrong with the implementation.
Computers as old as the MNAV did this right.


Andy

Andy > wrote:

>On Mar 17, 10:27*pm, Henryk Birecki >
>> Well... not exactly. You should always put MC=0 (setting it higher
>> would give you safety margin as you can slow down)
>
>Sorry, I don't acept that.
>
>As I said in an earlier post - with a head wind present, increasing
>the MC from zero will show reducing altitude required until it reaches
>a minimum and then increases again.
>
>Try it in simulator mode on any PDA based glide computer. If it
>doesn't do that there something wrong with the implementation.
>Computers as old as the MNAV did this right.
>

Yes, but you are talking about a different computation

>Andy

On Mar 18, 5:09*pm, Henryk Birecki > wrote:
> Yes, but you are talking about a different computation

My understanding is that Ramy concluded that to get best glide
distance into a headwind he should use a MC setting greater than
zero. Your reply was that was wrong and he should use zero MC
setting. I then disagreed and said any computer should show less alt
required at a MC greater than zero.

What different computations are being discussed here?

thanks

Andy

OK Andy, I'll bite. First of all Ramy was making a guess about what
and how GPS_LOG computes, not how to use MNAV.... Now for the lesson:

Let's start with what is MC setting. MC number is essentially expected
lift that you will find in thermals (assuming you reach them) Since
you are going to find lift you can trade it off for airspeed and
increase your XC speed. In classic cross country speed maximization
theory the speed to fly to maximize XC speed does not depend on wind,
so MC and airspeed that you fly at are in a one to one relationship.
Just use table lookup, or memorise it, no need for any compute power.

Computation that you are talking about is done by computationally
challenged (for good reasons) instrumentation, and computer programs
that emulate it. In that case YOU are telling the computer what speed
you are going to fly at by selecting MC setting. The MC in this case
has no relationship to the lift you will encounter in flight, but an
easy way to enter speed. Computer then calculates the easy part
(altitude you need). GPS_LOG is not computationally challenged. You
tell it what lift you expect and it gives you both altitude AND speed
optimised for maximum distance. In a thermal, altitude is reported
assuming that you will NOT find lift (nor sink) along the way, so as
you increase MC, altitude follows. During flight, both speed and
altitude are recomputed depending on Vz of airmass are flying through.
There are also other speed displays and altitude computation modes.

One of the GPS_LOG displays gives you maximum speed to fly to reach
destination given altitude you have, wind... It also displays
equivalent MC. This display is of particular use in the last thermal
before final glide. As long as you are climbing faster than the
displayed MC, you stay in the thermal. Once you no longer climb as
fast as MC indicates, you should leave.

Cheers,
Henryk

Andy > wrote:

>On Mar 18, 5:09*pm, Henryk Birecki > wrote:
>> Yes, but you are talking about a different computation
>
>My understanding is that Ramy concluded that to get best glide
>distance into a headwind he should use a MC setting greater than
>zero. Your reply was that was wrong and he should use zero MC
>setting. I then disagreed and said any computer should show less alt
>required at a MC greater than zero.
>
>What different computations are being discussed here?
>
>thanks
>
>Andy

Henryk,

Nice job with the lesson. For our club, I put together a basic cross-
country course. After reading your post, I am going to rewrite that
part of my lesson because your post makes it easier to understand.

Raul Boerner
LS6-b DM

wrote:

>Henryk,
>
>Nice job with the lesson. For our club, I put together a basic cross-
>country course. After reading your post, I am going to rewrite that
>part of my lesson because your post makes it easier to understand.
>
>Raul Boerner
>LS6-b DM

If you feel like sharing course materials on line I can probably host
them on my site.

Cheers,
Henryk

On Mar 19, 11:05*am, Henryk Birecki >
wrote:
> OK *Andy, I'll bite. *First of all Ramy was making a guess about what
> and how GPS_LOG computes, not how to use MNAV.... Now for the lesson:

Thanks for the explanation. I was not clear to me that you and Ramy
were discussing a feature of GPS_LOG.

Andy

Henryk Birecki wrote:
> OK Andy, I'll bite. First of all Ramy was making a guess about what
> and how GPS_LOG computes, not how to use MNAV.... Now for the lesson:
>
> Let's start with what is MC setting. MC number is essentially expected
> lift that you will find in thermals (assuming you reach them) Since
> you are going to find lift you can trade it off for airspeed and
> increase your XC speed. In classic cross country speed maximization
> theory the speed to fly to maximize XC speed does not depend on wind,
> so MC and airspeed that you fly at are in a one to one relationship.
> Just use table lookup, or memorise it, no need for any compute power.
>
> Computation that you are talking about is done by computationally
> challenged (for good reasons) instrumentation, and computer programs
> that emulate it. In that case YOU are telling the computer what speed
> you are going to fly at by selecting MC setting. The MC in this case
> has no relationship to the lift you will encounter in flight, but an
> easy way to enter speed. Computer then calculates the easy part
> (altitude you need). GPS_LOG is not computationally challenged. You
> tell it what lift you expect and it gives you both altitude AND speed
> optimised for maximum distance. In a thermal, altitude is reported
> assuming that you will NOT find lift (nor sink) along the way, so as
> you increase MC, altitude follows. During flight, both speed and
> altitude are recomputed depending on Vz of airmass are flying through.
> There are also other speed displays and altitude computation modes.
>
> One of the GPS_LOG displays gives you maximum speed to fly to reach
> destination given altitude you have, wind... It also displays
> equivalent MC. This display is of particular use in the last thermal
> before final glide. As long as you are climbing faster than the
> displayed MC, you stay in the thermal. Once you no longer climb as
> fast as MC indicates, you should leave.
>
Here's an example of why I like my computer to show me the classic MC
setting and speed to fly:

Final glide into a stiff wind, racing sunset. I have just enough
altitude to make the glide. Speed up i won't make it, slow down I won't
make it. I'll also get to the goal right at sunset, but cut very close.
To get my computer to show the right speed to fly to just make it in
this wind, I had to dial in an MC = 2 kt setting. If I understand right,
some think the computer should be set to MC=0 kts in this situation,
since I want the best L/D I can achieve. Sounds a bit simpler than
twiddling the MC knob to minimize altitude required. But both give the
same commanded speed to fly, we get there at the same time either way.

Now I hit a sweet little unexpected 1.5 kt thermal. It will drift me
backwards, but will get me higher so I can glide faster. What do I do? I
know from studying Reichman's Cross-Country soaring book, and John
Cochrane's lectures, that classic MC setting just happens to be the cost
of altitude where I am right now. That means if I can get altitude at a
rate greater than my MC setting, its a bargain and I should take some.
My net time will be reduced. If the lift is less than my MC setting, its
a bad deal, and I will loose time by stopping.

So since my computer is set to 2kts and thats enough to finish, I should
pass up 1.5 kts. It will just slow me down since it is less than my
current MC setting for this glide. The optimal action is to press on at
the classic MC=2 kts speed to fly.

But if my PDA is set to a wind adjusted MC=0, there is no easy, correct
way to make the decision. The wind adjusted MC does not have the same
meaning. It actually gives the cost of altitude assuming I can get it
without drifting with the wind. But this thermal is going to drift. If I
stop to climb until the new wind adjusted MC=1.5 gives me just enough to
get home at that setting, I will get there with a steeper, faster glide,
but it will be after sunset because I wasted some time climbing in lift
and drifting backwards such that my average speed through the air and
over the ground drops. No badge for me.

That does not mean the wind adjusted approach is wrong or won't work,
but its not as obvious what the correct strategy should be. Thats really
why most flight computers use classic MC speed to fly and don't try to
do wind corrections.

Now a computer that twiddles the MC for me on final glide might have
some value. I actually find it more intuitive to watch the altitude
required go up and down than the auto MC run up and down. But I want the
indicated MC to be the classic one, so I know what it means and what to
do with it.


ZL - wannabe ultimate competitor

I just did a little experiment; from Williams, I put in Redding, CA
which is due north. My B-100 read 93.3 s/m (Discus 1 polar), M/C 0.0 =
11,286

Then I dialed in a good headwind, 360/25 and started increasing the M/
C, the lowest altitude required occurred at M/C = 0.8 (18,035)

Then I dialed in 360/15 and the least altitude required occurred at M/
C = 0.4 (14710)

I believe this shows that we should not fly M/C zero except in very
light wind. Also I have found the Nimbus-3 and ASH-25 wont make the
glide that M/C zero says they should. Probably because of over
optimistic polar information from the manufacturer.

Another issue I found with the SN-10 happened on the last day at
Parowan, last year. We had a stiff S/W wind at 25 knots with 3 turn
areas. I has excess altitude as I entered the last turn area, which
was down wind from Parowan. I penetrated until the computer said I was
1000 feet over final glide home and then made my turn. As soon as I
finished the turn and centered up on the final leg, the computer said
I would just make it! I lost my 1000 foot cushion in the turn (wind
remained the same). What happened?

I believe the program in the SN-10 doesn't take into account the drift
correction angle which was considerable with the 25 knot cross-wind I
had on my final leg home. I was holding a good 10 degrees heading up-
wind as the ship tracked Parowan. This gave me much more of a head-
wind component and I believe the comuuter didn't compute on the 'real'
heading that would be required to track the destination into a stiff
cross-wind.
What do you thing?
JJ

JJ Sinclair > wrote:

>I believe this shows that we should not fly M/C zero except in very
>light wind.

Yes, as long as you have to use MC as a crutch to set speed so that
you get max glide, the above is absolutely correct. Downwind you
should set MC negative (if you can :) )


>I believe the program in the SN-10 doesn't take into account the drift
>correction angle which was considerable with the 25 knot cross-wind I
>had on my final leg home. I was holding a good 10 degrees heading up-
>wind as the ship tracked Parowan. This gave me much more of a head-
>wind component and I believe the comuuter didn't compute on the 'real'
>heading that would be required to track the destination into a stiff
>cross-wind.
>What do you thing?

I do not know what SN-10 does, but yes, cross wind component is very
important if you worry about wind.

Cheers,
Henryk

On Mar 20, 9:35 am, JJ Sinclair > wrote:
> Another issue I found with the SN-10 happened on the last day at
> Parowan, last year. We had a stiff S/W wind at 25 knots with 3 turn
> areas. I has excess altitude as I entered the last turn area, which
> was down wind from Parowan. I penetrated until the computer said I was
> 1000 feet over final glide home and then made my turn. As soon as I
> finished the turn and centered up on the final leg, the computer said
> I would just make it! I lost my 1000 foot cushion in the turn (wind
> remained the same). What happened?

Huh ? You gotta stop spinning the glider rounding the turn,
can't use the "turnpoint camera" excuse anymore yaknow...

> I believe the program in the SN-10 doesn't take into account the drift
> correction angle which was considerable with the 25 knot cross-wind I
> had on my final leg home. I was holding a good 10 degrees heading up-
> wind as the ship tracked Parowan. This gave me much more of a head-
> wind component and I believe the comuuter didn't compute on the 'real'
> heading that would be required to track the destination into a stiff
> cross-wind.

The SN10 certainly takes into account the wind direction (cross).
No idea what went on above. Sometimes the reserve jumps UP,
because we round turnpoints or AAT earlier than the nominal
turnpoint or AAT point, but a reserve going DOWN is really
very odd...

Please email me any further detail !
See ya, Dave "YO"

Hi Dave,
I just checked my trace for the last day at Parowan and guess what?
The computer didn't screw up, I did. I don't adjust the turn point
when flying a turn area task, I simply keep checking on final
'altitude required' by clicking off the turn point (which is the
center of the turn area). Well, I was tired, it had been a hard day at
the end of 8 hard days straight and I forgot to 'click it off'. If I
had turned early, the altitude required would have jumped up, but I
flew past it and.......................I better stop trying to figure
out what the SN-10 does, just concentrate on operating it correctly!
Sorry for the incorrect post, I do love the SN-10, really!
JJ

> The SN10 certainly takes into account the wind direction (cross).
> No idea what went on above. Sometimes the reserve jumps UP,
> because we round turnpoints or AAT earlier than the nominal
> turnpoint or AAT point, but a reserve going DOWN is really
> very odd...
>
> Please email me any further detail !
> See ya, Dave "YO"