Help calculating Speed To Fly for headwind and tailwind

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 he
http://www.soaringpilot.org/dokuwiki...l_glide_screen

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

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

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"

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