Help calculating Speed To Fly for headwind and tailwind

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

On Jun 2, 5:45*am, Nine Bravo Ground wrote:
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?

Last Monday's club contest task put us all in exactly that position.
The last turn of the area task was downwind of the finish and the
winds were quite strong. It was blue in the last turn area so most of
us made final glide altitude before entering the last area and went in
just deep enough to make the best use of the altitude. For me the
most difficult part of the puzzle is knowing what the winds will be on
the downwind leg and whether it will be the same on the up wind leg to
the finish. I have never flown with a glide computer that does a good
job of knowing the winds at all altitudes and uses that wind profile,
rather than a fixed value, for the final glide solution.

As I was already over min time and not wishing to landout because the
head wind on the last leg was stronger than expected I elected to turn
early and fly home fast. I have no doubt I could have earned a few
more points by going a bit further and flying slower on the last leg.

Andy