STF question

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

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.c...d_downwind.mht

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

John Cochrane

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