STF question

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

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
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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