STF question

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