STF question

I am using an oldeer Ilec SB-8 STF vario system, am very happy with it
and absolutely love the vario function (been an Ilec user for many
years) but in my Tetra-15 I am finally using a STF system, and have a
few questions.

Since this unit does not use GPS info , when I am in a headwind or
tailwind the P/S info is all the unit has available for speed
info...............yesterday I was in an area of extended strong sink,
along with a strong headwind............with no GS reference (per
GPS), what kind of accuracy does the SB-8 STF info have? And what can
I do to remedy this.

Another issue on that flight was a battery that dropped below
10volts............the STF needle was behaving rather badly!

Brad

On Thursday, May 3, 2012 11:48:50 AM UTC-7, Brad wrote:
I am using an oldeer Ilec SB-8 STF vario system, am very happy with it
and absolutely love the vario function (been an Ilec user for many
years) but in my Tetra-15 I am finally using a STF system, and have a
few questions.

Since this unit does not use GPS info , when I am in a headwind or
tailwind the P/S info is all the unit has available for speed
info...............yesterday I was in an area of extended strong sink,
along with a strong headwind............with no GS reference (per
GPS), what kind of accuracy does the SB-8 STF info have? And what can
I do to remedy this.

Another issue on that flight was a battery that dropped below
10volts............the STF needle was behaving rather badly!

Brad

Assuming that you are flying in accordance with standard MacCready theory, your speed-to-fly does not depend on the windspeed. The only time headwinds become important is on final glide and the adjustment is not large and can usually be safely ignored.

Mike

On Thursday, May 3, 2012 12:48:50 PM UTC-7, Mike the Strike wrote:
On Thursday, May 3, 2012 11:48:50 AM UTC-7, Brad wrote:
I am using an oldeer Ilec SB-8 STF vario system, am very happy with it
and absolutely love the vario function (been an Ilec user for many
years) but in my Tetra-15 I am finally using a STF system, and have a
few questions.

Since this unit does not use GPS info , when I am in a headwind or
tailwind the P/S info is all the unit has available for speed
info...............yesterday I was in an area of extended strong sink,
along with a strong headwind............with no GS reference (per
GPS), what kind of accuracy does the SB-8 STF info have? And what can
I do to remedy this.

Another issue on that flight was a battery that dropped below
10volts............the STF needle was behaving rather badly!

Brad

Assuming that you are flying in accordance with standard MacCready theory, your speed-to-fly does not depend on the windspeed. The only time headwinds become important is on final glide and the adjustment is not large and can usually be safely ignored.

Mike

I dare to challenge this common claim that STF does not depend on windspeed unless on final glide. The above claim may only be true when flying over flatland. It is not true when flying over mountain terrain, which is the type of flying most of us do in western US. When flying over mountain terrain, the lift tends to remain in the same spots and don't drift with the wind. It may be tilted at most. Also the wind is significant for terrain clearance, so you certainly need to fly faster in head wind if terrain clearance is an issue. And finally, thermals do not always drift as fast as the wind. As such, you should almost always add speed when flying up wind, the stronger the headwind the faster you should fly. How faster is another question, I believe most flight computers don't take wind in account, especially when not in final glide. Adding half the wind speed sounds about right, I usually bump up my MC setting by 0.5 for head wind of up to 10 knots, by 1 for 10-20 knots and so forth.
I'll be happy to hear other opinions.

Ramy

Note that I said if you are applying standard MacCready theory, the wind doesn't matter, especially on a closed course - you fly the same distance upwind and downwind.

However, thermals generally blow with the wind, but usually a bit slower than the wind velocity. This would suggest flying a bit faster into a headwind and a bit slower with a tailwind. It is widely suggested that if you are heading to an upwind turnpoint, you should minimize thermalling until after the turn.

I've tried analyzing cross-country flights with different assumptions on wind speed and its distribution and it really doesn't seem to make much difference for the range of speeds you'll normally see.

Sounds like a perfect analysis for John Cochrane!

Mike

On May 3, 5:50*pm, Ramy wrote:

I dare to challenge this common claim that STF does not depend on windspeed unless on final glide. The above claim may only be true when flying over flatland. It is not true when flying over mountain terrain, which is the type of flying most of us do in western US. When flying over mountain terrain, the lift tends to remain in the same spots and don't drift with the wind.. It may be tilted at most. Also the wind is significant for terrain clearance, so you certainly need to fly faster in head wind if terrain clearance is an issue. And finally, thermals do not always drift as fast as the wind. As such, you should almost always add speed when flying up wind, the stronger the headwind the faster you should fly. How faster is another question, I believe most flight computers don't take wind in account, especially when not in final glide. Adding half the wind speed sounds about right, I usually bump up my MC setting by 0.5 for head wind of up to 10 knots, by 1 for 10-20 knots and so forth.
I'll be happy to hear other opinions.

Ramy-


Ramy, I think you will find that your mountain clearance example is
really a "final glide" computation. You have a fixed point on the
ground you must reach at or above a certain altitude. So, this is
really a final glide, and not a basic speed to fly to achieve a
maximum speed over the ground for the soaring conditions computation.
Whether the thermals are drifting at the same speed as the free air
mass or not is really relevant. And John Cochrane has published an
analysis on this. As I recall, he said you need to be more picky
about taking only climbs that meet your MC setting when going upwind
(especially when getting close to a turnpoint) and can be a bit less
picky when going downwind towards a turnpoint.

Now, I have seen an instance when the computer said I could not get
home when I had MC 0 dialed in but could with a higher MC setting
entered. This was because I had about 50 knots wind on the nose, and
best L/D was at about 50 knots. I was 10 miles out at about 11000
above field elevation, in my 604. I dialed MC up to 4, it said to fly
about 80 knots, so now I had a ground speed and all of the sudden, I
had 6000+ feet of extra altitude. Again, this was really a final
glide case, and not a best speed over the ground case, assuming you
wil lclimb again. It showed that when you have MC set to zero, it does
not give you the coreect speed to fly to maximize your range if there
is wind.

So, speed to fly is only affected by a headwind when you are
approaching a set point you must reach or clear. This may be an area
with no lift downwind of it that you cannot clear because of wind if
you fly a true anticipated MC speed but could if you flew a higher MC
speed (see above example). Again, this is really a final glide issue,
as you have a set place you have to get to from the altitude you have
on hand. Sit down sometime with your polar and a calcualtor. I will
be very surprised if you can prove to me that flying faster than MC
optimum speed (using simple climb and glide theory, no bumps in
between) into a headwind will result in a faster speed over the ground
in getting from A to B than the optimum speed as defined by your polar
for a given MC setting.

Do any of the computers out there now have a mode that will give you
your speed to fly for maximum range if you are flying into the wind?
Or downwind? Or, is it up to us to know our polar, and have
determined best L/D speeds for different amounts of wind?

Steve

On May 4, 11:46*am, Steve Leonard wrote:
Do any of the computers out there now have a mode that will give you
your speed to fly for maximum range if you are flying into the wind?
Or downwind? *Or, is it up to us to know our polar, and have
determined best L/D speeds for different amounts of wind?

I'd be surprised if there any that don't do this. Even the MNAV that
was ancient when I was using it in my ASW19 did that. Set the wind
and then adjust the MC setting for minimum altitude required, then fly
the speed director.

Andy (GY)

On Friday, May 4, 2012 11:46:18 AM UTC-7, Steve Leonard wrote:
On May 3, 5:50*pm, Ramy wrote:

I dare to challenge this common claim that STF does not depend on windspeed unless on final glide. The above claim may only be true when flying over flatland. It is not true when flying over mountain terrain, which is the type of flying most of us do in western US. When flying over mountain terrain, the lift tends to remain in the same spots and don't drift with the wind. It may be tilted at most. Also the wind is significant for terrain clearance, so you certainly need to fly faster in head wind if terrain clearance is an issue. And finally, thermals do not always drift as fast as the wind. As such, you should almost always add speed when flying up wind, the stronger the headwind the faster you should fly. How faster is another question, I believe most flight computers don't take wind in account, especially when not in final glide. Adding half the wind speed sounds about right, I usually bump up my MC setting by 0.5 for head wind of up to 10 knots, by 1 for 10-20 knots and so forth.
I'll be happy to hear other opinions.

Ramy-


Ramy, I think you will find that your mountain clearance example is
really a "final glide" computation. You have a fixed point on the
ground you must reach at or above a certain altitude. So, this is
really a final glide, and not a basic speed to fly to achieve a
maximum speed over the ground for the soaring conditions computation.
Whether the thermals are drifting at the same speed as the free air
mass or not is really relevant. And John Cochrane has published an
analysis on this. As I recall, he said you need to be more picky
about taking only climbs that meet your MC setting when going upwind
(especially when getting close to a turnpoint) and can be a bit less
picky when going downwind towards a turnpoint.

Now, I have seen an instance when the computer said I could not get
home when I had MC 0 dialed in but could with a higher MC setting
entered. This was because I had about 50 knots wind on the nose, and
best L/D was at about 50 knots. I was 10 miles out at about 11000
above field elevation, in my 604. I dialed MC up to 4, it said to fly
about 80 knots, so now I had a ground speed and all of the sudden, I
had 6000+ feet of extra altitude. Again, this was really a final
glide case, and not a best speed over the ground case, assuming you
wil lclimb again. It showed that when you have MC set to zero, it does
not give you the coreect speed to fly to maximize your range if there
is wind.

So, speed to fly is only affected by a headwind when you are
approaching a set point you must reach or clear. This may be an area
with no lift downwind of it that you cannot clear because of wind if
you fly a true anticipated MC speed but could if you flew a higher MC
speed (see above example). Again, this is really a final glide issue,
as you have a set place you have to get to from the altitude you have
on hand. Sit down sometime with your polar and a calcualtor. I will
be very surprised if you can prove to me that flying faster than MC
optimum speed (using simple climb and glide theory, no bumps in
between) into a headwind will result in a faster speed over the ground
in getting from A to B than the optimum speed as defined by your polar
for a given MC setting.

Do any of the computers out there now have a mode that will give you
your speed to fly for maximum range if you are flying into the wind?
Or downwind? Or, is it up to us to know our polar, and have
determined best L/D speeds for different amounts of wind?

Steve

Yes, indeed my point is that you need to use "final glide" headwind computation not only when you are in final glide, but whenever terrain clearance is an issue, or when flying in mountain terrain and convergence, where the thermals are not drifting with the wind, but only tilted. In the places I fly, this means the majority of the time. This is why it is incorrect to claim that wind only matters during final glide, since final glide means just that - final glide... The whole MC theory is really more relevant for flatland flying with equally distributed lift and no terrain issues.... In mountainous terrain, if you always follow MC theory you may arrive at the next thermal below the peaks and spend significant more time climbing back, if you lucky enough to find the next thermal.

Ramy

The wind matters only when the ground matters. Sometime it does,
sometimes it doesn't!

Here is an old paper on the topic of gliding range:

http://tinyurl.com/7jewweg (5 pages, pdf)

Rick