Article on glide strategy

I wrote a new article on how to use computers to help judge glides.

http://faculty.chicagobooth.edu/john...ety_glides.pdf

or the first item here

http://faculty.chicagobooth.edu/john....htm#maccready

This will probably end up in Soaring sooner or later, but I always get
a lot of help from early readers. If it's not clear or you see
problems etc. let me know. (john dot cochrane at chicagobooth dot
edu0

John Cochrane

Good article, John. It deserves wider distribution. Thanks for the
analysis.

On Feb 27, 9:47*pm, Fred wrote:
Good article, John. *It deserves wider distribution. Thanks for the
analysis.

Anyone who flies out west knows about "rivers of sink". However, many
can't decouple the MacCready of the flight director from the computer
(SN-10 users, for example). I have typically used a height reserve
above my chosen MacCready setting (something like 1,000 feet for 25
miles). Flying dry in weaker winter conditions, I usually use a
MacCready of 3 to 4 and up that to 5 to 6 in summer conditions flying
ballasted.

I have also been experimenting with forecasting these "rivers of
sink". Even in the blue, there tends to be some minor wave activity.
If you have a local RASP, you can see these on the Boundary Layer Up/
Down Motion plot or on the HRRR plots of average vertical velocity.
We have seen quite good agreement between these forecast wave/
convergence lines and both lift and sink. For example, last Sunday we
had a line of cumulus form exactly where we had a forecast wave line,
running from southwest to northeast about 20 miles south of our field.

In the blue you can't see these lines, but it is useful to know which
way they are aligned. If you end up flying in a "river of sink",
chances are you are running down one of these wave lines and you need
to turn at right angles to it to get back in lift or zero sink. The
forecasts might not predict them in the right place, but usually get
the alignment right. Check the plots before you fly. Better yet,
print a copy and carry it with you.

Mike

I've been on a few of those sink streets too. Although some people
will say that I bring my own sink street with me on each flight.

From looking at your graph of altitude required to minimize the chance
of landing out it seems that the old rule I've heard of programming in
a 800 or 1000 foot get home altitude would allow you to program in a
slightly shallower Macready setting and still stay above the curve.
Then when you get into that last few miles where the curve starts to
drop you can speed up and finish at a lower altitude if you wish.

One nice thing in the Cherokee is that if you can see it you possibly
can't glide to it. I had several single digit glides at the Region 10
Contest. In fact John if you're looking for a river sink look at my
trace from Day 2 at Region 10 where I landed out 4 miles from the
finish. I don't think it was just the built in sink in the Cherokee
either as Dave Coggins reported seeing L/D's in the sub-10 range in
his Nimbus on the way back too.

When in Marfa i drew 10:1 circles which for the Cherokee was close
enough for easy math to the 1/2 your L/D method to make sure you can
make it home. Best I can figure that is about a Mc 9 or 10. I only
thought I was going to land out once during that week, see the old
adage of where there is big lift there is big sink.

Rivers of sink are pretty common.

Example: Uvalde typically has SW streeting and
task returns from SW. If bases are high its not
too hard (for me anyway) to get into the sink street
when descending on final glide.

You may want to stay way above glideslope (stay
with the clouds) longer than "MC reasonable".

Ask Ron how he almost landed 10 miles out on
last day of Open Nats last year...

Thanks John !
See ya, Dave "YO electric"

On Feb 27, 10:15*pm, John Cochrane
wrote:
I wrote a new article on how to use computers to help judge glides.

http://faculty.chicagobooth.edu/john...ocs/safety_gli...

or the first item here

http://faculty.chicagobooth.edu/john...ndex.htm#maccr...

This will probably end up in Soaring sooner or later, but I always get
a lot of help from early readers. If it's not clear or you see
problems etc. let me know. (john dot cochrane at chicagobooth dot
edu0

John Cochrane

Great article. Makes total sense. If we are using flight computers
to do the work of calculating arrival based an many factors such as
wind, polar, bugs, ballast and such, It doesn't make sense to me to
use something like L/D when L/D skips the wind and the polar info.
Arrival height takes these factors into account, but it seems MC
covers so many bases. It's the theory that describes how to maximize
lift/speed to fly, but can also describe reserve energy for safety
glides. I always think in terms of required MC to make the nearest
airport as I'm just starting to get into cross country soaring.

When I bought SeeYou, It took me a long time to sort out what critical
items (winds, reserve altitude, polar, bugs, ballast) were included in
calculations I saw in the various info boxes. When I discovered MC
included all of these items, plus the lift and speed to fly theory, it
seemed a no brainier to use required MC as my safety glide. With my
home airport as the goto, I would start thinking of heading home when
MC dropped below 8 based on conditions and distance from the field.
10 miles out with minimal lift, and MC 4 to target would make me
nervous.

I've only used SeeYou and XCSoar and they are not connected to our
club gliders.. I think these apps should have a Required MC to target
info box, and an option to replace arrival altitudes on airport tags
with required MC. Having a separate info box for ReqMC allows for
using the MC manual setting separately.

I hit a sink street last summer. The airport I was trying make it to
was aligned with the sink street based on the winds aloft. My options
were to continue in sink toward the airport, or turn away and hope fot
lift. I had something like MC 10 to make the airport with 800' safety
reserve, so I went direct and just made it. ... Aaron

On Feb 27, 7:15*pm, John Cochrane
wrote:
I wrote a new article on how to use computers to help judge glides.

http://faculty.chicagobooth.edu/john...ocs/safety_gli...

or the first item here

http://faculty.chicagobooth.edu/john...ndex.htm#maccr...

This will probably end up in Soaring sooner or later, but I always get
a lot of help from early readers. If it's not clear or you see
problems etc. let me know. (john dot cochrane at chicagobooth dot
edu0

John Cochrane

Finally! I've been arguing this with flight computer developers
(Winpilot, XCSoar) for years. STF MC, and safety MC are 2 different
things that needs to be decoupled. Use your STF Vario MC setting for
speed to fly, and keep a constant MC for safety (I use 4 which seem to
work for almost any situation) in your glide computer (i.e. PDA).
Problem is, that if you connet your 302 to winpilot/xcsoar (and
probably others) you could not decouple the two. The good news, is
that XCSoar 6.3 will give the option to decouple the two different MC
settings.

There is another solution though. Instead of setting a high safety MC
in your glide computer, you can degrade the polar using the bug factor
to achieve the same results. (typically 33% -50% degradation, depend
how aggressive you want to be). Probelm is, that some flight
computers, such as XCSoar, did not store this value, which means you
had to remember to set it before every flight. The good news is that
this is also addressed in 6.3, which will have persistent polar
degradation.

And last, this excellent article also demonstrate why just keeping a
safety altitude does not work, as it will be appropriate only for one
distance. The further you are the highest it would need to be. This is
why Safety MC, or polar degradation are better solutions, since they
are not depending on distance.

Ramy

On Feb 28, 9:52*am, akiley wrote:
On Feb 27, 10:15*pm, John Cochrane
wrote:

I wrote a new article on how to use computers to help judge glides.

http://faculty.chicagobooth.edu/john...ocs/safety_gli...

or the first item here

http://faculty.chicagobooth.edu/john...ndex.htm#maccr...

This will probably end up in Soaring sooner or later, but I always get
a lot of help from early readers. If it's not clear or you see
problems etc. let me know. (john dot cochrane at chicagobooth dot
edu0

John Cochrane

Great article. *Makes total sense. *If we are using flight computers
to do the work of calculating arrival based an many factors such as
wind, polar, bugs, ballast and such, It doesn't make sense to me to
use something like L/D when L/D skips the wind and the polar info.
Arrival height takes these factors into account, but it seems MC
covers so many bases. *It's the theory that describes how to maximize
lift/speed to fly, but can also describe reserve energy for safety
glides. *I always think in terms of required MC to make the nearest
airport as I'm just starting to get into cross country soaring.

When I bought SeeYou, It took me a long time to sort out what critical
items (winds, reserve altitude, polar, bugs, ballast) were included in
calculations I saw in the various info boxes. *When I discovered MC
included all of these items, plus the lift and speed to fly theory, it
seemed a no brainier to use required MC as my safety glide. *With my
home airport as the goto, I would start thinking of heading home when
MC dropped below 8 based on conditions and distance from the field.
10 miles out with minimal lift, and MC 4 to target would make me
nervous.

I've only used SeeYou and XCSoar and they are not connected to our
club gliders.. *I think these apps should have a Required MC to target
info box, and an option to replace arrival altitudes on airport tags
with required MC. *Having a separate info box for ReqMC allows for
using the MC manual setting separately.

I hit a sink street last summer. *The airport I was trying make it to
was aligned with the sink street based on the winds aloft. *My options
were to continue in sink toward the airport, or turn away and hope fot
lift. *I had something like MC 10 to make the airport with 800' safety
reserve, so I went direct and just made it. * ... Aaron

As a devoted SN-10 user, I would love to have a way of decoupling the
MC setting used to calculate alternates from the flight director
function. And of course also it would be great to be able to set the
reserve altitude for alternates as well. Maybe Easter Bunny Dave is
listening. Please, oh please, Easter Bunny!

Lynn

Ramy wrote:
This is why Safety MC, or polar degradation are better solutions,
since they are not depending on distance.

Safety MC is poor man's polar degradation. It doesn't make a lot of
sense, and will eventually be removed from XCSoar. We should have
implemented persistent polar degradation from the start.

Max