Article on glide strategy

Interesting approach.
I for myself use the MC for optimizing cruising, and I work with
required L/D for safety - exclusively.
The good thing about required L/D is that there are no assumptions
whatsoever to it, it's plain geometry.
In Alpine soaring (which I've been doing for the last couple of
thousand hours), with my 47:1 ship I feel safe with a required L/D
somewhere between 20 and 25, and unsafe above 30.
That still depends on meterological conditions (end of day vs high
winds...). I once had 20:1 in a 40:1 ship and I didn't make it :-(

At 15:23 07 March 2012, John Cochrane wrote:
On Mar 1, 9:36=A0am, Nine Bravo wrote:
Thanks John.

I tend to think about it slightly differently. On the one hand I think
ab=
out worst-case, inescapable sink for the conditions as measured by
minutes
=
times sink rate (result - a fixed number of feet that I am at risk of
losin=
g - I don't generally consider a string of sink occurrences - I assume
one
=
low probability sink patch is worse than multiple, higher probability
sink
=
patches and that multiple low probability events aren't likely enough to
wo=
rry about). On the other hand I think about the probability of being able
t=
o find lift to recover after hitting a sink patch - which is a function
of
=
altitude above ground.

On very short final glides the constraint is the rate of sink (not much
g=
lide time left), on longer final glides the constraint is the probability
o=
f finding decent lift to get back up. The pinch point for me tends to be
ar=
ound 25 miles out - chances of finding lift are declining, odds of a long
o=
f stretch of sink still decent. This seems roughly consistent with your
squ=
are root rule, though the math is different.

Your square root rule breaks down for me on very long final glides
where
=
I tend to optimize more around trying to transition away from climb and
gli=
de to cruise-climb in an effort to avoid thermal centering losses. This
tra=
nslates to an altitude buffer of maybe -1000 feet on a 50 mile "final
glide=
" that you are trying to "bump-up" to +1000 feet by the time you get to
25
=
miles out. Not sure if/how that figures in your analysis.

9B

Good thoughts. But just to clarify, the article is not about final
glides -- how to do it efficiently. The article is about safety
margins -- how to do it with x percent chance of landing out.

John Cochrane


Very good thoughs both John and Andy, I tend to think of it more like a
funnel. The further out the closer I can be to my actual glide ratio and
the closer in i need way bigger margin. The funnel idea makes me put the
"art of final glide" into a logrithmic equation in my head and is instantly
scalable and movable to alternate landing sites. Especially flying out west
where most of the airports that I fly at do not have any safe landing spots
close by and the last few miles need to have extra high safety margins.
Thanks for sharing

CH

In addition to considering the probability of a landout, you have to take into consideration the consequences of one. If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.

I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since!

Mike

On Mar 7, 12:08*pm, Mike the Strike wrote:
In addition to considering the probability of a landout, you have to take into consideration the consequences of one. *If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. *As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.

I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since!

Mike

Exactly. In the paper analysis, you adjust the "probability of not
making it" parameter according to the consequences of a landout. The
profile to follow over mowed fields, where the costs are inconvenience
or contest points, is very different than the profile to follow over
Zion. They're both square roots but the Zion profile is much higher up
-- in the range of MacCready settings you may never have used before.

That's one of the big points. We get used to Mc 3 or so glides and
that they almost always work out. Over Zion, that experience is not
good enough. To an earlier comment that experience trumps analysis,
well, you don't want to be the guy that learns about how often Mc 1
glides work out by experience!

John

On Wednesday, March 7, 2012 11:34:23 AM UTC-8, John Cochrane wrote:
On Mar 7, 12:08*pm, Mike the Strike wrote:
In addition to considering the probability of a landout, you have to take into consideration the consequences of one. *If your final glide is over a landscape of neatly mowed large fields, the consequences of a landout are trivial and you can push closer to the theoretical limits. *As Cliff says, many western sites have very poor options and carrying extra height, perhaps even more than mathematically indicated, might be wise.

I once got very very low over Zion National Park in Utah and have carried higher safety margins ever since!

Mike

Exactly. In the paper analysis, you adjust the "probability of not
making it" parameter according to the consequences of a landout. The
profile to follow over mowed fields, where the costs are inconvenience
or contest points, is very different than the profile to follow over
Zion. They're both square roots but the Zion profile is much higher up
-- in the range of MacCready settings you may never have used before.

That's one of the big points. We get used to Mc 3 or so glides and
that they almost always work out. Over Zion, that experience is not
good enough. To an earlier comment that experience trumps analysis,
well, you don't want to be the guy that learns about how often Mc 1
glides work out by experience!

John

Excellent article and excellent thread.
John, I am curious to hear your opinion about using higher MC vs degrading the polar (aka bug factor) for safety as some do instead or in addition.

Ramy

Excellent article and excellent thread.
John, I am curious to hear your opinion about using higher MC vs degrading the polar (aka bug factor) for safety as some do instead or in addition.

Ramy

I don't think using the bugs setting to calculate glides in real time
is that helpful. I never know what percent bugs means.

I do think it would be useful if our instrument makers could allow us
to input lift or sink. I'd like to input, "MacCready 3, 500 foot
reserve, and 100 fpm sink." Or when doing a final glide in Uvalde,
"MacCready 3, 500 foot reserve, and 100 fpm lift." (Clearnav has a few
emails from me on this!)

This simply shifts the polar curve up and down by the given lift and
sink, and would be easy for them to program.

For contest final glides, by keeping track of average netto in the
last few legs you could have an idea of lift/sink to be expected on
final glide, and then bump up / be cautious accordingly.

For safety reasons this would be very educational. You'd see directly
just how disastrous small bits of extra sink can be on your
glideslope. I also think many pilots would find it easier to take the
advice "assume 200 fpm sink all the way to your safest landing" than
they would to take the advice "input Mc 10 into your glide computer."
The former sounds reasonable, the latter outlandish based on cross-
country experience, yet they are the same thing.

Good point -- I'll add this to the article.

John

Super interesting. Thankyou!