Article on glide strategy

On Mar 1, 9:36*am, Nine Bravo wrote:
Thanks John.

I tend to think about it slightly differently. On the one hand I think about 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 losing - 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 worry about). On the other hand I think about the probability of being able to 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 glide time left), on longer final glides the constraint is the probability of finding decent lift to get back up. The pinch point for me tends to be around 25 miles out - chances of finding lift are declining, odds of a long of stretch of sink still decent. This seems roughly consistent with your square 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 glide to cruise-climb in an effort to avoid thermal centering losses. This translates 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

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