Article on glide strategy

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.

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