On Aug 25, 2:41*pm, Eric Greenwell wrote:
On 8/24/2010 9:32 PM, Ramy wrote:

On Aug 24, 5:16 pm, Darryl R
But honestly, I don't understand how many of you fly XC safely if you
don't degrade your polar. How do you determine you are within safe
glide from airports at any point in time? using your published polar
and a safety altitude margin? Good luck if you hit any sink or head
wind on the way unless you use a big altitude margin which will
significantly hurt your decision down lower. The suggestion to use bug
factor to degrade your polar is basically an implementation of the
common rule of thumb to use 50%-75% of your published polar to
determine arrival altitude.

Point #1: I think what you are doing is essentially the same as keeping
the bugs at "no bugs", but using a high MC setting to figure the "safety
glide". A high MC means a steep glide angle compared to 0 MC - there's
the "degradation" in the polar you are wondering about. I normally use a
4 MC for my "safety glide" computation, which gives an L/D of 70% of my
max L/D.

Point #2: In addition to the 4 MC setting, I usually carry excess
altitude above the 4 MC glide slope to absorb strong sink and
unexpectedly strong headwinds. Over friendly ground in moderate
conditions, 500' excess has proven adequate; in strong conditions over
unfriendly ground, it might be as much as 2000' excess. These numbers
are trimmed as the distance to the airport decreases, starting about
10-20 miles out, because my 1000' AGL arrival height begins to provide
the "sink absorption" buffer.

Of course, I don't use the 4 MC setting as my speed to fly if I have to
head towards my safety airport; instead, I use a 1 MC setting (or zero
MC if I'm truly desperate). My MC setting for the "safety glide" is
separate from my "speed to fly" setting on my Cambridge 302, which is
usually set at 1 (moderate conditions) or 2 (strong conditions).

The above MC and excess altitude settings have proved satisfactory for
all my gliders, from a Ka-6e to an ASH 26 E. Of course, the speeds flown
were quite different for each glider.

--
Eric Greenwell - Washington State, USA (netto to net to email me)

- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarmhttp://tinyurl.com/yb3xywl

- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what you need to know tinyurl.com/yfs7tnz

I generally do something similar to what Eric describes. In my
computer setup it's a lot easier to fiddle with the Mc setting than
adjust the bugs setting. The basic idea is to have a steeper glide
dialed and fly slightly slower until you establish that you are on
glidepath or better, but also to keep a constant arrival altitude
margin to account for the "2 miles of 10 kts down" scenario - for that
you need an arrival altitude buffer, not a glide angle buffer. If you
are way out on final glide you might start with a negative arrival
margin in an attempt to bump it up over time, but you need to get up
to glidepath by the time you are about 20 mile out or you will be out
of search range to find some lift to get up to glidepath.

9B