On Aug 23, 12:32*am, Darryl Ramm wrote:
On Aug 22, 7:10*pm, akiley wrote:



Hi All,

I'm aware of and use the math formula to get my rental Cirrus back to
home base, but I like to back it up with SeeYou mobile. *I know it's
recommended to use required LD to target making sure you have entered
a correct polar and safety altitude. *But this doesn't account for
winds does it? *If you are flying away from your target wondering how
far you can safely fly, you can't depend on required LD because big
headwinds can make this number useless. *As an example, I notice I've
got 25LD required to my home base. *I turn around and because of the
headwinds, I can only make 18LC. *Outlanding anyone.

I'm curious about MC required to target. *Wouldn't that be better to
use if you make sure all data is correct such as polar, winds, safety
altitude and make sure the correct target is activated. *This way, I
can wander away from my home field and I know if my MC doesn't fall
below about say 7 (which plays out to about 20 LD in no wind) I am
fairly assured of making it and that this MC will be wind aware. *Of
course it can't know about hitting lots of sink, but it seems a better
way for my type of non task, local soaring.

Before I finish, I would like to note that the MC to target NavBox in
SeeYouM doesn't always update very quickly if you change the winds
aloft manually. *For this problem, I scroll the MC value untill the
little glide slope type indicater on the left side of SeeYou centers,
then compare that MC to the required MC NavBox.

akiley

What is "the math formula". I am aware of many different math
formulas, including many for calculating/estimating glider performance/
navigation. But what are you using?

Thanks for the good advice Darryl,

My personal math formula is that, without winds, I want to be 15:1 L/D
from my home airport, (not a cross country pilot yet) with a 1000
foot safety buffer. So this is 400 feet per NM. So for 5 miles, it's
just 400x5=2000 feet AGL plus 1000 safety = 3000AGL. If there is a 5
knot headwind on the return home, I would divide the Cirrus 46kt best
LD speed by 41 to get 0.9. That .9 can be divided by my 400 feet per
NM to get 445 feet per NM for my new calculated L/D to target. I
guess there are other factors, but this is ballpark for me. When I
start to get 20 L/D from home I start to get nervous. But my formula
is roughly based on one half my best LD plus a bit. I can't remember
which book I got my math from, but it's airspeed plus or minus winds
divided by airspeed which gives you a number to modify your L/D. Then
divide 6000 by the modified L/D to get feet per NM required to reach
your target, then add any safety altitude you desire to that agl
altitude.

Required Mc is a kind of noisy number, especially if you think the
difference between two large numbers helps you much. It is sensitive
to high speed polar data and if you tried to fly it in a rental glider
with an unknown actual polar without a lot of experience at pushing it
is likely meaningless.

For recreational flying, unless you are racing with lots of
experience, I would focus less on twiddling Mc (or virtually twiddling
with SeeYou Mobile telling you its Mc estimate to goal) and more on L/
D achieved and L/D required as one data pair and on arrival height as
another. Arrival height factors in wind, uses the polar, bugs, Mc. Set
some sane low Mc near what you actually fly at. Pad the polar with
%bugs (start with max of 30% if new to XC) and have an arrival safety
height (at least your usual pattern height, more when starting). You
can try adjusting it at times and see what it does to your arrival
height but if you are at the stage it sounds like mostly leave it set
and don't go chasing large Mc numbers. Hide the navbox, there are
better things to look at. In fact hide almost everything, except the
two L/D boxes and arrival height and use the wind indicator on the
main map to check it looks sane. And forget the rest, including the
silly glideslope display, I cannot think of anybody who really uses
that thing (oops now we'll hear from them...).

Well, I've flown long enough to know not to trust electronics. I have
600 power hours using all sorts of navigators. Funny, when you use a
Garmin 396 on a computer, it sets magnetic variation to user set
instead of auto. SeeYou has quite a few bugs and gochas too. So my
primary is look at the down angle back to the airport.

I do monitor arrival height, but it's really just another way of
looking at MC as far as I can tell. The little glide slope indicator
is also sort of another way of looking at your MC to target. I figure
if I keep an eye on all of those, I'm less likely to trip over a bug
in SeeYou which I've found several of.

Required L/D to target tells you what you need to achieve. It makes no
sense to fold wind into that, its just the distance divided by the
difference in height. Achieved L/D tells you what you are getting
obviously with wind affects as well, all without any assumptions about
polars, mass, bugs, or wind. That is the beauty of working with L/D
required and achieved.

This is a good point, but since wind isn't factored into required L/D,
you don't know what your achieved L/D is unless you turn around and
head back to the airport. But it does seem to be safer and more
straight forward. And I suppose since you are always aware of the
winds, you can make a fairly accurate guess as to what you achieved L/
D is likely to be. If it's a straight headwind home at 5 knots, I
could just mentally modify what I expect to achieve.

But even better than asking on r.a.s. can you find a local
accomplished XC pilot(s) who can mentor you on all this stuff?

Yes, my club has several and I'm talking to them too. It's also funny
about gadgets in aircraft. My feeling is learn to use the autopilot
and whenever you can, learn navigators using simulators. Half
learning electronics is the most dangerous in my opinion. I enjoy
navigators, but I'm strict as to when and how to use them.

Darryl