SN10 Simple Glide Calculator Page questions

Hi,

I've been enjoying my SN10B for several years now and really love it.
Recently I have been trying to use the Simple Glide Calculator (SGC)
page to do side calculations during competitions (I used to do this
with my LS4 prayer wheel but gave it to a pilot that had no computer at
all - tried to get it back when he started beating me, but...). I
would like to use it to determine how much altitude it would cost to go
to some point and come back, or things like that. I really don't
need/want the dead reckoning feature (translate this as "I haven't
figured this part out yet"), but the static calculator should be very
useful.

I ran some tests to see if I understand how this page works. With the
glider sitting in my garage at about 900 MSL or so, and with the SN10
altimeter setting at 29.92, I got the following

On the altimeter setup page, I see 896 for altitude. On the SGC page,
if I set MC 0, Distance 0, Wind 0, and adjust the finish altitude such
that the glideslope result is 0, the finish altitude is 770 which is
about 125 ft different than the glider's current altitude. Where did
this go?

Do I understand correctly that if I wanted to go from my present
position directly upwind to a point 5 miles away, and then back again,
I could simply dial in 10 miles with a zero wind and adjust the finish
altitude window until I get zero in the glideslope window, at which
point the finish altitude window would show the altitude at which I
would expect to arrive back at the same point?

For a crosswind trip (again going 5 miles out and 5 miles back), can I
simply dial in a small headwind component as an estimate of the
decreased ground speed due to cross-track vector component (say 1 or 2
kt for a 10kt crosswind) and again leave the distance set for the
entire trip (5 miles out, 5 miles back)?

Back on the altimeter setting page, again with Q = 29.92, I adjusted
the finish altitude such that the glide slope result is -500. Since I
use the default 500 ft safety margin, this should result in a finish
altitude of 1396 but I actually get 1275, a difference of 121 ft. What
gives?

As I type this, I start to get the funny feeling that the SN10 knows
the *actual* barometric pressure, and the 121-125ft number is the
difference between my actual MSL altitude and the altitude I get with
29.92 dialed in. Is this correct?

Inquiring minds, yada yada yada ;-).

Frank (X3)

I've never used an S10 before, but I wonder that the 125 ft might be
the difference in TE between airspeed = 0 (in your garage) and airspeed
= best L/D at current MC?

-gil

Frank wrote:
Hi,

I've been enjoying my SN10B for several years now and really love it.
Recently I have been trying to use the Simple Glide Calculator (SGC)
page to do side calculations during competitions (I used to do this
with my LS4 prayer wheel but gave it to a pilot that had no computer at
all - tried to get it back when he started beating me, but...). I
would like to use it to determine how much altitude it would cost to go
to some point and come back, or things like that. I really don't
need/want the dead reckoning feature (translate this as "I haven't
figured this part out yet"), but the static calculator should be very
useful.

I ran some tests to see if I understand how this page works. With the
glider sitting in my garage at about 900 MSL or so, and with the SN10
altimeter setting at 29.92, I got the following

On the altimeter setup page, I see 896 for altitude. On the SGC page,
if I set MC 0, Distance 0, Wind 0, and adjust the finish altitude such
that the glideslope result is 0, the finish altitude is 770 which is
about 125 ft different than the glider's current altitude. Where did
this go?

Do I understand correctly that if I wanted to go from my present
position directly upwind to a point 5 miles away, and then back again,
I could simply dial in 10 miles with a zero wind and adjust the finish
altitude window until I get zero in the glideslope window, at which
point the finish altitude window would show the altitude at which I
would expect to arrive back at the same point?

For a crosswind trip (again going 5 miles out and 5 miles back), can I
simply dial in a small headwind component as an estimate of the
decreased ground speed due to cross-track vector component (say 1 or 2
kt for a 10kt crosswind) and again leave the distance set for the
entire trip (5 miles out, 5 miles back)?

Back on the altimeter setting page, again with Q = 29.92, I adjusted
the finish altitude such that the glide slope result is -500. Since I
use the default 500 ft safety margin, this should result in a finish
altitude of 1396 but I actually get 1275, a difference of 121 ft. What
gives?

As I type this, I start to get the funny feeling that the SN10 knows
the *actual* barometric pressure, and the 121-125ft number is the
difference between my actual MSL altitude and the altitude I get with
29.92 dialed in. Is this correct?

Inquiring minds, yada yada yada ;-).

Frank (X3)

Hi Frank ! Sorry I've been slow to respond but I've
been on the road and I wanted to explain this clearly.
Sit down and fasten your seat belt...

The ILEC SN10 calculates final glides for arrival
at the MC cruise speed. Simple glide page,
alternates, as well as normal task. That's not
always what everyone expects but does help
avoid subterranean arrivals.

Change MC and the deficit changes...

Hope that's clear !
Best Regards, Dave "YO"

PS: Thanks to gil who guessed correctly !