My concern was based on the principle that somebody
was developing an 'absolute' model which was based
on a limited number of parameters without considering
the wide range of factors that influencing launch distances
and climb rates.

If you're simply trying to establish a guideline of
height vs density vs temperature vs wind strength/direction
vs glider performance vs tug power to develop a minimum
runway length required, then fair enough - andI agree
that high temp and high altitiude is something that
we don't have an issue with in the UK. Nonetheless
we do have situations where runway length, tug power
and weight of glider are sometimes at a limit that
needs to be considered by the tug and glider pilot.
This comes down to a simple calculation.....if in
doubt, don't!

Gary

At 02:06 14 April 2007, Eric Greenwell wrote:
Gary Nuttall wrote:

As a glider pilot I act within both my and the glider's
limits. I trust my instincts that if something doesn't
feel right, I abandon the launch while it's still
safe
to do so. Maybe it's a US vs UK thing

Very likely - how often does a pilot in the UK have
to decide if an
airport at a 10,000 foot density altitude that has
never seen a towplane
is safe to use? Here in the USA, I'll bet we have this
problem much more
frequently.

but here in
the UK we take personal responsibility for our actions.

I think it's the same here, too. After all, the tow
can go bad for
several reasons besides a high density altitude tow
at an airport that's
never been used for towing! And, of course, it's not
dependent only on
the glider pilot: the towpilot should notice HE'S not
off the ground
early enough, and let the glider go so the towplane
pilot can deal with
the towplane's problem. In fact, the combination can
still be in trouble
even if the glider has taken off 'in time', because
the critical element
is the towplane taking off in time. It's hard for the
glider pilot to
assess this.

What I think Kilo Charlie and the others are trying
to determine is if
it's even worth taking a towplane to this potential
site. Without
experience at a similar site, looking for pertinent
numbers seems like a
better idea than just showing up and trying it.

If you're not sure that you have sufficient distance
to take-off then why would you trust a set of numbers
that say otherwise?

Perhaps because you've verified the table or equation
in other
situations, and added a margin for safety, and because
you are using a
towplane, towplane pilot, and glider pilot you trust
to handle the
situation, even if things go wrong.

I think the concept of calculating
takeoff runs is actually quite interesting but the
sheer number of variables involved make it an impracticable
exercise.

Nonsense. You aren't trying to precisely determine
takeoff runs, but
decide if the situation is 'safe enough'. They know
how the towplane
operates compared to it's POH values, and the addition
of the glider can
be calculated (it's just drag and weight, not a huge
number of
variables), so a sensible estimate can be determined.

--
Eric Greenwell - Washington State, USA
* Change 'netto' to 'net' to email me directly
* 'Transponders in Sailplanes' http://tinyurl.com/y739x4
* 'A Guide to Self-launching Sailplane Operation' at
www.motorglider.org