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At 05:00 12 March 2005, Marc Ramsey wrote:
Kilo Charlie wrote:
Don't argue with 9B re the logic of the math issue....trust
me....he's a
very bright guy and never leaves his calculator!
He is offering the
mathematical explanation of why cylinder finishes
may not be any safer. Of
course it supports my point so I'm thinkin' he's a
rad dude!

Hmm, I pegged him for a lawyer or politician, the numbers
may have some
basis in reality (assuming you fly in a vacuum), but
the logic is, uh,
'interesting'.

The point was pretty simple - I added the analysis
because without it I'm just a guy with an opinion (no
shortage of those here). So here's the point in simple
language: A low energy finish at 500' and 1 sm is
not significantly 'safer' than a low energy finish
at 50' over the airport. Either way you will be low
and slow in the pattern.

The numbers assume viscous, incompressible flow - gliders
can't fly in a vacuum. Bet you knew that. ;-)

9B

Andy Blackburn wrote:
The point was pretty simple - I added the analysis
because without it I'm just a guy with an opinion (no
shortage of those here). So here's the point in simple
language: A low energy finish at 500' and 1 sm is
not significantly 'safer' than a low energy finish
at 50' over the airport. Either way you will be low
and slow in the pattern.

Your analysis is flawed, because you miss one little point. Let's assume
we're comparing a finish gate adjacent to the center of the runway, and
a 1 mile radius cylinder centered on the runway. If you hold all other
factors constant, in particular the altitude at which you leave the last
thermal and the speed at which fly the final glide, if you pull up to
best glide at 1 mile you will always end up over the runway as high (if
your are already flying at best glide) or higher than if you pull up at
the gate. In other words, if you are low energy at 1 mile, you will
have as low or lower energy if you don't pull up until you reach the
gate, because you can't recover the drag you lose by flying faster than
best glide for the last mile.

Now, if you assume that you leave the last thermal when the computer
says final glide is made (or you leave with a constant offset from the
computer indication), then the 500 foot 1 mile case will require that
you climb higher, as it obviously takes less energy to get to the gate
at 50 feet (unless you are flying final glide at a speed where your L/D
is less than 10:1, which is ridiculous in modern gliders). You will
start a marginal final glide with more energy in the cylinder case, than
you will in the gate case. If your final climb is capped by the height
of the thermal, then you may have to opt for a rolling finish using a
cylinder, and still be able to make a gate finish, but you will be
making that final glide at essentially best glide, and have no energy
left to pull up after you go through the gate.

My point is also pretty simple. In no realistic case will you ever end
up with more energy for landing by delaying your pull-up until you reach
the airport. You will always end up with more energy over the airport
by making a final glide to 1 mile and 500 feet. This also means that
you have more margin for screw ups in the cylinder case.

The numbers assume viscous, incompressible flow - gliders
can't fly in a vacuum. Bet you knew that. ;-)

I do, but I'm not so sure you do. The exchange of energy implied by
your pullup from 50 feet at 150 knots that results in "something more
than 900'", either includes no losses for drag or you're doing a tail
slide at the top. If you have a trace where you actually manage to pull
up to 900 feet above your finish altitude, I'd love to see it...

Marc

With a center of airport 50' finish you still need to do a pattern including
up to three 90 degree turns after finishing (ignoring rolling finishes).
With a 500'/1 mile cylinder you can do a straight in or several possible
patterns, and you don't need to go to the center of the airport first, so
the distance is really 3/4 mile or much less to a downwind or base leg.
-Bob Korves

"Marc Ramsey" wrote in message
...
Andy Blackburn wrote:
The point was pretty simple - I added the analysis
because without it I'm just a guy with an opinion (no
shortage of those here). So here's the point in simple
language: A low energy finish at 500' and 1 sm is
not significantly 'safer' than a low energy finish
at 50' over the airport. Either way you will be low
and slow in the pattern.

Your analysis is flawed, because you miss one little point. Let's assume
we're comparing a finish gate adjacent to the center of the runway, and
a 1 mile radius cylinder centered on the runway. If you hold all other
factors constant, in particular the altitude at which you leave the last
thermal and the speed at which fly the final glide, if you pull up to
best glide at 1 mile you will always end up over the runway as high (if
your are already flying at best glide) or higher than if you pull up at
the gate. In other words, if you are low energy at 1 mile, you will
have as low or lower energy if you don't pull up until you reach the
gate, because you can't recover the drag you lose by flying faster than
best glide for the last mile.

Now, if you assume that you leave the last thermal when the computer
says final glide is made (or you leave with a constant offset from the
computer indication), then the 500 foot 1 mile case will require that
you climb higher, as it obviously takes less energy to get to the gate
at 50 feet (unless you are flying final glide at a speed where your L/D
is less than 10:1, which is ridiculous in modern gliders). You will
start a marginal final glide with more energy in the cylinder case, than
you will in the gate case. If your final climb is capped by the height
of the thermal, then you may have to opt for a rolling finish using a
cylinder, and still be able to make a gate finish, but you will be
making that final glide at essentially best glide, and have no energy
left to pull up after you go through the gate.

My point is also pretty simple. In no realistic case will you ever end
up with more energy for landing by delaying your pull-up until you reach
the airport. You will always end up with more energy over the airport
by making a final glide to 1 mile and 500 feet. This also means that
you have more margin for screw ups in the cylinder case.

The numbers assume viscous, incompressible flow - gliders
can't fly in a vacuum. Bet you knew that. ;-)

I do, but I'm not so sure you do. The exchange of energy implied by
your pullup from 50 feet at 150 knots that results in "something more
than 900'", either includes no losses for drag or you're doing a tail
slide at the top. If you have a trace where you actually manage to pull
up to 900 feet above your finish altitude, I'd love to see it...

Marc