Rounding a turnpoint in sink

Let's imagine the following scenario:

You are flying a comp and eager to win. This far you are really
doing well on today´s task.

You had a good climb some distance back and have left the main
gaggle behind. It looks like you might be in the lead. Now you
are approaching the second turnpoint and are bombing along
through the sink at, say 90 kts, roughly according to the
chosen McCready setting.

Now, when you reach the turnpoint, you need to turn 180 degs
and head back in the opposite direction. The turnpoint is in
the middle of a largish area of heavy sink.

What is the most effective technique for making the turn ?

a. Tight turn at the current speed (90 kts) and carry
on according to McCready.
b. Slowing down to thermalling speed, turning tight and
accelerating back to 90 kts. This would end up being
a kind of chandelle-like maneuver since you'd be
pulling up sharply, turning while still in the climb,
and diving in quick succession.
c. A half loop with a roll on top. OK, I suspect this
might be theoretically best, but let's suppose you
rule this out due to a load of loose objects in the
cockpit: maps, pen, camera, sandwiches, drinking
water etc. and you are not really into advanced
aerobatics anyway.
d. Carrying on for half a mile (or a mile ?) past the
turnpoint where a cloud is building and you expect
to find lift to turn in.
e. Anything else.

CV

From: CV e
What is the most effective technique for making the turn ?

a. Tight turn at the current speed (90 kts) and carry
on according to McCready.
b. Slowing down to thermalling speed, turning tight and
accelerating back to 90 kts. This would end up being
a kind of chandelle-like maneuver since you'd be
pulling up sharply, turning while still in the climb,
and diving in quick succession.
c. A half loop with a roll on top. OK, I suspect this
might be theoretically best, but let's suppose you
rule this out due to a load of loose objects in the
cockpit: maps, pen, camera, sandwiches, drinking
water etc. and you are not really into advanced
aerobatics anyway.
d. Carrying on for half a mile (or a mile ?) past the
turnpoint where a cloud is building and you expect
to find lift to turn in.
e. Anything else.

Any manuvering as in b or c will cost more than a plain efficient turn, option
a would be fine if there is a crosswind to the course, turn into the wind for
the shortest ground track around the turn. D is the best option for two
reasons: you know you have to fly back out through sink, so tanking up is good,
also if the wind is generally along the outbound course, you will be drifting
toward the next turn while climbing. This is based on the old logic of
arriving low at downwind turns and high at upwind turns. Another tactical
point: flying into the turnpoint on the outbound leg when the wind is a
headwind going in to the turn may mark lift to be used after rounding the turn.
E, anything else. Why were you flying so long in sink? Altering course to
left or right may have put you in better air.





-
Mark Navarre
2/5 black ace
LoCal, USA
remove brain to reply
-

Mark Navarre wrote:



Any manuvering as in b or c will cost more than a plain efficient turn, option
a would be fine if there is a crosswind to the course, turn into the wind for
the shortest ground track around the turn. D is the best option for two
reasons: you know you have to fly back out through sink, so tanking up is good,
also if the wind is generally along the outbound course, you will be drifting
toward the next turn while climbing. This is based on the old logic of
arriving low at downwind turns and high at upwind turns.

Do you mean "when turning downwind" and "when turning upwind"? If not
I'm gonna need some help understanding this.

Thanks,
Shawn

scurry wrote:
Mark Navarre wrote:
toward the next turn while climbing. This is based on the old logic of
arriving low at downwind turns and high at upwind turns.


Do you mean "when turning downwind" and "when turning upwind"? If not
I'm gonna need some help understanding this.

Looks like that is what Mark meant and it was a little confusing
to me too. It almost looked like he had mixed up the two and was
saying the opposite. With that interpretation it makes sense, but
is still easy to misinterpret.

I have usually heard this logic expressed as rounding downwind
turnpoints as high as you can and upwind ones as low as you dare.

CV

CV wrote:

scurry wrote:

Mark Navarre wrote:

toward the next turn while climbing. This is based on the old logic of
arriving low at downwind turns and high at upwind turns.



Do you mean "when turning downwind" and "when turning upwind"? If not
I'm gonna need some help understanding this.


Looks like that is what Mark meant and it was a little confusing
to me too. It almost looked like he had mixed up the two and was
saying the opposite. With that interpretation it makes sense, but
is still easy to misinterpret.

I have usually heard this logic expressed as rounding downwind
turnpoints as high as you can and upwind ones as low as you dare.

Thanks. That's how I've thought of it as well, but I wanted to be sure.

Mark Navarre wrote:
Any manuvering as in b or c will cost more than a plain efficient
turn,

That is not necessarily so, since a 180 turn at a high speed, even
if cleanly performed, will use up much more energy, and take longer,
than one at a lower speed.

It is not really possible to tell which is better simply based
on whether you are maneuvering or not. You'd need to do the math.
CV

I simulated this last year. The theoretical answer
between a and b in still air is to pull up and make
a tighter turn at thermalling speed. Making the turn
at cruise speed loses more altitude due to the higher
induced drag from turning at higher speed, which is
worse than the losses associated with a 1G pullup and
a lower energy turn. I recall the calculated difference
is about 100', so it's not a huge deal. I don't think
the answer changes with airmass movement.

With respect to option d, generally it's a good idea
to avoid voluntarily flying in sink. It's only worth
it if you think the savings in climb time will exceed
the additional 4 miles in higher sink (going from the
near side of the 1-mile cylinder to the far side and
back again).

For example, if you are getting 500 fpm in sink, at
90 knots over 4 miles it will cost you about 1200 feet.
If you anticipate a climb at 8 knots instead of 5 knots
then you'd need to climb about 3200 feet to make up
the time loss from having to climb the additional 1200
feet (breakeven math is: 3200ft/800fpm = 4 min. and
2000ft/500fpm = 4 min.)

This assumes that you don't need extra altitude to
avoid getting low on your egress from the sink area
and back to the last thermal you took. It also doesn't
take into consideration different McCready speeds that
might narrow the gap a bit. Also you need to consider
some probabilities: The probability that the sink will
continue and the probability that the thermal on the
far side will be that much better.

9B

At 14:30 21 July 2004, Mark Navarre wrote:

From: CV e
What is the most effective technique for making the
turn ?

a. Tight turn at the current speed (90 kts) and carry
on according to McCready.
b. Slowing down to thermalling speed, turning tight
and
accelerating back to 90 kts. This would end up
being
a kind of chandelle-like maneuver since you'd be
pulling up sharply, turning while still in the
climb,
and diving in quick succession.
c. A half loop with a roll on top. OK, I suspect this
might be theoretically best, but let's suppose
you
rule this out due to a load of loose objects in
the
cockpit: maps, pen, camera, sandwiches, drinking
water etc. and you are not really into advanced
aerobatics anyway.
d. Carrying on for half a mile (or a mile ?) past the
turnpoint where a cloud is building and you expect
to find lift to turn in.
e. Anything else.

Any manuvering as in b or c will cost more than a plain
efficient turn, option
a would be fine if there is a crosswind to the course,
turn into the wind for
the shortest ground track around the turn. D is the
best option for two
reasons: you know you have to fly back out through
sink, so tanking up is good,
also if the wind is generally along the outbound course,
you will be drifting
toward the next turn while climbing. This is based
on the old logic of
arriving low at downwind turns and high at upwind turns.
Another tactical
point: flying into the turnpoint on the outbound leg
when the wind is a
headwind going in to the turn may mark lift to be used
after rounding the turn.
E, anything else. Why were you flying so long in sink?
Altering course to
left or right may have put you in better air.





-
Mark Navarre
2/5 black ace
LoCal, USA
remove brain to reply
-

Andy Blackburn wrote:
I simulated this last year. The theoretical answer
between a and b in still air is to pull up and make
a tighter turn at thermalling speed. Making the turn
at cruise speed loses more altitude due to the higher
induced drag from turning at higher speed, which is
worse than the losses associated with a 1G pullup and

I would have thought the more significant factor is that
the time needed for a 180 deg change of heading is much
longer at a higher speed and you'd be travelling around
a longer distance as well, losing valuable seconds.

a lower energy turn. I recall the calculated difference
is about 100', so it's not a huge deal. I don't think
the answer changes with airmass movement.

It is not immediately, or intuitively clear to me whether
it changes or not. A key issue would be to minimise the time
spent in the sink area.

With respect to option d, generally it's a good idea
to avoid voluntarily flying in sink. It's only worth
it if you think the savings in climb time will exceed
the additional 4 miles in higher sink (going from the
near side of the 1-mile cylinder to the far side and
back again).

Hmmm. If I fly one mile out and one mile back I make
a total of two miles. I don´t follow the bit about
the mysterious "cylinder" that makes it 4, but it
sounds like a good idea to avoid it )

Anyways, what I meant to imply in the question, and
which I was perhaps not sufficiently clear about,
was not that you go out to that thermal to stop
and climb in it (you might want to of course if
it was strong enough and depending on wind as well,
but that was not part of the question) but merely
in order to perform the pullup and 180 deg turn
and subsequent acceleration in rising air rather
than in sink.

For example, if you are getting 500 fpm in sink, at
90 knots over 4 miles it will cost you about 1200 feet.
If you anticipate a climb at 8 knots instead of 5 knots
then you'd need to climb about 3200 feet to make up
the time loss from having to climb the additional 1200
feet (breakeven math is: 3200ft/800fpm = 4 min. and
2000ft/500fpm = 4 min.)

Well, I think it is not only about making up for the
height loss. There is also the time spent going past
the turnpoint, and returning to it. All that time will
be "wasted" in the sense that your progress on the
task is zero during that time.

This assumes that you don't need extra altitude to
avoid getting low on your egress from the sink area
and back to the last thermal you took. It also doesn't

Naturally. This was just a theoretical question about
TP-rounding technique as such. All sorts of other
factors and circumstances will of course be part of
the decision making in a real situation.

As somebody pointed out, a prevailing wind would of
course complicate the issue further as well.

Thanks for the answer.
CV

At 18:30 21 July 2004, Cv wrote:

Andy Blackburn wrote:
I simulated this last year. The theoretical answer
between a and b in still air is to pull up and make
a tighter turn at thermalling speed. Making the turn
at cruise speed loses more altitude due to the higher
induced drag from turning at higher speed, which is
worse than the losses associated with a 1G pullup
and

I would have thought the more significant factor is
that
the time needed for a 180 deg change of heading is
much
longer at a higher speed and you'd be travelling around
a longer distance as well, losing valuable seconds.

It's a bit of each. The bigger radius means that you
spend more time going sideways to the courseline, but
the sink rate associated with the higher speed also
contributes.


a lower energy turn. I recall the calculated difference
is about 100', so it's not a huge deal. I don't think
the answer changes with airmass movement.

It is not immediately, or intuitively clear to me whether
it changes or not. A key issue would be to minimise
the time
spent in the sink area.


Not much time difference between the two. The higher
speed turn has a longer path too, but the total time
to get turned around is only different by a couple
of seconds.

With respect to option d, generally it's a good idea
to avoid voluntarily flying in sink. It's only worth
it if you think the savings in climb time will exceed
the additional 4 miles in higher sink (going from
the
near side of the 1-mile cylinder to the far side and
back again).

Hmmm. If I fly one mile out and one mile back I make
a total of two miles. I don´t follow the bit about
the mysterious 'cylinder' that makes it 4, but it
sounds like a good idea to avoid it )

FAI AST racing tasks have a one mile radius (2 mile
diameter)around the turnpoint so you can either cut
short by a mile or go long by a mile so the total range
of distances is +/- two diameters or 4 miles. There's
not much point in extending beyond the turn cylinder
unless you think you will climb A LOT better because
distance beyond the far side of the cylinder doesn't
count towards the task. For FAI record tasks you don't
get any credit for distance past the turnpoint, so
you are losing the cruise time in addition to the incremental
climb time from the higher sink rate to extend past
the turn.


Anyways, what I meant to imply in the question, and
which I was perhaps not sufficiently clear about,
was not that you go out to that thermal to stop
and climb in it (you might want to of course if
it was strong enough and depending on wind as well,
but that was not part of the question) but merely
in order to perform the pullup and 180 deg turn
and subsequent acceleration in rising air rather
than in sink.


The pullup in lift versus sink would only amount to
half a turn if you didn't stop and climb. The thermal
would have to be a lot stronger or the additional distance
very short to be able to make up the altitude (and
in some cases time) you give away in half a turn.

For example, if you are getting 500 fpm in sink, at
90 knots over 4 miles it will cost you about 1200
feet.
If you anticipate a climb at 8 knots instead of 5
knots
then you'd need to climb about 3200 feet to make up
the time loss from having to climb the additional
1200
feet (breakeven math is: 3200ft/800fpm = 4 min. and
2000ft/500fpm = 4 min.)

Well, I think it is not only about making up for the
height loss. There is also the time spent going past
the turnpoint, and returning to it. All that time will
be 'wasted' in the sense that your progress on the
task is zero during that time.

See my previous comment explaining turnpoint cylinders.
If it's not a racing task your point is correct and
the higher climb rate needs to be even higher - for
every minute you extend you need to save a minute of
climbing PLUS make up for the additional altitude lost
in the sink to get there.


This assumes that you don't need extra altitude to
avoid getting low on your egress from the sink area
and back to the last thermal you took. It also doesn't

Naturally. This was just a theoretical question about
TP-rounding technique as such. All sorts of other
factors and circumstances will of course be part of
the decision making in a real situation.

As somebody pointed out, a prevailing wind would of
course complicate the issue further as well.

Thanks for the answer.
CV

Andy Blackburn wrote:
Not much time difference between the two. The higher
speed turn has a longer path too, but the total time
to get turned around is only different by a couple
of seconds.

OK, thanks.

FAI AST racing tasks have a one mile radius (2 mile
diameter)around the turnpoint so you can either cut
short by a mile or go long by a mile so the total range
of distances is +/- two diameters or 4 miles. There's

Right. That clarifies what you meant. By "turnpoint",
the way I used it in the question, I meant the point
that you actually have to round physically, whether
it be the near edge of a cylinder or the centre or
whatever.

not much point in extending beyond the turn cylinder
unless you think you will climb A LOT better because
distance beyond the far side of the cylinder doesn't
count towards the task. For FAI record tasks you don't
get any credit for distance past the turnpoint, so
you are losing the cruise time in addition to the incremental
climb time from the higher sink rate to extend past
the turn.

The pullup in lift versus sink would only amount to
half a turn if you didn't stop and climb. The thermal
would have to be a lot stronger or the additional distance
very short to be able to make up the altitude (and
in some cases time) you give away in half a turn.
....
See my previous comment explaining turnpoint cylinders.
If it's not a racing task your point is correct and
the higher climb rate needs to be even higher - for
every minute you extend you need to save a minute of
climbing PLUS make up for the additional altitude lost
in the sink to get there.

Yes, all this is clear. This could perhaps be summed up
by saying that to fly beyond the TP would only be worth
it if you find significantly stronger lift there than
you will along the task after the turn.

Thanks again.
CV