I wish I'd never got into this...

I'm sorry Kate but maths beats 'Feminine Logic' every
time.

If two gliders start at the same height & speed & accelerate
to the same speed then they'll lose pretty much the
same height & follow the same trajectory.

And before anyone else brings up golf balls & ping-pong
balls may I remind people that your average sphere
is a much much draggier shape than your average sailplane,
and the ratio of masses is way way way greater than
the ratio for a glider with / without ballast

At 00:18 16 September 2003, Glider Kate wrote:
Boys

You seem to be forgeting one or two things!!!

If two identical sailplanes with identical weight pilots
but with sailplane a) carrying water ballast and b)
dry. Set off in still air, side by side at the same
speed, say 45 knots and accelerate at the same rate,
to a new identical speed, say 100knots.

By the time they reach the new speed, sailplane a)
will accelerate faster and travel further and lose
more height than glider b).

No need for maths just a bit of feminine logic

Bye...............

Kate

And you miss the point, if I am on a final glide I don't care what height I
started from, what I am interested in is how fast I can make the glide to
the goal. The height I can pull up to determines how low I dare go near the
goal bearing in mind that I want to pull to a safe height for my approach
and landing. The point is that the ballasted glider will not only get there
faster, it will also pull up higher = ballasted glider wins.

Rgds,

Derrick.

Hi Udo,

When you ran the Sim, how much ballast were you carrying,
& what speed did you pull up to in each case?


At 02:30 18 September 2003, Udo Rumpf wrote:

The sink rate for the fully loaded glider(190 litres)
= 1.2m/s
for the none ballaste version 1.65m/s

Udo

check previous post.
Udo

When you ran the Sim, how much ballast were you carrying,
& what speed did you pull up to in each case?


At 02:30 18 September 2003, Udo Rumpf wrote:

The sink rate for the fully loaded glider(190 litres)
= 1.2m/s
for the none ballaste version 1.65m/s

Udo

check previous post.
Udo

When you ran the Sim, how much ballast were you carrying,
& what speed did you pull up to in each case?


At 02:30 18 September 2003, Udo Rumpf wrote:

The sink rate for the fully loaded glider(190 litres)
= 1.2m/s
for the none ballaste version 1.65m/s

Udo

Hi Derrick,

The principles are indeed simple.
The original post stated two gliders with & without
100kgs ballast starting at 100kts.

The heavy glider is indeed losing height more slowly
than the light one at this speed, (But the difference
is only about 1m/s).
This difference is only maintained for the duration
of the pullup (About 4-5 seconds) and will be diminishing
as the speeds drop off.
In addition we're not pulling up to a standstill 'cos
that's an untenable position for most gliders, in which
case the light glider gains an advantage because it
can fly a few knots slower than the heavy one.

So in the end I believe it's too close to call!!

(Cynically I believe that the original post of 100kts/100kgs
was deliberately chosen to make it too close to call)

Cheers

At 11:30 18 September 2003, Derrick Steed wrote:
In response to post number 39:

The math is a bit complex, but the physical principles
are simple.

1. In the glide the heavier glider goes faster for
the same glide angle
(read Frank Irving - he's a gliding aerodynamicist)

2. In the pull up the same is true (It's just a higher
wing loading

3. In the zoom subsequent to the pull up there are,
principally, two forces
at work:
i) that due to gravitational acceleration - this is
proportional to mass and so both gliders decelerate
at the same rate (if
this were the only factor then they would both zoom
to the same height
provided that they both started the zoom at the same
height and speed)
ii) that due to drag, this is primarily proportional
to speed
and not proportional to mass, the result is that the
heavier glider
decelerates at a slower rate than the light glider
and so goes further (=
higher).

Rgds,

Derrick.

Kevin,

If you are talking about 1m/s difference in sink rate, then I would think
that is a huge difference. Even unloaded, my gliders' polar shows me sinking
at about 2.5m/s at 100kts.

Rgds,

Derrick.

Hi Todd,

You seem to be of the opinion that in the accelerated
part at the start of the pull-up the ballasted glider
has a vast advantage over the un-ballasted one. Have
you any evidence to support this? I would have thought
that the effort required to accelerate the extra ballast
would give the advantage to the light glider at this
point - but yes that's just my opinion.

As far as I'm concerned the overwhelming maths here
is good ol' conservation of energy where speed is traded
for height & the two come out equal.

Yes there's some drag involved but the actual drag
forces are pretty small on modern sailplanes & the
time in which they have to operate is pretty small.


Three questions for you:-

1) How long do you think the pull-up lasts?
2) What sort of difference do you think there is in
the respective sink rates - during the pull, during
the climb & the push over at the top?
3) What sort of difference do you think there is in
the height gained?

Finally I've suggested a couple of times that someone
with a Duo / ASH25 / Nimbus D go & do the tests

At 15:42 18 September 2003, Todd Pattist wrote:
Kevin Neave
wrote:

The heavy glider is indeed losing height more slowly
than the light one at this speed, (But the difference
is only about 1m/s).

Where do you get that number? From the polar measured
at
1G? That's the wrong polar. The glider is not operating
at
1 G for much of the pullup.

This difference is only maintained for the duration
of the pullup (About 4-5 seconds) and will be diminishing
as the speeds drop off.

I regret to say that this analysis is bogus. It just
tells
us what would happen if the gliders flew side by side
for
4-5 seconds. Of course that difference is nominal,
but they
aren't doing that, they are flying at a varying G-load
through the pullup. You can't wave your hands and
ignore
that difference.

So in the end I believe it's too close to call!!

You have no basis other than your opinion. You need
to do
the math or the experiment. You've done neither.

Todd Pattist - 'WH' Ventus C
(Remove DONTSPAMME from address to email reply.)

OK, I forgot to engage brain before typing..

Your post states 190 litres of ballast, but not the
speed that you had left at the end of the pull-up

Cheers

Kevin

At 14:06 18 September 2003, Udo Rumpf wrote:
check previous post.
Udo

When you ran the Sim, how much ballast were you carrying,
& what speed did you pull up to in each case?


At 02:30 18 September 2003, Udo Rumpf wrote:

The sink rate for the fully loaded glider(190 litres)
= 1.2m/s
for the none ballaste version 1.65m/s

Udo

Kevin,
In case I missed it, booth gliders are allowed to fly / coast to the top
without
control input for recovery.
Udo

"Kevin Neave" k wrote in
message ...
OK, I forgot to engage brain before typing..

Your post states 190 litres of ballast, but not the
speed that you had left at the end of the pull-up

Cheers

Kevin

At 14:06 18 September 2003, Udo Rumpf wrote:
check previous post.
Udo

When you ran the Sim, how much ballast were you carrying,
& what speed did you pull up to in each case?


At 02:30 18 September 2003, Udo Rumpf wrote:

The sink rate for the fully loaded glider(190 litres)
= 1.2m/s
for the none ballaste version 1.65m/s

Udo