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

THANK YOU UDO!!!
I think your sim. has the rights equations planted within it's
program!!!!
I am the one who started this debate. Quite fun uh!!! For your info.
,since
french is mother tongue, I posted the same question on a French forum.
Hell
is raised over the pound too. But the balance is in favour of a tie (
no
advantage to the ballasted). We stated than any difference less than 50
feet
is a tie. (That,s what you can read on your alti. with minimum
precision).
Must of people rely on their intuition on this side " If
it's twice as heavy, it has twice the energy, and it will
coast to twice the height, since the same drag force is
acting to deplete it's kinetic energy reservoir of twice the
size.
" Do not forget that the heavy glider has a bigger job to do and its is
exactly equivalent to the extra energy!!!!!
More thoughts for your intuition
1. will you need a bigger force to change the direction of travel of a
heavier object than a light one.
2. Intuitively, the wings of the heavy will bend more. Is the force
needed
to bend the wings translate in lost enrgy for the glider?
More for your consideration, better L/D is no advantage when your nose
is
pointed to the zenith. Lifts acts perpendicular to axis, it is actually
pulling you away from optimal trajectory.
Lastly, using the original data, if the heavy glider go say 75' (25m)
higher, it takes (100kgs*9.81 m/s2*25 m= 25 000 Joules of energy to do
that
job. So conversely, it is the extra job done by drag (....what
else...)on
the dry glider. I cannot fit this in any equation!!!!MAGIC!!!!!!
BravoQuebec


"Udo Rumpf" a icrit dans le message de
...
I am not able to comment on the math questions,
instead I used the simulator.
I used the ASW27 model with and without water.
I selected a trim speed in both cases (empty and full) of 180 km/h
flaps stayed in original position through out the manoeuvre.
The pull ups were with max stick deflection till an optimum
trajectory
for
both gliders was achieved.
Once the trajectory was established the glider was allowed to
fly/coast to the top.
The results over many test runs showed a 12% advantage on average for
the
unballasted Glider.
Regards
Udo




While Udo doesn't state the numerical value of the difference, I bet it's
only 30-40 ft.

you are correct. it works out to about 39 feet.

Udo

"Andy Durbin" wrote in message
om...
(Chris OCallaghan) wrote in message
om...
Well, at least we've got everyone on the same theme now. It's the
drag. Why don't you guys in Phoenix do a little testing and we'll do
the same here at M-ASA. I think we all agree that the heavier glider
has a significant drag advantage at high speed, and will gain
additional altitude. But how much, exactly?


So far I have not seen anyone consider the fact that, at the same
(high) speed, the unballasted glider has a significantly higher sink
rate at the start of the pull up. The initial conditions are not the
same.


Andy (GY)

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

Udo

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

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.

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

Andy,

Sink rate and drag are the same thing (we need to burn something to
feed friction... it's our potential energy we're paying out). See my
other posts on AOA and wing loading. That's right, the initial
conditions are not the same, which is why we carry water and why a
ballasted glider will gain more height during a pull up from high
speed.

We're in the same place.


(Andy Durbin) wrote in message . com...
(Chris OCallaghan) wrote in message om...
Well, at least we've got everyone on the same theme now. It's the
drag. Why don't you guys in Phoenix do a little testing and we'll do
the same here at M-ASA. I think we all agree that the heavier glider
has a significant drag advantage at high speed, and will gain
additional altitude. But how much, exactly?


So far I have not seen anyone consider the fact that, at the same
(high) speed, the unballasted glider has a significantly higher sink
rate at the start of the pull up. The initial conditions are not the
same.


Andy (GY)

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

And the vertical kinetic energy converted to altitude (the only place
where we can take the energy in a glider) would be
h=v^2/(2g)
so for 1.2 and 1.65 m/s the altitude losses would be
0.073 and 0.139 m respectively. Not a signifigant factor I would say.

Plus, we would have to take into account the fact that in the end as
well (after levelling out) their sink rates will be different. But still
no signifigant effect...

Jere

jere at iki.fi

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.)