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

[Kevin Neave]

The original question stated a glider doing 100kts
with/without 100kgs of ballast.

Ignoring the stalling speed for the moment, Does anyone
out there think the pull-up takes more than 4 or 5
seconds?

Does anyone out there think that the sink rate of the
unballasted glider at 100kts is more than 1m/s greater
than the ballasted one?

Finally does anyone out there think that the difference
in height gain for the ballasted glider is more than
4 or 5 metres?

If you think the answer to the third is Yes but the
answer to the first & second is No, please let me know
why!!

And if your answer to 'Why ?' is ''cos I've done pull-ups
higher with ballast' I want to know exact entry speed,
exact exit speed, climb angle, exact amount of ballast,
and exact heights gained with & without ballast (preferably
backed up with a logger trace!).

(And NO comparing two different gliders, just 'cos
they have the same wing section & are doing rolling
manoeuvres during the pull-up doesn't count)

:-)

Kevin

[Kirk Stant]

Kevin,

What we appear to be seeing here is how strong preconceptions and/or
expectations can override actual events. I could have sworn I would
get a better zoom with ballast, but I now realize that it doesn't make
sense. And I know all about Gallileo's experiment - I surprized my 15
year old daugher with it just a few weeks ago. But not having put
much thought into it, I didn't connect the two. Amazing, you can
learn something on RAS, occasionally!

It would be fun to poll the general soaring population about this - I
asked a really good pilot friend about this and his immediate answer
was "Of course you will go higher with water".

Now, I still have a gut feeling that there are some other forces
acting that make it seem that a ballasted pullup goes higher - because
it sure feels like it does!

Must have something to do with Flat Earth Theory....

Kirk
66

[Marcel Duenner]

(Kirk Stant) wrote in message >...
> Kevin,
>
>I could have sworn I would
> get a better zoom with ballast, but I now realize that it doesn't make
> sense.


I'll double that.

I've just thought of another thing that might contribute to thinking
one gets higher with ballast. Maybe it's because with and without
ballast isn't normally flown in the same way. Is it possible that most
people tend to pull less g's with heavier gliders? I think I do.
I know that if you take a ASK-21 ant try a loop with an entry speed of
200km/h and you pull more than 4g you'll hardly make it. Same glider,
entry speed 180km/h and only 2.8g you'll go round really nicely. It's
got something to do with wasting to much energy with high g's.

Marcel

P.S. Now, before some smart-a.. tries to correct me on the ASK-21
figures - don't. It's been a while since I did it I could be slightly
off. The numbers given are only supposed to illustrate the point.

[Chris OCallaghan]

Kevin,

the problem isn't wholly ballistic. That is, it's not quite the same
as firing projectiles of different mass straight up at the same
initial speed. Remember, your sailplane has drag that varies with
speed. Additional mass reduces total drag at higher speeds. That's why
we carry water ballast when racing. Thus, during your pull up, the
ballasted glider will have less drag than the unballasted one, and
will therefore gain additional altitude. Is it alot? No. But
significant.

On a side note, Galileo never dropped objects off the tower of Pisa.
Though he did some work with inclined planes, he recognized that
friction would skew his results (think of dropping a ping-pong ball
next to a golf ball...), and Catholic dogma didn't leave much room for
fault when it came to heresy. Instead, he created a thought
experiment. He postulated that if heavier objects fall faster, a heavy
object tied by a string to a lighter object and thrown from a tower
should pull the lighter object faster, and the lighter object would
impede the acceleration of the heavier one. However, once tied by a
string, they were a single object of greater mass and should therefore
outpace the individual objects. This demonstrated the fallacy of the
argument for "greater attraction" and saved him the embarrassment of
having to demonstrate a flawed experiment to anti-empiricists.

[lance smith]

errr... sorry for this newbie post. I'm a PPL (regular ppl, no G : )
and am thinking about getting my glider rating. Anyways I don't
understand this ballast/no-ballast question. In a regular/powered
plane I can climb better without extra weight (i.e. passengers). I
don't see why this wouldn't hold true for gliders as well, am I wrong?
Is this a common fact/misconception/etc in the soaring crowd?

thanks,

-lance smith


(Kirk Stant) wrote in message >...
> Kevin,
>
> What we appear to be seeing here is how strong preconceptions and/or
> expectations can override actual events. I could have sworn I would
> get a better zoom with ballast, but I now realize that it doesn't make
> sense. And I know all about Gallileo's experiment - I surprized my 15
> year old daugher with it just a few weeks ago. But not having put
> much thought into it, I didn't connect the two. Amazing, you can
> learn something on RAS, occasionally!
>
> It would be fun to poll the general soaring population about this - I
> asked a really good pilot friend about this and his immediate answer
> was "Of course you will go higher with water".
>
> Now, I still have a gut feeling that there are some other forces
> acting that make it seem that a ballasted pullup goes higher - because
> it sure feels like it does!
>
> Must have something to do with Flat Earth Theory....
>
> Kirk
> 66

[Chris OCallaghan]

I reread this and want to add a few notes for clarity...

(Chris OCallaghan) wrote in message >...
> Pete,
>
> I got a server error, so this may be a duplicate. Here's one way to
> look at the ballast issue.
>
> Think of best L/D as a function of AOA. Now think of speed as function
> of AOA and load. Increasing the load increases speed without a change
> in AOA. Therefore, increasing load increases the airspeed at which we
> will achieve best L/D.
>
> Changing AOA, either higher or lower (slowing down or speeding up),
> results in higher drag (more induced drag if we slow down, more
> parasite drag if we speed up).

I simplified this too much. Increasing AOA from best L/D will reduce
total drag down to minimum sink speed. These values are getting closer
to one another among newer airfoils. However, the rule of thumb is
that best L/D occurs at the point where induced and profile (parasite)
drag are equal. Minimum sink occurs at the point where profile drag
equal about 1/3 the induced drag.


>
> Now, take identical gliders. One has a gross weight of 100. The other
> has a gross weight of 200. The first glider achieves best L/D at a
> given AOA. At a weight of 100, best L/D speed is 50. The second glider
> achieves best L/D at the same AOA, but because it is twice as heavy,
> its best L/D speed is 70 (square root of load factor increase). Glider
> two achieves the same L/D at 70 as its lighter twin achieves at 50. In
> order for the lighter glider to keep up, it will have to increase its
> speed by lowering angle of attack to a less than optimum value,
> thereby increasing drag.
>

ie, it will sink faster at 70.

Coefficent of drag changes with AOA. Additional load allows us to fly
the glider faster at optimum AOA, achieving a lower coefficient of
drag. Since induced drag becomes less important and profile drag more
critical as our speed increases, being able to maintain low C of D at
higher speeds is a significant performance bonus. At lower speeds,
increased induced drag due to loading is a detrement. (The rule of
thumb is that for lift less than 3 knots, don't bother with ballast.
For lift greater than four knots, start adding water. In between 3 and
4 it really doesn't matter, unless there is significant lift
streeting, in which case you speeds will be higher and increased
loading is justified.)

Back to the subject of the thread, I think it is clearer now that if
two gliders, one ballasted, one not, execute a pull up from high
speed, the heavier glider will achieve greater altitude because it has
less total drag throughout most of the maneuver, only becoming less
efficient than the empty one at low speeds, where there is
dramitically less available energy to convert into altitude
(diminishes with the square of the speed).

Counterintuitive, isn't it. But the proof is in the polar. Look at any
polor and note that the sink at at max wing loading at 100 knots is
quite a bit less than the corresponding sink rate at minimum wing
loading.

I think we were asked to ignore Reynolds numbers in this discussion.
So I will, except to say that at the speed we work with, it too is
significant (though second order compared to the effects discussed
above).


> Does this help?

[Bill Daniels]

"Chris OCallaghan" > wrote in message
om...
> I reread this and want to add a few notes for clarity...
>
> (Chris OCallaghan) wrote in message
>...
> > Pete,
> >
> > I got a server error, so this may be a duplicate. Here's one way to
> > look at the ballast issue.
> >
> > Think of best L/D as a function of AOA. Now think of speed as function
> > of AOA and load. Increasing the load increases speed without a change
> > in AOA. Therefore, increasing load increases the airspeed at which we
> > will achieve best L/D.
> >
> > Changing AOA, either higher or lower (slowing down or speeding up),
> > results in higher drag (more induced drag if we slow down, more
> > parasite drag if we speed up).
>
> I simplified this too much. Increasing AOA from best L/D will reduce
> total drag down to minimum sink speed. These values are getting closer
> to one another among newer airfoils. However, the rule of thumb is
> that best L/D occurs at the point where induced and profile (parasite)
> drag are equal. Minimum sink occurs at the point where profile drag
> equal about 1/3 the induced drag.
>
>
> >
> > Now, take identical gliders. One has a gross weight of 100. The other
> > has a gross weight of 200. The first glider achieves best L/D at a
> > given AOA. At a weight of 100, best L/D speed is 50. The second glider
> > achieves best L/D at the same AOA, but because it is twice as heavy,
> > its best L/D speed is 70 (square root of load factor increase). Glider
> > two achieves the same L/D at 70 as its lighter twin achieves at 50. In
> > order for the lighter glider to keep up, it will have to increase its
> > speed by lowering angle of attack to a less than optimum value,
> > thereby increasing drag.
> >
>
> ie, it will sink faster at 70.
>
> Coefficent of drag changes with AOA. Additional load allows us to fly
> the glider faster at optimum AOA, achieving a lower coefficient of
> drag. Since induced drag becomes less important and profile drag more
> critical as our speed increases, being able to maintain low C of D at
> higher speeds is a significant performance bonus. At lower speeds,
> increased induced drag due to loading is a detrement. (The rule of
> thumb is that for lift less than 3 knots, don't bother with ballast.
> For lift greater than four knots, start adding water. In between 3 and
> 4 it really doesn't matter, unless there is significant lift
> streeting, in which case you speeds will be higher and increased
> loading is justified.)
>
> Back to the subject of the thread, I think it is clearer now that if
> two gliders, one ballasted, one not, execute a pull up from high
> speed, the heavier glider will achieve greater altitude because it has
> less total drag throughout most of the maneuver, only becoming less
> efficient than the empty one at low speeds, where there is
> dramitically less available energy to convert into altitude
> (diminishes with the square of the speed).
>
> Counterintuitive, isn't it. But the proof is in the polar. Look at any
> polor and note that the sink at at max wing loading at 100 knots is
> quite a bit less than the corresponding sink rate at minimum wing
> loading.
>
> I think we were asked to ignore Reynolds numbers in this discussion.
> So I will, except to say that at the speed we work with, it too is
> significant (though second order compared to the effects discussed
> above).
>
>
> > Does this help?

I've followed this discussion with interest. I think there is one more
thing that might make a big difference and that is the particular glider's
airfoil. For example, comparing my Lark IS 28 with a G103 at the same wing
loading, the Grob zooms very well and the Lark is terrible. The difference,
I think, is that the Grob has a thick, high lift wing and typical smooth
fiberglass/gelcoat finish. The Grob wing just "grabs" the air better. On
the other hand, the Lark runs much better.

Bill Daniels

Bill Daniels

[Kilo Charlie]

I just entered this thread so apologize if having not read all the posts
this has been mentioned already. Let me quote Helmut Reichmann from Cross
Country Soaring pp 63-64:

"Starting in contests with full water ballast is always a good idea. If the
run through the gate is made at high speed, heavier sailplanes will gain
more height than light ones in the subsequent pullup".

Although I find it impossible to argue with the math presented might it be
that we are oversimplifying things? With all due respect a modern sailplane
is a long way from a rock or pendulum. I'm certainly not an engineer but
have flown for long enough and have done enough high speed pullups at the
finish and on course to feel fairly certain that the altitude gained is
substantially greater. But even more to the point if you don't believe this
then was Reichmann wrong? Maybe the translation was poor? Geez my bubble
is bursting! Send help!

Casey Lenox
KC
Phoenix

[Andy Blackburn]

If I look at my '27's polar and assume it takes about
0.25 nm to execute a pullup, I get less than 25 feet
of difference due to the higher sink rate at redline
for the unballasted case (at 154 kts the ballasted
L/D is 19 and unballasted L/D is 14).

This doesn't count for the losses associated with the
Gs of the transition to the pullup, but I would think
that you'd generate more induced drag to change the
(vertical) direction of the heavier glider.

I, too, have always flown with the belief that ballasted
gliders would get more altitude on a pullup from the
same speed - but I can't come up with any aerodynamic
or physics rationale to support it.

9B

At 04:00 15 September 2003, Kilo Charlie wrote:
>I just entered this thread so apologize if having not
>read all the posts
>this has been mentioned already. Let me quote Helmut
>Reichmann from Cross
>Country Soaring pp 63-64:
>
>'Starting in contests with full water ballast is always
>a good idea. If the
>run through the gate is made at high speed, heavier
>sailplanes will gain
>more height than light ones in the subsequent pullup'.
>
>Although I find it impossible to argue with the math
>presented might it be
>that we are oversimplifying things? With all due respect
>a modern sailplane
>is a long way from a rock or pendulum. I'm certainly
>not an engineer but
>have flown for long enough and have done enough high
>speed pullups at the
>finish and on course to feel fairly certain that the
>altitude gained is
>substantially greater. But even more to the point
>if you don't believe this
>then was Reichmann wrong? Maybe the translation was
>poor? Geez my bubble
>is bursting! Send help!
>
>Casey Lenox
>KC
>Phoenix
>
>
>

[Tony Verhulst]

For any fixed wing aircraft the max L/D speed (or minimum sink speed,
for that matter) changes with the square root of the difference of the
mass ratio. Assume that a glider and pilot weighs 700 pounds and has a
max L/D at 50 knots. Now add 400 pound of ballast for a total weight of
1100 pounds. 1100/700 = 1.57. the square root of 1.57 is 1.25 - so your
new best L/D speed is 50 x 1.25 = 62.5 knots.

Tony V.

[Derrick Steed]

I'd like a bash at this:

Can we get some assumptions out in the open first...

And before I get toasted, I am not being patronising, I just like to get
assumptions clear and out in the open.

1. throughout we stick to one glider type which is, e.g.
i) why? because it would be like comparing apples and oranges otherwise
ii) so, lets assume it is an asw23, or it is a pik20b, or ...
ii) consider two cases: ballasted (= greater mass) and unballasted
iii) otherwise, the configuration of the two gliders is identical except for
the amount of ballast they are carrying

2. potential energy = mass x gravitational acceleration x height, so
i) for a given height the ballasted glider has more potential energy than the
unballasted glider

3. kinetic energy = 1/2 x mass x speed x speed, so
ii) for a given speed the ballasted glider has more kinetic energy than the
unballasted glider

4. total energy = potential energy + kinetic energy
i) from the above we now know that, for a given height and speed, the
ballasted glider has a greater total energy than the unballasted glider

5. from the gliders polar and the basic arithmetic of ballasting we know that,
above a certain speed (i.e. the speed at which the sink rates of both the
ballasted and unballasted gliders are the same), the ballasted glider will be
travelling faster, for a given sink rate (= rate of energy loss), than the
unballasted glider, below that speed the unballasted glider will be losing
energy at a lower rate than the ballasted glider
i) this effect is due to the increase in the wing loading of the glider
ii) the same effect would apply (approximately, because the wing bending
would be somewhat different) to the unballasted glider in accelerated flight
(e.g. during a pull up)
iii) assuming the above, for a given speed the ballasted glider will be
sinking at a lower rate than the unballasted glider (e.g. the rate of energy
loss is lower) - don't believe me? look at the polar

6. provided we stay above that "certain speed" (which is determined by the
wing loading and so will be higher if the wing loading is higher)
i) for a given speed the ballasted glider will always be losing (potential)
energy at a lower rate than the unballasted glider
ii) this will be true regardless of whether the glider is in steady (i.e.
straight line) flight or in accelerated (e.g. turning or pulling up) flight
iii) in fact the difference in the rates of loss will be even greater in
accelerated flight

So far, so good (I hope). Now lets ignore the glide segment and just consider
the pull up and the subsequent zoom.

7. for two real gliders, of the same type, same configuration, one ballasted
more than the other, during the pull up
i) assuming the two gliders start at the same height and the same speed
ii) both gliders increase their wing loading in the same proportion to their
mass during the pull up
iii) I think that, given ii, they will follow the same pull up curve as a
result, but
iv) throughout the maneuvre, the ballasted glider will be losing energy at a
lower rate than the unballasted glider
v) so it should come out of the pull up higher and having lost less energy
than the unballasted glider (i.e. it will start the zoom faster than the
unballasted glider)

8. during the zoom (at zero g), if both gliders started at the same height and
speed
i) both will gain potential energy, and
ii) both will lose kinetic energy, but at a rate proportional to their masses
due to the effect of gravitational acceleration, and so
iii) the gliders would rise to the same height if they were in a vacuum
throughout, but
iv) they are not in a vacuum, they are gliding (probably at a reduced wing
loading), so
v) provided they are flying above that "certain speed", the ballasted glider
will be losing energy at a lower rate than the unballasted glider, and so it
will zoom higher

I admit this is a somewhat qualitative argument, so would someone like to put
figures on it?

Rgds,

Derrick.

[Chris OCallaghan]

Andy,

see my notes earlier in the thread. There's not much penalty for
pulling from high speed so long as you don't go to too quickly to high
AOA (bigger penalty in induced drag). 2g to 30 degrees nose up is
typical. The you ease off to 1g until you start push gently to
attitude at your desired exit speed.

Did your calculations include the losses to friction throughout the
manuever? Your altitude difference seems a little too low. I would
expect about a 20 to 30 foot difference for a pull from 100 knots to
60 knots, ie, a normal "test the strength of the core" pull up.

At any rate, if we get any decent weather, I'll be sure to make some
runs with a lighter glider and tender the real world results.

The original poster was looking for some real world feedback. Right
now all I can offer is that when ridge soaring, if I have water and
another 27/V2 is empty, I'll outpace him by at least 10 knots on a
hundred mile-per-hour day. Roughly 10 to 12 percent more speed at the
same sink rate/drag. A big spoonful of pure, sweet hubris.

[Andy Blackburn]

Hey Chris,

I used the factory polar to estimate altitude loss
at redline with and without ballast. The difference
over 0.25 nm was 26' (105' - 79'). As you bleed off
airspeed the difference in sink rate declines, so the
actual difference in altitude loss should be less than
25' - maybe more like 15'.

This ignores G-related losses, which should be low
if the Gs are low (i.e. not too radical a pullup).
Of course too gradual a pullup and you don't get maximum
altitude gain because the parasite drag will accumulate.

9B





At 23:00 15 September 2003, Chris Ocallaghan wrote:
>Andy,
>
>see my notes earlier in the thread. There's not much
>penalty for
>pulling from high speed so long as you don't go to
>too quickly to high
>AOA (bigger penalty in induced drag). 2g to 30 degrees
>nose up is
>typical. The you ease off to 1g until you start push
>gently to
>attitude at your desired exit speed.
>
>Did your calculations include the losses to friction
>throughout the
>manuever? Your altitude difference seems a little too
>low. I would
>expect about a 20 to 30 foot difference for a pull
>from 100 knots to
>60 knots, ie, a normal 'test the strength of the core'
>pull up.
>
>At any rate, if we get any decent weather, I'll be
>sure to make some
>runs with a lighter glider and tender the real world
>results.
>
>The original poster was looking for some real world
>feedback. Right
>now all I can offer is that when ridge soaring, if
>I have water and
>another 27/V2 is empty, I'll outpace him by at least
>10 knots on a
>hundred mile-per-hour day. Roughly 10 to 12 percent
>more speed at the
>same sink rate/drag. A big spoonful of pure, sweet
>hubris.
>

[Kevin Neave]

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
>
>
>
>
>

[Derrick Steed]

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.

[Chris OCallaghan]

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?

[szd41a]

>
> It's worth noting that the heavier glider has more energy
> stored kinetically and potentially. Whatever the difference
> in height is, it's due to drag, and heavier glider needs to
> give up less altitude for any given amount of energy
> required to overcome drag.
>
Since Newton was hit by the apple, it is very unfortunate that everywhere on
this universe, any system trying to carry a load away from the center of the
planet will have to work harder than one travelling light. This is intuitive
enough. I wonder what is catching here. For any ten feet of height, the
heavy system will have to work more than the light system. True, our
ballasted glider has more money in the bank at the start, but it will have
to spend more on the way up. I hope that money comparaison will help. There
is no way around this fact. Travelling towards the center of the planet is
another ball game. The reason that for a given angle of glide, the heavy
will go faster is that the weight being larger, it's component parrallel to
the direction of travel is bigger. That simple. This is the motor. Drag is
"induced". it is not running the show.!!
Hope this help.
Bravo Quebec

[szd41a]

> inclined ramp. There are two things slowing them and
> limiting the height they coast to, gravity and drag.
> Gravity has the same effect on both. Drag is higher on the
> heavy glider, but it has more kinetic energy, so the higher
> drag has less proportional effect on the distance up that
> ramp that the glider will travel.
>
Todd
Please admitt that the heavy has a bigger job to do. You just need more
energy to carry a heavy load up. Please stop denying that, or the buildings
around us will start to soar ;-))))
Bravo quebec

[Andy Durbin]

(Chris OCallaghan) wrote in message >...
> 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)

[szd41a]

> While Udo doesn't state the numerical value of the difference, I bet it's
> only 30-40 ft.
>
That makes sense, considering the expected 3-400' delta h. So this would be
a tie, and quite satisfying, considering that everybody I know in the
soaring world, including H. Reichman ;-)), believes that the ballasted
glider would go noticebely higher. I never tested it, but along with anybody
that did a little maths, I could not force-fit the extra ballasted height
into the equations. The debate is still on, and we deeply wish that someone
could run a "scientific" test. This is fun!!!
Bravo Quebec

[Udo Rumpf]

> > 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

[Udo Rumpf]

"Andy Durbin" > wrote in message
om...
> (Chris OCallaghan) wrote in message
>...
> > 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

[Kevin Neave]

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
>
>

[Kevin Neave]

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.
>
>
>
>

[Derrick Steed]

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.

[Udo Rumpf]

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
> >
> >
>
>
>

[Udo Rumpf]

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
> >
> >
>
>
>

[Chris OCallaghan]

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 >...
> (Chris OCallaghan) wrote in message >...
> > 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)

[Jere Knuuttila]

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

[Kevin Neave]

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.)
>

[Kevin Neave]

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
>> >
>> >
>>
>>
>>
>

[Udo Rumpf]

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

"Kevin Neave" > 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
> >> >
> >> >
> >>
> >>
> >>
> >
>
>
>

[szd41a]

Hello Udo

I collected some comments regarding your sims.

1.)In reality, the way you pulled in sim, it would have exceed glider's
limitation (too many g's), resulting in two broken wings!
2.)Since we always do a recovery around V=Min Sink, some argue that in this
case, it would demonstrate the advantage of the ballasted.
Any comments??
Bravo Quebec
"Udo Rumpf" > a écrit dans le message de
...
> Kevin,
> In case I missed it, booth gliders are allowed to fly / coast to the top
> without
> control input for recovery.
> Udo
>
> "Kevin Neave" > 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
> > >> >
> > >> >
> > >>
> > >>
> > >>
> > >
> >
> >
> >
>