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



 
 
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  #1  
Old September 11th 03, 08:27 PM
Kevin Neave
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Default I wish I'd never got into this...

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



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  #2  
Old September 12th 03, 02:19 AM
Kirk Stant
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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
  #4  
Old September 12th 03, 08:59 PM
Chris OCallaghan
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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.
  #6  
Old September 14th 03, 08:30 PM
Chris OCallaghan
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Default

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

(Chris OCallaghan) wrote in message . com...
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?

  #7  
Old September 14th 03, 09:34 PM
Bill Daniels
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Default


"Chris OCallaghan" wrote in message
om...
I reread this and want to add a few notes for clarity...

(Chris OCallaghan) wrote in message

. com...
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

  #8  
Old September 15th 03, 04:55 AM
Kilo Charlie
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Default

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


  #9  
Old September 15th 03, 04:05 PM
Andy Blackburn
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Default

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






  #10  
Old September 15th 03, 07:53 PM
Tony Verhulst
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Default



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.

 




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