Aerodynamics of carrying water

Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt

For the long answer, see
http://home.comcast.net/%7Everhulst/...t/ballast.html

The real short answer is - see the polar at the end of the above
article. One curve is with ballast, the other without.

The slightly longer answer is that a glider's best glide, for instance,
occurs at one speed. Increase the weight and that same best glide (more
or less) now occurs at a higher speed. You can go faster and maintain a
better L/D than you would without ballast.

Tony V.

Gene:

1: A glider's best glide ratio is unaffected by its weight.
2: However, a heavier glider flies and sinks faster at
the same glide angle than a lighter one.
3: When lift conditions are strong, the pilot accepts the
higher sink rates to achieve higher speeds over the ground.



Pete

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt



--

Peter D. Brown
http://home.gci.net/~pdb/
http://groups.yahoo.com/group/akmtnsoaring/

The best illustration used to be: Remember when you
rode your coaster wagon downhill alone and also with
a buddy in it? It always went faster with two people.

Who knows anything about coaster wagons anymore?

At 03:30 11 October 2005, Pete Brown wrote:
Gene:

1: A glider's best glide ratio is unaffected by its
weight.
2: However, a heavier glider flies and sinks faster
at
the same glide angle than a lighter one.
3: When lift conditions are strong, the pilot accepts
the
higher sink rates to achieve higher speeds over the
ground.



Pete

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more
years since
gliding. Unable to explain the 'why' of water ballast
to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt



--

Peter D. Brown
http://home.gci.net/~pdb/
http://groups.yahoo.com/group/akmtnsoaring/

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt

A ballasted sailplane has more energy at a given altitude and airspeed
than an unballasted sailplane. (potential + kinetic). At cruise
speeds the energy is dissipated predominantly to overcome parasite drag
which is independent of weight. Start with more energy, expend the
same to overcome drag, so give up less altitude. Edit to 25 words.

Andy

Vigorously throw pingpong ball.
Throw golf ball as vigorously.
Walk to pingpong ball. Stop.
Can you see golf ball yet?
Think. Ponder. Consider.
Massier = Energier
(25 words plus punctuation, not all words are real English)

Gary Emerson wrote:
We have a WINNER!

I think not (though I do like the answer :-) ). While "Massier =
Energier", the difference in golf ball / ping pong ball performance is
explained by ballistics (see "sectional density" and "ballistic
coefficient") and not aerodynamics. I am *not* an expert, i could be wrong.

Tony V.


Vigorously throw pingpong ball.
Throw golf ball as vigorously.
Walk to pingpong ball. Stop.
Can you see golf ball yet?
Think. Ponder. Consider.
Massier = Energier
(25 words plus punctuation, not all words are real English)

We have a WINNER!

wrote:
Vigorously throw pingpong ball.
Throw golf ball as vigorously.
Walk to pingpong ball. Stop.
Can you see golf ball yet?
Think. Ponder. Consider.
Massier = Energier
(25 words plus punctuation, not all words are real English)

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase=20
performance in gliders to argumentative airplane student.
=20
I need a simple explanation in 25 words or less.

Gene Whitt=20

For the long answer, see=20
http://home.comcast.net/%7Everhulst/...t/ballast.html

The real short answer is - see the polar at the end of the above=20
article. One curve is with ballast, the other without.

The slightly longer answer is that a glider's best glide, for instance,=20
occurs at one speed. Increase the weight and that same best glide (more=20
or less) now occurs at a higher speed. You can go faster and maintain a=20
better L/D than you would without ballast.

Tony V.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~

Did you mean to say in the same sentence that the L/D is the same, but
better?

The polar is magnified in the speed and sink directions by the square
root of the ratio of the weights, so that (simplistically) the L/D is
the same but occurs at a higher speed and sink rate. In other words, for
the same start height both the heavy and the light glider will hit the
ground at the same place, but the heavy glider will get there first (so
the golf ball/ping pong ball argument patently doesn't work).

And actually, the best L/D is slightly better on the heavy glider - the
reason is due to the fact that the heavy glider is operating at a higher
Reynolds number and has a slightly higher coefficient of lift as
consequence. Look at the LAK12 spec. and you'll see that it's advertised
L/D at max weight is 50 whereas at min weight it's 48, that's true of
other gliders too even if the manufacturer doesn't make a big deal of
it.


Rgds,

Derrick Steed

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt

Try this:

The important points are the cruise speed goes up, the sink rate goes
up, but the increased sink rate is insignificant on a strong day.

For why the glide angle remains the same with different aircraft weights:

* the best L/D occurs at one particular angle of attack (AOA is the
important parameter here)
* for low weights, this AOA will produce a low airspeed; at higher
weights, higher airspeeds

For why this is beneficial, given the glider won't climb as quickly in a
thermal at higher weights; for example, with a 20% weight increase with
ballast:

* you get a 10% increase in cruise speed
* you get a 10% increase in sink rate, but that's only 2.5% decrease in
the climb rate (unballasted sink rate of 150 fpm while thermalling) on a
600 fpm day

If you get these points across, you can generalize the "best L/D" to
other AOAs, and talk about how we don't fly at the best L/D on a good day.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA