Aerodynamics of carrying water

[Gene Whitt]

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

[Tony Verhulst]

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/GBSC/student/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.

[Pete Brown]

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/

[Nyal Williams]

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/
>
>
>
>

[Andy]

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

[[email protected]]

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)

[Tony Verhulst]

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)
>>
>

[Gary Emerson]

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)
>

[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=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/GBSC/student/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

[Eric Greenwell]

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

[Bill Daniels]

"Gene Whitt" > wrote in message
nk.net...
> 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
>
>

All good answers.

I would add that an examination of two polars for a particular glider, one
ballasted and one not, will show a crossover airspeed above which the sink
rate with ballast is less than without - very counterintuitive.

Most modern gliders flying in conditions with greater than 2Kt thermals will
spend 80% of their X/C time flying at airspeeds above the crossover point
where the sink rate is reduced by using ballast.

Flying fast with reduced sink is dramatic.

Bill Daniels

[Pat Russell]

On 11 Oct 2005 03:52:06 -0700, "Andy" > wrote:


> Edit to 25 words.

"A glider is a gravity-powered machine. The heavier it is, the more
power it has."

16 words, beat that!

-Pat

[Andy]

The polar documents the phenomenon, it does not explain it. Any
attempt to explain the use of ballast by reference to the polar should
leave any intelligent person asking - but why does the polar do that.
I believe the effect can only be explained by considering the system
energy exchange. Once that is understood the effect on the polar
family is intuitive.

[Andy]

But I need the same amount of power to go the same speed don't I? Why
does more power reduce my sink rate?

I think you'll need more than 16 words.


Andy

[Derrick Steed]

The polar documents the phenomenon, it does not explain it. Any
attempt to explain the use of ballast by reference to the polar should
leave any intelligent person asking - but why does the polar do that.
I believe the effect can only be explained by considering the system
energy exchange. Once that is understood the effect on the polar
family is intuitive.

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

but loses all the effects due to Reynolds number which is an aerodynamic
phenomena and the original question was about the aerodynamics, not the
dynamics.


Rgds,

Derrick Steed

[P.Corbett (ZZ)]

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
>
>
E=M(CxC)

Paul

[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=20
> performance in gliders to argumentative airplane student.
>=20
> I need a simple explanation in 25 words or less.
>
> Gene Whitt

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

buy glider, put water in, launch, do 100 knots, note sink rate, dump
water, land, fly, do 100 knots note sink rate, job done


Rgds,

Derrick Steed

[Jim Kelly]

Thermalling stores energy in the glider, but thermals stop at
cloud base. Water carried needs to be lifted too, thus more
energy can be harnessed.

jk


"Gene Whitt" > wrote in message
nk.net...
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]

Thanks everyone. Now for a 'yes' or 'no' answer to my follow-up
question.

Does the addition of weight have the same effect on airplanes with power off
and prop stopped as on gliders but at a less efficient level.?

Gene
Aside: I once dove a C-150 to 120 mph before stopped prop
would start the engine,

[[email protected]]

>>A glider is a gravity-powered machine. <<

A sled is a gravity-powered machine. A glider is solar
powered.........
==============
Leon McAtee

[Bruce Hoult]

In article t>,
"Gene Whitt" > wrote:

> Thanks everyone. Now for a 'yes' or 'no' answer to my follow-up
> question.
>
> Does the addition of weight have the same effect on airplanes with power off
> and prop stopped as on gliders but at a less efficient level.?

Of course.

Until you get up to Mach effects, at least.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

[Jack]

Jim Kelly wrote:
> Thermalling stores energy in the glider, but thermals stop at
> cloud base.

What do you call all that rising air in the cloud?


Jack

[iPilot]

The answer contains too many "if"-s

You have to understand, that gliders are in certain sense unique aircrafts.
Non-lifting parts give miniscule amounts of the glider drag (around 10%).
That is very different from other aircrafts (C-172 for example). As speed
increases (due to the higher wing loading), higher non-lifting drag in
motored aircrafts cancels most of the effect of reduced AOA. Provided that
such even excists with wings optimised for absolutely different flying.


Regards,
Kaido


"Gene Whitt" > wrote in message
nk.net...
> Thanks everyone. Now for a 'yes' or 'no' answer to my follow-up
> question.
>
> Does the addition of weight have the same effect on airplanes with power
> off and prop stopped as on gliders but at a less efficient level.?
>
> Gene
> Aside: I once dove a C-150 to 120 mph before stopped prop
> would start the engine,
>

[Andy]

I once gave a presentation on "speeds to fly when the engine quits" to
a power flying club. Not one person there had any idea that weight and
wind would be a factor but they could all quote the best glide speed
from the POH.

Did you know a DC-10 has a glide ratio nearly as good (bad) as a 2-33.
They get over 20:1 but it's at over 200kts.


Andy

[Eric Greenwell]

iPilot wrote:
>>Does the addition of weight have the same effect on airplanes with power
>>off and prop stopped as on gliders but at a less efficient level.?
>>

> The answer contains too many "if"-s
>
> You have to understand, that gliders are in certain sense unique aircrafts.
> Non-lifting parts give miniscule amounts of the glider drag (around 10%).
> That is very different from other aircrafts (C-172 for example). As speed
> increases (due to the higher wing loading), higher non-lifting drag in
> motored aircrafts cancels most of the effect of reduced AOA. Provided that
> such even excists with wings optimised for absolutely different flying.

For the same AOA, the percentages of drag from lifting and non-lifting
parts will be the same, so we can confidently adjust the polar of an
airplane due to weight changes just as we do for the glider. There
aren't any basic aerodynamic differences between a good glider and a bad
glider.

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

Eric Greenwell
Washington State
USA

[Bruce Hoult]

In article . com>,
"Andy" > wrote:

> Did you know a DC-10 has a glide ratio nearly as good (bad) as a 2-33.
> They get over 20:1 but it's at over 200kts.

Yes. Fuel is the major cost of long distance airlines. They save fuel
by reducing drag, which means getting the best possible cruise L/D.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

[Jim Kelly]

>>>
What do you call all that rising air in the cloud?
Jack
<<<

:-) Now, I can only afford to change words - not add words . .
.. so I'll try and get away with this modification (!):

Thermalling stores energy in the glider, but gliders stop at
cloud base. Water carried needs to be lifted too, thus more
energy can be harnessed.

Jim Kelly (ducking for cover . . .)

. . . you could keep your student enticed with stories of shear
wave . . . or cloud flying in UK . . . in the next lessons!

[Gene Whitt]

Y'all,
Many thanks for explanations that seemed to mean 'yes!'
Personal email from Pete started something. He found
my three million word web site interesting. I also tell
how I won WWII as a corporal on Tinian.
www.whittsflying.com
Gene Whitt

[Jack]

Andy wrote:

> Did you know a DC-10 has a glide ratio nearly as good (bad) as a 2-33.
> They get over 20:1 but it's at over 200kts.

Cite?


Jack

[Bruce Hoult]

In article >,
Jack > wrote:

> Andy wrote:
>
> > Did you know a DC-10 has a glide ratio nearly as good (bad) as a 2-33.
> > They get over 20:1 but it's at over 200kts.
>
> Cite?

Don't know about the DC-10, but the ancient 707 was about 19:1.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

[Paul Remde]

Hi Guys.

I think those numbers are very optimistic. I once heard the pilots of the
Boeing 767 that glided to a landing in Canada describe (at an SSA
convention) how they were surprised by how back the glide ratio was with the
engines out. I believe they said that the manuals numbers were actually
calculated with the engines at idle, or at an rpm setting that overcomes the
drag caused by the engines. They are very big drag devices when not
providing thrust.

Good Soaring,

Paul Remde

"Bruce Hoult" > wrote in message
...
> In article >,
> Jack > wrote:
>
>> Andy wrote:
>>
>> > Did you know a DC-10 has a glide ratio nearly as good (bad) as a 2-33.
>> > They get over 20:1 but it's at over 200kts.
>>
>> Cite?
>
> Don't know about the DC-10, but the ancient 707 was about 19:1.
>
> --
> Bruce | 41.1670S | \ spoken | -+-
> Hoult | 174.8263E | /\ here. | ----------O----------

[Andy]

A valid response of course. I was given this information years ago
when I worked in Douglas flight test. The numbers I was given were more
precise than I quoted but I don't remember exactly what they were.
MD11 and MD10 CFM don't include the data and that is all I have access
to now. Paul may be correct that this is for an engines idle case. I
expect to be spending a lot of time in MD-10 sims in the next few
months. If I remember I'll get a few data points for all engine out
glide.

Andy

[Bela]

Aerodynamics (polars) will show the affect of ballast but the good old
E=MCC explains it.
Bela

[Tony Verhulst]

Bela wrote:
> Aerodynamics (polars) will show the affect of ballast but the good old
> E=MCC explains it.
> Bela

No, it does not. The performance increase with ballast has very little
to do with the increase in energy due to the added weight. The reason is
purely aerodynamic.

Performance speeds increase by the square root of the weight change
ratio. If a 750 pound glider and pilot gets its best glide ratio of 40:1
at 50kts, adding 400 pounds water changes the weight to 1150, and so the
new best glide speed is sqrt(1150/750) x 50 = 61.9kts. The best glide
ratio of 40:1 stays the same (ignoring the secondary effects of Reynolds
number and coefficient of lift increases).

Tony V

[Tony Verhulst]

>.......(ignoring the secondary effects of Reynolds
> number and coefficient of lift increases).

Sigh, that should read "second order effects", of course.

Tony V.

[hannu]

"Bela" > wrote in message
ups.com...
> Aerodynamics (polars) will show the affect of ballast but the good old
> E=MCC explains it.
> Bela

Only if the glider is annihilated to form energy (say a collision with
similar antimateria glider) ;)

hannu

[Andy]

"Performance speeds increase by the square root of the weight change
ratio. "

Why do they?

Andy

[Tony Verhulst]

> and there's your answer, the new performance speed V2 at the
> new weight W2 and the original coefficient of lift/AOA/glide
> ratio is sqrt(W2/W1) times the old performance speed V1 at
> the old weight W1.


Thanks, Todd. For those interested in this stuff but who would prefer to
do without calculus, a nice book is "Flight Theory for pilots" by
Charles E Dole, published by Jeppesen Sanderson Training Products, ISBN
0-89100-432-7. The material that Todd mentions is the first part of
chapter 4, "Lift and Stall". Here's one source:
http://www.avmart.com/itemdetail.asp?pid=2253&cat=48 . A small review of
it on my web site at
http://home.comcast.net/~verhulst/GBSC/read_lst/aerodynamics.htm

Tony V.

[Ralph Jones]

On 11 Oct 2005 06:01:11 GMT, Nyal Williams
> wrote:

>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?
>
Think of a 400-pound skier.

rj

[Eric Greenwell]

Nyal Williams wrote:

> 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.

And I suppose it would go faster uphill, with two people pulling it
back: not a very good analogy, I think, since the heavy glider will
climb more slowly. "Going faster" isn't the issue, but instead, why
heavy is better than light. After all, both the heavy and the light
glider can fly the same speeds right up to Vne. It's because the heavy
glider can cruise faster for any given L/D, and (in strong conditions)
loses relatively little in the climb.

The aerodynamic reason it can cruise faster at any given L/D is that the
angle of attack determines the L/D, and the heavy glider needs extra
airspeed to generate the extra lift for that weight.

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

Eric Greenwell
Washington State
USA

[Rory O'Conor]

I cannot do 50 words, but how about this? - 750 words.
Rory

Why does a heavier glider have better performance?

Gliders fly through air and in the process create drag. Drag results in a
loss of energy. Due to the conservation of energy, if no energy is being
supplied from another source such as an engine or thermals, then either the
glider has to slow down and lose kinetic energy or the glider has to sink
and lose potential energy. In a steady glide, the glider maintains its
speed but loses some height and potential energy as a result of the drag.

The amount of drag created by a glider at a particular speed is not fixed. A
Cessna with a glide angle of 1:10 has a worse performance than a Nimbus with
a glide angle of 1:60. Not all 18m gliders weighing exactly 450kg and
flying at exactly 60 knots, will have the same performance. The amount of
drag is not predetermined.

There are two main types of drag: Induced drag and Parasitic Drag. Induced
drag is a by-product of creating lift. As the wings fly through the air,
they impart a slight downwash to the mass of air, which results in an
upforce on the wings. This lift is equal to the weight of the glider. It
varies with the angle of attack and the speed that the glider flies. In
essence, the same amount of lift can be created by either flying slowly at
large angles of attack or by flying fast at small angles of attack. A
ballasted glider is heavier, and will need more lift than the empty glider,
so it flies slightly faster at any given angle of attack to generate the
appropriate amount of lift. The relationships are not linear.

As the wings generate lift, they also generate induced drag in the form of
vortices in the air that the glider passes through. This drag results in the
glider losing some potential energy and sinking. The amount of induced drag
varies. It can be minimised by using longer wing spans, wing tips, shaped
wings and appropriate wing profiles, but it cannot be entirely eliminated.
The induced drag and vortices are particularly sensitive to the angle of
attack. At large angles of attack, the vortices are much stronger and the
induced drag much greater. So if you fly very slowly, the glider sinks
rapidly in a mushing stall, due to the induced drag. When flying fast, the
angle of attack is only a few degrees and the induced drag is less and has a
minimal effect on overall performance.

Because induced drag is a by-product of the generation of lift, a heavier
glider has more induced drag because it requires more lift. So at slow
speeds where induced drag predominates, the heavy glider has lower
performance, as shown by its greater minimum sink rate.

The second type of drag is Parasitic Drag. This is caused by air resistance
due to the shape of the glider as it flies through the air, and friction as
air molecules slide over the surfaces of the plane. This type of drag
becomes increasingly important, the faster the plane flies. In fact, the
parasitic drag increases in relation to the cube of the airspeed. A glider
whether empty or filled full of water, is the same shape and creates the
same amount of parasitic drag whatever its weight at the same speed. So
there is the same amount of energy to account for. However, the heavier
glider with more mass, has more potential energy and has to sink a smaller
distance to release the necessary energy.

There is a point, which happens to be the best glide angle when the
performance of the empty and fully ballasted gliders are the same. At this
point, 3/4 of the drag is due to induced drag and 1/4 due to parasitic drag.
The heavier glider will be flying faster. At speeds faster that this, the
performance of the heavier glider will be better than the light glider as
the parasitic drag predominates. There is also a speed, which is faster than
the best glide speed for the unballasted glider, but slower than the best
glide speed for the ballasted glider, where both gliders will have the same
sink rate at the same speed.

Thus the heavier glider may have better performance than the lighter glider
when gliding fast. How the overall cross-country performance of the
unballasted and ballasted gliders plays out, depends on other factors as
well, such as thermal strengths.

Rory 750 words.

[Ian Johnston]

On Tue, 11 Oct 2005 16:38:36 UTC, Pat Russell
> wrote:

> "A glider is a gravity-powered machine. The heavier it is, the more
> power it has."

So why don't two Grunau Babies side by side (you can join them with a
length of balsa if you want to) have better performance than one
Gruanau baby?

Ian

[Ian Johnston]

On Tue, 11 Oct 2005 18:31:13 UTC, Derrick Steed
> wrote:

> but loses all the effects due to Reynolds number which is an aerodynamic
> phenomena

Reynolds number is not an aerodynamic phenomenon. It's a dimensionless
quantity which is useful in characterising certain aerodynamic
phenomena, principally those which involve a laminar - turbulent
transition.

Ian

--

[Ian Johnston]

On Tue, 11 Oct 2005 22:51:26 UTC, "Jim Kelly"
> wrote:

> Thermalling stores energy in the glider, but thermals stop at
> cloud base. Water carried needs to be lifted too, thus more
> energy can be harnessed.

Doesn't cover the aerodynamics. The fact that you have to fly faster
to get the benefit is important.

Ian

[Derrick Steed]

> "A glider is a gravity-powered machine. The heavier it is, the more
> power it has."

So why don't two Grunau Babies side by side (you can join them with a=20
length of balsa if you want to) have better performance than one=20
Gruanau baby?

Ian

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

Now that _is_ a dumb question

The argument works for two aircraft which are identical in every respect
apart from their mass - two grunau baby's may be twice the mass of one,
but they also have twice the wing area, twice everything in fact
(except, possibly, the pilot) and so the argument doesn't work for that
situation.

Rgds,

Derrick Steed

[Derrick Steed]

> but loses all the effects due to Reynolds number which is an
aerodynamic
> phenomena

Reynolds number is not an aerodynamic phenomenon. It's a dimensionless
quantity which is useful in characterising certain aerodynamic=20
phenomena, principally those which involve a laminar - turbulent=20
transition.

Ian

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

Perhaps I should have said "Reynolds number which characterises certain
aerodynamic phenomena" and it is a fact that the slope of the lift
coefficient increases with increasing Reynolds number.



Rgds,

Derrick Steed

[Stewart Kissel]

Since in theory the point of carrying ballast is to
improve overall speed....I am curious as to the actual
performance improvement. The shift of the polar with
added ballast is rather straightforward, but not the
reduction in climb rate. Assuming something like
a 25% time of flight in climbing mode, my back of the
envelope calculations for a fully loaded modern ship
in strong condtions would gain somewhere around 7-8%
overall task speed improvement? Anyone smarter then
I care to refine this number?

[[email protected]]

It isn`t. It`s more complicated because interthermal speeds doesn`t
vary with the square root of % extra weight. In condor is a nice tool
which shows this very clearly.
Reality shows that filling a glider meens flying "a bit" slower than
sqrt(% extra weight), increasing range and the change for a real good
thermal.
The extra gain depend almost fully on the diameter of the thermal, with
1 m/s and huge thermals you can fly a racing glider almost full. And
what about the gain running cloud streets? Also not linear.

For huge thermals and weak cloudstreets of 1 m/s actual gain for an
Diana would be (theoretically) 40%.

Assuming 3 m/s in typical European (small) thermals shows a gain of
only 8%.

Life`s complicated ;-)





l/d max occurs when induced and pressure drag are the same, not at 75%
induced. (interference drag is (per definition) negociated)

When 75% of the drag is induced you`r flying at Vy-min (min sink)

One other point: the higher stall-speed is worth mentioning because of
difficulties with landing, thermalling, manouvrebility etc.

[Stewart Kissel]

Since in theory the point of carrying ballast is to
improve overall speed....I am curious as to the actual
performance improvement. The shift of the polar with
added ballast is rather straightforward, but not the
reduction in climb rate. Assuming something like
a 25% time of flight in climbing mode, my back of the
envelope calculations for a fully loaded modern ship
in strong condtions would gain somewhere around 7-8%
overall task speed improvement? Anyone smarter then
I care to refine this number?

[Ian Johnston]

On Sun, 16 Oct 2005 16:27:16 UTC, Derrick Steed
> wrote:

>> Reynolds number is not an aerodynamic phenomenon. It's a
dimensionless
>> quantity which is useful in characterising certain aerodynamic=20
>> phenomena, principally those which involve a laminar - turbulent=20
>> transition.

> Perhaps I should have said "Reynolds number which characterises certain
> aerodynamic phenomena" and it is a fact that the slope of the lift
> coefficient increases with increasing Reynolds number.

What do you mean by "slope of the lift coefficient"? With respect to
what?

Ian

[Derrick Steed]

>> Reynolds number is not an aerodynamic phenomenon. It's a=20
dimensionless
>> quantity which is useful in characterising certain aerodynamic=3D20
>> phenomena, principally those which involve a laminar - turbulent=3D20
>> transition.

> Perhaps I should have said "Reynolds number which characterises
certain
> aerodynamic phenomena" and it is a fact that the slope of the lift
> coefficient increases with increasing Reynolds number.

What do you mean by "slope of the lift coefficient"? With respect to=20
what?

Ian

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

the standard curve: plot lift coefficient against alpha (angle of
attack), for the same alpha flying at a higher speed increases the
Reynolds number, at this higher speed the slope is increased slightly
over what it was at the lower speed. It's a well known effect.


Rgds,

Derrick Steed

[Rory O'Conor]

Subject: Re: Aerodynamics of carrying water
Author: >
Date/Time: 17:30 16 October 2005
------------------------------------------------------------

>l/d max occurs when induced and pressure drag are the same, not at 75%
>induced. (interference drag is (per definition) negociated)

>When 75% of the drag is induced you`r flying at Vy-min (min sink)

------------------------------------------------------------

I stand corrected. Apologies.

http://www.av8n.com/how/htm/4forces.html#sec-powers

Rory

[Bruce Hoult]

In article >,
Rory O'Conor > wrote:

> Subject: Re: Aerodynamics of carrying water
> Author: >
> Date/Time: 17:30 16 October 2005
> ------------------------------------------------------------
>
> >l/d max occurs when induced and pressure drag are the same, not at 75%
> >induced. (interference drag is (per definition) negociated)
>
> >When 75% of the drag is induced you`r flying at Vy-min (min sink)
>
> ------------------------------------------------------------
>
> I stand corrected. Apologies.
>
> http://www.av8n.com/how/htm/4forces.html#sec-powers

Both the 50% and 75% numbers are "wrong" in the sense that either one
could be correct for some particular aircraft, but neither is correct
for all aircraft.


The correct statement is that l/d max occurs at the speed at which a
small change of speed (a small increase, say) causes an increase in the
parasitic drag and an exactly equal decrease in the induced drag. In
graphical terms, it is the point where the slope of one curve is the
same as the clope of the other, but one is going up and the other is
going down. And in the Fig 4.15 in in the link above that is pretty
clearly right around 65 knots or so.

And, yes, it appears that that is at the point, for that aircraft, where
about 75% of the drag is induced drag. But you could make some
modification to the aircraft that moved one (or both) of the curves up
or down (if you could do that without changing the shape of the curve)
and the minimum would still be at the same speed, but the proportion of
induced to total drag could be almost anything.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

[Chester D]

Bruce Hoult schreef:

> In article >,
> Rory O'Conor > wrote:
>
> > Subject: Re: Aerodynamics of carrying water
> > Author: >
> > Date/Time: 17:30 16 October 2005
> > ------------------------------------------------------------
> >
> > >l/d max occurs when induced and pressure drag are the same, not at 75%
> > >induced. (interference drag is (per definition) negociated)
> >
> > >When 75% of the drag is induced you`r flying at Vy-min (min sink)
> >
> > ------------------------------------------------------------
> >
> > I stand corrected. Apologies.
> >
> > http://www.av8n.com/how/htm/4forces.html#sec-powers
>
> Both the 50% and 75% numbers are "wrong" in the sense that either one
> could be correct for some particular aircraft, but neither is correct
> for all aircraft.
>
>
> The correct statement is that l/d max occurs at the speed at which a
> small change of speed (a small increase, say) causes an increase in the
> parasitic drag and an exactly equal decrease in the induced drag. In
> graphical terms, it is the point where the slope of one curve is the
> same as the clope of the other, but one is going up and the other is
> going down. And in the Fig 4.15 in in the link above that is pretty
> clearly right around 65 knots or so.
>
> And, yes, it appears that that is at the point, for that aircraft, where
> about 75% of the drag is induced drag. But you could make some
> modification to the aircraft that moved one (or both) of the curves up
> or down (if you could do that without changing the shape of the curve)
> and the minimum would still be at the same speed, but the proportion of
> induced to total drag could be almost anything.
>
> --
> Bruce | 41.1670S | \ spoken | -+-
> Hoult | 174.8263E | /\ here. | ----------O----------

Not true.

Drag varies with 1/Cl^2+V^2. converting to V means 1/V^2+V^2. Changing
that to 2/V^2+V^2 means a higher speed at which l/dmax occurs but both
are half. (Just try to plot it...)

Later more..

[For Example John Smith]

The beauty of the soapbox derby example--like Nigel's--is that it points out
that the glide slope is the same. With the added weight you have the same
glide slope but a higher speed.
The other way to make the soapbox derby car go faster was to reduce drag....
"Eric Greenwell" > wrote in message
...
> Nyal Williams wrote:
>
>> 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.
>
> And I suppose it would go faster uphill, with two people pulling it back:
> not a very good analogy, I think, since the heavy glider will climb more
> slowly. "Going faster" isn't the issue, but instead, why heavy is better
> than light. After all, both the heavy and the light glider can fly the
> same speeds right up to Vne. It's because the heavy glider can cruise
> faster for any given L/D, and (in strong conditions) loses relatively
> little in the climb.
>
> The aerodynamic reason it can cruise faster at any given L/D is that the
> angle of attack determines the L/D, and the heavy glider needs extra
> airspeed to generate the extra lift for that weight.
>
> --
> Change "netto" to "net" to email me directly
>
> Eric Greenwell
> Washington State
> USA