Aerodynamics of carrying water

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.

"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

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

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

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?

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.

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?

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

Subject: 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

In article ,
Rory O'Conor wrote:

Subject: 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----------