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.