Aerodynamic lift.
So let's the look at the leaf drop pencil
test one more time.
See it, nice drifting motion of the leaf.
Now in the previous tutorial, we talked about
Bernoulli's principle which says that when
we have say air that is moving, or the faster
the air moves, the lower the pressure.
So if the air is not moving, we have normal
atmospheric pressure, if it's moving the pressure
decreases, and the faster it moves, the lower
the pressure.
Now one of the most important applications
of this BernoulliÕs principle is the creation
of lift for a wing.
So if we have the airflow that goes fast over
the top of the wing, and slower underneath
the wing, then we have lower pressure on top
of the wing and higher pressure underneath
of the wing, this creates pressure difference,
because of this pressure difference, we have
a force and we call that lift.
Now, wings have shapes that promotes this
difference of airspeed, but even something
which is flat like a sheet of paper if it's
moving at an angle with respect to the path
of action is at an angle with the orientation
of a sheet of paper, then we can actually
have the same wing effect of airflow being
at different speed, above compared to below,
resulting this pressure difference.
Although we think of lift as being an upward
force, it is not necessarily always the case.
In fact, the airfoil on a race car actually
is an upside down wing.
And so instead of pushing up to lift the car,
it actually pushes down, this is so advantageous
for race cars because they don't want to be
flying up, they want to have a better traction
with the ground without adding weight.
So these airfoils are useful.
Getting back to the drifting leaf, as leaves
are falling, we actually have various forces
acting, there's of course always gravity,
there is the air resistance that tends to
slow the motion, and then we have finally
the force of lift, and that can on possibly
deflect the path of action, it can also change
the speed.
If the path of action changes so that the
leaf is rising, then that means that the leaf
is going to slow down as long as we donÕt
have a wind.
If there is a wind, then itÕs a different
situation and force of the wind can be affecting
path of action, and the timing and spacing.
Now it's a very similar situation with a sheet
of paper, even though the sheet of paper is
flat, we still have these three forces of
gravity, air resistance, and lift.
If the paper curls, then we have even more
likely to have a type of wing affect with
the airflow being different on one side to
the other, promoting the lift.
Now you realize that the force of air resistance
depends on the speed and the orientation of
a leaf and so does aerodynamic lift.
So because both of those vary with speed and
the angle, they are changing all the time,
and that leads to a rather complex timing,
spacing, and path of action for a leaf as
it drifts to the ground.
So there are some complexity, the best you
can hope for to create a believable animation
is not so much to focus on the timing, spacing,
and path of action in of themselves, but to
imagine or actually feel the forces that would
be acting in terms of the gravity pulling
down, air resistance of slowing the motion,
and then lift potentially deflecting the path
of action as well as the speed.
So I feel a bit like Obi Wan, telling you
to use the force here, but if you try to animate
by feeling the forces that are acting and
then try to reproduce that timing, spacing,
and path of action, you're more likely to
be successful in animating this type of motion.
So let's look at the paper drop pencil test
because I want to mention another aspect of
this type of motion.
So right at the end, if you see the settle
as it slides on the ground, you notice the
effect whichÉhere we will see in this video
as I slide these sheets of paper on smooth
surface.
So you see this type of smooth gliding motion
that occurs right when the paper is just off
the surface of the lab table.
As an additional type of lift, which occurs
when we have something like a smooth sheet
of paper, very close to a smooth surface.
And this is called ground effect lift.
So the presence of this lower surface of the
table enhances the amount of lift that we
get so there's additional high-pressure a
cushion that is produced between the paper
and the table.
And so that cushion,that high-pressure just
supports the paper as it slides over the surface.
You'll typically see this with the leaf because
the shape of the leaf is too irregular, and
you won't see it of the surface is rough like
the grass or dirt.
One more example of lift, which is not present
with the leaf drop or paper, but I want to
mention because it's type of lift created
when an object is spinning.
So if we have, say, a ball that is spinning
then the spinning motion causes the airflow
to be faster on one side, where the flow goes
in the same direction as the spinning of the
surface, and the air flows slower on the other
side.
So the side where the air is going faster,
is lower pressure and then the other side
is higher pressure, this gives us a pressure
difference, and that leads to a force and
this is called the Magnus force.
And this affect is called the Magnus effect.
An example of this is when we have baseball
that's thrown and the baseball has a spin.
Depending on the orientation of the spin,
this can either cause the baseball toÉtend
to rise or tend to drop faster than it would
,under simply gravity.
Or if the spinning is in a vertical axis then
this thing cause the ball to be deflected
left to right or right to left, and this is
a curveball.
So in the summary, aerodynamic lift is a force
created by pressure differences arising from
Bernoulli's principle, so whenever airflow
is at different speed on different sides of
an object, say, we have this pressure difference
and a resulting force.
The force of lifts can either be in the upward
direction like a airplane wing, or downward
as the airfoil of a racecar, or sideways as
the curve ball.
The ground effect lift is an enhanced lift
that occurs when a flat object is flying just
over a smooth surface, like sheet of paper
over smooth table.
And finally, the Magnus effect is a special
type of lift that is produced by an objects
are spinning motion, the spinning motion causing
a difference of airspeed one side of the object
to the other.
Anyway, those are the basic forces that occur
as an object is moving through the air.
So hopefully that helps you animating that
type of motion.
