(bright music)
(airplane engine roaring)
- [Narrator] The Secret of Flight,
a series of programs on aerodynamics.
Program seven, The Problem of Drag.
Your host is Dr. Alexander Lippisch.
- Well, if we want to talk about drag,
aerodynamic drag today, you must be aware
that this is one of the most
intricate problem in fluid motion.
Many years of theory and scientific work
was spent on this question,
but we have still today basic problems
which are not well explained.
Now, if you think about the drag of a body
which moves through
the air, you would say,
all right, the drag is
caused by the friction
of the air against the body.
All right, but what is friction?
Well, if you look it over,
friction is the resistance
between two bodies moving
against each other.
Here's the air against the body.
All right.
Then the resistance is the drag,
so there you are again.
And what we have to do is
we have to explain friction,
aerodynamic friction.
Let's see how we could do it.
Well, we think for instance of,
all of the theories are stolen.
And think of this motion.
Now, you might realize that if the stone
goes through the air,
there might be some air particles
which are still attached to the stone.
So, the stone makes
molecules go through the air.
A certain motion in the air.
It propels a column of air
which is behind the stone.
And runs then with the stone.
So, well, if I can explain
it to you on a blackboard.
It would be probably this.
Here's the stone,
and the thing is moving
in some, this direction.
Then, we have a certain amount of air,
which is still attached to the stone
due to the fact that little particles
stick on the surface.
And when the stone is there,
then there is a column
of air behind the stone.
Which is filled with air,
which is moving also in this direction.
The stone is moving in this direction,
where the air is at rest.
You can actually feel this
if you point and stand there
while a train goes by
or a car.
You can see it
in the dust cloud,
which is behind such a car that goes
over some of our old gravel roads.
But you can see it even better.
If we look at the smoke tunnel.
Now, I don't have a stone
in the smoke tunnel.
I have a nice round circular cylinder.
But it is,
a good presentation
and a good example to look at this.
Here it is.
Now,
there
we see this
piece of air which is more
or less attached to the body.
But we see one other thing.
Very peculiar.
This is an oscillation
in the wake of the body.
You know, we call this part,
this air, which is moving with the body,
we call this the wake.
Let's write it here.
The wake of the body, the
wake in the flow of air.
And in this wake, we have here apparently
a very distinct oscillating motion.
Now,
we might not see it clear
with this normal speed.
But we can see it much better if we take
of this flow picture a high speed movie
and then look at the high speed.
And there it is.
Now look at this.
Isn't that fascinating?
This regular motion
of a generation
of
large velocities,
which are now coming off sporadically,
one at a time from the upper or the lower
corner of this round body.
Such a phenomenon was first investigated
by Professor von Karman,
and that's why we call this wake,
which we call also a vortex
street, the Karman street.
Now, the generation of these vortices
certainly needs lots of energy,
and therefore the drag is
not only here tension drag,
it's actually a very
small part of the drag.
But the main part of this drag
is caused by the
generation of the vortices.
Because there's energy
needed to make such vortices.
And this energy must
correspond to the drag times
the velocity of this motion
of the body through the air.
Now, if you want to stop
this, what could we do?
Well, you could do one thing.
You could probably make a
plate flexor on the body,
which then separates
both parts of the flow,
and in this way then outputs oscillation.
So let's try this in this tunnel.
Now we have here that
cylinder with air plate in it,
which we call a baffle
plate, so that we would stop
the oscillating motion of these vortices.
You can see it here very well.
You see now this part is mainly uniform,
and there is no oscillation.
But still at the wake far
behind the oscillation begins.
It actually is something
which has some similarity
to the scuttling of a flag.
And if you wanted to
reduce now this large wake,
which is still in here, and
which gives us a dead air zone,
which are, again what cause
drag, we have to fill it out.
And
make this part of the body.
That means you have to do something here.
So what you would come finally to a round
piece which has a flaring behind.
And if you smooth this out a little bit,
then you get a shape which
is approximately like this.
And which we call a streamlined shape.
You will see this as our
next test in the tunnel.
And let's just put it in there.
It takes just a few minutes to mount
this streamlined section,
which you see here.
You see?
This is now
derived from
originally a round part,
which is then a little bit
elliptical flared out
here, but mainly has a long
and smoothly decreasing tail end.
This is much more important than this.
Because we have seen on the round part
that the flow here on the
front part was all right.
We had only trouble at the rear.
So if we make a blunt nose,
this wouldn't bother us.
But we have to flare it
out nicely and make a quite
long tailpiece on it so
that the air can follow
such a surface smoothly like this.
Now, let's look at this in the tunnel.
Start the blower and the lights.
And there we are.
You see, now,
I might adjust this a little bit.
You see that we have now stopped
the vortex motion almost completely.
And we have only a wake
which contains now here
air which moves with a lower
velocity than the outer one.
Because due to the
friction on this surface,
the air which has narrowed to the surface
has flowed down and has lost,
has lost a certain part of its momentum.
On the other end we see that a streamline
which is near to the
body now suddenly begins
to jiggle and to flare out.
We call this the transition
from a smooth liminal flow
to a turbulent flow condition.
Which we may see even
much better if we look now
to pictures which we have made in our 3D,
three dimensional tunnel.
Now, before we see these pictures
on the three dimensional tunnel,
I probably have to explain to you
a little bit how this tunnel works.
You see the tunnel which we have here
is a two dimensional
tunnel, we call it this way
because we have just a
narrow strip of smoke flow
which is closed in by
two narrow glass walls.
Now, a three dimensional tunnel
is a large cross-section,
which is actually somewhat rectangular.
And then we mount the model
from one side of the wall.
And here we have the flow through it.
So this is kind of a box enclosed
where the flow goes through.
And the model itself, which
you will see in these pictures,
are, for instance this straight wing,
you'll see just a half
of a straight rectangular
aircraft wing, which is now painted black
so that we can see the
smoke lines very well.
And we release the smoke
on this leading edge.
You see, we have very
fine holes all along here.
And part of the holes along this tip.
And we blow now the smoke into the model
through this tube, where
we've also mounted the model
so we can change its angle of attack.
Another model which you will
see is this swept wing model,
which is mainly the same
kind of construction.
You see, with the fine holes
on the leading edge,
the smoke is introduced
from the side into the hollow
interior of this model.
Now let's look at one of these pictures.
We first see
the straight wing mounted in the tunnel.
And we see it with normal speeds.
When you see it with normal
speeds, you don't see much.
But when you look at this picture now,
where we have a high speed
movie or slow motion picture,
with about 1500 frames per second,
then we see suddenly
the transition between
the smooth flow at the
beginning of the surface
and the turbulent flow
which is then at the rear.
At the swept wing, we have, in some way,
a different condition,
because due to the swept back,
the flow is not uniform
all over the surface.
But at different stations of the wing,
we have different conditions.
Which we see very well in
one of the next pictures.
This is Cessna.
And there you see this wing from above.
And you see that the
transition at the outer part
begins much earlier than at
the inner part of the wing.
Now here you see it from the side view.
And I think you should
observe this picture very well
which shows us so clearly that we have
first waves on the surface.
And then these waves overroll
and finally break up,
like the breakers on a soft flow.
And then the turbulent motion
over the surface of the wing begins.
Actually, this is not falling.
It still sticks to the surface,
but with this turbulent motion.
Now, to see this even better,
and to explain it more clearly
we have put a large straight
wing in the three D tunnel,
and you see here a picture
of this large tunnel,
large wing mounted in the tunnel.
You see it here from the side view.
And it is just a picture
to see how it is arranged.
When we now look more close on this model,
then we see again this wave motion,
which is very distinct.
And
actually is one of the most
intricate problems of surface flow.
Why is it so?
Why are waves formed?
And why are then these waves suddenly
broken up into a turbulent motion?
When we want to see this more clear,
we can now look at one smoke ribbon.
So before we see this
picture of the smoke ribbon
on the surface of the
wing, I will explain it
a little bit to you because it's,
in some way, difficult to see.
On another way, this picture
is a very important one.
And actually is the first visual
observation of such a process.
You see we have the
surface of the wing here.
And we have a small smoke ribbon
which just floats above the surface.
Now, while it floats over,
part of it is decelerated.
It's slowed down due to the friction.
So this stays back, and finally,
the outer part rolls over and forms here
first waves, it rolls over, forms waves,
and then rolls over
completely, forms a vortex,
which now this attaches
and goes through here.
And instead of now continuing as a vortex,
due to conditions of instability,
this thing explodes and forms a big bubble
of turbulence, a turbulence bubble
which goes over the surface.
Now when you see this
picture, you must think
that since this is white smoke,
and we have a black surface
which is very smooth,
you see also the mirage,
the reflection of this
white smoke on the surface.
So actually you see the smoke
itself and its reflection.
So don't bother.
Now there's a picture.
Only the upper part is the smoke,
and the lower part is the
reflection on the black
surface of the body.
Here you see it very clearly.
This picture was taken with
3,500 frames per second.
And we are actually very proud
that we achieved this picture.
I'm very thankful to the
photographic department
of the Color Trading Company
who helped us with this.
And to my assistant, Mr. Schroeder,
who made all the
arrangements for this test.
Now, if you see all this,
and you see these friction on the surface,
on this breakup of the bottom layer,
then you think, what could
you probably do to improve it,
and to prevent first a large turbulence,
on the other hand, to
reduce the total drag
due to friction on such a surface.
Well,
first of all, you could do this.
Since the lower layer on
the surface is retarded
and is practically dead
air, you could suck it away.
That means you could take a surface
and have a little slot in it.
And when then this layer,
which is retarded here,
comes to this point,
you suck it away, and,
flow which has no momentum
comes back on the surface.
In this way you would reduce
the tendency of the
surface flow to break up.
Another method would be
to do the reverse thing.
That means you would make a
slot here which would blow
a certain amount of fresh
air into this layer.
And in this way propel it.
Now we have here a model in
the tunnel where we might
show you how surface suction works.
This model is a streamlined section.
And we have slots in it.
And through the slots we suck.
Now watch it.
When we turn the section on, there it is.
(airflow screeches)
Then at once you see that the
wake becomes much smaller.
And you see also very
distinctively the steps
of this streamline, they
are part of the dead air
which is on the surface,
is sucked into the model.
Now we have a Boeing
model in the wind tunnel.
And I have made this extremely blunt.
That means I have made
a streamlined section
which is really reduced to a
very small size and very big
so that you might see how
such action really works.
When we want to try now
to turn on the blowers,
then you will see that
we can attach the flow,
and we get a very nice and
smooth flow over this surface.
Even if I give angle of attack,
then you see that the flow
still holds on such a very
narrow curvature of the body.
You see in this way you can
reduce the drag considerably.
The transition from this state
with the large vortices and the section,
we have also taken a slow motion picture,
which you see now here.
And you might observe very
clear the sudden reduction
of this wake and then the
closing of the streamlines.
But when you think this
vortex formation is mainly
the major source of drag,
and if we look at a wing,
at a full wing with all
its span in the tunnel,
then you might observe
another cause of vortices,
which we will deal with
in the next lecture,
but I might show you just a few
examples of this type of a flow.
To show you this formation
of vortices which make drag
due to lift, we have here
a model in the wind tunnel
which represents a small wing.
And you will see that if I increase
the angle of attack and
the lift of the wing,
then you see suddenly a formation
of large vortices from the wingtips.
Look at this here.
Why is it so?
Well, it's due to the fact
that we have underneath
of the wing a higher
pressure than above the wing.
So on the wingtips, this
pressure differential
must equalize by flowing
around the wingtips.
And this flowing around the wingtip
with a translation of the forward motion
gives you a spiral motion
which forms this vortex.
Now if I give negative lift on this wing,
that means if I turn it this way,
then you see the vortex
rolls the other way.
But you can see this
formation even better if we go
to a model which shows us only
the wingtip in the tunnel.
Now you see this vortex
formation on the tips
of the wing cannot be done by nothing.
You have to put new energy into the flow
which makes the thing rolling.
And this energy has to be provided
by the propulsion system of the aircraft.
This drag, which is
caused by the wingtips,
by this flow due to lift,
is called induced drag,
because it is induced by lift.
Now we have this wingtip
model here in the tunnel.
And when we give angle
of attack to the model,
then you will see very clear
how such a wingtip forms.
I think it's quite
exciting to see how nicely
even such a flow line stays together
while it turns violently around.
You see it is no friction there.
It's just a rotational motion
which is introduced here into the flow.
Let's give it some more angle of attack.
You see this holds for
quite a while together.
But this is probably the normal position
as it is formed on the wingtip.
Now, in the next program,
we will have many examples
of this type of flow phenomenon.
Not only on a straight
wing, but on the modern
sweptback wing, on other wings.
And I will show you quite some exciting
pictures on this method.
I hope you will not miss this program.
On the other hand,
we will have pictures from
the three D tunnel too.
So until the next program, goodbye,
and (speaks foreign language)
(bright music)
- [Narrator] You have just
seen The Problem of Drag,
the seventh in a series of programs
explaining The Secret of Flight.
Your host has been Dr. Alexander Lippisch,
director of the aeronautical
research laboratories
of the Collins Radio Company.
This program was produced for
the Educational Television
and Radio Center by the
State University of Iowa.
(airplane engine roars)
(bright music)
