How do airplanes actually fly? - Raymond Adkins
By 1917, Albert Einstein had explained
the relationship between space and time.
But, that year, he designed
a flawed airplane wing.
His attempt was based
on an incomplete theory of flight.
Indeed, insufficient and inaccurate
explanations still circulate today.
So, where did Einstein go wrong?
And how do planes fly?
Though we don’t always think of it
this way, air is a fluid medium—
it’s just less dense
than liquids like water.
Things that are lighter than air
are buoyant within it,
while heavier objects require an upward
force, called lift, to stay aloft.
For planes, this force is mostly
generated by the wings.
One especially pervasive false
description of lift
is the “Longer Path”
or “Equal Transit Time” explanation.
It states that air molecules traveling
over the top of a curved wing
cover a longer distance than those
traveling underneath.
For the air molecules above to reach
the wing’s trailing edge
in the same instance as those
that split off and went below,
air must travel faster above,
creating a pocket of lower pressure
that lifts the plane.
This explanation has
been thoroughly debunked.
Air molecules floating above and below
the wing don't need to meet back up.
In reality, the air traveling
above reaches the wing’s trailing edge
much faster than the air beneath.
To get a sense of how lift
is actually generated,
let's simulate an airplane wing in motion.
As it moves forward, the wing affects
the movement of the air around it.
As air meets the wing’s solid surface,
a thin layer sticks to the wing.
This layer pulls the surrounding
air with it.
The air splits into pathways
above and below the wing,
following the wing’s contour.
As the air that’s routed above makes
its way around the nose of the wing,
it experiences centripetal acceleration,
the force you also feel
in a sharply turning car.
The air above therefore gathers more
speed than the air traveling below.
This increased speed is coupled with a
decrease in pressure above the wing,
which pulls even more air
across the wing’s upper surface.
The air flowing across the
lower surface, meanwhile,
experiences less of a change
in direction and speed.
The pressure across the wing’s
lower surface is thus higher
than that above the upper surface.
This pressure difference results
in the upwards force of lift.
The faster the plane travels,
the greater the pressure difference,
and the greater that force.
Once it overcomes the downward force
of gravity,
the plane takes off.
Air flows smoothly around curved wings.
But a wing’s curvature is
not the cause of lift.
In fact, a flat wing that’s tilted
upwards can also create lift—
as long as the air bends around it,
contributing to and reinforcing
the pressure difference.
Meanwhile, having a wing
that’s too curved or steeply angled
can be disastrous:
the airflow above may detach
from the wing and become turbulent.
This is probably what happened
with Einstein’s wing design,
nicknamed “the cat’s back.”
By increasing the wing’s curvature,
Einstein thought it would
generate more lift.
But one test pilot reported
that the plane wobbled
like “a pregnant duck” in flight.
Our explanation is still
a simplified description
of this nuanced, complex process.
Other factors, like the air that’s flowing
meters beyond the wing’s surface—
being swept up, then down—
as well as air vortices formed
at the wing’s tips,
all influence lift.
And, while experts agree that
the pressure difference generates lift,
their explanations for how can vary.
Some might emphasize the air’s behavior
at the wing’s surface,
others the upward force created
as the air is deflected downwards.
However, there's no controversy
when it comes to the math.
Engineers use a set of formulas
called the Navier-Stokes equations
to precisely model air’s flow
around a wing
and detail how lift is generated.
More than a century after Einstein’s
foray into aeronautics,
lift retains its reputation
as a confounding concept.
But when it feels like it’s all going
to come crashing down, remember:
it’s just the physics of fluid in motion.
This video was made possible with support from Marriott Hotels. With over 590 hotels and resorts across the globe, Marriott Hotels celebrates the curiosity that propels us to travel. Check out some of the exciting ways TED-Ed and Marriott are working together, and book your next journey at Marriott Hotels.
This video was made possible with support from Marriott Hotels. With over 590 hotels and resorts across the globe, Marriott Hotels celebrates the curiosity that propels us to travel. Check out some of the exciting ways TED-Ed and Marriott are working together, and book your next journey at Marriott Hotels.
