Airplane, Car, Relative Motion
If you are riding in a car and a radio controlled toy
airplane is flying outside, parallel to the car and you fly it
into an open window, would it just hover next to you?
Under normal conditions, the airplane would not continue to fly
but would settle in the seat next to you. The airplane requires
a relative wind moving over the wing to create lift. While flying
next to you, it has the same relative wind as your car. When it
moves into the car, it would now experience a relative wind based
on the air held in the car. It would not be sufficient to maintain
flight. If the car were large enough (really big) the plane could
accelerate in the inside air (moving as fast as the car and airplane)
and develop a flying speed. It would then need to remain this speed
within the car to continue flying.
The key to creating lift with an aircraft wing is relative wind. If
you find this interesting, you might want to explore wind shear and
the effect on aircraft.... particularly landing and departing aircraft.
Sounds great - and a great plot for a story, but unfortunately not
possible. The plane will travel relative to the air in which it is
flying. Inside the car the air is relatively calm, being pushed along
by the back window and protected by the windscreen. As soon as the
plane is inside the car it will fly at speed RELATIVE TO THE AIR IN
THE CAR and therefore relative to the car itself. I for one would
not wish to be in the car when you try this stunt - it could get
very messy and painful!!
Tennant Creek High School
Here is my mental experiment. I thought of two things:
If the radio controlled airplane is flying at the same speed the car
was traveling, it might be tempting to think that if it took a little
jog through the window, it would just hover there. However, the little
airplane still has to obey the laws of Physics including the whole
Bernoulli thing, you know, lift. As long as the air is passing over
the wings, the little guy has lift and can fly. Supposing it could
make it through the car's slipstream without getting off course or beat
up and get through the window, I think it would fall on the seat next
to you. Inside the car, you are behind a windshield, this will prevent
the airplane from gaining or maintaining lift. Now it might be
momentarily fly forward into the windshield as it moves through the
car's interior, but it will run out of room real fast.
Now I though,t how might this happen? I think you would have to be in
some sort of open-air carriage. A convertible has a windshield and I
am not real sure how far the air is interfered with. It might work.
Then I thought of something like a car going slowly, not highway speeds.
A horse drawn carriage would work. I am pretty sure you could get the
airplane to hover quite near in the carriage as long as the horse was
not bothered by the noise of the plane.
So those are my two answers to your thought experiment. If you ever do
test it, yo willl have to let me know how it all turned out. ( Please
don't try to drive and operate the RC plane, get a designated driver or
No, if a toy airplane flying along the side of a moving car flew in the
car’s window, it would be flying around inside the car. It would not
just be hovering because the airplane; because
The toy airplane only experiences the wind stream resulting from its flight
regardless of whether the wind stream is moving or not.
On a calm wind day
the ground speed of the toy airplane outside of the car would simply be
equal to its air speed: Its speed in the air.
Inside the car, ( if the toy airplane is still facing forward in the car)
the toy airplane’s ground speed would be its air speed in the car plus the
speed of the car over the ground.
The airplane needs air to flow over its wings to generate lift. The
lift due to the wind flowing over the wings is what allows the
airplane to fly. The air in the car is moving at (close to) the same
speed as the car. Inside the car it would no longer feel the wind, and
therefore would no longer generate lift. So no, it would not hover
next to you.
Also, as a practical concern, there is a lot of unpredictable air flow
around the car - as the plan approaches the car, the buffeting winds
close to the car would likely cause the plan to lose control and
Hope this helps,
As you slid your RC plane through the window into the car,
it might seem to hang next to you for a briefly as it started to
fall and accelerate forwards.
When you let an object go and it falls, does that seem smooth or abrupt to you?
Both points of view are OK, but either way,
that's about how fast the plane would start to move upon entering.
What is changing, as seen from the airplane's perspective, is the airspeed around it.
It goes from 60 mph to 0 mph in a second or less.
So then the wings cannot have any lift, so it stalls and falls.
And the air-drag disappears, so the propellers' steady thrust
gradually starts to accelerate the plane forwards.
It is not clear whether the propeller's thrust is more at 60mph airspeed or 0 mph airspeed,
but the drag just went from 100% of thrust to 0% of thrust,
so even with no change in thrust,
there is suddenly plenty of unbalanced force to cause acceleration.
I the car were huge enough for a plane to fly around inside,
say a blimp with an open hole in the side,
then the plane would eventually be flying 60mph within the blimp,
and burst out through the nose of the blimp.
After that the plane would find itself in 120 mph airspeed,
and accelerate backwards due to excess drag, until
it was flying ahead of the blimp at 60mph matching speed..
My point is that changing airspeed suddenly
ends the equilibrium of thrust and drag that the toy had before.
The plane will accelerate (positive or negative) to reach
a new equilibrium at 60 mph perceived airspeed
(or whatever speed you were flying at).
If the RC toy was helicopter instead of a plane,
it might do a bit differently in the vertical direction.
but the onset of forwards acceleration would still happen,
unless the helicopter adapted rapidly.
Modern RC helicopters might just adapt that fast,
because they all have on board electronic gyro-sensors
adjusting their attitude and thrust faster than the operator can,
and they could easily be given accelerometers too.
So then there would be some size of forwards lurch upon entering,
after which a good hover could commence.
Einstein might say that this is a good gedunken (thought) experiment.
If you think of the reasons why the plane might hover, you might state
the fact that the plane is traveling the same speed as it was outside
of the car and so it should continue to hover right next to you.
However, if you start to explore the reason why a plane can fly in
the first place, then you will discover that if you were lucky, the
plane would fall nicely into your lap and you would be able to fly
your plane another day. It would not in fact hover because its
ability to fly has been removed.
Planes fly because of the difference in pressure between the air above
the wing and the air below the wing. The shape of the wing is such
that it is pretty straight and flat on the bottom of the wing and it
is curved on the top of the wing. As a plane moves forward, the wing
"cuts" into the air and half of it must go above the wing and half
below the wing. The half that goes above the wing has to travel farther
because of the curvature. Without getting too technical, this causes
a lower pressure on the top of the wing and allows the higher pressure
below the wing to push the plane up. It is the shape of the wing and
the fact that wind is traveling so fast over and under the wing that
allows it a plane to fly. If your model plane was to fly into your
car through the window, it would stop the air from traveling so quickly
passed the wing. If you were in a convertible car the wind would still
be there and so it would still be able to fly.
The plane would fall to your lap. The wings of a plane do not “know”
how fast the ground appears to move. Flight is based on how fast the
air around the wings appears to move. If the plane moves fast through
the air, or if the air moves fast past the plane, then the plane flies.
After entering the car, the plane and the air around it are moving
together. From the plane’s “point of view”, the air is not moving.
A plane can fly while moving backwards relative to the ground, provide
the air is moving backwards much faster than the plane. A simpler
example is a kite. If the wind blows strongly, the kite flies in the
moving air without moving along a ground. If there is no wind, you can
fly a kite from a moving car. The kite flies while moving quickly
through the air.
Dr. Ken Mellendorf
Illinois Central College
The airplane gets its lift, the force that keeps flying from the speed of
the air moving over its wings. The air inside the car would be going at
the same speed as the car and the airplane. When the airplane came in the
window it would be moving at the same speed as the air inside the car. No
air would move over the wings. It would lose lift and crash.
A similar thing happened a number of years ago for real. A Delta jet was
landing at Dallas-Fort Worth airport. A strong wind from a thunderstorm hit
the plane from the rear and stopped the air moving across the wings from in
front. Even though the plane was moving at a pretty good speed, the wings
acted as if it were standing still. The plane lost lift and crashed. Even
though the plane was moving at a pretty good speed, the wings acted as if
it were standing still. The plane lost lift and crashed.
The same thing would happen to your plane as it came in the window - it
Hope this helps.
OSUIT, Okmulgee, OK
Click here to return to the Physics Archives
Update: June 2012