Can a Plane on a Treadmill Take Off?

"Greg Copeland" wrote

Sorry....the plane would not fly as you described. For if your
statement were true, we would not need wings...only a engine and a
prop. We must have air flow over the wings to generate lift. Propwash
does not generate enough lift, especially for planes wihch have a
centrally located engine/prop between the wings.

Nitpicking aside, I suspect that everyone agrees that in order for the plane
to take off it must move forward along the conveyor. Since the prop applies
a force to the plane which acts independent of what the conveyor and the
wheels are doing, the plane can definitely move forward, and therefore it
can take off.

Tie a rope to the plane and to your car which is parked in front of the
conveyor and not on the belt. Start the conveyor and run the belt at any
speed you wish. The plane sits still on the conveyor as the wheels spin
away. Now, if you drive your car forward the plane will move forward along
the conveyor at the speed that you are driving your car forward, regardless
of how fast the conveyor belt is moving. The conveyor cannot keep the plane
from moving forward, it can only spin the plane's wheels. The example says
that the belt moves backwards at the same speed that the plane moves
forward, but that doesn't mean that the plane must be standing still.

Get rid of the rope and the car and use the prop and the engine to pull you
forward along the conveyor (because it pulls you by exerting a force on the
air) and voila, you're flyin' the friendly skies.

BDS

Nitpicking aside, I suspect that everyone agrees that in order for the plane to take off it must move
forward along the conveyor.

Which is exactly my point! If you have a motoroized conveyor which
always reduces the plane's forward movement to zero, no lift is
generated, preventing the plane from flying. In other words, the plane
generates lift by moving air over its wings. It moves air over its
wings by moving forward. If you zero out forward movement, by a
motorized conveyor, resulting in a zero delta, no lift is generated.
it's a question of the plane making forward movement.

Specifically, as it relates to your reply, while prop wash would indeed
produce some lift over the wing, it would not be nearly enough to
obtain take off....which is why we have wings. Thusly, if forward
movement is zero and you're full throttle, you're not airbound....which
is exaclty the same thing as a plane with no wings.

"Greg Copeland" wrote
Nitpicking aside, I suspect that everyone agrees that in order for the
plane to take off it must move
forward along the conveyor.

Which is exactly my point! If you have a motoroized conveyor which
always reduces the plane's forward movement to zero

It doesn't say that the plane has no forward movement - it says that the
conveyor moves backwards at the same speed at which the plane moves
forward - that does not prevent the plane from moving forward.

Let's do the old Einstein thought experiment. Let's presume a motorized
conveyor belt that is being rotated towards the departure end of the runway.
Let's also presume frictionless wheel bearings in an aircraft sitting at the
departure end of the runway on the conveyor belt.

What happens to the aircraft? Nothing. THe airplane remains motionless
because the aircraft wheels, which are rotating, do not impart any force to
the aircraft to make it move in any direction (F=ma). To a bystander
sitting on the taxi light at the end of the runway, the conveyor belt is
moving left to right, the wheels are spinning in a counterclockwise
direction, and the aircraft itself is motionless.

Now introduce wheel bearing friction. The aircraft will begin to slowly
move left to right as a function of how much friction there is. Fire up the
propeller and give it just enough throttle to overcome wheel bearing
friction. Again, the observer on the taxi light sees the aircraft
motionless.

Now give it full throttle. Not only do we now have enough thrust to
overcome wheel bearing friction, we have more than enough to launch the
aircraft successfully into the air.

If ya can't see this, I give up.

Jim




"BDS" wrote in message
t...

"Greg Copeland" wrote
Nitpicking aside, I suspect that everyone agrees that in order for the
plane to take off it must move
forward along the conveyor.

RST Engineering wrote:
Let's do the old Einstein thought experiment. Let's presume a motorized
conveyor belt that is being rotated towards the departure end of the runway.
Let's also presume frictionless wheel bearings in an aircraft sitting at the
departure end of the runway on the conveyor belt.

What happens to the aircraft? Nothing. THe airplane remains motionless
because the aircraft wheels, which are rotating, do not impart any force to
the aircraft to make it move in any direction (F=ma). To a bystander
sitting on the taxi light at the end of the runway, the conveyor belt is
moving left to right, the wheels are spinning in a counterclockwise
direction, and the aircraft itself is motionless.

Actually, that isn't true. You don't need wheel bearing friction to
apply a horizontal force to the wheel at the contact point. The wheel
has inertia and accelerating the wheel will cause a reaction at the
contact point with the belt and the aircraft will begin to move along
the direction of the conveyor. This force will go to zero once the belt
reaches a steady-state speed, but the aircraft will continue to move
along with the belt.

Now if the wheels have no mass as well as no bearing friction... :-)


Matt

Which is exactly my point! If you have a motoroized conveyor which
always reduces the plane's forward movement to zero, no lift is
generated, preventing the plane from flying.

That's not what the original problem stated. And we have explained at least
ten times why the conveyor belt CANNOT prevent the plane from moving
forward. My attempt, an earlier post:

You are taking the statement 'a conveyer belt that moves in the opposite
direction at exactly the speed that the airplane is moving
forward' to mean that somehow there is a force being applied to the mass of
the aircraft, equal and opposite the thrust generated by the propellor. The
only place the treadmill can exert any force an the airplane is the only
place the treadmill is touching the airplane: the wheels. Any motion of the
treadmill belt will be translated into rotation of the wheels. This will not
prevent the aircraft from moving forward, through the air and taking off.

Greg,

If you have a motoroized conveyor which
always reduces the plane's forward movement to zero,


Nowhere does it say that in the question.

--
Thomas Borchert (EDDH)

On the contrary, the problem clearly states that the conveyor is moving at
the speed that the plane is moving (but in the opposite direction). If the
plane is not moving, the conveyor would not be moving. If the conveyor is
moving, the plane MUST be moving. If it's moving fast enough, it will take
off.

--
Bob (Chief Pilot, White Knuckle Airways)


"Thomas Borchert" wrote in message
...
Greg,

If you have a motoroized conveyor which
always reduces the plane's forward movement to zero,


Nowhere does it say that in the question.

--
Thomas Borchert (EDDH)

"Bob Chilcoat" wrote in message
...
On the contrary, the problem clearly states that the conveyor is moving at
the speed that the plane is moving (but in the opposite direction).

The statement "the conveyor is moving at the speed that the plane is moving"
(the one you say the problem states) is a very different statement from
"which always reduces the plane's forward movement to zero" (the statement
Thomas responded to).

How is your post contrary to Thomas'?

Pete

Not true. The original problem statement was this:

"An airplane on a runway sits on a conveyer belt that moves in the
opposite direction at exactly the speed that the airplane is moving
forward. Does the airplane take off?" (Assuming the tires hold out, of
course.)

I restated it, but either version clearly implies that the airplane must be
moving for the conveyor to be moving. Sure if the plane isn't moving, it
won't take off. But if the conveyor is moving, the plane must be moving in
the opposite direction, because that's what the problem says. The problem
never said that the conveyor somehow moves in a way that cancels the plane's
forward motion.

As many have stated, unless the pilot is applying the brakes, so that there
is friction between the wheels and the belt, the movement of the conveyor
has nothing to do with the speed of the plane. All the problem says is that
the conveyor is moving in the opposite direction at the same speed as the
plane.
--
Bob (Chief Pilot, White Knuckle Airways)


"Peter Duniho" wrote in message
...
"Bob Chilcoat" wrote in message
...
On the contrary, the problem clearly states that the conveyor is moving
at the speed that the plane is moving (but in the opposite direction).

The statement "the conveyor is moving at the speed that the plane is
moving" (the one you say the problem states) is a very different statement
from "which always reduces the plane's forward movement to zero" (the
statement Thomas responded to).

How is your post contrary to Thomas'?

Pete