Can a Plane on a Treadmill Take Off?

"alexy" wrote

Reread the stated problem:

"a conveyer belt that moves in the
opposite direction at exactly the speed that the airplane is moving
forward."

All it is, is a trick question, aimed at testing your reading and
comprehension ablility. The plane moves off in exactly the same manner as
on a regular runway. The moving belt is a distracter.
--
Jim in NC

Of course! the plane does take off! revelation!! if you kept the
throttle low enough to equalize drag on the wheels from the belt, you
wouldnt go anywhere. Firewall it, youre outta there! like a touch n go,
if you have the throttle retarded when you touch down, your in a
similar state, wheels spinning, add throttle you accelerate, even
though the runway increases the speed of the wheels. doesnt matter if
you start out at zero KIAS, KTAS, GS or any other. the prop applies a
force to the surrounding air, the wheels spin freely underneath you, of
a take off occurs... ok, its so simple. if the conveyor matches the
wheel speed so what, youre off still. if it matches air speed, so what
youve got rollers to get you off that spinning track! cool, i get it
now... my bad ....

As posed by the question, I agree that the belt is a distracter. However, it
is not irrelevant as the thrust applied to achieve flying speed must
overcome not only the normal tire rotation friction but twice that. That the
plane will fly presumes that there is enough excess thrust to do just that.

--
-------------------------------
Travis
"Morgans" wrote in message
...

"alexy" wrote

Reread the stated problem:

"a conveyer belt that moves in the
opposite direction at exactly the speed that the airplane is moving
forward."

All it is, is a trick question, aimed at testing your reading and
comprehension ablility. The plane moves off in exactly the same manner as
on a regular runway. The moving belt is a distracter.
--
Jim in NC

Actually, I believe you are incorrect. That passage only implies the
belt will move inversely proportional to the wheels on the plane;
meaning the delta is conantly zero. They never actually state the
plane is moving forward. Thus, with a wheel speed of zero, the delta
is still zero. Therefore, I would suggest the only correct answer,
based on that sole passage is, "no".

Now, if we look at the actual question posted on straight dope, the
answer is, "maybe", for many reasons. Having said that, we can
*easily* tear up the explanation provided on straight dope. Simple
fact is, Cecil makes a huge assumption which causes his whole house of
cards to come tumbling down. Simply stated, there is nothing which
precludes the belt from being motorized. Given a sophisticaed enough
implementation, one can absolutely state, the plane would never leave
the ground; assuming no head wind is involved.

Long of the short, there is simply not enough information to provide an
exact answer, other than maybe. If the belt is not motorized, AND the
plane is moving (positive delta), yes, the plane will fly. If the belt
is motorized, and it is intelligent enough to ensure a belt/wheel delta
of zero, no, the plane will never fly.

"Greg Copeland" wrote:

Actually, I believe you are incorrect. That passage only implies the
belt will move inversely proportional to the wheels on the plane;
What problem are you reading? The one I saw said "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." I didn't see
anything about wheel speed mentioned or implied. If you inferred it,
I'd suggest that is a product of your inference, not of the author's
implication.

meaning the delta is conantly zero.
What delta?
They never actually state the
plane is moving forward.
That's true. One does have to make the radical assumption that the
question is not about a plane parked on a non moving treadmill.

Thus, with a wheel speed of zero, the delta
is still zero. Therefore, I would suggest the only correct answer,
based on that sole passage is, "no".

Now, if we look at the actual question posted on straight dope, the
answer is, "maybe", for many reasons. Having said that, we can
*easily* tear up the explanation provided on straight dope. Simple
fact is, Cecil makes a huge assumption which causes his whole house of
cards to come tumbling down. Simply stated, there is nothing which
precludes the belt from being motorized.
In fact, I would argue that it would need to be motorized to go
backwards at the same speed at which the plane is moving forward. In
what way does that "cause the whole house of cards to come tumbling
down"?

Given a sophisticaed enough
implementation, one can absolutely state, the plane would never leave
the ground; assuming no head wind is involved.

Long of the short, there is simply not enough information to provide an
exact answer, other than maybe. If the belt is not motorized, AND the
plane is moving (positive delta), yes, the plane will fly. If the belt
is motorized, and it is intelligent enough to ensure a belt/wheel delta
of zero, no, the plane will never fly.
Of course, if it did that, it would assure that the plane did not move
forward, thus meaning that the belt would not move forward, which is
back to your rather uninteresting parked plane analysis. I think it
far more likely that the problem intended what it said, that the
conveyor moves backward at the same rate the plane moves forward,
rather than your different problem statement having to do with wheel
speed.


--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

There will be airspeed felt by the plane in the area of the propellor
wash. Out of the prop wash, since the plane is not moving relative to
the surrounding air, there will be no relative wind felt by the
airplane. The airplane MIGHT take off, but it will not be a normal
takeoff, at least not for a normal airplane with 36' wingspan and a 6'
propellor. I really don't think there is enough information to know the
answer for sure. Too many unknowns.

This is one of these "frame of reference" problems.

"Doug" wrote in message
oups.com...
There will be airspeed felt by the plane in the area of the propellor
wash. Out of the prop wash, since the plane is not moving relative to
the surrounding air, there will be no relative wind felt by the
airplane. The airplane MIGHT take off, but it will not be a normal
takeoff, at least not for a normal airplane with 36' wingspan and a 6'
propellor. I really don't think there is enough information to know the
answer for sure. Too many unknowns.

This is one of these "frame of reference" problems.

Re-read it, Doug. The plane is not getting its motion from the wheels, so
it matters not what the wheels or wheel speed is doing.
--
Jim in NC

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.

"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.