Debate!

[eno]

I wonder wether this one will go to 8 pages.  :-? :-?

[expat]

I wonder wether this one will go to 8 pages.  :-? :-?

God, I hope not.

Matt

[Brett_Henderson]

This debate is troublesome, because it's far more complex than it appears.

If the wheels are truly just "free-wheeling".. you could turn the conveyor on (any direction) while the engines were shut down and the plane would stay put while the wheels just spun. We know that wouldn't happen.

We also know that with the engines shut down, the conveyor could gradually and gently accelerate the plane to take-off velocity and "launch" it like a glider (or could it ?

[Jakemaster]

I still don't quite understand the nature of the conveyer.  If it is say a treadmil, very short and going the same speed as the plane so the plane is like a runner on a treadmil, then no, it won't get any airflow.  If its just a really really long conveyer, then yes the plane will take off assuming the engines can run at takeoff thrust long enough and the wheels can withstand the speeds.  

[EGNX]

Right i'll try to go into more detail...

The conveyor belt is x miles long....
The conveyor belt counteracts the motion of the wheels. i.e As the wheels roll forward the conveyor moves back.
Try not to think about the aircraft limitations, including tire limitations and engine endurances just yet...

All you need to picture is any aircraft (I just said a 747 for ease) on a conveyor where the motion of the aircrafts wheels are counteracted by the conveybelt, (Sort of like a person on a tread mill) But will the aircraft ever take-off?

Some posts here are along the right lines but missing some key points....

[expat]

Right i'll try to go into more detail...

The conveyor belt is x miles long....
The conveyor belt counteracts the motion of the wheels. i.e As the wheels roll forward the conveyor moves back.

Call me sad, but I went to work on my off day to experiment. I fly electric RC amongst other things. However for scale it works. I went down to the baggage handlers and got the use of a baggage loader (we are a small airport with long periods of not much happening :-/). At one end I placed my aircraft, a Multiplex Mini-Mag and started the engine. The baggage handler, very bemused, at my signal turned on the loader. As we are only interested if we have airflow to produce lift, I also hung some cotton down from the wings to check for air flow. We now have is a scale version of the question. If we agree for the sake of scale that the maximum speed of the loader equals the maximum thrust of my prop, as in the original (see above), I advanced the throttle to equal the loader and.............the aircraft (model) stayed in the same place as expected (rolling road effect). The cotton hung straight down from the wings showing no airflow over the wings, showing that flight would not take place.

As in Myth Busters I await the......yes but..........and you forgot.............

Matt

[Chris_F]

This question is subtly different from last time.

Last time the conveyor in question went the same speed as the plane, but in the opposite direction.

[Hagar]

[quote]This question is subtly different from last time.

Last time the conveyor in question went the same speed as the plane, but in the opposite direction.

[expat]

This question is subtly different from last time.

Last time the conveyor in question went the same speed as the plane, but in the opposite direction.  So the plane still had airspeed but the wheels were turning twice as fast since the conveyor was running opposite.  This time the conveyor is counteracting the wheel speed meaning the wheels aren't moving.  So the conveyor is traveling WITH the airplane.  The plane has airspeed but no ground speed (as opposed to a ground speed of twice the air speed).

Still, it makes no difference what the ground speed is.  It's the airspeed that's critical and since the airplane has air speed in both examples it will take off.  The confusion is that people assume the runway runs fast enough to keep the plane motionless yet this isn't stated in the problem statement therefore it shouldn't be assumed.

Now, if we're going to go ahead and state the assumption with the problem statement then the rolling runway would have to run fast enough to keep the 747 motionless.  That would be very fast indeed (and rely on friction and other rolling losses in the wheel/tire), but we're talking hypotheticals.  So if you can run the runway fast enough to counteract the thrust from the 747 and keep the airplane motionless then you have your answer... the plane won't take off.

Yet nothing is said about the plane being kept motionless by the runway, only that the runway moves relative to the ground at some speed.

EGNX stated:

"The conveyor belt counteracts the motion of the wheels. i.e As the wheels roll forward the conveyor moves back."
This gives the rolling road of running machine effect.

Matt

[Brett_Henderson]

I know the answer (when the question is properly stated), but I'm getting confused again reading this thread..  I'm staying out of it  

[Chris_F]

[quote][quote]This question is subtly different from last time.

Last time the conveyor in question went the same speed as the plane, but in the opposite direction.

[Hagar]

[quote]It's the vagueness of the "counteracting the motion" statement.

[EGNX]

People are starting to look to deep and not concentrating on the basic fundementals...

To be even more simplified...

The aircraft is on a very long ( As long as you want ) tredmill...
As the wheels rotate forwards the treadmill counteracts this and begins to move backwards.
(Imagine your on a treadmill where it responded to how fast you were going so as you speeded up so did the treadmill.)

So now, if you understand that, will the aircraft be able to take-off?  :P

[dcunning30]

You guys are way overcomplicating this conundrum.  My admonition for you is as follows:



The answer is rather obvious, everyone should see it's clearly 42

[Jakemaster]

Right i'll try to go into more detail...

The conveyor belt is x miles long....
The conveyor belt counteracts the motion of the wheels. i.e As the wheels roll forward the conveyor moves back.
Try not to think about the aircraft limitations, including tire limitations and engine endurances just yet...

All you need to picture is any aircraft (I just said a 747 for ease) on a conveyor where the motion of the aircrafts wheels are counteracted by the conveybelt, (Sort of like a person on a tread mill) But will the aircraft ever take-off?

Some posts here are along the right lines but missing some key points....

I still don't get it.

Lets get away from planes for a moment...

Say I take my car out to a similar conveyer belt strip.  Lets say I start rolling at 10 miles per hour.  So the belt goes at the same speed to make it so that while my wheels may be going 10 miles per hour, the car isn't moving.  Right?

Now I speed up, say, 100 miles per hour.  Is the belt now moving at 100 miles per hour (or the equivalent speed so that my car doesnt move)?

What I'm saying is if the belt ALWAYS matches the planes speed ie the plane never moves no matter how fast the wheels turn, then NO it will not take off.

But if the belt only goes at one speed or at a certain speed, then YES the plane will takeoff assuming it can move faster than the belt therefore getting airspeed.

THAT is what I don't understand.