Author Topic: plane on a conveyor belt? (Read 19874 times)

lukster · « **Reply #195 on:** January 22, 2007, 01:14:33 PM »

Hi Casca, I think the premise is that the belt will continue to increase it's speed so long as the plane is moving forward. Assuming no translated forces due to friction, other than what is required for contact between the tire and belt ( yes, an impossibility), the belt may reach a speed approaching infinity in it's effort to prevent the plane from moving forward. If friction is not a factor then the plane should still accelerate forward gaining airspeed.

While I was joking about the black hole earlier I meant it as truth. As an object with mass (like the surface of the conveyor belt) accelerates, approaching the speed of light, it's mass approaches infinity.

Hmm, reread the original question, I think you're right Casca, the wheel moves forward up to say 70mph before taking off, the belt moves in the opposite direction at 70 mph making the wheel rpm equivalent to only 140 mph. I guess I was confused about wheel speed being forward relative to it's axis rather than rotational speed.

parin · « **Reply #196 on:** January 22, 2007, 02:16:37 PM »

Jeez guys everyone knows only the LA7 and Spit XVI would be able to fly. And all other aircraft would flounder on the belt!

But seriously the rope example makes it pretty clear to me.

lukster · « **Reply #197 on:** January 22, 2007, 02:21:54 PM »

This might be where some of the confusion and disagreement comes in. The "plane's wheel speed" statement is ambiguous. It could be interpreted to mean forward relative to a fixed point in space or rotational speed. Like this:

Golfer · « **Reply #198 on:** January 22, 2007, 02:34:22 PM »

It doesn't matter...the plane flys.

Take a model airplane and put it onto a treadmill with its engine on.

Power up the treadmill will concurrently powering up the airplane.

Make sure nobodys standing in front of the treadmill cause they'll get thwacked by an airplane.

If you put a real airplane on a real big treadmill and leaned it gently against some retaining blocks on the trailing edges of the wings to keep it from rolling backward. Turn on the treadmill...the wheels spin.

Turn on the engines. Lets say they're jet engines. Power up. gently...even though the wheels are spinning the engines are taking air from in front of the airplane and squirting it out the back of the airplane. The plane is moving in relation to the fluid (air) around it.

You could taxi the airplane at a normal speed in relation to a fixed point next to the giant treadmill regardless of how fast that treadmill is spinning. You could takeoff (provided the tires don't go flinging in 360 different directions) normally from said treadmill with no ill effects.

lukster · « **Reply #199 on:** January 22, 2007, 02:37:56 PM »

It matters because if you spin the belt fast enough the bearings will heat up acting as brakes. Try taking off with the brakes fully engaged.

Golfer · « **Reply #200 on:** January 22, 2007, 03:05:15 PM »

oh for the love of god...

Where's my icon of the smiley smashing his head against a brick wall?

lukster · « **Reply #201 on:** January 22, 2007, 03:24:02 PM »

Quote

Originally posted by Golfer
oh for the love of god...

Where's my icon of the smiley smashing his head against a brick wall?

Spin your treadmill up to 2 -3 hundred miles per hour and put the plane on it. Go ahead and stand to the rear if you think it'll take off.

eskimo2 · « **Reply #202 on:** January 22, 2007, 03:53:04 PM »

Quote

Originally posted by lukster
Spin your treadmill up to 2 -3 hundred miles per hour and put the plane on it. Go ahead and stand to the rear if you think it'll take off.

Actually, that would only nudge it back, once the wheels get to 2 – 300 MPH the plane would start moving forward (provided the tires don’t blow, bearings overheat, etc. What must happen it the treadmill is going 300 MPH at 1 second, 600 MPH at 2 seconds, 900 MPH at 3 seconds, etc. The treadmill and aircraft wheels must be accelerating at a fantastic rate for this to work.

Want another twist? If we change the wheels to be mass-less, the conveyor and wheels would go to an infinite speed instantly and the plane would take off normally!

Golfer · « **Reply #203 on:** January 22, 2007, 03:55:07 PM »

Quote

Originally posted by lukster
Spin your treadmill up to 2 -3 hundred miles per hour and put the plane on it. Go ahead and stand to the rear if you think it'll take off.

I'd be glad to. I'd happily sit in the airplane as well provided the tires have no speed limitation on them.

lukster · « **Reply #204 on:** January 22, 2007, 03:58:39 PM »

Quote

Originally posted by eskimo2
Actually, that would only nudge it back, once the wheels get to 2 – 300 MPH the plane would start moving forward (provided the tires don’t blow, bearings overheat, etc. What must happen it the treadmill is going 300 MPH at 1 second, 600 MPH at 2 seconds, 900 MPH at 3 seconds, etc. The treadmill and aircraft wheels must be accelerating at a fantastic rate for this to work.

Want another twist? If we change the wheels to be mass-less, the conveyor and wheels would go to an infinite speed instantly and the plane would take off normally!

We're talking about a model airplane with wheels designed to rotate at maybe 40 mph. Golfer said the speed of the conveyor didn't matter but it does. If 200-300 isn't enough try 2-3 thousand. Massless wheels and belt must by definition have no ability to influence the motion of the other.

lukster · « **Reply #205 on:** January 22, 2007, 04:04:02 PM »

Quote

Originally posted by Golfer
I'd be glad to. I'd happily sit in the airplane as well provided the tires have no speed limitation on them.

It's your model plane, contruction of the tires is up to you. You want to try a real plane? Try spinning the belt to 10,000 mph and see if the planes wheels can keep up. It's obvious we're talking friction here and the wheel's ability to convert the movement from the belt into a pure rotational speed. If the engine is powerful enough to overcome the wheel friction then yes the plane will accelerate and takeoff. Let's not mix theoretical and practical or at least if we do let's not confuse them.

Golfer · « **Reply #206 on:** January 22, 2007, 04:05:40 PM »

Quote

Originally posted by lukster
We're talking about a model airplane with wheels designed to rotate at maybe 40 mph. Golfer said the speed of the conveyor didn't matter but it does. If 200-300 isn't enough try 2-3 thousand. Massless wheels and belt must by definition have no ability to influence the motion of the other.

do you realize that you just solved the agrument for us with your statement?

The wheels are a lubricant if you will...they allow the airplane to stay put and use its own propulsion (thrust) to make it move without being influenced by the treadmill underneath it.

Golfer · « **Reply #207 on:** January 22, 2007, 04:06:44 PM »

Quote

Originally posted by lukster
It's your model plane, contruction of the tires is up to you. You want to try a real plane? Try spinning the belt to 10,000 mph and see if the planes wheels can keep up. It's obvious we're talking friction here and the wheel's ability to convert the movement from the belt into a pure rotational speed. If the engine is powerful enough to overcome the wheel friction then yes the plane will accelerate and takeoff. Let's not mix theoretical and practical or at least if we do let's not confuse them.

You used both of those underlined sections in the same paragraph for your agrument?

lukster · « **Reply #208 on:** January 22, 2007, 04:09:54 PM »

Quote

Originally posted by Golfer
You used both of those underlined sections in the same paragraph for your agrument?

Well, a conveyor belt capable of speeds to 10,000 mph is theoretically practical.

Bearing friction may not even be the largest source of drag. Take a model plane for example, spin the wheels to fast and they will likely wobble. A wobble takes them out of line with the direction they are "traveling" inducing more drag on the tire in the direction of the supposed belt. Yes, I'm bored.

eskimo2 · « **Reply #209 on:** January 22, 2007, 08:33:03 PM »

Here is a glimpse into how a treadmill pushes a wheel back as it accelerates. Note the set-up:

The fire extinguisher is an anchor (overkill, I know) for the rubber band that is tied to a wire that is looped through the axel of the wheel. To keep everything aligned, the wire goes through tubes that are taped to the green stool.

The wheel is resting on the belt sander. When the sander is turned on, the sander and the wheel gain RPM for less than ½ a second. During this time, the wheel shoots to the right, stretching the rubber band. When the sander and wheel stop accelerating and the RPM become constant, the wheel is no longer gaining significant energy from the belt and the rubber band pulls the wheel back to the left where it spins merrily in a steady state of energy.

Watch the Movie (1.2 MB ~ 3 seconds)

The acceleration of the wheel stretched the rubber band in the direction of the treadmill (belt sander). This is an example of how a treadmill of unlimited speed could load energy into a wheel of unlimited strength (and through a perfect bearing) through rotational acceleration. Since the force is only applied to the bottom of the wheel where it contacts the treadmill, it is not balanced. A vector of the force is applied to the axel in the same direction of the belt. Note that it will not prevent the plane from moving if it only accelerates for ½ a second. The acceleration (increase in RPM) must be constant, and must be massive.

Watch the movie and imagine things on a much greater scale.