It doesn't matter how fast the conveyor turns. It cannot affect the forward motion of the plane. It can only make the wheels of the plane spin faster than they normally would prior to take off. During the acceleration run leading up to take off, the wheels' only purpose is to reduce friction and stabilize the vehicle for long enough to gain sufficient speed, and therefore lift, to take flight.

 

So, how come when Burt says basically the same thing I did people agree, but when I said there was too much to assume, people said I wasn't getting the big picture?

hmmmm?

 

Maybe because of wording... maybe because of fate. But most of all because I was not here to interpret your post to everyone like I was for Burts.

Why couldn't either of you have just said... "The conveyer neither aids to, nor renders the flight capabilities of any given wheeled air craft. If it was flight ready it would take off. If it wasnt flight ready it would not." ?

 

If an elevator breaks loose and falls from the 20th floor to the bottom ,, if you could time it and jump at the last second would you live because u would be in mid air....givin the elevator didn't smash into a 1000 pieces.

 

What if the plane is trying to take off on pontoons upstream in a fast moving river.

 

only if you can out accelerate the downward movement. so no you'll smash to the ground with the elevator because you cant jump up at 60mph.

 

I have seen this done before. Bugs Bunny does something similar with a cabin that gets blown about 2 miles high into the air. While he is inside the cabin.

He just stands on the porch while the cabin falls back to earth, and he steps off when the cabin is at about 1 foot from the ground.

So yes, it it works for Bugs Bunny.......

 

He will still only have to reach ___ mph ground speed to achieve lift off.
However the water surface traveling under the plane will be ground speed plus the speed of the current. He will require slightly more power to over come the friction resitance of the water pushing on the toons. The faster the current, the more power required.

 

Bugs also has an "Air Brake" handle that can stop a B52 in a nose dive in mid air 1 foot from the gound so he can step off to safety too.

 

De-Ja-vu or however that's spelled.....haven't we had this conversation before?

 

I think so.
If memory serves me it was in the "Just Ask Kobi™" thread.

 

Your right. At any rate, kids, I wouldn't try any of Bugs' stunts at home. He is highly trained in the art of breaking the laws of physics. Most Hanna-Barbara cartoon characters are.

 

So, it seams like we are starting to all come together and agree that the airplane will take off. This just happens to be the correct answer. I will attempt to walk through a logical explanation.

"A plane is standing on a runway that can move (some sort of band conveyer).

To most people this seems simple, however we seem to have a few that cannot understand this. To those that say you never stated that the plane could take off, I have but one thing to say.... IT'S AN AIRPLANE, THAT'S WHAT THEY DO!! If it could not take off it would not be called an AIRplane.

The plane moves in one direction, while the conveyer moves in the opposite direction.

This is where most people's confusion starts. It is a setup to catch people with the next sentence. Some people start thinking about a car or motorcycle on a dyno some people start thinking about the treadmill at the gym. This however is completely wrong. Something on a dyno is anchored in place with the driving wheels on a rotating surface; the same goes for a treadmill. Your feet supply the propulsion by pushing against the ground.

This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)."

Here is where most people get confused. Notice it never states that the conveyor stops the plane from moving. However, most people automatically assume that the negative motion of the belt cancels out the forward motion of the aircraft. From here, the most common mistake is to assume that the airplane does not move, and therefore can not generate lift. From here things just get ugly, people start coming up with all sorts of reasons to back up their original idea.

What really happens is that, the airplane starts his takeoff roll the aircraft's propulsion system pushes against the air, not the ground. As has already been stated numerous times, the wheels of an aircraft do nothing other than stop the paint from being scratched and help to slow the aircraft down when the pilot uses the brakes. Other than that, they do nothing. If you can't visualize this, think about when an aircraft breaks ground. At this point, the only thing that the aircraft is in physical contact with is the surrounding air yet it still can accelerate.

Now, most people will agree with this part. Yet many will still scream that the wheels are being pushed opposite the aircraft's motion creating a relative velocity of 0 so the aircraft is not moving, no movement no lift. The only part of that statement that is correct is the last four words. The wheels on an aircraft are free to spin around an axle. This negates any affect that the moving runway has on the aircraft. Consider the reaction forces that act on the entire system. First, you have the aircraft's power plant supplying thrust against a rigid body, the airframe. This works to push the aircraft along the runway. Secondly, there are the landing gear trunions. These support the weight of the aircraft while it is on the ground and serve as an axle for the wheels. Then you have an independent power source spinning the belt as well as the runway surface acting normally against the wheels and trunions to support the weight of the aircraft. Now we can move to the reactionary forces. There is the aircraft's propulsion system acting against the air to provide thrust. This yields positive force acting on the airframe. We can ignore the normal forces that support the weight of the aircraft as they are equal and opposite and cancel out. Then we come to the force the runway exerts against the airplane. We have already negated the vertical forces, so we are left with horizontal forces generated by the moving belt. The belt acts on the tires pushing them in a clockwise motion (assuming the plane is headed in a left to right motion) causing them to rotate around the axle. This yields very little (nearly 0) negative force to counteract the positive force of the aircraft power plant. Therefore, a positive force plus a nearly zero negative force yields an almost unchanged positive force. So, the aircraft's forward motion is unhindered by the runway, creating movement. So now we all can chant, "movement equals lift, lift equals flight." The only thing that happens is that due to the aircraft moving forward spinning the wheels and the runway moving opposite spinning the wheels the wheels will spin at twice the aircraft's speed.

Clear as mud???

Joe

 

Thank you Joe. Even though I was wrong in my assumption about the wheels moving in the opposite direction upon take off and landing, I knew it had to do with something with the wheels. Also figured weight would be a factor. So I guess by saying yes, it will take off, I was right.

LT

 

So, essentially- for the "Twice as fast" notion to be valid- the "opposite" movement of the conveyer essentially turns the wheel twice as fast as they normally would turn, but still support the forward motion of the aircraft...

You're right- the term "Opposite" had me thinking the conveyor was moving in such a manner that it's "opposite" movement cancelled out the forward motion of the wheels. But, opposite movement in this case- would stand to support the forward movement, only twice as fast... If that notion is valid.

If that notion is valid, then yes- it will fly... Something akin to the going fast on a 10-speed, and putting it in 6th gear... You can pedal faster, but you won't move any faster than you were already going.
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But, I agreee with Closer & XLT- there is a lot of vagueness in that paragraph- but one thing we can all agree on is this.

For a normal airplane, on earth, flight cannot be accomplished without forward momentum via thrust (or pull) and lift.

Only for a VTOL aircraft is this not true. But, as long as an aircraft can attain forward movement at the appropriate speed, lift can be developed, and the plane will fly.