The old airplane/treadmill revisited

[woodturner]

And now a new theory of relativity: We can't go faster than light, or energy as we know it today, because as we accelerate out mass expands to infinity at which point we become light, or pure energy. As we know, nothing can go faster than it's self, ergo we can not go faster than light.

FWIW, that's a "working theory" - that is, the terms of the equation we have discovered to date result in the equation indicating mass is infinite at the speed of light.

Scientists have discovered subatomic particles that travel faster than the speed of light and disintegrate when they slow down to the speed of light. So the precise statement is that current theory suggests that objects become infinitely massive as they approach the speed of light. Put another way, objects can travel faster or slower than the speed of light, they just can't travel at or too close to the speed of light.

[jking]

Planes are not powered by their wheels. Unless the brakes are locked, rollers, treadmills, conveyors, etc will have negligible effect.

I think the only thing this thread might prove is to not take anything on mythbusters as fact. I've seen more than one show where they started out with a flawed premise, added a flawed experiment, & came to a flawed conclusion. For them to portray themselves as anything other than entertainment is laughable.

[woodturner]

IDo you think the seaplane would have problems taking off heading upstream? Of course not. The prop is pushing against air, not water.

Good analogy. In that case, if the wind is strong enough or the plane is moving fast enough relative to the water to achieve the required air speed over the wing, the plane will take off.

If the plane is stationary relative to a fixed point on the earth and there is no wind, however, the plane will not take off.

[Mr__Bill]

FWIW, that's a "working theory" - that is, the terms of the equation we have discovered to date result in the equation indicating mass is infinite at the speed of light.

Scientists have discovered subatomic particles that travel faster than the speed of light and disintegrate when they slow down to the speed of light. So the precise statement is that current theory suggests that objects become infinitely massive as they approach the speed of light. Put another way, objects can travel faster or slower than the speed of light, they just can't travel at or too close to the speed of light.

[]
Can't be infinitely massive without infinitely large because without getting larger we would become a black hole, the black hole sucks in all light and you really can't go faster than something your collecting along the way. Mind you, I'm not talking about the stuff out there that is traveling many times the speed of light, I'm talking about me and you going faster than light.

Now about that seaplane on the conveyor belt. It can't take off because the plane doesn't have enough power to slide the floats along the dry belt so it doesn't move and the belt doesn't move and eventually the plane runs out of gas. For that matter a plane with wheels won't take off either because as it moves forward on the conveyor belt it runs out of belt before reaching a takeoff airspeed. It falls off the end of the belt and crashes on it's nose.

Now let me tell you about this guy I know who mounted his stationary bike on his treadmill, he didn't take off.
[/]

Bill
in the fog on the Sunny Oregon Coast

[JeffG78]

Somehow, this seems to fit.

http://www.totalprosports.com/wp-con...-all-wrong.gif

[Mr__Bill]

Somehow, this seems to fit.

http://www.totalprosports.com/wp-con...-all-wrong.gif

Laughing, dang where is that ROTFLMAO icon when you need it!



Bill

[cgallery]

Okay, duh, I get it.

The plane takes off.

Sorry everyone.

Sometimes it takes me longer to think things through than the rest of you.

[chopnhack]

sorry to say but your thinking is off. the engine puts no power on the wheels at all. The wheels do offer a restriction due to friction, the engines power is delivered to the air that it moves. Air is about 13 cubic ft per lb so the prop is moving a mass times the pressure difference between the leading edge and the trailing edge of the prop this = engine HP. the only way to keep a plane from moving is to anchor it with the equivalent weight of the air the prop is moving or that the jet engine is thrusting. Most airplanes due not move as much air with the engine as they weigh so they need forward motion. and the brakes are sufficient to keep them stopped at full power. The military planes that will go straight up have more thrust then they weigh.

I understand that planes don't take off due to their engines turning their wheels, but rather from the engines propelling them forward until their forward speed is sufficient to create buoyancy beneath their wings, what I was getting at was that if the plane has no forward motion, i.e. it does not cover distance over time, then it does not do any work. If the plane was on rollers and you stood next to the plane, if the speed of the conveyor was perfectly matched to the plane's forward speed then their would be no true forward speed, it would not cover any distance and you could stand next to it at full speed. The plane is obviously not going to take off, no more than you could take off by standing next to it. At least in my thinking

[JeffG78]

The plane does move forward - period. The treadmill has no effect on the plane. The faster the treadmill moves, the faster the tires spin, but it doesn't keep the plane from accelerating forward and taking off.

[leehljp]

If the plane was on rollers and you stood next to the plane, if the speed of the conveyor was perfectly matched to the plane's forward speed then their would be no true forward speed, it would not cover any distance and you could stand next to it at full speed. The plane is obviously not going to take off, no more than you could take off by standing next to it. At least in my thinking

This is where the conveyor (and including roller concepts) messes with the mind. The thrust is not on the wheels. Put the plane on rollers, gun the throttle to full thrust and it will jump OFF of the rollers, - unless it is held back by cables or braces. Same for conveyors. The thrust will pull it forward no matter how fast the wheels are spinning. And Jets, with enough thrust will scoot forward even with full brakes or 1000 mph converyor, so a roller or moving conveyor will not hold it back.

The concept would be different if the vehicle were a car where forward thrust was dependent on the wheel thrust. Air thrust (jets and props) are independent of wheel speed.

[LCHIEN]

sigh, there are so many people who can't do system level analysis.

First there are several red herrings.
One the wheels. Assume the wheels are with frictionless bearings
Second the conveyor belt. Assume the conveyor belt is narrow, the top moves in the opposite direction of the bottom.
Consider your frames of refernence. It should be the air and the surrounding ground where the observer is located.
The air does not move significantly with the conveyor belt. Its tempting to think of the comvenyor belt as ground because the plane is initially sitting on it but its not.
The air is stationary with respect to the ground, or close to it with mild winds.

Now the question is what moves relative to what?
The wheels are completely free, the center moves with what its attached to, the outer circumference stays in contact with the surface its against with weight (friction) and turns at 2 Pi omega times the radiusdivided by the relative linear speed.

The airplane, propellor or jet moves forward relatiev to the ground and the observer standing on the ground because it pushes against air and gets the equal and opposite reation.

The conveypr belt is a completly independent system. The given is that the sensor automatically moves teh suirface in the opposite direction of the airplane.

While the airplane is on the conveyor belt the wheels will turn twice as fast as they would if it was on a stationary runway. THis is becasue theairplane has a velocity V and the runway has an equal but opposite motion at -V so the relative motion is 2V.
Hnce the wheels turn twice as fast as normal. Until the plane lifts off. It will because the relative speed of the plane to the air creates bernoulli lift and it flies.
As it rises the weight on the wheels will go to zero and the friction to zero and the wheels will jsut spin at the last 2V it had in contact. The wheels will spin forever in a frictionless bearings. because there is equal airflow on the top and bottom of the wheel, netting zero force.


Then there are those of you who will never be convinced...

[woodturner]

[]
Can't be infinitely massive without infinitely large because without getting larger we would become a black hole, the black hole sucks in all light and you really can't go faster than something your collecting along the way.

That is consistent with my understanding of the current theory.

However, the belief is that the current theory is incomplete, so we are really sure exactly what happens at the speed of light, and really aren't certain exactly what black holes are and how they are made.

[woodturner]

Until the plane lifts off. It will because the relative speed of the plane to the air creates bernoulli lift and it flies.

If the air is still and the plane is not moving relative to the ground, it's no different than a plane sitting on the ground motionless - it cannot take off. As originally stated, the problem is misleading because it implies that the plane is stationary to the ground, when it really is not. The point is that the conveyor cannot match speed quickly enough so that it cannot hold the plane in a stationary location, so the plane accelerates and moves, achieving air speed.

OK, I confess - I wanted to make sure this was correct, and we happened to be testing model airplanes in the wind tunnel yesterday, so I tried it (with the model airplane). As expected, when the plane remained stationary relative to the ground, the plane did not take off.

Bottom line, the airplane needs sufficient air flow over the wings to achieve takeoff. That can be due to wind or due to travel along the ground - but without achieving the air flow over the wings, it won't take off.

Think about an airplane that stalls - it's completely free of ground contact, but air flow has reduced to the point that the wing stalls.

[LCHIEN]

yeah, we're assuming the plane moves forward relative to the air because its engines push against the air. So it will take off.
The wheels free spinning totally decouples the plane from any conveyor belt and or ground friction effects.

[cgallery]

The point is that the conveyor cannot match speed quickly enough so that it cannot hold the plane in a stationary location...

The conveyor can be moving infinitely fast and as long as the plane's hub resistance is able to be overcome with forward thrust of the engine(s), then the plane will move forward and take off.