[Davesrose]

Quote:

Originally Posted by Rav /img/forum/go_quote.gif Nope, the engine will only provide a finite amount of thrust, be it jet or prop. Once at full power that's it, you're not getting any more. Just because a propellor blows air over the wing, doesn't mean it can lift itself.

But we're in hypothetical land

You said in hypothetical land that tires can keep on lasting....lets say you have huge fan like propellers over the wings that have infinity horsepower

Enough to create a drag under the wing. Silly, I know....but this is hypothetical

 

[bhd812]

Of course the plane can lift off, the wheels have nothing to do with the actual lift of the plane off the ground if anything they allow the engines to build up thrust in the level of time not length.

same goes for how a plane lifts off a air craft carrier, only the jet in this case can build up thrust faster and does not need as much time a non-jet plane would.

not sure why i find this so easy and why everyone is talking about it here, i guess this may be one of those "how you think" questions..

 

[Davesrose]

Quote:

Originally Posted by bhd812 /img/forum/go_quote.gif same goes for how a plane lifts off a air craft carrier, only the jet in this case can build up thrust faster and does not need as much time a non-jet plane would. not sure why i find this so easy and why everyone is talking about it here, i guess this may be one of those "how you think" questions..

Inertia is a very difficult concept, because it's hard to relate too....well unless you're in an auto accident and suddenly find yourself flying

And actually, prop planes take off from an air craft carrier's catapult system too. It gets flung at the same speed as a jet (assuming they are similar weights). The airplane's own propulsion doesn't really get underway until after the catapult slings it. What the catapult is doing is providing the airplane more forward thrust then what it could do on its own (and would need more distance to reach proper acceleration). Lets look at the treadmill again. If you run at the same pace as the treadmill, your torso won't be moving right? If the treadmill goes forward, and you walk forward, you will go a faster speed relative to the ground (kind of like those airport walkways). That's essentially the catapult. But in this analogy, we're saying that for as much as you are walking forward, the belt is moving backward. Thereby keeping you still. An airplane gets a bit more complicated because it has wings....now can you generate enough of a vortex to lift the wing, if there is no forward inertia? That is the big if.

 

[bhd812]

good good..

here's another way I look at it..

how do you get a paper plane to fly? thrust..
if you put wheels on a paper plane and put it on a treadmill would it life off? no cause the thrust from your hand is not there.

now if you run on a treadmill and throw a paper airplane will it lift off? yes..

just like a plane on a Conveyor belt, as long as you have the thrust of the engine (or your hand for a paper airplane) you will lift.

 

[Davesrose]

Quote:

Originally Posted by bhd812 /img/forum/go_quote.gif now if you run on a treadmill and throw a paper airplane will it lift off? yes.. just like a plane on a Conveyor belt, as long as you have the thrust of the engine (or your hand for a paper airplane) you will lift.

See, you're changing the conditions

What you're doing is adding more forward thrust by throwing the paper airplane yourself. Oh, I got a real mind trip for relativity. Lets say you get on a treadmill with an RC airplane right in front of you...going the same speed as you. Now if the condition was that the treadmill is only matching the speed of you, you could push the plane and give it more forward thrust. On the ground, it would look like you and the airplane are staying still untill you gave that airplane a push....the plane would then move forward for however long you pushed.

However, if the treadmill is keeping speed constant to the RC plane, it would accelerate its reverse thrust when you add more speed to the RC plane....its matching your added thrust to the plane. Assuming that you can compensate for the difference in acceleration (because inertia is now acting on you), it would look as if you were going backward as the plane is staying still.

Freaky isn't it? These are some interesting brain teasers

 

[Febs]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif However, if the treadmill is keeping speed constant to the RC plane, it would accelerate its reverse thrust when you add more speed to the RC plane....its matching your added thrust to the plane.

You are assuming that the increased speed of the treadmill exerts a force on the plane that is equal to the force exerted by the thrust of the engines. But it doesn't.

 

[Davesrose]

Quote:

Originally Posted by Febs /img/forum/go_quote.gif You are assuming that the increased speed of the treadmill exerts a force on the plane that is equal to the force exerted by the thrust of the engines. But it doesn't.

No, my example is that the treadmill is having the same reverse speed as the forward speed of the plane (not the engine's thrust). If you add more acceleration to the plane yourself, the treadmill is going to add an opposite, yet equal counter acceleration. I am trying to give examples of inertia....since this is what can keep the airplane stationary to the ground in this hypothetical

 

[Febs]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif No, my example is that the treadmill is having the same reverse speed as the forward speed of the plane (not the engine's thrust). If you add more acceleration to the plane yourself, the treadmill is going to add an opposite, yet equal counter acceleration.

Speed does not equal acceleration.

If you exert a force on the plane (through the thrust of the engine), it will accelerate the plane. The speed of the treadmill may increase, but that does not impose a force on the plane equal to the force that you have added with the engine. The wheels of the plane may spin faster. Perhaps there would be a very small force exerted on the plane as the result of rolling friction, but it will not be anywhere near the force exerted in the opposite direction by the engine thrust. Thus, the plane will move forward.

 

[Febs]

Here's another way to say this: The engine is pushing the plane. Because the wheels on the plane are spinning, no matter how fast the treadmill goes, it does not push back.

 

[Davesrose]

Quote:

Originally Posted by Febs /img/forum/go_quote.gif Speed does not equal acceleration. If you exert a force on the plane (through the thrust of the engine), it will accelerate the plane. The speed of the treadmill may increase, but that does not impose a force on the plane equal to the force that you have added with the engine. The wheels of the plane may spin faster. Perhaps there would be a very small force exerted on the plane as the result of rolling friction, but it will not be anywhere near the force exerted in the opposite direction by the engine thrust. Thus, the plane will move forward.

No, but they're very related...acceleration is the change in speed over time.

Lets stick with my example still...with a moving object on a treadmill. When the object is moving at the same speed (but oposite direction) as the treadmill it negates any distance traveled. The same is true for anything with free floating wheels on this belt. A plane's engines would normally provide it with forward acceleration. But our hypothetical belt is reversing in at the same constant (speed and acceleration), thereby negating any change in distance. If you say that the forward thrust has more thrust then the opposite force as our belt, yes, obviously the plane is getting more thrust and move forward (and also break the condition about no forward inertia). Now if we had a car trying to do the same thing, it would burn a lot of rubber because it's now causing friction as the wheel surface is going in the direction of the car and opposite to the belt. That's the main difference with a car trying to keep the same speed as the belt vs a free floating wheel from an airplane.

 

[Davesrose]

Quote:

Originally Posted by Febs /img/forum/go_quote.gif Here's another way to say this: The engine is pushing the plane. Because the wheels on the plane are spinning, no matter how fast the treadmill goes, it does not push back.

The wheels are making contact, and if they're freely suspended on the airplane's axle, they'll be going in the same direction as the treadmill. They keep contact because of the pull of gravity. Since gravity continues to stay constant to the plane, whatever forward thrust the engine puts out....this magic treadmill can match in both acceleration and speed.

The wheels are like your feet on a treadmill. If your acceleration is greater then the treadmill, you move forward. if your acceleration is less then the treadmill, you'll go back. If you keep your acceleration to the treadmill, you won't move. You're saying that if you change acceleration in mid stride, that that will put you forward. It won't if the treadmill changes its acceleration to the same speed.

Anyway, there's a lot of absolute variables here....in theory it would be easy to make an airplane stay in place on a conveyor belt. In reality, I think it pretty easy to say it would take off

 

[Mrvile]

Here is a good way of putting it into "realistic" terms for you guys who still can't understand:

Say you are wearing rollerskates. You are on a treadmill. The treadmill rolls back at the same speed you are moving forward. You have a rope in your hands attached to a stationary object (like a rock) a few feet in front of the treadmill. Are you still able to pull yourself forward?

YES

It's as simple as that.

The plane's engines act as your arms, and the rope is the air going through the engines. The engines pull the air, which creates thrust and propels the plane forward. The wheels start spinning backwards wildly, but who cares, they don't affect the plane at all. As the plane continues to move forward it moves faster and faster until it pulls enough air over the wings to take off.

GG

 

[bhd812]

Quote:

Originally Posted by Mrvile /img/forum/go_quote.gif Here is a good way of putting it into "realistic" terms for you guys who still can't understand: Say you are wearing rollerskates. You are on a treadmill. The treadmill rolls back at the same speed you are moving forward. You have a rope in your hands attached to a stationary object (like a rock) a few feet in front of the treadmill. Are you still able to pull yourself forward? YES It's as simple as that. The plane's engines act as your arms, and the rope is the air going through the engines. The engines pull the air, which creates thrust and propels the plane forward. The wheels start spinning backwards wildly, but who cares, they don't affect the plane at all. As the plane continues to move forward it moves faster and faster until it pulls enough air over the wings to take off. GG

my paper plane was much better cause...

A: I thought of it all my own..
B: everyone loves paper planes, unless you had your eye poked out when you were a child..
C: I thought of it all my own..

you see I being Me am always rite by default..

 

[Davesrose]

Quote:

Originally Posted by bhd812 /img/forum/go_quote.gif you see I being Me am always rite by default..

Well obviously I'm right because my assumptions are totally based on absolute theory and not grounded in reality.....just like the original question

 

[Febs]

Quote:

Originally Posted by Mrvile /img/forum/go_quote.gif Say you are wearing rollerskates. You are on a treadmill. The treadmill rolls back at the same speed you are moving forward. You have a rope in your hands attached to a stationary object (like a rock) a few feet in front of the treadmill. Are you still able to pull yourself forward?

To use the same analogy, Daverose, how would you answer this question:

Say you are wearing rollerskates. You are on a treadmill. The treadmill rolls back at the same speed you are moving forward. You have a rope in your hands attached to a stationary object (like a rock) a few feet in front of the treadmill. Suddenly, the front of the treadmill is lowered so that you are facing downward at a 45 degree angle. Will you roll forward?

Suppose the speed of the treadmill increases. Will you still roll forward?