A plane on a runway, how smart is r3vlimited?

[E30 Wagen]

What I'm getting at is that the skateboard analogy is flawed because you are not accounting for the amount of force being place on the skateboard.

Place a scale between the hand and the back of the skateboard and there will be an amount of mearsure force there.

Okay, so let's put a compression scale in between the skateboard and your hand (which is holding it from behind). Unless the wheel bearings are really crappy, I doubt that the measured force will be significant. The most important point to remember is that the treadmill is moving and the skateboard isn't, which of course means we have breached the requirements of the problem. However, let's say that we take our hand and push the skateboard forward at the same rate the treadmill is going in the opposite direction. Of course the scale will compress more and record a higher value, but again I believe that number will be a very small increase. This small increase in force is equivelant to extra amount of thrust required of the plane's propellers in order for it to move foward. The skateboard problem illustrates that if we set the amount of thrust of the plane's propeller to a constant value, and the speed of the moving runway high enough, then the plane could stand still for the amount of friction of the wheel bearings is too great for the thrust of teh plane's propeller to overcome. Again however, this does not satisfy the requirements of the problem for the speed of the plane and the speed of the belt are not equal.

If you read some of my posts you would understand that there is alot more to just saying.. "Oh the jet flys with air speed and the wheels are not relative." which is completely wrong.

What I had discussed is that in the real world the weight of the plane, the amount of possible thrust, and wheels size would play a significant roll. The question is not HOW a plane is able to fly the questions is will it take off.

wheel size has nothing to do with it, unless you consider that a bigger wheel adds more weight to the plane itself. However, this means that the wheel will spin slower, decreasing friction. I bet that a perfect ration of wheel size to wheel weight could be discovered to optimize the least amount of drag. I maintain:

And again, I am sure that the increased friction of the spinning wheels is next to nothing and that we could throttle the propeller up to a higher RPM and the plane will be able to get up to speed enough to take off.

The conclusion is that if the plane is properly built and balanced with enough thrust to overcome the backward thrust of the conveyor it could fly. My second conclusion is that there are very few or maybe no planes built that could overcome this feat.

Remember, you can't really say that the conveyor belt has a "backward thrust." This equivocates the force of the plane's propellers, or jet engines or whatever, with the force of the belt, which is not what the problem advocates. I made that same mistake too until I reread the problem a couple more times. Anyway, consider how heavy a 747 is. For it to even take off on stationary ground, I imagine that the wheel bearings on that plane are extremely efficient, and the jet engines extremely powerful. I suppose that if you take a plane from the middle of the century, it would struggle, but with today's airplanes, I'm confident that many could do without much increased effort of the plane's engines.


Just thought of another example that might help explain. Actually, I know it works to explain this because I think many of us have seen this i real life:

The conveyor belt would be like a big trailer that you would tow with a truck. Without the trailer, the engine can run at a lower RPM because there is no resistence (weight from the trailer). However, when you're towing a trailer, the truck's engine has to work harder and run at a higher RPM in order to maintain the same speed as it did without the trailer.

edit: ^that example isn't a good one, ignore it.

[benDASHDASH]

This is more like it for Ben. :D



EDIT - If I had Pshop at work I would have made the sign say "Magic BTM" (BTM is Ben's initials).

someone find me a good lol gif

[DarkWing6]

I seriously can't believe we're on Page 25 already, the only discrepancy between whether or not the plane will lift off is due to how you are calculating the speed. If the original question had said that the conveyor belt would move at exactly the same speed as the planes INDICATED GROUND SPEED then no, the plane would never take off. It would remain stationary and neve move. However, if (because it's an airplane, and ground speed is there for moot) you say the conveyor will move at the same speed as the planes ACTUAL FORWARD SPEED, airspeed for example, then yes, the plane will be moving forward and given the right conditions will generate enough lift to take off. Friction will be a small factor but the point is the plane would be moving forward. I don't want to hear another goddamn thing from any of you until you can agree HOW YOU ARE MEASURING THE PLANE'S SPEED.

Have fun...

If you were to measure the planes speed as ground speed the air speed would have to be very small to a point that it is trying to have the plane at a stand still (like my earlier question of how you could make the plane stand still with the belt moving).

Just look at it this way. The plane's speed it is measured by air speed. If it were measured at ground speed obviously the plane would not take off because 100mph one direction ground speed and 100mph the other direction ground speed are going to equal zero. The question is designed to actually make us think. Air speed with the plane and ground speed with the belt. The plane will take off.

[RobertK]

[QUOTE=E30 Wagen;623116]Okay, so let's put a compression scale in between the skateboard and your hand (which is holding it from behind). Unless the wheel bearings are really crappy, I doubt that the measured force will be significant. The most important point to remember is that the treadmill is moving and the skateboard isn't, which of course means we have breached the requirements of the problem. However, let's say that we take our hand and push the skateboard forward at the same rate the treadmill is going in the opposite direction. Of course the scale will compress more and record a higher value, but again I believe that number will be a very small increase. This small increase in force is equivelant to extra amount of thrust required of the plane's propellers in order for it to move foward.[quote]

Like I said there are many coefficients that make the skateboard scenerio and the plane a very bad comparison. (ie. weight, wheel size in proportion to the skateboard, etc.) If you used that analogy to prove your point to a group of physics professors you would get eaten alive.

The skateboard problem illustrates that if we set the amount of thrust of the plane's propeller to a constant value, and the speed of the moving runway high enough, then the plane could stand still for the amount of friction of the wheel bearings is too great for the thrust of teh plane's propeller to overcome. Again however, this does not satisfy the requirements of the problem for the speed of the plane and the speed of the belt are not equal.

Problem is how do we determine the speed of the plane? By how fast it actually is moving across the conveyor OR by the amount of thrust relative to what the plane would normally need to take off the ground?

My argument is that of course the plane could take off if given enough thrust to overcome the drag produced by wheel friction

wheel size has nothing to do with it, unless you consider that a bigger wheel adds more weight to the plane itself. However, this means that the wheel will spin slower, decreasing friction. I bet that a perfect ration of wheel size to wheel weight could be discovered to optimize the least amount of drag.

Err wheel size would have EVERYTHING to do with it in a real world scenerio.

Remember, you can't really say that the conveyor belt has a "backward thrust." This equivocates the force of the plane's propellers, or jet engines or whatever, with the force of the belt, which is not what the problem advocates. I made that same mistake too until I reread the problem a couple more times. Anyway, consider how heavy a 747 is. For it to even take off on stationary ground, I imagine that the wheel bearings on that plane are extremely efficient, and the jet engines extremely powerful. I suppose that if you take a plane from the middle of the century, it would struggle, but with today's airplanes, I'm confident that many could do without much increased effort of the plane's engines.

Even if the conveyor could match the amount of thrust the plane did to inifinity eventually the plane would fly because the wheels to work to aleviate some of the backward thrust. Energy would be lost in the form of heat at the tires and the wheel bearing.

Just thought of another example that might help explain. Actually, I know it works to explain this because I think many of us have seen this i real life:

The conveyor belt would be like a big trailer that you would tow with a truck. Without the trailer, the engine can run at a lower RPM because there is no resistence (weight from the trailer). However, when you're towing a trailer, the truck's engine has to work harder and run at a higher RPM in order to maintain the same speed as it did without the trailer.

Another thing to think about though is that one of the reasons the truck has to work harder is due to added weight and friction produced by the surface area of the tires.

[DarkWing6]

Forget all these examples.

Since the wheels are free spinning the plane is basically flying from the start, which is why it is judged by air speed and not ground speed. There is just a certain amount of air speed it needs to lift off. The only thing the wheels and the belt are going to do is cause a slight amount of drag from friction, so we can basically not even worry about that.


What about this do you not understand?

[E30 Wagen]

Forget all these examples.

Since the wheels are free spinning the plane is basically flying from the start, which is why it is judged by air speed and not ground speed. There is just a certain amount of air speed it needs to lift off. The only thing the wheels and the belt are going to do is cause a slight amount of drag from friction, so we can basically not even worry about that.


What about this do you not understand?

+ 13858574373954784303933

Oh, and whether or not the tires will explode (they won't).

edit: We don't give a rat's ass how big the wheels are! All we know about them is that they will be spinning twice as fast as normal. We aren't measuring the speed of the wheels in mph, but in rpms.

[Fidhle007]

If you were to measure the planes speed as ground speed the air speed would have to be very small to a point that it is trying to have the plane at a stand still (like my earlier question of how you could make the plane stand still with the belt moving).

Just look at it this way. The plane's speed it is measured by air speed. If it were measured at ground speed obviously the plane would not take off because 100mph one direction ground speed and 100mph the other direction ground speed are going to equal zero. The question is designed to actually make us think. Air speed with the plane and ground speed with the belt. The plane will take off.

Thank you! You understand what I'm saying!

Forget all these examples.

Since the wheels are free spinning the plane is basically flying from the start, which is why it is judged by air speed and not ground speed. There is just a certain amount of air speed it needs to lift off. The only thing the wheels and the belt are going to do is cause a slight amount of drag from friction, so we can basically not even worry about that.


What about this do you not understand?

Yet again, thank you! Just look at my earlier car example, it's just a matter of figuring out where you are measuring the speeded from, and in this case it's airspeed. The plane will take off.

[DarkWing6]

:up:

[Justin B]

+1 on top of that 29736017162 or whatever it was. Seriously.

It does not matter how big or small the wheels are, it does not matter how much speed it needs to take off, the question is simple and everybody is overthinking it with everything else that they know of in this world.

Come on people. What if the problem was to calculate exactly at what angle and where on the string the nail will hold up a painting in a picture frame, if the mounting points were not quite right. Given the weight, size and mounting points of the frame, you should be able to calculate it without too much problem.

Of course the way this thread is going, if applied to this problem you would start to get overcomplicated. What size string is it? What is it made of and what is its resistance to slide over the nail, how slippery is it? Is there more glue or staples holding the frame together on one side than the other? If thats fine, you'll get into whether or not the painting is real or not and how much paint is caked on to the microgram on each side and exactly where, and demand that the painting be removed and weighted at every point. Of course thats not even enough complication, you'll want to know what angle the painting was held above the nail, or originally offset, so you could factor in how much it will try to center itself. Oh, what about gravity? What about the resistance of the frame against the wall, what is the frame made from, what finish is on the wall, and what angle is the nail set at. My point is that you need to look at the problem, interpret it in an ideal situation like everybody is that is arguing that it will fly, and call it done.

Same idea people, you're adding way more to the problem than is required, just like I said would happen earlier in the thread. Just like what I said above, now you're trying to argue wheel sizes. Serioulsy, what the hell is your brain thinking people? It tries to think realistically, while the problem obviously is not 100% real. It's IMPOSSIBLE to get a exact solution as to exactly what will happen for any sort of different plane (they will act slightly differently, but they will all fly), unless you do it in real life. The problem states everything it wants factored in, everything else is ideal, or it will turn into what it has, like my picture frame with everything factored in.

It takes off and flies.

[Dave]

[joshh]

LMAO...perfect picture for this thread.

Air speed........lol. So if a plane is traveling at 300mph and they have a tail wind of 100mph....does that mean the plane is really only traveling at 200mph...air speed right.
In relation to the ground of course....

[Justin B]

Well thats a little bit different, but as far as the airplane is concerned, it must be travelling in relation to its surrounding air. That is why its called air speed. Ground speed doesnt help it at all if its a lightweight plane that only needs to cruise at 70mph airspeed to float, and while its doing say, 80 mph ground speed, and wind picks up from 0 to 30mph (in which the case the plane will speed up at least most of that since its engines will feel as if it is only pulling through 50mph, but for this we'll assume it doesnt), the plane is onlymaking 50mph worth of lift and it will fall, assuming that the wind doesnt push the airplane forward and the engines dont pick up the extra slack in the wind resistance forward for this reason. In your case, If the plane is going 300 with a tail wind of 100, the wings are only producing lift as if it were flying at 200. The wings need that air to flow over the wings to create the lift, and the engines really are only working hard enough to push it at 200mph. Here's an example, its unrealistic to get a fans gust of air to catch up to a model airplane, but make a paper airplane. Toss it and turn on a fan that will blow right into its path. The plane will speed up but it didnt recieve any more thrust from your toss, it just has its own inertia to keep going and was doing the same airspeed as before the gust from the fan hit it (whether it just passed over the top of a fan or not, it doesnt matter, it's simulating wind vs no wind and how the plane will react).

[Fidhle007]

If the plane's airspeed indicator reads 300 mph (240 kts) and you have a 100 mph tailwind (80 kts) that means you are actually traveling over the ground at 400 mph (320 kts.)


I think...

[equate975]

The plane would take off, this is why.

First off, what is the point of landing gear with wheels? Think about this, lets say a plane (747 for argument sake) is trying to take off, but its just laying on its belly with no landing gear or wheels. Whats going to happen when it tries to take off? The plane won't, why? Friction, Imagine those engines that normally propel the plane down the runway on nice rolling wheels trying to push a metal hog on its belly scraping the ground, the plane would never get up to speed to take off. Imagine pushing your car in neutral vs. pushing your car while its on the ground with no wheels on, you probably wont move it anywhere by hand.

Second, planes don't just "lift off" and fly, again for arguments sake lets say this plane has to get up to 150mph to take off. This is how it would work: This plane on a normal runway, would be at the end. Assuming 0 wind (and I am sure lacking a few variables here but again just to demonstrate a point bear with me) the plane would have to get up to 150mph, to get 150mph of wind across its wings to get enough lift to leave the ground.

So this is why we have wheels, just to take away friction to get the plane up to 150mph as quickly and as efficiently as possible.

And we can all agree on that wheels DON'T propel the plane at all (like the rear wheels of a honda ;) ) They are there just to hold the plane up.

Now, exchange that runway for a treadmill. It's supposed to be setup like this: If the plane were going 150mph the belt would go 150mph in the opposite direction. We all can agree on that.

So what causes a plane to fly? Lift, we all again can agree for this plane we need 150mph of wind across the wings to cause sufficient lift to allow the plane to take off.

Put the plane at one end of the runway, the belt is still since the plane is not moving.

Now as the plane starts to thrust its engines and try to get up to speed, the belt spins faster and faster. However, its really irrelevant how fast the belt spins.

Like I said before, the point of the wheels is not have friction on the plane so it can go down the runway as fast as possible, and just hold the plane up.

So, the airplane on the belt, its still being held up by the wheels (although they are spinning really fast now) and it still does not have any abnormal friction of drag, it would be the same as if it were on a runway.

The airplane would be able to engage its engines and take off since the wheels have no effect on the speed of the plane, again they are only there to hold the plane up and reduce friction so the plane can actually take off.

Think of it like this. What about those planes that land and take off in water? Lets put one of them on the belt. But under the plane lest say it has an infinite amount of Petroleum Jelly or something, assuming this would not cause enough friction to prevent the plane from taking off what would happen? The plane again would engage its engines and just slide on the jelly until it got up to enough speed to take off.

This is super hard to explain, but you have to think of the wheels and the plane as 2 separate things.

Like I said, the speed of the belt is irrelevant. It could be going at 1mph or 10,000 mph, the wheels would still spin, they would still hold the plane up, and it would still not have any friction to allow the plane to take off.

[JasonC]

here is another picture for this thread