Can Plane Take Off

Paen said:

The answer is no.

Done.

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Idiot. The answer is YES. The plane will take off.

Paen was referring to the fact that I"m one arming it in place. No, it will not take off in that situation.

bcware said:

is there a difference between "pulling" air and "pushing" via jet propulsion? i would imagine at low speeds propeller powered planes, which tend to sit on a tripod set of wheels with the nose pointed upward, would do nothing but spin the wheels. As the plane picks up speed and just before it takes off, the plane straightens out and the 3rd wheel in the rear comes off the ground. I would think that a propeller based plane with this tilted tripod setup would do nothing more but lift it?s 3rd wheel in the rear off the ground once the propeller spins fast enough to level the plane. A jet, however, ?pushes? and doesn?t pull, they also sit relatively flat don?t they? Do the physics change completely? The wheels on the plane don?t generate any force; they just provide a means to transfer the power to the ground until speed is sufficient to generate lift. I just feel like the wheels must ?push? off something to get the plane going. Intuitively the physics of a plane and car don?t seem all that different until the plane actually leaves the ground.

I wanna see the physics guys? I haven?t taken enough physics to say anything useful here.

edit:

a plane uses the same friction to move until it reaches sufficient speed to generate lift. Exactly the same way a car does, it just generates it?s power to move via a different source. There is no drive train connected to a plane?s wheels because it isn?t necessary, we can ?drive? forward by pulling air instead of pavement, but the same driving force is transferred to the wheels which push off the ground and move the plane forward. The fundamentally seems exactly the same as a car to me. the only difference is that when the plane moves fast enough a second set of physics kicks in and lift is generated. i think the dyno-plane thing is possible and wouldnt fly, but obviously it"d be an extremely dangerous equilibrium to maintain. pull the wings off a plane and what does it do? it"s a propellar powered car isn"t it? i think the key to fully understanding this problem is breaking this problem into different distinct pieces.

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first off not all propeller planes have a tail wheel. in fact most have a setup similiar to this.



second off you could put ski"s(for landing in the snow) or floaties(for water landing) and a similiar styled plane can still take off. in both cases there is no way for the propeller to apply a force to the surface the plane is on which will make the plane move. think of the propeller like a giant screw. when its turning its pulling itself through a substance(air) which causes it to pull forward, similiar to putting a screw in wood. this is what gives prop planes thrust. all these landing mechanisims act as a means to reduce friction to a near 0, friction is actually what they try to avoid not utilize

TheWand said:

sheesh, 12 pages and still growing.

and i"m still amazed at how many people believe it cant take off.

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Amen...

Just a question to all the idiots out there that say it won"t take off...

Imagine you attach a rope to Superman. The other end of the rope is attached to said plane. In this scenario, we won"t start the engines of the plane and instead, we"ll let Superman do all the pulling as he flies away, pulling the plane behind him. Will the plane take off or not?

Here"s yet another scenario. Let"s say instead of taking off, we"re trying to land this plane on said conveyor belt. According to the moronic theory, such plane should come to an instant and immediate stop upon its wheels making contact with the conveyor belt.

Christ...

Let it die. Please. this thread is just another annoying, pointless cockfight.

The plane does take off.

My scanner is a couple years old, and hasn"t been used in just as long. I"ll look for another one somewhere tomorrow to get the free body diagrams to y"all.


1) Friction

The plane takes off thanks to friction, or rather the occasional lack of it. There are two places with friction, despite how we want to picture it: between the plane and belt, and between the belt and its inner components (motor, wheels it"s wrapped around, etc).

If the friction is higher between the wheel and belt, then the plane literally yanks the belt in the direction it wants to go. The plane ultimately will have more force than the belt on occasion, because it is the belt that reacts to the plane. All a pilot needs to do in this case to take off is hit the gas.

If the friction is higher between the inner mechanics of the belt, then the plane is pulled back some, but due to the reactionary nature of the belt, it will still ultimately lose. The plane will fight it until it takes off. If it"s acceleration isn"t sufficient, then the pilot just pushes the stick forward more.


2) The plane"s taking off does depend on the pilot.

...but then again, it does so when there is no belt, as well. The pilot has to make sure that the plane"s engines kick the belt"s ass.


3) If the pilot sucks, and the two sources of friction are equal, but the plane still takes off.

Such an equillibrium is always uneasy. The whole thing will oscillate with the plane"s wheels spinning out until the plane takes off anyway.




One interesting thing we found is that whenever the plane"s engines give it a good kick in the ass, and it overcomes the belt, then the wheels actually spin faster than they are driven to spin by the force of the plane"s thrust.

All this means is that the wheels spin out often, and the belt adds to their angular momentum. At these points, the wheels don"t matter at all.


In other words, those who have said that the plane lifts off have been exactly right, and almost entirely right about the reasons why. The only difference between their hypothesis and reality is that friction can"t be ignored, because it is ultimately the dynamics brought about by the friction itself that defines the system.

When I add the diagrams, I"ll just edit them into this post as links. Failing my being able to figure out why this POS I have isn"t working, I"m pretty sure there"s a scanner at the library here.

The friction between the plane and the belt is almost non-existant though. You keep talking about the wheels and the belt, but this does not exert a direct force against the plane. The transmission of force is from the wheels to the plane. The Coeff on the wheels relative to the plane because of ball bearings is so small that the force exerted by the belt to the plane is not enough to counteract the force of the engines forward.

Or, looking at the whole picture...

Since the velocity of the upper side of the belt relative to the ground is equal to that of the plane relative to the ground in the opposite direction (note that the question does not say that the belt is attemping to exert force high enough to stop the plane from moving), the force exerted on the plane is equal to the thrust of the plane multiplied by the Coeffs of all points of friction between the plane and the belt, all squared. Unless the Coeffs of all those points are all 1, the plane will have more force opposite the belt than the belt applies back to it; therefor it will take off.

Except that you know, plane engines don"t work in a vacuum, cause they need "oxygen." I know, wild eh?

As far as an electric engine, what the fuck are you even talking about? An electric engine to spin the wheels of a plane until it took off, and uh, then what?

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http://www.megahobby.com/Modeling_Zo...en_planes.html
http://www.treehousetoyshop.com/cata...l.php/pid=1531

Gee, learn something new every day eh? Nothing in the post said ANYTHING about what TYPE of plane was being discussed, just that it was moving forward on a conveyor...

The original blurb:

An airplane is on a conveyor. The conveyor is set to match the speed of the airplane in the backwords direction. However fast the plane moves, the conveyor moves just as fast.

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BTW, some planes (like the F15 for example) have a thrust-to-weight ratio of > 1. This means they need no lift from air moving over their wings to "fly" (control is another story) and can actually accelerate straight up. Most planes, including the two I linked above (grin) can"t do that however, and depend upon lift generated by air moving over their wings for (positive) vertical motion. They typically use horizontal motion (provided by an engine (OR A RUBBER BAND...) moving a prop, or exhaust gasses from a jet, etc.) to generate the afore mentioned airflow over their wings. The difference in airspeed (and the corresponding difference in air pressure) over the top and bottom of the airfoil (wing) is what generates lift. Thus only speed differential that actually matters to a plane (with a non-positive thrust-to-weight ratio, with respect to vertical motion, etc. etc.) is between it and the air it"s moving through. If it"s not moving thru air (or other compressible medium for the pedantic!) then it"s not generating any lift, and that means it"s not going up. In a vacuum, (most) planes don"t fly.

All I was doing in my original, and apparently poorly received post was to point out, in what I thought was a humorous way, that the original query was insufficiently specified to give a simple "yes" or "no" answer, without making quite a number of not necessarily obvious assumptions. Doing a lot more of that here. Or something like that.

So- Will the plane be able to take off? The answer is... it depends.

RdT.

Grayson Carlyle said:

Unless the Coeffs of all those points are all 1, the plane will have more force opposite the belt than the belt applies back to it; therefor it will take off.

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*smacks forehead*

? ? 1, always

Only in the pretty much nonexistant case that both coeffs equal one would the more advanced model even be needed. The back force is divided twice, and the forward force acts independant of friction. The plane will always take off.

Jesus, that"s so much simpler. This semester must have melted my brain. I need a vacation.

TheWand said:

I have yet to see one person who believes it will fly change their mind and say it wont.

When I originally saw the question, I also thought the plane couldnt move.. but after I realised the whole illusion the conveyor creates, I couldnt believe how stupid I was. It really is 6th grade simplicity.

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Yeah, it is 6th grade simplicity.
...and I originally said that it would take off, then it wouldn"t then that it would, finally after going through all the techinically unneccessary steps, I agree that it takes off.

I wonder if all physics students find themselves over thinking everything. I"d feel bad, except that our professor didn"t even realize that about the coeff of friction.

One day, long from now on a business trip to Vegas, I"m going to end up using calculus to pay a prostitute.

? ? 1, always

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For this scenario, yes.

Universally and Generally, apparently no.

I was taught, even in post-secondary courses, that ? ? 1, always, but...

http://www.school-for-champions.com/...n_equation.htm

says apparently not

Many students and teachers mistakenly think that ? must be less than 1. That is incorrect, since Fr could be many times the normal force.

One extreme example is if you glued an object to another. The resistance to moving the objects would be very large and the coefficient of friction would also be very large. If the glue was so strong that they could never be slid against each other, then ? would equal infinity.

The reason people think that ? must be less than 1 is probably since most listing of coefficients of friction have values less than 1. That is because most materials of interest usually slide relatively easy on each other.

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I"m not going to act all smart or anything, because I just learned that yesterday.

I realize I just linked a site called "school-for-champions", but you can google it and other sites back it, like Wikipedia.

Good and true observation, Frawdo

Fixed:

? ? 1, always when the two bodies in question are free to move against eachother



...this is pretty much the first time I"ve seen anything having to do with physics sound dirty

I knew there just a little something that was tripping you up, making the problem seem more complex. I couldn"t understand all the attention to friction. I guess it was the coefficient.

I was going to ask, in your model, did you account for the fact that the Normal force on the plane decreases as the plan accelerates and gains velocity? So, at the point of lift-off the Normal force is actually 0, and thus friction would also be 0.

That"s something I"ve been mulling over, but I"m not sure how to apply it in terms of the math involved.

Anyways, the plane flies

I question the possibility that a coefficient of friction may be greater than 1. It is a measured value of the normal force against an opposing force.

The only way it could be greater than 1 is if when you applied a force, the object you are applying it to applied more force back (Force of Friction/Normal Force). Since the Force of Friction is derived from the Normal Force and no other forces, it cannot get the energy to apply a greater force back to it than was applied. It would require producing energy from a source other than the Normal Force.

Imagine pushing a car from the back end, only to have it go towards you. Not only would this be a source of infinite energy, if you were to pit 2 such forces against each other they would blow up the universe.

The only possible way for a coefficient of friction to be 1 is if it the object receiving the force were perfectly hard and perfectly elastic; losing no energy via heat or molecular bonds.

If you take the original equation of Fr = u*N, if u were greater than 1, it"s quite obvious that you would be getting more energy out of the system than you put in.

Frawdo said:

I was going to ask, in your model, did you account for the fact that the Normal force on the plane decreases as the plan accelerates and gains velocity? So, at the point of lift-off the Normal force is actually 0, and thus friction would also be 0.

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Yes, but not exactly. The normal force decreases as the lift force increases, not necessarily as the velocity goes up.

I question the possibility that a coefficient of friction may be greater than 1. It is a measured value of the normal force against an opposing force.

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It comes out as infinity in the case of two objects that are bonded, but it technically isn"t if the two objects can be pulled apart. That"s just one of the things that"s assumed for convenience when it doesn"t impact the system.

For example, if you have a weld, and you know that there will not be enough force present in your calculations to break that weld, then you can represent this as there being an infinite coefficient of friction between one object and the other.

Of course, that"s kind of moot because it would be easier to just treat the welded objects as one object instead.

To be honest, I can"t think of an instance off the top of my head where it"s not just simpler to treat the two objects as one, but I wouldn"t doubt that there is an instance where an infinite friction could perhaps be useful in deriving equations.

How does it come out as infinite on a bonded object? u = Ff/N

For it u to be infinite, N would have to be 0. But Ff cannot exceed N, so even if N were 0, 0/0 is not infinite.

In a bonded object, Ff isn"t infinite and neither is N=0. Force out = force in. Even in a bonded object you have loss of energy due to molecular bonds being stressed. Enough force over enough time will break all those bonds and the object will fall apart, but not necessarily at the point of bonding. All that is beside the fact that if you were to push on a solid object, the force returned is nearly equal to the force applied.

A bonded object would have a coefficient very near 1, but not surpassing it.