Qhimm.com Forums
Off-topic forums => Completely Unrelated => Topic started by: Jari on 2006-12-31 12:57:24
-
Ok, here's a question for you, imagine the following:
A plane is sitting on a huge conveyor belt. This belt will act as a runway for the said plane. Problem is that the belt is set up to match the speed of the plane, but it will run in opposite direction. Think of a treadmill, if you have trouble imagining what I mean.
Here's the question: will the plane be able to take off?
-
No. A plane must be able to lift from the ground, and that can't happen if there's no air presure under the wings (?)
but it makes you think alittle :mrgreen:
Edit: Nice avater jari :D
-
It would take off. The wheels under the plane are roll freely, so their isn't the same reaction as say an automobile. Their would only be slight friction, but the forward inertia/thrust would easily compensate for that.
-
Im going to say no. Because its not going anywhere.
-
It would take off. The wheels under the plane are roll freely, so their isn't the same reaction as say an automobile. Their would only be slight friction, but the forward inertia/thrust would easily compensate for that.
Look it doesn't matter if the weels are spinning at lightspeed, the plane would not take off. It's the air under the wings that creates the lift. The engines just pushes it forward to get the air pressure to be able to take off :)
The plane isn't moving at all, the conveyor belt is just meaking the weels move and will not let the plane be able to get enough lift for it to take off.
I may be wrong, but as far as my thinking, it would just stand still on the ground, with full thrust on the engines.
-
The plane may not be physically moving from the point of view of the observer, but the force of the planes kinetic energy is still acting on the free moving air. Unless this is in some crazy vacuum. :-D
-
depends on the plane
-
depends on the plane
That's a safe answer. :-D
-
Yes. if it's a Harrier, it would just take off, (but, it's rare to se a harrier actually take off verticaly, the thrusters can't handel the heat off the engines while taking off. so they are saving the "cooling" when they are going to decent verticaly.)
-
Forgive me, if I'm brief but something that happened in this thread earlier (it seems to be gone now, but it's so sad because it's the third time this person has done something like this - so I have no other option than to assume that it was an intentional attempt to ruin this) really pissed me off. For the same reason I'm cutting this thing short.
Otokoshi has the exactly right answer - it will take off just as usual, except that the wheels will be spinning twice as fast. Entirely regardless of what kind of a plane it is. Only thing that could possibly prevent it from taking off is the increased rolling resistance of the tires, and increased friction of the bearings - since they are spinning twice as fast as they normally would. This is a much smaller force than the aerodynamic drag however, so no plane should have much difficulty overcoming it.
As the plane's engines try to move it in relation to the air surrounding it (which certainly is not traveling with the belt's surface), it doesn't actually matter what the belt does - how fast it moves or which way - or what the wheels are doing, as long as they are not locked.
There is another version of the question, with a slightly different wording, and that is much, much more tricky; the question specifies that the belt is trying to match the speed of the wheels (well, the speed the wheel's road surface is traveling around and around - wheel itself of course travels just as fast as the plane), and as far as I can see it, this could lead to a case where belt's speed reaches infinity soon after the start. But since this question was talking only about the speed of the plane, it's not a problem here.
And yes, if someone is wondering, the speed of the plane and the speed of the belt's surface are both in relation to a stationary outside observer - or "real world", if you prefer. The question obviously wouldn't work, if they were for example measured in relation to each other, or something like that.
Here (http://ooine.com/index.cfm?CommentID=154)'s one explanation and here (http://www.straightdope.com/columns/060203.html)'s another (that explains the "matching the wheel speed"-issue, as well).
Reason I asked this was because this seems to generate massive amounts of debate, and even some real life pilots seem to misunderstand the simple laws physics that make their planes fly:
http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/
http://forum.physorg.com/index.php?showtopic=2417
http://www.airliners.net/discussions/tech_ops/read.main/136068/1/#1
http://www.r3vlimited.com/board/showthread.php?t=79439
:-D
PS. On the other hand, car with a wings attached to it (nevermind that such a thing could never actually really fly - without a propeller or a jet engine or some other alternative means of propulsion, since wheels can't power it when airborne) would never lift off. Simply because cars, unlike planes, use their wheels to generate motion and as such have to do it in relation to the belt's surface, not air. So, car would just sit on the belt, never moving, because the belt would simply match any attempt to go forward.
-
Ahh fair enough. Im not into physics and whatnot so I was mainly guessing.
Also, what do you mean "something happened"? Have there been dissapereing posts again?
-
Don't worry about it, someone just went and blurted out the right answer right at the start.
I'm going to give him the benefit of a doubt, and assume that he actually had not read the post where I first mentioned this question, told the correct answer, and that I was going to post it here... and that while it was okay for the people in the know to answer, I would not like it if they explained it - given that I had already explained it to them.
I actually do believe that there was no ill will intended, but this person has done this before - more than once - which is what pissed me off.
Anyway, no big deal. :)
EDIT: This is also why the actual voting is locked - I was more than little peeved at that point. One of the admins must have intervened afterwards, but they didn't re-open the poll.
-
What the hell? I posted my answer in spoiler tags that apparently don't work on this board anymore. I didn't realise that at the time and I would have corrected it following your first PM however I've been away for New Year so sorry? And when have I done this before?
-
Actually, the thing might be even more clever than I originally thought.
For planes it should work exactly as described - that part is right. Plane will care very little about the belt, since it tries to move itself in relation to the air surrounding it.
But for cars... actually, I think that a car might be able to accelerate as well. See, it states that the belt will match the speed of the car (just imagine that it's a car now, instead of a plane). So, if a car is moving forward at 50 mph, the belt is moving 50 mph to the opposite direction. Thing is... that only means that the wheels of the cars are spinning at a rate that corresponds to it going 100 mph - because that's exactly what it's doing - relative to the belt.
Right?
If it was trying to match the speed of the wheels - or rather the speed of the surface of the wheel - car would never move (for a plane this might be much more trickier, like I said before...). But since it does not try to do this...
Jedimark: Oh, when have you done this before? Remember that quiz by Nori? You know, the one which she asked the replies be given as PMs, instead of writing them on the thread? And certain PM from me, that I thought was meant as private, not for people to read?
And you of course were totally incapable of both, previewing the post, or checking the thread after you posted? Both of these things would have revealed that tags indeed did not work.
I'm willing to assume that you don't do these things out of malice, but just because you type before you think. And yes, I'm mad because of it - mostly because you don't seem to learn at all. Shall we leave at that, or do you want to argue?
-
I don't want to argue with anyone but I think we should leave it there before it gets even more trivial than it already is.
-
To Jari and Otokoshi,
You have to remember that thrust is a vector quantity, with both speed and direction. If the conveyor speed is exactly matched to the aircraft speed but reversed direction, then in fact the two vectors (aircraft and conveyor) cancel each other out. Bernoulli's principal, which is what governs the "lift" of aircraft wings, states that the airflow has to be significantly different between two sides to produce the "lift" nessesary to allow an object flight. In most cases, an aircraft requires 100 (for small)- 175 (cargo/passnger) miles per hour forward air speed (FAS) to acheive enough airflow/lift to maintain flight. However, on a conveyor system like you suggest, you cancel out the forward direction needed. Discounting the airflow to/from the engines, the aircraft would experience no more airflow over the craft then if it were standing still, hence it could not "take off".
-
We are aware of what makes planes fly - at least I am, and I'm fairly sure that Otokoshi is, too.
But can you explain how the conveyor manages to exert force to aircraft to stop it from moving, through a set of wheels that are freewheeling under the aircraft? Other than the rather minor (compared to aerodynamic drag, for example) force exerted via the higher than normal rolling resistance of the wheels and friction at the bearings (since they are spinning at twice the speed they would on a normal takeoff).
Do note that the question does not say that the conveyor would be trying to keep the plane still - it isn't. It's merely running to the opposite direction at the same speed the plane is moving, making the plane's wheels spin twice as fast.
Yes, the plane needs to move to take off - well, in relation to air at least. And it will move, just like it would on a regular runway. It will accelerate bit slower, due to the reasons I mentioned, but it will take off.
-
To Jari and Otokoshi,
You have to remember that thrust is a vector quantity, with both speed and direction. If the conveyor speed is exactly matched to the aircraft speed but reversed direction, then in fact the two vectors (aircraft and conveyor) cancel each other out. Bernoulli's principal, which is what governs the "lift" of aircraft wings, states that the airflow has to be significantly different between two sides to produce the "lift" nessesary to allow an object flight. In most cases, an aircraft requires 100 (for small)- 175 (cargo/passnger) miles per hour forward air speed (FAS) to acheive enough airflow/lift to maintain flight. However, on a conveyor system like you suggest, you cancel out the forward direction needed. Discounting the airflow to/from the engines, the aircraft would experience no more airflow over the craft then if it were standing still, hence it could not "take off".
I believe Jari might have already explained it well enough, but I'll just beat this to the ground. :-D
As I said earlier, the plane my not be theoretically moving on the ground, but the air is still circulating as a normal takeoff procedure. Whatever the aircraft's FAS and so on is comes down to simple physics. As I mentioned earlier, it may not be moving on the ground, but the potential energy gained by the plane will achieve lift capable for takeoff. This is the simplest explanation I could find.
Kinetic energy is the energy by virtue of the motion of an object. It is defined as the work needed to accelerate a body from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. Negative work of the same magnitude would be required to return the body to a state of rest from that velocity.
With the free rolling wheels of the plane, the only negative work being done is the slight friction of the wheels contact with the belt. The air would act as normal, the airflow would remain the same going to/from the engines...unless this is done in a vacuum of course. :-D
Edit:
I hope this all makes sense, I'm still recovering from the New Years party I threw. *grabs more Aspirin*
-
As I said earlier, the plane my not be theoretically moving on the ground, but the air is still circulating as a normal takeoff procedure. Whatever the aircraft's FAS and so on is comes down to simple physics. As I mentioned earlier, it may not be moving on the ground, but the potential energy gained by the plane will achieve lift capable for takeoff. This is the simplest explanation I could find.
Edit:
I hope this all makes sense, I'm still recovering from the New Years party I threw. *grabs more Aspirin*
Heh. Actually, I do believe that it indeed is moving on the ground. Or relative to the ground - if one wants to nitpick that it's actually on a conveyor and not on ground. :P
I agree with Ooine (http://ooine.com/index.cfm?CommentID=154)'s view:
If you were watching from the sidelines, the take-off roll would look identical to any other that you may have seen. The only difference being the rate at which the wheels are spinning. They would be spinning approximately twice as fast.
And since the air is assumed to be still (relative to that very same ground) or at least not moving more than it would due to regular wind - which planes have to deal with in on daily basis, the plane is also moving through air, thus eventually gaining sufficient airspeed for takeoff.
Hmmm... perhaps it would help if I'd use numbers - all of these are relative to ground, or a stationary observer, if you wish:
- plane moves 150 kts one way
- belt moves 150 kts the other way
- air is still
- thus plane moves 150 kts relative to air
- and 300 kts relative to the belt
-
The thing is, the kinectic force is being transfered. The aircraft is pushing down on the conveyor and forward. The conveyor is push up towards the aircraft and back. A little ASCI art might show this better:
<=======Aircraft
||
||
\/
/\
||
||
Conveyor=======>
Normally, the aircraft is only dealing with the up-down forces (along with inertia) so it can direct it's energy to forward momentum. However, this shows that an opposite running conveyor would, in fact, negate that forward momentum. Unless the conveyor slows down or stops, the plane has a net sum of 0 mometum for forward motion. Therefore, it is ,in a sense, standing still in relation to it's surroundings (except the conveyor itself). It would be just like being on a ten-speed bicycle on top of a treadmill. Do you think you would feel a breeze if it was running full speed? No, because you are not moving in relation to the room, only in relation to the treadmill surface. And, in this case, the aircraft is not moving in relation to it's surroundings, merely in relation to the conveyor.
-
However, this shows that an opposite running conveyor would, in fact, negate that forward momentum.
How? How does it show that? How is the belt affecting the plane in any way? It doesn't even touch it. Only thing the belt can directly affect, are the wheels of the aircraft... and in case you haven't noticed, they can turn freely.
It would be just like being on a ten-speed bicycle on top of a treadmill.
Okay, have you noticed that the aircrafts actually move by pushing air? Air. Not belts. They are not wheel driven, like cars or bikes (which, like I said earlier, actually could accelerate from the belt as well, provided that the situation matched the one described in the question).
The aircraft is very much accelerating in relation to air, just like they always do.
-
Hm, I begin to wonder what happens if there is only a limited amount of air in front of the plane... does it create vacuum? Will the plane take off? Or will it start dancing Macarena? :P
Okay, enough of the jokes.
I agree with Jari's, Okotoshi's and Ooine's view.
Initially, I thought that the plane would not lift up, too. However, as you said, the belt does not affect the air, no matter how fast and in which direction it moves. Therefore, I see no reason why the plane should not take off...
I would like to hear (/read) halkun's view on this topic, as he has been working on an aircraft carrier some time ago...
- Alhexx
-
Now, this is going to sound a strange question, but where is this conveyor belt setup? Inside or outside?
I guess this is just to try and straighten things out in my mind.
The way I see it, the plane only can take off with the air moving around it, right. Now the only way it can get enough lift, is by going fast AGAINST the air current.
If its on a conveyor belt, it is not going anywhere, therefore surely there is little air resistance being created, therefore no lift, therefore no airbourne....ness....
Meh, Ramblings.
-
If its on a conveyor belt, it is not going anywhere...
But it is going somewhere--in the direction the turbines/propellers are pulling it.
Think of it like this: You have Rollerblades on and you are standing on a treadmill that matches the speed of you in the opposite direction. You are holding a rope being towed by a car (in front of the treadmill) and the car stars moving.
You're saying that the treadmill is going to pull you backward, and therefore there can be no positive net gain in velocity.
Jari is saying that the wheels/treadmill have no effect whatsoever on your forward movement and your wheels will just be spinning twice as fast when you reach the end of the treadmill.
-
However, this shows that an opposite running conveyor would, in fact, negate that forward momentum.
How? How does it show that? How is the belt affecting the plane in any way? It doesn't even touch it. Only thing the belt can directly affect, are the wheels of the aircraft... and in case you haven't noticed, they can turn freely.
The wheels ARE important, because they allow the aircraft to ROLL and accelerate forward. They redirect the energy and inertia of the craft. Otherwise, you wouldn't need them. You could just throttle up until you reached the proper turbine rate, then release the brake for an almost instantaneous take off.
Why do you think runways are so long? Why do you think aircraft carriers have catapault launchers? If a conveyor system worked like you think, it would not be a lot cheaper (not to mention SMALLER) to use, than a 1/2 mile long deck with 3 x 50 ton, steam-driven catapault?
It would be just like being on a ten-speed bicycle on top of a treadmill.
Okay, have you noticed that the aircrafts actually move by pushing air? Air. Not belts. They are not wheel driven, like cars or bikes (which, like I said earlier, actually could accelerate from the belt as well, provided that the situation matched the one described in the question).
The aircraft is very much accelerating in relation to air, just like they always do.
Aircraft do not move by PUSHING AIR. It is the SHAPE of the aicraft moving THROUGH the air that produces lift. The engines burn fuel with air to produce thrust, put they're not simply 'moving air'.
The engines transefer kinetic energy to the airframe (via reactionary force), which then transfers down to the wheels, and then the runway itself. The shape and structure of the wheels allows the aircraft to move forward (which tranfers kinectic energy away from the airframe). As the aircraft rolls forward, the air begins to flow over the surfaces of the airframe. Once the aircraft reaches a certain speed (which varies from one aircraft model to another) the airflow level produces enough lift to support the airframe in flight.
However, if you take away the forward speed (via the conveyor belt), then the nessesary airflow from the airframe accelerating through the air never happens.
Again, this is why aircraft carriers are designed the way they are. The aircraft are connected to the catapault. The aircraft throttles up his engines. Then the catapault pulls the aircraft forward (at a vastly accelerated rate) so it is at the proper airspeed/airflow at the end to maintain flight (0-175 MPH in 2 seconds).
-
If a conveyor system worked like you think, it would not be a lot cheaper (not to mention SMALLER) to use, than a 1/2 mile long deck with 3 x 50 ton, steam-driven catapault?
Umm... I don't think you quite understand this whole concept. You still need to reach the same speed you normally would to take off (i.e. the conveyor belt would have to be the same length as a normal runway), the only thing that would change is the speed of the wheels which have no effect on flight whatsoever (beside keeping the plane from grinding along the ground).
-
GGRRRRRRRR!!!!! :x :x :x
You people still don't get it... All the wheels do is allow the aircraft to move (and accelerate) down the runway. The forward movement is the important part. A plane on a conveyor belt (in the opposite direction, at the same speed) has NO forward movement in relation to the surrounding air. Hence, there is NO airflow over the wings and NO lift.
Which part are you not getting?
Edit: I was just reading the 'Oinnes' post on this subject. The "solution" section is correct on the first 2 points. However, the third states:
Thrust acts according to Newtons Third Law of Motion - every action has an equal and opposite reaction. The thrust of the engines is acting against the air.
(emphasis mine)
The engines are NOT pushing the air. They transfer kinetic energy to the AIRFRAME, in the opposite direction of the "thrust" caused by the turbine engines. Turbine engines are NOT designed to simply move air. They draw in air, which is mixed with fuel, compressed, then ignited. They use turbine engines in a lot of different things, including large generators and the M1 Abrams Main Battle Tank.
The wheels allow the airframe to move in the opposite direction of the "thrust" from the engines. The wheels are NOT supplying any motive force, they simply allow the aircraft to roll over the surface. HOWEVER, some kinetic force is transfered to the ground, which lessens somewhat as it picks up speed (until the airflow "lift" cancels it out entirely). This is because GRAVITY is trying to pull/hold the mass of the aircraft down to the ground. The friction between the wheels/ground is irrelevent.
Because the wheels are free-wheeling and we have assumed zero friction at the hub, it follows that the conveyor belt, no matter how fast it is moving, CANNOT EXERT ANY FORCE on the aircraft with respect to forward motion! There is no force in our experiment that can oppose the thrust vector of the aircraft.
If the conveyor belt cannot exert any relevant force on the aircraft, you can completely ignore it. Ergo, the aircraft takes off as if nothing unusual is happening.
Here, somebody went wrong big time. The conveyor belt, by moving in the opposite direction to the "thrust vector" of the aircraft, CANCELS OUT the forward motion of the aircraft, in relation to the surrounding air. It's NO DIFFERENT than a person, bike, or car "keeping pace" on a treadmill. The method of force tranferance doesn't matter, the results are still the same. ZERO relative movement to the surrounding air.
It isn't the conveyor or treadmill exerting force on you.. it is YOU (or the object) exerting force on the conveyor. That's an important distinction, because the conveyor (when not turned on) is no different from any other stationary surface. It's only when it's in motion, that it adds or subtracts to our relative speed, depending on our direction in relation to its movement. The airframe is only trying to move relative to the ground (because of gravity and inertia). This gives it forward speed, and in addition, airflow over the airframe. But because it is on a conveyor which is running in the exact opposite direction, and same 'apparent' speed, this would in fact cancel out the forward speed of the aircraft.
-
The wheels allow the airframe to move in the opposite direction of the "thrust" from the engines. The wheels are NOT supplying any motive force, they simply allow the aircraft to roll over the surface. HOWEVER, some kinetic force is transfered to the ground, which lessens somewhat as it picks up speed (until the airflow "lift" cancels it out entirely). This is because GRAVITY is trying to pull/hold the mass of the aircraft down to the ground. The friction between the wheels/ground is irrelevent.
This is paradox...
Here, somebody went wrong big time. The conveyor belt, by moving in the opposite direction to the "thrust vector" of the aircraft, CANCELS OUT the forward motion of the aircraft, in relation to the surrounding air. It's NO DIFFERENT than a person, bike, or car "keeping pace" on a treadmill. The method of force tranferance doesn't matter, the results are still the same. ZERO relative movement to the surrounding air.
What you are saying is that the aircraft does not try to pull air from in front of it to behind it. But it does.
And that's the main differerence between a car and an aircraft. The car tries to pull the street by rotating its wheels.
I do not say that the plane takes off because it pulls air, but it moves because of this. The shape of the aircraft makes it fly as soon as there is enough airflow.
In your theory, saying that there is no movement relative to the air surrounding the plane when the plane does not move (as you claim), that would mean that the air does not move either. But it does, since the engine of the aircraft is pulling it. That's a contradiction.
- Alhexx
-
RPG already explained this, but since I had written this, long, long time ago...
Now, this is going to sound a strange question, but where is this conveyor belt setup? Inside or outside?
You mean in a building or outside in the wild? Well, I suppose that if we are talking about a real, life-sized aircraft, it would have to be outside. As not only the conveyor has to be slightly longer than the minimum take off run the plane would need normally, attempt to fly a real aircraft inside a building would most likely face some challenges - namely walls and ceiling. :P
Other than those things, it really doesn't matter. Or, we can assume that it's a R/C model plane - small and slow enough one to be flown inside a large building. Then it can be inside as well. :)
The way I see it, the plane only can take off with the air moving around it, right.
True.
Now the only way it can get enough lift, is by going fast AGAINST the air current.
True.
If its on a conveyor belt, it is not going anywhere, therefore surely there is little air resistance being created, therefore no lift, therefore no airbourne....ness....
But it is going somewhere, that is the whole point of the question. :) The belt has no way of stopping it from going somewhere - at least with the current rules of the question.
RPG's example about rollerblades, treadmill and a car is very good, though. This is the very same thing; plane is using method of propulsion that doesn't give rat's ass about the conveyor and the wheels underneath it take care of any effect the belt might otherwise have, so if that example is easier to grasp than planes and conveyors....
The wheels ARE important, because they allow the aircraft to ROLL and accelerate forward.
I'm glad that we agree on this.
Why do you think runways are so long? Why do you think aircraft carriers have catapault launchers? If a conveyor system worked like you think, it would not be a lot cheaper (not to mention SMALLER) to use, than a 1/2 mile long deck with 3 x 50 ton, steam-driven catapault?
Like RPG said, I don't think that you understand the question. If the conveyor system worked like I think, it would work almost exactly like a regular runway.
Certainly, you could reverse the belt's direction, engage wheelbrakes of the aircraft, and use the belt as a catapult. But that is not what the question is about.
But I don't really see how you can think of three or four mile long conveyor belt capable of running at 200 kts as a cheaper or smaller than a catapult (or even physically possible, for that matter)? Because the belt is not any shorter than a regular runway, actually it should be bit longer to counter for the energy losses in rolling resistance of the wheels.
Aircraft do not move by PUSHING AIR. It is the SHAPE of the aicraft moving THROUGH the air that produces lift. The engines burn fuel with air to produce thrust, put they're not simply 'moving air'.
I said nothing about what produces lift.
If you don't want to call it 'moving air', sure. That's semantics, but it makes no difference; the point is that a jet engine's (or propeller's) frame of reference is the air surrounding it - unlike for example car's, which frame of reference is the road underneath it. Or to be specific, in car's case the road is the reference point for the wheels, since they do the job of actually propelling the car forwards - just like engines do for aircraft.
What is important here that the aircraft's engines do not care about the conveyor. They are propelling the airframe forwards with a force that is not dependent on what the belt is doing.
However, if you take away the forward speed (via the conveyor belt), then the nessesary airflow from the airframe accelerating through the air never happens.
But the whole point is that the belt is not capable of that. Not unless you engage the wheelbrakes on the aircraft... and why would you do that, if you are trying to take off?
If the wheels are able to rotate freely, only thing the belt can do is make them spin faster.
Again, this is why aircraft carriers are designed the way they are. The aircraft are connected to the catapault. The aircraft throttles up his engines. Then the catapault pulls the aircraft forward (at a vastly accelerated rate) so it is at the proper airspeed/airflow at the end to maintain flight (0-175 MPH in 2 seconds).
And usually the carrier is also sailing headwind, at full speed to give the plane additional airspeed - some 30 knots plus the wind, actually. See, some of us know quite a bit about planes and aviation, some of us might even be interested in them.
That paragraph of yours is not really related to the question at hand in any way, though.
All the wheels do is allow the aircraft to move (and accelerate) down the runway.
I'm glad that we agree on this. :) But I have to ask, why do you claim otherwise in the very same paragraph?
You say that the conveyor has the power to stop the plane from moving. Okay, how does it do this?
- Wheels are the only part of aircraft that touch the belt. Do we agree on this?
- Thus the wheels are also the only part of the aircraft the belt can affect directly. Rest of the aircraft is affected through the wheels, so to speak. Do we agree on this?
- So, when the belt is trying to keep the aircraft from moving, like you seem to think, it has to somehow do this through the wheels. Do we agree on this?
- Those wheels turn freely, allowing the aircraft to move and thus accelerate on a runway. Like you said. Do we agree on this?
- Which means that when the belt is trying to keep the aircraft still by running to the opposite direction, only thing it actually achieves is to make the wheels spin faster. Do we agree on this? If not, please provide an answer about how the belt can stop the plane, by merely affecting its wheels.
The engines are NOT pushing the air. They transfer kinetic energy to the AIRFRAME, in the opposite direction of the "thrust" caused by the turbine engines. Turbine engines are NOT designed to simply move air. They draw in air, which is mixed with fuel, compressed, then ignited. They use turbine engines in a lot of different things, including large generators and the M1 Abrams Main Battle Tank.
Now you are confusing things. While you can indeed have a gas turbine outputting power through a shaft, like in M1, jet engines are different. They are somewhat simpler, and yes, they infact do propel the aircraft by the thrust of their exhaust gas. Or in layman's terms; by pushing hot air out of their ass.
You should seek a certain Top Gear (I'm fairly sure that it was a Top Gear) episode on Google video. They demonstrate the power of 747's jet engine by keeping the plane still on the runway with wheelbrakes, while having *one* engine running at full throttle for a short period of time (can't have more than one, or for long periods of time, as the brakes, wheels and landing struts can't take the stress). Then they drive a car across the runway, some distance behind the 747. Regular family car swerves and tilts rather badly when the airflow hits it. However, good old Citroen CV2 - being both light and having somewhat highish center of gravity - actually flips over in the wind.
So, yes, jet engines are very much pushing air.
And while we are at it; while internal combustion engine only makes a shaft turn, a propeller attached to that shaft is... you guessed it, pushing air. :) That's on a prop aircraft, of course.
Not that this would really matter, since we mean by "pushing air" the very same thing you mean by "transfer kinetic energy to the AIRFRAME, in the opposite direction of the "thrust" caused by the turbine engines". It's just semantics.
The wheels allow the airframe to move in the opposite direction of the "thrust" from the engines. The wheels are NOT supplying any motive force, they simply allow the aircraft to roll over the surface.
Indeed.
HOWEVER, some kinetic force is transfered to the ground, which lessens somewhat as it picks up speed (until the airflow "lift" cancels it out entirely). This is because GRAVITY is trying to pull/hold the mass of the aircraft down to the ground. The friction between the wheels/ground is irrelevent.
Indeed. This is called rolling resistance, or sometimes rather curiously rolling friction, in layman's terms. And trust me, compared to aerodynamic drag, this is a very small force indeed, especially at higher speeds. Me and Otokoshi have been mentioning this since our very first posts, we are aware of it - it's just not nearly enough sufficient to slow the aircraft considerably, much less stop it.
Volkswagen actually had a nice demonstration; they were showing off their Touran SUV with a V10 diesel engine. They towed a B747 with it. :P
So, the rolling resistance isn't that massive. Granted, the Touran was somewhat modified - for one it was carrying a great deal of extra weight, but it does show that a powerful SUV is capable of overcoming the rolling friction of 747's tires. And even the static friction, which is - as I'm sure you know - greater than the rolling friction.
Because the wheels are free-wheeling and we have assumed zero friction at the hub, it follows that the conveyor belt, no matter how fast it is moving, CANNOT EXERT ANY FORCE on the aircraft with respect to forward motion! There is no force in our experiment that can oppose the thrust vector of the aircraft.
If the conveyor belt cannot exert any relevant force on the aircraft, you can completely ignore it. Ergo, the aircraft takes off as if nothing unusual is happening.
Here, somebody went wrong big time. The conveyor belt, by moving in the opposite direction to the "thrust vector" of the aircraft, CANCELS OUT the forward motion of the aircraft, in relation to the surrounding air. It's NO DIFFERENT than a person, bike, or car "keeping pace" on a treadmill. The method of force tranferance doesn't matter, the results are still the same. ZERO relative movement to the surrounding air.
OMG. Of course the method matters. Especially when in this case the belt has NO method at all to transfer any force directly to the plane. Except through the wheels, and for the umpteenth time; the rolling resistance is a very small force compared to aerodynamic drag.
And yes, it is very much different from your examples, because of that very reason.
It isn't the conveyor or treadmill exerting force on you.. it is YOU (or the object) exerting force on the conveyor. That's an important distinction, because the conveyor (when not turned on) is no different from any other stationary surface. It's only when it's in motion, that it adds or subtracts to our relative speed, depending on our direction in relation to its movement. The airframe is only trying to move relative to the ground (because of gravity and inertia). This gives it forward speed, and in addition, airflow over the airframe. But because it is on a conveyor which is running in the exact opposite direction, and same 'apparent' speed, this would in fact cancel out the forward speed of the aircraft.
No. Again and again, no.
The engines are pushing the airframe in reference to air. And while there indeed is rolling resistance - like me and Otokoshi haven been saying since the beginning - between the wheels of aircraft and the belt, it's not nearly significant enough to prevent the plane from taking off.
The reason Ooine makes the assumption that there's no friction at the wheelhubs - or rolling resistance for that matter - is because it's not significant enough to be an issue. And thus the whole point of the question is easier to understand if you ignore it.
EDIT: Removed one unnecessarily repeated sentence from one of the quotes.
EDIT2: It's actually Touareg, not Touran. I blame Volkswagen and their stupidly similar names. :P Anyway, couple of links about it: 1 (http://www.auto-power-girl.com/cars-2007/volkswagen-specifications/volkswagen_touareg_tows_boeing_747-1335), 2 (http://www.carpages.co.uk/volkswagen/volkswagen-touareg-23-11-06.asp).
-
You should seek a certain Top Gear (I'm fairly sure that it was a Top Gear) episode on Google video.
Yeah, it is :-).
Boeing vs Mondeo (http://video.google.com/videoplay?docid=9211793766758129558) (3 min)
-
the answer is no that plane could not take off unless it is moving in a forward motion
thats something you learn if you pass your restricted pilots license its actually one of the questions you have to answer in your BAK(basic aeronautical knowledge) test
-
We couldn't agree more with BAK. Now you just have to tell us how BAK explains the plane not moving. :)
PS. I'm glad that BAK does tell that, since it's true. However, I'm slightly alarmed if it actually took someone that long to figure out why planes fly. I knew it when I was nine years old. :-D
I also love condescending people - I've seen people who claim to be pilots and people who claim to be engineers fall flat on their face when it comes to this question. Just trust me; what makes planes fly is not some occult science, actually I'm willing to bet that most adults in western countries are quite aware of it - it just seems that pilots and the like think too much of themselves to admit this. :P
-
hey i just was this puzzle for you checked it out answered, didn't take that long at all. but i did lie it is possible to make a plane fly without it moving, all you need is sufficient air flow around the wings
-
What you say?
Not about the airflow - we know that too - but in general. No matter how much I try to to rearrange your words, they totally fail to make sense to me.
-
I do that alot