December 11th, 2007, 04:05 PM
okay, so I read this on another forum and it generated some good debate.
There is an airplane on a run way. The run way is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the run way senses the wheels moving, the run way will move match the speed of the airplane's wheels but in the opposite direction.
Can this airplane take off?
Discuss.
December 11th, 2007, 04:18 PM
The plane should be able to take off. The conveyor is irrelevant as the turbine engines are pushing the air, not the ground.Originally Posted by nist7
the conveyor belt will move with the wheels of the airplane. no matter how fast the turbine moves, AIR HAS TO BE MOVING OVER THE WINGS FOR LIFT TO OCCUR
Air must move past the wings and along the natural direction that the airplane travels to provide lift.
Now...
If it was the wheels which were driving the airplane, then the conveyor could move in such a way that the plane remains still relative to the ground.
However, it is the turbine that provides the thrust, and the turbine will push the plane relative to the surrounding air. Thus, the plane will have no problem moving down the runway and, hence, will be able to take off.
Good question.
Cheers,
william
N.B. The wheels and conveyor will end up in a runaway-type scenario.
Good point
How the heck does anybody check "NO" in the above question in the first place??? Sheesh...............turbines provide the thrust, not the wheels. The wheel swill rotate way faster than usual, but if they're not destroyed in the process this plane WILL take off of course.
I say NO.
The problem description is vague but it appears the situation is that the airplane has no forward velocity, relative either to the ground or the air. Thus there is no lift on the wings. No matter how powerful the thrust of the propellers the airpane cannot lift off the belt.
Hi Steve,
Take another scenario - that of a wheel-driven car.
Ask: What do the wheels "push" against?
Back to the airplane...
Ask: What does the turbine "push" against?
Cheers,
william
William, you ask "what does the turbine push against?"
Well, it forces air backwards. And Newton's third law says the airplane is driven forwards.
But the craft is on a conveyor belt that goes just as fast as the airplane but in the opposite direction, neutralizing the plane's forward thrust.
As I said, the description is vague. We are not actually told whether the airplane is able to make forward progress... or not. But my interpretation is that the drag of the belt on the wheels is enough to keep the plane in the same spot (relative to the ground). Frankly, I don't know if this will actually be the case but if it is, there is no forward motion, no lift on the wings, and no takeoff.
The turbines push the air, right? so, if the mass of the airplane wants to go forward, and the free moving wheels are going backwards, wouldn't it be like pulling a rug from under a wheel that has a turbine on it? wouldn't the wheel just keep going faster and faster and faster, (the rug and wheel racing eachother to catch up as the airplane keeps trying to go faster and faster?) each time the plane moves forwards, the wheels move a little faster and a little faster. The airplane does not stop moving forwards, the turbines push the gigantic body of the plane at a constant rate. finaly, at some point one of a few things will happen.
1) the wheels smash into a million pieces pecause thayer going to fast
2) The friction of the wheels becomes so great that it overpowers the tremendous pushing force of the airplane's turbines
3) the turbine overpowers the friction and accelerates slowly, and in a couple of days and a couple of miles later it reaches take-off speed
- I belive that the wheels would break. If they didn't the ever incresing speed would create enough friction to mess everything up. The wheels would be going realy fast though.
If the conveyor belt reacts to the speed of the wheels then the plane has to be moving forward as speed of wheels increases. If the wheel speed is increasing then the plane must be moving. If the conveyor belt is as long as a runway the plane will take off.
Are the brakes on or off
Right, as the engine thrust begins to move the plane forward the runway will move but as the plane is moving the runway will simply accelerate towards infinity or until the friction of the wheel bearings is enough to overcome the thrust and halt the plane, at which point the runway will instantly stop and guess what happens to the plane?
With brakes on the engines take off and the rest of the plane stays where it is.Are the brakes on or off
The friction will slowly slow down the plane, because this question does not allow for mechanical failures (such as melted, seising parts)
as it slows down, the friction drops, and the plane speeds up again.
How fast will the wheels be going?
in mph?
in rpm?
how fast will the plane be going in the small window that the plane speeds up and slows down?
this is realy complicated stuff, so... I'l give you most of the facts you'll need (we might not be able to get them all)
4 engines
Engine thrust is 63,300 pounds (28,710 kg) on the current model.
its maximum takeoff weight is 910,000 pounds (412,770 kg)
16 wheels
unknown radius
unknown wind resistance
(you'll need to find both of those to do the math, obviously)
all aluminum parts
it doesn't need to be perfect, what is a close estiment?
can anyone do the math?
Will the wheels accelerate indefinately, or will they just move at twice the speed backwords.
Yes, the turbines are pushing air and not ground. however, I belive that the wheels must move (relative to the ground ) for any motion to take place.
If the turbines push the airplane to a speed of 180mph (the take-off speed for one of those airplanes) than the wheels would be moving twice the speed as the airplane, only 360 mph (180 for forward motion, 180 because of the runway) for the plane to experiance any forward motion at all, the wheels must mobe faster than therunway. They can't... the runway will always keep up. they will always be racing eachother. The turbines, however will be pushing the plane forwards. If the plane stops accelerating, it will stand still. If the plane accelerates, the ramp accelerates to. There becomes a physical imposibility. How is it that a plane can move forwards, (a motion that requires the wheels to move faster in one direction than the ramp in another) while the ramp keeps up with him? the movement forwards is what causes the motion of the wheels. As soon as the motion happens, will it not be almost immediately counteracted by the motion of the ramp? The acceleration of the wheels to a tremendous speed would be almost instant. What happens now? The problem is becoming more and more complicated!
Forward motion is made imposible! as infinity is introduced to the equation by the first motion (that motion wich cannot exist) we jump out of the relm of physics, and into the realm of impossibility.
This is a much wordier reitoration of Steve F's last post.
There motion is imossible, because of a variable that depends on motion, there are no possibilities. (unless a 180mph huricane came by and swept the airplane off of it's feet. without desroying it)
Imagine what happens to the wheels as they reach the speed of infinity.. The outside will suddenly move at c. as the inside(still conected to it) tries to reach it. one item now has parts moving at the speed of light and under the speed of light at the same time. they travel in differant dimentions. The outside of the wheel ceases to be matter. another impossibility, but it's fun to imagine, isn't it?
nist7:
I'm interested to know what results other forums produced. could you give us some of their hyperlinks?
The aiplane will not take off
Just for the heck of it, imagine the plane has skis or vats underneath. So if the tires are not destroyed in the process the plane will take off.
That's a big if, because the moving runway will be traveling at an infinite speed before the airplane can go anywhere at all.
Twaaaaang has a point though.
(now assuming that mechanical failures are accetable) The wheels would almost instantly fail as soon as the airplane tried to move. the airplane falls on the skids. If we say that the conveyor belt moves exactly the same as the wheels, the belt is stoped when the plane hits the ground.
If we say than it mst slow down...the pilot has a surprise in store for himself.
Remember though, that I don't belive that any motion is possible at all. the wheels can't move because of the mathematical paradox: wheel vs. belt vs. thrust + infinity (I don't wanna explain it again.)
The belt has to respond to the speed of the wheels. The wheels have to move from one point to another to create speed. The belt cannot ever respond immediately to keep the plane motionless. If the plane is motionless because the belt can keep up with the speed of the wheels then the belt shouldn't even be running. zero speed means belt is off. Therefore the plane will continue to move forward. If It has enough fuel it will continue to gain speed and eventually lift off.
how fast can the belt respond?
In real physics (a belt like this is impossible anyways) no, the belt can't respond immediately. It will be a race. Will the wheels accelerate fast enough to break down before 180mph?
In the world of thought experiment though, shaderwolf would be right.
The turbines provide thrust yes, but the forward momentum is cancelled out by the runway moving in the opposite direction, so the body of the aircraft will remain stationary relative to the body of air, thus no lift will occur and the control surfaces will be utterly useless.
NO THE PLANE WOULD NEVER TAKE OFF !!!!
The plane would be using only half optimal thrust at desired landspeed relative to the runway, not enough thrust though to take off. Any more thrust would incur a landspeed unfit for the tyres..............also, by which stage, there wouldn't be enough runway: the plane would drive off the conveyor belt. Yet, with the conveyor presumably moving in an opposite direction, leading to an infinitely long runway, all the thrust of the jet would be counteracted by the conveyor belt. The bottom-line is airflow dynamics. Does the movement of the conveyor belt actually produce a type of wind effect in the surrounding air? What is the surface of the conveyor belt like in order to determine if it would produce a windflow?
Let aside the impossibility to construct such a runway (do you know how much energy it takes to accelerate a 3 km strip out of solid concrete?? And don't mind the friction between wheels and runway and assuming there's no aerodynamic problem with a surface moving that fast -THE TURBULENCE, OH....THE TURBULENCE!!!!!) this is about the relative motion of the plane in repect to the air, nothing else. An Newtons laws still hold, let's not forget this. So action - reaction. The tidbit with the moving runway has nothing to do with it. The plane will fly.
Where has it been established that the plane WILL IN FACT move with respect to the air? I think in the real world the plane would actually move forward and eventually take off but in the hypothetical world of mind-games, perfect conveyer belt runways and perfect indestructible airplane wheels I'm not so sure. The thrust of the plane provides a force parallel to the ground but I don't see how that force necessarily translates into forward motion. Since there's also a downward force (gravity) the spinning wheels in my mind would take all of that energy and just spin faster. Hence no motion WRT air and no plane taking off
The statement of the problem says the runway matches the speed of the wheels in the opposite direction, so you have to assume that is true, no matter how great an engineering feat that would be. It didn't say the airplane was anything extraordinary, so it has to be assumed to be made of ordinary materials. As soon as the plane starts to move, say 1 inch per hour, the moving runway will react and move backwards at some great speed, whatever it takes. The bearing friction will increase with increasing speed due to fluid friction. At some point the rubber tires will fly off. The bearings will burn out and the wheels will be ground down to a nub. When there are no longer any wheels for the moving runway to sense, then it would stop moving, presumably. Could the plane take off with its landing gear, without wheels, scraping on the runway. I don't think it would have enough thrust if it's an ordinary plane.
Remember. to have forward motion, the wheels must move faster than the runway, wich counteracts the wheels perfectly. So, the wheels can't actualy "move" forward at all. When you add the fact that the plane is trying to move forwards, It turns the problem into a paradox. How can the wheels cause the belt to move, If they can't move forwards because the runway moves when they do? The plane moves from hereto here
Wait, I want to change my answer. It says the runway matches the speed in the opposite direction, so that means if the speed is 200 mph forward, the runway speed in 200 backward not an infinite number. So the wheels move 400 wrt the moving runway, the air speed in 200 and the plane takes off.
I just deleted my post:The conveyor move in the opposite direction and speed of the wheels, not the plane.Wait a minute. What about shifting the frame of reference? Let’s say the take-off speed of an airplane relative to the air is 120kmph. Imagine a plane traveling at 120kmph, with the runway at 0kmph and the air at 0kmph with respect to an observer on the ground. Now, imagine the conveyor moving at -60kmph, the plane moving at 60kmph and the air moving at -60kmph, with respect to an observer on the ground. Both planes should be able to take off.
For our plane the conveyor would be moving at -120kmph, the plane at 120kmph, the air at 0kmph, but the wheels would cover 240kmph on the conveyor, relative to an observer on the ground. Would it not be able to take off?
The thing about the conveyor matching the speed of the wheels, would imply that this is with only the wheels and the conveyor belt in that reference frame.
nist7, we haven't heard from you at all. I want to hear what other forums said.
I'm agreeing with KALSTER here, by the way. And say that if that is true, shaderwolf is right. The problem creates a paradox. Physics fails! The universe as we know it ceases to exist! The wheels travel faster than light! A gravity vortex opens, and sucks up the solar system. Then the guy that made the dang experiment is sent to hell for breaking space.
Wow! More people voted that the airplane wouldn't take off than those who voted it would.
Silly mortals.
Can you prove it does? Does the air through the turbines create the lift like a moving plane? Even when its stationary.
The plane will not be stationary once the turbines push the AIR, which will move the plane FORWARD until it reaches rotation and LIFT'S OFF. The wheels can spin till they fall off, it doesn't matter. The wheels are NOT providing the propulsion.
Hold on everyone.
Reread the initial statement and question.
“There is an airplane on a run way. The run way is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the run way senses the wheels moving, the run way will move match the speed of the airplane's wheels but in the opposite direction.
Can this airplane take off? “
No where is it said that the plane had to be turbine driven. Let us say that the plane in question has a solid rocket power system.
Now what should the answer be?
Square one.
There needs to be a better definition of the airplane in question. What type of propulsion system does it have? Once given the type of system then we can give any configuration of that system. Keeping things in the real world, apart from the moving runway, then we can better define the situation with which the airplane could achieve flight.
The forces that must be dealt with are: thrust, drag, lift, and weight. Given that the moving runway (drag) will counteract the forward motion (thrust) then those two cancel each other out. Now the only two remaining parameters to deal with are lift and weight.
One scenario could be a very efficient turboprop power system. Should the system move enough air over the airfoil then in theory it could produce enough lift to overcome the weight and allow the airplane to separate from the runway.
There are some questions that need answering here.
1) - Do the wheels have a maximum possible rate of spin?
2) - Does the conveyor have a maximum speed?
3) - Could the plane take off if it's wheels were locked and unable to spin? (IE. how strong are its engines)
If we take the scenario at face value, then #3 becomes the only important question.
The wheels are not providing the propulsion but I still do not see why that necessarily means the plane will move. We all agree there's a force at the rear of the plane - why can the force not be translated down to spinning the wheels as opposed to actually moving the plane forward if there is no limit to how fast the wheels can spin?The plane will not be stationary once the turbines push the AIR, which will move the plane FORWARD until it reaches rotation and LIFT'S OFF. The wheels can spin till they fall off, it doesn't matter. The wheels are NOT providing the propulsion.
I mean I think people can see my point better if you picture instead of a strong thrust and normal gravity, picture very very strong gravity and a weaker forward thrust. Would the plane in this situation still move forward or would the thrust merely spin the wheels against the perfect conveyer belt?
I'm sure we have had this question before
Specify the plane... An airliner passenger jet. Boeing 474.
The plane would move. No doubt. The wheals would be destroyed almost instantly.
Boeing 747- crash
F-35- no prob.
wright flier- you wish
Well I ticked yes for this. The forces in question are thrust and drag. The thrust is given by the rate of change of momentum of the fluid stream out of the turbines (Newtons third law). This is of course independent of any interaction between the wheels and the moving runway.
The drag will be given by the equation:
F = μR
Where:
F = Drag
R = Reaction force = mg =Weight (Newtons third law)
μ = Frictional co-efficient
So long as the thrust outweighs the drag, the plane will move forward (relative to a stationary point).
My outlook is that as the planes starts in motion, its wheels will get faster, and in turn so will the runway, in turn causing the wheels to go faster still. The speed of the wheels and the runway will increase exponentially towards infinity at the same (exponential) rate.
However the mutual increase in speed will not affect the drag of the plane, because its weight, and hence reaction force are completely independent of this. (In a real-life situation, the only thing that will change is the frictional coefficient, which will be negligible anyway).
So overall, the thrust WILL outweigh the drag, the plane will move forward, air will pass over the airfoils, and lift will be generated. Again, as more lift is generated, the Drag will decrease further (as given by above equation, due to both reaction force AND frictional co-efficient decreasing), and in turn the plane will go faster and more lift will be generated.
The only assumption I am making here is that the wheels will be able to withstand the turning speeds up to the point the plane finally leaves the runway.
Remember though, that high speeds like that will also contribute to gigaantic amounts of friction in the wheel bearings. I'm not challenging bit4bit, so far, your right. What effect will this have on the situation though?
Read the post again. You might feel a little
Have a look at bit4bits post. An excellent description.
My replies operated on the presumption that that would be impossible.
Everone, make the decision.. are the wheels and such made out of some extraordinary super unbreakable material?
Read this: "When the run way senses the wheels moving, the run way will move match the speed of the airplane's wheels but in the opposite direction."
What speed is the runway sensing, relative to what frame of reference, and what is the control action and setpoint?
I don't think you can get from this that the runway goes infinitely fast in the opposite direction - just fast enough to equal the speed of the plane. That's the reason the plane takes off.
If I could photo this event at the airport, how fast would I have to run on a treadmill while holding my camera (a treadmill, one that matches my foot speed similarly to the observed airplane event) for me to take off "if I had a pair of wings on my back that at a certain speed of airflow dynamics allowed me to also "take-off""?
Of course if I were running with a jetpack on my back, the jetpack would provide "lift", pointed in the right direction, of course, but standard aeroplanes arn't designed that way, are they. I would shoot off my treadmill, would I not, if it were not for the fact the horizontal jet-turbine linear velocity provided for the treadmill reaction under my feet keeping me in a very noisy situation, right?
Please click onto my website below (www) to get an exact link to "Boeing"......maybe you can ask them what actually is responsible for lift: is it, for instance, airflow over/under wings, OR, is it the type of tyres they use on their craft..........?
(can I use the LOL feature yet)
OK, forget the LOL feature.
If the jet enginecan be tilted in a way to create a vector lift, SURE, there may be a type of take off.
And, yeah, all of a sudden the plane is going to go from 0mph to 200mph, right, when it does take itself off the treadmill, right?
That would be the case if the runway were sensing the speed of the plane itself.What speed is the runway sensing, relative to what frame of reference, and what is the control action and setpoint?
I don't think you can get from this that the runway goes infinitely fast in the opposite direction - just fast enough to equal the speed of the plane. That's the reason the plane takes off.
From what I understood, the OP was referring to the rotational speed of the wheels, which translates to the instantaneous velocity of the wheel at the point of contact with the runway. Supposing it were a stationary runway, then the only significant variable affecting this velocity is the thrust of the plane.
It's different if the runway itself is moving though. Due to the frictional contact between the runway and tyre, the velocity of the runway (relative to stationary ground) in the opposite direction to the planes motion will ADD to the rotational velocity of the wheel.
This means that whatever the speed of the wheel, the control system will aim to spin the runway at the same speed. When it does so, it will grip the tyre of the wheel, and add to the rotational velocity it was initially trying to match! Then of course it will attempt to spin even faster, and you end up with this control loop.
I don't think it is accurate to say that the wheel and runway will spin infinitely fast, but they will accelerate towards infinity, up to the point they lose contact, where the wheel and hence the runway will slow down. The reason I express it as 'towards infinity', is because we aren't given any maximum speeds for the wheel or runway.
In a real control system the reaction of the system to process its changing inputs into an output won't be instantaneous, but the affect will be the same...except the actual acceleration of the wheels/runway wouldn't be so smooth.
Unless the wheel is slipping the instantaneous velocity of the wheel at the point of contact with the runway is zero.
The airplane would stand still I think. As the conveyor belt would move same direction
as the wheels, the airplanes speed would double. The thrust of the engines was to negate
due to the lack of friction on the ground. The trust will not move the airplane forward,
hence there wasn't being lift.
No because this doesn't create any lift on the wings. Unless you include the possiblity of extremely high winds in the opposite direction of the plane. The thrust only creates the forward motion against air to provide lift. Not lift itself.
Ok so bit4bit probably had the best way of explaining it. I retract my last post in favor of my new hypothesis. The airplane will take off. I have no time to explain it but the frictional force will be smaller than the force of the propulsion what ever that may be. Then the plane will start to move and eureka we have lift off. I will post some math up on this later. Sorry I was wrong on my first post.F = μR
Where:
F = Drag
R = Reaction force = mg =Weight (Newtons third law)
μ = Frictional co-efficient
So long as the thrust outweighs the drag, the plane will move forward (relative to a stationary point).
First clarify what is meant by "wheels moving". Does it mean forward motion or rotation?There is an airplane on a run way. The run way is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the run way senses the wheels moving, the run way will move match the speed of the airplane's wheels but in the opposite direction.
Can this airplane take off?
If it means forward motion, this is independent of contact between the tires and the runway (neglecting wheel bearing friction) so the plane will take off.
If it means rotation, as soon as the slightest rotation is sensed by the monitor the conveyor moves backwards at twice the tangential velocity of the wheel. The sensor senses the increased wheel rotation speed, so increases the conveyor speed again. The wheels accelerate almost instantaneously towards light speed, but the tires melt so the plane can't take off.
I disagree with this interpretation because according to the problem statement the runway will move to match the speed of the wheels in the opposite direction. If we are measuring the wheel speed as say clockwise rotation, then the runway will try to make them turn counterclockwise. To do that, it has to go the same direction as the airplane is moving, until the rotational speed is zero. The plane takes off.First clarify what is meant by "wheels moving". Does it mean forward motion or rotation?There is an airplane on a run way. The run way is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the run way senses the wheels moving, the run way will move match the speed of the airplane's wheels but in the opposite direction.
Can this airplane take off?
If it means forward motion, this is independent of contact between the tires and the runway (neglecting wheel bearing friction) so the plane will take off.
If it means rotation, as soon as the slightest rotation is sensed by the monitor the conveyor moves backwards at twice the tangential velocity of the wheel. The sensor senses the increased wheel rotation speed, so increases the conveyor speed again. The wheels accelerate almost instantaneously towards light speed, but the tires melt so the plane can't take off.
But if the rotational speed is zero the runway would stop moving; so you're right, the question has to be interpreted in terms of forward motion, not wheel rotation.I disagree with this interpretation because according to the problem statement the runway will move to match the speed of the wheels in the opposite direction. If we are measuring the wheel speed as say clockwise rotation, then the runway will try to make them turn counterclockwise. To do that, it has to go the same direction as the airplane is moving, until the rotational speed is zero. The plane takes off.First clarify what is meant by "wheels moving". Does it mean forward motion or rotation?There is an airplane on a run way. The run way is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the run way senses the wheels moving, the run way will move match the speed of the airplane's wheels but in the opposite direction.
Can this airplane take off?
If it means forward motion, this is independent of contact between the tires and the runway (neglecting wheel bearing friction) so the plane will take off.
If it means rotation, as soon as the slightest rotation is sensed by the monitor the conveyor moves backwards at twice the tangential velocity of the wheel. The sensor senses the increased wheel rotation speed, so increases the conveyor speed again. The wheels accelerate almost instantaneously towards light speed, but the tires melt so the plane can't take off.
Hello GenerationE!
What you are saying was not subject to that matter. If the engines produce greater thrust the wheels spin faster though
the conveyor belt was running faster as well. the plane will not take of cause its still standing on the same position
seen from a fixed remote point.
Also, whether the bearings will become so hot the wheels will jam, as a sample, was not asked for as far as I do
understand the problem.
The whole site would go up (off ) in a big blast anyway.
What I do find interesting was the rotation of the wheels and the direction to which the conveyor belt
runs to respectively the rotation of the rolls the belt does rest on.
The belt self would move to two directions seen closely. The upper part to the right for example, the
lower to the left.
If the lower part would move right the upper will move left.
If this was meant ( the direction(s ) of the belt ) or the direction of the rolls was not coming clear in
the problems formulation, fully.
Steve
Wow, I would not have expected so many responses to this seemingly simple question. It turns out to be a good question after all.
As a side note, I am also intrigued by a certain (predictable) psychology that tends to play out with controversial problems (that alone is worth scanning through all the responses): When the controversy is strong enough to create a feeling of despair, people start nitpicking on the semantics of the original question or completely dismiss the hypothetical framework by calling it unrealistic. Let's not succumb to that practice, yet. The fact that boundary conditions are abstract or hypothetical does not invalidate an otherwise well-posed problem.
We all agree: to create lift, we need air motion over the wings. If the aircraft stands on the ground (no motion relative to the airport frame of reference) and if we had a strong enough wind in the right direction (against the aircraft's nose), we could simply turn on the engines to counter aerodynamic drag (the aircraft is still not moving relative to the airport), and make use of the lift created on the wings to counter gravity and take off vertically.
Usually we don't have that much wind. In the other extreme, with no wind at all, the air is quiessent in the airport frame of reference. To create air motion over the wings, the aircraft will have to move relative to the standing air (duh!) and in this case it just happens to also move relative to the airport.
In all the previous responses (unless I missed something) you guys have pretty much tied the question of "lift" or "no lift" to the question whether the airplane is moving relative to the airport or not. This implies that you are assuming the air does not move relative to the airport (no wind). Now here's the problem with that assumption: If the conveyor belt runway moves at significant speed (about the speed of a regular aircraft at take-off), there will be wind! Don't get upset, yet. I'll explain...
First, let's assume (hypothetically) that we can create a situation where the aircraft eninges are just being turned on, but the aircraft does not move relative to the airport frame of reference. According to Newton, the only way to do this is to have a perfect force balance in this reference frame. This means that the engine thrust must be compensated by a force of equal strength and opposite direction. In the case of zero wind, what would such a force be? Maybe the rolling friction imposed on the weels by the conveyor belt. Let's assume, hypothetically, that this is possible, and the aircraft doesn't move because of this force balance created by its engines and the friction of the very fast conveyor belt. I'd like to make the point that even in this case we will eventually have lift!
When air moves over a solid body (such as a wing, or the Earth's surface) there is something called a viscous "boundary layer" due to the viscosity of the air. If you have ever been on a high building, you probably know from experience that winds are usually much stronger at higher altitude, than they are on the ground. This is partially due to the fact that the wind is slowed down by trees and other things on the ground but it's even the case on a smooth surface. That's because, due to viscosity, the air "sticks" to the ground, i.e. right on the surface the relative velocity between ground and air is zero! If you measure a profile of the velocity as a function of altitude (at a steady wind), you'll see a more or less parabolic variation from zero velocity on the ground to a maximum velocity somewhere far above. I think you get the point of this lengthy but simplified explanation of what we call "boundary layer". Now back to the aircraft, standing in the airports frame of reference on a fast conveyor belt. No significant winds (at first).
Remember, the relative velocity between ground and air is always zero. This is also the case on the conveyor belt, meaning, the air sticks to the belt and is actually moving along! Given enough time (seconds), we will have a nice (inverse) boundary layer with air moving at a high speed on the ground, and somewhat slower but still at significant speed on the altitude of the wing, and even far above. We now have air moving over the wing and therefore lift! How much lift? Just as much as a regular aircraft running down the runway in quiessent air. This new situation is entirely equivalent to the airplane moving down a runway, except its happening in a different reference frame (kind of like an outdoor wind tunnel).
If you thought you could cheat on nature I have to disappoint you. You may (or not?) be able to use a conveyor belt to switch from one reference frame to another, but the forces will be pretty much the same, even in that (hypothetical) case.
The conveyor might create enough wind locally to lift the plane a few feet off the runway, but as soon as it got above that local wind, or to the end of the runway, down she comes. I wouldnt call that a take-off.
Then as soon as it lifts up a bit, the engines are free to propel the airplane forward. It could even behave like fighter jets on an aircraft carrier, held back and then released suddenly?The conveyor might create enough wind locally to lift the plane a few feet off the runway, but as soon as it got above that local wind, or to the end of the runway, down she comes. I wouldnt call that a take-off.
You know where you got stuck? It's often said an airplane flys hence the wings provide lift. But,
what's about the forward move provided by the turbines bucket or the propeller.
I wrote this cause the space shuttle and other space vehicles fly in regions where the (this ) lift
(due to the atmosphere ) was actually not provided. Nevertheless the shuttle as well as other
space vehicles do fly in that atmosphere - less space environment as well.
There must be the same effect which later provides lift on top of the wings. So the plane, our
plane, the one creating the thrust first will fly cause the turbine was able moving the airplane
forward until the proper speed was reached. You could blow as much air as you wanted the
plane won't take off.
When changing the angle in which you'll blow the wind, well, you'll be out of the problems scope
again. That's my interpretation of the subject at least.
And, when talking about a ( meaning one ) direction referring to the conveyor belt it's being
incorrect.
I will agree it's not of importance meaning the plane - belt issue, but there are two directions
actually to which the belt does run at the same moment. Don't wanna argue about the issue.
Steve
You are missing a critical point here. What the propellers or jet does, is propel the plane forward so that air can move over the wing to create lift.The propellers or jet creates horizontal movement, while the lift created by the wings apply a force to counteract gravity, vertical movement . It cannot lift off without wind moving over the wings, whether it gets blown over it, or is caused by the propulsion of the jet.There must be the same effect which later provides lift on top of the wings. So the plane, our
plane, the one creating the thrust first will fly cause the turbine was able moving the airplane
forward until the proper speed was reached. You could blow as much air as you wanted the
plane won't take off.
The engines don't provide the lift on an aircraft, as Kalster pointed out the wings do this job. The engines only provide the forward movement for the wing to generate lift. If the aircraft is stationary then there will be no air moving over the wings, unless you are in a wind tunnel and the aircraft is thedered. Then the plane would lift off the ground but only because there is air moving over the wings.
If there is no Thrust (or forward movement) then there is no lift, drag or weight acting on the aircraft (the four forces of flight would equal 0). In other words, the plane is not flying.
Copyright: www.cartage.org.lb
Forward movement is the product of thrust, Drag is the product of forward movement, Lift is the product of forward movement, Weight is the product of lift. As you can see, the key here is forward movement. Without it, the plane may aswell be a brick.
As I said earlier the ONLY acception to this would be inside a wind tunnel where the air can be moved while the aircraft remains stationary.
P.S. As soon as the pilot applied power to the engines the plane may nudge forward by a small amount (or untill whatever is controlling the treadmill realizes the plane has started moving) but it will go no further. But it may very well oscillate backwards and forwards untill the speed of the treadmill matches the maximum power output of the engines.
I was talking about an effect, which in particular will make the aircraft fly. Not just lower pressure
or lift in general. I mean what was really going on. It's being off topic now, but how would you want
to explain the shuttle (or other space vehicles ) flying in space where was no atmosphere?
Also, you both will agree the aircraft needs to have been accelerated to a certain velocity until
taking off. this forward thrust was a result of the interaction of the propa or the turbines blades and
the air. In our sample. Was that being right so far?
This blade - air interaction was previous actually before the air takes affect on the wings. That's
all I wanted to say.
The interaction of the blades and the air accelerating the plane to its actual take off velocity had
to be the same as the wing air interaction which lifts the plane up afterwards the take off speed
was reached.
And than, I mean it was being also the least slightly different if the plane would fly itself or if you'd
blow air to provide lift. Although, surly ending up to be the same effect again.
Steve
Spacecraft orbiting a planet are actually in a constant state of free-fall as are the planets around the sun. Go deep enough into space (into an intergalactic region lets say) and there is no or very little gravitational forces acting on the craft. There is no up, down, left or right in space.
It is correct that the blades of the engines generate lift but only in the forward direction. The blade of a prop for example is nothing more than a small wing. It spins through the air causing low pressure infront of the blades, sucking the aircraft forward (<< only true for props). Jet engines work a bit differently to this. But, if the aircraft is stationary it will not reach it's V2 (takeoff safety speed) speed. The props or jet engines do not have enough power to instantly accelerate the plane to its V2 speed which would be required. The only craft that I can think of that can fly from a stationary position are helecopters, VTOL aircraft and rockets (though rockets are not technically flying). A choppers blades are basically the same as the prop on an aircraft, only they are designed to direct the lift upwards (just like a wing on a standered plane).
I agree, but the speed of the treadmill is cancelling out any forward movement produced by the props.
Not true. A planes prop has to be overpowered to have a constant "bite" on the air. This is why prop planes never broke the sound barrier. Pilot's found that, as the approached MACH 1, their prop's were actually stalling or loosing their "bite" on the air. But your argumaent above really depends on the aircraft in question...
I think that the key thing to remember here is that the engines do not provide the lift nor do they blow air over the wings. The simple fact of the matter is if the plane is not moving (or air is not moving over the plane) then the wing cannot do their job, which is to lift the main fuselage of the plane and keep it flying.
What treadmill? :-)
January 4th, 2008, 11:20 AM
This is the first post.
I know. Unless the bold conveyor belt.
Sorry, a bit of bad comunication on my part. In my posts when I say treadmill I mean converyor belt.
You see. Initially, that has been the main issue I referred to.
Anyways, I do feel
you didn't understand me. I don't think the plane flys because off the propeller or engine thrust. But,
they also don't fly because of lift on the wings since they have to be accelerated to a take off velocity
depending on the plane (weight and construction ). So the lift on the wings was not something static,
but some streaming by of air, that has to stream steadily.
That same streaming by of air (on the props or turbines blades ) was resulting in thrust which moves
the plane forward (in the sample ). Engines cause the stream themselves whereas the wings are
rather passively ending up in the same streaming. This was, by the way, same with helicopters and as
well as sailplanes. If your too slow you'll get a stall warning, and that's it. Or if the helicopters engine
fails the helicopter succumbs towards the ground. That's of a some importance in times of space travel.
Btw, I don't see any evidence for the 'planets fall constantly towards some gravitational entity'.
Steve
edited:
Sorry the server was down.
You're making this so complicated. The engines push the airplane forwards relative to the air... The body of the plane will move. The forwards motion... how will that be counteracted by the treadmill/conveyor belt? We earlier had a post saying that the frictional force in the wheels will not be enough to stop the plane. He used math. Imagine a real conveyor belt that is moving continuously. Take a toy car, and hold it in place on the conveyor belt so that the wheels only move. Now, as you move the car in the opposite direction of the conveyor belt, your hand is working as the engines would. The car is moving forwards, and the wheels while you spin them are going even faster than the conveyor belt. For an object to move forwards, the wheels must move faster in the opposite direction than the conveyor belt. Our conveyor belt matches the speed of the wheels. The body will continue moving forwards as the conveyor keeps catching up with the wheels. As the engines push the plane forwards slowly, the acceleration of the conveyor belt increases exponentially. It does not take much time for the wheels to reach a speed that they cannot handle. Either they will fail, or they will slip. If they fail, the plane will crash. Most likely that is what will happen. We are now dealing with huge speeds, frictional coefficients, and amounts of heat on the joints. However, if the wheels slip because the friction becomes to much for them to handle, it will start over again. All of this happens in the first .0005 seconds of movement, because the plane is continuing to accelerate. The plane's wheels cannot move forward because the conveyor matches. Once again, they must be moving faster than the belt to move forwards. Since forwards motion is impossible because the wheels cannot move faster than the conveyor belt, the problem in effect, becomes a paradox. The wheels can't move forwards because they can't move faster than the conveyor belt. The belt cannot move because the wheels cannot move. The plane is moving forwards though, so the wheels must move forwards n the conveyor belt. They can't though. The conveyor belt matches their speed. That is the answer to the problem. There will be no agreements no "yes because" no "no because" That is why this is such a good question for the forum. Since it is a paradox it is not possible. No one wants to admit that, so we get a great conversation, with lots of guessing. The chasing of wheel and conveyor belt cannot even happen. No, I'm not just giving up. I believe that the person who posted this has been laughing their head off (if they posted it for the reason I think they did) you haven't heard them recently have you. They purposely introduced a paradox for some personal reason. It might have even been a psychological test. It might be a college student doing this for a class. They might have just wanted a good laugh. Admit it, you've been duped. Anyone want to explain what I said in simpler terms? Just don't think so much into the problem. What color is the sky? Blue? Actualy, it's everything else (If you know simple light physics, you know what I mean). Come on. It's an argument no one can win.
If the conveyor belt always cancels the motion of the wheels by moving as fast (relative to the air) as the wheels move (relative to the air), then the only way the plane can take off is for the engines to pull so hard that it skids off the runway.
The wings have to reach something like 80 mph relative to the air around them in order to provide enough lift to pick up an average plane. It's the force of the wings pushing against the air in front of them that does this, not the force of the engines blowing air behind it.
Solid point. Especially since we now know that it's a normal plane, and not one with infinity powerful engines.The conveyor might create enough wind locally to lift the plane a few feet off the runway, but as soon as it got above that local wind, or to the end of the runway, down she comes. I wouldnt call that a take-off.
sorry, I haven't read most of this but what about a harrier? Lift is not created by air flowing over the wings. Well, not at first.
This was very true. A very good point. A flying vehicle I didn't think on, at the moment. And, an other good
sample therefor how outdated theoretical physics often was these days.
A Harrier has jets that can turn to face downward, so it lifts up. Nothing unusual there. In our experiment, the plane is of the usual type, so the Harrier has no baring on the discussion. I don't see how you can say theoretical physics brakes down? Just so you understand, steve, if a plane stands still somewhere with the jets or propellers turned off and you had a large enough fan and blew wind across the wings, it would rise into the air.
I mentioned VTOL (vertical takoff and landing) aircratft in one of my previous posts. All the harrier does is direct it's jet exhauts down. For every action there is an equal and oppisite reaction.... The engine produces more pounds of thrust than the weight of the harrier so it lifts off. It has nothing to do with the wings.
It's getting more off topic now.
This is true. You can see this effect in fotage of some of the early nuclear explosion tests. WWII fighter aircraft (that they used to test the effects on) parked on the ground literally rise into the air when the shockwave hits them.
Yes, sure it would rise into the air I think. But that was not a clue to the initial problem, which btw. I think
I, as well as other, I have answered.
Theoretical physic will explain the lift as that was why an airplane does fly. Was engine thrust covered at
all by theoretical physics? I'm not sure. What I thought and already asked but not did recive an answer
about was if a turbine driven airplane would take of flying when there was just the engines fans (intake and
the other(s ) ) runing - electrically driven - and not the hot exhaust gases streaming out.
Theoretical physics are to challenge due to the fact neither the wings of the harrier nor a helicopters rotor
blades lift the vehicle up.
So it was flying I understand, but with no clue why, theoretically. Spinning this thread further, also answers
at why an airplane does fly won't be correct. Not at least entirely.
About the jet engines.
The hot gases only provide about 30% of the total thrust. The main fan at the front of the engine takes care of the other 70%. This basically acts just like prop. So, its fesible to think that, if the plane were stripped out of all non-esential items, then there would be enough thrust to get the plane rolling by driving the fans electrically.
Depending on the weight of the plane at the time, most pilots never use 100% of the power for take off. It's normally between 80-95% N1.
Just on the helecopter. When you say "rotor", are you referring to the tail or main rotor?
The one(s ) on top.
Helicopter blades work exactly the same way as the propellers of an airplane (and a fan). Look at the way they are curved, so that air that is in the path of the blade as it comes down gets pushed to the back (forward in a fan's case). When the propellers turn fast enough, the plane moves forward and the helicopter goes up. The propellers actually suck the plane forward and the helicopter into the air. With the plane, when it goes fast enough, the pressure on top of the wings is sufficiantly low enough relative to the bottom, that (while moving forward) it takes off. I think the flaps on the back of the wing, when curved, exerts a force on the plane in a downward direction. Then the pilot (or the computer) keeps it in ballance with the lift from the wings so that the plane stays at the same hight. With the helicopter, it tilts its nose down and tail up so that the resultant force in an upward direction exactly equals the force of gravity on the helicopter and the result is forward movement.
I guess a provided a clue to the specific problem. Or am I ignored?
Steve
I have no idea what you are talking about.
That's quite odd. Do you have an idea about what other are saying, if they do so?
What are you laughing about?
Have you got images of you?
The plane does not have to move forward to lift off. I will give you an example. A plane sitting on the runway without tie-downs. With enough nose on wind the plane will lift off the ground, usually with some backward motion first. This usually results with the plane being flipped over on its back or to one side or the other. This has happened more than once at many airports around the world.
What this means is that the plane does not have to be moving forward, however there must be a sufficient amount of wind movement over the wing to create lift.
In the initial proposition the configuration of the plane was not specified. If a prop, fan, or some other wind moving device was positioned such that airflow over the wings was sufficient to create lift then the plane would lift off without ever moving forward.
This in effect is nothing more than a wind tunnel.
This has been a hypothetical question and to get hung up on ideas such as tires or wheel a bearing not withstanding the speed is ridiculous.
It's less ridiculous than to blow wind. To blow wind was not specified in the initial post.
The plane was said to have propulsion engines on board.
Also, tires and bearings can become an issue supposed the experiment does run long
enough.
Some folks got hooked after a while of really kneeling in to something, you know.
I believe this thread is starting to around in circles now...
I don't. Even that was not the case, I thought.
Well I don't see why my post on the fourth page has no bearing. If the friction of the tires against the runway is not as great as the thrust the plane will move forward anyway.
Another thing that everyone keeps saying is that the acceleration will tend toward the speed of light almost instantaneously. Imagine if the plane and conveyor belt speeds are controlled by the same accelerator and just have oppossite directions. Say when the pilot pushes his stick forward its also the same instrument that tells the conveyor belt to move backwards. If the point where the wheel mets the conveyor belt has zero velocity which it does, then you will not accelerate towards the speed of light. The planes engines and the conveyor belt are both not capable of producing those speeds anyways.
While in your car driving down the road the point at which the tire mets the road has a velocity of zero. the top of the tire at the same time has twice the velocity of the center of the wheel. That is physics.
One more thing I think some people don't fully understand what provides lift.
http://en.wikipedia.org/wiki/Lift_%28force%29
The bernoulli principle equation is on that page. Specifically look at the role of preasure in the equation.
I still say that with some given parameters then it is possible and with some it is not.
Hello!
But what about the tires rotation? There was no limit to the speed of the rotation(! ) and the
conveyor belt therefore will negate any possible forward motion of the plane (the belt will adjust
to all speed of the wheels in opposite direction ).
Steve
That's exactly what I've been trying to say all along! Thank god you put it in plain englishHello!
But what about the tires rotation? There was no limit to the speed of the rotation(! ) and the
conveyor belt therefore will negate any possible forward motion of the plane (the belt will adjust
to all speed of the wheels in opposite direction ).
Steve
I see what your saying. But then aren't we getting into theory. I'm talking about everything be taken into account. You cannot build such a conveyor.Hello!
But what about the tires rotation? There was no limit to the speed of the rotation(! ) and the
conveyor belt therefore will negate any possible forward motion of the plane (the belt will adjust
to all speed of the wheels in opposite direction ).
Steve
What I am saying is that the friction, which in this case the wheel is not slipping, will not over power the thrust.
frictional force= frictional contant*Mass*Gravity < than Force of thrust = Mass of plane*acceleration.
Thats why I am saying the conveyor were talking about in the scenario is unrealistic. But if we were able to build it then yes you would be right.
This is my last post on this topic because there are clearly lines drawn. nobody is budging on their position.
Some folks might have to understand, there are problems one can cope with in theory.
And if you are willing to alter something for the good one has to start the right point with.
The belt was not an high tech instrument. Only the belt and cylinders. As soon as the plane
will want to move forward the belt will start rolling the other direction with, theoretically,
no limits. The belt would not need to have any supporting gadgets. I think one could build
such a test rig, but what for. Surly, you'll have some (unexpected ) information left, but
what price for?
It's like to study wolfs traveling. If you want to know how waterproof the radio transmitter
was put it into a water tank. Same results on what the outcome could be.
IrishStu,
thanks many times,
Steve
OK for those of you who think that you can stop the plane from taking off, look at it this way :-
I have a ten ton block of LEAD mounted on four wheels, I then haul this to the top of an 80 degree slope. THis slope consists of a conveyor which sense the speed of the wheels and rotates in the opposite direction (just like your runway), now I let go....
Just to make the maths easier the wheel bearings are friction free and there is no upper limit to how fast the conveyor may move.
Does the trolley run down the slope or stay near the top???
For those of you who think it will stay at the top I have set a table and chairs at the bottom for you to sit at and watch, one way or another in a few seconds time there will be no more argument..... :wink:
| « Prisms Vs. Trichroic beam splitters | Math behind the maryland experiment » |
| Bookmarks |
Bookmarks |
LinkBack URL
About LinkBacks
