Hill William
Lets Go Mountaineers
If I ever get hurt real bad, I want a wheelchair with wings. 
Riddle me this batman
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K
Kipper
Here you go MA.
DVINNY
2-time 10K winner
But the plane will still take off just fine. Even with the covers over your head.
LOL.
LOL.
DVINNY
2-time 10K winner
OK,
if the guy that is doing the indoor skydiving was wearing a pair of roller blades, and they were on a vertical conveyor that was moving in the opposite direction, would he now no longer be able to skydive? would the roller blades scooting along that conveyor make him fall on his face?
I guess so with your theory.
The question is whether I can sustain the velocity needed to initiate flight
If he got his lift from a jet pack, needed wings to fly laterally away from the wall, and gravity acted horizontally, then ...yes I believe so. :blink:
DVINNY
2-time 10K winner
FROM A GOOGLE SEARCH
Airplane on a Conveyor Belt
A riddle was proposed on the Neal Boortz show today:
If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?
Astoundingly, Neal and the rest of his crew took the position that the plane would sit there stationary! Good God? this man is a pilot and has a law degree! I could understand a random high school dropout being fooled by this, but a pilot?
Then I googled the riddle, and found a thread on Airliners.net that has been raging on, with the vast majority of people taking Neal?s position? that the plane would not be able to take off.
Their argument is this, to quote one poster:
Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane?s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane? or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the ?conveyor belt? is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal?s logic, the car should push back on your finger with the same force that you are exerting on the paper? but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we?ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force? the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move
Airplane on a Conveyor Belt
A riddle was proposed on the Neal Boortz show today:
If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?
Astoundingly, Neal and the rest of his crew took the position that the plane would sit there stationary! Good God? this man is a pilot and has a law degree! I could understand a random high school dropout being fooled by this, but a pilot?
Then I googled the riddle, and found a thread on Airliners.net that has been raging on, with the vast majority of people taking Neal?s position? that the plane would not be able to take off.
Their argument is this, to quote one poster:
Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, its providing an exactly CANCELLING force pushing the aircraft back.
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane?s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane? or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the ?conveyor belt? is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal?s logic, the car should push back on your finger with the same force that you are exerting on the paper? but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we?ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force? the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move
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DVINNY
2-time 10K winner
Dude,
if I could do horizontal gravity, we'd all be RICH.
DVINNY:
Are you trying start from scratch again????
The key to the puzzle is how consistent the air resistance immediately behind the plane is at providing a reaction for the forward thrust. I'm not convinced it's a fixed value because the plane is not allowed to move forward into "fresh" air. answer "dunno" see scenarios 1 through 3 above.
time for a little horizontal bop.
Are you trying start from scratch again????
The key to the puzzle is how consistent the air resistance immediately behind the plane is at providing a reaction for the forward thrust. I'm not convinced it's a fixed value because the plane is not allowed to move forward into "fresh" air. answer "dunno" see scenarios 1 through 3 above.
time for a little horizontal bop.
E
EdinNO
I think this is it:
The two sides are looking at the definition of the situation differently.
1. Some look at it as the plane is totally stationary in that however it tries to move, the conveyor counteracts it. That in mind, the plane wouldn't move or fly.
2. Others look at it as the conveyor moves backward say 150 MPH relative to the ground, the plane moves forward 150 MPH relative to the ground and the wheels move forward 300 MPH relative to the ground. In this case, the plane will fly as normal.
Its all in the way question is perceived. I have been on both sides of the fence. Now I can see the second scenario as being more correct. But if the givens of the question were as assumed in scenario 1, it might not move or fly. This would assume that something (a cable, a large arm, etc...) were holding back the plane.
I think it will fly. Now quit boggling my feeble mind with this kind of stuff. Jessh! I'm just recovering from the April exam!
Ed
The two sides are looking at the definition of the situation differently.
1. Some look at it as the plane is totally stationary in that however it tries to move, the conveyor counteracts it. That in mind, the plane wouldn't move or fly.
2. Others look at it as the conveyor moves backward say 150 MPH relative to the ground, the plane moves forward 150 MPH relative to the ground and the wheels move forward 300 MPH relative to the ground. In this case, the plane will fly as normal.
Its all in the way question is perceived. I have been on both sides of the fence. Now I can see the second scenario as being more correct. But if the givens of the question were as assumed in scenario 1, it might not move or fly. This would assume that something (a cable, a large arm, etc...) were holding back the plane.
I think it will fly. Now quit boggling my feeble mind with this kind of stuff. Jessh! I'm just recovering from the April exam!
Ed
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DVINNY
2-time 10K winner
For what it's worth. The others in my office all disagree with me and think that it won't move.
Sometimes its lonely being correct. (just kiddin)
Sometimes its lonely being correct. (just kiddin)
cement
gray haired dude
at 110 posts, this rivals the +1 fest stickied above. what is the next question, oh riddler?
DVINNY
2-time 10K winner
Regardless. You're screwed.
E
EdinNO
I would say yes to the last question- you would add the amount of PE (potential energy) you had stored in you.
Maybe also look at the impact loading equation.
Ed
Maybe also look at the impact loading equation.
Ed
civengPE
Well-known member
Ok Guys. I emailed my friend who is an aerospace engineer that designs and tests missiles for the air force. Here is what he says. (it wil take off normally)
I've never heard of this riddle, but it's interesting, and after some thought...I've decided it's a trick question and depends on your assumptions. I'll TRY to give you a quick explanation but I don't think I can. Forget about the conveyor belt for a second and let's establish the physics. If you sum vertical forces, for an airplane to takeoff, the lift generated just has to barely overcome the weight. We know that lift is proportional to dynamic pressure which is equal to 1/2*density*V^2. That velocity is the velocity of the median or in our case air particles (could be any gas) relative to the wing. Period end of story, if air particles don't reach a certain speed relative to the wing then no lift. So reading the problem again, if you assume the airplane is not moving in some inertial reference frame (the ground) then the airplane won't do shit. Propeller airplanes will pull some air over the wings but not nearly enough to counteract weight and we're assuming there's not a 300 knot headwind.
But now lets look at the horizontal components, thrust and drag. As any good PE and pilot should know, airplanes create thrust from Newton's 2nd law of equal and opposite forces. Propeller airplanes take a whole lot of air (big mass) and push it back just a little bit (small velocity), while jet aircraft take a little bit of air (small mass) and push it back really fast (big velocity) so that pushes the airplane forward. So our airplane can create thrust but now drag. If we assume we're not moving in the ground frame then there is zero aerodynamic drag, but even if we are moving it's not a large number. And now we come to our conveyor belt, the only other horizontal forces would be drag forces on the wheel, which I'll put all into rolling friction (axle, wheel deformation,etc) which once again ain't much. So we've established that we can create a shit load of thrust and the only force to counteract it comes from rolling friction. So if you assume rolling friction is constant vs rolling speed it would be impossible to speed the conveyor up enough to give it a ground speed of zero, but if you assumed rolling friction increases with speed then it would be "theoretically" possible to move the conveyor fast enough to zero out it's speed, but obviously you'd blow out any real world tire due to heat well before that happens. And that speed is certainly faster than an equal and opposite speed of the airplane.
So again, if I take the question literally, because it states that a plane is moving in one direction while the conveyor is moving in the equal and opposite and it states that this is accomplished by measuring the plane's speed and matching it with the conveyor, then if you assume the velocities are measured in the ground frame, then simply matching the plane's ground speed wouldn't do much. Therefore, yes it would take off (assuming your tires stay intact). Man, I hope that was clear.
I've never heard of this riddle, but it's interesting, and after some thought...I've decided it's a trick question and depends on your assumptions. I'll TRY to give you a quick explanation but I don't think I can. Forget about the conveyor belt for a second and let's establish the physics. If you sum vertical forces, for an airplane to takeoff, the lift generated just has to barely overcome the weight. We know that lift is proportional to dynamic pressure which is equal to 1/2*density*V^2. That velocity is the velocity of the median or in our case air particles (could be any gas) relative to the wing. Period end of story, if air particles don't reach a certain speed relative to the wing then no lift. So reading the problem again, if you assume the airplane is not moving in some inertial reference frame (the ground) then the airplane won't do shit. Propeller airplanes will pull some air over the wings but not nearly enough to counteract weight and we're assuming there's not a 300 knot headwind.
But now lets look at the horizontal components, thrust and drag. As any good PE and pilot should know, airplanes create thrust from Newton's 2nd law of equal and opposite forces. Propeller airplanes take a whole lot of air (big mass) and push it back just a little bit (small velocity), while jet aircraft take a little bit of air (small mass) and push it back really fast (big velocity) so that pushes the airplane forward. So our airplane can create thrust but now drag. If we assume we're not moving in the ground frame then there is zero aerodynamic drag, but even if we are moving it's not a large number. And now we come to our conveyor belt, the only other horizontal forces would be drag forces on the wheel, which I'll put all into rolling friction (axle, wheel deformation,etc) which once again ain't much. So we've established that we can create a shit load of thrust and the only force to counteract it comes from rolling friction. So if you assume rolling friction is constant vs rolling speed it would be impossible to speed the conveyor up enough to give it a ground speed of zero, but if you assumed rolling friction increases with speed then it would be "theoretically" possible to move the conveyor fast enough to zero out it's speed, but obviously you'd blow out any real world tire due to heat well before that happens. And that speed is certainly faster than an equal and opposite speed of the airplane.
So again, if I take the question literally, because it states that a plane is moving in one direction while the conveyor is moving in the equal and opposite and it states that this is accomplished by measuring the plane's speed and matching it with the conveyor, then if you assume the velocities are measured in the ground frame, then simply matching the plane's ground speed wouldn't do much. Therefore, yes it would take off (assuming your tires stay intact). Man, I hope that was clear.
All these aerospace guys and noone has yet described the ambient air pressure in the immediate vicinity of the plane.
civengPE: I follow with what your friend is saying and agree. However, I see two keys to the problem that no one has answered and I have said repeatedly I don't know the answer.
1) How is the thrust of a commercial jet engine affected by ramping up to speed at a fixed location in the atmosphere? The answer would be a comparison between the thrust curve for an engine mounted in a stationary test fixture comapared to the thrust curve for a moving plane taking-off. the difference is that the test scenario has to draw its intake air from the reservoir in front of it while the moving plane draws from an undisturbed reservoir (it is constantly moving into a "new" section).
2) Assuming the thrust is unaffected. Will the jet engines draw enough air flow over the wings to initiate flight.
Think about this. As long as the plane is on the ground, the forward thrust of the plane will cause it to roll (it ain't airborne).
As long as its rolling the wings aren't cutting through the atmosphere because the plane is not advancing forward because its on the conveyor.
The only air passing over the wings is air flow past the plane caused by the engines. This airflow has to reach critical velocity over the wings to cause the plane to lift off of the conveyor and advance forward in the air. Once this happens then the plane flys normally.
If there is not enough airflow over the wings then the plane won't take off and it will try to move forward off of the ground and sit and spin its wheels on the conveyor until it runs out of gas.
If I had to pick I'd say: I don't think the engines can do number 2 so I don't think the plane will take off. This is especially ttrue for a plane like a jet fighter where the intake is at the nose and the exhaust is at the rear with no path through the turbines over the wings.
I can't see any other scenario happening.
It's all about the airflow over the wings caused by the engines on a stationary plane and no one has addressed that.
civengPE: I follow with what your friend is saying and agree. However, I see two keys to the problem that no one has answered and I have said repeatedly I don't know the answer.
1) How is the thrust of a commercial jet engine affected by ramping up to speed at a fixed location in the atmosphere? The answer would be a comparison between the thrust curve for an engine mounted in a stationary test fixture comapared to the thrust curve for a moving plane taking-off. the difference is that the test scenario has to draw its intake air from the reservoir in front of it while the moving plane draws from an undisturbed reservoir (it is constantly moving into a "new" section).
2) Assuming the thrust is unaffected. Will the jet engines draw enough air flow over the wings to initiate flight.
Think about this. As long as the plane is on the ground, the forward thrust of the plane will cause it to roll (it ain't airborne).
As long as its rolling the wings aren't cutting through the atmosphere because the plane is not advancing forward because its on the conveyor.
The only air passing over the wings is air flow past the plane caused by the engines. This airflow has to reach critical velocity over the wings to cause the plane to lift off of the conveyor and advance forward in the air. Once this happens then the plane flys normally.
If there is not enough airflow over the wings then the plane won't take off and it will try to move forward off of the ground and sit and spin its wheels on the conveyor until it runs out of gas.
If I had to pick I'd say: I don't think the engines can do number 2 so I don't think the plane will take off. This is especially ttrue for a plane like a jet fighter where the intake is at the nose and the exhaust is at the rear with no path through the turbines over the wings.
I can't see any other scenario happening.
It's all about the airflow over the wings caused by the engines on a stationary plane and no one has addressed that.
Another view is that the problem would be the same if you tethered the plane so it couldn't move forward until it lifted off of the ground, and didn't block air flow fore and aft. (take the wheels and conveyor right out of the picture. Would it take off?
I really need to do some work today.
I really need to do some work today.
civengPE
Well-known member
I have an easeir one. This was one of those bonus questions on an exam in college.
You have a helium filled baloon in your car while traveling down the highway. What happens to the baloon if you suddenly slam on your brakes?
:MIG:
You have a helium filled baloon in your car while traveling down the highway. What happens to the baloon if you suddenly slam on your brakes?
:MIG:
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E
EdinNO
HEre is one way civeng123 put it that I think aids in the discussion:
On the plane issue. Since your a boater, let me try this with you.
You're in your boat with an icechest full of beer and three or four half naked girls. Below your boat (under water) is a long conveyor belt.
If you were to drop a wheel attached to your boat to the conveyor and turn on the conveyor would anything happen?
If you hit the gas, would you go. Of course. you see your boat is imparting it's thrust to the water not the air.
I think this will become clearer after many beers. That always helps me!
MA, I was with you for a while, but now have corssed over to the dark side!
Forget about lift for a minute. You know when a plane is taxiing, you feel the quick shots of thrust that get ti rolling- especially from a stop? Well this thrust will get it trying to roll. The conveyor will try to conteract this. Ultimatey, the thrust against the air by the jets will get the plane going forward, say up to 150 mph. It just has to. That's what jets do. By problem definition, the conveyor is hauling butt backwards, at 150 MPH. Now the plane is moving 150 MPH relative to the ground and 300 MPH relative to the conveyor. Granted, it may take a little longer to accomplish this because now you have more friction in the wheels pushing backwards.
Another quandry is wht happens if the plane is sitting still on the conveyor and then the conveyors starts rolling backwards at 150 MPH. THEN, the plane starts up its jets? Would it ever reach take off? I am thinking that the thrust of the jets would eventually negate its negative 150 MPH speed relative to the ground, get it to 0 MPH relative to the ground, then as the air stabilizes around the plane it would evnetually make it up to 150 MPH relative to the ground (and ambient air).
I am assuming 150 MPH is the magic number to get it to fly. Just pulled it out of the air- no pun intended!
Ed
On the plane issue. Since your a boater, let me try this with you.
You're in your boat with an icechest full of beer and three or four half naked girls. Below your boat (under water) is a long conveyor belt.
If you were to drop a wheel attached to your boat to the conveyor and turn on the conveyor would anything happen?
If you hit the gas, would you go. Of course. you see your boat is imparting it's thrust to the water not the air.
I think this will become clearer after many beers. That always helps me!
MA, I was with you for a while, but now have corssed over to the dark side!
Forget about lift for a minute. You know when a plane is taxiing, you feel the quick shots of thrust that get ti rolling- especially from a stop? Well this thrust will get it trying to roll. The conveyor will try to conteract this. Ultimatey, the thrust against the air by the jets will get the plane going forward, say up to 150 mph. It just has to. That's what jets do. By problem definition, the conveyor is hauling butt backwards, at 150 MPH. Now the plane is moving 150 MPH relative to the ground and 300 MPH relative to the conveyor. Granted, it may take a little longer to accomplish this because now you have more friction in the wheels pushing backwards.
Another quandry is wht happens if the plane is sitting still on the conveyor and then the conveyors starts rolling backwards at 150 MPH. THEN, the plane starts up its jets? Would it ever reach take off? I am thinking that the thrust of the jets would eventually negate its negative 150 MPH speed relative to the ground, get it to 0 MPH relative to the ground, then as the air stabilizes around the plane it would evnetually make it up to 150 MPH relative to the ground (and ambient air).
I am assuming 150 MPH is the magic number to get it to fly. Just pulled it out of the air- no pun intended!
Ed
Oh boy:
Ed using your boat example. The connection between the wheel and the boat is rigid and can take vertical load. The water is low enough so that there is vertical load on the wheel. The boat is only partially floating and is ultimately supported on the wheel. Turn on you motor. The boat wants to mnove formard but the conveor is moving so the wheel (and support and your boat) aren't moving up stream either. The water is moving under your boat and your speed realtive to it increases as the motor goes faster. as you go faster the boat starts to come up on plane (actual boating term). As it goes up on plane the wheel lifts off the conveyor and away you go up stream.
The question is: can you get the water under the boat moving fast enough to get the boat on plane?
If not then you won't go anywhere. Which you really don't care about with beer and naked girls.
Same problem, same answer.
Ed using your boat example. The connection between the wheel and the boat is rigid and can take vertical load. The water is low enough so that there is vertical load on the wheel. The boat is only partially floating and is ultimately supported on the wheel. Turn on you motor. The boat wants to mnove formard but the conveor is moving so the wheel (and support and your boat) aren't moving up stream either. The water is moving under your boat and your speed realtive to it increases as the motor goes faster. as you go faster the boat starts to come up on plane (actual boating term). As it goes up on plane the wheel lifts off the conveyor and away you go up stream.
The question is: can you get the water under the boat moving fast enough to get the boat on plane?
If not then you won't go anywhere. Which you really don't care about with beer and naked girls.
Same problem, same answer.
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