Amused
Elite Member
- Apr 14, 2001
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Um, no. What part of: The plane uses air to push itself, and not its wheels can you not understand? What the ground is doing is 100% irrelavant so long as the wheels can withstand the extra speed.
Why do you people keep confusing the drivetrain of a car (thrust is transfered through the wheels) with that of a plane (thrust is transfered through the air, making the wheels merely a friction reducer)?
Dear Mr. Amused,
There is no point in replying to spider07 or smackdown because they are just trolling. The thread has 20 pages of people saying and explaining why the airplane will take off and they are both still stuck in wonderland where the plane doesnt take off. Apparently they are the only 2 people out of hundreds who know anything about airplanes or treadmills. Please don't feed the trolls.
Sincerely,
randay
So.. my take on this..
If the plane is actually advancing along the runway at the proper speed, regardless of what the wheels/treadmill (or skis/ice or whatever) is doing, I think most people will agree the plane will take off.
If the treadmill can make the plane not advance along the runway and stay in one position then the plane cannot take off.
So if everyone will agree on those two points then you can proceed to argue over the nature of the question and what the wheels will do, etc., and stop telling people that planes need air over the wings to lift. We know this.
If the plane is actually advancing along the runway at the proper speed, regardless of what the wheels/treadmill (or skis/ice or whatever) is doing, I think most people will agree the plane will take off.
If the treadmill can make the plane not advance along the runway and stay in one position then the plane cannot take off.
So if everyone will agree on those two points then you can proceed to argue over the nature of the question and what the wheels will do, etc., and stop telling people that planes need air over the wings to lift. We know this.
exdeath
Lifer
- Jan 29, 2004
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Hmm... one engine on a 747 produces about 55,000 lbs of thrust or about 245,000 Newtons, per engine.
4 engines = 1,000,000 N.
A 747 has 5 landing gear trucks with 4 wheels each for a total of 20 wheels. Assuming a bearing on each end of the axle of each wheel, that?s 40 bearings (2 per wheel).
The rolling friction in each bearing would have to be 25,000 N (5,620 lbs) in order to keep the plane from moving forward.
I've done the work this far, now someone else compute the friction of a single wheel bearing with the weight of the 747, first at normal take off ground speed (180mph), then at twice the take off ground speed (360 mph). Then calculate what speed the wheels would have to be turning in order to generate the required 25,000 N of resistance in each wheel (far faster than possible due to the plane leaving the ground before that happens)
A 747 weights about 800,000 lbs, so each bearing is holding 20,000 lbs of the aircrafts weight (normal force in each bearing). Don't forget to relate wheel circumference when figuring revolutions per second in the bearing and divide by 2 bearings per wheel, one on each side of the wheel hub, etc.
I think you'll find that yes; bearing resistance is miniscule and CAN be safely ignored, even at wheel speeds impossible to reach due to the plane lifting off. FYI if the 747 begins lifting off at 180 mph the wheel speed will never exceed 360 mph on the conveyer before losing contact with the conveyer. Assume the bearings can't melt and lock up (if they did they wouldn't stop the plane, you would just have the sliding wheels dragging the plane now instead of the rolling bearings, still easily doable with the given thrust). If the bearings could go as fast as you wanted without siezing, you might compute that it would require a wheel speed of 6,000 mph or something to generate the required 25 KN of friction, however the wheels will never see beyond 360 mph before the plane loses contact with the conveyer.
sao123
Lifer
- May 27, 2002
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If you read my post a few pages ago... one of the original sources of this question explicitly says that the question "as stated" does not prohibit the plane from moving forward. You guys need to re-read and understand the wording of the question.
The original question does not say (explicitly or implicitly) in any way shape or form that the plane can not, will not, or does not move forward.
Facts are not determined by a vote. Anyone says the plane takes off is either reading the question wrong or doesn't understand the physics of a wheel.
the question doesn't say if they conveyor belt tracks the speed of the plane with reference to ground or with reference to ground. A speed with out a reference point is worthless. With out it stating one way or another then there are two equal valid answers.
Amused
Elite Member
- Apr 14, 2001
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Are you saying an unpowered wheel is physically incapable of rolling at 180-280 MPH?
Fact: For the plane to be able to take off, the wheels MERELY have to be able to roll at twice the normal speed of a takeoff. 180 MPH for a prop plane, and 280 MPH for a jet. For you to claim the plane cannot take off means you are claiming that an unpowered wheel cannot roll at 180-280 MPH. Which is, of course, completely absurd.
Would you mind telling me why you are arguing the absurd?
Notice the OR in the above quote.
I'm making the claim that if a plane is on a treadmill that matches the speed of the airplane with reference to the treadmill surface that the plane can not move. This is simple from the definition of the problem if the speed are matched then the net speed of the plane with reference to ground is the speed of the plane on the treadmill - speed of treadmill. This will always equal zero and there will be no movement of the plane.
Again the plane staying put has nothing to do with friction between the wheel and plane. It has to do solely the wheels gaining massive amounts of kinetic energy.
exdeath
Lifer
- Jan 29, 2004
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Just out of curiosity, have you ever taken any kind of physics course? I'm thinking not.
BTW, the treadmill cannot 'grip' the plane to pull on it like that, because the wheels roll and slip independantly of both the plane and the treadmill. If I was running from you with a greased rope trailing behind me, you could pull on the rope as fast and hard as you wanted and it would just slip out of your hands faster and have no net effect on stopping me... wheels = grease.
Do the wheels create frictional drag on the plane? Sure they do... about as much as the greased rope creates when it slides through your hand. The amount of negligible.
sao123
Lifer
- May 27, 2002
- 12,653
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it doesnt matter whether the question states the point of reference....
the ONLY valid reference that the speed of the plane could be measured against is the air. Because that is the medium with which the planes propulsion system interacts with.
The ground is an invalid frame reference, the treadmille is an invalid frame of reference.
And I say the treadmill is the only valid frame of reference because when one talks about speeds on a treadmill the treadmill is always used as the frame of reference.
If you place a thread on a treadmill of stupidity. Whenever the thread goes forward the treadmill of stupidy matches its progress but in the opposite direction, can the thread ever die?
- Mar 20, 2000
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the whole thing depends on how much force the treadmill can put through the wheels.
if you've got 10,000 lbs of thrust from the turbines, you're going to need 10,000 pounds of force coming through the wheels to keep the plane from moving.
if you've got 10,000 lbs of thrust from the turbines, you're going to need 10,000 pounds of force coming through the wheels to keep the plane from moving.
Originally posted by: KK
It must have been that new guy "uselessengineer" that made you change you mind.
I have a lethargic feeling on people.. so that they dont feel like keeping up the fight
Wow you really are trying to hard to look like an idiot.
Are you trying to tell me that I can't mark a point where the plane is at time 0 wait 1 second and measure the distance between the original mark and the current location of the plane?
Come on show me how that distance calculation would fail.
exdeath
Lifer
- Jan 29, 2004
- 13,679
- 10
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For those who think bearing resistance is large enough to negate the engine thrust try this to get a feel for bearings and rolling friction...
part one)
Place a heavy skateboard on a treadmill and place a scale behind it which prevents it from falling off the treadmill.
The measured weight on the scale indicates the force that the skateboard is being pushed backwards by the treadmill through its rotating wheels (ie: the force on the scale is the bearing and wheel friction force).
Try it at various speeds and record a scale reading for each speed, and graph and observe what large changes in treadmill speed do to the scale reading (ie: test bearing efficiency at various speeds).
Also compare these measurements with the weight of the skateboard itself.
If the wheels are in good condition and use high quality well maintained and lubricated ball bearings, I predict the force felt on the scale will be very small compared to the weight of the skateboard. This makes sense because you can easily push an object multiple times your own mass if it is on wheels, say, like a large tool box or a car, even if you couldn't lift it straight up. I also predict that relationship between treadmill speed and scale reading is non-linear, that is, doubling the speed of treadmill will not result in doubling the force on the scale.
Part two)
Next fix a pully to the front of the treadmill and attach a string to the front of the skateboard and hang weights over the pulley. The weight required to hold the skateboard in place without the scale should be approximately equal to the scale reading from step one at a the same treadmill speeds.
Now repeat the above step with each speed/scale reading combination, but apply extra weight on the end of the string beyond whatever the scale reading at that speed was in part one. Observe that the skateboard should accelerate against the treadmill now. Record the extra weight used at each speed setting. Time the speed it takes the skateboard to cover a known distance up the treadmill and the extra weight used.
Part three)
Now repeat the pulley/weight experiment on a fixed table top using only the difference in weight from the last two steps. ie: if 5 lbs held the skateboard in place in part one and you used 7 lbs to accelerate it against the treadmill in part two, only use 2 lb of suspended weight on the stationary surface. Compare the time it takes the skateboard to cover the same known distance from part two. It should be roughly the same, within measurement tolerance. Maybe slightly longer due to overcoming rotational inertia of the wheels at rest when not on the treadmill.
Note that the velocity of the skateboard in parts two and three are approximately the same, but the wheel speeds differ. This should show that the rolling resistance in the wheels and bearings is negligible.
part one)
Place a heavy skateboard on a treadmill and place a scale behind it which prevents it from falling off the treadmill.
The measured weight on the scale indicates the force that the skateboard is being pushed backwards by the treadmill through its rotating wheels (ie: the force on the scale is the bearing and wheel friction force).
Try it at various speeds and record a scale reading for each speed, and graph and observe what large changes in treadmill speed do to the scale reading (ie: test bearing efficiency at various speeds).
Also compare these measurements with the weight of the skateboard itself.
If the wheels are in good condition and use high quality well maintained and lubricated ball bearings, I predict the force felt on the scale will be very small compared to the weight of the skateboard. This makes sense because you can easily push an object multiple times your own mass if it is on wheels, say, like a large tool box or a car, even if you couldn't lift it straight up. I also predict that relationship between treadmill speed and scale reading is non-linear, that is, doubling the speed of treadmill will not result in doubling the force on the scale.
Part two)
Next fix a pully to the front of the treadmill and attach a string to the front of the skateboard and hang weights over the pulley. The weight required to hold the skateboard in place without the scale should be approximately equal to the scale reading from step one at a the same treadmill speeds.
Now repeat the above step with each speed/scale reading combination, but apply extra weight on the end of the string beyond whatever the scale reading at that speed was in part one. Observe that the skateboard should accelerate against the treadmill now. Record the extra weight used at each speed setting. Time the speed it takes the skateboard to cover a known distance up the treadmill and the extra weight used.
Part three)
Now repeat the pulley/weight experiment on a fixed table top using only the difference in weight from the last two steps. ie: if 5 lbs held the skateboard in place in part one and you used 7 lbs to accelerate it against the treadmill in part two, only use 2 lb of suspended weight on the stationary surface. Compare the time it takes the skateboard to cover the same known distance from part two. It should be roughly the same, within measurement tolerance. Maybe slightly longer due to overcoming rotational inertia of the wheels at rest when not on the treadmill.
Note that the velocity of the skateboard in parts two and three are approximately the same, but the wheel speeds differ. This should show that the rolling resistance in the wheels and bearings is negligible.
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