Facebook Follies

Almost everyone is saying it can't take off :mad2:
 
Initial Fix said:
Can someone post a synopsis of the FB post? I don’t do social media
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There's a mixture of people saying it can't take off, and people explaining why it can (the latter including myself). There are also people assuming that the problem statement means that the engine is not being used, which would be silly.
 
Palmpilot said:
There's a mixture of people saying it can't take off, and people explaining why it can (the latter including myself). There are also people assuming that the problem statement means that the engine is not being used, which would be silly.
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I'm not saying that people are stupid, but some people really just aren't very smart.
 
The problem is that the description is not clear. if the treadmill is moving fast enough to keep the plane from moving forward relative to the wind (which in my opinion, is the only scenario where the question even makes any sense), then unless its a plane that has enough prop blast to provide sufficient lift for flight with no forward motion relative to the air (because that’s what we just agreed the treadmill is doing, preventing the plane from moving forward relative to the air), then it can’t fly.

if you’re saying that a plane can out power a treadmill and fly anyway (like what they did on mythbusters), then I say the spirit of the question is violated and it’s all nonsense anyway, because of course it will fly if it goes fast enough to overcome the force of the treadmill. But then the Question is just “can a plane out power a treadmill”. And that’s a pointless questionl and is not what was intended by the thought experiment IMO
 
Katamarino said:
³He said, on a social media site.
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I have never considered specialty forums to be social media. Social media to me is more like shouting on the street corner where anyone might hear it, where this is more like talking to the local zoning board.

That said, the only way a plane wouldnt take off from a treadmill is if the wheel bearings seized before reaching Vr since the wheels are going to be spinning twice as fast as normal.
 
Salty said:
if the treadmill is moving fast enough to keep the plane from moving forward relative to the wind
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So the rolling resistance of the tires exceeds the thrust available from the engine? That's some mofo serious speed. Them tires gonna blow and the airplane ain't gonna fly on account of what the tires being gone.
 
Capt. Geoffrey Thorpe said:
So the rolling resistance of the tires exceeds the thrust available from the engine? That's some mofo serious speed. Them tires gonna blow and the airplane ain't gonna fly on account of what the tires being gone.
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Perhaps, but the question is a thought experiment, and as such is pointless if the treadmill isn’t holding the plane still. That’s the whole friggen point of the question.

Now if you want to argue that the questions premise is flawed, that has nothing to do with launching A plane frankly.
 
Uh oh, now I've gone and done it! :eek2:

Salty said:
The problem is that the description is not clear. if the treadmill is moving fast enough to keep the plane from moving forward relative to the wind (which in my opinion, is the only scenario where the question even makes any sense), then unless its a plane that has enough prop blast to provide sufficient lift for flight with no forward motion relative to the air (because that’s what we just agreed the treadmill is doing, preventing the plane from moving forward relative to the air), then it can’t fly.
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Unless the wheel bearings are seized up, I'm not convinced that it's possible to run a treadmill fast enough to prevent the propeller's thrust from moving the airplane forward. The rolling resistance of the tires may increase with speed, but if the treadmill ran fast enough to cause rolling resistance to equal full-throttle thrust, the tires would probably catch fire!

Salty said:
if you’re saying that a plane can out power a treadmill and fly anyway (like what they did on mythbusters), then I say the spirit of the question is violated and it’s all nonsense anyway, because of course it will fly if it goes fast enough to overcome the force of the treadmill. But then the Question is just “can a plane out power a treadmill”. And that’s a pointless questionl and is not what was intended by the thought experiment IMO
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Assuming the wheel bearings are in good working order and the brakes are not being applied, the only rearward force that the treadmill applies to the airplane is due to rolling resistance of the tires. A person can single-handedly apply enough force to push a small airplane into its tie-down spot, but cannot apply enough force to stop an airplane at full throttle from moving forward. That proves that normal rolling resistance will not be enough to overcome the thrust.

Making assumptions about unstated intent of the thought experiment is arbitrary. That especially applies if the assumed intent violates laws of physics.
 
Salty said:
Perhaps, but the question is a thought experiment, and as such is pointless if the treadmill isn’t holding the plane still. That’s the whole friggen point of the question.

Now if you want to argue that the questions premise is flawed, that has nothing to do with launching A plane frankly.
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The problem statement doesn't actually say that the plane is held still.
 
Palmpilot said:
The problem statement doesn't actually say that the plane is held still.
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Then whats the point of the treadmill? might as well ask if a plane in a blender can take off.
 
Salty said:
Then whats the point of the treadmill? might as well ask if a plane in a blender can take off.
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In my opinion, the point of the treadmill is to confuse people. It's a masterful job of trolling.

A couple of years ago, one of the Mythbusters guys posted an interesting discussion of how this question generates so much disagreement:


If anyone's interested, the full Mythbusters episode is Season 5, Episode 28. It's available on various streaming services, including Discovery.
 
Palmpilot said:
There's a mixture of people saying it can't take off, and people explaining why it can (the latter including myself). There are also people assuming that the problem statement means that the engine is not being used, which would be silly.
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Thanks!

Now I’m tracking. Did folks not watch the myth busters episode? It’s not worth reposting their results
 
IT WONT TAKE OFF!
ONE OF THE STIPULATIONS IS THAT THE SPEED OF THE TREADMILL MUST EXACTLY MATCH THE SPEED OF THE WHEELS. THE ONLY TIME THE SPEED OF THE WHEELS WILL MATCH EXACTLY THE SPEED OF THE TREADMILL IS IF THE AIRSPEED IS ZERO.

Sorry I don't know why that was in all caps but okay pretend I shouted all that.

If the treadmill is set to go at 100 mph then the wheels of the airplane must spin at 100 miles an hour which means the airspeed of the aircraft will be zero.

If a plane is going to take off of the treadmill and the treadmill is set at 100 miles an hour and the airplane has to be doing 70 to take off than the wheels of the airplane are doing 170 while the treadmill is set at 100 which breaks the stipulation.

The only time the plane' s wheels will be moving at the exact same speed of the treadmill as if the airspeed is exactly zero.

At an airspeed of exactly zero, planes don't fly.

A lot of pilots think that people who think it can't take off are stupid because they think those people think the wheels have something to do with the propulsion. This is not what this is about everybody knows the wheels don't drive the plane. But if the stipulation exists that the wheel speed has to be the same speed to which the treadmill is set your plane is not moving through the air.
 
The statement stipulates a "giant" treadmill. If that refers to width, the plane can't take off without leaving the treadmill. If it refers to length and there's enough length to accelerate to take off speed then it can. That assumes that there's a negligible amount of friction at higher wheel RPMs, which would require a slightly longer treadmill length.
 
A plane can take off of a treadmill only if it's wheels are allowed to spin faster than the treadmill.
 
SixPapaCharlie said:
A plane can take off of a treadmill only if it's wheels are allowed to spin faster than the treadmill.
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What limits that? The wheels only have to add enough additional RPMs to match the speed of the treadmill. If the treadmill is moving at 10 mph and the airplane takes off at 100 mph, the aircraft wheels only need to spin at 110 mph. The added friction is likely negligible with that difference.
 
I was shocked to hear a pilot think the plane couldn't take off from a treadmill when the Mythbusters episode originally aired.
To see that pilots still think today that a treadmill would prevent a plane from taking off (normal mechanical condition, not seized wheel bearings and stuck brakes) is the equivalent of hearing people claim the Earth is flat.
Yes, there will be a small amount of drag from wheel/bearing friction. No, it's not even remotely close to what it would take to prevent the plane from accelerating to takeoff speed. The airspeed of the plane will still be 100 mph, even if the treadmill moves back at 100 mph. The wheels will spin at 200 mph, but that's (almost) irrelevant.
 
dmspilot said:
The wheels are irrelevant. With enough power you can take off with the brakes locked.
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On a 100mph treadmill, facing backwards.
 
SixPapaCharlie said:
A plane can take off of a treadmill only if it's wheels are allowed to spin faster than the treadmill.
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The wheels are not connected to anything that will prevent them from spinning faster than the treadmill.
Although I had a classmate in college ask (while watching a fighter jet taxi past us) how is power sent from the engine to the wheels. The pilot giving us the tour answered (without missing a beat): "through a crankshaft".
 
SixPapaCharlie said:
A lot of pilots think that people who think it can't take off are stupid because they think those people think the wheels have something to do with the propulsion. This is not what this is about everybody knows the wheels don't drive the plane. But if the stipulation exists that the wheel speed has to be the same speed to which the treadmill is set your plane is not moving through the air.
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The scenario as posted in the Facebook thread did not contain that stipulation. It had even more ambiguity than the original problem statement that I saw in the 1990s! With either version, different people make different assumptions about what the premise really is, and then they answer the question without, in most cases, making it clear what assumptions they're making about the premise.

Facebook Treadmill Scenario.png
 
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SixPapaCharlie said:
A plane can take off of a treadmill only if it's wheels are allowed to spin faster than the treadmill.
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I don't understand what you mean by "allowed to spin faster than the treadmill." Are you talking about the tires losing traction with the surface of the treadmill?
 
Palmpilot said:
I don't understand what you mean by "allowed to spin faster than the treadmill." Are you talking about the tires losing traction with the surface of the treadmill?
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The original premise that started all of this stated that the wheels of the plane must match the treadmill speed exactly. I believe that was stipulated in the mythbusters as well.
I didn't read this current facebook variety and assumed it was the original one.

Of course a plane can take off from a treadmill.
But if given the constraints of the original logic problem which included the constraint that the wheel speed must match the treadmill speed, it simply cannot.

This is the wording of the original problem that started all of this mess:
"Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"
 
SixPapaCharlie said:
The original premise that started all of this stated that the wheels of the plane must match the treadmill speed exactly. I believe that was stipulated in the mythbusters as well.
I didn't read this current facebook variety and assumed it was the original one.

Of course a plane can take off from a treadmill.
But if given the constraints of the original logic problem which included the constraint that the wheel speed must match the treadmill speed, it simply cannot.

This is the wording of the original problem that started all of this mess:
"Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"
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The only reason it couldn't take off is because as the airspeed increases above zero, the wheel/treadmill speed would instantly approach infinity and something down there would explode. I guessing the tires first.

That can't be the intent of the question. It's only purpose is to trap people who don't understand how airplanes work.
 
Jim K said:
The only reason it couldn't take off is because as the airspeed increases above zero, the wheel/treadmill speed would instantly approach infinity and something down there would explode. I guessing the tires first.

That can't be the intent of the question. It's only purpose is to trap people who don't understand how airplanes work.
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See, that's what I have always wondered. Was this shallow enough to trick people that aren't thinking about to he propulsion source or is it a much deeper problem which leads to the instant infinity problem.
I assumed (as someone that has known what moves an airplane since I was 5 and had the little rubber band powered ones) that it was somewhat common knowledge and this was deeper.
OR is it super deep with built in trolling designed to laugh at the people that think planes move like cars AND ask the deeper question.



copter.jpg
 
SixPapaCharlie said:
This is the wording of the original problem that started all of this mess:
"Imagine a 747 is sitting on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?"
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it Seems the question mythbusters, and many here are answering would be better phrased like this:

Imagine a plane on a conveyor belt, as wide and long as a runway. The conveyor belt is designed to run at or greater than the max speed of the plane in level flight, moving in the opposite direction. Can the plane take off?
 
PaulS said:
lol, the speed of the wheels is meaningless to the wing flying. This thread cracks me up.
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Actually, I know what Bryan is saying, and I think people are missing the premise.

Conditions: Calm. Plane facing west, treadmill moving east.

Put a plane on a treadmill, also put a speedometer on a wheel. The only way that the speedometer reads the same speed of the treadmill is when the forward motion of the plane is 0. Any eastward movement by the plane and the treadmill speed will be greater than that of the speedometer reading. Any westward movement of the plane will result in a speedometer reading higher than that of the treadmill. If the treadmill is moving at 70kts, and the speedometer will say 70kts, and the airspeed says 0, the engine is providing just enough thrust to counter any friction loss. However, any extra thrust providing any forward airspeed will result in the speedometer reading more than what the treadmill is moving at - in which case the experiment is now void because the treadmill is NOT matching the speed of the wheels.
 
dbahn said:
What limits that? The wheels only have to add enough additional RPMs to match the speed of the treadmill. If the treadmill is moving at 10 mph and the airplane takes off at 100 mph, the aircraft wheels only need to spin at 110 mph. The added friction is likely negligible with that difference.
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A runway is a treadmill moving at speed zero. The wheels are spinning faster than the treadmill when you start your takeoff roll. You have now broken the conditions of Bryan's premise.
 
SixPapaCharlie said:
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EdFred said:
Actually, I know what Bryan is saying, and I think people are missing the premise.

Conditions: Calm. Plane facing west, treadmill moving east.

Put a plane on a treadmill, also put a speedometer on a wheel. The only way that the speedometer reads the same speed of the treadmill is when the forward motion of the plane is 0. Any eastward movement by the plane and the treadmill speed will be greater than that of the speedometer reading. Any westward movement of the plane will result in a speedometer reading higher than that of the treadmill. If the treadmill is moving at 70kts, and the speedometer will say 70kts, and the airspeed says 0, the engine is providing just enough thrust to counter any friction loss. However, any extra thrust providing any forward airspeed will result in the speedometer reading more than what the treadmill is moving at - in which case the experiment is now void because the treadmill is NOT matching the speed of the wheels.
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Yes, fair enough, there are a few versions of this going around, I should have asked him to clarify before I posted. But even with that premise, the treadmill could variably match the AC speed and keep the wheels steady. The other issue with the premise is that once the aircraft begins producing thrust, it's not going stay stationary with the treadmill since the wheels are unpowered and only negligibly affect the motion of the AC.

The version I used to argue with a guy on was if an airplane landed at X speed on a conveyor going X speed in the opposite direction, would the airplane stop instantly as it touched down. He said absolutely, that was always an interesting discussion.
 
PaulS said:
Yes, fair enough, there are a few versions of this going around, I should have asked him to clarify before I posted. But even with that premise, the treadmill could variably match the AC speed and keep the wheels steady. The other issue with the premise is that once the aircraft begins producing thrust, it's not going stay stationary with the treadmill since the wheels are unpowered and only negligibly affect the motion of the AC.

The version I used to argue with a guy on was if an airplane landed at X speed on a conveyor going X speed in the opposite direction, would the airplane stop instantly as it touched down. He said absolutely, that was always an interesting discussion.
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Yeah, the arguments really need to be based on very specific conditions and premises and what the word "is" is.
 
What if you run the treadmill then slam on the brakes? :stirpot:
 
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