- #106
A.T.
Science Advisor
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The whole thread in five words.sophiecentaur said:... but I read it as ...
The whole thread in five words.sophiecentaur said:... but I read it as ...
If you chase back the sub-thread to which you responded, you will find.sophiecentaur said:I think many of the loopy contributions have not ever stated their full case. 'It stands to reason' is never a full case.
@willem2 proceeded to perform a computation containing actual physics and arrived at a figure for how fast the conveyor would need to accelerate rearward so that the friction required to spin up the wheels to match the conveyor speed would match the forward thrust of the aircraft engines.willem2 said:Actually, If you accelerate the conveyor fast enough, it can stop the plane!
To which you responded that you did not see how it followed.jbriggs444 said:result is in the right ballpark and results in the wheels reaching their rated max speed in a fraction of a second.
@willem2's post seems to be on-point in a rebuttal of this claim. [Arguably a rebuttal which was already accounted for under the "obvious physical constraints" exception].Barring obvious physical constraints, such as tire maximum ratings, the conveyor direction and speed has no bearing on whether the plane can take off or not.
We are clearly at cross purposes here. I realize you are not a total loony (just enough to want to contribute as regularly as I do on PF!) by the sentence:jbriggs444 said:Looks to me like it follows.
If you are suggesting that means to drag the wheels back so fast that they lock up and drag the plane backwards then what you are claiming will follow.RandyD123 said:The conveyor best is designed to exactly match the speed of the wheels, but run in the opposite direction
I think that sub-thread you reply to is not about guessing what "matching" means anymore, but simply asking what would the belt have to do, in order to stop the plane based on rotational inertia of the wheels.sophiecentaur said:But I do not understand what 'matching' means, in your interpretation.
Sorry, I missed this last line of the post.jbriggs444 said:None of this should be read as disagreement with your (@sophiecentaur) sound advice to state assumptions first and calculations after.
That could be true but what a fruitless conversation, in the light of almost absolute ignorance of the values of all the variables involved. I would totally agree that the experiment is a nonsense but it is scaled down very easily with a low speed prop aircraft and the Physics point is proved.A.T. said:I think that sub-thread you reply to is not about guessing what "matching" means anymore, but simply asking what would the belt have to do to stop the plane, based on rotational inertia of the wheels.
Here's the op:sophiecentaur said:I think we should demand that answers should include how the OP has been actually understood by the contributor
RandyD123 said:Imagine a 747 sitting on a large conveyor belt, as long and as wide as the runway.
The conveyor best is designed to exactly match the speed of the wheels, but run in the opposite direction.
CAN THE PLANE TAKE OFF?
Reductio Ad Absurdum.sophiecentaur said:Sorry, I missed this last line of the post.
But I still take issue with the requirement to accelerate things in a fraction of a second. The air speed of the plane is the variable that determines what the conveyor needs to be doing.
The problem I have with that interpretation of the intent of the question is the equivalent of asking that if you put a car on a treadmill that travels backwards at the same speed as the car is moving forward relative to the ground, can the car move forward? It is a bit nonsensical, because the car moves forward as one of the initial postulates of the problem. It just seems more reasonable that the problem is assuming that the treadmill moves backwards at the same speed as the tread of the wheels move relative to the car or plane. Now you can reasonably ask whether or not the car or plane can move forward.sophiecentaur said:From OP:
How does that follow? The wording in the OP is a bit vague but I read it as meaning that "match" means the conveyor surface always goes backwards at the same speed as the wheels are going forward (i.e. the bearings and the rest of the plane).
But it isn't th3e same thing at all. In the car scenario, the motion of the car is determined entirely by the power delivered via the wheels. Forces on the wheels are an essential part of the propulsion. If the 'road' moves backwards at the tangential speed of the wheels, the car goes nowhere. That is just not the case with an aeroplane.Janus said:The problem I have with that interpretation of the intent of the question is the equivalent of asking that if you put a car on a treadmill that travels backwards at the same speed as the car is moving forward relative to the ground, can the car move forward?
You can't include them in the same model. What drives the car forward? What drives the plane forward?Janus said:the same speed as the tread of the wheels move relative to the car or plane.
But isn't that point? In the case where the treadmill moves at the same speed as ground speed, the results are the same for car and plane. When it's the same speed as tangential speed of the wheel, then obviously the car doesn't move relative to the ground as it is propelled by the wheels. The plane is not propelled by its wheels and the problem becomes more reliant on the constraints applied. Since the intent of the question is to present a perceived conundrum. It does not seem likely that the scenario with the trivial answer is what was meantsophiecentaur said:But it isn't th3e same thing at all. In the car scenario, the motion of the car is determined entirely by the power delivered via the wheels. Forces on the wheels are an essential part of the propulsion. If the 'road' moves backwards at the tangential speed of the wheels, the car goes nowhere. That is just not the case with an aeroplane.
You can't include them in the same model. What drives the car forward? What drives the plane forward?
I suggested earlier that people in doubt should draw a free body diagram. Is the force on the plane in any way dependent on the wheels (in the absence of friction losses)?
I was all ready to drop this and I read your post. It's just not right.
This is just more over analysis. You can keep changing the goalposts for another 119 posts but who will it help? The original question was obviously about the simplest case. Why not deal with just that?Janus said:the problem becomes more reliant on the constraints applied.
DaveC426913 said:I can't believe this is still under discussion.
It doesn't matter what the wheels are doing - it's not a car.
A plane gets its forward motion via the air, using props or jets.
Once the thrust of the engines rises, the plane will accelerate with respect to the air - and eventually gain lift via air, as usual - no matter what the wheels are doing.
You can do whatever you want with the conveyor - move it as fast as you want - till the wheels blow, and the landing gear is worn down to nubs - the plane is still going to move forward, given sufficient thrust.
Full stop.
"If you’re desperate to tell me that I’m wrong on the internet, don’t bother.OCR said:Now...
Carry on.
[Edit to add a final clarification:]
OCR said:[End of edit to add a final clarification:]
Now...
Carry on.
But... but... with a landing gear worn down to nubs, will the thrust be sufficient to reach take off speed within the length of a normal runway?DaveC426913 said:...till the wheels blow, and the landing gear is worn down to nubs - the plane is still going to move forward, given sufficient thrust.
Yes you CAN! This is PFDaveC426913 said:I can't believe this is still under discussion.
Then we no longer have wheels and the question no longer applies.A.T. said:But... but... with a landing gear worn down to nubs, will the thrust be sufficient to reach take off speed within the length of a normal runway?
Oh my stars & garters - XKCD has a forum??OCR said:Oh, and BTW... has anybody looked here ?
And with that, I think we can call this thread done. Thanks for helping out, folks.OCR said: