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But you're assuming friction increases with speed, which isnt the case. Imagine sitting on your bike on a treadmill and holding the sides of the treadmill. If you turn on the treadmill at 5mph you only need to hold on gentle to keep yourself in place (totally ignore the bike can be pedalled!!). You can pull youself forward with very little extra effort. Increase the treadmill to 50mph and you still only need to hold on with same force to remain stationary as [i]friction is constant[/i].
Take same situation with the plane - it would use only a very small proportion of its power to remain stationary, regardless of whether the treadmill was turning at 5mph or 500mph. That means that the rest of its thrust can be used for forward momentum, and as we all know - airspeed is what makes the plane fly...
Andrew you're the only one who's still not realised you're talking crap! How long will it take?!
Isn't this the 3rd time this topic has come up over the years?
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Andrew,
In some respects you are right. If as the question sometimes states the treadmill matches the speed of the wheels then the plane will always have a ground speed of zero (where the ground is everything that is not on the treadmill) and so is unlikely to have any significant airspeed. In practice this is impossible to set up but like all good theoretical questions practical considerations don't come into it. There was a joke above about considering perfect spheres in a vacuum. It's that kind of thing and is the trick part of the question for those not paying attention.
However, there is another version of the question which leaves out the details about the speed of the wheels and instead refers to the speed of the plane. In this case the treadmill is an irrelevance as the wheels will just spin faster than the treadmill allowing the plane to move forwardand gain enough airspeed to take off.
jonba,
the wheels and conveyor are an irrelevance in both versions of the question. Thrust, an external force to the wheels and conveyor still acts upon the aircraft.
Assuming 0mph windspeed, the groundspeed of the plane (the speed of the conveyor) would be 2 x airspeed.
If you overtake the last person in a race, then you are in what position?
1st place - you're lapping them.
If you overtake the last person in a race, then you are in what position?
Same as you were before, because you weren't in the race.
Andrew you're the only one who's still not realised you're talking crap! How long will it take?!
Probably depends how much encouragement we give him.
Isn't this the [s]3rd[/s] 495th time this topic has come up over the years?
Yes, about right!
the wheels and conveyor are an irrelevance in both versions of the question. Thrust, an external force to the wheels and conveyor still acts upon the aircraft.
Assuming 0mph windspeed, the groundspeed of the plane (the speed of the conveyor) would be 2 x airspeed.
This. As said previously, if the conveyor belt is moving at the same speed as the plane then it'll take off at 150mph as normal, but the wheels will be doing 300mph. It's completely irrelevant.
andrewh is making me laugh.
😀
Quality, wether intentional or not.
I think njee bike on a treadmill was a good example.
If you stand by the side of a treadmill and lower a bike on to it. You wont feel any force pushing you backwards. As the wheels will turn.
I suppose it would theoretically be possible to run the treadmill so quickly that that enough friction builds up in the hubs that the wheels cant match the speed on the treadmill and then you would feel the backwards force.
Not trolling, but have a sneaking suspicion that I am being a bit dim somewhere...
In this case the treadmill is an irrelevance as the wheels will just spin faster than the treadmill allowing the plane to move forwardand gain enough airspeed to take off.
That is kind of the point. If the treadmill accelerates to match the speed of the wheels then it doesn't move forwards and so gets no air-speed.
All the wheels do is stop the bottom of the plane getting scuffed so why would the speed of the treadmill have any effect on the plane taking off?
Must resist....
Can it take off IF the belt matches the speed of the wheels
is equivalent to
Can it take off with zero groundspeed
yes, it *could* easily take off, but the wheels would be spinning faster than the belt, which breaks the constraint (as happens in the vids).
Yes, its both stupid and impossible, but there you go.
Everyone else understood this from page 1 onwards...
"This. As said previously, if the conveyor belt is moving at the same speed as the plane then it'll take off at 150mph as normal, but the wheels will be doing 300mph. It's completely irrelevant."
If conveyer belt is moving at 150, then the plane will be moving at 150 relative to the treadmill, the wheels at 150 again realative to the treadmill. Relative to fixed point on ground aircraft is not moving, no airflow, no lift. No ground effect as speed over groud is required (not speed on conveyer belt). Jeez, why is there even a debate on this!!? Conventional plane will not take off if forward thrust matches speed of conveyer belt. Plane needs airflow over wings to generate lift.
Thrust vectoring assets are very different as they can genertae their own lift.
To quote Scroobius Pip -
"I've seen the world; I've seen the good and the shitty bits;
And all I've got to say is god damn, y'all are ****ing idiots."
😀 😀 😀 😀 😀
I thought we solved this last time by saying that we needed an enormous fan in front of the plane, rather than a conveyer belt?
dont make me find the old thread
Conveyor belt: designed to run in the direction of travel.
Treadmill: Designed to run against the direction of travel.
Basically the wheels and treadmill are an irrelevance: The way the plane picks up forward momentum is by pushing against the air pressure, not against the friction of the ground; the wheels are there to merely aid the movement of the plane, not to propel it. Therefore, the plane will take off regardless of what the treadmill is doing as the treadmill has no effect on the air pressure surrounding the plane.
As many others have said, the only effect the treadmill would have is to make the wheels spin faster than if the plane was taxiing on an ordinary runway.
Goddamn it druid, you bad bad man!
Here's another way for wallace to look at it, just in case he's not trolling.
The treadmill is stationary, the plane is stationary, the jets go on full thrust and it begins to accelerate. when it gets to 100mph, the treadmill suddenly switches on, instantaneously going at 100mph. what happens to the aircraft's speed relative to the air around it and the non-treadmill ground? Does it carry on accelerating? Or suddenly stop or start to go backwards?
You can have the airstrip made up of a bunch of treadmills in a row, all going at different speeds forwards and backwards, the plane will accelerate over them. The tyres will be under a bit of strain, but the plane will still accelerate in relation to the air and the earth (not treadmill) until the airflow over the wings generates more lift than the weight of the plane.
Or:
Can a plan land on a treadmill? Or a conveyor belt?
I'm still not sure what the question is.
Bit late to this and I struggle with long sentences, if the plane can take off coz "it's airspeed over the wings produced by the engines" then why don't planes just sit on a short runway with their wheel brakes on give it some beans and take off from stationary? Could save a lot of money building the next airport.
The myth busters episode doesn't seem right because the plane moved forwards, the "conveyor belt" was running backwards but the thrust from engines carried the plane forwards.
I'll stick with my original "no it won't" stance.
That's the spirit!!
Nedrapier - I do not disagree with you. In your scenario it will still accelerate.
However, the scenario in question dictates that the belt will match the speed of the wheels, therefore speeding up, and the thrust increases, therefore the aircraft will stay stationery, with the thrust used up to keep it stationery. No flight.
On running treadmill, you generate no forward momentum, so if you jump up, you would not leap forward relative to ground.
do the treadmill/wheels have to accelerate infinitely? as the plane WILL move forward and take off, the wheels must always go faster than the treadmill, so it must speed up to compensate.
No, if my thinking is right the thrust from the engines ignores the treadmill altogether, the plane will roll forward however fast the treadmill is going and (presuming it's a short treadmill and a plane that needs a lot of airspeed) fall off the front of it. Still won't take off but not for the reasons you say.therefore the aircraft will stay stationery, with the thrust used up to keep it stationery.
...I think.
However, the scenario in question dictates that the belt will match the speed of the wheels, therefore speeding up, and the thrust increases, therefore the aircraft will stay stationery, with the thrust used up to keep it stationery. No flight.
That's one of the scenarios. The one that can't work. It can't even start. It could in a normal car, or with legs in the running example, where forward motion is generated by force through the wheels/trainers onto the treadmill.
But with the plane, where the wheels move ONLY because the plane starts to move, it just can't hold, unless the plane, the wheels and the treadmill are all stationary. Or the plane is on a treadmill which is on the back of a truck, and the truck is accelerating to match the plane in order to stay underneath it.
See my earlier post about sitting on your bike on a treadmill - takes very little force to hold yourself in place as you only need to overcome friction therefore very little of the planes thrust is used to overcome friction, most is used to push it forward.
And friction is a fixed force and doesn't increase with speed, despite what scu98rkr suggested about running the treadmill faster to creat more friction...
There we go. The alternate scenario, where the treadmill matches the speed of the tyres, can only continue to hold true if the treadmill itself (the whole thing, including the mechanism) moves forward to match the speed of the plane as its prop or jet accelerates it through the air to... TAKE-OFF!
Boom.
I can't help looking back in. I see we've reached the 'arguing over the semantics of the question phase'.
miketually - Member
Yet again, I can't help looking back in. I see we've once again reached the 'arguing over the semantics of the question phase'. Again.
FTFY!
😀
actually, we've gone beyond that. Either that or we haven't.
Answers thus:
a) Yes
b) Yes, or no depending.
The semantics are irrelevant, it's yes regardless! Jesus wept.
The plane will not take off on the spot, that's stupid, it would still require a 10,000ft conveyor belt, just as it would a runway.
As aP said, the wheels just stop the bottom of the plane getting scratched! The engines are pushing the air backward to accelerate, by what mechanism does the conveyor belt stop this happening? All it's doing is opposing the motion of the wheels.
Another example for you then... Ice. Car on solid ice, zero friction (for arguments sake). Foot to the floor. Wheels spin, car doesn't move.
Aircraft on same, frictionless surface... engines spool up, plane moves forward. Ta da!
the semantics* are very relevant, I didn't see the original question/thread, I thought the scenario was short conveyor belt, stationary plane in which case the plane obviously would not take off (not from the conveyor belt anyway)The semantics are irrelevant
If the scenario is runway length conveyor belt then yes of course it will take off.
*fairly bloody important hypothetical details so possibly not semantics
The question is "Will a plane take off from a conveyor belt?".
Some people interpret this as 'can it gain speed on the spot then magically lift off', which is of course a no. However, there are no stipulations about the size of the conveyor belt.
As has happened here, people say "there are 2 questions, one where you mention the conveyor belt moving in the opposite direction to the plane, at the same speed", but that is totally irrelevant. To turn it around... there is no circumstances under which a conveyor belt can stop a plane taking off.
Anyway, we've gone full circle, in my very first post I said that it could be done, but would require a runway-length conveyor belt.
[i] To turn it around... there is no circumstances under which a conveyor belt can stop a plane taking off[/i]
That statement is wrong in so many ways I wouldn't even know where to start with it 😀
Why, pose me a situation whereby a conveyor belt would stop a plane taking off?
And I don't mean 'by having one the size of the plane and putting it in the sea', because then it's a lack of land, nothing to do with the conveyor belt!
luggage conveyor truck putting a hole in the fuselage?
Big pile of conveyor belts half way down the runway?
I remember the original discussion.I only read the first few posts of the first page and my brain went hurty.
But then I'm a girl, so the only relevant issue is "what colour is the aircraft?".
Big pile of conveyor belts half way down the runway?
😆
luggage conveyor truck putting a hole in the fuselage?
That's the truck, and planes can still fly with holes in them.
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whilst not quite stationary, this is worth a watch
In order for the plane not to move forwards the treadmill will need to generate a force on the plane equal to and opposite to that produced by the thrust of the engines. This would mean that right from the start of take off thrust being applied (and I think TOGA thrust can be spooled upto in 2-3 seconds) the friction of the wheels rotation to transmit this force would need to be huge. This might be possible in a model plane with low take off speed and small wheels and iffy bearings but for a full sized commercial plane it isn't going to happen .
Why, pose me a situation whereby a conveyor belt would stop a plane taking off?
Well you've already ruled out my factitious answer based on lobbing it in the engine intake, however..
Because I was bored I did some fag packet calculations based on a Cessna Mustang twin engined light business jet.
2 engines 6.49Kn thrust = 6490*2 = 12980N
Max Takeoff weight = 3930KG = 38553N
Calulation of frictional coefs
Fr = Cr. Fv
Cr = Fr/Fv
= 12980/38533 = 0.33
Rolling resistance in sand is about 0.33
So if you're conveyor belt was covered in deep sand it would't even move in the first place, but well assuming that the belt has the same properties as tarmac.
Rolling resistance of stationary tire on tarmac is approximately
C = 0.005 +1/p(0.01+0.0095(v/100)^2)
p = pressure in Bar (~ 6 bar for this aircraft) v = speed in kmh)
so at 0kmh C =0.005+1/6(0.01) = 0.006
at 100kmh = 0.00825
at 1000Kmh = 0.16
at 1500Kmh = 0.36
So with a ground velocity of 15000kmh we have a rolling resistance that requires a force to overcome which is greater than the combined thrust of the engines.
So if the belt is moving backwards at 1500kmH the plane will not be making any forward progress.
Of course at the tires will have exploded by that point, somewhat increasing the friction. 🙂