Principles of flight?

[Bridget]

Velocity "distance travelled per unit time" so the velocity in your equation would be the distance over the travelator, not the ground, so wouldn't be zero...

correct me I know what I said was bullshit, lol, my commonsense says it couldn't take off but I just don't know anymore

[vvega]

im sorry guys but you still have yet to explane where the energy of the thrust has gone

and why the travelator is a brake
the wheels have no motors of there own
so how do they move
ffs get a toy car and push it on your had and tri to work it out



ps here is some advice ill give you for free
this question was asked on a forum of aeronaughtticalengineers and they came up with..........

im very confident the plane will fly
in fact
id put money on it

v

[V8MOFO]

Are we all agreed that it will 'not' take off ?

[vvega]

the plane will have velocity
the travelator will not stop it from moving forward

if what some of you were saying was true roller bearings would not work


v

[V8MOFO]

Velocity "distance travelled per unit time" so the velocity in your equation would be the distance over the travelator, not the ground, so wouldn't be zero...

correct me I know what I said was bullshit, lol, my commonsense says it couldn't take off but I just don't know anymore

no, the ground has nothing to do with a plane in the air. The earth could be moving at 1 million miles an hour for all it cares. The velocity in that equation is the of the AIR.

[Bridget]

blah blah blah who gives a shit anyway? The only way everyone will be satisfied is if the experiment is carried out.

[no_8wire]

im sorry guys but you still have yet to explane where the energy of the thrust has gone

and why the travelator is a brake
the wheels have no motors of there own
so how do they move
ffs get a toy car and push it on your had and tri to work it out



ps here is some advice ill give you for free
this question was asked on a forum of aeronaughtticalengineers and they came up with..........

im very confident the plane will fly
in fact
id put money on it

v

energy of the thrust has gone

Yes the thrust moves the plane forward...which in turn moves the wheels as the plane is connected to the wheels. how ever the belt is moving in the opposite direction at exactly the same speed.
So the thrust force pushs it forward, while the force/velocity of the belt cancels it out.

like this.
5 + -5 = 0

and why the travelator is a brake
the wheels have no motors of there own
so how do they move
ffs get a toy car and push it on your had and tri to work it out

The belt is NOT a brake. It is simply moving in the opposite direction as however fast the plane is going rendering the total resultant force nil.
Yes the wheels have no motors, they move when the plane moves

[vvega]

the travelator is irelevant its only there to confuse you
i now see why so may people fail physics

if the wheels were the forum of popruslson it would be revelant but because its not it has nothing to do with it

the oinly way to sop the plane movin ids to make a brake

ie tie it down
or apply the brakes on the wheels and use firiction to hold it still
the traveltor offers no friction
in fact it says thw faster the wheels move the faster it will move
in order to do that iit has to move
and as soon as it dose that
you have forward momentum and its game over

likw i side push a car off a dyno and you will see how it worls

v

[Cahuna]

Thinking this whole thing through again I think the most interesting question isn't "will the plane fly" so much as "is it possible to build a conveyor that exactloy matches a planes' wheel speed" I raise the question because:
* If a conveyor matches the speed of the planes wheels in the opposite direction then that means that the net ground speed of the plane must be zero. BUT
* Because an aircraft uses thrust to move the wheels are free to just spin and do whatever they like. They are effectively (almost) frictionless. Thus when thrust is applied it is irrelevant what is below the plane for the wheels to roll on or what speed it is doing in which direction, the plane will move with the thrust.
* But, in that case, for the plane to move forward its wheels must be moving at a different speed to the conveyor belt.

So, because I don't think it is possible to stop the plane from moving forward when thrust is applied I think it must be impossible to build a conveyor that can always exactly match the wheel speed of an aircraft. Just like using your hand to push a toy car against the belt on a treadmill - my intuition tells me it isn't possible for the treadmill to stop me from pushing the car forward, thus it is impossible for the treadmill to always exactly match the wheel speed of the car. Ditto for aircraft.

So I think that as the question is worded it is not possible for the plane to fly because having the conveyor travel at the same speed as the plane wheels would result in a zero ground/air speed for the plane. BUT I think that the conditions described in the question are impossible to create as it is not possible for the conveyor to counteract the force of the aircraft engines, thus it is not possible for the conveyor to always match the wheel speed.

My money is still on my original answer: the plane will fly. And the physicists who created a question using constraints that are not physically possible in the real should be shot

[vvega]

not to mention the planed wheels would explode at teh rotationa speeds were taking

v

[matt dunn]

[That force of thrust no matter how large, has to be converted through the wheels in order for the plane to move. And how will the plane go forward if the all of that thrust is cancelled out by the conveyor.

So if a plane is cruising at 200kmh flying along above ground,
and they they go full throttle,
how does the plane accelerate without the wheels on the ground.


The question that you must answer here first is,

Will the plane move if it is on the conveyor?

Then you can answer the question will it fly?

[vvega]

[That force of thrust no matter how large, has to be converted through the wheels in order for the plane to move. And how will the plane go forward if the all of that thrust is cancelled out by the conveyor.

So if a plane is cruising at 200kmh flying along above ground,
and they they go full throttle,
how does the plane accelerate without the wheels on the ground.


The question that you must answer here first is,

Will the plane move if it is on the conveyor?

Then you can answer the question will it fly?

its not hard matt
but like all exam questions its setup to confuse

[Bridget]

as an exam question there is no straight out yes or no, it's the reasoning the person gives behind their answer.

[V8MOFO]

vvega, you have made me believe that the plane will move over the travelator. Oh and for the record, I never failed physics, hell I never even did physics, except it science...

you all agree that travelator speed goes to infinity? Good, mass of travelator goes to infinity, gravitational forces become infinite and BAM, no lift is sufficient for take off

[pidge]

Lets get back to first principles here.

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

What have here is:
Initial conditions (t=0)
Vw=0 (wind speed)
Vc=0 (aircraft speed)
Vb=0 (converyor belt speed)

All measured with respect to a fixed point off the conveyor belt.

We will assume that
The wheels of the aircraft are free to rotate without friction.
The aircraft has jet/rocket or propulsion.
Theaircraft a mass of 10,000kg
The full-power thrust of the engines is 100,000 N (newtons, or m.kg/s^2)

Just after t=0, the pilot powers up the engines to full thrust.

Immediately the aircraft beings to move forward with an acceleration of 10m/s^2 ( about one "gee").

Now please note that the wheels are free to turn. The brakes on the aircraft are not on.

For the aircraft to not move, something has to push back on the aircraft with as much force as the engines are providing. 100,000N.

Where could that come from to prevent the aircraft taking off?

Things that come to mind are:
- air drag
- wheel friction (let's assume there is some)

Even if there was wheel friction, it would be neglibile (i.e. maybe its 10,000N worth of drag - changing the acceleration on full thrust to just 9 m/s^2). The friction due to the wheels is likely to be speed dependant as the bearings heat and expanding, increasing the amount of friction. But since the wheels don't melt off the landing gear of aircraft (normally!), we can assume that the drag due to the wheels is negligble.

Let's examine air drag. First, on observation. Under normal circumstances, the only times an aircrafts engines are at full throttle is on take off, and on landing with the reverse-thruster flaps (the first time I saw those working on a 737 it was a bit of shock to see the back of the engine "fall off"). To maintain cruising speed, the engines do not go at anything approaching full thrust (not sure how much throttle though!).

So air drag is very unlikely to stop the aircraft from taking off.

The conveyor belt seems to be red herring to confuse and/or confound.

Oh, and btw, jet engines do not push against the air. The exhaust from the engine is what is being "pushed" against (conservation of momentum -it's called a "reaction" drive). Throw 44g (1 "mole") of CO2 gas backwards at 200kmp/h (momentum of 2.44 kg.m/s), you'll move forward a bit too. Throw enough lots of CO2 (and water and air), you'll move a 10,000kg aircraft as well. Which is what jet engines do.

[vvega]

i was talking prop wash pidgie
i worked on aircrat all day everday up till recently so i have a good understanding of aircraft engines

[RomanV]

lets see now, The whole point of the travelator is that the plane doesn't move. Otherwise there is no point to the question.
I think what you are missing, is that this is a physics question on how lift works, not a question on how planes work.

No no.... you just assumed that.
That is the trick to the question!

Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

If the plane was stationary.... then the travelator would not move AT ALL, would it? Think about that.
So the wheels would never get up to ANY speed.

The plane DOESNT stay stationary! If there was no conveyor underneath, then the speed of the wheels is equal to the speed of the plane relative to THE EARTH. However since there is a travelator underneath, the speed of the wheels DOUBLES, while the speed of the aircraft stays the same as it would otherwise.

So the plane is going at 50kph, while the wheels are spinning at the effective speed of 100kph.

THE PLANE STILL MOVES, OTHERWISE THE WHEELS WOULDNT BE SPINNING AT ALL.

Read that 5 times over.

If the plane doesnt stay stationary, then of course it can take off.

[pidge]

i was talking prop wash pidgie
i worked on aircrat all day everday up till recently so i have a good understanding of aircraft engines

Actually, the same thing. The prop wash is the result of the prop pushing the air back, which in turn pushes the prop (and the aircraft) forward. I think my comment was directed as someone else though.

Paraphrase: props "pull" aircraft through the air, and jets "push"

Action and reaction are equal and opposite

[RomanV]

And for those of you yet to grab the concept....

Answer me this question.

If the plane WAS moving fowards at 100kph on flat ground, the wheels would be spinning at 100kph..... Do you agree?

If the plane was going 200kph, the wheels would be spinning at 200kph.

Now if the plane is travelling at 100 kph, and there is a travelator moving in the other direction, then the speed of the wheels will be 200kph.
But the plane is only going 100kph.

If you put a conveyor belt under the plane, the plane is still moving fowards at 100kph, but now the wheels are spinning at a speed of 100kph for the plane speed, + 100kph for the travelator speed in the opposite direction.

So the plane travels at 100kph.... and the wheels rotate at 200kph.

As per the original statement.

[V8MOFO]

And for those of you yet to grab the concept....

Answer me this question.

If the plane WAS moving fowards at 100kph on flat ground, the wheels would be spinning at 100kph..... Do you agree?

If you put a conveyor belt under the plane, the plane is still moving fowards at 100kph, but now the wheels are spinning at a speed of 100kph for the plane speed, + 100kph for the travelator speed in the opposite direction.

So the plane travels at 100kph.... and the wheels rotate at 200kph.

As per the original statement.

I agree with that, but this theory only works if you assume that the plane can accelerate in the first place. Add thrust to the plane traveling at 100kmph ( with the wheels going at 200kmph relative to the belt ), this increases the planes speed ( at the wheels ) and then increases the belt's speed.

Lets turn the equation around*, A giant round wheel ( which represents the plane wheel ), stick a car/truck/bike ( anything with wheels ) on the top with the same setup. However fast the wheel goes, the *car* matches the speed / acceleration / time of acceleration. Will the car go anywhere when these figures change. ( but still staying the same as each other )?