Imagine a plane is sitting on a treadmill

The plane might as well be able to float a little off the surface of the ground/runway/treadmill like a Landspeeder thing from Star Wars. Forget the wheels.

The biggest misconception it seems is that the treadmill will always go twice as fast as the wheels are spinning on the plane. Which is, of course, impossible as long as the wheels aren't slipping. The treadmill speed is determined by the plane speed relative to the earth and the air.

If the plane moves forward (relative to the control tower) at 5 km/h, the treadmill moves backward at 5 km/h.

• The plane wheel speed indicator would read 10 km/h.
• The airspeed indicator would read 5 km/h.
• The lift generated would be exactly equal to a plane moving down a stationery runway at 5 km/h.

The wheel speed indicator would constantly read double the airspeed indicator. Assuming the plane has good tires, it will take off at its normal takeoff airspeed. The only difference will be that the wheels will be spinning faster than usual.

To take this one step further, if a plane takes off at 100 km/h (airspeed), but has a 100 km/h tailwind, it will need to travel down the runway at 200 km/h (wheel speed) in order to take off.

Yeah, let's just throw out all those stupid physics equations and build every plane by trial and error.

Add "some" distance is all it takes? You're really backtracking now.

crack engineers?

i've heard of mechanical, electrical, civil, chemical, industrial, genetic, aeronautical, marine, biological, and sanitation engineers; but never crack engineers...

I knew that those NASA people were smoking something!

This thread is just a penis extension for all you blokes who think you have knowledge of Physics but TBH I can't be arsed and yet know the answer with my elementary IBH Physics as mentioned previously. Please continue... and DrivinWest will be happy to be the initiator of the fastest growing thread.

I have got to say this is one heck of an amusing thread.
Just to add my 2 cents.
The only force the conveyor can apply to the plane is via the friction of the tires on the conveyor combined with the friction of the bearings in the tires. Thus, as all good physicists do, assuming friction is negligible the plane will take off as normal.

Best example: with friction taken as negligible, take the plane as stationary, and start the conveyor moving.
the conveyor can move as fast as it wants, but without friction it can impart *no* force upon the
plane. So as fast as the conveyor moves the plane stays at rest.

Basically the wheels provide an "almost" frictionless way for the plane to move while on the ground. So unless the friction from the tires and bearings is quite significant, the plane will take off as normal.

Thanks for the fun DW

My understanding of the problem was that the conveyor belt acted to keep the plane stationary (before take-off) with respect to all objects at that elevation other than the conveyor belt. Therefore the forward velocity of the plane (and it's wings) was exactly offset by the conveyor belt's reverse motion. This meant that the airspeed over the wings was 0 - hence no lift could take place. I didn't arse about with practicalities since the subject line starts with the word "Imagine".

The funny thing is, whenever I give good examples that should try and make the "non believers" see whats going on, they seem to be ignored. Its almost like the arguments you see online about creationism Vs evolutionism. You explain something concisely to the creationists and they just ignore it and quote back exerpts from the bible to counteract you...

So, one more time, here are some things I want the non-believers to consider...

1. Other than relatively insignificant wheel & tyre rolling resistance, what force is there that stops the plane speeding up relative to a stationary object. This question has been poised MANY MANY times but no non-believers have actually explained it in a way that fits any physical laws. You must remember that the brakes are OFF. Just saying "but the treadmill counteracts the forward motion" is not a sufficient answer as that does not explain where the actual forces of counteraction are. Where are these magical counteraction forces??

2. What would happen if the plane was being towed by another entity? Say for example, a helicopter with a towrope... (Sorry, I could not resist a diagram of my own)

This could in fact be any other entity like a winch at the end of the runway. Question: Would the plane move forwards? I want the non-believers to answer this one before I go on

3. For you people saying that the plane will be stationary wrt the tower but the treadmill will be going backwards at such-and-such speed, consider this? Why is the treadmill moving at all? You say yourselves that the plane is stationary so surely the treadmill would be stationary too? Huh? How does it decide how fast to move if the plaine isnt moving? Huh? The question clearly states "The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction." - if the treadmill is to move at all, the plane HAS to be moving along.

4. ARRRGH

5. I am really surprised that this relatively simple question can cause such a divide between people, its really kinda cool - and frustrating at the same time.

6. I had loads more points and retorts but tbh, I cba going through all 17 pages to find them...

You've said that twice now but I guarantee that those in the wrong are thinking the exact same thing - that they learned this in grade school and the others are complete morons. So what's your position? Will it fly or won't it and why?

I'll just bother with these 2:

Here is DW's question:

Answers to your questions:
1. The only force is the bearing wheel friction - this is the real world (not the imaginary world the question asks for) - but if the conveyor belt were to match the speed of the plane exactly then no forward motion relative to a non conveyor belt object (i.e. the air) could take place. This is why I agreed with the poster who said it depends on whether the air moves with the conveyor belt or not since in such a case there would be reverse motion of air over the wings creating lift.

2. Of course - all that is neccessary for the plane to take off is air moving backwards over the top of the wings creating pressure differential.

I saw this elsewhere and was so entertained by it I had to ask the question here!

No. This is where the flaw in your logic is. Imagine a plane flying East at 300mph over a conveyor belt moving West at 300mph. You will agree that both are moving in opposite directions in relation to a static observer. Now the plane's undercarriage touches the conveyor belt as it lands. What happens? Does the plane suddenly stop moving i.r.t the observer? No it doesn't - the wheels start to spin and the plane continues moving westwards until it stops.

The fact that the conveyor belt is moving in one direction does not decrease the speed in which the plane is moving in the other directions. The friction is the same as that of a none moving runway.

@ SP
Agreed - completely - but in your situation there was already forward motion of the plane with respect to the only object that matters in the question of whether or not there is lift force on the wings - the air. This is ALL that matters.

I've bothered to look through about the 1st 7 pages of the thread now - and it seems to me the different solutions are because of different understandings of the conveyor belt design:

My interpretation - the conveyor belt acts in such a way to prevent true forward motion - that is it prevents air being pushed back over the top of the wings. No lift off. This means of course that the wheels friction are exerting a force equal but opposite to the thrust which is unrealistic.

If the conveyor belt were designed in any other way then sure plane takes off. Once again it's a question of imagination.

Guys, the whole question is flawed, as has been pointed out by me and others way back.

The concept of the conveyor belt only makes sense if the forward accleration of the plane comes from torque in the wheels. Instead the forward acceleration, and most importantly the forward acceleration relative to the air comes from the thrust of the jets. The wheels provide an almost neglible, braking effect.

Imagine if the converyer belt started moving forwards at 300 mph. And the planes brakes were off. then the plane would stay almost stationary, and the wheels would spin in reverse, because the intertia of the plane coupled with its wind resistance would far exceed the forward excleration applied by the forward moving conveyer belt applied to the wheels.

So why are you still trying to find a solution to a flawed question?

I QUIT!! I thought this thing would be dead by now. No luck. But before I quit i want to address the above.

1. The answer is in the TEXT of the question. I am certain this is where the argument lies so... The question posed is whether a plane on a belt, each moving at the same speed but in opposite directions woiuld in fact take off. I read this to mean that the plane is stationary with respect to anything not on the belt. For example you could put the plane on the belt, take tie downs and attach the plane to the ground around the belt and then start it all up, the plane isn't moving. Said another way, the belt is going to travel at whatever speed the plane is travellig on the belt, the only thing moving are the wheels.

2. Doesn't matter, the parameters of the questions are clear, belt plane same speed, if the plane starts to make progress on the belt (move forward to a non-belt entity) the belt must incvrease speed to compensate and bring the plane back to the original starting point.

3. WHAT??? Plane starts up and belt matches any increase in speed by the plane.

4. Agreed, i am sick of of this thread, but its better than deep fried crack rings

5.

How does what you said make the question flawed? With what you said you are effectively agreeing that the plane will take off

edit: I cba with this any more.. nite

As stated in the question: the engines are running at take-off thrust, not accelerating to counteract the conveyor belt's influence and the mass of the plane adding to the friction which is not possible to simply dismiss as inconsequential.


Under the premise of the question, I don't think the runway will be long enough.


Yeah, lets throw out gravity and friction because we only want to obey some laws and not others.
Where have I backtracked?

sGb i think when the plane and conveyor move with SAME speeds but opposite directions, there would only be wheel spin(twice angular velocity compared to the case of stationary conveyor) but no translational motion occurs. As soon as thrust increases to make the plane speed more than conveyor, then for sure plane moves forward.

So the answer is YES take off happens but only if plane can move faster than conveyor.

But the belt cannot prevent forward motion. To do this it would have to move the plane backwards at the same speed. It cannot do this as the wheels of the plane will spin. The belt is moving backwards at the same speed as the plane is moving forwards, but the plane is not being propelled backwards by the belt at the same speed.

OK, I said I quit.. but hey.. Lets take this one a little further...



OK, what if you replace the tow rope between the helicopter and the plane with a solid structure? Would the plane still move forwards?

wtf - do you think this runway actually exists

What does the plane have to do to achieve forward motion?

1. Overcome inertia of the plane itself - conveyor belt has no influence on this
2. Overcome gravity - ditto
3. Overcome friction in wheel bearings - ditto
4. Overcome friction of wheels on tarmac/belt - ditto

So what is preventing the plane moving forward at normal take off speed. No - it is not being moved backwards by the belt, because the wheels are spinning.

Think of it this way, the plane is moving at takeoff thrust (not speed) and the pilot goes down a normal runway with the brakes slightly applied (equaling the friction caused by the conveyor belt runway constantly matching the speed of the plane), will the plane reach take off speed without compensating for the friction that quite a few would like to ignore by applying extra acceleration on the runway that is of set length?

But you said the plane is not moving wrt a stationary object. How does the belt know how fast to go? It has no feedback from the engines for it to "guess" how fast to turn!

The question is not flawed.

@ SP
THe belt does not have to move the plane backwards with respect to the air , it must simply keep the place stationary with respect to the air. Once again IMAGINE.

you say it yourself in your fourth statement - the belt is moving backwards (with respect to the air over the plane's wings) at the same speed as the plane is moving forwards (with respect to the belt) - therefore no movement of air over plane's wings = no lift.

Is the helicopter moving forward with respect to the air ? Answer me this first. meh. If yes then yes.
And wrt this question of "how does the belt know" WTF ? Imagine.

If thrust is just equal to belt friction then resultant of the forces is zero=no movement. If thrust is boosted more, then resultant is positive toward plane movement direction and plane moves.

I reckon the increased friction of the wheels spinning twice as fast would NOT be enough to overcome the thrust, no. So it would speed up.

Anyway this can be taken out of the equation simply by using a plane that has a take off speed of say 100mph but has overspecced wheels that can do 200mph easily.

But as the wheels were not specifically mentioned in the original question, you cannot base the answer on this. All you can do is give an answer and state an assumption about the wheels. Go look at post number 6 in this thread and you will see what I mean

btw, you do know that the primary resistive force when cars go fast is wind resistance and not rolling resistance from the wheels/tyres, don't you?

How the hell do you think the belt/tyre/wheel friction is going to be even remotely near the quantity of thrust??? of course the thrust will be much more and of course it will move.

The helicopter is pulling forwards wrt the air, yes.

So you say yes.

OK, now mount the helicopter, no, 2 helicopters onto the wings in a pointing forwards way such that they are pulling forwards wrt the air. It wont be very aerodynamic for the wings, granted, but go with me here...

Will the plane still move forwards?

Then why is it stated in the original question that the plane and conveyor move with same speed?

That's the sensibilist thing you've said yet eurovol!

Now, given that a (very strong) person can pull a plane, I would estimate the rolling resistance to be around 500-1000 N. And a jet engine gives 200000 N of thrust. No contest. The end.

No is the helicopter MOVING forwards with respect to the air ? Fuck sake man that's what i asked.

Trying to imagine a physical impossibility is like trying to imagine the fourth dimension. If a plane is generating enough thrust to move forwards at 300mph then a belt moving backwards at 300 mph would be unable to keep the plane stationary or prevent it from moving forwards through the air and generating lift.

Think about it this way. The friction of the belt pushes the tyre backwards, but because the wheel spins around an axis the whole plane is not affected by this backwards force. The backwards motion of the conveyor belt has no (or negligible) affect on the forwards motion of the plane.

Its pulling forwards, so yes it is. To restate you, when the helicopter was pulling the plane with a towrope and I asked if the plane would move, you said "2. Of course". What has changed by merely making the towrope rigid?

was that an engineer bitch slap?

i still cant envisage the damm thing taking of, but i'm no grease monkey i guess. i did sit on the u-bahn and think about it though. and after my brain had melted i gave up... I guess this is why engineers are kept in dark windowless rooms at the back of the building away from customers. 'Cos as much as i try to imagine the "grease monkey" version the plane never reaches v1 let alone v2.

Go back to post #311 and look at the force diagram of the plane. Unless you want to add any more to that diagram (I'm open to suggestions) then the plane will move forwards. It's simple physics that you do at school, F=ma and all that.

Topic about physics - 20 pages.
Topic about porn - 3 posts.
Weird.

@ SP
You must have had a shit childhood to not be able to imagine a physical impossibility. I'm not gonna go in circles with you - you can re-read my posts and watch some Yank movies to help with your imagination.

No it's not you silly sod. Man pulls box , box's inertia plus frictional forces >> tension in pulling mechanism = no movement. Fer chrissakes you ask a question which I'll answer but you won't do the same.

@ sgb - same thing - in the question posed the engines are ALREADY at take-off thrust , NO forward motion is taking place , the conveyor belt is theoretically offsetting the engines. It's IMAGINATION.

fun discussing a completely theoreticaly odd mechanical question. 20 days
fun discussing porn. 3 minutes

simple.

fuck me, did someone get out of bed the wrong side this morning. The only thing going round in circles are the wheels of the plane. I have a very fertile imagination, thanks, but it is beyond even my powers to imagine you ever being able to understand why ypu're wrong. Why don't you go yank off to a star trek movie and re-read your own posts.

@ SP
Post #393 . QED. You lose.

Well if the helicopter is attatched to the plane with a tow rope or solid structure, it can only move forwards if the plane moves forwards too. You said the plane moved forwards in the original tow-rope premise, obviously implying that the heli is moving forwards as well.

Only when I changed the tow rope to a solid structure (lets call it a metal bar for a towrope) did you suddenly ask if the helicopter moves. The answer is, the helicopter is exerting a force, the same force as when it was pulling with the tow rope. The helicopter will only move if it is strong enough to pull the plane. Lets assume it is strong enough to pull the plane so the helicopter does move.

So.. Yes, the helicopter moves forward, does the plane move forward too? (This seems so redundant it hurts).

I dunno if I can be bothered dragging this one out after all, I said go with me but its just going on and on...

Yes, if you read the post I quoted, you would see that to prevent the plane moving the conveyor would need to accelerate at around 40g continuously, forever. So the plane remains stationary, the belt would be moving at 2000, 4000, 6000, 8000 mph etc increasing this quickly for ever. How on earth is that "matching the speed of the plane"?

Precisely the point - the helicopter needs to be moving , not simply exerting a force.

Imbeciles! All of you! Now do some work

F=ma dude. If an object it at rest (eg a plane at 5% throttle on a moving conveyor) and you apply an extra force to it (eg 95% increase in throttle) you get an acceleration. It's not rocket science.

I'm taking it that the resultant net force = 0 . In other words that the frictional forces cancel out the forces of the jets - I'm ACCEPTING that this is silly but I'm reading the orginal question to imply that the conveyor belt precisely cancels out any movement w.r.t. the air. For those with poor imaginations - the fucking conveyor belt is as big as a runway - so you all should start saying "uh fuck off DW , let me stop you right there , cos our imaginations can't stretch that far."

OK, well at least we agree on the physics then.

EDIT: And I think the plane would take off, even if the conveyor went that fast. Because after about 2 seconds the wheels of the plane would explode, the plane would fall onto the conveyor (which is now spinning backwards at 10000s mph) and the plane would be catapulted backwards, surely taking off a bit in the process


It doesn't say anything about cancelling out though, it says the conveyor moves at the same speed as the plane, it is not the same thing. If the plane goes forward at 10mph, the conveyor goes back at 10mph, all that does is make the wheels spin faster, the plane still can go forwards.

This pantomime may last 20 pages, but it also only has three unique posts:

1) Ohhh, yes it will!
2) Ohhh, no it won't!
3) Boo, hiss!

I'm off to work. Have fun!