Airplane and Conveyor Belt Debate

[AJerman]

Figured I'd just copy and paste over to here the explanation I gave over at Neowin. I actually voted no at first, then realized quickly my mistake. Then I brought it down to my father who is an aircraft mechanic, and my grandfather who loves planes and asked them. I actually had to spend about 30 minutes explaining to them how it would still take off before they understood what I was saying. It's a fun one to throw out there though.

Wow, I have to say that this is a very good one, more of a good one in the way that it causes great conversation. I initially voted no, but I want that vote back, I was wrong. It will and it won't take off. I didn't read every post before this, so I'll just post this to explain what would happen rather than reply to anyone.

So first off, it will take off because an airplane get's it's forward motion, it's thrust, from it's jet engines pushing air. No matter what the wheels on the ground do, a jet engine at full throttle CAN NOT sit still without being strapped to the ground or having some sort of friction greater than the power it is providing (which is quite a bit and wheels cannot cause anywhere near enough friction to do this). It's going to push the air and move forward, and if the ground is moving the same speed as the plane then the wheels will end up spinning at double the speed, but the plane will continue to move forward with the same thrust as it would no matter what the runway is doing. That's simple enough if you know a little physics to figure out. If the wheels are providing no power to be turned into forward motion, then the runway moving cannot change the amount of thrust you are going to have.

The problem is that we, in general, think of thrust as something that comes from what is making contact with the ground. Our feet pushing against the ground make us go forward. The wheels on a car spinning against a stationary ground make a car go forward. However, on a plane, the wheels that are on the ground don't make the plane go forward, the jet engines sucking in air, compressing it, mixing it with fuel, igniting it, and shooting it out of the back, produce the thrust and forward motion of a plane. Therefore the wheels have nothing to do with moving forward, and what the ground is doing has nothing to do with it either.

Also, the plane will take off in roughly the same distance as it would on a stationary runway. With the wheels spinning twice as fast, there will be a small increase in friction, however this wouldn't cause much difference in the long run.

But now seems like a good time to say why it likely WON'T take off. We're going to use 180 mph as the take off speed for our jet (which is similar to that of a 777). This means that the wheels will be going 360 mph at take off. At this speed the wheels, bearings, and tires are unlikely to hold up and will end up grounding the plane. But hey, our question is theoretical, so let's ignore these issues, or assume our plane in question has been reinforced to withstand any of the above factors. Besides, where are you going to get a conveyer belt this big anyway?

 

[chroot]

The question has already been proposed here, and the conclusion is that YES, the plane will take off, despite the very imprecise wording of the question. It's already been beaten to death on this forum.

https://www.physicsforums.com/showthread.php?t=101259

- Warren

 

[vincentm]

airplane question

This question has caused quite a ruckus on the other forum i post on, so i decided to post it here since a lot more members here are more competent in there physics and math.

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

 

[Doc Al]

This has been discussed to death here as well: https://www.physicsforums.com/showthread.php?t=105314

 

[HallsofIvy]

Airplane lift is due to the motion of the wings relative to the air (that's why airplanes prefer to take off and land into the wind). If the wings are unmoving relative to the air, the airplane will not "take off" no matter what the speed relative to the ground is.

 

[maverick6664]

airplane question: revisited

As https://www.physicsforums.com/showthread.php?t=105342 is locked, I post here again.

As I have a pilot's license, this is interesting.

The answer is queer; because it's impossible to make such a conveyer. So the answer is "cannot be determined." Airplane's engines are made so the airplance accelerates "relative to the air", not to the ground. See the propellers or jet engines blow winds backwards. So such a conveyer cannot "stop" the airplane assuming we ignore the friction of wheels of the airplane. That airplane will accelerate as usual relative to the wind (so called "airspeed") regardless of the conveyer, and will finally take-off.

But this will be against the intention of the conveyer, because it's supposed to "stop" the airplane. The answer is "this condition is impossible, so doesn't have an answer."

Yes, an airplane's lift is produced by the speed to relative wind to the airplane. So if the advancement of the airplance is stopped in other ways than that conveyer, it cannot take-off, but in this experiment, the airplane will advance relative to wind regardless of the conveyer.

 

[Danger]

There's a very good reason why that thread, and others on the same subject, are locked. We're all pretty much sick of the topic by now. The plane will take off, and there isn't too much point in beating it around any more.

 

[russ_watters]

Yes, the only thing that can prevent the plane from taking off is bad wording. So what's left to discuss...?

 

[raid517]

Airplane Question

Someone sent this to me, and apparently it caused quite a ruckus on another forum. I couldn't make any sense out of their responses, so I decided to put it to the great minds on this forum. I didn't find it anywhere, so sorry if this is already on here. The Question:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Please post a reason.

I am really looking for a conclusive answer. My instinct is to say no, but I would like this confirmed by some hard physics and maths if possible.

GJ

 

[fargoth]

ok, so you got an airplane that doesn't move... what's your point?
i didnt get what reason you want...
EDIT: (im wrong here )

 

[Homer Simpson]

There was some thread on this I saw a couple weeks back. Basically, it the wheels will turn however fast they want. As soon as the jets fire up the plane will move relative to the stationary ground. Any treadmill beneath it will just make the wheels spin faster. This would only be a problem if the wheels were motorized to make the plane move, but that's not the case.

On the other hand, If the treadmill was running at the same speed in the same direction as the jet as it fires up, the jet would take off with no movement of the wheels.

You don't need math or physics for this type of stuff, just think about it.

 

[Danger]

I seriously think that if this question shows up one more time, I'll scream.

 

[raid517]

Well this is true and this is what I thought. But I thought that this was a good place to shoot it in the head for once and for all.

Everything I know of says that it's a stupid question and of course the aeroplane wouldn't take off - but when you have 50 different guys challenging your assumptions, you can begin to doubt yourself.

GJ

 

[Danger]

Take a look at the other threads on the same subject, which mercifully have been locked. The plane will take off.

 

[fargoth]

oh yeah, i forgot about the fact that the wheels arent motorized, if the jets are what causing the movement the airplane would move... though i think that if the wheels spin too fast there would be lots of heat generation and the plane would move slower. (but if the wheels are ideal, it just means they would move twice as fast)

 

[raid517]

Point me to these other threads please?

GJ

 

[Danger]

Look at 'Classical Physics' in the post entitled 'Airplane question: revisited'. There's a link in the OP to another.

 

[deadmoon]

plane

hi
if you had a plane on a treadmill would it take off?
the treadmill is moving in the oppsite way of the plane at the same speed

thanks

 

[Danger]

Welcome, deadmoon. Unfortunately, you introduced yourself with a question that has absolutely been done to death here. If you check back a few pages, you'll see a very thorough examination of the subject. Bottom line: yes.

 

[brewnog]

KILL IT! LOCK IT! QUICK! KILL IT! KILL IT TIL IT DIES FROM IT!

Argh!

 

[russ_watters]

The answer is yes, unless the question is so poorly worded as to be meaningless. See this thread: https://www.physicsforums.com/showthread.php?t=101259&highlight=treadmill

 

[Doc Al]

And this: https://www.physicsforums.com/showthread.php?t=108477

 

[R1ch]

Plane on a moving runway

Hi all I've just signed up to the forum.
This question seems to have got over the net a bit so sorry if its a re-post

A plane is sat on a runway with some sort of conveyer belt, when the plane moves the conveyer moves in the opposite direction matching the planes speed exactly. Can the plane take off?

 

[russ_watters]

Sorry, we've discussed this ad nauseum and won't go over it again. Do a search of the forum for the topic.

In short, though, the problem is so poorly worded as to not have a definite answer. Regardless, an airplane takes off when the airflow over the wings is fast enough to create enough lift to make the plane fly. That's it. It doesn't matter what the wheels are doing at the time (the plane can even be completely stationary if it has a strong enough headwind).

 

[Kaldarin]

Will the plane fly off ...?

If an airplane is placed on top of a conveyer belt which turns in the oposite direction of the plane with the exact speed as the plane, will it ever take off?

 

[Pengwuino]

This has been discussed over and over and over and over and over and over and the answer is basically yes with the qualification that the question is worded too poorly to be worth anything. Please search the general physics area.

 

[ZapperZ]

I'm almost going to insist a 1-year moratorium on this question.

https://www.physicsforums.com/showthread.php?t=101259

Zz.

 

[kmarinas86]

Conveyor and Plane

Sorry, this question is too fun too resist.

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

Well, to understand how the system works, let's change the settings first:

Instead going 100% of the speed in the opposite direction, have the conveyor belt go 700 knots in the opposite direction. The implications of this are obvious now since the air would be flowing backwards on the wings not forward, therefore the plane will be affected by this movement.

Another thought experiment. Put a Honda civic on a steel treadmill. If it has a spoiler, is the spoiler being used? It isn't. The only wind being generated is from the exhaust and the contact of the air with the moving surfaces of the vehichle and the steel treadmill. The body as a whole is not generating a lift or downforce as if it were traveling fast on the road.

 

[Gokul43201]

This exact question has been discussed in an earlier thread in Physics.

Read the discussion there :

https://www.physicsforums.com/showthread.php?t=101259

 

[russ_watters]

...and will not be discussed again.

The question isn't fun, it's dumb. The only reason there is any doubt as to the answer is because the question is poorly worded and unrealistic (how can the conveyor exert a force on the plane?).

 

[MrsScruffles]

Throwing Myself At Your Scientific Mercy

I'll hope you'll all excuse the intrusion, but I've fled here from another forum in the hope that you will be able to settle a debate that has been raging for far too long. The original question that has causes such furore is this:

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not?

I did try searching your forums, and saw a similar question with a car, but not a plane. I'm sat in the "no" branch, because I believe that there are no aerodynamic forces at work because essentially the airplane is stationary, and only the wheels are in motion, so you wouldn't get lift. However, the oppposing argument seems to be that the force from the jet engines would cause lift off regardless. I don't think any of us have a good enough grasp of physics to ever formally end this debate. Our formally peaceful forum has turned into something out of Gullivers travels, but instead of everyone divided on which way to eat a boiled egg we're going crazy over a hypothetical plane.

Twenty two pages later and the argument is still going.

So can someone kill this question dead for me?

 

[Pengwuino]

What forum is this?! We've received this question like 20 times!

 

[Mk]

Yeah. What Forum Is It! Do We Have To Wipe Their A** For Them?? I think we need to do a DNS attack on them. And SPAM, yes Spam...

 

[MrsScruffles]

Sorry, as I said, I did do a search and couldn't see it. The question I found was about cars. And as far as I know, I think it's quite viral, and been spread from place to place. Sorry for wasting your time.

 

[fasterthanjoao]

the plane isn't actually moving wrt anything other than the belt so there's no lift from air? surely.. hmm..