[Frost]

This is actually a simple question, with one correct answer (as long as you don't read more into the question than is actually there). However, the sheer, enormous number of people who get it wrong (>50%) is baffling. So, what do you think is the correct answer?

[Tesseract]

Which part of the wheels is the conveyor belt moving in the opposite direction to? Top or bottom?

[Quicksilver]

No--the plane would be stationary with respect to the ground. If the plane can't get sufficient airflow moving around its wings, then it can't take off.

[Tesseract]

Only if the conveyor belt is moving in the opposite direction to the top of the wheels.

[Blackshawk]

If this were a car, then the assumption that it would hold in place is absolutely correct. But an airplanes wheels are more just to keep the plane balanced and reduce as much friction as possible – what really makes a plane fly is air moving over its wings, not how fast its wheels are moving on the runway.

[MILBOP DIPPLEBERRY]

I got this question wrong a long time ago because I didn't know anything about airplane wheels.

[Tesseract]

Blackshawk, on September 8th 2007, 01:49 PM, said:

what really makes a plane fly is air moving over its wings, not how fast its wheels are moving on the runway.

Yes. But if the plane is stationary relative to the ground, then it's stationary relative to the air as well (ignoring wind, but a stiff breeze isn't enough to make a plane take off when it's sitting on the ground).

[teflon]

noone said we couldnt have gale force winds now did they??

i reckon this would be doable in a small plane, not a jumbo cos of the amount of air, but a small one could just about manage it in a messed up attempt at take off...

so whos up for doing this experiment for real??

[Lemon Lime]

I know not many of the people on these boards may actually work out, but when you are running on the treadmill, are you getting wind in your face? no. and without air there is no lift, no lift, no flying.

[MILBOP DIPPLEBERRY]

Also, if this were possible, we would probably have high-tech conveyor runways at airports to save space.

[Tesseract]

For those of you following along at home, my initial request for clarification was a red herring. If the conveyor belt is defined to be moving in the opposite direction to the bottom of the wheels, then stationary is the only possible state for the system to be in. The proof of this is left as an exercise for the reader.

[Janichsan]

They plane would fly. I'll just rip-off the explanation of someone else,because I couldn't phrase it better:

Quote

But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

[Huntn]

Well, this question did make me think and I'll probably still get it wrong!

If you look at a car on a treadmill, it could be registering 100 mph, the wheels are turning at 100 mph, but it's stationary, so there is no wind moving over that car. To make things simpler lets say the wind is calm. Aircraft require wind over the wings to generate lift so as power is added, thrust would normally push the aircraft along the ground, through the air mass, which it does by means of it's wheels. (Ever hear the joke about why it's taking 100% power to taxi after you land? Cause you forgot to drop your landing gear for landing...) So in this case I imagine as the aircraft starts to move forward based on it's wheels rotating, the conveyor kicks in countering the wheels, and prevents the aircraft from accelerating into the airmass because the treadmill moves in the opposite direction. What caused me to ponder is that with an aircraft, thrust is generated by force out the back of the engine, not powered wheels. But I still have to believe that the aircraft would be standing still similar to an aircraft that has the brakes set. You can run the power up to 100% on most aircraft with the brakes set they are not going anywhere.

It might be easier to picture this situation by adding just enough power to taxi the plane at 5 mph and your sitting next to the conveyor on stationary ground, in a lounge chair observing. In this case if the conveyor was moving at 5 mph in the opposite direction, the plane would be stationary. The wheels are rotating at 5 mph, the conveyor is running at 5 mph, no movement. Ok lets bump up the power so the aircraft would taxi at 10 mph. Now the conveyor increases to 10 mph, still no movement. If you take this example to full takeoff power, it seems to me that the aircraft would still be sitting there. One final example. Lets say you're God and your holding this airplane in the sky stationary, but it's engines are at 100%. Then you let it go, initially would it fly? No, it would fall possibly tumbling, but by virtue of accelerating through the air mass, eventually there would be enough wind over the wings to allow it to fly normally. What does this have to do with this specific example, err mostly likely nothing. I just like fantasizing about being God.

A related question would be, lets say your aircraft is sitting in a giant wind tunnel as long as a runway and as the airplane adds power and moves forward the wind increases the same amount as the airplane's speed but in the opposite direction of aircraft movement. Would it move? Would it be able to take off? And would you notice anything unusual about this aircraft in relationship to the ground?

Answer:

Spoiler

The aircraft would move down the runway because the aircraft's thrust is much greater than the wind, although it would be a short takeoff roll. And once airborne it would hover in place.

[Huntn]

Janichsan, on September 8th 2007, 06:52 AM, said:

They plane would fly. I'll just rip-off the explanation of someone else,because I couldn't phrase it better:

Quote

But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

Where did this quote come from? I like my 5 mph example. I don't think the aircraft would be moving.

[Bernie]

Regardless of whether the plane can take of or not, it can still have a very minor accident causing the passengers to sue for huge amounts of money.

[Lemon Lime]

Bernie, on September 8th 2007, 06:38 AM, said:

Regardless of whether the plane can take of or not, it can still have a very minor accident causing the passengers to sue for huge amounts of money.

sadly this is true...

[Frost]

Just a clarification: The wording in one part is actually a bit poor (but I replicated the question exactly as posed), that being, "The conveyer belt is designed to exactly match the speed of the wheels." The best way to address it would be to say there are potentially two ways to read it: the right way, and a way that ends in a physics paradox. If you end up at a physics paradox, that's not the right meaning.

And Janichsan, get off Google! Un-IDDQD.

[bobbob]

So when you're on a treadmill, you don't have wind going over you... because you're inside a gym.

I think it would be like a US aircraft carrier. Those crafty Yanks didn't think to add a ski jump or use only short/vertical take-off aircraft like the Brits, so they have to drive the carrier into the wind to get enough lift to take off. Given enough windspeed the planes would take off, whether the carrier was actually moving forward or not, since the speed on the ground is obviously and absolutely irrelevant. They're not inside a gym, after all.

Quote

as the airplane adds power and moves forward the wind increases the same amount as the airplane's speed but in the opposite direction of aircraft movement

Do people not know how planes fly? Do they hold a kite and keep running with the wind, hoping it takes off this time?

[Janichsan]

Quote

And Janichsan, get off Google! Un-IDDQD.

I had this discussion earlier in another forum - we came basically to the same answer as I posted here.

Frost, on September 8th 2007, 07:28 PM, said:

Just a clarification: The wording in one part is actually a bit poor (but I replicated the question exactly as posed), that being, "The conveyer belt is designed to exactly match the speed of the wheels."

That's basically the important part: a plane does not get driven forward by its wheels - unlike a car. So the speed of the wheels is irrelevant.

[Tesseract]

But consider the following. When a plane is on the ground, the speed of the bottom of the wheels relative to the ground is 0, assuming no slipping. The speed of the top of the wheels is twice the speed of the plane, and the speed of the middle of the wheel is the same as that of the plane as a whole. But the ground in this case is a conveyor belt, which is moving backwards at either 1 or 2 times the speed of the plane (depending if it is made to match the top or middle of the wheels). So the plane does indeed move relative to the belt, but the belt is moving backwards, meaning that the plane either stands still or goes backwards. This is an unavoidable consequence of the fact that the speed of the plane and the speed of the wheels are tightly linked while it is on the ground.

Obviously if you actually tried this, the belt motor would quickly become unable to keep up, but in the question it is simply defined to do it.