Plane on a conveyor belt

[eddie_van_earache]

my gawd my head hurts.. can someone post a nekkid chick or something.... eeesh...

[dg]

Ralph, I've been looking for the best wording of the problem I could find, and I think this is it:

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

The question leads many people to the quick assumption that the conveyor will be able to keep the plane in place. This is the whole point of the argument. The other side (the one I'm on, along with George, RobRR, Zeegs & others) argues that a conveyor can't keep the plane from moving forward, because it's wheels aren't what's propelling it. The prop or jet engine is propelling it, and the wheels are just free-spinning supports that roll across the conveyor. We believe that the plane is going to be able to accelerate forward no matter how hard the conveyor tries to keep up. Here's the best analogy I've seen:

Imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily. Your arms pulling on the rope are the same as the propeller or jet engine.

No one is arguing that the plane can lift off if it stays in place on the conveyor (at least I hope not). We are arguing that the conveyor can't keep the plane in place.

If the question were worded like this:

"Assuming a plane is kept in place on a conveyor belt as it tries to move forward, can it take off?"

it would be very easy to say no, but that's not how the question is worded.

[Ralph E.]

Ok, so the thrust will overcome the wheel speed, which will in turn create airflow and lift at some point...

Interesting. I'll be watching tonite for sure.

[fuel0707]

If the question were worded like this:

"Assuming a plane is kept in place on a conveyor belt as it tries to move forward, can it take off?"

it would be very easy to say no, but that's not how the question is worded.

Actually, a Harrier can takeoff without a ground roll. And depending on one's definition of an airplane, the V-22 is another example of a fixed-wing aircraft that can takeoff without a ground roll.

[rjohnstone]

Imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily. Your arms pulling on the rope are the same as the propeller or jet engine.

Bad analogy.
The rope being attached to the wall is not the same as trying to propel yourself forward.
You just introduced another mechanical component into the equation.

Try this instead.
Get on a treadmill with roller blades on with a high powered fan strapped to your back. See if you can move forward.

I don't if you will or won't, but that would be about the closest way to simulate the equation.

[Evilution]

For people who still say the plane will take off.

Why are aircraft carriers so bloody huge, why don't they just have a conveyor on a short deck?
If that worked they could mount a second prop above the original one but blowing the other direction to match the first, the plane wouldn't move but it would be flying, infact it would be hovering. Obviously that wouldn't happen and the plane would fall from the sky.

[Bengal]

Now this is making me dizzy...

Those that believe that the plane will fly do so because they believe the belt CAN'T keep the plane still. That's the arguement. Not if it will fly standing still. Everyone knows that it can't. What is being debated is if the belt can hold the plane still or not. That is the question. Can the moving belt keep the plane stationary...

It took me a while to figure that out as well. Sounded perfectly clear to me that it won't fly. I'm assuming that the belt will keep the plane still. That is the question, can the belt hold the plane in place. That's the question. It gets all mucked up when they bring in the word "fly" into it. Flying really has nothing to do with it. The real question is can the belt keep the plane stationary...

Of course those that think it will fly believe the belt cannot hold the plane stationary. Those that think it won't assume the belt will keep the plane stationary. There is the debate. It has nothing to do with flying, the flying is a by-product of the exercise...

I still think we will have no difinitive answer after the program aires in 4 hours....

[Newc]

The original question still remains "Can an airplane on a conveyor belt achieve lift?"

As it makes its way around the internet it gets revised and expanded and even reworded completely to lean towards the answers of each camp.

[dg]

Bad analogy.
The rope being attached to the wall is not the same as trying to propel yourself forward.
You just introduced another mechanical component into the equation.

Try this instead.
Get on a treadmill with roller blades on with a high powered fan strapped to your back. See if you can move forward.

I don't if you will or won't, but that would be about the closest way to simulate the equation.

Both analogies show that the plane's thrust is independent of the wheels, and all the wheels are there for is to reduce friction. That's all I'm trying to get across. As long as your fan is good & strong, that's cool w/ me!

[fuel0707]

Why are aircraft carriers so bloody huge, why don't they just have a conveyor on a short deck?

Since about the beginning of the jet age, aircraft carriers have had catapults. That's because jets cannot reach flying speed in the relatively short length of an aircraft carrier's deck. If you look at the layout of a carrier, you'll see that a good portion of the deck space is allocated for aircraft recovery.

Check out this description of aircraft carriers: aircraft carrier link. There is a little section on the catapults and it mentions that modern fighters need to reach upwards of 150 mph in about 300 feet. If it could be done more efficiently with a conveyor belt, I'm betting that modern aircraft carriers would be so equipped.

[SeventhSon]

For people who still say the plane will take off.

Why are aircraft carriers so bloody huge, why don't they just have a conveyor on a short deck?
If that worked they could mount a second prop above the original one but blowing the other direction to match the first, the plane wouldn't move but it would be flying, infact it would be hovering. Obviously that wouldn't happen and the plane would fall from the sky.

saying the plane will take off on the conveyor belt does NOT imply it needs a shorter runway.

the plane will need the same amount of runway and the same amount of speed.

what we are saying is that the conveyor belt moving at a speed equal to but opposite of the plane's take off speed simply does not do anything to hold the plane back. in other words, it does NOT have the net effect of reducing the plane's forward motion to 0. in other words, it's a red herring.

not saying i'm any kind of genius...in the last thread i was one of the last man standing for saying the plane wouldn't fly. i believe LouSiffer finally convinced everyone in that thread.

[SeventhSon]

The original question still remains "Can an airplane on a conveyor belt achieve lift?"

As it makes its way around the internet it gets revised and expanded and even reworded completely to lean towards the answers of each camp.

the original question has always been if you match the backwards speed of the conveyor belt with the forward speed of the plane will you prevent the plane from taking off. the answer to that has always been NO.

the question is sometimes modified to ask: is it possible to move the conveyor belt with a backwards speed sufficient to stop the plane from taking off. the answer to that is, YES, but only theoretically possible.

the first question is more interesting because most people intuitively assume that the net forward speed of the plane in that scenario will be 0, but that is not the case.

[Ralph E.]

The speed of the plane isn't zero, but the lift which is needed to take off is not generated IMO. If I understand the whole thing, theoretically the plane will never "move" from it's starting spot, but rather accelerate thru the propeller while the belt offsets forward movement by going the other way. If this is indeed the principles, I don't see how lift can be achieved.

For example, if I drive my car at 40 mph and I stick my hand out the window, I would get alot of wind buffering my hand. Now if I was driving my car on a belt at 40mph but not moving anywhere, my hand would not meet any resistance when I stuck it out the window. No wind, no lift IMO.

p.s. my hair hurts from thinking about this so much today

[mm2002]

For example, if I drive my car at 40 mph and I stick my hand out the window, I would get alot of wind buffering my hand. Now if I was driving my car on a belt at 40mph but not moving anywhere, my hand would not meet any resistance when I stuck it out the window. No wind, no lift IMO.

p.s. my hair hurts from thinking about this so much today

Bingo! Good analogy.

[dg]

What we're saying is that the conveyor won't act on the plane in the same way that it acts on the car in your example. The car uses its wheels for propulsion. The plane gets its thrust from the propeller or the jet engine, and the wheels are simply friction-reducing supports that will easily move across the conveyor even if you speed it way up. This is the key reason why the plane can overcome the conveyor belt. It should only take just a litle bit more throttle than usual to overcome a little bit of extra friction.

-edit- and I'll add again that no one is arguing that a plane kept in place can achieve lift (conventional plane, no wind-tunnel, etc., etc.). We're saying that a conveyor can't stop a plane from moving forward and achieving lift in its normal way.