[kc311v2]

I read this off another forum and I can't find the link, but the question went like this:

Imagine an airplane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the airplane take off?

[Dakar²]

I'd think not, airplanes use the lift of the wind under the wings to take-off. If they aren't actually moving forward they wouldn't achieve that.

Or did I miss something?

[Severed Hand of Skywalker]

I wouldn't think so as you need air traveling over the wings to take off not just thrust.

[centerchannel68]

No, it wouldn't take off. Imagine the same scenario on a regular runway with a 300mph tailwind. It wouldn't generate lift.

[Dakar²]

This wasn't much of a brain tickler.

[TETENAL]

Do airplanes have engines attached to their wheels like cars that push them forward on the runway? If so, then the airplane would stay in place and could not take off.

Or do they have propellers or jet engines that push the airplane forwards against the surrounding air? Then it wouldn't matter what happens to the wheels and the plane would take off regardless.

[besson3c]

Here's a variation of this brain tickler...

If this plane was able to go twice as fast on the ground as your average airplane but didn't have any wings, could it take off?

[voodoo]

Originally Posted by kc311v2 

I read this off another forum and I can't find the link, but the question went like this:

Imagine an airplane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the airplane take off?

Yes it would take off, it would need a longer runway and possibly higher rating motors than on a normal runway, but it would take off without much problem.

Why? Imagine you're pushing a model airplane on a conveyor belt with the same properties as above. You could move the plane forward, even if the belt matches the speed of the wheels of the plane.

Airplanes aren't powered through the wheels, so this would just add more friction, but the plane would take off.

V

[Severed Hand of Skywalker]

Originally Posted by TETENAL 

Do airplanes have engines attached to their wheels like cars that push them forward on the runway? If so, then the airplane would stay in place and could not take off.

I know on smaller planes they do not have motors and I think it is the same for jets as they sill rev the motors to move or are even taxied by those trucks.

[Dakar²]

Originally Posted by voodoo 

Yes it would take off, it would need a longer runway and possibly higher rating motors than on a normal runway, but it would take off without much problem.

Why? Imagine you're pushing a model airplane on a conveyor belt with the same properties as above. You could move the plane forward, even if the belt matches the speed of the wheels of the plane.

Airplanes aren't powered through the wheels, so this would just add more friction, but the plane would take off.

V

If the engines give thrust and the airplane moves forward through the traction the tires have with the pavement, if you remove the traction by moving the runway back at an equal rate, wouldn't that negate the forward movement and therefore and wind required for lift?

[Calimus]

Originally Posted by Dakar² 

If the engines give thrust and the airplane moves forward through the traction the tires have with the pavement, if you remove the traction by moving the runway back at an equal rate, wouldn't that negate the forward movement and therefore and wind required for lift?

A plane needs no traction on the ground to take off. How would a plan on skis, or a sea plane with pontoons take off? If anything the wheels make it harder for the plane to move forward.

[Gossamer]

Originally Posted by Dakar² 

If the engines give thrust and the airplane moves forward through the traction the tires have with the pavement, if you remove the traction by moving the runway back at an equal rate, wouldn't that negate the forward movement and therefore and wind required for lift?

When the question says the conveyer belt matches the 'speed' of the wheels, it needs to be clarified: linear speed or rotational speed.
I think the plane would take off. If the conveyer belt constantly matched the linear speed of the wheels, the speed of the conveyer belt would initially be zero when the plane is at rest. When the thrust from the jets causes the plane to move, the conveyer belt would adjust to keep the speed of the wheels at zero. So the plane would still be accelerating with respect to the airport, but not the runway.

[Dakar²]

Originally Posted by Calimus 

A plane needs no traction on the ground to take off. How would a plan on skis, or a sea plane with pontoons take off? If anything the wheels make it harder for the plane to move forward.

Hmmm...

My basic point is, no forward progress equals no wind equals no lift.

[missingbite]

If there is no friction for the wheels to move, the airplane will not takeoff. I have about 10,000 hours flying jet a/c. Trust me, it won't go anywhere until there is enough friction from the ground for the wheels to move. If when you apply thrust the a/c tries to move forward but can't b/c the wheels move forward while the belt moves backwards at the exact speed, the a/c can't get wind to flow over the wings which is what creates lift.

[Gossamer]

Originally Posted by Dakar² 

Hmmm...

My basic point is, no forward progress equals no wind equals no lift.

Originally Posted by missingbite 

If there is no friction for the wheels to move, the airplane will not takeoff. I have about 10,000 hours flying jet a/c. Trust me, it won't go anywhere until there is enough friction from the ground for the wheels to move. If when you apply thrust the a/c tries to move forward but can't b/c the wheels move forward while the belt moves backwards at the exact speed, the a/c can't get wind to flow over the wings which is what creates lift.

But you're assuming the conveyer belt undoes what the plane is trying to do. If the conveyer belt matches the linear speed of the wheels and maintains it at zero, it would actually be easier to take off because there would be no friction.

[Dakar²]

Originally Posted by Gossamer 

linear speed or rotational speed

[::maroma::]

Can these really be considered brain ticklers? They seem more like engineering test questions to me. My brain isn't giggling, its annoyed.

[Dakar²]

Yeah, its beginning to look like it's either so straightforward its dumb, or technical so its pointless for a layman.

[BRussell]

I don't know, but I bet it could if it was one of these planes.

[Dakar²]

Imagine if that flew through a cloud made of cheese directly above a giant plate of spaghetti.

[Calimus]

The key is whether or not the axle of the wheel moves forward. I think we can all agree that the axle of the wheel must move forward(with respect to stationary objects around it). If this were a magical non real-world situation where you could move the treadmill at say... 300mph backwards, friction from the wheels pushing the plan backwards was enough to counteract the engines pushing on the air, then it would not take off. In essence the treadmill would act like a super strong guy pushing against the landing gear holding the plane in place.

The problem in reality is that I think the amount of friction required on the wheels to counteract the engines would probably cause so much heat as to destroy the wheels and foil the experiment.

So to conclude, theoretically, in some computer simulation, the plane would not take off. In the real world, you couldn't make it work.

[Gossamer]

Originally Posted by Dakar² 

Linear speed is the velocity of the center of gravity of the tire. So if a car is moving at 60mph the linear speed of each of its tires is 60mph. Rotational speed is the speed that it's rotating. So if a car is moving at 60mph and the radius of the tire is 10 inches, the rotational (angular) speed is 176 rotations per second, or 10,560 rpm (if my math isn't flawed).

[Gossamer]

Originally Posted by Calimus 

The key is whether or not the axle of the wheel moves forward. I think we can all agree that the axle of the wheel must move forward(with respect to stationary objects around it). If this were a magical non real-world situation where you could move the treadmill at say... 300mph backwards, friction from the wheels pushing the plan backwards was enough to counteract the engines pushing on the air, then it would not take off. In essence the treadmill would act like a super strong guy pushing against the landing gear holding the plane in place.

The problem in reality is that I think the amount of friction required on the wheels to counteract the engines would probably cause so much heat as to destroy the wheels and foil the experiment.

So to conclude, theoretically, in some computer simulation, the plane would not take off. In the real world, you couldn't make it work.

If the wheels were capable of turning fast enough to account for a 300mph treadmill it would be okay.

[Dakar²]

Originally Posted by Gossamer 

Linear speed is the velocity of the center of gravity of the tire. So if a car is moving at 60mph the linear speed of each of its tires is 60mph. Rotational speed is the speed that it's rotating. So if a car is moving at 60mph and the radius of the tire is 10 inches, the rotational (angular) speed is 176 rotations per second, or 10,560 rpm (if my math isn't flawed).

...do they somehow act independent of each other?

[Dakar²]

Sky Captain spotting in the PL, hopefully he'll weigh in here.

[missingbite]

Originally Posted by Gossamer 

Linear speed is the velocity of the center of gravity of the tire. So if a car is moving at 60mph the linear speed of each of its tires is 60mph. Rotational speed is the speed that it's rotating. So if a car is moving at 60mph and the radius of the tire is 10 inches, the rotational (angular) speed is 176 rotations per second, or 10,560 rpm (if my math isn't flawed).

Agreed, but put a freestanding wheel with an axel through it on a treadmill. Turn the treadmill on at 5 MPH, and the wheel will go nowhere. Speed the wheel up through an outside force and IF the treadmill speed meets the wheel speed, the wheel will not move forward. If the wheel will not move forward, the a/c can't move forward to produce airflow over the wing to produce lift.

[missingbite]

Originally Posted by Gossamer 

Linear speed is the velocity of the center of gravity of the tire. So if a car is moving at 60mph the linear speed of each of its tires is 60mph. Rotational speed is the speed that it's rotating. So if a car is moving at 60mph and the radius of the tire is 10 inches, the rotational (angular) speed is 176 rotations per second, or 10,560 rpm (if my math isn't flawed).

Originally Posted by Calimus 

The key is whether or not the axle of the wheel moves forward. I think we can all agree that the axle of the wheel must move forward(with respect to stationary objects around it). If this were a magical non real-world situation where you could move the treadmill at say... 300mph backwards, friction from the wheels pushing the plan backwards was enough to counteract the engines pushing on the air, then it would not take off. In essence the treadmill would act like a super strong guy pushing against the landing gear holding the plane in place.

The problem in reality is that I think the amount of friction required on the wheels to counteract the engines would probably cause so much heat as to destroy the wheels and foil the experiment.

So to conclude, theoretically, in some computer simulation, the plane would not take off. In the real world, you couldn't make it work.

Exactly

[centerchannel68]

Originally Posted by TETENAL 

Do airplanes have engines attached to their wheels like cars that push them forward on the runway? If so, then the airplane would stay in place and could not take off.

Or do they have propellers or jet engines that push the airplane forwards against the surrounding air? Then it wouldn't matter what happens to the wheels and the plane would take off regardless.

No plane drives the wheels with gearing like a car. They just use the engine's thrust to move it forward. The lack of aviation knowledge in this place is kinda suprising.

[Gossamer]

Originally Posted by Dakar² 

...do they somehow act independent of each other?

It depends on friction. Nail your brakes on ice at 60mph. Your wheel's linear speed is still 60mph even though it's not rotating. Or floor it from a stand still. Your rotational speed can be 10000rpm while linear speed is zero.

Originally Posted by missingbite 

Agreed, but put a freestanding wheel with an axel through it on a treadmill. Turn the treadmill on at 5 MPH, and the wheel will go nowhere. Speed the wheel up through an outside force and IF the treadmill speed meets the wheel speed, the wheel will not move forward. If the wheel will not move forward, the a/c can't move forward to produce airflow over the wing to produce lift.

But what if the treadmill is used to maintain the wheel's rotational speed at zero, but does nothing to affect the linear speed?

[d4nth3m4n]

i've never seen this thread hit a forums and be less than 10 pages.

short story- it'll take off.

long story- who cares how fast the wheels spin, the engines don't act on the wheels directly and wheel bearing friction is negligible in terms of keeping the plane from accelerating. think about pushing a rollerskate forward on a treadmill.

[Dakar²]

Originally Posted by Gossamer 

It depends on friction. Nail your brakes on ice at 60mph. Your wheel's linear speed is still 60mph even though it's not rotating. Or floor it from a stand still. Your rotational speed can be 10000rpm while linear speed is zero.

Ok. SO I guess the next question is, are we assuming this fantasy treadmill isn't perfect in keeping those two synched?

[Gossamer]

Originally Posted by d4nth3m4n 

i've never seen this thread hit a forums and be less than 10 pages.

short story- it'll take off.

long story- who cares how fast the wheels spin, the engines don't act on the wheels directly and wheel bearing friction is negligible in terms of keeping the plane from accelerating. think about pushing a rollerskate forward on a treadmill.

But what if the treadmill keeps the linear speed at zero by matching the rotational speed?

[centerchannel68]

Originally Posted by voodoo 

Yes it would take off, it would need a longer runway and possibly higher rating motors than on a normal runway, but it would take off without much problem.

Why? Imagine you're pushing a model airplane on a conveyor belt with the same properties as above. You could move the plane forward, even if the belt matches the speed of the wheels of the plane.

Airplanes aren't powered through the wheels, so this would just add more friction, but the plane would take off.

V

Dude, you don't know what your talking about. Groundspeed doesn't matter in an airplane..it's all about airspeed. So hypothetiically, if you put a huge FAN in front of an airplane that blew a lot of air at 300mph, the plane could take off standing 'still'. I've seen an ultralight actually take off backwards into a stiff headwind.... ie, it was facing into the wind, and the little engine it had could not move the plane forward, but the airspeed was such that the plane went into the air, and when the pilot pulled back the throttle, he was actually traveling backwards, very slowly, but still climging and flying.

[Gossamer]

Okay, here are the two possible cases.

One:
The treadmill matches the rotational speed of the wheel. The plane has zero linear velocity with respect to surrounding objects, and it is unable to generate lift with the wings. It does not take off.

Two:
The treadmill matches the linear speed of the wheel. Even though the wheels are not turning, the plane is moving quickly with respect to surrounding objects, and can take off.

The question is not specific enough.

/thread

[centerchannel68]

Originally Posted by Gossamer 

When the question says the conveyer belt matches the 'speed' of the wheels, it needs to be clarified: linear speed or rotational speed.
I think the plane would take off. If the conveyer belt constantly matched the linear speed of the wheels, the speed of the conveyer belt would initially be zero when the plane is at rest. When the thrust from the jets causes the plane to move, the conveyer belt would adjust to keep the speed of the wheels at zero. So the plane would still be accelerating with respect to the airport, but not the runway.

It would never get any windspeed. It would not generate lift. This is a simple idea. No wind= no lift.

Airplane engines provide thrust so it can gain enough AIRSPEED to generate lift.

[missingbite]

Originally Posted by Gossamer 

It depends on friction. Nail your brakes on ice at 60mph. Your wheel's linear speed is still 60mph even though it's not rotating. Or floor it from a stand still. Your rotational speed can be 10000rpm while linear speed is zero.



But what if the treadmill is used to maintain the wheel's rotational speed at zero, but does nothing to affect the linear speed?

An airplane through its own power can't get linear speed without wheel friction. If the wheels can't move forward with or without rotaional speed (i.e. wheel locked but moving forward) the a/c can't generate lift by having airflow over the wing.

[centerchannel68]

Originally Posted by Gossamer 

Linear speed is the velocity of the center of gravity of the tire. So if a car is moving at 60mph the linear speed of each of its tires is 60mph. Rotational speed is the speed that it's rotating. So if a car is moving at 60mph and the radius of the tire is 10 inches, the rotational (angular) speed is 176 rotations per second, or 10,560 rpm (if my math isn't flawed).

It's still the same speed. You're just looking at it two different ways...but it's still the same motion, at the same speed.

[d4nth3m4n]

Originally Posted by Gossamer 

But what if the treadmill keeps the linear speed at zero by matching the rotational speed?

see, but it's not keeping the plane from moving, it's just matching the [accelerating] speed of the planes wheels. it REACTS. the plane will gain speed no matter how [amazingly] fast the wheels are spining.

da?

[Gossamer]

Originally Posted by missingbite 

An airplane through its own power can't get linear speed without wheel friction. If the wheels can't move forward with or without rotaional speed (i.e. wheel locked but moving forward) the a/c can't generate lift by having airflow over the wing.

So planes can't take off on ice? If a wheel is locked, but on ice, the plane is fully capable of propelling itself and taking off.

 

Didn't you see the /thread?

[olePigeon]

Doesn't a plane use wind for lift? If it's stationary (even if it's a jet plane), you still need airflow to generate lift. The plane can generate all the thrust it wants (with as little or as much friction on the wheels you want,) but if there's no air flowing over the wings, it's not going to fly anywhere.

[Dakar²]

I have a feeling this thread is about to reach critical mass.

[olePigeon]

Originally Posted by centerchannel68 

It would never get any windspeed. It would not generate lift. This is a simple idea. No wind= no lift.

Airplane engines provide thrust so it can gain enough AIRSPEED to generate lift.

Exactly. This is a non-issue. You can't fly a plane without lift. Period. If your relative speed is 0 with no headwind, you're not going anywhere, I don't care how fast the wheels are turning or how much thrust is being generated by an engine.

[d4nth3m4n]

Originally Posted by Gossamer 

 

Didn't you see the /thread?

 

you're not the last word on anything, get off of yourself, jackass

[olePigeon]

Originally Posted by BRussell 

I don't know, but I bet it could if it was one of these planes.

If it was one of those planes, one the propellers would randomly quit and it'd come crashing into the ground.

[missingbite]

Originally Posted by Gossamer 

So planes can't take off on ice? If a wheel is locked, but on ice, the plane is fully capable of propelling itself and taking off.

 

Didn't you see the /thread?

Re-read my post, an a/c can takeoff with a locked wheel, or no wheels (seaplane) but the wings have to move forward (linear) in order for it to become airborne. If the treadmill moves the entire a/c forward and the a/c gets to enough speed for lift to overcome drag it will get airborne. That is not what I think the teaser is asking.

[Gossamer]

Originally Posted by d4nth3m4n 

 

you're not the last word on anything, get off of yourself, jackass

Lighten up

Originally Posted by missingbite 

An airplane through its own power can't get linear speed without wheel friction. If the wheels can't move forward with or without rotaional speed (i.e. wheel locked but moving forward) the a/c can't generate lift by having airflow over the wing.

Originally Posted by missingbite 

Re-read my post, an a/c can takeoff with a locked wheel, or no wheels (seaplane) but the wings have to move forward (linear) in order for it to become airborne. If the treadmill moves the entire a/c forward and the a/c gets to enough speed for lift to overcome drag it will get airborne. That is not what I think the teaser is asking.

I agree with your second post, but not the first one.

[Dakar²]

Originally Posted by Dakar² 

I have a feeling this thread is about to reach critical mass.

Originally Posted by d4nth3m4n 

(you're not the last word on anything, get off of yourself, jackass)

At least I was right about one thing in this thread.

[Calimus]

I hate to jump into this again since it's one of those problems where there are so many variables that depending on how you look at it, it can go either way, but I thought of another way to look at it. Imagine the acceleration of the treadmill precedes the acceleration of the plane.

If a plane sits on a treadmill that is stopped, the treadmill begins to move backward. The plane will move backward because of the friction of the bearings on the wheels. Now the plane starts up it's engine to start rolling forward(with respect to the treadmill, stationary with respect to the surroundings). It's pushing forward at the same speed the plane is being pushed backward by the friction of the wheels(say both forces are still fairly small at this point, 2-3mph in either direction) since the two are equal, the plane will roll, but now be stationary with respect to the surroundings, but moving on the treadmill. The whole crux of the problem is whether or not the treadmill is able to go fast enough to generate enough friction on the wheels to counteract the forward push of the plane engines. As I stated above, in the real world, this probably wouldn't work, in an ideal computer model, it probably could. It might be that the treadmill would need to move at some insane speed 20-30x the speed the plane needs to take off, but it could keep the plane in place(theoretically).

[centerchannel68]

Ugh. Some of you people are so dense. Here's an idea. Buy a foam airplane. Put some little wheels on it, maybe a plastic hotwheels car. See if it flys. Now tie a string to it, and put it on a treadmill. I promise you IT WILL NOT FLY, because it needs AIRFLOW OVER THE WINGS in order to fly, period.

Anybody who says otherwise just does not understand the very basics of why an airplane flies.

[Zeeb]

I have never been good at "story problems" and I have no idea, but this sounds like a good problem for the Mythbusters. Someone write them.