Airplane/Conveyer Belt Question

see above.

actually, picture this.

Two identical planes, with big velcro rollers on the bottom of each instead of wheels, are belly to belly, but facing opposite directions. Assume no gravity.

If you pushed both of their engines into full throttle at the same time, would they move, or would they stay stationery?

They would move. The rollers would not pose any impediment to their forwards movement, even though there is a roller under them with the exact opposite speed and acceleration.

i’m confused by the wheels going forward making it impossible to stay still on a treadmill part. the treadmill goes backwards, so the wheel goes…forwards? the treadmill goes forwards, so the wheel goes…forwards? whah?

in my mind the question is, if i were pedaling on a bicycle on a treadmill, would it be 100% impossible to be stationary even if the treadmill was perfectly operated by a computer to speed/slow/go backwards? wouldn’t this be the equivalent of a treadmill being made out of perfectly smooth ice, resulting in the wheel going nowhere? but the engines make it go forward, so the treadmill has to go backwards but then i’m back to, is it impossible for the treadmill to make it go backwards/stay still…

That’s the wrong question. You’re not pedaling a bicycle on a treadmill. You are not fighting the motion of the treadmill. Rather, you are COASTING on a bicycle with a ROCKET strapped to your back. You may as well not be attached to the bicycle at all, and the only reason why you are is because we have gravity and you have to have something under you lest you leave a big red skid mark on the ground.

but to sim a treadmill the plane on the bottom would be facing the same horizontal direction as the one on top wouldn’t it? if the top part of the roller was going the opposite direction of the top plane to make it stand still the bottom plane would have to be heading in the same direction as the top plane.

if that were the case, they’d launch off together in unison, attached at the hip with their velcro rollers. :)

this makes more sense.

but assuming gravity, wouldn’t there be a magic point where the rocket going at full blast and the treadmill going backwards at x fraction of lightspeed make me stand still?

still trying to think this one out…

On the face of it, I’d say no. Because we’ve all seen a commercial with some new car driving on a conveyer belt, and it doesn’t go anywhere.

However, cars get their forward motion by friction between the ground and tires. Planes do not. The jets would push the plane forward at speed X. With the conveyer belt were moving backwards at speed X, you’d have the tires spinning at speed 2X but neither hindering nor contributing (significantly) to takeoff.

Think of a vertical analogy: put a conveyeyer belt under the space shuttle and have its wheels on it: wouldn’t matter because it’s the rocket moving the shuttle upwards, not the wheels. It “sounds” better in the vertical analogy, IMHO.

This is the crux of the question, how you interpret this line. If you’re measuring the speed of the plane by how fast the wheels are rotating (like how your speedometer tells you how fast your car is going) then the plane will not take off. Arguably, that’s the wrong way to measure the speed of the plane though, it should be measured by its movement relative to a separate location in space. You can get all stupid about how the whole planet is spinning and flying through space and what the heck is speed relative to anyway, but for common sense purposes, the speed of the plane, conveyor or not, is how fast it “looks like” it’s going to someone standing next to the runway, not how fast its wheels are actuallly revolving. When there’s no conveyor, the speed of the plane and what the wheels’ revolutions are reading for distance travelled happens to match up, but that’s just lucky for us (and cars).

The plane will fly.

It’s time to call the Mythbusters.

And Cecil’s definitive reply.

Here’s how I look at it: suppose an F/A-18 were making a carrier landing at 120 kts, but something goes wrong and the trap didn’t engage. No problem: as we all learned from the Jane’s sim, the pilot just applies more thrust, climbs away for a bit, and comes around again.

Now suppose the jet fighter were making the same approach, but for some reason the carrier crew replaced the trap with a conveyer belt running 120 kts in the opposite direction. Heck, run the belt at 300 kts even! The landing gear makes contact, and what happens? I think it’s pretty obvious the jet would roar past the tower as before, again apply thrust, and fly away.

There’s just no way a conveyer belt would apply any substantial braking force to the landing jet - which means there’s no way it can impede a plane that wants to accelerate.

Wait, I know what the problem is: the scenario ranvarian’s friends posited is possible only in theory, which is how all physics riddles get away with this type of bullshit. Until motherfuckers start popping out giant conveyor belts that magically match forward thrust for jet engine aircraft, at any rate.

Also, if Roger is right and it’s all due to friction, I’m taking a big dump in ranvarian’s friend’s mouth the first chance I get. A big, steaming, post Baja Fresh dump. With green sauce.

You’re not going to even pretend to back that up?

See, this way of interpreting it leads to the plane taking off. Engines thrust x mph left, conveyor pushes x mph right, wheels spin at 2x mph, (give or take negligble friction) and away the plane goes. Most people who think it doesn’t take off don’t think of the conveyor in terms of matching forward thrust of the engines (which is basically the actual velocity of the plane, give or take more negligble stuff like wind and air resistance), they think of it in matching rotational speed of the wheels.

Let me say again: air resistance is not negligible, it is how planes fly.

The plane will remain stationary on the conveyor belt until the engine generates sufficient thrust to overcome the friction holding the wheels to the conveyor belt. After which point it will move forwards regardless of the conveyor belt and take off.

Apparently the real reason it would fly is that the source of the plane’s acceleration is not the wheels, but the engine. So the plane wouldn’t stand still, and would eventually move forward in relation to the air, generating the required lift.

Or something.

Yeah, but his point is that there will be air resistance, because the plane will move foward through the air despite the conveyor turning under its wheels. A car would not, because its only source of forward power is friction between the wheels and the road: take that away (by moving the road backwards in tandem with the wheels), and the car goes nowhere. A plane, on the other hand, gets forward power by pushing back on the air behind it (via its engines), not by pushing against the road underneath it. So to the plane, it doesn’t matter how fast the conveyor is moving, the plane will still move foward and take off.

Here’s another way of thinking about it: instead of a conveyor, imagine a frictionless road (like the ultimate oil-coated ice skating rink). The car can’t move forward, obviously – the wheels will just spin in place. But the plane still can, and if you could keep it pointed straight, you could take off.

What an incredibly badly worded physics problem. There are two ways to interpret it - first, the conveyor belt spins to match the groundspeed. That is, wheels are moving forward at 10mph, belt moves backwards at 10mph. The net force that gets applied to the actual body of the airplane is a small amount of drag generated by the friction from the wheels. Thus, the conveyor belt in this configuration isn’t gonna keep the plane from moving forward and generating lift under its wings, so it takes off.

The scenario I guess the problem is trying to make you think about is, if the plane is generating thrust like crazy, but somehow this magic conveyor belt is keeping it from moving forward, will it take off? I suspect, though I’m sure as hell not going to do the force vector calculations to figure it out, that you could prove that such a situation is actually physically impossible, and thus such a gedanken experiment is meaningless. But yeah, if the plane doesn’t move forward, it doesn’t take off. (Edit - barring obvious exceptions like VTOL planes)

Hawkeye- It’s not that badly worded. Ranvarian can’t be held responsible for people assuming that it’s a magic conveyor belt that keeps the plane still for no reason. It isn’t even a physics question, it’s a “How planes work” question. They don’t go fast because their wheels spin, their wheels spin because the giant honking enginges on the wings make the plane go fast.

Well, my answer to “how do planes work?” would probably be along the lines of “Physics!” But I used to be a science nerd, so maybe it’s just me.