So, you have a plane on a conveyor belt...

[Rina]

I heard a rumor that this company (conveyor belt company in LA) would consider engineering something like that for fun, or at least its a content - http://www.mrosupply.com

heres what they said

https://www.mrosupply.com/page/airline-conveyor-belt/

[Wjolcz]

I never liked that Mythbusters episode. Seems obvious that you need speed to fly.

If the air is still (doesn't move) around the plane and the conveyor belt adjusts to the speed of the plane rolling, it won't fly. It will just stay still with it's propeller pulling it and the wheels rolling faster and faster. It's the airflow under and above the wings that matters.

EDIT: maybe if the propeller is big and powerful enough to give the wings the airspeed they need to produce lift, then it might work.

Edited October 1, 2015 by Veeltch

[Lukaszenko]

There is a plane parked on a conveyor belt. The plane fires up it's engine and attempts to take off. The belt runs in the opposite direction. Does the plane take off?

How fast does the belt turn in the opposite direction? 1 mph? 40? 5000? As fast as the plane?

Not that it really matters. A plane sitting on conveyor is just as affected by it as a plane flying over the conveyor: not very much. The plane doesn't interact with the conveyor, after all, it interacts with the air. Yeah the wheels might spin faster than normal, but who cares. In fact, during takeoff that's the function of wheels: to decouple the plane from the ground, and only use the ground for vertical support.

What if we stipulate that the conveyor moves as fast as the plane??? Again, doesn't matter, because by this requirement the plane HAS to move in order for the conveyor to move....as in, it has to move relative to the ground/ air for the conveyor to move. That's the stipulation. In fact, even a car could move under this requirement, albeit its speedometer would falsely read 2x its actual speed.

Of course, this assumes that we measure the airplane's (or car's) and the conveyor's speed relative to the air/ ground. But what if we measured it relative to the conveyor?? First, that's stupid because why the hell would we do that? Second, that's stupid because both the conveyor's and the plane's speeds are both variables in the question. You wouldn't measure the plane's speed relative to a variable, whose very value depends on the plane's speed. It's circular. It would become a mathematically impossible situation, basically saying that a = b + a.

So basically I cannot see any realistic situation where the conveyor would or could keep the plane from taking off because, again, they are decoupled from each other. The best we can do is spin the conveyor so fast that it destroys the wheels (and therefore the plane), but that's just going out of the bounds of the question and getting into the realm of indestructible conveyors (it's easier to engineer a fast-spinning wheel than a fast-moving conveyor).

[Tex_NL]

...

EDIT: maybe if the propeller is big and powerful enough to give the wings the airspeed they need to produce lift, then it might work.

A propeller that big and powerful would easily overpower the brakes. And besides that; pointing that propeller upwards would basically turn your airplane into a helicopter.

[shynung]

A propeller that big and powerful would easily overpower the brakes. And besides that; pointing that propeller upwards would basically turn your airplane into a helicopter.

We have one already.

This is the V-22 Osprey, a military transport aircraft designed to have the cruising performance of a turboprop aircraft and the hovering capability of a helicopter.

[Jovus]

This is the V-22 Osprey, a military transport aircraft designed to have the cruising performance of a turboprop aircraft and the hovering capability of a helicopter.

And the safety rating of Jebediah's favorite rocket.

Sorry, I know it's mostly untrue. I couldn't help myself.

[sgt_flyer]

And the safety rating of Jebediah's favorite rocket.

Sorry, I know it's mostly untrue. I couldn't help myself.

Well - they had to use a lot of triple redundancy all over it + a mechanical linkage of the 2 engines because it's both a bad plane and a bad helicopter for emergency unpowered landings (bad glide ratio and very bad autorotation capabilities) - at least, the redundancy is there to prevent most of those scenarios.

Now, regarding the conveyor belt problem, i wonder how it would affect the power needed by a plane that start and accelerate after the conveyor belt (and the plane in this case) started moving in the opposite direction of the plane's flight, VS having the plane & the conveyor belt starting & accelerating at the same time. - even with the limited friction, having to compensate for the negative speed for the first case will still need quite more power

[lajoswinkler]

God, not this crap again.

[Shpaget]

I don't get it why is this even debated. All that matters for a plane is airspeed. Airplane does not care about the motion relative to the ground, nor does it care if it's wheels are spinning and how fast. Planes can fly backwards relative to the ground if there is enough head wind.

If the conveyor moves the plane backwards, plane just needs to accelerate to match that speed and then it's the same as when starting from the resting state. Energy requirements are exactly the same as the energy required to accelerate forwards to the conveyor speed, which is not even worth mentioning considering the fact that it takes only seconds for aircrafts to take off, compared to hours that planes are capable of staying in air.

Why did Adam and Jamie even take on this "myth"? It's asinine.

[Frybert]

It's asinine.

Quoted for truth.

[K^2]

I don't get it why is this even debated. All that matters for a plane is airspeed. Airplane does not care about the motion relative to the ground, nor does it care if it's wheels are spinning and how fast.

It's debated because people, including Myth Busters, don't understand simple things like constraints of motion and coordinate systems.

If you take a car moving at 60mph on a highway and put it on a conveyor belt running 60mph backwards, guess what? Car will still move forward. Because for a car, like an airplane, the dominant force on a freeway is aerodynamics. And its wheels can spin at way more than 60mph.

The whole "myth" comes from people insisting measuring car's speed by its speedometer, which is based on wheel rotation. If we put the same constraint on the airplane, it won't move either. Sure, the "conveyor" will have to accelerate at a mechanically impossible rate, quickly disintegrating the landing gear, but if your constraint is that wheel speed matches belt speed, then then the object cannot be moving forward by definition. Be it a plane or a car. This isn't even physics. It's just mat

However, such constraint is practically impossible to enforce for both plane and a car. No matter how fast the belt is moving, if the driver is gunning the throttle, the car will either be moving forward, or you'll destroy the car's engine/transmission/wheels. Not unlike the case with the airplane.

[sgt_flyer]

Which is why i asked - in order to try to get something a bit more useful - how much more power would it require for the plane to go from -60mph to +60mph (airspeed of course) if the conveyor belt was set in motion Before the plane

(I know well that If the plane goes in motion and accelerate at the same rate as the conveyor belt, the only thing that'll change is that simply the wheels are going to spin faster as they are freely rotating. (The plane might lose a bit of power due to friction, but it'll stay negligible)

[GeneralVeers]

There is a plane parked on a conveyor belt. The plane fires up it's engine and attempts to take off. The belt runs in the opposite direction. Does the plane take off?

Yes. Because the plane will pretty quickly skew to one side or the other, and roll off the side of the conveyor belt.

What?? Why are you guys staring at me like that?? It's what happened when the Mythbusters tried to test this one.....

Jokes aside, every single test in the Mythbusters episode was a fail. The problem with the cloth conveyor belt being that the weight of the plane was holding that section of the "belt" in place, and stretching of the cloth (which wasn't visible on camera) was vastly reducing the amount of backwards force applied to the plane.

With an ideal conveyor belt that didn't do any of that? It would have to be a conveyor belt that ran at a ridiculous speed <insert Spaceballs joke here> because in order to generate more backwards force than the plane's forward propeller force, the belt would have to be moving faster than the plane's normal takeoff speed. Probably a lot faster, because I don't have any idea how much braking friction a plane's wheels generate when a plane touches down for landing and the wheels are "coasting" on the runway. Also I don't have any idea what's the maximum ground speed of a plane speeding down a runway with the wheels still on the pavement. The conveyor belt would have to be faster than that.

[Camacha]

The whole "myth" comes from people insisting measuring car's speed by its speedometer, which is based on wheel rotation. If we put the same constraint on the airplane, it won't move either. Sure, the "conveyor" will have to accelerate at a mechanically impossible rate, quickly disintegrating the landing gear, but if your constraint is that wheel speed matches belt speed, then then the object cannot be moving forward by definition. Be it a plane or a car. This isn't even physics. It's just mat.

You are making this way too complicated. The difference does not come from people incorrectly taking the speedometer as the basis, but from the fact that cars are wheel driven and planes are not. Push against conveyor, cancel each other out. Push against something static (including air), move forward. People do not get that.

With an ideal conveyor belt that didn't do any of that? It would have to be a conveyor belt that ran at a ridiculous speed <insert Spaceballs joke here> because in order to generate more backwards force than the plane's forward propeller force, the belt would have to be moving faster than the plane's normal takeoff speed. Probably a lot faster, because I don't have any idea how much braking friction a plane's wheels generate when a plane touches down for landing and the wheels are "coasting" on the runway. Also I don't have any idea what's the maximum ground speed of a plane speeding down a runway with the wheels still on the pavement. The conveyor belt would have to be faster than that.

That comparison falls short, since the engines on a landing plane are pretty much idling, while those on one trying to take off are at full blast. A quick search does not turn up which system would win if both used to their maximum extent.

[Frybert]

People do not get that.

I'm amazed those people don't die of shock when they see a plane keep moving after its left the runway.

[Camacha]

I'm amazed those people don't die of shock when they see a plane keep moving after its left the runway.

When it is airborne, it is obvious. When it is on the ground, people apparently are so used to ground driven vehicles like they see every day, it is not apparent to them. Some of them, at least.

[GeneralVeers]

That comparison falls short, since the engines on a landing plane are pretty much idling

And at that point, how much drag is being exerted by the wheels? (or, more accurately, friction in the wheel bearings)

In order to prevent the plane from taking off, that force has to be greater than the force exerted by the engines.

[Camacha]

And at that point, how much drag is being exerted by the wheels? (or, more accurately, friction in the wheel bearings)

I do not think that comparing the landing to a take-off is a fruitful endeavour. The loads are rather different.

[Tigermisu]

I remember completely derailing my physics class when I asked this last year, haha

[0111narwhalz]

Oh, for- You all have a physics simulator sitting right on your desktop! Boot up KSP, build a rolling airstrip, and take a plane off of it, backwards. You'll find that (gasp) the plane takes off! Unless it runs out of runway. In which case, build a bigger runway. Just keep in mind that your speedometer measures ground speed, which in KSP is the same as airspeed. You can get conveyor ground speed by targeting the rolling thingy.

[K^2]

You are making this way too complicated. The difference does not come from people incorrectly taking the speedometer as the basis, but from the fact that cars are wheel driven and planes are not. Push against conveyor, cancel each other out. Push against something static (including air), move forward. People do not get that.

You are falling into the same fallacy as everything else. Pushing against moving things takes a bit more power, but it doesn't cancel anything. A car on a moving treadmill will drive off JUST AS EASILY as an airplane.

There is absolutely no difference between an airplane and a car as far as this problem goes. For both, it is a stupid problem. For either one in idle, a slowly moving conveyor will keep them in place. For either one at full throttle, no conveyor speed will keep them put. It's seriously that simple. Only people who understand nothing about basic physics of motion, which is by far most people, keep trying to see a difference.

[Tigermisu]

I made this simple video.

https://dl.dropboxusercontent.com/u/12852456/Video_1443743476.mp4

Left one is a car powering its rear wheel at roughly the same speed of the conveyor.

Right one is a "plane" with a 20kg mass and a 2 thrusters outputting 0.1N, which was the lowest I could assign them to.

This proves that the plane does take off regardless of the treadmill.

[Shpaget]

Which is why i asked - in order to try to get something a bit more useful - how much more power would it require for the plane to go from -60mph to +60mph (airspeed of course) if the conveyor belt was set in motion Before the plane

If it takes x amount of energy to accelerate from 0 to 60, it takes 2x to accelerate from -60 to 60.

Energy, not power.

In practice, the energy requirements are actually less for deceleration from -60 to 0 than acceleration from 0 to 60 due to drag that is helping to decelerate.

In either case, total energy spent on this deceleration is negligible compared to total energy spent on a standard flight. Take off acceleration, even for monster like a 747, lasts about 45 seconds. Compare the fuel consumption in those seconds to the fuel consumed on a 5 hour flight and you get the perspective (yes, even including the higher engine power at take off than at cruise).

[K^2]

Left one is a car powering its rear wheel at roughly the same speed of the conveyor.

Right one is a "plane" with a 20kg mass and a 2 thrusters outputting 0.1N, which was the lowest I could assign them to.

This proves that the plane does take off regardless of the treadmill.

That's a very strange interpretation of the "myth". Why did you make the "car" always turn the wheel at constant speed, but have the "plane" just apply maximum thrust regardless? Have the "car" apply constant torque on its wheel and try again.

[KerikBalm]

I am so tired of this poorly formed and IMO stupid question.