Plane on a Conveyor - Does it feel like Adam & Jamie didn't follow the spirit of the myth?

Posted by MrMusAddict@reddit | mythbusters | View on Reddit | 32 comments

The Mythbusters seem to have interpreted the myth as:

Is it possible for a plane to take off if it's on a conveyor belt being pulled backward at take-off speed?

Which is starkly different than my interpretation:

If a plane is being pulled backwards on a conveyor, and the plane thrusts forward only enough to perfectly counter-act that backwards momentum (therefore causing the plane's global speed to be 0 mph), will the plane take off since it's going take-off speed relative to the conveyor belt?

The infographic at the beginning of the episode seems to reinforce my interpretation; it shows a plane moving forward at the exact speed of the conveyor belt in the opposite direction, resulting in the plane being stationary relative to the world.

However, in both their small scale test and the full scale test, the plane was given enough thrust to overcome the speed of the conveyor and take off anyway since it had enough speed relative to the world to take off anyway.

I interpret this result to be "We've proven that a plane can take off, since it's going double the take-off speed relative to the conveyor, which is still the normal take-off speed relative to the world.", which seems to ignore the spirit of the myth?

I understand what Adam and Jamie said about how the momentum of a car comes from it's wheels whereas the momentum of a plane comes from its propeller, and therefore the plane can over-power the momentum of the conveyor since the wheels are effectively not a factor.

But here's where I'm getting tripped up; if the plane was off while on a conveyor, it'd be moving backwards at take-off speed. Therefore, it should be true that there is some amount of thrust required to make that plane stationary relative to the world (like the bowling ball out of the truck myth). And then from that point, I believe the spirit of the myth is; "Would the amount of thrust that perfectly counters the backwards momentum of the conveyor be enough to make the plane take-off, since relative to the conveyor belt the plane is travelling take-off speed?"

I guess I would feel a lot more satisfied if the pilot gradually thrusted up only enough to keep his plane stationary, to prove that the plane would not take off.

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WaitForItTheMongols@reddit

Imagine you're standing on a skateboard on a treadmill. You're holding a rope tethered to the wall in front of you so you don't move. The skateboard's wheels are spinning like crazy but you aren't going anywhere. The wheels are very well greased, so even lightly hanging onto the rope is enough for you to stay in place.

If you pulled on the rope tug-of-war style, would you be able to move forward? Of course you would! With exactly the same ease as you could without the treadmill, because the free-spinning wheels mean you don't actually get propelled by the treadmill.

In the same way, the plane has no difficulty sliding forward despite the thing being pulled out from under it. It can still propel because its wheels just whizz away.

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zidacs@reddit

If you pulled forward, then your wheels are traveling faster than the treadmill, invalidating the problem statement. This is a purely theoretical relative motion problem that creates a paradox.

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Lecture_Green@reddit

The wheels will always be travelling faster than the treadmill because there is no instantaneous communication. You've got some kind of sensor on the wheels that has to communicate with the program controlling the treadmill. So you've got transmission delay, and then whatever time it takes for the program to process, then further transmission delay sending instructions to the treadmill motors, and then further delay as they accelerate to match.

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zidacs@reddit

The the wheels travelled faster than the treadmill, then the premise is invalidated. This is a thought problem that cannot exist in real life.

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Lecture_Green@reddit

You're right- in the actual scenario, the treadmill matches the speed of the spinning wheels, and the plane still moves forward, dragging its wheels and losing some material as it goes. The treadmill has to match the speed of the wheels, but the wheels dont actually have to match the speed of the plane.

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Uncle-charlie5555@reddit

No. The treadmill speeds up. But you still move forward. It never says constant speed. The question is stupid.

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StormCruzzer@reddit

NO! This is a HORRIBLE example! It completely misconstrues the problem.

The problem states that the belt will instantaneously match the speed of the wheels. In your example, the speed of the treadmill is consistent. If the treadmill matched the speed of the skateboard wheels, the harder you pulled, the faster the treadmill would go, and you would not move anywhere.

Their entire experiment in MythBusters was horribly planned out and performed. It is impossible to match the speed of the plane and the speed of the tarp. And the weight of the plane was NOT simply supported by the tarp. The plane's wheels were making contact with the ground underneath.

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Lecture_Green@reddit

Except it doesn't work that way. The force of friction, as a percentage, actually DECREASES with speed, so the faster the treadmill moves, the smaller the fraction of energy lost. Friction is at its greatest when stationary. This is why when you push on a heavy box, gradually increasing your force, it doesn't move for a while and then you suddenly hit a threshold where it slides forward quicker than one would expect.

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ProfessionalRun5006@reddit

People do this with cars and skateboards all the time. From experience I can tell you what actually happens is you pull yourself off the skateboard and it hurts like hell.

MasterFubar@reddit

The propeller pulls against the air, not the ground. As long as the wheels are allowed to turn freely, the movement of the ground under the airplane is irrelevant.

This is not true. The propeller pulls against the air, yes. That is correct. But in order for the plane to move forward, the wheels must rotate. If the wheels rotate, the conveyer belt moves backward instantaneously at the same speed. So the plane starts moving relative to the conveyer belt, but not relative to the ground and surrounding air (since the air is responsible for providing lift). Without forward movement relative to the ground, you cannot have air forced under the wings to provide lift. See my reply to the OP above to understand why.

Thrullx@reddit

Mark Rober just released a video about this myth. Turns out, according to him, a plane won't take off. This is exactly what I suspected would happen even though the Mythbusters disagreed. https://youtu.be/g6tlNyr5sl8?t=680

travelingwhilestupid@reddit

So there are two forces on the plane. One pushing the plane forward (the jets), and one pulling it back (the wheels). You're assuming that they are equal and opposite, therefore net motion is zero.

I'm no expert, this is just a guess. I think you've made a mistake. The friction coefficient between the wheels and the ground isn't that high. There's a reason race cars have spoilers - the drag pulls down on the car, increasing the friction between the tires and the tarmac. A plane doesn't have big wheels relative to its size. It can't stop quickly like a race car, it can't produce the same breaking force, but the jets produce much more force. And that's with down pressure when they land (that's why they land forcefully and use the wings to push down). Assuming not much initial speed, and thus relative wind, and assuming the slats aren't engaged, very soon the wheels will just start to slip. As the plane speeds up at all, any upwards force reduces the friction coefficient of the tires. and once the plane takes off even a millimeter, it's game over.

KevinEarle7164@reddit

100% correct!

Wojan@reddit

For me the movement of the wheels IS relevant, since that is how the problem is defined. I understand the plane will take off because the plane motor pushes the plane forward and makes the wheels rotate. However there is also some forward friction (the speed of plane is not fully translated to the rotation of the wheels - plane moves forward faster than the wheels rotate). So the conveyor belt, which matches only the speed of wheels rotating doesn't actually match the speed of the plane, since the plane moves faster than its wheel rotates. Should there be no forward friction allowed (and my understanding is that this is not how planes work), the plane would stay in place - but that's hypothetical, possible in a world in which we can just remove some physical law, in real scenario plane will always take off.

Xntrik_shaman@reddit

I agree, the whole point of the myth was that the conveyor belt is supposed to MATCH the rolling speed of the wheels...not stay at a constant rate (whatever the take off speed is)

While I realize that the aircraft gets its thrust from the propellers/jets and not the wheels it still must roll across the ground until the forward momentum is enough to create lift across the wings.

If you counteract that rolling speed it would require an absurd amount of thrust to take off, essentially it would need to immediately become airborne (like a space rocket). However normally planes must gain speed along the ground allowing the lift to build up to that critical liftoff speed.

They really need to retest this myth with a conveyor belt that speeds up as the wheels increase their rolling speed.

JustANormalHat@reddit

the plane will take off regardless, what the wheels are doing has nothing to do with what the plane is doing

Chirrup58@reddit

But surely this myth is about lift? A plane needs air flowing over its wings to generate lift and therefore take-off. A plane on a conveyor belt has no movement of air over the wings, therefore no lift, therefore no flight. Even if the plane is matching the speed on the conveyor belt, there's still no air movement -- right?

the air is still flowing, the conveyor belt has no effect on the air, the wheels have nothing to do with the plane gaining lift

I never said the wheels had anything to do with lift, they don't-- that's my point.

If the plane is matching the speed of the conveyor belt (or treadmill), then relative to the rest of the world, the plane is stationary. The plane is effectively just running its wheels in place. Therefore there is no movement of air over the wings to generate lift, ergo no lift, ergo no flight.

The plane wouldn't be stationary!!! This isn't how planes work!

Say I'm on a skateboard on this magical conveyor belt. And I'm holding a rope to keep me in place. If I pull myself on the rope, do I move forward? Even if the belt speeds up?

Of course I do.

Pulling the rope = thrust.

superlibster@reddit

They only proved a Cessna can do it. And that’s because the plane is very light and propellers generate upward thrust from their angle and actually forces air under the wings to produce lift. This would not work for a larger plane.

Imagine if a truck was tethered to a glider pulling it down a runway. The glider would take off. Now put the truck and the glider on a conveyor belt. It would not take off. And I think that’s the true principle of the myth.

If planes could realistically take off from a conveyor belt, aircraft carriers would look a whole lot different.

DangerSwan33@reddit

You should watch the Tested question Adam just posted on YouTube.

https://www.youtube.com/watch?v=xUjcHW7SHaI

In this, he explains why the question itself is basically flawed, and semi-misleading, and therefore causes your very problem, but the actual act of putting an airplane on a conveyor belt means the plane takes off.

I see all of your responses in this thread, and they all boil down to one thing - you're willingly ignoring the physics in favor of what you want the physics to be.

You think that they ignored the spirit of the myth because you think that the spirit of the myth is that an airplane on a conveyor belt would go 0 mph, and you want to know if that airplane would take off. Of course it wouldn't (if all other variables, such as headwinds, are controlled for).

But an airplane on a conveyor belt will not go 0mph, no matter how badly you want it to. It will not do it. It WILL NOT go 0mph. It won't. It won't, and that's the answer. An airplane on a conveyor belt matching the airplane's speed will still accelerate forward at the same speed it always would have. This is the fact, and it's an indisputable one, for all of the reasons outlined in the episode, in Adam's explanation above, and in this thread itself.

What you're struggling to overcome - and that's fine, because it's confusing - is the thought that an airplane is a car with wings, which it is not.

An airplane's power does not go to its wheels. That would be a car with wings.

Now, you're correct - a car with wings, capable of flight, on a conveyor belt matching its forward speed, would not take off. It would not be able to achieve wind speed.

However, the true spirit of the myth - and the messageboard arguments that sparked the myth - was all centered around whether or not an airplane would be able to achieve wind speed on a conveyor, and the answer is a resounding yes - the conveyor makes zero difference on the airplane's ability to accelerate, because it does not use the ground to accelerate.

You’re misrespresenting the spirit of the myth. This is a purely theoretical problem as evidenced by the problem statement. If the plane moved forward then the wheels are moving faster than the conveyor, invalidating the premise. This is very simple to understand, and relative motion is taught in highschool physics. The plane stands still relative to the ground and does not take off.

Shevster13@reddit

It depends what you are using as the reference point for calculating speed. You are using the starting point on the belt. That is a unusual definition, and one that would get you marked down in most physics exams, because you would also have to measure the speed of the conveyor from the same reference frame, meaning the conveyor would have a speed of zero.

The standard way to be measuring the speed is a fixed starting point outside of both the plane and the conveyor (aka a spot on the ground). In which case it would be completely possible to have the conveyor and wheels traveling at the same speed, in opposite directions from that starting point.

relative motion only works if you have an external reference frame.

Ryverious@reddit

The myth was poorly explained and executed. The plane had to compensate for the ground moving under it (essentially pulling it backwards) at the same speed it was needing to go forwards to take off. If they looked at the engine output, they would have seen that the engine was working harder to compensate... essentially having to work as if it was getting up to twice the speed. Though not quite twice the speed would be necessary, as the wheels would eventually just spin against the belt as the speed increased and the amount of friction decreased

Infamous-Humor9895@reddit

I think I am in agreement with MrMus Addict. The conveyor belt thought experiment is supposed to keep the airplane's motion RELATIVE TO THE GROUND at 0 mph (Keep in mind that in the thought experiment, the air is also not moving. There is no 25 mph headwind. So can we say that the airplanes motion is relative to the ground and the air?). But in the Myth Busters full-size experiment, the airplane is moving forward relative to the ground as evidenced by the plane's relationship to the line of traffic cones. The conveyer didn't match the airplane's speed. The edited video makes it difficult to calculate the airplane's speed realtive to the ground, but it looks to me as if the airplane is accelerating (it is also not clear to me how fast the plane is moving relative to the ground when its tires no longer touch). I totally understand that the airplane's thrust is generated by the propeller screwing through the air, but unless that propeller can launch the airplane from a standstill, like a helicopter, the plane will have to gradually ratchet up its speed by rolling down a runway until the wings, pushing into the air, can create lift. This particular airplane is able to take off is when it is running into the air at 25mph. Unless the conveyer belt is able to pull the atmosphere backwards at 25mph and not just the atmosphere behind the propeller, but also all the atmosphere in front of the wings, the plane shouldn't be able to take off. A 25mph stream of air needs to be flowing over the tops of the wings to create lift. If the conveyer belt is doing its job, the airplane's wings - the whole airplane - should not be moving. To argue that the plane can take off without moving relative to the ground is to demonstrate that airport runways are a silly waste of resources

rnatty@reddit

Adam just did a video about this: https://www.youtube.com/watch?v=xUjcHW7SHaI

MrMusAddict@reddit (OP)

I actually just watched that video, but it only reinforces the concept that I understand which is; the thrust through air makes the wheels irrelevant. But, I also understand that's only true in an idealistic frictionless world. There should be some amount of thrust from the plane, greater than 0 but less than 20mph, which would make the plane stationary relative to the world.

And then my interpretation from the myth is; would that plane take off since it's going take-off speed relative to the conveyor.

JimmerUK@reddit

The answer is no.

A plane needs airflow over the wings to take off, it has nothing to do with the speed of the wheels.

If there were fast winds blowing directly at the plane, then it could take off, but the conveyor belt is irrelevant as it could just be parked up normally and do it.

HighwayNovel@reddit

I didnt like that the "conveyor belt" they made wasnt really a conveyor belt. The wheels still had contact with the ground under the belt. Just me venting. Just watched this episode lol

Drive_Hound@reddit

I'm with you here. The experiment is supposed to be that the wheels are going the same speed as the conveyer. But they don't factor in the thrust which brings the wheels to a higher speed than the conveyor. I understand the wheels have nothing to do with a plane flying, But the purpose of this thought experiment is to see if a plane at liftoff thrust, can take off while staying in the same place. To simplify this, it's like taking the wheels off completely and setting a plane on it's belly and expecting it to tke off from the trust of its engine.