Mythbusters vs. The Moon

[Lindley]

That's true, but irrelevant to the case of a moving treadmill. Friction is obviously important, which is exactly why planes have wheels, but rolling friction is such a minimal component of the hindrance to lift-off. Although rolling friction may increase as wheel rotation speed increases, it will still be a very minimal backward force compared to things like wind resistance.

No, it won't, actually, and that's the entire point. Rolling friction is constant regardless of speed. And a small constant at that. That would, in fact, be the reason why wheels are so awesome.

A sea plane first gets up on the step before it can take off. It has to hydroplane on the surface of the water first. Until it gets up out of the water it can't take off.

Technically it doesn't; it's merely that due to the properties of air and water, lift has an easier time raising the plane to hydrofoil than lifting it out of the water entirely. Same principal, just different critical speeds. If the plane managed to reach the liftoff speed before it was fully out the water it would take off immediately, but that doesn't happen in a practical situtation.

 

[Lindley]

Come on! Not drag in an aerodynamic sense, drag as in too much resistance to forward motion.

The technical term is friction. Static friction before the plane starts moving, kinetic friction once it starts making headway.

Personally, I find the whole debate about the ability of a plane to take off without wheels is rather beside the point. Just say the plane has a magical frictionless underbelly if you need to, in order to make the point.

 

[Carpe Occasio]

Come on! Not drag in an aerodynamic sense, drag as in too much resistance to forward motion.

The technical term is friction. Static friction before the plane starts moving, kinetic friction once it starts making headway.

Personally, I find the whole debate about the ability of a plane to take off without wheels is rather beside the point. Just say the plane has a magical frictionless underbelly if you need to, in order to make the point.

The point is a plane rests on a surface before it takes off. That surface can impact it's ability to reach takeoff speed. A piece of fabric lying on the ground however won't have enough impact to prevent takeoff, regardless of whether it's moving in the opposite direction. I've flown Cessna's and any flight instructor will tell you the same thing.

 

[Daedalus12]

The Wright Brothers plane took off from rails. It wasn't lying on the ground. A Cessna lying on the ground will develop too much drag.

1. There is no such thing as ground drag. Drag is a technical term which refers to only fluid resistance.

2. Wright Brothers didn't have engines available to them that can produce 100,000 pounds of thrust.

Come on! Not drag in an aerodynamic sense, drag as in too much resistance to forward motion. And what does 10,000 pound thrust engines have to do with anything? The Wright Brothers didn't take off from the ground. The plane moved along rails to expedite achieving takeoff speed.

Drag by definition is of the aerodynamic sense.

If you have an engine that produces 100,000 pound of thrust and can be attached to the Cessna then it'll definitely take off with no gears. Assuming a 20,000 pound engine and a 3000 pound Cessna then the sliding friction force generated by the Cessena fuselage sliding along the concrete runaway would be at most around 18,000 lb. This leaves about 80,000 lb of net thrust left which is ample to get the plane to takeoff speed.

 

[Christopher]

All this quibbling over technical details of the surface is missing the point of the original myth. The fundamental question is, would a backward-moving treadmill cancel out the forward thrust of a plane's propellers? The ability of the surface to hold up the plane is irrelevant to the core question, which is about whether a forward force can be cancelled by a backward force. And the fundamental flaw that makes it a myth is the assumption that the treadmill can generate a force that will counter the plane's thrust. That's a false assumption because the plane's thrust has nothing to do with the ground, only with the air. All this hairsplitting over friction with the treadmill and melting the wheels and so on is an attempt to shift the goalposts, to redefine the original premise in order to avoid admitting that it's been falsified. The original, pure concept behind the question is about the fundamentals of thrust versus counterthrust, not the minutiae of extreme permutations of the premise like burning the wheels off with a superfast treadmill.

The one flaw in the Mythbusters' test is that, although they busted the conditions of the myth, they failed to replicate the results, which is supposed to be an essential component of every Mythbusters experiment. After proving that a treadmill under the wheels could not counter the forward thrust of a plane's engines, they should've investigated what it would take to counter that thrust. And the logical answer is, a wind tunnel. Since a plane's thrust is purely a thing of the air, having nothing to do with the surface the plane is resting on, the one thing that actually would achieve the desired cancelling effect is an airflow in the opposite direction. Maybe if the Mythbusters had demonstrated that, if they'd put a plane in a wind tunnel and showed that airflow, not a treadmill, was what you needed to counter a plane's thrust, it would've cleared up some of the confusion.

 

[Lindley]

The point is a plane rests on a surface before it takes off. That surface can impact it's ability to reach takeoff speed.

Yes, it certainly can.

A piece of fabric lying on the ground however won't have enough impact to prevent takeoff, regardless of whether it's moving in the opposite direction.

Absolutely. The key thing is that even if the entire runway is moving in the opposite direction, it won't affect its ability to reach speed----only the friction properties of the runway affect that, not its movement (if any)! Rolling friction is constant regardless of speed, so the runway would have to be sufficiently bumpy for kinetic friction to become a dominant term before runway movement would have any appreciable effect.

Of course, if the plane is moving backwards at the *start*, it'll take longer to get off the ground; same effect as if there were a tailwind. But the myth in question specifies a stationary start, with acceleration of the treadmill proceeding from there, so that's not a problem.

I've flown Cessna's and any flight instructor will tell you the same thing.

So have I.

 

[Carpe Occasio]

1. There is no such thing as ground drag. Drag is a technical term which refers to only fluid resistance.

2. Wright Brothers didn't have engines available to them that can produce 100,000 pounds of thrust.

Come on! Not drag in an aerodynamic sense, drag as in too much resistance to forward motion. And what does 10,000 pound thrust engines have to do with anything? The Wright Brothers didn't take off from the ground. The plane moved along rails to expedite achieving takeoff speed.

Drag by definition is of the aerodynamic sense.

If you have an engine that produces 100,000 pound of thrust and can be attached to the Cessna then it'll definitely take off with no gears. Assuming a 20,000 pound engine and a 3000 pound Cessna then the sliding friction force generated by the Cessena fuselage sliding along the concrete runaway would be at most around 18,000 lb. This leaves about 80,000 lb of net thrust left which is ample to get the plane to takeoff speed.

But let's be realistic. You're not going to strap engines like that onto a Cessna 172.

The point is a plane rests on a surface before it takes off. That surface can impact it's ability to reach takeoff speed.

Yes, it certainly can.

A piece of fabric lying on the ground however won't have enough impact to prevent takeoff, regardless of whether it's moving in the opposite direction.

Absolutely. The key thing is that even if the entire runway is moving in the opposite direction, it won't affect its ability to reach speed----only the friction properties of the runway affect that, not its movement (if any)! Rolling friction is constant regardless of speed, so the runway would have to be sufficiently bumpy for kinetic friction to become a dominant term before runway movement would have any appreciable effect.

Of course, if the plane is moving backwards at the *start*, it'll take longer to get off the ground; same effect as if there were a tailwind. But the myth in question specifies a stationary start, with acceleration of the treadmill proceeding from there, so that's not a problem.

I've flown Cessna's and any flight instructor will tell you the same thing.

So have I.

So we agree.

 

[Lindley]

I suppose. I'm just not sure why you brought up the thinness of the material in the first place, since it has no effect on the outcome.

 

[Carpe Occasio]

I suppose. I'm just not sure why you brought up the thinness of the material in the first place, since it has no effect on the outcome.

I wasn't focusing on that. I was just saying that it wasn't really a treadmill. And regardless, even if they built a real, giant treadmill, it wouldn't have changed the results.

 

[Daedalus12]

But let's be realistic. You're not going to strap engines like that onto a Cessna 172.

It was a thought experiment only. Obviously 80,000 lb of thrust would most likely rip the wings off the Cessna 172.

 

[Carpe Occasio]

But let's be realistic. You're not going to strap engines like that onto a Cessna 172.

It was a thought experiment only.

Fair enough.

 

[RobertVA]

But let's be realistic. You're not going to strap engines like that onto a Cessna 172.

It was a thought experiment only. Obviously 80,000 lb of thrust would most likely rip the wings off the Cessna 172.

If you attached an engine with a fraction of that much thrust to a Cessna 172 it could fly without wings! And I'm having fond memories of GE's commercial that featured a powered up 1903 Wright flyer!

 

[Meredith]

I think people are still arguing about the airplane on a conveyor belt myth that reran last night. Although seeing how some past discussions went, both subjects might be better debated in the Neutral Zone.

I think they should have tethered a plane with a cable to keep it stationary and then see if it is held in place if the engines would be powerful enough to pick it up off of the ground.

"Keeping it stationary" is the "expected" end result of the treadmill anyways.


Treadmills work only for things that push against the ground, like a person walking or a car that get's it's thrust from the apply energy to the ground. Unless the airplane has a car-like drivetrain to help it build up takeoff speed then a treadmill is going to do diddly squat to stop an airplane. But Meredith, planes push air against the ground to achieve lift! No, they don't they create lift by moving air faster over the tops of the wings than the bottom of the wing. Forward thrust is provided by the intertia imparted by the propeller pushing the air itself not what the air ends up hitting once it has passed the propeller.

You would need a "treadmill for a plane" which would be a huge fan mounted on the runway behind the plane blowing air from the back to the front of the plane.

Apples vs. Oranges.

Ground Locomotion vs. Air Flow Locomotion.

 

[Q2UnME]

Hmmm. I really thought the whole purpose of Mythbusters was an excuse for Adam and Jamie to blow shit up....

Q2UnME

 

[Trekker4747]

I think they should have tethered a plane with a cable to keep it stationary and then see if it is held in place if the engines would be powerful enough to pick it up off of the ground.

That's not the essence of the question!

OF COURSE if you tether the plane to a wall to "keep it stationary" the plane won't go in the air anymore than it would sitting on the runway on a calm day! The plane needs airflow over the wings to obtain lift. If it's standing still, no airlfow over the wings. No airflow over the wings, no lift!

"Keeping it stationary" is the "expected" end result of the treadmill anyways.

It's "expected" only because of how people want to read into the wording of the myth. The myth is, often, worded in such a way to lead you down a path of false conclusiions. It's like a trick question, a trick "thought experiment" - if you will.

It's like if I asked you what the six sense are, I'm asking you that to "fool" you into thinking there's six senses -as opposed to five. Someone doesn't have to come-up with a sixth sense to "prove" to you there's only five.

There's nothing, NOTHING in Heaven or Earth that can make a treadmill keep a plane stationary. Positively nothing. Short of moving so fantasticly fast that it eventualy destroys the plane's gear and the plane itself before it can take-off and that's not quite the same thing.

The only thing that could be done is to make an "air treadmill" -as Christopher suggested- to counteract the way the plane moves forward and I'm scratching my head trying to figure out how even that would work and may require me to invest more thought in it.

And, Chirstopher, that was a good point on bringing up they didn't "duplicate the results." I suspect they probably figured coming up with something that could do it was beyond the budget/capabilities of themselves and the show or they just didn't have time. -Result revisit like the JATO-Chevy?



(Which they still need to re-visit thanks to the failure of the rockets they bought.)

 

[jkladis]

The only thing that could be done is to make an "air treadmill" -as Christopher suggested- to counteract the way the plane moves forward and I'm scratching my head trying to figure out how even that would work and may require me to invest more thought in it.

It exists and has a name. Relative wind.

Same concept, exact same problem with friction, only it's against the skin of the airframe. This is an argument that can not be won absolutely, I wish everyone would see that side.

If it were not for friction, the airplane would never move backwards to begin with. You can't ignore a basic condition of the experiment.

 

[Small White Car]

The myth is, often, worded in such a way to lead you down a path of false conclusiions. It's like a trick question, a trick "thought experiment" - if you will.

Which is what I've always said which is why I STILL fail to see why the Mythbusters did it.

"Oh, it's a trick question with no answer that can't be tested. Well, let's test something that sounds similar." What's the point? At least with the moon-thing everyone can agree on what's being tested.

 

[Trekker4747]

Which is what I've always said which is why I STILL fail to see why the Mythbusters did it.

Well they did it to show people in practicality how it'd work.

For being an epic question that rips internet forums in half they didn't "Mythbuster it up" enough. A tarp dragging on the ground against an ultralite doesn't this experiment make, IMHO but I guess they couldn't necessairly build a working 3/4-mile long treadmill runway and plop a 727 on it.

But for some people they need to see something. And when you ask most "normal" people this question (ask your mother or grandmother) and they'll likely initially think the plane won't go but then just explain to them the plane pushes off the air and use an verbal example of them on rollerskates on the treadmill with a rope tied to a wall.

Some people, though, may just need to see it.

And in the "true" wording of the "myth"/thought experiment it's just asking if the treadmill can prevent the plane from moving forward. Not that the treadmill IS preventing the plane from moving forward -can it take off?

The latter cannot happen and cannot be tested on this planet -possibly in this universe.

 

[PlixTixiplik]

The only thing that could be done is to make an "air treadmill" -as Christopher suggested- to counteract the way the plane moves forward and I'm scratching my head trying to figure out how even that would work and may require me to invest more thought in it.

It exists and has a name. Relative wind.

Same concept, exact same problem with friction, only it's against the skin of the airframe. This is an argument that can not be won absolutely, I wish everyone would see that side.

Well, it's actually completely different as a headwind or tailwind have nothing to do with friction and everything to do with airspeed over the wing. In fact, a strong headwind will shorten takeoff distance and make it easier to lift off. So yes, you could give an airplane a fast enough tailwind that it could not take off (tailwind on the order of 700-800 km/h; of course weaker tailwinds will still increase lift-off distance) but it's not friction that's preventing takeoff.

If it were not for friction, the airplane would never move backwards to begin with. You can't ignore a basic condition of the experiment.

No one is ignoring friction, it's just that rolling friction is so miniscule that it could not possibly stop the plane from moving forward. As long as the wheels are able to turn, the treadmill is irrelevant.

-MEC

 

[Small White Car]

Which is what I've always said which is why I STILL fail to see why the Mythbusters did it.

Well they did it to show people in practicality how it'd work.

Remember this one:
If a rooster is on the top of a roof and lays and egg, which side will it roll down?
A: Roosters don't lay eggs!

So what would they do there? Try to make a Rooster lay an egg and bust the myth when they can't? Or say "it has to be a chicken to get an egg" and put a chicken up there and declare that "left side of the roof" is the answer?

I find both of those scenarios as equally pointless as the airplane thing.

I prefer that they stick to real questions, not trick-questions. The moon one, as silly as it seems, is good hard science. Why DON'T stars show up in the photos? That's something real and demonstrable. I'm looking forward to this one.