Airplane on a Conveyor - Post-Mythbusters-Mortem

[KennyFlys]

Anyway, I think part of the problem was the manner in which this was presented. Couple other folks seem to agree.

It seems that way.

Watch out for the lightning!

 

[Greg Bockelman]

My response is the same as to Chuck. I don't know how I can explain my opposing view any better than I have.

You're going to have to try, Ken because nobody knows what the hell you are talking about.

For example, the throttle advances only to the point that equals the resulting speed equivalent to the conveyor. If a fixed prop is determined to effect 25 MPH with 2000RPM then that should be a calibrated setting. Then, calibrate the conveyor to be at a speed of 25MPH.

What result do you think this gives? I am trying to understand but if you don't TRY to explain it there is no way I can. In the real world, what is going to happen given your statement above, is that the conveyor belt will move one direction at 25 MPH reference the ground and the airplane will go the opposite direction at 25 MPH reference the ground. If the takeoff speed is less than 25 MPH, the airplane WILL fly.

 

[KennyFlys]

Reading the post mortem comments has led me to the conclusion that there is a basic disagreement of what the question is to begin with. And it's really hard to agree on an answer when you can't agree on the question.

That's part of my complaint. The premise seems to have changed (I've explained that; more than once). Hence, my issue of no calibrated power application or speed monitoring nor an overall view of the entire experiment.

 

[KennyFlys]

You're going to have to try, Ken because nobody knows what the hell you are talking about.

What result do you think this gives? I am trying to understand but if you don't TRY to explain it there is no way I can. In the real world, what is going to happen given your statement above, is that the conveyor belt will move one direction at 25 MPH reference the ground and the airplane will go the opposite direction at 25 MPH reference the ground. If the takeoff speed is less than 25 MPH, the airplane WILL fly.

Ok, help me what you don't understand about the following... I'm sincerely at a loss to explain this better than I have...

For example, the throttle advances only to the point that equals the resulting speed equivalent to the conveyor. If a fixed prop is determined to effect 25 MPH with 2000RPM then that should be a calibrated setting. Then, calibrate the conveyor to be at a speed of 25MPH.

All I can add is these settings are arrived at through calibrated and verified means.

 

[Ken Ibold]

For example, the throttle advances only to the point that equals the resulting speed equivalent to the conveyor. If a fixed prop is determined to effect 25 MPH with 2000RPM then that should be a calibrated setting. Then, calibrate the conveyor to be at a speed of 25MPH.

Here's where your problem lies. The "resulting speed" will NEVER be equivalent to the conveyor. They could be calibrated to have the same number and even the same units -- your 25 mph -- but they are not equivalent because the conveyor power setting that yields 25 is measuring groundspeed and the airplane power setting that yields 25 is measuring airspeed. Apples and oranges. Only the wheels will feel the difference; they will rotate at 50 mph.

 

[Greg Bockelman]

Ok, help me what you don't understand about the following... I'm sincerely at a loss to explain this better than I have...


All I can add is these settings are arrived at through calibrated and verified means.

OK, So 2000 RPM gives you 25 MPH. Set the treadmill to 25 MPH. What do YOU think is going to happen? THAT is what remains unclear to everyone.

 

[TMetzinger]

My own view of the experiment is that I was disappointed. I thought the "myth" involved an airplane with wheels turning as fast as but no faster than a treadmill heading in the opposite direction at or above takeoff speed - a scenario where, at least in one version of the theory, you have an airplane incapable of developing sufficient air speed for takeoff..

Regardless of the actual velocity, when a wheel is in contact with a treadmill, you always have "wheels turning as fast as but no faster than a treadmill heading in the opposite direction".

If they wanted to, they could have had the airplane chasing AFTER the truck, and had the airplane wheels turning in REVERSE during the takeoff roll, as long as the truck pulled the belt faster than the airplane's liftoff speed. Hell, they should have done that and it might have FINALLY gotten the point across that the wheels have NOTHING to do with the airplane taking off, except to reduce rolling friction.

 

[KennyFlys]

Here's where your problem lies. The "resulting speed" will NEVER be equivalent to the conveyor. They could be calibrated to have the same number and even the same units -- your 25 mph -- but they are not equivalent because the conveyor power setting that yields 25 is measuring groundspeed and the airplane power setting that yields 25 is measuring airspeed. Apples and oranges. Only the wheels will feel the difference; they will rotate at 50 mph.

Huh? That bold line confuses me.

I understand one is "groundspeed" and the other is "airspeed." No argument there.

But, let us try a slightly different comparison...

Let's say you apply the required power (calibrated) for reaching that 25MPH rotation speed... BUT, you're on a conventional runway. Obviously, like any other takeoff roll accelerating to Vr, you're going to lift off.

Now, that same runway is moving backward at the same speed as Vr. It's still gonna fly with the same power setting? I still have my doubts.

 

[KennyFlys]

OK, So 2000 RPM gives you 25 MPH. Set the treadmill to 25 MPH. What do YOU think is going to happen? THAT is what remains unclear to everyone.

See my later post for another view... but, if the conveyor speed is identical to Vr, it will not lift off.

 

[Greg Bockelman]

See my later post for another view... but, if the conveyor speed is identical to Vr, it will not lift off.

Ken, the propeller is pulling the airplane through the AIR. If you set your hypothetical RPM to 2000 to give you the 25 MPH as you stated in your example, the airplane will move in that direction relative to the ground and the conveyor belt will move the opposite direction at 25 MPH. The wheels will be spinning the combined rate of 50 MPH.

You seem to think that the conveyor belt is capable of holding the airplane in place. It is not. The airplane will accelerate. If you don't believe that, then we are done.

 

[KennyFlys]

You seem to think that the conveyor belt is capable of holding the airplane in place. It is not. The airplane will accelerate. If you don't believe that, then we are done.

I have a hard time believing it with the "experiment" that was conducted on top of a changed premise. Sorry.

 

[Greg Bockelman]

I have a hard time believing it with the "experiment" that was conducted on top of a changed premise. Sorry.

Exactly WHAT premise was changed?

 

[stingray]

Huh? That bold line confuses me.

I understand one is "groundspeed" and the other is "airspeed." No argument there.

But, let us try a slightly different comparison...

Let's say you apply the required power (calibrated) for reaching that 25MPH rotation speed... BUT, you're on a conventional runway. Obviously, like any other takeoff roll accelerating to Vr, you're going to lift off.

Now, that same runway is moving backward at the same speed as Vr. It's still gonna fly with the same power setting? I still have my doubts.

Why do you have doubts Ken it happened last night. The runway (tarp) was going backwards the same speed or greater than Vr. The pilot said and I quote " I thought it was funny because it took off just as easily as it did on the runway."

I have saids all along Ken that it is easier to see the answer when you put the plane in motion, same as you would each time you take off in real life. Now use any means you like to show that the belt stops that plane in any way from moving forward. Just look at it that far not whether it takes off or not, but what force can the belt apply to the plane to stop it's forward motion. Don't worry about what speed its going or where you are measuring it from, just what force can stop the plane.

Dan

 

[stingray]

Exactly WHAT premise was changed?

If you go to the Red board there is a link there that goes to a blog of some sort. It reads there that the belt matches the speed of the tires. It is coupled to the mythbusters, but the Mythbusters got it right matches the speed of the plane.

Kinda like 10 people reading something then explaining it to someone else. They add things that are not in the original quote.

Dan

 

[KennyFlys]

Exactly WHAT premise was changed?

I give up. It has been stated before in this thread and in at least one other previous thread.

 

[Greg Bockelman]

I give up. It has been stated before in this thread and in at least one other previous thread.

I didn't see it. If you don't want to reference it, so be it.

And you are still wrong. At least I have gone out of my way to try to prove it. You haven't done much to prove you are right except to mention things that no one understands. Then you get all bent when people try to understand.

 

[KennyFlys]

I didn't see it. If you don't want to reference it, so be it.

And you are still wrong. At least I have gone out of my way to try to prove it. You haven't done much to prove you are right except to mention things that no one understands. Then you get all bent when people try to understand.

I'm not bent over anything. I don't believe it was scientifically proved with clearly calibrated controls. I honestly do not know what else I can add to make my position any clearer.

 

[Greg Bockelman]

I'm not bent over anything. I don't believe it was scientifically proved with clearly calibrated controls. I honestly do not know what else I can add to make my position any clearer.

Whatever. I'm outta here.

 

[Greg Bockelman]

Sorry about severely hijacking the thread, Chuck. Didn't intend for it to go this far.

 

[Greebo]

Guys, I did understand this, but didn't understand the manner in which this was presented.

The plane accelerates as normal. If one was standing on the ground next to the treadmill observing, the plane would accelerate as if the treadmill wasn't there while the wheels spin the other way.

What screwed me up was the Myth Busters folks seems to say to me, the plane would stay still relative to the ground because the treadmill was going the other way.

No, they did NOT say that. They said thats what happens with *cars*. With planes, by using the model car, they clearly showed that when you disconnect the motor from the wheels, the car can be held still relative to the ground with almost no force, and to push it forward on the treadmill requires only a tiny fraction more.

So, part of the reason I wasn't on board with this thing in my mind was the way I perceived it was being explained. Perhaps I fixated on the wrong point. But I didn't think they showed the plane would stay still relative to the ground; and they didn't. It accelerated normally.

It is physically impossible to build a treadmill capable of moving so fast that it would cause a plane with free spinning wheels operating normally to be unable to move forward by normal use of its propeller.

Anyway, I think part of the problem was the manner in which this was presented. Couple other folks seem to agree.

The presentation was the only presentation that could have been done. No other interpretation of the question is testable thanks to the laws of motion and energy.

I don't know how to explain it any better than I have.

I'm asking you to be patient and repeat yourself from the beginning.

Yes, an external rope (static hold) verus internal power limited to within the confines of the conveyor.

Ah, I think I see.

Ok - here's the disconnect, Ken: The airplane is not connected to the conveyor.

It never was and it never will be.

The airplane moves using forces completely disconnected from the conveyor. The rope is air, the propeller is the arms pulling the rope, and the conveyor belt is functionally irrelevant.

I have a hard time believing it with the "experiment" that was conducted on top of a changed premise. Sorry.

See no, Ken, it wasn't.

The premise you subscribe to defies the laws of physics. It is impossible to create and impossible to test. The only way a conveyor belt could stop a plane with free spinning wheels from moving would be if that conveyor belt could move at hundreds or even more probably thousands of miles per hour (or more) - thats the ONLY way you could build up enough drag in the wheels to slow the plane significantly and counter the forward thrust of the propeller.

Problem being that if the wheels spin that fast, the bearings will burst into flames from heat resistance.

I'm not bent over anything. I don't believe it was scientifically proved with clearly calibrated controls. I honestly do not know what else I can add to make my position any clearer.

If you believe the premise is that the airplane will not be able to take off using the exact same power setting, you're still mistaken. If you first read THAT question, then you read a bad question. Why?

Because airplanes take off at full power. ALWAYS. Full throttle. You don't set the plane for X rpm and take off, you set full power. You use the full capacity of the engine, every single time.

At full power, the airplane used by Mythbusters took off at 25mph relative to the ground. That measurement was also irrelevant. It is airspeed that matters.

At full power, on a conveyor belt moving 500 miles per hour, the airplane will still achieve necessary AIR SPEED to lift off.

Period.

---- Moderator note --- I merged my 6 replies into one reply because I was reading, replying, then moving on. ANd it made me look like I was a spammer. (Abuse of power is so fun...)

 

[alaskaflyer]

The propeller is pulling the plane through this pressurized goop we call air. The treadmill spinning the other direction simply makes the plane's wheels turn faster, but so long as they don't fall off, the propeller is still going to pull the plane through the air in the correct direction, until it reaches takeoff speed and lifts off.

 

[KennyFlys]

Can't anyone just accept me as being wrong in their eyes while I'm simply not yet convinced I am wrong? Goodgriefgollygeewhiz!

 

[midlifeflyer]

Regardless of the actual velocity, when a wheel is in contact with a treadmill, you always have "wheels turning as fast as but no faster than a treadmill heading in the opposite direction".

I can't see why that would be true. This is not a free-wheeling treadmill, it's one set to operate at specific speed. Are you saying that a treadmill going 25 MPH with a car with an independent source of power going 50 MPH and moving on it on it is an impossibility?

 

[TMetzinger]

I can't see why that would be true. This is not a free-wheeling treadmill, it's one set to operate at specific speed. Are you saying that a treadmill going 25 MPH with a car with an independent source of power going 50 MPH and moving on it on it is an impossibility?

Sorry, I should have been clearer - when only one of the two surfaces is under power, they are always equal and opposite.

But when both surfaces are under power at different speeds, having no relative motion is an impossibility.

 

[jason]

I'm not bent over anything. I don't believe it was scientifically proved with clearly calibrated controls. I honestly do not know what else I can add to make my position any clearer.

Ken, I think the reason that we're struggling with this line of thought is that, if you understand the logic behind the results, there is no point in repeating the experiment with more calibrated controls. The physics would play out regardless of the level of calibration or the conditions.

Can't anyone just accept me as being wrong in their eyes while I'm simply not yet convinced I am wrong? Goodgriefgollygeewhiz!

That's the thing. I can. I can accept two people that just don't see eye to eye on a particular topic. What I do struggle with, however, is the fact that anybody can have dozens of his friends telling him that he's not looking at something the right way and that person can't sit back and say "ya know what, I'm talking to some really sharp people here...I can't be the only one right about this. Perhaps I need to revisit my stance." That's not to say that you're not sharp as well. I just can't comprehend a the thought process that would allow me to believe myself correct in the face of such overwhelming opposition and reasoned arguments...especially if I considered those debating with me to be my friends.

The frustrating part is that you're not working through it step by step with us. You're taking part in the discussion on your terms. When somebody tells you why your logic is faulty, you walk away from that part of the discussion.

This isn't meant as an attack, Ken. I'm just trying to explain to you why others are frustrated by your stance on this topic.

 

[KennyFlys]

This isn't meant as an attack, Ken. I'm just trying to explain to you why others are frustrated by your stance on this topic.

I'm not taking offense by anything said. There's no reason for me to so. I've got plenty of things up the line for me to rattle my brains and heart on before I get there over this issue.

I've always said it's plausible. I'm just not buying it at this time with the existing Mythbusters "experiment" done. I've previously indicated several reasons why. It probably never will be done with the kind of calibrated controls I'd like to see unless someone manages to get a couple hundred million out of the feds (US Taxpayers) for "aeronautical research."

But, life goes on and I have this weekend to be more concerned with.

 

[stingray]

I've always said it's plausible. I'm just not buying it at this time with the existing Mythbusters "experiment" done. I've previously indicated several reasons why. It probably never will be done with the kind of calibrated controls I'd like to see unless someone manages to get a couple hundred million out of the feds (US Taxpayers) for "aeronautical research."

Ken here is an experiment that you can try at home with little funds. No aeronautical research needed.

You will need two treadmills and a rope along with a pad the same height as the treadmills.

Raise up your plane and put the two treadmills under the main wheels facing backwards. Put the pad under the nose wheel so the plane is level.

Tie down the tail with the rope. use around 10 feet.

Start up the plane and use just enough throttle to make the rope straight but not tight. Now turn on the treadmills slow at first then faster. Watch the rope. Does it stay straight or does it droop? There is your answer, as you will see the belt cannot put force on the plane to make it go backwards (The rope would droop). You can turn the treadmills up to 70 mph and the plane will just sit there with the rope straight.

Now all you have to do is understand what happens when you give it more throttle, Zoom Zoom.

Dan

 

[KennyFlys]

Ken here is an experiment that you can try at home with little funds. No aeronautical research needed.

You will need two treadmills and a rope along with a pad the same height as the treadmills.

Raise up your plane and put the two treadmills under the main wheels facing backwards. Put the pad under the nose wheel so the plane is level.

Tie down the tail with the rope. use around 10 feet.

Start up the plane and use just enough throttle to make the rope straight but not tight. Now turn on the treadmills slow at first then faster. Watch the rope. Does it stay straight or does it droop? There is your answer, as you will see the belt cannot put force on the plane to make it go backwards (The rope would droop). You can turn the treadmills up to 70 mph and the plane will just sit there with the rope straight.

Now all you have to do is understand what happens when you give it more throttle, Zoom Zoom.

Dan

Dude, I don't have one treadmill, let alone two. But, it's ok. I'm all "conveyored" out.

 

[mattaxelrod]

OMG I can't believe I'm wading into the treadmill discussion. But I read a couple explanations elsewhere and they made so such sense I just had to share.

One way to think about it is pretending you're on roller skates facing the other way on the treadmill. To keep you from going backwards, someone has to apply just a little bit of force to your back to keep you in place. Like even a finger would do it. To get you to move forward, all they have to do is push just a little bit. (That has already been sort of explained with the truck and the rope I think).

OK, how about this--you have a plane in flight in the air. Imagine taking a treadmill and levitating it up in the air until the moving belt makes contact with the plane's wheels while it's flying. The plane doesn't stop flying! Just the belt and the wheels are moving--having nothing at all to do with airspeed.

 

[midlifeflyer]

Ken, I think the reason that we're struggling with this line of thought is that, if you understand the logic behind the results, there is no point in repeating the experiment with more calibrated controls. The physics would play out regardless of the level of calibration or the conditions.

If you understand the logic behind the results, the empirical experiment wasn't needed to begin with.

X: A is true.
Y: I don't agree with you.
X: If you understood the underlying principles you would agree. But I'll prove it empirically.
[runs experiment]
X: See what I said would happen happened.
Y: I don't agree that you set up the experiment properly. You didn't account for (a), (b),and (c).
X: Those don't matter. If you understood the underlying principles you would agree.

I'm not agreeing with Kenny, but I think that you are entering into a bit of a tautology.

 

[Henning]

Ok, Mythbusters has done the test. They put a plane on a makeshift conveyor and got the conveyor going faster than the plane's takeoff speed, in the opposite direction. The plane flew, without a hiccup.

So - now that you've seen it for yourself, what is your position?

I can't believe you started this thread, did you take your sado-masochist pills this morning or something?

 

[Greebo]

I can't believe you started this thread, did you take your sado-masochist pills this morning or something?

What kind of silly question IS that?

I'm a server admin!

I don't need to take pills, I'm already a masochist!

 

[ReverendSlappy]

Though I've never chimed in on the topic before, I'm squarely in the #4 (and #1, obviously) camp. But it's obvious this whole debate* reminds me of something my Mom would say to me as a kid when I'd say I didn't believe something that was true: "It doesn't matter if you 'believe' it or not. It's a fact." "Belief" or "convincing" or anything else that's similarly subjective is irrelevant with respect to the factuality of something; often it's laughable.

*Note: I shouldn't really use the word "debate" here, as there's only one side of the issue that is -- unequivocally, demonstrably, and utterly infallibly -- right.

 

[Dave Siciliano]

Bet your Mom never ran for President (duckin and runnin)!!

Best,

Dave

 

[flyingcheesehead]

Kenny,

I have a couple of different scenarios that I just thought of, no treadmill/conveyor required.

1) Say you have an airplane that will take off at 50KIAS, and cruises well above 100KIAS. You get on a big runway, trim the plane for 100KIAS so that you can keep it on the ground until it reaches 100KIAS. (Assume zero wind.) Will it take off?

2) Same airplane, but this time you get to the runway threshold and the weather changes, so you now have a 40-knot headwind right down the runway. You add enough power to keep the plane from being blown backwards and release the brakes, and simply use the power of the engine to keep the plane in a stationary spot on the ground. The wind increases to 45 knots, so you have to add just a bit of power again to keep the plane from being blown backwards. You continue to keep this equilibrium intact. The wind increases to 50 knots. What happens?

3) Now, you go out in the same 50-knot winds again (you crazy bastard) but this time you go to the opposite end of the runway and add full power. When you reach 50 KIAS, you pull back. What happens? When your airspeed reaches 50 KIAS, what is your groundspeed(/tire speed)?

Now, explain why the treadmill, in your mind, is different than scenario #3.

 

[KennyFlys]

Kenny,

<snip>

Now, explain why the treadmill, in your mind, is different than scenario #3.

I can't. I've moved on to simpler things like Chuck's rental property investment.

 

[Ghery]

I want option one AND option four!

I'll vote for that!

 

[ReverendSlappy]

Bet your Mom never ran for President (duckin and runnin)!!

Best,

Dave

No, thank God. If she had, I'm sure I'd be even more messed up than I am now, without that.

 

[Missa]

Hell, they should have done that and it might have FINALLY gotten the point across that the wheels have NOTHING to do with the airplane taking off, except to reduce rolling friction.

Technically.... Wheels don't reduce rolling friction. They reduce friction to the rolling friction of the wheels instead of the sliding friction seen by early aircraft with just skids. <Dam Enginners>

 

[bluesky74656]

Best analysis of the problem I've seen in a long time (ever?):

Airplane on a Treadmill Explanation