Wouldn’t have helped, other than the aside about the speed of light. Although having seen it, I can agree that it’s true, the argument based on the rotational acceleration of the wheels didn’t occur to me until after I had read (part of) the previous monster thread (I would give more specific credit here, but I can’t remember who first presented that argument). At which point Cecil was also aware of that argument, of course.
Can somebody help me with the terminology?
BF#1 and such, specifically. I’ve followed the threads lightly but I guess I was absent when the basic stances got abbreviations.
[quote=Cecil]
But the assumption is false. While the conveyor does exert some modest backward force on the plane, that force is easily overcome by the thrust of the engines pulling the plane ahead. The plane moves forward at roughly its usual speed relative to the ground and air, generates lift, and takes off. Many people have a hard time grasping this (although it can be easily demonstrated in the lab), but eventually they do, smack their foreheads, and move on. We’ll call this Basic Realization #1.
Message-board discussions of this question tend to feature a lot of posters who haven’t yet arrived at BR #1 talking right past those who have, insisting more and more loudly that the plane won’t take off. Then there’s a whole other breed of disputants who, whether or not they’ve cracked the riddle as originally posed, prefer to reframe it by proposing progressively more esoteric assumptions, refinements, analogies, etc. Often they arrive at a separate question entirely: Is there a way to set up the conveyor so that it overcomes the thrust of the engines and the plane remains stationary and doesn’t take off?
There is a disconnect here. And unfortunately Cecil responded about the planes speed. Not the wheel speed. Cecil actually mis-quoted the original question. I still believe that Cecil is correct in the interpretation of the question.
I personally have learned a lot in these threads. And on the SDMB in general. I hope we can keep it up.
[side note] I think I might have said BF#1 at some point. Ummm…. Brain Fart. Is what I was thinking of. Basic Realization is what is meant.[/side note]
I might be misunderstanding you, but if you lock up the wheels, there should be enough friction between the tire and pavement to keep the plane in place. Thrust-to-weight ratio is almost always lower than the tire friction coefficient (maximum friction force-to-weight ratio).
I think I remember someone explaining in one of the other threads that pilots really do this at the end of the runway before takeoff–spool up their engines with the brakes locked, I mean.
Right, at least with reciprocating engines. If the plane is stationary to start with the brakes will hold it still against full engine power.
I believe this is also true of jets. At least it was for the 707. On a takeoff from Frobisher Bay airport our SAS 707 went to the very end of the runway. Ran all engines up to full power and then released the brakes so al to make the takeoff run as short as possible. Even then it was just enough.
I take back everything I said then. Sorry.
Cecil wrote this in the column of 03-Feb-2006:
This is not true. The plane will not take off. As for interpretation of the question, I am interpreting it as shown in the picture in the above link. The airplane, when on the treadmill, will not be moving forward at all in relation to the real ground, just like when you’re on a treadmill at a health club. But it will be moving forward in relation to the treadmill, so as to not get thrown off the back of the treadmill.
Forget about wheel friction, or infinite speed conveyor belts and all that. The plane won’t fly not because of some arcane technicality like wheel ball bearings. It won’t fly because Cecil’s little insertion of “–and more importantly the air–” is wrong. Read his above full quote again. The problem with Cecil’s answer is the assumption that the plane has any speed in relation to the air when on a treadmill. It does not. If you’ve ever been on a treadmill, you know this is true.
It is right in that the plane is moving forward relative to the ‘ground’ – with the
‘ground’ being the treadmill. Whether the treadmill is just large enough to hold the plane, or is 500 miles long and 700 miles wide, the plane would be moving forward relative to it, yes.
But the plane is simply not moving forward in relation to the air, as Cecil claims. Again, going back to the health club treadmill: When you run faster in a treadmill, is there any more air rushing past you? No, there is still no air at all rushing past you because you are completely still in relation to the air, no matter how fast you are running in relation to the treadmill. The idea that the plane on a treadmill is moving forward relative to the air is wrong. Anybody who has been on a treadmill knows there is no increase or decrease in air blowing by. Because of this, the plane will not take off.
Outside on the airplane treadmill, the air will be still, or maybe there is a 5mph headwind or a 10 mph tailwind. And whether the plane is revving its jets or propellers enough to counteract a 50 mph or 500 mph treadmill so that the plane is stationary to the real ground, there is still only still air, or perhaps a 10 mph tailwind.
Without that air moving over the wings, there is no way to generate lift and the plane won’t take off. Cecil’s column seems to imply that it is the engines, whether jet or prop, ‘suck’ air over the wings, and this will cause the plane to take off. No. The engines provide forward thrust, and with enough forward movement through air, the pressure differential between the top and bottom of the wings is enough to generate lift and elevate the plane.
Here’s an easy experiment. You need a real bike, a treadmill, and an umbrella. First, grab the umbrella, and put your bike on the treadmill and get pedaling up to a good speed, say 10 mph. Now, open the umbrella over your shoulder, so it acts like a parachute. Note how it doesn’t make any difference at all. Now, take your bike and umbrella and go outside, and again pedal up to 10 mph. Open the umbrella as before.
That huge force that tears the umbrella out of your hand and causes you to crash? That’s the air. That’s the same force that would lift the airplane if it wasn’t on a treadmill and had no speed in relation to the air.
Now, if the question was assuming there was some super duper plane that had some super duper engine that could actually suck so much air toward it that it would be enough air to generate lift on the wings, then ignore all of this. But nobody’s said that is the case, and I don’t think it would be reasonable to think so.
Also, notice how the force generated to keep the runner / plane / bicycle stationary to the real gound while on a treadmill is irrelevant. Doesn’t matter if it’s a jet engine or your own legs or just a rope connecting you to the front of the treadmill. In all cases, there is no air. That little em-dash-enclosed snippet, “–and more importantly the air–” is false, which is why the plane won’t fly.
There’s nothing at all wrong with your interpretation–in fact, that’s the interpretation I find more interesting, myself.
But other people interpret the question as meaning that the speed of the plane with respect to the ground matches the speed of the treadmill with respect to the ground. That’s where Cecil gets his statement that "if the plane’s forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. " And there’s nothing wrong with that interpretation, either.
The things that you’re discussing–airspeed and so forth–are differences between the two interpretations. So it’s not that anyone thinks there’s wind on a treadmill, it’s that they’re interpreting the question differently than you are.
The latest explanation from Cecil for BR#2 was a mistake, I think. In stating this problem, I think that the mass and friction of the wheels should be totally ignored so there is only one answer.
I asked a few people this question a couple months ago and no one got it right initially except one person. This person is a pilot. He immediately realized that the speed of the wheels (and hence the conveyor belt) had nothing to do with the problem. He quipped something that provided additional insight from another aspect of this type of problem - “think about taking your plane off from an ice lake - the speed of the wheels doesn’t make a difference.”
turf
MonkeyMonkey.
You overlooked the fact that you can ride the bike faster than the treadmill is moving. In that senario (which is the one that many of us are talking about) the bike (plane) will be moving relative to the ground, air will flow over the wings and lift will be created.
"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?"
I think people are missing a few basic ideas here. Cecil’s answers were based on this specific question.
Now, let’s pretend that this magic treadmill is, say 1/4 mile long.
Next, we need to remember that the WHEELS on the airplane has breaks, but to motor to drive them forward. That’s why they are towed into hangars. They roll freely.
The treadmill is matching the speed of the airplane, NOT the wheels. (This is the part I think everyone isn’t getting. Reread the question.)
If the plane is moving forward at 80 miles per hour, the belt is moving back at 80 miles per hour, and the wheels (freely turning) is probably rolling at 160 miles per hour. Like ball bearings or the wheels of a skateboard. The only thing that would change this is friction.
The wheels are effectively negating the treadmill. If the wheels were motorized and were required to aid in the acceleration of the plane, then yes, it wouldn’t fly.
I can’t believe that I registered just to point this out. Cripes. :eek:
but no motor to drive them forward
whoops, fixed that.
Me either 
This has been explained at least a dozen times in many different scenarios in 3 different threads. The problem is new people come along, don’t read the threads and start making old assertions that have already been proven wrong. (I’m refering to the assertion against BR#1 that the plane would not take off)
Nice, except I said exactly the same thing in post #17 and it was roundly ignored by people too interested in not understanding each other.
Fair enough, you did too.
I think you were more circumspect in your discussion of the word “speed” though. And I was a lot more scathing about the people who arrivate at the “mile-long runway” interpretation!
Here’s a thing. Why does the illustration of this problem clearly show a very short treadmill…?
Harrier jets work work somewhat like helicopters, in the sense that the engines have defelector panels that direct the exhause downward, exerting (a la Newton’s law) a reverse-force upward, pulling the plane upward along with themselves. Years ago, the DoD experimented with Jet(and rocket) -Assistes Take Of (JATO/RATO) to give some of that Newtonian force to assist the normal runway-roll take-off, shortening the take-off run, not replacing it the way helicopters and VTOL airraft do.
Yes, the forward momentum is generated by the power plant (be it propellor or jet engine) not the wheels on the TARMAC. But, (and this is a big but) the wheels allow the aircraft to move forward, creating the lift forces due to movement of air over and under the wings. If something (i.e., the treadmill moving under the wheels, prevent the wheels from grabbing ground in front of the aircraft, the aircraft cannot move forward and no air will move over-and-under the wings and no lift will be created. This situation is different a comparable situation involving an air-boat (like those used in the Everglades) trying to move up a river flowing as fast as the big propellor could drive the boat forward. The boat could skip over the water, whether the river is moving or not. The standard aircraft cannot do that because the wheels have to grab the surface to allow the aircraft to move forward.
Two problems with the video:
- They aren’t moving the paper towels backward as fast as the skateboard is moving forward (I think you can see that the skateboard also moves forward faster after they run out of paper towel)
- the pwper towels (unlike the theoretical treadmill) are just a surface under the floor, upon which the skateboard is really moving) but even with this caveat, one can notice the skateboard hestitating in its forward motion before it can actually grab the underlying floor.
Nice to see someone really trying, though.
You would think a video like that would help ‘prove’ that Cecil isn’ t in the wrong here
[/QUOTE]
Two problems with the video:
- They aren’t moving the paper towels backward as fast as the skateboard is moving forward (I think you can see that the skateboard also moves forward faster after they run out of paper towel)
- the paper towels (unlike the theoretical treadmill) are just a surface under the floor, upon which the skateboard is really moving) but even with this caveat, one can notice the skateboard hesitating in its forward motion before it can actually grab the underlying floor.
Nice to see someone really trying, though.
You would think a video like that would help ‘prove’ that Cecil isn’ t in the wrong here
[/QUOTE]
The key difference is that a plane moving at an airspeed capable of generating sufficient lift to stay aloft is a big difference from one trying to get up to that speed.
Do you imagine that if the wheels-up plane were to lower its gear while flying low over a treadmill that it would be stopped in mid-flight?
[/QUOTE]
The subject wasn’t ‘could you generate enough friction on an aircraft to keep it from moving’.
The subject was would a treadmill affect an aircraft’s ability to fly.
Here is the large item that everyone is working around…
Air is full of stuff (gas molecules, particles, whatever you want to call it). That mass we call air is what an aircraft reacts against.
Engines don’t blow themselves through the air…they move the mass of air through the engine…essentially, pulling itself through the air (props act the same in this sense to jets).
It is also a mass of air that flows over/under the wings and creates lift…as the aircraft pulls itself through.
In order for you to negate the thrust (other than bolting the acft to the runway) would be to create a large TAILWIND. This would cause the air molecules to not pass of the wings, and therefore…the aircraft would not lift off.
All aircraft takeoff calculations are based on wind and air density (temperature, pressure altitude). The more the headwind/denser air the more thrust the aircraft can produce/lift occurs at lower temperature/aircraft takes off at lower speed less distance.
If you make a runway slippery, all it does is increase the stopping distance…it does nothing to the takeoff distance. If you make a runway uphill, then part of the thrust from the engines is being used to climb the hill, so it takes longer to takeoff (less relative thrust).
So yes the aircraft will takeoff. If you want to increase the amount of rolling resistance to an infinite amount then you can stop the aircraft from accelerating…but it has nothing to do with the treadmill.
Now if you locked the brakes on the aircraft, then put the giant treadmill on reverse…then released the brakes…you would significantly shorten the takeoff roll (the navy calls it a catapult)
So the issue is the mass of the air…not how fast the tires are spinning…besides…the tires would disintegrate around 250 knots anyway.