No, it doesn't. I'm sorry you've typed as much as you did, I'm sorry that it was all for nothing, I'm sorry that something is blocking your ability to think rationally Come on dude ... so the original topic didn't "word" itself, from a physics standpoint, absolutely correct. If you'd just read the thread like eveyone else did, you'd have found out in page 2 the original "joke" wasn't worded perfectly. The idea here is to understand what they're trying to fool people into discussing. The fact remains that if the engines push the plane to 10mph ... say taxy'ing speed, the treadmill will be running at 10mph in the opposite direction, so the wheels will be traveling at 20mph. By the time the plane takes off (lets say 200mph), the treadmill will be moving 200mph in the opposite direction and the wheels will be traveling at 400mph. Whatever friction the wheel bearings create at 400mph, I'm sure would be overcome by the thrust of an Civic, much less the thrust of a jet. So please drop this friction crap Since when do free-spinning wheels not allow an object to move? Again, I'm sorry that you're so ABSOLUTELY SET in your mind frame, that you cannot possibly comprehend the possibility that the guy who wrote this "joke"/"question" whatever-you-want-to-call it might not actually have a PHD in physics and Literature so as to make sure he worded it 100% correctly. Do you really not understand what the question was originally trying to trick people into thinking about? Bill in Brooklyn
Rich, I think it is clearly implied that the conveyor speed relative to the wheel must ALSO match the conveyor speed relative to the earth. (Though I do agree with you, again, with the basic premise that wheel speed has nothing to do with plane acceleration) Therefore you must specify rotational or translational. So just to be clear once again: Do what you want with freely spinning (or not) wheels, and conveyors and the plane can still take off.
Bill, I think bpu699 understands more than you are giving him credit for, he knows like most of us still here that the wheels don't accelerate, or hold back the plane. And that the original question is a conundrum, and infinite speeds as regards the original question are valid. The fact that he wants to discuss these things is OK though regardless of what you think the original question "should have" been.
Hi Teak, I'll elaborate a little on what I meant with the above. The centre of the wheel can move forward at, say 10mph or 180mph (take off speed) in relation to the Earth, and the in the translational sense the centre of the wheel could be matched by the conveyor doing 10mph (or 180mph) in relation to the earth. This is nice and consistent and doesn't break constraints and I know you agree with that. In the rotational sense, according to the above, the wheel is spinning as if its doing 360mph (we agree on that, too) but I (respectfully) don't agree that this also breaks the constraint. There CANNOT be a plausible, implied constraint that says the conveyor speed relative to the wheel must ALSO match the conveyor speed relative to the earth, to go with the (stated) constraint that the conveyor belt moves backwards in relation to the earth. Just think about this for a second: Plane moving forwards at 10mph on a normal runway means a rotation speed of X (depends how big the wheels are). If the conveyor is to maintain the same rotational speed of X the wheels of the plane as the plane moves forward, the conveyor simply has to stay still, otherwise the conveyor belt would be 'modifying' the rotation of the wheels. The constraint that says the belt *moves* backwards means that the extra constraint that you have implied cannot exist. An object can't possibly have the same speed in relation to one reference point and the same speed in relation to something that itself is moving in relation to that first reference point. Now we're disagreeing on how we agree that the plane takes off!! Will this ever end?! Cheers! Rich. I have to hit the sack soon, it's late here (again!)
Hi Teak, (Sung to a well known Eagles melody) On a conveyor runway, cool wind in my hair [or is there?] Warm smell of jet engines rising up through the air Up ahead in the distance, I saw a bright landing light My head grows heavy from the debating... I have to stop for the night There it stood on the runway; I read the problem well... And I was thinking to myself, 'This should be Easy but this could be Hell' Then it lit up the engines and did it move away? There were voices in the terminal, I thought I heard them say... Welcome to the Airplane Physics Question... Such a lovely 'plane such a lovely 'plane ...but the thread's a pain Plenty of angst on the Airplane Physics Question... Any time of year, it'll still be here Engines by Pratt&Whitney, wheels by Mercedes Benz I don't agree with ev'rybody here, but they're still friends How they argue the problem: It flies!, It does NOT! Some care about the theory, some don't give a jot. So I called up the Captain, 'Will the airplane belly flop?' He said, 'We haven't had the no-burst tyres yet from [you guessed it] our friends down at Dunlop' And still those wheels under us are frantic'lly spinning away, I dream of engines and force at night Just so I can say... Welcome to the Airplane Physics Question! Such a lovely 'plane But the thread's a pain... Fightin' it out at the Airplane Physics Question Do the wheels have gears? Ask some engineers... A guy walks down a train, A rocket pack on ice Hostess said 'They're just analogies here, and not worth quoting twice' Whilst in their flight-deck, The pilots aimed at east The belt whipped backwards just as fast But it just can't slow the beast Last thing I remember, I was Sitting in my seat I had to ring the hostess bell For a sandwich I could eat 'Relax,' said the pilot, 'The others, they may scoff.' You can spin the wheels any way you like, but this baby'll STILL TAKE OFF!' That was fun!! G'night, Rich.
Ok, it will fly. I am converted, and here is what made me see the light. Think of a battery powered toy plane hanging from a string. You turn it on and it begins going round and round. No touching the ground, it just goes, pushing against the wind. Does that help?
Enlighten me. I've heard evry argument on all the boards and have spoken to engineers and pilots about this. I tink I get it. I may state it simply but I understand the problems the question presents.
Been thinking about this a little bit. Bpu699 has a very valid point: The plane moves forward because of thrust from the engine. The tires are attached to the plane. The tires rotate, but because the conveyor exactly matches their speed at any moment in the opposite direction, they do not move forward. The tires rotate in place, relative to both the conveyor AND the ground. So how can the plane move forward if the tires don't? If the tires move forward even one trillionth (sp?) of an inch, then the conveyor belt is not moving at the same speed in the opposite direction. The plane will move forward, but only after the tires/landing gear snaps off and it becomes a giant, malfunctioning Segway??! Wether it takes off then depends on the strength of both engines and airframe?!
The tires will simply rotate twice the speed of the plane. Speed of plane on take-off + speed of the conveyor. The tires are not powering the plane, they can rotate at any speed they want.
You guys are using two different definitions of wheel speed. Is it wheel speed in relation to the ground, or rotational speed?
Ok apologies for being fick here but how do you define the 'opposite direction of rotation'? "...designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.." Someone has suggested a theory to me that says the belt rather than doubling the speed of the wheels actually stops them (i.e. the belt moves in the other direction to what I had assumed) and so the plane effectively takes off without the wheels turning at all (the belt cancelling out the wheel rotation but not the forward movement of the plane). My head hurts.
Now, how does your answer change, depending on whether the airplane is front wheel drive or rear wheel drive?
For those who think it won't fly (as I did) please explain why my "plane on a string analogy" doesn't help:
It is amazing to me how intelligent people allow their thoughts and opinions to run away, thereby leading to incorrect opinions and knowledge. First of all the plane will fly [and lets not pursue a bunch of "what ifs"(friction, bearing failures, conveyor concepts, etc), as I do not believe the question was meant to be that deep] . However, for some of the people who continue to struggle with this concept, I read a recurring misunderstanding. They are focusing on the wheels and the conveyor (yes, this has been described many times and I am not going to repeat any of the previous explanations - simply, the wheels in this question have nothing to do with the flight of the plane, other than assisting in keeping the nonrotating parts of the aircraft from touching the conveyor). Specifically, the conveyor direction and the wheels rotation are in the same direction. The "opposite" movement of the conveyor is actually in the same rotational direction as the wheels. Therefore, for those of you who are stuck on the conept that there is no net rotation of the wheels are misuderstanding that the conveyor is not providing a opposite effect on the wheels; it is merely causing the wheels to "spin" twice as fast. A major componet of intelligence (I believe) is the ability to listen and take in information, and use that information (even if it is conflicting with your current opinion or knowledge) to help make the correct decision (or answer). For those of you continuing to argue that the plane will not fly, take a step back and look at the facts, unerstand those facts, and there will be no more arguement.
Yes, I now have to follow my own advice. I just re-read the original question and realized that I misunderstood the motion of the conveyor. The conveyor is indeed moving in a direction counter to the rotation of the wheels. Therefore, there would be no net rotation of the wheels. However, the plane would still lift off and fly (even with no net rotation of the wheels).
YES!! The 2 guys above got it right!!! After my arguments in page 3, My answer still remains IT WILL FLY, but for completely different reasons!! There will be no wheel spin like the gentleman above suggested. None At all! The question says It will spin in the opposite direction of the wheel. WHAT IS THE OPPOSITE DIRECTION????? The opposite direction is not what everyone thinks, as in moving "backwards". Its actually moving forwards, because the surfaces are inverted! So the conveyor belt will actually push the plane forward not backwards!! AHHHHH! I just did an experiment with a coke can on a book. Moving the book forward is COUNTER rotation to the coke can, even if the can is moving forward. Moving the book BACKWARDS like everyones been thinking for the past 30 pages is indeed moving IN THE SAME ROTATION NOT THE OPPOSITE. THERE YOU GO! The conveyor belt actually moves the plane forwards because its matching the wheel speed in the OPPOSITE direction not the SAME direction. I feel really stupid now. Nevertheless, the plane will fly.
I took the liberty of doing up a simple diagram in illustrator to demonstrate the point. Image Unavailable, Please Login
Ok I read the first 11 or 12 pages and then just started every other post from then on out. I'm still a bit undecided at the moment but leaning towards it won't fly. For the record Im sticking to what I believe the is the spirit of the problem, not trying to find a word issue. So I'm taking the problem to mean: 1. Assume plane has a fastest possible speed in shortest amount of time it can go before it takes off. 2. Conveyor belt is preprogrammed to match this(it's not reacting to the plane, its set to mimic the planes best case scenario so the plane 'on the ground' can't magically pull ahead). 3. While the wheels don't drive the plane, the wheels are rolling to match its best case scenario. Now are the people who are saying the plane will fly assuming it managed to break from the start point on the ground or is it doing this : Image Unavailable, Please Login
I took the liberty of drawing up this simple diagram in illustrator to demonstrate the point So the plane WILL indeed take off. Because the question states that the conveyor belt is moving in OPPOSITE rotation to the wheels. Therefore the plane will be moving forward with no wheel motion. (since the conveyor belt instantly matches the speed of the wheels) Image Unavailable, Please Login
Of course the airplane will fly. It does not matter that the wheels are moving forward/backward or not at all. The conveyer can be going forward/backward/stopped for all the airplane cares. Flight is only dependant on airflow over the wings. The best example I can think of is to look at float planes operating on a river. In effect their "runway", the water, is moving all the time. A pilot can land or take off with the current, or against it, won't matter to the airplane. Of course he will always try and land or takeoff into the actual wind to shorten takeoff and landing distance. Wheels, floats, skis, only serve to keep the airplane off the ground, snow or water! Hope that helps.
No the aircraft will not take off, since the relative speed of the plane with respect to the air around it is 0 (assume no wind). If it was taking off, what would happen just at the moment that the wheels lifted off the ground, would it experience a jerk movement forward because of the difference in speed? This does not make sense!
One more example, I have seen a video of a pilot land and takeoff his Super Cub on the roof of a van. The van motored down the road at about 60MPH which is well within the Super Cub performance envelope. Net rotation on the wheels at touchdown and takeoff??? Zero. This does not get any simpler, direction or speed of wheel rotation is independant of flight.
