Ok so if a plane is accelerating for takeoff, lets just say it's going 150mph down the runway. And just for clarity lets say the plane is traveling east going 150mph. All of the sudden the runway starts moving at 150mph west. Would'nt the plane just stop? The only way for the plane to move forward anymore is if it's speed is > than the runway's speed. But that can't happen because the runway = the plane's ground speed.
Picture the thrust as being a rope attached to the nose of the plane. If you're pulling it down the runway via the rope and the runway starts moving, you'll still be pulling the plane. The problem is that this whole thing gets phrased and interpreted very differently. Some people think the runway moves with the plane, causing the wheels to remain stationary; in this case the plane STILL moves with respect to its environment and takes off. If the runway moves opposite of the plane at the same speed, you'll just have the wheels spinning along at 300mph when the plane is going 150.. short of friction losses being unsurmountable the plane still takes off. The tricky one is where the conveyor supposedly matches the rotational speed of the wheels perfectly; here the problem becomes theoretically unsolvable because obviously the wheels can't spin at an infinite speed, neither can the runway move at an infinite speed.. this is where people tend to get hung up.
oh, i see now. EDIT: Wait, but in that scenario, i'm not moving, so the plane can move forward. I think, im just getting more and more confused.
Just think of it like this: the wheels just serve to prop the plane up, not to move it around or propel it. The plane gets lift from moving relative to the air; whatever occurs on the ground is mostly irrelevant.
oh ok....now this makes sense, atleast for me....I mean there are planes that don't even have wheels and fly just fine...
only way a plane can lift off is when the underside surface area (body, wing, etc,...) has higher pressure than the upperside dont matter what the wheel speed is but in order to generate the pressure diff the air has to flow in order for the air to flow it has to be moving forward or wind is blowing against it or wind is sucking pass it No I dont believe it will take off even jet engines suck enough air and push it out back
This reinforces you guys thoughts...i just doubt anyone will click on the link. The implicit assumption is that if the conveyor belt's speed backward exactly counteracts the airplane's "speed" (whatever that means) forward, the plane remains stationary relative to the earth and, more importantly, to the air. (We assume the winds are calm.) With no wind moving past its wings, the plane generates no lift and can't take off. 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? The answer is yes. Understanding why is Basic Realization #2. The conveyor doesn't exert much backward force on the plane, but it does exert some. Everyone intuitively understands this. To return to the analogy in my original column, if you're standing on a treadmill wearing rollerblades while holding a rope attached to the wall in front of you, and the treadmill is switched on, your feet will initially be tugged backwards. Partly this is due to friction in the rollerblade wheel bearings, but partly--this is key--it's because the treadmill is accelerating the rollerblade wheels and in the process imparting some angular (rotary) but some linear (backward) momentum to them. You experience the latter as backward force. Eventually the treadmill reaches a constant speed and the rollerblade wheels cease to accelerate. At this point you can easily haul in the rope and pull yourself forward. But what if the treadmill continues to accelerate? Different story. In principle it's possible to accelerate the treadmill at a rate that will exactly counteract any forward force you care to apply. (This is a departure from the original question, which said the conveyor belt compensated for the plane's speed,, not its force.) The only mathematics needed to demonstrate this is the well-known physics axiom F = ma--that is, force equals mass times acceleration. Given that the conveyor exerts some backward force F on the plane, we simply crank up the acceleration as much as necessary to equal any forward force F generated by its engines. Result: The plane stands still and doesn't take off. Welcome to BR #2. You may say it's impossible to build a constantly accelerating treadmill, that eventually we run into the limitation imposed by the speed of light, etc. True but irrelevant--BR #2 has an intrinsic elegance that transcends such practical concerns. Why didn't I bring it up in the first place then? You've got to be kidding. It took an entire column to get BR #1 across, and a second one to convey (I hope) BR #2. One fricking thing at a time.
Actually, i thought of this problem in a different perspective. Consider different frames of reference and relative motion. 1) Your frame of reference is a point on earth, off the plane and the conveyor belt. Lets call this frame 1. Air is stationary in this frame. I believe most people are analyzing the problem from this frame. In this case, the only time when the conveyor belt will match the speed of the wheels exactly will be if the plane is stationary. Now, the thrust will make the plane move forward. The conveyor belt can not stop the plane. So the wheels start to spin on the belt. The belt speeds up which causes the wheels to spin even faster. The conveyor belt will never match the speed of the wheels. However the plane will continue to move forward. The plane will take off. 2) Your frame of reference is on the plane. Lets call this frame 2. Thrust will now cause frame 1 (the rest of the world) to move backwards. Note that with respect to frame 2, the plane is stationary. Also note that no matter how fast the conveyor moves, it will never match the speed of the wheels with respect to frame 1. However, with respect to frame 2, the conveyor will ALWAYS match the speed of the wheels. Regardless, air will be moving backwards with respect to frame 2. This will cause lift and the plane will fly.
I always thought that every aircraft has to take off into the wind, isn't that correct?. I am going to ask a few buddies of mine that work as ATC's. Mark
the question says that the the belt is designed to exactly match the speed of the wheels in the oppostie direction...this is a hypothetical question as well as an imaginary one. that being said b/c of the design of this imaginary belt being able to match the wheel's speed exactly the plane would not be able to take off b/c it is not moving forward. thrust can only easily move the plane forward because the wheels are designed to roll obviously. the plane can not take off if you stick to the perameters of the original question.
unless you're a helicopter in which case you beat the wind into submission and no you do not HAVE to take off into the wind it just helps conserve fuel and runway space
Also, isn't there the thrust to weight ratio that is involved?. I was in Phase Maintenance on F-16 C's and I always remember the pilots talking about that... Mark
NO, it would be like running on a treadmill, you could get velocity speed maybe:dunno ~now to read the rest of the thread~
Ok at first I thought I was A lil smart from milo's 1st comment #1 oh well so much for that. I think its true It all depends on how you look at it.but going from the statment it would seem simple but oh no....bet this is one of those questions thats better answered after a blunt lol. peanutbutterjellytime break was right on time!!
i didn't read the thread, but based on what you said... " The conveyer belt is designed to exactly match the speed of the wheels at any given time" then it wouldn't seem be moving and there would not be enough lift from the needed airflow around the wings.
The wing is shaped in a way so that wind and wind ONLY will allow lift and for that matter the SR71 was designed in a way so that the body created lift as well from wind. No wind passing above and below the wing = NO fly. At lockheed is a low speed and a high speed WIND tunnel to test aerodynamics or lift. How fast the wheels spin have nothing to do with the plane flying.
Match the speed in which direction? If it's going the same direction the plane is, the wheels will stand still but the plane will still move relative to the environment, generate lift and take off. It's the other scenario that gets everybody all confused.
Doesn't this ^^^ Contradict the original problem statement, though? Your perspectives seems to make an interesting assumption: That there is a lag between the acceleration of the wheels and the acceleration of the conveyor. Now, if that's the case, then sure, I agree that the plane takes off. If, however, the conveyor and the wheels' speeds are matched continuously, with no lag (instantaneously), I don't see how the plane can ever move forward. Forward motion would be created through two different situations: 1) The wheels spin faster than the conveyor. Given the problem statement, this isn't possible. 2) The wheels break free from the conveyor. Again, unlikely given a typical environment. This forward motion is what would create the airflow over the wing to generate lift. I just don't see how that happens. Can you elaborate? Taking the bicycle on the treadmill with rope... analogy: Once the bike and the treadmill have begun rotation, pulling on the rope will create a faster rotation in the bike's tires. The only way rope can be pulled in is if the tires go faster (effectively, although it's really the force of friction being exceeded by the tension in the rope) than the treadmill. Right? What am I missing? So, to me that says that the thrust of the plane gets negated by the conveyor. Can you give me another example to help me understand this?
^^^ ignore the frictional forces between the wheels and the conveyor. In this case, the solvability of the problem depends on your frame of reference... read what i have written about frame #2 and tell me if it makes sense
Yeah, I read Frame #2 and I still don't see it. In order for the plane to move forward, the wheels seem to HAVE to spin faster than the conveyor. ...and, according to the problem, that can't happen...
To state some definitions: Frame of reference: The point in space that you assume is stationary. Relative motion: If you are standing on the road, then to you, cars are moving. However if you are sitting in a car, then the world is moving and you are stationary. You might think you are stationary on earth. However, with respect to the solar system, the earth is moving, and so are you. wrt= with respect to Frame 1: On the earth (Off the plane and the conveyor. Air is stationary in this frame) Frame 2: On the plane. The plane is always stationary wrt this frame. The rest of the world could be moving Now consider 2 examples: (a) The conveyor is stationary wrt frame 1: Thrust will make the plane move forward wrt frame 1. Assume the plane is moving with speed s. However, wrt frame 2, the plane is stationary and the world, including the conveyor, is moving backwards with speed s. Since there is relative motion between the conveyor and the wheels, the wheels spin forwards with speed s. Hence the conveyor speed and the wheel speed is matched wrt frame 2. Now air is also moving relative to the plane. Hence the plane will take off (b) The conveyor is moving with arbitrary speed v wrt frame 1, in the direction opposite to the motion of the plane: Thrust again causes the plane to move forward with speed s wrt frame 1. However, wrt frame 2, the plane is stationary and the world is moving backwards with speed s. Also wrt frame 2, the conveyor is moving backwards with speed (s+v). Due to relative motion between the wheels and the conveyor, the wheels will spin forwards with speed (s+v). Hence the conveyor speed and the wheel speed is equal wrt frame 2. Wind is moving over the wings of the plane at speed s (wrt frame 2). Hence the plane will take off. Cliff notes: if you choose frame 2 as your point of reference, the conveyor speed will always match the speed of the wheels, no matter what the speed is. The plane will take off.
Thanks for the explanation, but I still just cannot understand how or why thrust will move the plane forward.
wrt frame 1, thrust from the engine produces a backward force... every action has an equal and opposite reaction... ignoring parasitic forces (drag, friction, etc.) the plane reacts by moving forwards... the only thing that will keep the plane from moving would be a force that is equal and opposite to the thrust...
Wind has little to do with lift; a wing flies because there is a vacuum above it. As a wind goes thru the air it causes air to pass directly under the wing but the air crossing over the top is slightly above the wing allowing it to lift. Speed is the factor not wind (not that speed doesn’t cause wind). Mark