I now think it'll fly, I'm just not sure at what point that actually occurs. I'm sure we've all seen a R/C plane being launched by hand and I'm assuming the same concepts apply here. I'm just confused on how thrust translates into lift (air resistance) in this senario-- when does the hand "let go". More to the point, if this is a viable method why isn't it employed in any airport around the world or even aircraft carriers?
you're half way there, the car is indeed rolling on the wheels. in the toy car scenario i described, you would be moving the car about indipendent of the speed of the wheels it rides on. you are the external force i.e. jet thrust that causes the car to move forward although the paper is travling backward with the wheels moving along with it. if you go grab a toy car and try it you'll see it happen
the lift needed for a plane to take off is based on 'air speed' not ground speed. so even with a moving runway, the plane needs to cover a specific distance to atain the air speed needed for lift.
yeah but see my other statement so in this case, if you move the car FORWARD, then it's breaking the parameters of the test, that being that the forward motion of the wheels IS GREATER than the backward motion of the conveyor. if someone can prove to me this is not the case then hey, but i don't see it happening.
for those not sure on how a plane "lifts" it is based on negitve and positive air pressure. the shape of the wing generates a positive air pressure below and a negitive pressure above, this causes a vaccum on the surface of the wing and viola 'lift' edit: i'm keeping this very simple
i just think what people aren't getting is the 'trick' to the question, that if the conveyor moves exactly the same speed negative to the wheels, then obviously no relative earth motion happens. as to why i'm discussing this:
ash buddy, go grab a toy car. place it the paper and watch the wheels. even though the car may stand still or move by you pushing it the wheels and paper will travel past each other at the same matched speed. when we all get crazy and do a burnout, the wheels are travling faster then the ground hence the nice acrid smell. you pushing the by thrust of hand will not cause the wheels to turn faster then the ground, or in his case the conveyor.
Well, yeah, but not because it's a paradox or something like that - just because in the real world there are limits on everything. But lets say we could build a treadmill that could go infinitely fast and plane wheels that could rotate at infinite speeds without disintegrating.... well, I guess the planes engines spool up, the wheels start to turn, the treadmill moves it back, but the plane starts to move forward, because of thrust, not wheel speed, so the treadmill would instantly hit maximum speed, which would be something less than the speed of light, since thats a physical limit that cant be exceeded, so with the treadmill moving at the speed of light, the plane would keep acceelrating and take off The acceleration of the treadmill to max speed would be instant assuming the treadmill could instantaneously change its speed and react to wheel speed on the plane. So I guess there is a logic answer and a nit-pick answer. Logic answer is the plane can still accelerate and take off regardless of something moving beneath the wheels in the opposite direction. Super technical answer is the scenario is impossible because of lots of reasons, not the least of which is that no such device to match wheel speed is possible to create, and even if it was, it would break the wheels and itself apart at speed, and even if those werent factors, there is a limit on maximum velocities provided to us in physics, the speed of light, so by definition the treadmill cannot "match wheel speed" - therefore the scenario as stated could never happen unless you take real world constraints such as the lag of the conveyor matching wheel speed, etc.
The answer is that it is physically impossible for the conveyor to match plane wheel speed, therefore the plane would take off.
no, air speed and ground speed are very different things. to atain air speed you still need to cover distance. even with the conveyor system moving the plane will move forward and lift off, whilst travling the same distance whether or not the ground was moving.
yep i get that, but i think you might be missing the point that the conveyor should be moving relative to the EARTH the same speed that the wheels want to go the opposite direction. it isn't a case of wheels relative to conveyor, it's wheels to earth vs conveyor to earth, if both these match (as implied by the question) then no motion occurs. if you PUSH the car forward, then the relative speed to the earth is GREATER than what the paper is going in the opposite direction.
Sitting in a hotel in Seattle - gotta get to bed soon. When I get back from my trade show tomorrow I bet it will be 10+ pages
i wondered how on earth that would happen when i saw it in the first post like 2 hours ago but yeah having contributed 15 posts myself, i see that it's not about us finding an answer, it's about us all trying to convince the other that our theory is correct, and as always, mine is!
Here is the forum which i think the question was initially brought up: http://www.pprune.org/forums/showthread.php?threadid=200511 The proffesional pilots rumor network. " Here's where everyone who went wrong did so right from the start. The question, 'Will the aircraft take off?' is not a trick question. It's a very straight question. The answer to which is yes. Why is it Yes? Because, in fact, the belt, regardless of speed or contol system, cannot prevent forward or reverse movement of the axle. Why is that so? Because it can only cause rotational movement of the wheel around that axle and that axle is the origin of all points around the wheel and we only ever need one of those points to be in use at any given time for the wheel to serve its purpose. The bit with the weight on it, the bottom of the tyre. Yeah but that point's being moved backwards I hear you scream. Not so. Why? Well, that point on the tyre is stationary relative to the surface on which it is running. Always stationay regardless of the speed of the belt and/or aircraft. If it wasn't the tyre would be skidding. If the contact patch is stationary for any given speed, forward or reverse, it remains stationary for any applied opposing or assisting surface speed. Right up to the point at which the belt, in the assisting direction, would provide sufficient airspeed for rotation without the wheel even turning. You see how the contact patch is stationary now? So, belt speed has no effect over contact patch speed since there is and, unless the wheel stops rotating and skids, never will be any. It's a zero that remains a zero. It's not a variable. The axle/aircraft speed and belt speed are variables. If a point that allows motion is in fact stationary, it stands to reason that it cannot be limited. It doesn't need to move so cannot be countered. The axle from which the contact point originates is also on that zero line and hence uninfluenced by any goings on outside the zero point. The aircraft attached to that axle is, likewise, unaffected." And Here are some quotes from another forum: http://www.physicsforums.com/showthread.php?threadid=101259 "There are four forces that govern an airplane at any given time: lift, weight, thrust and drag. Lift and weight oppose each other but can be disregarded in this question because all we are talking about it whether or not the airplane can accelerate in reference to the surrounding air. Once we get acceleration and airflow we can get the lift and weight but we don't need to talk about that now. The throttle of the airplane is advanced and propeller/jet of the airplane produces thrust. We now have a forward vector, say 500 pounds of force, for a small cessna. In order for the plane to remain stationary an equal and opposite force needs to be introduced. Everybody follow? This opposite force would be drag. We need to account for 500 pounds of drag. Right now the only drag is the force of friction in the wheel bearing. Has anyone here ever tried to push a cessna? It's not very hard...maybe 50 pounds of force at the most. So now we have 450 pounds of force acting in the forward direction. The conveyor belt itself does not impart any friction or drag to the airplane. It will accelerate, gain airspeed, and take off Let's think of it a different way. Let's say the plane is landing. It approaches the runway at 100 knots and is 1 foot above the ground. As posed in the original question the conveyor belt is moving in an equal and opposite direction. So it's moving 100 knots backwards. As soon as the plane touches down, what happens? nothing...it continues its rollout as if the runway were stationary. It doesn't just automatically come to a stop. The airspeed indicator of the plane would read 100 but the wheel speed would be 200. (apologies to Youens at flightinfo.com) People cannot separate a car/bike/walking on a treadmill with an airplane. Same thing would happen if you put the airplane on ice and hit the throttle, it will still accelerate at the exact same rate as on a runway (not a safe operation however). Try that with a car/bike/your foot and you'll get drastically different results." "the conveyor belt means nothing. as the plane goes forward the conveyor belt goes backwards with the same amount of force, but since the forward moving power comes from the engines mounted somewhere on the wings/fuselage and not the wheels, and the wheels merely rotate freely on an aircraft, the plane will go forewards while the wheels just spin twice as fast as they usually would." "The question is slightly unclear: if the conveyor "tracks the plane's speed" and keeps it exactly zero, then obviously, no, the plane would not take off as it would be stationary with respect to the air. I don't see what could possibly be difficult to understand about that. The way the question is worded though, it could mean to say that the plane moves forward at 150kts, while the conveyor belt moves backwards at 150kts, for a rotational speed of 300kts for the wheels. In that case, as long as the wheels don't burn off, the plane would take off." Just some food for thought..
here is the original question again. where is the mention of the earths rotation? did i miss that addition?
from what i can see the whole theory is based on a fallacy. if it's not preventing forward movement, then it's not travelling at the same negative speed. simple.
Please raise your hand if you're absolutely giddy at the prospect of watching various 'net heads build dangerous contraptions to prove each other wrong. First one to register planeconveyorbelt.com wins!
To be even more precise than in my first post, unless there is a fan, a car on a dyno WON'T make any use of its wings/spoilers to generate downforce. It's the difference of speed between the car and the air surrounding it that will create the drag, not the speed of the wheels alone. Same if it's jet powered. Wheels will convert the thrust into rotation. If the thrust is strong enough, it could leave instantly the rolling ground if there is too much friction on it (meaning the conversion of wheel acceleration to rolling ground acceleration wouldn't be instant), but this would require a lot of power.
