sputnik Posted November 11, 2006 Share Posted November 11, 2006 Do you think a conveyor belt would stop these guys? USAF Thunderbirds Not an explanation of the problem, but it's pretty cool. Quote Link to comment Share on other sites More sharing options...
bodcaw boy Posted November 11, 2006 Share Posted November 11, 2006 I saw this on another forum I frequent and found some of the commentary interesting. I don't know the answer although I've got an opinion. It WAS interesting seeing the answers/thoughts some people gave (2 pages worth). I'm copying it exactly like he had it there a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction. the question is will the plane take off or not? (ps its been debated to death on other forums, its always fun to see how people present the theory behind there answer) well no...........the grass has to move too......geesh..... boy!! Quote Link to comment Share on other sites More sharing options...
Doc Robin Posted November 12, 2006 Share Posted November 12, 2006 The aircraft will not fly until the air flow about the airfoil produces enough lift to get it off of the ground. If it would work the US government would have spent Billions of dollars on it by now. Bill Quote Link to comment Share on other sites More sharing options...
mas Posted November 12, 2006 Share Posted November 12, 2006 Huh??? ...Some talk of real systems where there is friction, some talk of ideal systems where various sources of friction, etc. are ignored. Thus far the problem simply posits that the conveyor upon which the plane sits is moving and that the wheels are turning sufficiently fast to allow the plane to maintain a stationary equilibrium. This could be achieved in an ideal environment neglecting friction in the wheel bearings. In this state of equilibrium, the motion of the plane is 0, and the velocity of the air over the wing surface is 0. There is no lift. (note, this totally ignores how the system came into being! This is significant, for if the plane were simply sitting on the conveyor from rest, it would also be moving at the same rate in the same direction as the conveyor!) Thus far, no one has shown that the air flow relative to the wing is sufficient to cause sufficient lift. And THAT is the only factor that is functionally relevant here. If additional force is applied to the plane sufficient to cause the plane to move forward at a rate greater than the conveyor, the rate of the conveyor and the plane will cease to be equal, and the plane will move relative to the conveyor at a greater rate (and at a rate >0 relative to the surrounding ground surface and air) and perhaps achieve sufficient velocity relative to the air (assuming the air is stationary and its velocity = 0) such that sufficient lift is created by the airflow over the wing surface to allow flight. The only factor the conveyor plays here is in the fact that the problem states that the rate of the conveyor is offset by an equal movement of the wheels, rendering a change in position moot. It is much like saying that a man walks forward at 5 mph while the conveyor under him moves backwards at a rate of 5 mph, resulting in no positional change in the person. In order for the person to change position forward, either he or some external force must be applied to increase his velocity to an amount greater than the equilibrant 5 mph velocity of the conveyor. Likewise, sufficent force still needs to now be applied to the plane to accelerate it to a velocity greater than the velocity of the oppositely moving conveyor sufficient to create sufficient velocity of the plane relative to the stationary air such that suffient lift is generated. (example in still air: v(conveyor= 5mph) = v(plane=5mph) -> no movement; V (conveyor=5mph) <V(plane5mph) + (AdditionalVelocity caused by Force=Xmph) -> net movement of plane at the total rate of Xmph Situation as stated: air = 0 mph plane wheels <-- 5 mph conveyor rate --> 5mph ----------------------- net motion = 0 displacement Situation with additional acceleration: air = 0 mph plane wheels <-- 5 mph additional acceleration to plane <-- X mph (additional applied force necessary to accelerate the plane to a greater velocity) converyor rate --> 5 mph ---------------- net motion <-- X mph displacement relative to stationary air By the way, you must still reach a critical velocity such that X is sufficient to generate the necessary lift in order for the plane to fly! (Man this is no fun without rich text or an equation editor! []) If no additional force is applied to the plane (and we assume that the conveyor and the plane's wheels will continue to function in the same manner), it will continue in equilibrum with zero net change in position. This is no different than if the plane is standing stationary on the stationary ground! And it is actually no different that saying that the earth and a plane sitting upon it (and the atmosphere, neglecting local winds) are hurtling through space at X mph!! Can the plane fly? Only here the earth and the plane and atmosphere are travelling at the same rate of travel, thus they appear stationary relative to each other. Again, it is the differential rate of movement of the wing relative to the air that generates lift. Or it is the same as saying a plane sits on an aircraft carrier travelling at 20 knots. (And thus they are both travelling at 20 knots. But since the relative difference is zero, they remain stationary relative to each other. Can the plane take off? (The only difference here is that wind velocity and direction becomes another real factor and can either contribute or actually impede the planes attempt by flowing with, or in opposition to the plane, thus subtracting or adding effective relative velocity and force). But sure planes can fly, provided an accelerating force is provided by the catapult and the engines to increase velocity of the air flow relative to the wing surface sufficient to generate the required lift. Bottom line, only the differential rate of flow between the air and the wing surface is a factor in the plane's ability to fly. Sufficient force is still necessary and must be applied in order to accelerate the plane to a velocity relative to the flow of air over the wings generating sufficient lift to fly the plane. It is the differential force and movement that makes the difference! Look up relative motion in any physics book! If you want to move the plane sufficient for it to generate sufficient lift to fly, apply additional force to accelerate the plane to a velocity greater than the conveyor in opposite direction. Or, you could turn the plane around and use the velocity of the conveyor plus whatever force is necessary is required to accelerate the plane to a critical velocity sufficient to generate the necessary lift by accerating the plane's wings relative to the air. This is a case of simple vector addition! Quote Link to comment Share on other sites More sharing options...
Champagne taste beer budget Posted November 12, 2006 Share Posted November 12, 2006 a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction. the question is will the plane take off or not? I was thinking about this yesterday as I was doing some work outside. I believe some clarification is needed before the question can be answered. "Will the plane take off"? is significantly different than "Can the plane take off"? As worded in the original question, it will not, since there is no mention of the plane having a functional engine, fuel for said engine or someone at the controls capable of operating said plane. If there is an engine, fuel and a pilot, then yes, tha plane Can take off. Imagine you're inside a car, big horsepower, big tires, on a smoothly frozen lake. You can spin the tires all day and not necessarily move the car forward. Wheels spinning, car not moving. The tire to ice interface is your only source of forward proplusion, and the friction between the two is not high enough to gain any traction. Now, say you have a buddy that works at the Air Force surplus/demolition store who just happens to let on that he can get ahold of a jet engine. [6] I don't care if there is Crisco, KY, jelly, three 55 gallon drums of silicone on top of the ice and someone has coated your tires with Teflon, you strao that puppy to the roof of the car and fit "Ignition", that car is gonna move across the ice. If you had previously strapped some wings to the roof in the correct configuration to allow lift, that sucker would fly, too. Those of you saying the conveyor belt plane will not take off, even with an engine, fuel and driver, wouldn't mind sitting in this car on the lake, then, right? We know it can't move since the wheels are just spinning, correct?[] Quote Link to comment Share on other sites More sharing options...
Jay481985 Posted November 12, 2006 Share Posted November 12, 2006 In that instance, the game of tug of war is not equal. Everything before was, as the conveyor belt and such were pretty much even. The Jet plane has the advantage and pushes alot more then the conveyor belt. Quote Link to comment Share on other sites More sharing options...
Champagne taste beer budget Posted November 12, 2006 Share Posted November 12, 2006 Who said the original plane wasn't a jet? Aren't there some big prop planes that produce more thrust than a small jet? No one ever stated that the conveyor belts surrounding air currents were the only thing expected to allow the plane to take off. All of which is why I stated the original question if faulty since it does not give enough information to produce any correct solution. Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 12, 2006 Share Posted November 12, 2006 There's nothing wrong with the wording of the problem. Don't get bogged in irrelevant minutia. Propellor, jet, or rocket propulsion - it doesn't matter. The speed of the conveyor doesn't matter - it can be spinning at any speed you wish. The number of flight attendants doesn't matter either. The only thing that matters is the resolution of forces acting on the plane. As mas said earlier, it's a case of simple vector addition. The only horizntal force acting on the plane is thrust generated by the engine. So, without a force to oppose the engine thrust, the plane accelerates in the direction of the force imparted by the thrust and achieves sufficient velocity relative to the air to achieve lift and fly away. I think folks are getting confused into thinking that the plane is somehow held stationary or the conveyor imparts a force on the plane the same way it would on a car. A car moves by transferring force to the tires. The planes tires do not impart or transmit any horizontal force on the plane so there is no force to counteract the thrust. The plane's landing gear is essentially a rollerskate. The frictional force (traction) imparted on the tire by the conveyor acts tangentially on the tire and only spins the wheels - it does not transfer through the wheel bearings to the plane. Any frictional forces from the tire deformation and the wheel bearing are negligible compared to the engine thrust. Once again, no horizontal force is transmitted from the spinning conveyor to the plane to oppose the engine thrust so the plane accelerates normally and takes off with madly spinning tires. Think of standing on a treadmill while wearing rollerskates. Once you got used to it, it would be pretty easy to maintain your position. Now, if someone hands you a rope that is attached to the wall beyond the treadmill and you pull on the rope, you'll move toward the wall independent of the speed of the treadmill. The thrust from the plane is equivalent to you pulling on the rope and the plane moves normally, independent of the speed of the conveyor. Quote Link to comment Share on other sites More sharing options...
Seadog Posted November 12, 2006 Share Posted November 12, 2006 It will take off, with the wheels spinning very, very fast. Unless of course the wheel bearings are bad (seized) to the point where the thrust of the engine cannot overcome the frictional forces in the wheels, but in that case, the airplane would not take off with or without the conveyor belt scenario. Quote Link to comment Share on other sites More sharing options...
bodcaw boy Posted November 12, 2006 Share Posted November 12, 2006 you need air to flow over the wings...it don't matter how fast the wheels go....did i get it??[:S] boy!! Quote Link to comment Share on other sites More sharing options...
jacksonbart Posted November 12, 2006 Share Posted November 12, 2006 Is the the stewardess serving alcohol to the pilots? Quote Link to comment Share on other sites More sharing options...
sheltie dave Posted November 12, 2006 Share Posted November 12, 2006 It depends if the forward vector is a mechanical force or a static force. IF the plane is a prop or jet, AND it has a forward vector greater than stall speed, then YES it will take off. If it is a static force forward vector, ie is pulled by a rope, then it will not take off, as the vector force will stop as soon as it is released. A glider generally needs a lift factor 1.3 times greater than its weight, IIRC. As others have noted, the Bernoulli forces are the critical element of the answer. Driven or static, there has to be positive lift generated on the underside of the plane wings. Quote Link to comment Share on other sites More sharing options...
Moderators dtel Posted November 13, 2006 Moderators Share Posted November 13, 2006 Well the people on this forum must be smarter than the people on the forum you speak of ? That's a much harder question to answer. Thought it was answered correctly in the first half of page one. speakerfritz which was the first reply had it correct.IMHO Ok so I must retract my first statement on page one, this is now 4 pages long.[] Quote Link to comment Share on other sites More sharing options...
PhilMays Posted November 13, 2006 Share Posted November 13, 2006 After this is resolved, can we PLEASE solve the age old question of the Woodchuck and the amount of wood said woodchuck could chuck? Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 13, 2006 Share Posted November 13, 2006 Good Lord! As long as it isn't dragging it's tail too much and the cheapo plastic wheels spin freely, even a rubber band powered plane will take off from the conveyor. The conveyor is irrelevant - it just spins the landing gear wheels. I'd love to see someone try to explain how the conveyor imparts a force on the plane that opposes engine thrust and somehow holds the plane stationary (or even slows it down). Quote Link to comment Share on other sites More sharing options...
PhilMays Posted November 13, 2006 Share Posted November 13, 2006 Good Lord! As long as it isn't dragging it's tail too much and the cheapo plastic wheels spin freely, even a rubber band powered plane will take off from the conveyor. The conveyor is irrelevant - it just spins the landing gear wheels. I'd love to see someone try to explain how the conveyor imparts a force on the plane that opposes engine thrust and somehow holds the plane stationary (or even slows it down). So what you are saying is the the plane will not stay stationary and that the thrust will propel the craft forward to create airflow for wing lift. Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 13, 2006 Share Posted November 13, 2006 So what you are saying is the the plane will not stay stationary and that the thrust will propel the craft forward to create airflow for wing lift. Yes, the plane will move forward independent of the action of the conveyor. The landing gear wheels just spin faster as Dr. Who pointed out on page 1. Quote Link to comment Share on other sites More sharing options...
WS65711 Posted November 13, 2006 Share Posted November 13, 2006 Forget about all that. What happens if you're riding in the back of a bullet train (a really fast one) travelling forward at 3000 fps. You open the back door and fire a gun (muzzle velocity 3000 fps) straight out the back of the train. Will the bullet simply fall to the ground? Quote Link to comment Share on other sites More sharing options...
sputnik Posted November 13, 2006 Share Posted November 13, 2006 .....Will the bullet simply fall to the ground? Yes, but would you hear the shot? Quote Link to comment Share on other sites More sharing options...
Jay481985 Posted November 13, 2006 Share Posted November 13, 2006 Forget about all that. What happens if you're riding in the back of a bullet train (a really fast one) travelling forward at 3000 fps. You open the back door and fire a gun (muzzle velocity 3000 fps) straight out the back of the train. Will the bullet simply fall to the ground? no since the speed you are at is the reference point. technically it is the 0 because the bullet is already moving the same speed as the train in relation before the shot. Just like if you were in a falling elevator, you jump the last second, you still have the speed of the elevator and you to overcome. Will you hear it, no! sound does not have the reference bounds and hence why supersonic jets cannot hear their own sonic booms, once sound is created it does not have a speed it was already traveling. Also I am confused, was the conveyor belt moving to the speed of takeoff from the plane or is the plane's engines on and fighting the conveyorbelt? Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.