Can Plane Take Off
- Thread starter TheWand_foh
- Start date
I think the anger stems from the fact that we"re at page 18 and, regardless of the multiple explanations, people are still questioning whether it will fly or not.
Nevertheless,
The thing you have to wrap your head around is that the conveyor belt, moving at the same speed as the plane, would have no way to impart a force large enough to stop the plane from moving.
Nevertheless,
Ok, up to this point you"re ok.
This is true. However, the thing is that horizontal acceleration is not prevented.
No, a plane without wings would in fact move along the conveyor belt. If our hypothetical conveyor belt had an end, it would reach it.
The thing you have to wrap your head around is that the conveyor belt, moving at the same speed as the plane, would have no way to impart a force large enough to stop the plane from moving.
Jesus christ.
Look, place a plane on (with it"s engines turned OFF) a conveyor belt. Conveyor belt on at 10mph. The plane"s wheels rotate at 10mph. The plane does not move.
Turn engines on. Equivilent thrust to push the plane at 200mph. The plane"s wheels now rotate at 210mph. The plane accelerates forward at 200mph. Said plane takes off.
Where"s the fucking mystery?!
Far too much pseudo-science on this messageboard.
Look, place a plane on (with it"s engines turned OFF) a conveyor belt. Conveyor belt on at 10mph. The plane"s wheels rotate at 10mph. The plane does not move.
Turn engines on. Equivilent thrust to push the plane at 200mph. The plane"s wheels now rotate at 210mph. The plane accelerates forward at 200mph. Said plane takes off.
Where"s the fucking mystery?!
Far too much pseudo-science on this messageboard.
Good lord. I"ll agree that the question is not worded very well. But the people arguing that it won"t fly are having some serious problems understanding what is going on here.
First: let"s clarify the whole "conveyer belt moving at opposite the speed of the airplane" statement. The only way this makes sense is to assume this means the conveyer belt moves at the exact speed that the plane would be moving at were it on a regular, "immobile" runway, given it"s thrust. Thinking of it as instantaneously matching it"s speed relative to the surroundings is a complete paradox. Think about it, such a contraption could never even get started. Consider this situation: the conveyer belt has managed to slow the plane to a halt (ie not moving relative to the surroundings)... well, doesn"t this mean that the conveyer belt (as well as the plane) are both moving at 0 mph? It"s nonsense, and obviously not what the question intended.
Second: let"s get the whole weight/friction thing out of the way. Think of the airplane sitting still, with zero thrust being applied by the engine/prop. If the conveyer belt started at this point, with no thrust being applied by the engine/prop, then yes, the plane would move backwards with the conveyer belt. This is because the plane is not generating enough thrust to overcome the friction of the runway-tire-plane connection. This is similar to the "if you dropped the airplane onto a conveyer belt moving 100 mph" statement.
However the people arguing that the plane won"t take off like to make this out to be some monumental force to be overcome. It isn"t. It is TRIVIAL for an airplane to generate enough force to break through this barrier. You can go to an airport and see it done all the time, it"s called "going from 0 mph to >0 mph". If it wasn"t trivial, think about how much energy/fuel/etc it would take to get a plane to accelerate to ~200 kph. The force needed to overcome the friction barrier between the wheels and the ground is like a whimpering fart compared to the amount of thrust the engines/prop routinely generate to move the airplane fast enough to fly through the air.
Now if you understood all that, and still don"t see why the plane would take off, you really are an idiot.
Let"s run through the whole situation (this is mostly a rehash of some good posts in the physics thread link btw)...
Initial conditions: Plane is generating 0 mph worth of thrust (yes I know mph isn"t really a measure of thrust but I think you know what I mean). Conveyer belt is spinning at 0 mph. Plane is stationary with regards to the surroundings.
Now let"s assume that the engines are turned on very gradually, such that if the plane was on a normal runway, it would be stationary for a split second before beginning to move. Say it takes 1 "unit" of thrust to push the plane hard enough to break the bond of friction between runway and wheel.
Plane is generating 0.5 units of thrust. Conveyer belt is spinning with -0.5 units of thrust. Plane is moving BACKWARDS at some very miniscule speed.
The thrust increases to exactly 1 unit of thrust.
Plane is generating 1 unit of thrust. Conveyer belt is spinning with 1 unit of thrust. Plane is STATIONARY. AT THIS POINT AND ABOVE THE WHEELS CAN BE TREATED AS FREELY SPINNING.
Now the thrust goes above 1 unit. The thrust of the conveyer belt goes above 1 unit. If this were a car, with powered wheels, it would remain stationary with regards to the surroundings. A car is moved BY it"s wheels. The wheels turn, and the friction between them and the road moves the car forward. We"re not talking about a car. We"re talking about an airplane using a jet engine or a prop. The engine/prop sucks in air from the front and spits it out the back. With the (negligible) friction barrier broken, the runway and the wheel don"t matter AT ALL. The plane will accelerate porportional to how fast the engine/prop pulls it through the air, and eventually move through the air fast enough to generate lift on the wings.
A good analogy ripped from the physics forum link...
Instead of a plane on the conveyer belt, its you, you"re wearing rollerblades, and you have a (forward firing) jetpack on your back. You will move forward. This is literally the EXACT SAME situation as far as physics is concerned.
It feels really redundant writing out a post this long after so much has been said, but some people are just so wrong I can"t stand it.
First: let"s clarify the whole "conveyer belt moving at opposite the speed of the airplane" statement. The only way this makes sense is to assume this means the conveyer belt moves at the exact speed that the plane would be moving at were it on a regular, "immobile" runway, given it"s thrust. Thinking of it as instantaneously matching it"s speed relative to the surroundings is a complete paradox. Think about it, such a contraption could never even get started. Consider this situation: the conveyer belt has managed to slow the plane to a halt (ie not moving relative to the surroundings)... well, doesn"t this mean that the conveyer belt (as well as the plane) are both moving at 0 mph? It"s nonsense, and obviously not what the question intended.
Second: let"s get the whole weight/friction thing out of the way. Think of the airplane sitting still, with zero thrust being applied by the engine/prop. If the conveyer belt started at this point, with no thrust being applied by the engine/prop, then yes, the plane would move backwards with the conveyer belt. This is because the plane is not generating enough thrust to overcome the friction of the runway-tire-plane connection. This is similar to the "if you dropped the airplane onto a conveyer belt moving 100 mph" statement.
However the people arguing that the plane won"t take off like to make this out to be some monumental force to be overcome. It isn"t. It is TRIVIAL for an airplane to generate enough force to break through this barrier. You can go to an airport and see it done all the time, it"s called "going from 0 mph to >0 mph". If it wasn"t trivial, think about how much energy/fuel/etc it would take to get a plane to accelerate to ~200 kph. The force needed to overcome the friction barrier between the wheels and the ground is like a whimpering fart compared to the amount of thrust the engines/prop routinely generate to move the airplane fast enough to fly through the air.
Now if you understood all that, and still don"t see why the plane would take off, you really are an idiot.
Let"s run through the whole situation (this is mostly a rehash of some good posts in the physics thread link btw)...
Initial conditions: Plane is generating 0 mph worth of thrust (yes I know mph isn"t really a measure of thrust but I think you know what I mean). Conveyer belt is spinning at 0 mph. Plane is stationary with regards to the surroundings.
Now let"s assume that the engines are turned on very gradually, such that if the plane was on a normal runway, it would be stationary for a split second before beginning to move. Say it takes 1 "unit" of thrust to push the plane hard enough to break the bond of friction between runway and wheel.
Plane is generating 0.5 units of thrust. Conveyer belt is spinning with -0.5 units of thrust. Plane is moving BACKWARDS at some very miniscule speed.
The thrust increases to exactly 1 unit of thrust.
Plane is generating 1 unit of thrust. Conveyer belt is spinning with 1 unit of thrust. Plane is STATIONARY. AT THIS POINT AND ABOVE THE WHEELS CAN BE TREATED AS FREELY SPINNING.
Now the thrust goes above 1 unit. The thrust of the conveyer belt goes above 1 unit. If this were a car, with powered wheels, it would remain stationary with regards to the surroundings. A car is moved BY it"s wheels. The wheels turn, and the friction between them and the road moves the car forward. We"re not talking about a car. We"re talking about an airplane using a jet engine or a prop. The engine/prop sucks in air from the front and spits it out the back. With the (negligible) friction barrier broken, the runway and the wheel don"t matter AT ALL. The plane will accelerate porportional to how fast the engine/prop pulls it through the air, and eventually move through the air fast enough to generate lift on the wings.
A good analogy ripped from the physics forum link...
Instead of a plane on the conveyer belt, its you, you"re wearing rollerblades, and you have a (forward firing) jetpack on your back. You will move forward. This is literally the EXACT SAME situation as far as physics is concerned.
It feels really redundant writing out a post this long after so much has been said, but some people are just so wrong I can"t stand it.
if the wheels had to spin for acceleration to take place, how would the plane accelerate once it is in the air?
the only purpose of the wheels is to remove friction from the plane and the runway
It does affect it, in proportions though. And to clarify on the wheels, the HARDER wheels will coast FARTHER than softer. Harder wheels have LESS rolling resistance than soft ones. The soft wheels under pressure will create a bigger ?footprint? on the surface. That larger footprint means more surface contact, meaning increased resistance.bcware said:
Close though ?
I used to skateboard in high school. Idealy you might be right, but in reality soft wheels go MUCH MUCH MUCH longer than hard wheels. Long boards have very soft wheels. The boards you see with people doing all sorts of crazy tricks tend to have hard wheels. The wheels are made of urethane i"d think and even under immense pressure "expansion" is negligable.Bonzai Da Gnome said:
Longboard wheels are not only much softer but much wider as well.
the wheels must spin for acceleration to take place until sufficient speed is generated and lift occurs. the reason they "must" spin is because if they didn"t the plane would drag across the ground and friction would prevent it from ever getting enough speed to take off.Metranon said:
anyway, i"ve been thinking of this problem as a plane sitting on a dyno. a "conveyer" belt would never work by any stretch of the imagination because there is no way to control the plane and keep it in a perfectly straight line. the trick to keeping the plane on the dyno would be to accelerate very slowly because even powerful cars can jump a dyno, nevermind a plane. to me it seems very reasonable that a plane could be maintained in equilibrium on the dyno. it all boils down to what your goal is, do you WANT the plane to jump the dyno or do you want to keep it on? if i attached wings to my 1200 bhp dodge viper would it take off while it"s on the fucking dyno? NO, it wouldn"t do anything. But i could make my dodge viper fly if i gave it wings and drove it outside, and this has been demonstrated by planes that are powered by cars. We have rocket powered cars that drive with jet engines and we have combustion engines, the two generate forward motion in different ways but drive exactly the same.
What?s more, I don?t believe that a plane that can ?beat? the conveyer belt would ever just instantly take flight. Even balls to the wall the plane has no relative motion and thus no air going over its wings until it breaks free of the conveyer belt/wheel system. The plane, if it broke free, should drive straight off the edge of the conveyer belt, roll across the ground for a 100 feet and THEN take off.
So then you acknowledge that the plane would move forward if it had no wheels, but the friction would keep it from taking off. And you realize that its friction that is holding the plane back right?bcware said:
Well the whole point of the wheel is to reduce the friction, at what point the thrust of the engine only has to overcome the friction of the wheel spinning, which in relation to thrust, is almost 0.
yeah i can see how it would beat a flat conveyer belt system. i"d like to see some diagrams though, i want to know how the mass of the plane applied to the wheel works while running on a flat conveyer belt system because i"m still not totally clear on that.Zuuljin said:
Perhaps I can make use of the dyno example. See the car is propelled forward by "pushing" off the ground. When you put a car on a dyno, the rollers in the back are what the wheels are "pushing" off of, but since they spin with the tires, the car go nowhere. Obvious yes. Now, the cyclinders on the tires are spinning equally with the speed of the tires correct? Ok, now imagine a big tractor comes along, gets behind the car, and pushes it forward. Will the car move forward and jump the dyno? YES! Because the force of the tractor (thrust of air from plane engine) is completely unrelated to the tires on the dyno (or tires of the plane on the belt).bcware said:
So no matter how fast your car spins its tires, the tractor can still push it off the dyno. Noone here is saying the plane stays in 1 spot then magically jumps off the belt in flight. What we are saying is the belt is irrelevent and the plane goes forward, speeding up until its going fast enough to take off. In which case it would run right off the belt, onto the ground, then take off.
Here"s the simplest, best worded explanation in the thread for folks who don"t know much about physics.Zaptruder said:
Good analogy, Zaptruder!
Eomer
Trakanon Raider
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Again, the easiest way for people to understand where they go wrong is to consider how the conveyor belt would apply a force to the airplane to counteract it"s thrust if said plane is on freely spinning wheels.
I"ve asked this at least two or three times now, and still haven"t gotten an answer. Because there isn"t one.
I"ve asked this at least two or three times now, and still haven"t gotten an answer. Because there isn"t one.
I think it depends wholly on the pilot, screw the conveyor belt for the time okay?
If the pilot doesn"t pull the throttle back... its going no where, duh!!!
TRICK QUESTION.
If the pilot doesn"t pull the throttle back... its going no where, duh!!!
TRICK QUESTION.