| hondacuraworld |
Imagine a propeller powered plane is sat on the beginning of a massive conveyor belt-type arrangement, as wide and as long as a runway, and intends to take off.
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
:hmmm: |
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| dj-mdx2 |
I'm not an engineer but I'll hazard a guess...
If the plane has enough propeller or jet power to provide lift, and the wings are the appropriate shape, I believe the plane can take off. Aren't a plane's wheels non-powered anyway? |
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| mdxx3 |
^^ oh yah that's right huh?!
(For a moment I was thinking the plane will fall off the conveyor belt.:1pat: ) |
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| Ceenit |
I'll hazard a guess in the opposite direction.
No the plane will not take off. For a plane to fly (or take off) it relies on the aerodynamics of the wind against its wings for lift.
While the propeller may create enough thrust to make the conveyor belt move, there would be no 'aerodynamic lift' to provide the upward thrust needed to make the plane leave the ground and fly. |
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| Ceenit |
| One additional thought, the conveyor belt would not need to be as long as a runway to serve it's function. :rolleyes: |
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| one4gatr |
My guess would be no. If I understand the question correctly the conveyor belt is essentially negating the forward speed of the plane so in theory I assume the plane would maintain its position on the conveyor as it is unable to generate enough forward speed to allow the wings to provide adequate lift.
But what do I know I work in the banking industry... |
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| mdx99 |
| No lift off, 1-1 =0, opposite speed of conveyor belt will balance forward speed of wheel. No wind equal to no uplift at wing.:eek: |
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| G. COLTON |
Yes!!! The conveyor belt has nothing to do with it.
Just think about airplanes with pontoons and skis.
G |
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| Ceenit |
I still say 'no' and have another example to help illustrate.
Your in your work-out room running on your treadmill and your holding a paper airplane in your hand. When you start jogging faster on your treadmill, you drop the airplane.
Will it soar into the room? Nope, it drops because it has no lift to take it to flight.
Someone should take this into Mythbuster. It would be cool to see them bust the myth with full scale model.:D |
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| dj-mdx2 |
quote:
Originally posted by Ceenit
I still say 'no' and have another example to help illustrate.
Your in your work-out room running on your treadmill and your holding a paper airplane in your hand. When you start jogging faster on your treadmill, you drop the airplane.
Will it soar into the room? Nope, it drops because it has no lift to take it to flight.
Someone should take this into Mythbuster. It would be cool to see them bust the myth with full scale model.:D
I did some belated couch research on this. Apparently, this conundrum has been making the rounds on the net before.
This link provides some decent thought experiment answers. Please note a similar analogy to a treadmill, albeit on different terms. |
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| gmc74 |
quote:
Originally posted by Ceenit
I'll hazard a guess in the opposite direction.
No the plane will not take off. For a plane to fly (or take off) it relies on the aerodynamics of the wind against its wings for lift.
While the propeller may create enough thrust to make the conveyor belt move, there would be no 'aerodynamic lift' to provide the upward thrust needed to make the plane leave the ground and fly.
I concur.
A plane gains speed to increase the air flow over and under the wing, causing lift. If there is no air creating lift (because the plane is stationary) then it just sits there. |
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| gmc74 |
quote:
Originally posted by G. COLTON
Yes!!! The conveyor belt has nothing to do with it.
Just think about airplanes with pontoons and skis.
G
Since this plane has wheels, not skis or pontoons, and is sitting still, I do believe that is crack in that there pipe... |
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| dj-mdx2 |
quote:
Originally posted by gmc74
Since this plane has wheels, not skis or pontoons, and is sitting still, I do believe that is crack in that there pipe...
Somehow there is a flaw in your logic. The plane is NOT sitting still - it can move relative to the conveyor belt and may be able to generate enough velocity to achieve lift. Maybe. |
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| one4gatr |
quote:
Originally posted by dj-mdx2
Somehow there is a flaw in your logic. The plane is NOT sitting still - it can move relative to the conveyor belt and may be able to generate enough velocity to achieve lift. Maybe.
I ran this by my dad who was a navy avaitor longer than some of us have been alive and to boot pretty good with physics. Without going into a long discertation he agreed that with the information provided the plane would not take off due to the lack of forward movement. But to that end he also stressed many details are lacking to properly answer the question such as the size/shape of the wings, the amount of thrust provided by the engine, etc.
He did offer though that there may be an assumption that turbulence from the prop combined with the counteraction of the belt could possible generate lift however without knowing the amount of thrust produced by the engine conceivably you might have lift but no forward motion.
Hopefuly Tim will chime in soon and let us know. |
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| one4gatr |
quote:
Originally posted by dj-mdx2
I did some belated couch research on this. Apparently, this conundrum has been making the rounds on the net before.
This link provides some decent thought experiment answers. Please note a similar analogy to a treadmill, albeit on different terms.
Excellent link. So based on how Tim worded his question we are all right... good stuff |
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| hondacuraworld |
I came across a discussion on this on a motorcycle forum. There was no definitive answer there, either.
But that link really does make sense of it :) |
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| G. COLTON |
There is a very definitive answer here. The "Link" seems very clear to me.
Maybe it would help you to think of it this way. Imigane that instead of the conveyor belt the airplane was sitting on a runway that had a perfectly frictionless surface. As far as the wheels of the aircraft are concerned the results are the same. The wheels would not turn as the airplane moves forward and flies into the air.
In fact, the airplane would be able to attain liftoff speed faster than normal because it would not have to overcome the friction of the wheel bearings.
Where some of you are having problems is that your standard frame of reference is vehicles like an automoble. In these vehicles the wheels are the part of the vehicle that are providing the motive power. In the airplane the motive power is provided by the jet/rocket engine and/or the propeller. These are what interact with the air mass to provide the propulsion.
Remember that for an airplane, all that the wheels/pontoons/skis/etc. do for that aircraft is to provide a relative small friction bearing surface that keeps the body of the aircraft from lying on the ground.
G |
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| gmc74 |
I hope mythbusters does a segment on this one, it is interesting, but I don't buy that guys theory. I read an aeronautical engineers post on another site that stated it wouldn't -
"Whether by push or pull, in the real world it's not the engines that generate the Bernoulli/Navier-Stokes airflow over and under the wing to create lift. Maybe there is some lift, but my intuition says it's too turbulent and wouldn't amount to much. It's the whole wing moving through the air with substantially laminar flow that creates the lift" |
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| hondacuraworld |
Think of it this way.....
A roller skate is placed on a sloped treadmill, gravity acts on the skate to pull it down the slope, the treadmill instantly reacts to MATCH the wheel speed. What happens? We conclude the results are a constant never ending acceleration of the treadmill to stay up with the wheel speed.... which cannot happen in reality. How much treadmill speed and acceleration that would be required to keep the skate in place is the question. The only way the treadmill can act on the plane is by applying a large amount of friction.
It would be in a constant state of acceleration as it compensates for wheel speed... |
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| Prodigy7 |
If it's a normal prop plane, the answer is clearly no. If it's one of these then the answer is definitely. |
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| highcountrymdx |
As they say, "Everything is relative!"
The airplane will not fly.
Don't confuse speed relative to the surface that the airplane is sitting on to the air that surrounds it. Consider the airplane's airspeed indicator in all of this. In this conveyor belt example, what will the airspeed indicator read? If the air is calm, no matter how fast the airplane is moving relative to the surface below the plane, its airspeed indicator will always read 0. Note that an airspeed indicator measures energy. No energy, no fly.
Let's consider a special case. Say we have an aircraft sitting on a fixed runway that is infinitely long. This particular day, this airplane is experiencing a tailwind of 200 kts. (Extreme example, I know.) As the aircraft reaches a speed across the ground of 200 kts, the airspeed indicator in the cockpit reads 0. You can haul back on the control column all you want, but the aircraft will remain on the ground. Assuming, just for this discussion, the tires have no speed limit, and the airplane normally takes off at an indicated speed of 120 kts, the aircraft would have to accelerate to a ground speed of 320 kts to become airborne.
Note the exceptions: One can design an aircraft that can mount enough engines so as to supply enough airflow over the wings that will provide enough lift to become airborne (See above). In jets, consider the Harrier. It can become airborne with no forward speed at all! But these are special cases.
Again, it's all relative. |
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| dj-mdx2 |
| It would be a very expensive test to simulate using a real conveyor belt. I guess if you could devise a contraption that will hold a plane stationary (simulating the effect of the belt) and yet measure the amount of upward velocity you could determine if the plane generates enough upward force to overcome drag and gravity. Or maybe someone with enough tech know-how can simulate it on computer. But that would be really difficult since lift is still a mysterious entity that even physicists have a tough time explaining. The Bernoulli principle explanation has its loopholes, since it doesn't explain inverted flight. Newtonian physics may explain it better. But that's a different thread entirely.:) |
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| G. COLTON |
This is really amazing. Have some of you actually read the LINK that was posted?
G |
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| one4gatr |
quote:
In fact, the airplane would be able to attain liftoff speed faster than normal because it would not have to overcome the friction of the wheel bearings.
Wow... this is an interesting debate. The more I think about it the more I believe the question cannot be answered as posed. As my father suggested we dont know the amount of thrust being produced nor the size/shape of the wing and the weight of the plane.
To put it into a different perspective think of the space shuttle. With enough thrust behind the shuttle it is great enough to overcome the force of gravity (at least as long as the fuel supply lasts). But if you put on a bunch of bottle rockets it sits on the pad.
Airplanes become airborne at different rates of speed relative to their size, shape, power, etc... A Piper Cub for example will become airborne much faster than a C130 even though both are "prop" driven.
I respectfully disagree with the assertion that the plane would "fly" without forward motion though. The engines provide the forward motion which allow the wings to provide the needed lift to get the airplane airborne. The way the question is worded says the forward speed of the plane is negated by the reverse speed of the conveyor. If planes were able to generate the required lift simply due to their engines there would be no need for runways. (with the exception of Harriers and the like but the still require enough power to lift verticle and slowly redirect the flow to produce adequate forward motion to keep the plane airborne) |
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| nightguy |
| If the plane could fly, then I wouldn't need a fan blowing on me during a treadmill workout. It would be easier to take off from cities with mountains around them. And aircraft carriers wouldn't need catapults. |
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| highcountrymdx |
dj-mdx2
Inverted flight, in fact, in no way violates the Bernoulli principle. Where did you hear that?! |
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| G. COLTON |
This is a problem in reading comprehension, NOT a physics problem. Many of you on here need to return to school and retake reading comprehension.
NO WHERE in the original post does it say that the aircraft is motionless in reference to the ground. It says that the conveyor belt has a speed such that the WHEELS of the aircraft are motionless.
The ONLY thing that the wheels of the aircraft do for the aircraft is keep the belly of the aircraft from touching the ground. The wheels HAVE NOTHING to do with the propulsion of the aircraft. Pontoons, skis, etc. do the same thing. Pontoons and skis are always motionless in relation to the aircraft.
Go back and reread the original post and think about what it really says.
Then you will see that there is nothing that prevents the aircraft from flying in whatever is it's normal manner.
Now that I have seen how much fun this post is on this forum I am going to post it on a boating forum and see how many people there catch on. There are several astute pilots there so it should be solved fairly quickly.
G |
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| mdx99 |
Look! if this conveyor theory can fly an airplane, then all the aircraft carrirers in the world would have this type of conveyor short runway instead of hard runway, don't you think?? :1:
I am no mechanical engineer only a dumb structural guy! |
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| highcountrymdx |
G
I think the problem in interpretation is, in fact, yours. I humbly suggest you go back and reread the original setup.
Consider the wheels. Are they moving, stationary, or rotating? For the purposes of flying (generating lift), their condition is irrelevant. They might help the airplane in movement over the ground surface, and to manage energy transfer to the aircraft structure on landing, etc.
But what's relevant to leaving the ground is the energy state of the wind field over the wings, which is zero. Get it? This is a gross simplification, I know, but an aircraft produces lift as a function of wing design (coefficient of lift), speed of air over the lifting structure, and angle of airflow, relative to the chord of the wing (angle of attack). In this case, again, no airflow, no lift.
Keep your eye on the prize. Don't be distracted.
Please note, although not necessarily astute, I have flown for 40 years, have 24,000 hours in the air, and have not yet left the ground, or flight deck, with zero airspeed, excepting the few hours buzzing around in a helicopter. NB: even a helicopter needs air movement over its rotors (think rotating wings) to fly. |
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| mdx99 |
The key is that if the conveyer belt's speed is constantly matching the speed of the plane. That means the plane will have to constantly accelerate to maintain a certain speed because of the fiction between the belt's material & the rubber of the wheel. I don't see any motion here. No matter how much thrust the engine is translating into spinning of the wheel & being counteracted by the equal but opposite movement of the conveyor belt, there is no movement & therefore no uplift below the wing to lift off.
Come to think of it this way, what if the conveyor is going in the same direction as the plane, then we may only need a shorter runway as the belt will speed up the plane without additional thurst. I wonder if this theory will work on our next generation aircraft carrier. You hear this here first!:2: |
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| highcountrymdx |
| Do you mean this? |
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| mdx99 |
quote:
Originally posted by G. COLTON
In fact, the airplane would be able to attain liftoff speed faster than normal because it would not have to overcome the friction of the wheel bearings.
G
Back to static of equilibrium. When the conveyor belt is running in the opposite direction versus that of the wheel, it's actually creating more friction between the 2 surfaces thereby reducing the speed of the plane NOT increasing it. Frictional forces are in opposite direction. Principle of mechanic 101.:confused: |
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| mdx99 |
quote:
Originally posted by highcountrymdx
Do you mean this?
YES! where is the belt??:2: We should get a patent for this & go to the Pentagon. |
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| G. COLTON |
quote:
Originally posted by mdx99
Back to static of equilibrium. When the conveyor belt is running in the opposite direction versus that of the wheel, it's actually creating more friction between the 2 surfaces thereby reducing the speed of the plane NOT increasing it. Frictional forces are in opposite direction. Principle of mechanic 101.:confused:
If the wheel is not moving there IS NO friction.
G |
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| G. COLTON |
quote:
Originally posted by mdx99
The key is that if the conveyer belt's speed is constantly matching the speed of the plane. That means the plane will have to constantly accelerate to maintain a certain speed because of the fiction between the belt's material & the rubber of the wheel. I don't see any motion here. No matter how much thrust the engine is translating into spinning of the wheel & being counteracted by the equal but opposite movement of the conveyor belt, there is no movement & therefore no uplift below the wing to lift off.
Come to think of it this way, what if the conveyor is going in the same direction as the plane, then we may only need a shorter runway as the belt will speed up the plane without additional thurst. I wonder if this theory will work on our next generation aircraft carrier. You hear this here first!:2:
Now that I read some of this I can see that some people not only need a course in reading comprehension they need a course in high school physics.
G |
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| Ceenit |
G,
I'm a pretty good reader and have better than average intelligence when it comes to physics. But I'm definitely not infallible.
Can you point out in a fact based discussion if you think airspeed is a requirement for takeoff? Also, where then would the airspeed be derived from in this scenario?
I think there is a difference in interpretation if the plane is actually moving. I'd rather come to agreement by clarifying the assumptions and data versus questioning the education level of others. :) |
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| gmc74 |
quote:
Originally posted by G. COLTON
Now that I read some of this I can see that some people not only need a course in reading comprehension they need a course in high school physics.
G
If a plane were pushed forward, it would move the distance that the wheels would take it. If, upon pushing the plane forward, you "pulled the ground back" with the same force, the plane would stay in the same place.
The propeller is going to pull at the plane, but the plane needs to gain speed to create lift.
To me, this would be the same as hooking the back of a plane to a tree, pulling the line tight, and giving it full throttle. It isn't going to start to take off, because it isn't moving, thus creating zero lift.
The wheels (or skis, or pontoons) are used to transfer the energy to the ground, if that energy is negated, then you won't be able to gain speed. If you can't gain speed, you can't get air flow over the wings, no airflow, no lift. No lift, you have yourself a car with wings.
I am pretty sure it is you who should spend a little time hitting the books. Why don't you go and get a credible source for your theory, before telling everyone else that they are idiots and need to learn how to read.
This isn't the first thread that I have found you making an a$$ out of yourself, why don't you learn how to get along with people rather than fight with them. |
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| nightguy |
quote:
Originally posted by G. COLTON
Now that I read some of this I can see that some people not only need a course in reading comprehension they need a course in high school physics.
G
The people that built my treadmill are going to be soooo upset. :) |
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| G. COLTON |
quote:
Originally posted by nightguy
The people that built my treadmill are going to be soooo upset. :)
Why?
Do you try to fly airplanes from your treadmill?
G |
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| nightguy |
quote:
Originally posted by G. COLTON
Why?
Do you try to fly airplanes from your treadmill?
G
Because I always outrun it even though it's moving in the opposite direction at the same speed. |
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| mdx99 |
[QUOTE]Originally posted by G. COLTON
Now that I read some of this I can see that some people not only need a course in reading comprehension they need a course in high school physics.
G [/QUOTE
You are hopeless!! It's okay to be wrong but to be wrong & NOT knowing that you were wrong is hopeless & dangerous. BTW, which school you went to? |
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| mdx99 |
quote:
Originally posted by G. COLTON
If the wheel is not moving there IS NO friction.
G
Try pull your car into one of those smog check station where the wheels is running at high speed on a set of rollers while they are checking the exhaust. Now, the wheels are moving, right? but the car is not moving, dude! |
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| mdx99 |
quote:
Originally posted by hondacuraworld
Imagine a propeller powered plane is sat on the beginning of a massive conveyor belt-type arrangement, as wide and as long as a runway, and intends to take off.
The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
:hmmm:
BTW, I just emailed a professor at UCLA's aerospace engineering department. I will keep you guys updated when I get a reply from him. Since about 50/50 of the members in this forum have opposite answers, it will be interesting to get an opinion from the true expert. I don't think HS physic taught that. Stay tuned! |
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| G. COLTON |
quote:
Originally posted by mdx99
Try pull your car into one of those smog check station where the wheels is running at high speed on a set of rollers while they are checking the exhaust. Now, the wheels are moving, right? but the car is not moving, dude!
????????? And your point is???
G |
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| nightguy |
quote:
Originally posted by G. COLTON
????????? And your point is???
G
Let's say the shop that houses the dynamometer is in a business district where the speed limit is 30 mph. You rev up the engine and get the tires spinning at 70 mph. The speedometer shows that the car's speed is 70 mph.
A patrol officer shows up at the door with a speeding citation. She says with radar, she clocked your car traveling 40 mph over the speed limit.
Should you argue the ticket ? |
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| oceanMDX |
The plane would have little difficulty taking off.
Consider this thought experiment: Imagine that the wheel bearings were absolutely frictionless. If the engine of the plane remained off, and the treadmill was turned on in an attempt to make the plane move backwards, the wheels would turn but the plane would not move (zero velocity relative to the ground) because no force is being applied to the plane itself (no force to counter its inertia). You can increase the speed of the treadmill all you want and only the rotational velocity of the wheels will change. Since the wheel bearings are frictionless in this case, no force is transmitted to the plane so it doesn't move. Now run the engine and spin up the propeller. This force (thrust) makes the plane accelerate forward until the plane takes off.... no matter how fast the treadmill moves.
Now in the real world, the wheel bearings do have some friction. However, even when the treadmill accelerates (velocity continually increasing), only a small negative force is transmitted to the plane (which tends to make the plane move backwards). The positive (forward) force generated by the prop is much larger than the negative force created by the treadmill (thrust >> drag). If this were not the case, the plane couldn't take off even on a normal runway. The resultant net force (adding the vectors) would clearly be positive, so the plane would accelerate forward until airborne.
Recall: a= F/M (acceleration = force/mass) |
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| oceanMDX |
The only way that the treadmill could apply enough negative (backward) force on the plane to prevent it from accelerating forward would be if you applied the plane's brakes.... that's also the case on a normal runway.
Bottom line: whether the plane is powered by prop or jet, a treadmill running backwards has little affect on the plane's forward acceleration or its ability to fly.
You guys that think the plane couldn't take off should release the park brake and try again. :D |
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| one4gatr |
quote:
Originally posted by oceanMDX
The only way that the treadmill could apply enough negative (backward) force on the plane to prevent it from accelerating forward would be if you applied the plane's brakes.... that's also the case on a normal runway.
Bottom line: whether the plane is powered by prop or jet, a treadmill running backwards has little affect on the plane's forward acceleration or its ability to fly.
You guys that think the plane couldn't take off should release the park brake and try again. :D
Ok... So I put a R/C prop on a C130 and that puppy is taking off right? The treadmill does impart some effect on the plane. The way the question is worded is the treadmill matches the speed of the wheels... thus negating the "forward" movement of the plane (in theory). The way I see it a treadmill moving backwards (east to west) would actually rotate the wheels forward (counter clock wise). But the weight of the plane would keep it static.
As I mentioned in my earlier post if enough thrust is applied you can get anything airborne. The question as posed is impossible to answer. We really dont know the amount of thrust required to get said plane airborne. It could be feasable to match the speed of the treadmill but not have enough thrust to generate liftoff. |
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| gmc74 |
quote:
Originally posted by one4gatr
Ok... So I put a R/C prop on a C130 and that puppy is taking off right? The treadmill does impart some effect on the plane. The way the question is worded is the treadmill matches the speed of the wheels... thus negating the "forward" movement of the plane (in theory). The way I see it a treadmill moving backwards (east to west) would actually rotate the wheels forward (counter clock wise). But the weight of the plane would keep it static.
As I mentioned in my earlier post if enough thrust is applied you can get anything airborne. The question as posed is impossible to answer. We really dont know the amount of thrust required to get said plane airborne. It could be feasable to match the speed of the treadmill but not have enough thrust to generate liftoff.
I agree, until you can create lift, the prop just pulls the plane along the ground, but it isn't going anywhere because the ground is moving backwards. |
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| G. COLTON |
Well, I guess boaters have a better reading comprehension and physics background. It did not take that group long to come to agreement on the answer.
Some of the reasoning on this board is unbelievable!!!
G |
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| oceanMDX |
quote:
Originally posted by one4gatr
[B]
Ok... So I put a R/C prop on a C130 and that puppy is taking off right? The treadmill does impart some effect on the plane. The way the question is worded is the treadmill matches the speed of the wheels... thus negating the "forward" movement of the plane (in theory). The way I see it a treadmill moving backwards (east to west) would actually rotate the wheels forward (counter clock wise). But the weight of the plane would keep it static.
This is getting bizarre. No one was making strange modifications to try to support their argument until you with this C130..... you could have suggested we put the prop on backwards just as well.
Look, the speed of the treadmill, the speed of the wheels, nor the price of jet fuel in Norway is relevent here. The only thing to work out is the resultant net force acting on the body of the plane.... I already explained how that works.... it doesn't matter much even if the treadmill is moving at 300 mph.... the drag imparted to the plane (through its wheel bearings) will be small relative to the thrust the prop (normally equipped) can generate.
quote:
As I mentioned in my earlier post if enough thrust is applied you can get anything airborne. The question as posed is impossible to answer. We really dont know the amount of thrust required to get said plane airborne. It could be feasable to match the speed of the treadmill but not have enough thrust to generate liftoff.
The question is very elementary.... you only have to apply the simple principles of Newtonian Mechanics.... that's the problem here, too many of you aren't doing that. As long as thrust exceeds drag, the plane will accelerate forward until it's airborne. You can't generate much drag on an airplane through the wheels until its brakes are applied.
The amount of thrust that any normally equipped airplane has already would easily overcome whatever (small) additional drag on the plane that a treadmill could exert.... you seem to forget that the wheels have free-spinning bearings.... they are designed to minimize friction.
Any normal airplane (prop powered or jet) would have no difficulty taking flight with this treadmill arrangement. |
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| oceanMDX |
quote:
Originally posted by gmc74
I agree, until you can create lift, the prop just pulls the plane along the ground, but it isn't going anywhere because the ground is moving backwards.
Actually, the prop creates forward thrust.... it doesn't simply "pull the plane along the ground"..... what happens to the ground is largely irrelevent to the plane. It's not a car, it's an airplane we're talking about. A car moves by applying a force to the ground through its wheels, an airplane does not.... its power moves the air. The car is "connected" to the treadmill in a way that a plane is not.
Tell me, if the wheel bearings of the plane where perfectly frictionless, how would spinning the treadmill affect the plane - with its engine off - other than spin the wheels? Do you think that the plane would move? The answer is the plane wouldn't move because the bearings isolate the plane from any force coming from the treadmill. If I jacked both wheels of a Cessna 152 off the ground, I could easily spin a wheel with my hand... and it would have little affect on the rest of the plane.... you could spin the other wheel... so the two of us would spin both wheels like crazy and if someone came along and powered up the engine the plane would accelerate forward and run right over both of us. :2: |
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| oceanMDX |
quote:
Originally posted by G. COLTON
Well, I guess boaters have a better reading comprehension and physics background. It did not take that group long to come to agreement on the answer.
Some of the reasoning on this board is unbelievable!!!
G
The problem is, some people try to predict how matter would behave while ignoring the revelent physics. |
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| dj-mdx2 |
| If only our younger generation were so passionate about science as the rest of the, er, senior population here, the future would be so much brighter.:2: |
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| nightguy |
quote:
Originally posted by oceanMDX
The problem is, some people try to predict how matter would behave while ignoring the revelent physics.
You guys are funny.
Please tell me why this treadmill theory isn't applied at airports around the world ! |
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| gmc74 |
quote:
Originally posted by oceanMDX
Actually, the prop creates forward thrust.... it doesn't simply "pull the plane along the ground"..... what happens to the ground is largely irrelevent to the plane. It's not a car, it's an airplane we're talking about. A car moves by applying a force to the ground through its wheels, an airplane does not.... its power moves the air. The car is "connected" to the treadmill in a way that a plane is not.
Tell me, if the wheel bearings of the plane where perfectly frictionless, how would spinning the treadmill affect the plane - with its engine off - other than spin the wheels? Do you think that the plane would move? The answer is the plane wouldn't move because the bearings isolate the plane from any force coming from the treadmill. If I jacked both wheels of a Cessna 152 off the ground, I could easily spin a wheel with my hand... and it would have little affect on the rest of the plane.... you could spin the other wheel... so the two of us would spin both wheels like crazy and if someone came along and powered up the engine the plane would accelerate forward and run right over both of us. :2:
I can begin to see what you are saying, and in theory it seems fine to me.
I keep going back to the fact that you can't create lift with out movement, and you can't create movement if the force of the propeller isn't being put to the ground.
I guess I will have to wait for Mythbusters... |
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| oceanMDX |
quote:
Originally posted by nightguy
You guys are funny.
Please tell me why this treadmill theory isn't applied at airports around the world !
Treadmills - running in a direction to try to move the plane forward - aren't used because they don't have much affect on the plane.... just like I have been telling you.... the US Navy uses catapults, not treadmills. :2: |
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| oceanMDX |
quote:
Originally posted by gmc74
I can begin to see what you are saying, and in theory it seems fine to me.
I keep going back to the fact that you can't create lift with out movement, and you can't create movement if the force of the propeller isn't being put to the ground.
I guess I will have to wait for Mythbusters...
Your first point is valid.... you can't produce much lift by the simple backwash of the prop over the wings because the air over the wings is too turbulent.... you need laminar airflow over the wings to produce much lift, and that is created when the "relative wind" is great enough.... generally, that means that the plane has to be able to accelerate forward and gain enough speed.
Your second point is totally false... you have no difficulty creating movement (of the plane) without the engine (or prop) power being applied to the ground.... that's the whole advantage in this context of what a plane has over a car. |
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| mdx99 |
quote:
Originally posted by oceanMDX
If I jacked both wheels of a Cessna 152 off the ground, I could easily spin a wheel with my hand... and it would have little affect on the rest of the plane.... you could spin the other wheel... so the two of us would spin both wheels like crazy :2:
I think that's how I spinned the wheels of my MDX after I jack up the car to change tires. Maybe I can add wings to the X' & put a turbo engine in the rear so it can pop up in the freeway & fly.....:2: |
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| gmc74 |
quote:
Originally posted by oceanMDX
Your first point is valid.... you can't produce much lift by the simple backwash of the prop over the wings because the air over the wings is too turbulent.... you need laminar airflow over the wings to produce much lift, and that is created when the "relative wind" is great enough.... generally, that means that the plane has to be able to accelerate forward and gain enough speed.
Your second point is totally false... you have no difficulty creating movement (of the plane) without the engine (or prop) power being applied to the ground.... that's the whole advantage in this context of what a plane has over a car.
I just don't see it, I have taken many physics courses, but I am definitely not a plane person. I will have to wait and see if mythbusters can do this one.
At best, I think it would be more like a plane with skis (rather than wheels) trying to take off on a concrete runway, which most likely wouldn't work. |
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| oceanMDX |
quote:
Originally posted by gmc74
I just don't see it, I have taken many physics courses, but I am definitely not a plane person. I will have to wait and see if mythbusters can do this one.
At best, I think it would be more like a plane with skis (rather than wheels) trying to take off on a concrete runway, which most likely wouldn't work.
I'll take one more crack at explaining it to you. You have to realize that only a force applied to an object can make it move/change velocity (accelerate). Now, imagine the plane is sitting on the treadmill with a spring scale (like the ones fishmen use to weigh a fish) attached to the front of the plane to measure the maximum backward force created by the treadmill when it is running. The plane's engine is off while we measure just how much backward force the treadmill can apply on the plane through the entire speed range of the treadmill..... since the plane's wheel bearings are not totally frictionless, there will be some small backward force applied to the plane. We are holding the plane in position with the spring scale and measure the maximum backward force (drag) at, say, 30 pounds.... it's not a huge plane. Now, we stop the treadmill and place the spring scale at the back of the plane to measure the maximum thrust generated by the prop under full engine power. This forward force (thrust) could be, say, 100 pounds. We already understood that thrust from the prop (under max. power) had to be greater than the drag generated by the runway surface through the wheel bearings because the plane is able to take off from a normal runway. Since thrust is greater than even the maximum drag that the treadmill can cause, the plane will experience a minimum force of 70 pounds forward. The plane has to accelerate forward until it reaches takoff speed no matter how fast the treadmill is going...
Bottom line... no matter what the treadmill does, it can't stop the plane from taking off.:4:
Now if we redid the drag measurement (using the spring scale) with skis on a rubber treadmill, the reading would be huge because there would be so much friction between the skis and the treadmill... the measurement might be 300 pounds? The plane couldn't move relative to the treadmill with the prop under full power.... we would need a much more powerful engine/prop combination (to exceed the 300 pound drag) to make the plane fly. |
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| nightguy |
| So since a glider has no engines it won't fly ? :confused: |
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| oceanMDX |
quote:
Originally posted by nightguy
So since a glider has no engines it won't fly ? :confused:
A glider is towed aloft by the power of another plane. When it is released from the tow rope, it is already flying. It is moving forward and has laminar airflow and upward moving currents of air (thermals) keeping it aloft. When it lowers its nose, it gains speed and that creates more lift.
...without the other plane, yeah, a glider will just sit on the ground. Did you sit in one and wonder why it wasn't taking off? :o |
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| gmc74 |
quote:
Originally posted by oceanMDX
I'll take one more crack at explaining it to you. You have to realize that only a force applied to an object can make it move/change velocity (accelerate). Now, imagine the plane is sitting on the treadmill with a spring scale (like the ones fishmen use to weigh a fish) attached to the front of the plane to measure the maximum backward force created by the treadmill when it is running. The plane's engine is off while we measure just how much backward force the treadmill can apply on the plane through the entire speed range of the treadmill..... since the plane's wheel bearings are not totally frictionless, there will be some small backward force applied to the plane. We are holding the plane in position with the spring scale and measure the maximum backward force (drag) at, say, 30 pounds.... it's not a huge plane. Now, we stop the treadmill and place the spring scale at the back of the plane to measure the maximum thrust generated by the prop under full engine power. This forward force (thrust) could be, say, 100 pounds. We already understood that thrust from the prop (under max. power) had to be greater than the drag generated by the runway surface through the wheel bearings because the plane is able to take off from a normal runway. Since thrust is greater than even the maximum drag that the treadmill can cause, the plane will experience a minimum force of 70 pounds forward. The plane has to accelerate forward until it reaches takoff speed no matter how fast the treadmill is going...
Bottom line... no matter what the treadmill does, it can't stop the plane from taking off.:4:
Now if we redid the drag measurement (using the spring scale) with skis on a rubber treadmill, the reading would be huge because there would be so much friction between the skis and the treadmill... the measurement might be 300 pounds? The plane couldn't move relative to the treadmill with the prop under full power.... we would need a much more powerful engine/prop combination (to exceed the 300 pound drag) to make the plane fly.
I totally get what your saying, and I think this is a perfect way of explaining it.
I definitely see your point, and though it goes against what my brain sees, I understand how it could potentially get the speed to take off. Now the question is, can it do it in the same distance? |
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| oceanMDX |
quote:
Originally posted by gmc74
I totally get what your saying, and I think this is a perfect way of explaining it.
I definitely see your point, and though it goes against what my brain sees, I understand how it could potentially get the speed to take off. Now the question is, can it do it in the same distance?
Good!
The answer to your question involves determining whether or not drag (from the wheels) increases significantly as the rotational speed of the plane's wheels increases. I suspect that the drag would increase to a small degree as the wheels sped up.... if that's true, it would take a bit longer for the plane to become airborne than if the plane was on a normal runway. However, as the plane is travelling down the runway gaining speed, less weight is applied to the wheel bearings because the wings are starting to generate lift... and that will reduce friction in the wheel bearings and tend to reduce drag. I would think that this last factor is more important to drag than the former. In other words, I would guess that drag on the plane due to the wheels on either type of runway would decrease as the plane built up speed. Of course air drag on the wheels increases with the square of the air velocity.... that's why faster planes retract the wheels once in flight.... but that's beside the point we are making with the treadmill. |
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| gmc74 |
Something else I was thinking of, is that the potential for the plane to dive nose first is pretty high
I have a friend who is an aeronautical engineer, I will be posing this question to him tomorrow. I will let you know what he thinks. |
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| gmc74 |
Ok, I can't let this go.
I was thinking about it some more...
Let's say for arguments sake, you were in the plane on the treadmill, with the plane's brake engadged, and the engine fired up. The plane would be stationary (this would be true on the tarmac or the treadmill).
I think we agree so far...
If you release the break, the plane will want to go forward (i.e. the wheels can now move) but it won't, because as the wheel spins forward, the treadmill moves back wards the same amount.
Now, if the engine had enough power to pull the plane with the brakes on, then the tires wouldn't rotate, and thus the conveyor would not spin, so the plane would move forward, but very slowly because of the friction caused by the wheels skidding across the ground.
The plane, if it could move forward, would not move forward fast enough to create enough wind over the wing to create lift.
I think this goes back to my original post about transfering the power to the ground, you can't do it efficiently enough to take off in this scenario. |
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| oceanMDX |
quote:
Originally posted by gmc74
Something else I was thinking of, is that the potential for the plane to dive nose first is pretty high
I have a friend who is an aeronautical engineer, I will be posing this question to him tomorrow. I will let you know what he thinks.
The plane can't dive nose first because the front landing gear prevents that.... it holds the nose up, and there is no excessive downward force on the nose in any case. |
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| oceanMDX |
quote:
Originally posted by gmc74
[B]Ok, I can't let this go.
I was thinking about it some more...
Let's say for arguments sake, you were in the plane on the treadmill, with the plane's brake engadged, and the engine fired up. The plane would be stationary (this would be true on the tarmac or the treadmill).
I think we agree so far...
Yes. Of course when the treadmill is turned on, with the brakes still on, the plane would move backwards.
quote:
If you release the break, the plane will want to go forward (i.e. the wheels can now move) but it won't, because as the wheel spins forward, the treadmill moves back wards the same amount.
The plane won't "want to go forward" .... it will go forward. Again, you are completely wrong on this point... as the plane moves forward, the wheels will move faster than the treadmill... it is impossible for the treadmill to keep up with the speed of the tires if the plane moves forward, which it must. The operator of the treadmill can turn the treadmill to its maximum speed, yet the plane will move forward so the wheels of the plane will actually go faster than the treadmill. Although the question isn't worded exactly this way, we both understand what the question meant... that was: can running a treadmill backwards at any speed prevent an airplane from taking off? The answer is obviously no!
It's like you wearing a pair of roller skates on a treadmill. No matter how fast the treadmill is going, you will always be able to pull yourself forward with a rope. Of course the moment you start moving forward, the wheels of the roller skates will be moving faster than the treadmill. The drag on the skate wheels caused by the treadmill (pulling you back) is much less than the force that you can apply to the rope.... so you can move forward | | | |
