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Thought Experiment

Discussion in 'Issues Around the World' started by joseftu, Dec 11, 2006.

  1. Greg

    Greg Full Member

    BTW, some planes don't even have wheels: ski planes and float planes. FWIW.
     
  2. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    You may not have read the nine pages...but you're on the same page as most of us anyway!
    :)
     
  3. mikeky

    mikeky Member

    This actually a good example to get across Leon's point about friction: put that ski plane in a fast flowing river and try to take-off going upstream, and it will never get off the ground (or water, in this case) if the water is flowing fast enough.
     
  4. ethics

    ethics Pomp-Dumpster Staff Member

    I already commented on that, along with water, snow, ice, etc... I am tired of repeating myself.

    I also already stated what WOULD change my mind if I was wrong.
     
  5. Stiofan

    Stiofan Master Po

    So change it already! :lol:
     
  6. ethics

    ethics Pomp-Dumpster Staff Member

    You know what irks me more than anything? No one has tackled this in the scientific community. I've never seen such a legitimate topic get so hot out there and still, no one produced an experiment, even with a radio controlled plane and scaled down with numbers.
     
  7. tke711

    tke711 Oink Oink Staff Member

    Maybe it's because there is nothing to prove since it's so obvious that the airplane takes off. :p

    No, seriously though, your right. This thing is all over the net and it would be nice if someone would put it to bed.
     
  8. Greg

    Greg Full Member

    Well give me a reference to anything you think Leon posted that needs debunking and I'll either support him or debunk him. I'm sure you understand why I'm not interested in reading all the 9 intervening pages. Not that I'm any authority but this is all freshman physics. Knowing Leon I suspect he was correct in his posts because I know him as a cautionary person and I doubt he would step out on thin ice.

    Nevertheless, this "thought experiment" may be tentative to people without technical education, it's plain potatoes for those of us in science related jobs, particularly those that require college degrees in hard science.

    However I think it's an interesting conundrum and I'm happy to provide any enlightenment where I am able. Again, I'm not willing to read the entire thread (perhaps a negative trait) but if you want to sic me on an individual post, gimme the post number and I'll respond.

    This is simple science. Newton (IIRC) figured out the basics, Da Vinci got the concepts, and the Wright Brothers developed the practical applications. If we had the Internet 50 years ago we could have carried on the same discussion and nothing in science has changed in that half century, at least nothing related to the OP.
     
  9. Greg

    Greg Full Member

    Well give me a reference to anything you think Leon posted that needs debunking and I'll either support him or debunk him. I'm sure you understand why I'm not interested in reading all the 9 intervening pages. Not that I'm any authority but this is all freshman physics. Knowing Leon I suspect he was correct in his posts because I know him as a cautionary person and I doubt he would step out on thin ice.

    Nevertheless, this "thought experiment" may be tentative to people without technical education, it's plain potatoes for those of us in science related jobs, particularly those that require college degrees in hard science.

    However I think it's an interesting conundrum and I'm happy to provide any enlightenment where I am able. Again, I'm not willing to read the entire thread (perhaps a negative trait) but if you want to sic me on an individual post, gimme the post number and I'll respond.

    This is simple science. Newton (IIRC) figured out the basics, Da Vinci got the concepts, and the Wright Brothers developed the practical applications. If we had the Internet 50 years ago we could have carried on the same discussion and nothing in science has changed in that half century, at least nothing related to the OP.

    The single thing that is important to an aircraft attaining sufficent lift to become airborne is the speed of the air over the wings. In most instances this is due to ground speed but the essential factor is airspeed over the wings, the factor that provides the lift to take the aircraft aloft. In most instances, again, this is due to the thrust of the engines moving mass backwards and the "every action there is an equal but opposite reaction" concept. This is the same principle that explains why rockets are able to shoot into space and keep going even though there is no air.

    Leon, "You know what irks me more than anything? No one has tackled this in the scientific community."

    The reason the scientific community hasn't tackled this is because it was explained 50, 100, or more years ago (although we did not have the aircraft example until the Wright Bros.). It is the same reason why the scientific community does not address witchcraft, astrology or werewolves. It's lore, not science.

    Science is busy doing science. We don't got no time to waste on folk lore. We don't got no time to waste on pseudo-science, Scientology, or Guided Evolution or whatever the latest pseudo-science name they have to counter the scientific theory of evolution as founded by Darwin.

    You wanna have your witchcraft, your superstition, your religious beliefs, then you're welcome to them. My advice is to not put that on your resume if you're applying for any job related to hard sciences.

    I'm sure there's disharmony in this discussion and I appologize in advance if I've stepped on anybody's toes. If you can't run with the dogs then stay under the porch. The topic of the OP is nothing related to present day science. This was all figured out centuries ago except for the aircraft example which had to wait for the Wright Bros. to develop a practical aircraft.

    Meanwhile I wish I had the money to afford to rent modern day aircraft. Astronomical insurance rates (because of the potential risk) has driven the costs of renting aircraft for common persons such as myself
    into the realm that only very well-to-do persons to afford. Sadly I'm not one of them. I haven't been off the ground in probably a dozen years, not even on commercial airlines, because the costs are prohibitive.

    You'll never know the freedom to fly unless or until you learn to fly an airplane. It's the best fun you can have with your clothes still on. (Many pilots have said this. We pilots are all thick with one another. Nothing will bring you closer than another person than two pilots trading flying stories on the tarmac. At least nothing short of sex.) We pilots are a fraternity you cannot join until you've soloed.

    The speed of the wheels has absolutely nothing at all to do with the aircraft leaving the ground. It's solely due to the speed of the air over the wings. The differential pressure of the air traveling over the long surface of the top of the wing opposed to the higher pressure of the air traveling over the shorter surface of the underside of the wing provides the lift. Look up "Bernoulli" and "Venturi" if you want to grasp this concept. It's anti-intuitive but it's well explained in the literature.

    As an OT comment, science has proven that it is impossible for bees to fly. Go figure.
     
  10. Greg

    Greg Full Member

    It doesn't need putting to bed. It was asleep 50 years ago. The reason it's all over the net is because any idiot can buy a computer and rent an Internet connection.

    Even worse, Google (et al.) has indexed all this crap and something.edu has little more value than my-vanity-page.com to the search engines.

    Real scientists are busy developing real science. Meanwhile for every scientist there's 100 idiots barely smart enough to write HTML or master Frontpage. We all have printing presses now, and Babel is ascendant.

    You wanna understand this stuff, get a hard science degree or spend a lot of time reading the proper books at the library. There is no controversy here, except amongst uneducated amateurs.

    Well there you have it. I've probably offended half of the posters. I better crawl back into the woodwork.

    This is not a scientific controversy. It's an educational controversy.
     
  11. Kluge

    Kluge Observing your world for over 50 years

    Agree with the latter, not so sure about the former. The Rolling Wind Tunnel is why.
    The introduction of details takes the debate away from the thought experiment. Neglecting friction and so on makes the problem too simple, adding the details makes the problem too hard. In the design department, it's called 'job security'. Not the kind that protects secrets, but the the kind that keeps designers employed year after year. :noworthy:
     
    1 person likes this.
  12. ethics

    ethics Pomp-Dumpster Staff Member

    Yeppers. This is where I think majority of the divide is. I think some are discounting too much details. I am approaching this from as much realism as possible and to me, the problem is not simple at all--although I thought it was from the beginning.
     
  13. Greg

    Greg Full Member

    James, your "rolling wind tunnel" has nothing to do with an aircraft taking off. The aircraft will fly if there is sufficient air speed to create the necessary lift and depart the ground. In my example of the chain around the tail, even then the aircraft could leave the ground if the wind speed was sufficient, although of course it would go nowhere except the chain's length off the ground (adjusted for angle).

    Leon, the friction of the wheels against the ground and the friction of the wheels against the bearings mounting them to the landing gear is negligible compared to the thrust of the jet engines. You know that plane would go nowhere if the wheel bearings were frozen or corroded solid, but in a real situation I'll hazard a guess that the frictional force of the wheels holding the aircraft back is perhaps 1-2 percent of the engine's thrust.

    The problem is simple to anybody with a hard science college degree, or at least anybody who took college physics. I spent IIRC a year and a half solving physics problems that make the conundrum of the OP seem like a grade school mathematic word problem.

    Anybody who disagrees with my analysis is welcome to point out any incorrectness and I'll be happy to attempt to explain it further, but just because you don't understand the physics doesn't mean that scientists, or even B.S. college graduates have any doubt about the correct answer. This is like the evolution controversy. Some lay persons and religious followers doubt Darwin's theory because they say it's just a theory. Real scientists specializing in that field have no doubt at all that Darwin was correct.

    So believe whatever you like, and I apologize to anybody who feels that I've come off this with haughtiness. I solve scientific problems much more complicated than this every day at work. (Well, when I'm working I do that.)

    The plane is going to move forwards if the engine's thrust is greater than the friction of the tires as long as nothing else is holding it back. It will leave the ground when the lift exceeds the weight of the aircraft. It doesn't matter at all what the wheels are doing unless their friction is great enough to overcome the engine's thrust. The wheels could be spinning backwards at 600 mph but if the airspeed reaches the speed that we pilots call V<sub>r</sub> ("velocity to rotate" i.e. pull up the nose) the pilot will pull back on the control wheel and the aircraft will leave the ground.

    (V<sub>r</sub> is calculated as V<sub>s</sub> stalling speed plus an adequate safety margin. You might enjoy this explanation although it does not directly address the OP. However much of the math is there if you want to read it.)

    The aircraft in the OP is going to leave the ground if the speed of the air traveling over the wings exceeds stalling speed as long as the pilot pulls up on the elevators at the correct time, and as long as the thrust sufficiently exceeds the frictional counter force of the wheels and the resistance of the air.

    So believe what you like but there is no controversy here, only a difficult problem for those who are not knowledgable in this field. However, it's been interesting debating the OP. It's a good conundrum. :)
     
    1 person likes this.
  14. jfcjrus

    jfcjrus Veteran Member

    I may be wrong, but from your comment, I'd suspect that you've never gone fishing via a float plane into remote areas?

    Floatplanes often land on rivers (moving water, unlike a lake).

    And, after they've picked you up, when they're taxiing away from shore into the river's current, you'd not argue with anyone that would predict that you're going to be swept to your doom, as the raging river current hurls the floatplant downstream.
    But, as soon as the pilot aligns the plans upstream and applies power, you take off as easy as if you were on a glass smooth lake.

    Sorry, I can offer no scientific proof as to why this is so, just observation. ;)

    Regards,
     
    1 person likes this.
  15. Greg

    Greg Full Member

    Jfcjrus is correct. Float planes are difficult to steer until they have built up sufficient airspeed for the rudder to work. At zero speed the only wind comes from the propeller and that has very little authoritivity compared to the turbulence of the water. (I believe some float planes are equipped with small water rudders too, linked to the pilot's controls, to assist at low speeds. Not sure though. I fly only the small, conventional Cessnas and Pipers with tricycle landing gears. Oh yeah, and I never had to take off from a moving belt! ;))


    Let's turn this experiment around. Same airplane, same belt, but this time the belt is going in the other direction, traveling forwards in the direction of intended flight.

    Would the aircraft take more time to take off, less time, or the same amount of time that it would on a conventional runway? This is an essay question. Have at it! :)
     
  16. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    Excellent, Joe. Observation is scientific proof, and this one works perfectly. The river flowing backwards is perfectly analogous to the conveyor belt rolling backwards--and as you say, in both cases, the plane isn't slowed down.
     
  17. cmhbob

    cmhbob Did...did I do that? Staff Member

    Ah HA!

    This finally made it click for me!
     
  18. ditch

    ditch Downunder Member

    Well, maybe not perfectly analogous Joe, despite the fact that the plane takes off in both cases. The fast flowing water for a start is moving against the floats at a constant speed, regardless of the speed of the plane. In the OP the conveyor's speed was perfectly matched to the opposite direction of the wheels as the plane attempted to move forward and only moved when the wheels moved.

    That, and the fact that the floats are partially submerged in the water as opposed to the wheels sitting on the conveyor and only making minimal contact, causing a significant amount of drag, make the two situations substantially different IMHO.

    What the second example does I think, is offer further proof that the conveyor is irrelevant, just as the speed of the rapids is in the second example, as a reason why the plane cannot take off.

    Which is pretty well what you are saying.:)
     
  19. joseftu

    joseftu ORIGINAL Pomp-Dumpster

    Yep. But thanks for the corrections, anyway. Good points.
     
  20. Steve

    Steve Is that it, then?

    At the risk of inflaming matters.....

    I stopped reading Joe's "thought experiment" thread a while back so I have no idea where anyone's opinion stands as of now. I did take the liberty of contacting the High-Powered Rocket Society at the Massachusetts Institute of Technology, asking if anyone there would care to tackle providing an answer, one way or the other.

    Here's one email I received:

    Did you ever get an answer from anyone at the MIT rocket society?
    If not, let me know. I've got an aero/astro degree from MIT, used to be on the staff there, and was a member of the society when I was there. I have an answer for you, but I'll not waste either of our time writing down the explanation if you're already set.

    The short answer is that the jet will, absolutely, take off.
    Details can follow if you need 'em.​

    Then, after replying to her, she sent this:

    Hmmm. Well, let's see. For an aircraft, here are some base
    principles:

    Lift on wings is produced by air moving past the wing. (Or by the wing moving through the air, you can think of it either way, that's just a frame-of-reference thing.)

    We'll assume, just to keep things simple, that the air and the earth around the aircraft are not moving. (In reality, both do, and can affect aircraft, though the earth's movement only gets important when you're dealing with something like the Space
    Shuttle!)

    Jet engines work, basically, by flinging mass backwards with great force, and Newton's third law says that there must be an equal and opposite reaction, which thrusts the plane forward.
    (Rockets also work this way, it's just a question of what they use to make the explosion that sends the gas backwards so quickly. Jets need air to work; rockets provide their own
    propellant.)

    A jet engine works regardless of whether the aircraft is on the ground or in the air, i.e. the state of the aircraft's wheels'
    contact with the ground is immaterial to whether the engine will push the plane forward. (This is a key basic concept here; if you don't believe this - which would be silly, going, as it does, against observed reality, as it's clear that a plane's engines work when the plane is not touching the ground - well, then the rest isn't going to make any sense.)

    So in the thought experiment, as soon as you turned on the engines, the plane would begin to move forward with respect to the stationarey earth and air. The conveyor belt, magically calculating how fast it needs to go (equal to the translational speed of the wheels, i.e, the speed of the aircraft itself) would begin moving backwards at that speed. Assuming that the wheels are doing their job - that is, turning, thus reducing the wheel/ground friction to something that the engines could overcome - then all that would happen is that the wheels would be spinning twice as fast as they would in real life.
    (Translational speed of the aircraft *plus* the speed of the belt moving backwards.) But the plane would, itself, move forward.
    Once it reaches the necessary velocity for takeoff, that is, it's moving fast enough to generate enough lift to counteract its weight, it would do so... It's inevitable if you believe that a jet has the ability to push its vehicle forward with respect to the unmoving world around it.

    I think the source of the confusion must be that everyone has an
    automobile- or human-power-related gut reaction to this thought experiment. If you were in a car on such a conveyor belt, of course, you'd never move with respect to an observer standing on the non-moving ground. Most people are used to thinking about vehicles like cars, rather than airplanes, and I think that just terminally confuses their ability to think this one through.
    Humans naturally have *no* ability to move themselves the way a jet engine lets you move, so there's no gut feeling to go on, and the usual gut reaction, the car/human-powered one, is just wrong here.

    I don't know if that helps; if it's just rehashing the same argument that folks aren't responding to, then I'm not sure what else to try!

    Anne​

    That's pretty much the same things I recall being said in the thread but now it's coming from an MIT erospace engineer, so maybe that makes a difference.

    Now, which one weighs more, a pound of lead or of feathers? :)
     

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