Physics Question for you.....

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  • tjmac44
    Forum Newbie
    • Nov 2006
    • 76
    • Omaha, Nebraska

    #46
    3 forces involved. Forward force of the plane, opposite force of the conveyor and downward force(gravity).
    Wont fly. Dont forget about gravity acting on the plane, the downward force. The upward force, lift of the turbulence created by air moving over the wing, must overcome gravity. This wont happen because the planes movement, forward force, is cancelled by the conveyors opposite force.
    Todd

    Grounded in fly-over country.

    Comment

    • BigguyZ
      Veteran Member
      • Jul 2006
      • 1818
      • Minneapolis, MN
      • Craftsman, older type w/ cast iron top

      #47
      The Air plane will not fly! OK, I think we can all pretty much agree that the plane's speed realative to the ground means nothing. It's the plane's speed in relation to the air around it. It takes a large mass of air passing over/under the wing for the Bernouli Effect to create enough of a pressure differential for the upward lift (caused by a higher pressure below the wing than above) to overcome the force of gravity.

      Now, the engines themselves move air. That is, after all, how they create thrust. However, they do NOT move enough air by themselves to create enough lift to allow the plane to take off. In a prop type engine or a Jet engine (though especially a jet engine) they do not suck air over the entire wingspan. That's why when a plane moves forward through the air, which for all intensive purposes is static in relation to the ground, there is a greater amount of wind current over the entire wingspan than when the only airflow is generated by the flow of air through an engine.

      It won't happen.

      Comment

      • leehljp
        Just me
        • Dec 2002
        • 8429
        • Tunica, MS
        • BT3000/3100

        #48
        Originally posted by mmgross144
        In response to:

        "THRUST does NOT make the plane fly! The PURPOSE of Thrust is to get enough speed up so that there is a fast flowing of wind/air OVER the WINGS. It is WIND Speed over the Wings that create lift for flying.

        Thrust will not make it "fly" until there is enough airspeed (wind) over the wings.

        Since the natural winds are hardly controlable, nor convenient to our flight requirements, artificial wind must be generated, BY THRUST, to acheive sufficient airspeed.

        OK, sorry to belabor the subject. Have a wonderful holiday season!
        Are we forgetting about gliders? When the tow cable is released, gliders don't drop straight to the ground.

        Some of you Canadians will remember the commerical jet about 15 years ago in which there was a mix up in liters/gallons for the American pilot. He ordered gallons/ they put in liters. The plane was somewhere near 30,000 feet or so and the engines quit - fuel starvation. No Fuel. They glided 50 or 60 kilometers and did a dead stick landing safely at an abandoned airfield where there was a lot of Sat/Sun afternoon crowd, jsut missing them.

        Same thing when a 747 about 15 - 20 years ago flew through a erupting volcano cloud at night in SE Asia. No one knew the remote volcano was erupting. Pilot thought it was high clouds at 30,000+ feet. Engine failure and they glided something like 100 miles to Singapore or somewhere.

        The Point: Air speed over wings create lift even when engine thrust is long gone. No engines to "pull" it forward. Although engines provide thrust to get it up to "wind speed', the thrust does not produce lift except in a guided rocket like situation - in which case it is not "flying" in the manner of Bernouilli. Thrust does not produce lift on a plane. It produces speed. When ennough WIND speed over the wings is reached, it will fly. Drop a well balanced light hand glider from the roof of a house and it will be flying in 6 to 8 feet due to wind speed over the wings.

        As Stytooner said, in a hurricane and plane can take off and land due to wind speed over the wings - as long as there is not direction change of plane or wind.

        Some might argue that gravity and huricanes provide the thurst. Gravity will provide "thrust" in the sense that it is causing accelleration downward. A hurricane's wind on the other hand will provide lift without the thrust. Wings provide the lift at a given wind speed, at which point "flying" takes place.
        Last edited by leehljp; 12-10-2006, 02:26 AM.
        Hank Lee

        Experience is what you get when you don't get what you wanted!

        Comment

        • Stytooner
          Roll Tide RIP Lee
          • Dec 2002
          • 4301
          • Robertsdale, AL, USA.
          • BT3100

          #49
          I think that all the guys that say it will fly have one thing in common. They all think the plane will be moving in relation to the Flight tower, the runway lights and that stray golf ball lost by one of the pilots.

          The scenario and the reason for the inclusion of the conveyor is to cancel out forward movement of the plane and its wings includings its engines. Sure, air is flowing through the engines. It's being sucked in at the the front and blasted out the back. This is the force that is keeping the conveyor going. It actually has nothing to do with gravity or bearings at all. Its forward speed that is lacking because of the belt. Its a theoretical scenario of course, but theoretically there can be no acceleration of the wings if the belt keeps them from moving in a forward direction. Sure the wheels are rolling, but the plane ain't moving.

          This is my last shot at this.
          Say there is a tug on this conveyor. Attached to this tug with a long release line is a glider. The glider is sitting on the ground behind the conveyor. The driver puts the tug in gear and tries to drive forward. That's thrust at the wheels, but he ain't going anywhere because of the conveyor. He now has it floored at 100 MPH on his speedo and it still isn't going anywhere. The belt is going just as fast as his wheels are, but the other way. What's happening with the glider? Nothing. Still just sitting in the same spot.

          Now the glider pilots starts yelling at the tug driver. The driver slows down the wheels on the tug until he's comletely stopped. He wants to back up and disconnect the glider. He puts the tug in reverse and starts the wheels rolling backward. This conveyor belt starts to go the other way. Dang, he still ain't moving. Its not gravity nor friction thats keeping him in position on the conveyor. Its an equal and opposite reaction to the energy of his wheels which in this case is directly proprtional to the direction they are turning.

          You guys ever seen a machine that checks your speedometer for accuracy?
          It's a set of rollers that you pull the car up on. Both drive wheels set in between these two rollers. When you put the car in gear, the wheels turn these rollers. No matter how fast the speedo says the car is going, the front wheels ain't moving.
          This is the type scenarion That I think Mike intended when he set up the scene. A force that was acting against the thrust to virtually hold the plane still w.r.t. the ground, not the conveyor bellt.

          If a bird was on this conveyor, it could easily take off at any speed due to them using their entire wing span to develop thrust and lift. They don't need speed of forward motion because they use speed of flapping wings to create lift and thrust.
          Put an albatrose on this conveyor though and he will have problems. He won't be able to run fast enough to develop the forward motion needed to help him fly. He is much more like an airplane. He can't initially flap his wings fast enough to lift his body and uses forward speed to help him get the needed air flowing under his wings. With the coveyor holding him basically in the same spot on the tarmack, he's not lifting, not flying, he's running in place flapping his wings and looking even more awkward that usual.
          Lee

          Comment

          • mmgross144
            Forum Newbie
            • Oct 2006
            • 24
            • Dumfries, VA
            • Ryobi BT3100

            #50
            Interesting Quoting

            Hank,
            Just wondering, are you a journalist? I reflect on you last post on gliding and the portion that you quoted from a previous post of mine (which was in part pulled from your prior posting), in which you did not include an interior paragraph where I addressed gliding...so I'll restate it here for posterity's sake:
            "Of course, but wings without thrust are just nifty sun-shades. Wings cannot lift off by themselves without an outside force, Wind. Wings, properly balanced can glide if given sufficient airspeed to generate lift, but they won't do it from the ground by themselves."

            Your examples of gliding stem from aircraft that had already achieved lift, via their own thrust (airliners) or external thrust the glider's tow aircraft. According to the law of inertia, an object at rest will stay at rest unless acted upon by an outside force, and an object in motion will remain in motion unless acted upon by an outside force (loose translation).

            Your already flying wings will continue to fly until the outside forces, wind friction & gravity, overcome the lift (especially in airliners) forcing the craft down. Wings that have not achieved lift/flight will remain pinned by the same gravity until sufficient force is exherted upon them to generate the motion needed to gain the needed airspeed to gain lift.

            So, how's Tokyo this time of year? (your post says that you are there.)

            Comment

            • gjat
              Senior Member
              • Nov 2005
              • 685
              • Valrico (Tampa), Florida.
              • BT3100

              #51
              I re-read the question and have changed my answer.

              The plane will fly.

              The plane motor (let's make it a jet), moves the body of the plane including the wings. The plane will move and take off like normal if it's on a non-moving runway. It will also take off on the conveyor belt because, the plane will still move through the atmosphere as observed by it's spcatial relationship to the Tower, FBO, and stray golfball.

              The conveyor will only make the wheels spin faster and has no relationship with the movement of the plane body and wings through the atmosphere.

              Final answer. Am I a millionaire, Regis?

              Comment

              • niki
                Senior Member
                • Nov 2006
                • 566
                • Poland
                • EB PK255

                #52
                OK, after re-evaluating the question, and second (or more) thought, I'm changing my mind, it will fly.

                Yes, the tires will speen like crazy and we shall need a little bit more thrust or power to overcome the drag but at the end we shall takeoff.

                Newton's Third Law states that for every action there is an equal and opposite reaction.

                Thrust is created just by accelerating an air mass, the reaction to this acceleration is the force to the "opposite direction".
                It can be created even in a vacuum (in space), if we shall shall supply the required fuel and air to operate the engine (I'm talking about Jet engine).

                The conveyer can adjust it's speed to the tires speed but cannot oppose the foreward thrust created by the engine.

                The conveyer is put in the question just to decieve me, and it did.

                niki

                Comment

                • Stytooner
                  Roll Tide RIP Lee
                  • Dec 2002
                  • 4301
                  • Robertsdale, AL, USA.
                  • BT3100

                  #53
                  I gotta be thinking to simple on this.
                  Any forward motion of the plane is countered by backward motion of the belt. No matter how big, how fast, how strong the engines are or what the tire pressure. It could be skid mounted too. Gravity is keeping the plane on the belt of course. It is the forward motion of the entire plane that is being negated by the belt. It ain't flying.

                  He didn't say the belt is turning in relation to thrust or how fast the wheels are turning. It's moving a millimeter back for every millimeter the plane moves forward, but also at the same time.
                  Lee

                  Comment

                  • LarryG
                    The Full Monte
                    • May 2004
                    • 6693
                    • Off The Back
                    • Powermatic PM2000, BT3100-1

                    #54
                    Last night, sitting in the local community theatre, waiting for the play to begin, I suddenly understood what the "the plane will fly" contingent was arguing and was ready to admit they're right. This morning, I'm not so sure.

                    First, an analogy that illustrates why the conveyor moving backwards matters:

                    A river flows at five MPH. If you're in a rowboat, paddling upstream at five MPH, the current carrying you downstream will exactly cancel out your forward progress. Your boat will remain stationary relative to the river's banks, because your propulsion system -- the oars -- are "connected" to the current.

                    Now tie a rope to the boat, get out onto the river bank, and walk upstream at five MPH. Now your propulsion system is independent of the river's current. While it is true that you will have to expend sufficient effort to overcome the current, the amount of work you're doing doesn't matter. All that does matter is the end result. If you are able to walk upstream at five MPH on the river's bank, the boat tied to the other end of your rope will also move upstream at the same five MPH. It must.

                    The key phrase in the foregoing is you will have to expend sufficient effort to overcome the current and that, I think, is what we don't know about our hypothetical airplane: will its engines produce enough thrust to first overcome the conveyor's carrying it backwards AND THEN accelerate it to take-off speed? Or rather, can its engine produce enough thrust to do this? I don't think they can, and here's why:

                    Contrary to what some are claiming, there's a lot more to overcome than just the small amount of friction in the plane's wheel bearings. There's the weight of the plane; there's also the drag it will produce once it begins moving forward through the air surrounding it.

                    If you don't believe the plane's weight enters into this, consider this: if the plane was sitting with its engines off, and the conveyor started running at 100 MPH backwards, what would the plane do? Obviously it would move right along with the conveyor, at 100 MPH, backwards. Granted, if the wheels were not chocked or otherwise restrained, and the conveyor instantly accelerated to 100 MPH, the plane might initially stay right where it is -- think about the trick of jerking a table cloth out from under the dishes -- but soon the spinning wheels would coast to a stop and the plane would be carried rearward on the conveyor at 100 MPH.

                    If the pilot started the engines and opened the throttles, the plane would begin to attempt moving forward, since its propulsion is independent of the conveyor's influence (i.e., the rope tied to the boat). But since the engines were started when the plane was moving backwards at 100 MPH, the first thing it must do is generate enough force to cancel out that backwards motion. When that balancing point is reached, and the plane is back to being motionless relative to the fixed ground adjacent to the moving conveyor, its wings will also be motionless WRT the surrounding air. Motionless wings generate zero lift; and until the wings begin generating lift, the plane's full weight will still be pressing down on the conveyor, and the conveyor will be attempting to carry it backwards. The plane's weight is "connected" to the conveyor in the same way the boat's hull is "connected" to the river current.

                    If the plane is able to take off at 100 MPH, it engines at this point it is able to counteract the conveyor's rearward motion are producing only half the amount of thrust needed (100 MPH to get even, with another 100 MPH needed to take off).

                    And therein lies the rub. No matter how much thrust the engines produce to move the plane, the conveyor will compensate by speeding up, serving to keep the plane's wings motionless in the surrounding air. If the wings are motionless in the air, they are producing no lift. And as long as the wings are producing no lift, the conveyor is supporting the plane's full weight and is therefore attempting to carry it backwards with it. The faster the plane goes, the more behind it gets.

                    The plane won't fly.
                    Larry

                    Comment

                    • backpacker85
                      Established Member
                      • Sep 2006
                      • 148
                      • Dickson, TN
                      • BT3100-1

                      #55
                      OK, I'm seeing that everyone agrees with the principle of this thing, that the "flight" of the plane is based on it's forward progress through the air. The only points being disputed now seem to be whether there will in fact be any forward progress made by the plane, and that can't be determined given the facts in the original question (and the "conveyor" may be the red herring).

                      If the assumption is that the plane is truly on a "conveyor", the only effect the conveyor would have is on the wheels of the plane, not its relative motion through the air.. THE PLANE WILL FLY.

                      If the assumption is that the "conveyor" is somehow able to counteract the plane's forward motion through the air, THE PLANE WILL NOT FLY.
                      This is the part I have trouble with.

                      To counteract its forward motion through the air, the plane would have to be firmly anchored to the "conveyor". In effect, the plane may as well be anchored firmly to solid ground!!

                      If you work the problem backwards assuming the plane will NOT fly, you are saying that there is no forward progress being made through the air; and if there is no forward progress being made, there is also no backwards motion being supplied by the conveyor, since it is assumed to move at equal speed but in an opposite direction than the plane through the air. The whole setup is sitting motionless.

                      Sounds like the only way the plane WILL NOT fly, is if it is firmly anchored down.
                      Ken W.
                      _____________________
                      "If you can't fix it right, fix it so no one else can fix it right."

                      Comment

                      • Stytooner
                        Roll Tide RIP Lee
                        • Dec 2002
                        • 4301
                        • Robertsdale, AL, USA.
                        • BT3100

                        #56
                        I agree with Larry on this.
                        I do see both points now and almost changed my mind as well, but thats only when I don't take the initial statement at face value.

                        "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

                        The question is:

                        Will the plane take off or not? Will it be able to run up and take off?"



                        It is the plane's speed, that is being taken away by the conveyor's speed. Not the thrust of air through the engines, but the actual forward progress of the plane itself. Given the information we have and reading as little into it as possible, the plane will not take off.


                        A fighter jet on an aircraft carrier that is faccing into the wind sitting on deck ready to take off is having its forward progress arrested by the catapult. Even with the after burners on at full throttle, its still just sits there. It's only when they let it go, that it gets enough forward movement to take off.
                        Will the conveyor counteracting on a planes forward movement, it cannot move forward, therefore it cannot takeoff.

                        That is my main point. The conveyor is matching the plane's speed which ultimately gives it no speed at all.
                        Lee

                        Comment

                        • niki
                          Senior Member
                          • Nov 2006
                          • 566
                          • Poland
                          • EB PK255

                          #57
                          I would like to explain why I changed my mind.

                          Take a matchbox car, tie a string at the front and tie a fish-scale to the string.
                          Put the car on your belt sander, switch it on, and measure the force required to hold the car in place, let's say that you measured 2 lbs, this force is the product of the tire and bearing friction.
                          now, increase the belt speed, you shall still read 2 lbs (maybe a little more) but the tires are turning faster.

                          Now, install on top of the car roof some propeller or jet engine that can produce 3 lbs or more. 2 lbs of thrust will be required to overcome the tire/bearing friction (without any connection to the tire speed) and the rest of the thrust will produce forward motion relative to the ground, or more correct, to the space.

                          The wheels, are only medium between the plane and the belt, they are not tieing the plane to the belt and are free to rotate. They produce some drag force, but ones we overcome this drag force, the plane is like in the free air.

                          Hope it make sense
                          niki

                          Comment

                          • jackellis
                            Veteran Member
                            • Nov 2003
                            • 2638
                            • Tahoe City, CA, USA.
                            • BT3100

                            #58
                            There's an analogue to the airplane example, which is a float plane on a river where the current moves from tip to tail of the airplane at exactly the same speed that the airplane moves relative to the current. In this case, the force of the current on the airplane and the airplane on the current exactly counteract one another. Otherwise, the airplane would start moving.

                            The speed of the treadmill "runway" is probably irrelevant. What is relevant is the force exerted by the treadmill runway against the airplane's wheels in the direction that's perpendicular to gravity (horizontal). A wheel airplane will only move on that treadmill if it's wheels skid. With enough power and a very very long runway, the airplane would fly, but it would be an ugly thing to watch.

                            Comment

                            • niki
                              Senior Member
                              • Nov 2006
                              • 566
                              • Poland
                              • EB PK255

                              #59
                              I think that all the confusion comes from the wording of the question:

                              The conveyer DOES NOT track the AIRPLANE SPEED (that is measured relative to the air and the conveyer does not know it), the conveyer is tracking the TIRE SPEED that has nothing to do with the TAS (True AirSpees) or IAS (Indicated Air Speed).

                              I just Googled "airplane takeoff" and found the same question in the following sites:





                              And even a video on YouTube
                              Myth is: 737 passenger jet airplane gains lift on a conveyor belt moving opposite to plane's forward motion...What Do You Think?...Now simply put it- THE PLA...









                              Comment

                              • Stytooner
                                Roll Tide RIP Lee
                                • Dec 2002
                                • 4301
                                • Robertsdale, AL, USA.
                                • BT3100

                                #60
                                There is no mention of tire speed in the original statement. Its tracking the plane's speed period. Ground speed, airspeed all the same. Speed is speed or in the case, the nulifying of forward speed with equal speed in the opposite direction at the same time.

                                To actually acheive any forward speed on the plane, you would at least have to slow down the conveyor belt. This would at least allow some forward movement of the plane and therefore at least some speed.
                                No speed, no go. No go, no fly. No fly, no good.

                                If ya'll want this plane to fly, you will have to adjust the conveyor and therefore the original statement.


                                I have a question for Russian Wolf. Did you have an answer for this when you popped it to us?
                                Lee

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