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yeah i guess iw asnt clear. i was thinking in terms of like the treadmill is lets say 100 yards long then the regular runway is there. of course in any situation an airplace unless propeled via a pulley *aircraft carriers* needs their specified length of runway for take off.Quote:
Originally Posted by plv
Length of the treadmill has nothing to do with the debate, even though I know what you are saying and you are correct. The only thing that matters is if the plane will move forward AT ALL, if the answer is yes than than the problem is solved.
The answer is yes.
Ummm...... stupidity is everywhere in here.
Any given wing design, in order to make enough lift, has to have a certain amount of airflow over it. So if this airplane is sitting on a treadmill, and the treadmill is moving with the equivilant of 100mph (planes wheels spinning at 100mph) but is remaining stationary, it will not lift. Now in order to make lift (and say something none of you idiots have said) the aforementioned aircraft would have to propell itself to whatever neseccary airspeed to create enough lift for flight. What seems also to be forgotten is that any given jet engine only creates so much thrust. If the treadmill is moving at 100mph, there will be a % of thrust needed to maintain momentium as gravity will try to pull the plane in the direction of the treadmill.
The reason the plane would slow down is because gravity pulling the jet down creates friction where the tire meets the ground causing the plane to slow down. Just like brakes dont stop a car. Brakes cause a weight shift toward the front of the car causing more friction on the front tires which slows the car down. Thats why you can have big ass raceing brakes and shitty tires and be out braked by a car with stock brakes and sticky tires. Same with the plane. Gravity will be pulling the plane "down" causing this same friction, which will in turn, cause the jet engines to be operating at a certain percentage, reqiring more thrust to exceed the stationary effect. The only way the plane would take off is if the "treadmill" was long enough for the plane to reach the appropriate AIRSPEED and if the engines had enough thrust left to propel them to the needed speed.
If you were to able to speed up the "treadmill" and the plane at the same rate until the planes propulsion system reaches 100%, the aircraft would NOT lift off. Remember, lift is a direct result of AIRSPEED (rate at which how fast air moves over the wings reguardless of thrust or ground speed). You "test is very poor because you are "propelling" the object yourself, rather then it doing it on its own.
Edit: I see what you are saying here, but you are thinking into it too much. The question is will the plane create lift or not. The answer is clearly yes.
No, the plane only creates lift as airspeed increases. It will not create lift no matter how fast the treadmill is moving or how much thrust the jets are making if the plane is "stationary" in position on the treadmill.Quote:
Originally Posted by fcman
Also, the video example is using a rocket, not a jet engine. a rocket like the one used only has X amount of thrust from ignition until fuel is spent, no more, no less. In that example, the rocket created enough thrust to exceed the stationary speed. Now if the treadmill was moving at a much higher rate, it would not be able to exceed the thurst needed to move forward relative to the treadmills speed.
bottom line - airspeed creates lift, only way to create lift is to move air over a wing. Now, if you were to put some super fan infront of it to move enough air (artificial airspeed) to create lift. Once the wings are outside the area of moving air, it will loose lift unless it can move fast enough forward to match the needed airspeed outside the area of moving air the fan has created. THRUST HAS NOTHING TO DO WITH LIFT. Hince a glider which gets artificial thrust from a tow plane... after that, there is no thrust, but still makes lift due to gravity pulling it down. As gravity pulls it down , it moves forward fast enough to make lift over the wings. Like a sailboat can move perpindicular to wind and keep going. The wind puched on the sail which has a deflection to move the air toward the rear and propells it forward.
ibtheplanetakesoff
The plane wouldn't be stationary on the treadmill though. No matter how fast that treadmill moves, the plane will move forward in relation to an observer not on the treadmill, thus creating airspeed. Because a plane's wheels are free-rolling, the speed of the wheels/treadmill are irrelevant to the question. If you get past that the only thing this question is truly asking is "Can an airplane take off?"Quote:
Originally Posted by DropTopDrifter
If you really want, I can draw it up and work some formulas for you with Boeing 747 specs. It would be nice practice for my Aerospace Engineering final on Thurs. ;)
Quote:
Originally Posted by NSCredondo
actually, it'd really be going 300mph, but because of the situation, its stationary
Quote:
Originally Posted by fcman
So you're saying that if the treadmill is moving at , lets say, 1000mph, the jet can still take off?????
It cant, there will be too much friction created by the wheels... the jets would be able to make enough thrust to over come airspeed.
If this was the theory, then where are there not huge treadmills on aircraft carriers?? Oh, it wont work.
Look at a Harrier Jet for examples of thrust vs airspeed and lift. It creates upward thrust from nozzles forcing thrust down. To go forward, it slowly turns the nozzles back to make forward thrust. It it went too fast, it would not make enough forward thrust to create lift (airspeed) and fall down. It balances upward thrust and forward thrust until enough lift is created.
So lets review again. Thrust overcomes friction caused by gravity(weight) to create airspeed over the wings to finally make lift.
http://en.wikipedia.org/wiki/Lift_%28force%29
http://en.wikipedia.org/wiki/Thrust
wrong, it wouldnt be moving at all.Quote:
Originally Posted by Stormhammer
Speed is the rate of motion, or equivalently the rate of change of position, many times expressed as distance d moved per unit of time t.
ok... here is the sole question.
How does a treadmill DIRECTLY create lift?
Answer to 1000mph question, that depends on the coefficient of friction on the wheels. But since a VW Touareg (522lb-ft of torque) was able to tow a 747, we can assume the Cf is almost negligible. If a car can move it on stationary ground, how are you gonna tell me that 4 Rolls-Royce Turbofan engines (each with 63,300lb max thrust) can't move it with a very small amount of added resistance.Quote:
Originally Posted by DropTopDrifter
Tread mill doesn't do anything other than add a small amount of resistance, requiring a little more thrust.Quote:
Originally Posted by DropTopDrifter
Whether the treadmill/conveyor is moving at 1mph or 500mph the plane will still propel forward just as it would on any runway. Remember, a plane is NOT powered by its wheels therefore a treadmill/conveyor has ZERO effect on an airplanes forward motion. http://forums.importatlanta.com/showthread.php?t=69925Quote:
Originally Posted by DropTopDrifter
Speed doesn't have anything to do with change of position, it is a scalar quantity. Velocity is a vector, which is what you described.Quote:
Originally Posted by DropTopDrifter
Quote:
Originally Posted by fcman
+1 for that. :goodjob:
Quote:
Originally Posted by DropTopDrifter
no - if a speedometer could register unlimited mph -a nd the treadmill is going 150mph one way, and the tires are moving 150mph the other way - the speedometer would register 300mph ( cars aren't free rolling )
its the same concept if two oncoming cars are approaching each other at 50mph and impact - the impact is a 100mph impact cause of both combined forces. It'd only be a 50mph if one was stationary and not moving.
well i fell i answered it in different ways. like mentioned, too many variables. Gravity = weight = rolling resistance. How much is there? i cant answer that. The question sounded like the plane was stationary, but the "it will move forward, but will take more effort" i also stated was right. So having a tread mill moving under the plane, yes it could take off, but there would be a point at which there would be too much resistance. It would help to know how fast the mill was moving, to what type of plane it is, what type of engines and the weight of the plane.... so i may have taken it to the extreme side :)
The measurement of tq (ft/lb) vs thrust is completely irrelivent though.Quote:
Originally Posted by droptopdrifter
There is virtually zero additional resistance. The only thing differant is the wheels will be spinning twice as fast (or the speed of the plane + the speed of the treadmill/conveyor) The treadmill/conveyor has no effect what so ever, never ever ever, on the plane.Quote:
Originally Posted by DropTopDrifter
It doesn't matter if it's an F-22, an ultralight powered by a 5hp briggs & stratton, or even a guy on rollerblades with a fan strapped to his back...no matter what the speed of the treadmill/conveyor is it will still move forward with the same amount of power & effort as any other time.
To hold a skate board in place on a treadmill how much "force" to you need? Your pinky finger alone will prevent it from falling off the back of the treadmill right? Now how much force do you think it will take to roll it off the front? Now imagine being on skates on a treadmill, it would be really easy to hold yourself from rolling backwards right? What if someone came up behind you and kicked you in the ass bobby bouche style, would you not fly forward with ease? That kick is no differant than the trust created from a planes engine, they are propelled by that thrust and has nothing to do with the free-wheeling wheels.
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