videogames101
Diamond Member
- Aug 24, 2005
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no, you plane is moving with all it's weight on the wheels, staying stationary, aka no air, aka no lift, aka, your wrong =) we'll see in 1/2 hour tho....
no, you plane is moving with all it's weight on the wheels, staying stationary, aka no air, aka no lift, aka, your wrong =) we'll see in 1/2 hour tho....
You're assuming that the plane can even HAVE a velocity. It can't. If it does then you have broken the very stipulation of the problem.
Put a plane on a treadmill with no thrust, the plane retreats. Are you still saying the treadmill cannot apply an opposing force on the plane?
Well we can pretend the wheels are independent and their a fairys making it fly it doesn't make it true. If you look at any airplane wheels you while notice an axel that attaches to the plane making them one system.
Or any P&N thread that has to deal with Israel...come in, lay a smackdown of %$it and than flake off.
@ Thread
They will screw it up and the plane will not take off (Even though it should)
videogames101
Diamond Member
- Aug 24, 2005
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I'll take it. 1000 to 1 odds, everyone here is a witness. I put forward 5 bucks.
One little flaw in your analysis. A plane moves because the engine(s) push air out of the exhaust, which results in moving air beneath the wings. The rate in which it pushes air out has nothing to do with the rate in which the ground moves.
That's where you're wrong. The engines apply force at the frame, but the treadmill applies force at the wheels, and can only apply angular force. If the original problem specified massless wheels and no friction, there is NO way the treadmill could keep the plane stationary.
As for the "speed of the wheels" bit, let me specify a simpler to understand control system.
Put a position sensor next to the runway. Hook up your control system to this sensor. Attempt to keep the plane in one place by applying control inputs to the treadmill. It's possible if the treadmill is sufficiently powerful, and if we have wheels of finite (nonzero) mass and/or friction at the hub. If we throw out these constraints, it's impossible. If the plane is kept in one place, it will NOT take off.
If the control system is matching the ground speed of the plane (relative to stationary earth), the plane WILL take off, no ifs, ands, buts, or fancy assumptions necessary.
There are two ways to understand the control system. One way allows the plane to take off, the other does not, but requires an immensely overpowered treadmill to work at all. Otherwise it's like trying to stop a boat from launching by squirting a Super Soaker at the bow.
Yeah, I'd like to get in on that action too. I'll put in $10.
Of course, he'll probably try to weasel out of it by claiming the second interpretation of the control system, and claim that their treadmill wasn't powerful enough.
Right, this from the same myth buster that tested if a frozen bird was different from an unfroze bird on glass that wasn't rated for bird strikes. They are going to find that the plane takes off because that is the easy result but not the right result.
videogames101
Diamond Member
- Aug 24, 2005
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are you implyign air is forced under the wings by motors? lol, air is forced under my the movement of the plane, and the plane starts with no movement, while moving on WHEELS. so, as the wheels spin, the treadmill goes opposite, no movement, no air, no lift.
LOL, I love these threads. When most people think of a plane on a treadmill they immediately recall seeing commercials, etc, of cars on a treadmill. That's not the correct comparison to make. Cars exert force through their wheels to the ground to move forward and brake. Planes exert force through their turbines to the air to create forward thrust. Because planes don't exert thrust through their wheels, the plane will move forward on a treadmill and the wheels will spin faster than they normally would. The plane would take off ignoring friction created by the ball bearings of the wheels and assuming a treadmill going near C in the opposite direction, etc.
Imagine this: A car traveling down the road at 50mph. Suddenly, the road itself moves backwards at 50mph and the car is stationary. This is what most people think when they think moving object on a treadmill.
To relate this to a plane, imagine a treadmill at 30k feet. The plane is traveling 150 knots with landing gear down and locked. Imagine we set the treadmill going in the opposite direction at 150 knots and then placed the treadmill in front of the plane. As the plane passed the treadmill and the landing gear made contact, would the plane suddenly lose all lift and become stationary? No, it would slow down because of friction exerted by the stationary treadmill but the engines would force the plane through the treadmill because the plane doesn't derive it's forward thrust from the wheels. I'm not sure this is the best way to think of it but perhaps it is better than thinking of a plane sitting on the ground.
Imagine this: A car traveling down the road at 50mph. Suddenly, the road itself moves backwards at 50mph and the car is stationary. This is what most people think when they think moving object on a treadmill.
To relate this to a plane, imagine a treadmill at 30k feet. The plane is traveling 150 knots with landing gear down and locked. Imagine we set the treadmill going in the opposite direction at 150 knots and then placed the treadmill in front of the plane. As the plane passed the treadmill and the landing gear made contact, would the plane suddenly lose all lift and become stationary? No, it would slow down because of friction exerted by the stationary treadmill but the engines would force the plane through the treadmill because the plane doesn't derive it's forward thrust from the wheels. I'm not sure this is the best way to think of it but perhaps it is better than thinking of a plane sitting on the ground.
I didn't no wheels with mass was a "fancy assumptions" and here I've been driving my car around all this time with out massless wheels. Do you know where I can get some of your magical tires so that my plane will get better gas millage.
How many times do I need to repeat there is no difference between a car and a plane. The only time it would matter is if we were talking about placing them on rollers.
Lets make this simple.
Do you agree that a car on a stop treadmill will accelerate?
Do you agree that in that case the a force is applied to the body of the car?
Then why do you thing that once the treadmill is started a force is no longer applied to the body of the car?
If you don't think that what makes the plane any different then the car?
The plane has a velocity. The engine(s) moves air, which has nothing to do with the speed of the ground. The whole experiment can start out with treadmill going backwards, as long as the engines can start, then the plane will move forward.
When there is no thrust, the plane and the wheels behaves as one system, instead of two.
Read my post, I said that massless wheels was a fancy assumption.:roll:
You still fail to understand the problem. Go do an order-of-magnitude calculation on the thrust provided by 4 standard-size jet engines, and the acceleration necessary to impart an equal force on standard-weight, standard-size aircraft wheels via the angular momentum and friction terms. You'll find it's shockingly high. Clearly the original creator of this problem intended the first definition of the control system, but worded it poorly. Attempting to stop the plane using the second (somewhat bizarre) definition is purely a thought experiment, since it would be impossible to perform in real life.
- Jan 2, 2006
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Skeeedunt
Platinum Member
- Oct 7, 2005
- 2,777
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I think this finally makes sense. If the wheels are frictionless, the plane takes of. Otherwise, the conveyor can move the wheels so fast that the downward force of the wheels' rotational inertia is so great that it counteracts the thrust of the engine against the air. Am I right am I right?? Do I get a cookie??
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