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Post by elwyman on Feb 2, 2006 17:13:25 GMT -5
The answer is obviously no! As someone said at the start of the thread, the plane doen't move relative to the ground and its airspeed is therefore 0 - if it doesn't move there is no uplift on the wing to make it fly. Unless it's a harrier jet. It's like a treadmill, as TP said. Me roller skates moving pavement under me me remaining stationary with wheels spinning. me with big jet pack switched on me flying up the pavement byeeeeeeeeeeeeeeeeeeeeeeeeeeeee Wear a crash helmet 'cos you'd bang your head as you fall off the back of the conveyor. ;D The jet pack would just maintain your position on the conveyor. Getting back to the plane, it might "kangaroo" up the conveyor, and if the conveyor was infinitely long, it might eventally gather enough speed to take off? Just a gut feeling, and assuming the conveyor speed lags slightly behind the plane speed. If astrophysicists don't know the answer, what chance do we have. Perhaps NASA should do an experiment!
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Post by traprain on Feb 2, 2006 17:56:43 GMT -5
let thrust sufficient to hold aircraft stationary (wheels spinning) while runway zips backwards at takeoff speed = x
let thrust sufficient to achieve normal take-off speed = y
set thrust to x+y
take-off
what's so hard about that?
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Post by Willie Gunn on Feb 2, 2006 18:23:20 GMT -5
If a train is going at 99 miles an hour west and hits a fly flying east at 1 mph. The two collide and the train continues west with the fly attached. The fly must have stopped as it changed direction(east to west) as it was in contact with the train the train must have stopped as well.
I lost track with the plane.
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Post by John Gray on Feb 2, 2006 18:29:58 GMT -5
Surely the wheels and the conveyor belt are the aeronautical equivalent of a red herring!
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Post by Cothi on Feb 2, 2006 18:41:48 GMT -5
Surely the wheels and the conveyor belt are the aeronotical equivalent of a red herring! Or even a red arrow....
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Post by mikeconnor on Feb 2, 2006 22:05:00 GMT -5
The plane will take off, because its thrust is independent of the wheels or the conveyor belt.
Its engines ( jets)thrust against, or (props) pull it through the air surrounding it, and this causes it to accelerate independently of whatever it is standing on. Or whether that surface is moving.
TL MC
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Post by The Famous Grouse on Feb 2, 2006 22:44:12 GMT -5
That would be incorrect, Mike. Airplanes fly because of lift, not thrust.
Lift is caused by the wing (airfoil) moving through the air at a speed great enough to generate enough lift to carry the weight of the airplane.
Let's say an airplane needs to move at 60 mph to generate enough lift to fly.
If the conveyer belt is moving at 60 mph, and the airplane's engine is generating enough thurst to keep it in a constant position on the conveyer belt, there is no air moving across the wing. The plane is sitting still relative to the surrounding air.
The airplane needs to be pulled or pushed with enough thrust to move the wing through the air at 60 mph. Therefore, using the example above, the airplane would have to generate enough thrust to move at 60 mph just to stand still on the conveyer belt + an additional 60 mph to generate the air movement across the airfoil required to lift the airplane.
Grouse
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Post by mikeconnor on Feb 2, 2006 22:52:36 GMT -5
Airspeed = the speed of an aircraft relative to the air in which it is flying. As soon as the aircraft is thrust forward by its engines it has airspeed, and the airspeed is what causes the lift over the wings. Everything else is irrelevant. What the wheels are doing, or some mythical conveyor belt, is of no consequence to the airspeed. This is easily proven. Rather surprising that there is so much discussion on it. People are confusing groundspeed = the speed of an object relative to a fixed point on earth, with airspeed, which is the speed of an object relative to the air surrounding it. Also, what the wheels are doing is completely independent of engine thrust. On a conveyor belt moving backwards, the plane will still move forwards in air due to engine thrust, but the wheels will spin faster as long as they have contact with the belt. The ONLY way to prevent a plane from taking off, would be to use an airstream around the whole plane which moved in its entirety, in the opposite direction to the thrust of the engines. Also, a plane does not have to move relative to the ground in order to take off, there merely has to be a sufficiently large airstream over its wings. That is how a wind tunnel works. The plane does not move relative to the ground, the air does. This lifts the plane. Furthermore, in a strong headwind for instance, the plane will take off sooner, because the airstream across its wings provides sufficient lift sooner. The groundspeed of an aerofoil is basically irrelevant. The airspeed is of paramount importance. Whether the ground moves or not is thus also irrelevant. The speed of the aerofoil relative to the air surrounding it is the only factor which has to be considered here. The energy equations for adiabatic flow of an ideal gas are carried out thusly; www.mathpages.com/home/kmath282/kmath282.htmNo conveyor belts or wheels are involved. For further visual clarification, imagine a flying boat taking off up river. The river is moving in the opposite direction to the plane. This is quite irrelevant, as the plane continues to be thrust upriver by its engines, until such time as it generates enough airspeed to give it the required lift for takeoff. It is important to remember that the ONLY relevant factor in this "problem" is airspeed. This is generated by engine thrust against the air itself. What the wheels or floats or anything else not connected with the acceleration of the aerofoil in air are doing is absolutely irrelevant. Just to get everybody excited, imagine a plane standing on a conveyor belt with its brakes full ON, and its engines OFF, and the conveyor belt accelerating forwards. What happens? The plane eventually takes off, because it was fixed ( by virtue of its wheel brakes) to the conveyor belt, and the forward speed of this belt induced airspeed, which caused lift across the planes wings. THIS IS BECAUSE THE THRUST WAS PROVIDED BY THE CONVEYOR BELT! Now the same again, but no wheel brakes. What happens? The plane will move and accelerate a little, but probably not much, the wheels will simply spin as the air resistance to forward motion overcomes the friction of the wheels on the conveyor belt. TL MC
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Post by ScottRods on Feb 3, 2006 0:09:19 GMT -5
Is thrust in a jet engine the equivalent of torque in a car engine. ??
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Post by mikeconnor on Feb 3, 2006 0:13:26 GMT -5
Is thrust in a jet engine the equivalent of torque in a car engine. ?? NO! Torque = The moment of a force; the measure of a force's tendency to produce torsion and rotation about an axis, equal to the vector product of the radius vector from the axis of rotation to the point of application of the force and the force vector. A turning or twisting force. Thrust = The forward-directed force developed in a jet or rocket engine as a reaction to the high-velocity rearward ejection of exhaust gases. TL MC
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Post by Willie Gunn on Feb 3, 2006 2:29:16 GMT -5
The question is flawed so you cannot answer it. The conveyer belt would make no difference. If the question asked would a plane with wings take off, drive coming through the wheels, you would be right in saying no. Flawed question, a moderators way of increasing posts/interests in a flagging forum
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Post by mikeconnor on Feb 3, 2006 2:39:34 GMT -5
There is no point in e-mailing me on this, it is a simple matter.
A car is propelled forward because its wheels are driven around by a motor. These wheels exert a force on the ground.
A plane is propelled forward because it is thrust forward by its engines. These engines exert a force on the air around them.
What the wheels on a plane do is irrelevant. Normally, at takeoff, the pilot will keep the wheel brakes on until his engines have developed sufficient thrust, in order to shorten the takeoff distance. Otherwise, the only function the wheels have, is to allow the aircraft to be rolled and steered on the ground.
Run on a treadmill. You stay in the same place, because the belt keeps pace with your steps.
Strap a jet engine on your back, get on the treadmill, regardless of what it does, and you will fly through the wall, because the force which is now propelling you forwards is completely independent of your feet, or the treadmill.
The conveyor belt, the wheels, whatever, are all completely irrelevant to the problem posed.
A jet engine has no contact with the ground, ( and neither does a propeller), they propel whatever they are attached to by thrust against the surrounding air.
It is quite irrelevant what the ground ( or the conveyor belt) does.
The second point which people are obviously not grasping, is that airspeed has nothing at all to do with groundspeed.
Groundspeed, is the speed at which something is travelling relative to a fixed point on earth.
Airspeed is the speed at which something is travelling relative to the surrounding air.
THERE IS NO CONNECTION BETWEEN THESE TWO DEFINITIONS.
Airspeed is required in order to lift an aerofoil. What actually moves, either the aerofoil, or the air, or both is irrelevant.
What the groundspeed may be at any particular point during this operation is completely irrelevant.
There is no point in discussing this, as they are quite simple facts, which anybody ought to be able to grasp immediately.
It is also quite irrelevant how many people answer polls on the matter, or who maintains something different. None of this changes the simple facts.
In the problem posed, the aircraft will take off quite normally when it reaches its lift off speed.
TL MC
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Post by MarkH on Feb 3, 2006 2:40:23 GMT -5
Flawed question, a moderators way of increasing posts/interests in a flagging forum Are you saying that Chris started this thread to increase posts/interests in this forum?
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Post by Windknot on Feb 3, 2006 2:43:39 GMT -5
Mike,
I'm NEVER getting in any plane you built! :-)
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Post by mikeconnor on Feb 3, 2006 2:50:15 GMT -5
Mike, I'm NEVER getting in any plane you built! :-) It would not surprise me to learn that you already have. I worked for Airbus for quite a long time, building several models of airbusses, the transall, and a number of other aircraft. I have also worked on a large number of other aircraft, both the mechanics and the electronics. TL MC
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