DivideBYZero
Lifer
- May 18, 2001
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Lets assume that there is no wind otherwise it is just stupid.
But what law of physics is being violated.
Actually if one ignores the laws of physics(or math, or whatever) and hypothesizes on what would actually happen in the wheel speed/infinite acceleration scenario, the airplane still takes off. Although it would be taking off backwards... the airplane cannot have infinite thrust and the treadmill can, since it accelerates infinitely, therefore the airplane would be moved backwards at an infinite speed and fly off the treadmill then crash shortly afterwards.
Originally posted by: randay
Actually if one ignores the laws of physics(or math, or whatever) and hypothesizes on what would actually happen in the wheel speed/infinite acceleration scenario, the airplane still takes off. Although it would be taking off backwards... the airplane cannot have infinite thrust and the treadmill can, since it accelerates infinitely, therefore the airplane would be moved backwards at an infinite speed and fly off the treadmill then crash shortly afterwards.
If your just going to troll you should leave the thread.
OMG, this thread is insanity. Good grief, the plane takes off, people. Now whats this about somebody's ass and peanut butter?
okay i dont understand what the hell is going on and i read this thread...
airplanes NEED air going over the wings to fly. if that need is fulfilled than the plane cannot and will not take off. no matter how much those huge engines thrust the plane forward it its force will just be matched by the treadmill but in the opposite direction. the myth states the treadmill matches. meaning engines thrust to 50 mph treadmill go in opposite direction 50 mph engines thrust to 50,000mph treadmill go in opposite direction 50,000mph. the total NET speed will always be 0mph. now with calm wind and 0mph how is a 164,800lb (empty weight) plane suppose to get airborn. the wheels can spin at a speed of infinity only to be matched by treadmill...stop focusing on friction, wheels, axels, rc planes, all that bs...no AIR over the WINGS no LIFT.
air over wings=lift...theres no other way to get lift on an airplane.
and like i said the myth state the treadmill will always match the speed of the airplane...meaning u can slam on the brakes or put the thrust to 100% or whatever u want to do but the treadmill will alwasy match the speed of the plane.
so lets look at these formulas. if any of these are wrong please correct me.
air over wings=lift
no air over wings=no lift
please prove me wrong.
airplanes NEED air going over the wings to fly. if that need is fulfilled than the plane cannot and will not take off. no matter how much those huge engines thrust the plane forward it its force will just be matched by the treadmill but in the opposite direction. the myth states the treadmill matches. meaning engines thrust to 50 mph treadmill go in opposite direction 50 mph engines thrust to 50,000mph treadmill go in opposite direction 50,000mph. the total NET speed will always be 0mph. now with calm wind and 0mph how is a 164,800lb (empty weight) plane suppose to get airborn. the wheels can spin at a speed of infinity only to be matched by treadmill...stop focusing on friction, wheels, axels, rc planes, all that bs...no AIR over the WINGS no LIFT.
air over wings=lift...theres no other way to get lift on an airplane.
and like i said the myth state the treadmill will always match the speed of the airplane...meaning u can slam on the brakes or put the thrust to 100% or whatever u want to do but the treadmill will alwasy match the speed of the plane.
so lets look at these formulas. if any of these are wrong please correct me.
air over wings=lift
no air over wings=no lift
please prove me wrong.
Amused
Elite Member
- Apr 14, 2001
- 57,351
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Very easily. The treadmill is irrelevant to the airspeed of the plane because the plane moves itself by pushing air behind it, NOT by force through the wheels.
You are applying the drive train of a car to this problem. How fast the ground is moving is completely irrelevant to the airspeed of the plane (given the UNPOWERED wheels can withstand the extra speed). If it were, a plane would gain speed on a regular runway, take off, lose thrust and be forced to land again.
The wheels ONLY exist to reduce friction. They do not drive the plane... so how fast the ground is moving is, again, totally irrelevant. This is why planes on skids, pontoons and skis can take off.
Here's what I believe will happen:
Plane is sitting still, engines start up and allowed to warm up (not providing any thrust).
Pilot cleared for takeoff and opens up the throttle to full.
Planes moves forward, conveyor tries to compensate so increases speed as quick as it possibly can.
Despite wheels turning stupidly fast, the plane will still move forward because of massive thrust.
After a short while the grip of the tyres will give up and you'll be seeing loads of smoke and some horrible noises.
Assuming gear can take this, the plane will continue forward (you can take off in a large aircraft with the brakes on, so long as you don't want to use your gear/tyres again).
Aircraft moves faster and faster with same amount of smoke and screeching going on (tyres are beyond maximum grip).
Aircraft takes off.
Pilot wonders how he's going to land, perhaps considering using some kind of conveyor belt to belly down onto...
Smack down sits in the corner and has a little cry.
Plane is sitting still, engines start up and allowed to warm up (not providing any thrust).
Pilot cleared for takeoff and opens up the throttle to full.
Planes moves forward, conveyor tries to compensate so increases speed as quick as it possibly can.
Despite wheels turning stupidly fast, the plane will still move forward because of massive thrust.
After a short while the grip of the tyres will give up and you'll be seeing loads of smoke and some horrible noises.
Assuming gear can take this, the plane will continue forward (you can take off in a large aircraft with the brakes on, so long as you don't want to use your gear/tyres again).
Aircraft moves faster and faster with same amount of smoke and screeching going on (tyres are beyond maximum grip).
Aircraft takes off.
Pilot wonders how he's going to land, perhaps considering using some kind of conveyor belt to belly down onto...
Smack down sits in the corner and has a little cry.
exdeath
Lifer
- Jan 29, 2004
- 13,679
- 10
- 81
You don't measure engine power in mph...
Just picking random numbers out of the air, this is how it works:
Engines provide say 100,000 Ns of constant force forward.
Treadmill goes backwards as fast as it wants, rotating the wheels of the plane.
Normal forces between the plane and axles and contact between the plane and axles via bearings results in frictional drag on the plane from the axles as the wheels experience high speed rotation on the treadmill.
This drag force is the same force which slows your car down when you coast... however most of that slowing is from air resistance. If you coasted you car in a vacuum, how far would you roll before coming to a complete stop? Quite far actually. Clearly the rolling frictional drag of the wheels and axles is insignificant. In reality, the friction forces in the bearings cancel themselves out as the bearing rotates. Ie: the friction force on the top of the bearing moving foward is negated by the bottom of the bearing moving backwards. It's not perfectly balanced since the top half experiences more normal force due to bearing weight and the back of the bearing is being pushed against the rear of the housing, so there IS a net drag force backwards that the axle exerts on the body it's fixed to, but it's close enough. The majority of actual loss of speed comes from the gradual conversion of a fixed amount of kinetic energy into lost heat energy. If you roll long enough to come to a stop, the total heat released in the axle bearings, the measured heat from the tires, the internal energy absorbed IN the tires that has softened the compound, etc. would be equal to the initial kinetic energy at the start of the coast. But for the sake of continuity, lets continuing calling this loss of kinetic energy 'frictional drag'
The force of frictional drag felt by the axles is maybe, lets throw out, 50 Ns, a force which the plane has to overcome normally when taking off anyway, along with air resistance, etc. The plane can be moving forward at only 50 mph and the treadmill can be moving backwards at 600 mph. The drag force may then be 60+ Ns at the axles due to increased friction, still nowhere near the 100,000 Ns of the engines constant thrust.
100,000 Ns >>>>>> 50 Ns, so the plane has a net acceleration foward, which creates air speed and it flys as normal.
Here is yet another example despite my claiming to try one last time before...
Take a toy car with sufficient mass with good working wheels (ie: wheels that spin freely and don't stick when you try to spin them and let them freely spin)
Launch the car across a table with a table cloth as hard as you can and yank the table cloth out as fast as you can (ie: like the 'magic trick').
Observe that you only spin the wheels faster, as evidenced by the brief high pitch 'wzzz' of the wheels spinning faster the moment you pull the table cloth. The toy will still continue forward, only in this case under its own forward inertia and without a constantly applied pushing force. A heavy car is needed because this experiment relys on momentum to maintain forward movement in place of an external acceleration force.
Amused
Elite Member
- Apr 14, 2001
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I don't have to. Imagine the treadmill was ice and the plane had skis. The only question here is: Can the plane's wheels take the extra speed? Since a jet must be at about 140 MPH to take off, the wheels must be able to withstand a speed of 280 MPH. Not hard for them to do if properly maintained. If not, simply beef them up a bit.
Either way, the plane takes off.
Amused
Elite Member
- Apr 14, 2001
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I did. Read the whole post.
The wheels simply have to be able to withstand twice the speed of a normal takeoff. On a prop plane, about 180-190 MPH. On a jet, about 280MPH. Not hard at all especially since landing gear is usually over-engineered for safety.
Okay. 19 Pages for this topic and it just goes back and forth.
A plane's wing works through the means of air pressure. A wing provides lift because as it sails through the air, the pressure on the underside of the wing is greater than that over the top side of the wing. Lift is created. If the wing does not move, then the pressure differential will no develop and lift is not created.
If there is a plane creating thrust, and unless there is sufficient counter force provided on the wheels (which there isnt), then the plane will gain velocity until sufficient airspeed is obtained to lift off. Matching the plane's wheels velocity would do nothing but increase the rotational velocity on the wheels.
Bottom Line: A plane cannot use a treadmill to take off, unless you lock the wheels on the treadmill and propel the plane forward with it (catapult the plane to the airspeed required).:shocked:
A plane's wing works through the means of air pressure. A wing provides lift because as it sails through the air, the pressure on the underside of the wing is greater than that over the top side of the wing. Lift is created. If the wing does not move, then the pressure differential will no develop and lift is not created.
If there is a plane creating thrust, and unless there is sufficient counter force provided on the wheels (which there isnt), then the plane will gain velocity until sufficient airspeed is obtained to lift off. Matching the plane's wheels velocity would do nothing but increase the rotational velocity on the wheels.
Bottom Line: A plane cannot use a treadmill to take off, unless you lock the wheels on the treadmill and propel the plane forward with it (catapult the plane to the airspeed required).:shocked:
Amused
Elite Member
- Apr 14, 2001
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Your name fits you.
The plane does not use it wheels for thrust, therefore what the ground is doing is totally irrelevant. It gets its thrust by pushing air behind it. The wheels only exist to reduce friction on the ground. This is why planes on skis and pontoons can take off without wheels: Because the ground is irrelevant.
The only question is, again, this: Can the UNPOWERED wheels withstand twice the speed of a normal takeoff? If not, the wheels break and the plane does a belly scrape. If they can, the plane takes off with ease.
exdeath
Lifer
- Jan 29, 2004
- 13,679
- 10
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If anyone wants do take the time to do a free body diagram try this:
Take a 747 for example. Based on the weight, tire size, etc. you can calculate the normal force and the dynamic friction coefficients of all the landing gears if they were sliding and not rolling; ie: brakes applied.
Even in this condition, you'd see that the force of the engines thrust is far greater than the sum of frictional forces provided by all the locked up landing gear tires combined.
Provided the landing gears were balanced such that no torque is induced on the body of the plane as it moves forward with locked up brakes (ie: and thus diving the nose into the ground, steering it off the runway, etc) the plane could easily take off.
Then consider the rolling friction in the bearings, metal on metal with lubricant, with the wheels freely spinning. At any given velcity, rolling friction in a axle bearing will always create far less frictional drag than sliding friction of rubber tires on asphalt.
Therefore if we compute that the planes engines have more forward force than the friction from locked up tires, clearly the wheels rolling at high speed is even less of a problem, this includes treadmills.
But because the speed of the treadmill increases slowly, the treadmill will act on the wheels by pulling the plane backward. You cant just ignore bearing resistance. So when the plane has revved up to 200mph, the treadmill is pulling backwards on the aircraft at 200mph. It wont go anywhere.
Amused
Elite Member
- Apr 14, 2001
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Wrong.
The wheels simply need to be able to withstand twice the speed they normally do at takeoff. If the minimal bearing friction of twice the speed is able to keep a plane from moving, than the friction from normal speed would keep it from taking off.
Why is this so hard for people to understand? What the ground is doing is, for all intents and purposes, irrelevant.
And as stated in the problem the plane simply cannot advance. Impossible. Can't happen. If it did then it doesn't obey the rules of the scenario/problem.
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