Physics Puzzle
- Thread starter chilluk
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You've contradicted yourself - previously you said that there was no friction.big-all said:
I haven't seen any calculations that show the relative proportions of drag from air and from friction, so I have to be a little suspicious of your claim that one is "hundreds" more than the other.big-all said:
In any case, the air resistance, to the wings of the plane moving forwards, is the same whether or not the plane is sitting on a conveyor belt, and whether or not that conveyor is moving.
If it's the same in two scenarios that are being compared, then it can be discounted. That doesn't make it non-existent, merely irrelevant.
Oh, I really don't think so!big-all said:
I have not the first clue what the b*ggery b*llocks you're talking about here.big-all said:
If you could frame your posts using the following generally accepted terms, it would be immensely helpful:
force
magnitude
direction
momentum
velocity
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For an aircraft to take off (i.e. have lift) it must have forward momentum through the air which is theoretically still. This forwards force is caused by the engines thrust pushing the aeropane forwards against the still air (not the moving runway). This force needs to overcome the air resistance and any other forces acting against it for the plane to move forwards. Now comes the tricky bit when you consider what was being said about the runway moving, it depends on how you calculate the speed, wether it is done relative to a still point (the air or terra firma) where the wheels will spin at twice the rate which they would normally or wether it is calculated between the wheel and the moving runway. It is an impossible concept for the runway to be moving at the opposite but equal speed to the wheel and for the aircraft moving forwards, the only time this holds true is when the aircraft is stationary so this is where the treadmill falls down. As soon as the wheel moves forward say 1m, the treadmill will have to move backwards 1m to compensate causing the wheel to move 2m which means at the same time the treadmill has to move 2m to compensate etc etc etc....
Spark123 said:
The thing is though, the wheel is a free-spinning object at the bottom of the plane, so moving the wheels circumference round by 1m does not necessarily equate to moving the plane back by 1m.
Imagine the plane is on the runway, but tethered to fixed ground points either side of the runway on non-moving bits of earth.
Start up the runway, and the plane will move backwards a bit, but then the tethers will take up the slack and will in effect act as a force moving in the forward direction. The conveyor is still moving, the wheels are spinning, and the plane stays still.
Now, replace the tethers with thrust. Once the thrust is greater than the forward force those tethers were exhibiting, its going to go forward, regardless of what the conveyor belt and wheels are doing. They will both spin faster, but they do not relate to the position of theplane, as the backwards-direction force (caused by friction between the wheels and the conveyor) is going into spinning the wheels round, not into moving the plane backwards.
It is kind of the same difference as you would get between having a car in neutral on a moving conveyor belt, and a car in 1st gear. The Car in neutral would not move backards at the speed of the conveyor belt, even though the wheels were turning at the same speed as the conveyor was moving underneath it.
I think there's only one way to resolve this.
Now, am I right in saying that empip has some access to planes ?
And am I right in saying that Crafty1289 has some access to conveyor belts ?
And its about time we all had a DIYNot day out, isn't it ........
Now, am I right in saying that empip has some access to planes ?
And am I right in saying that Crafty1289 has some access to conveyor belts ?
And its about time we all had a DIYNot day out, isn't it ........
Softus said:Oh, I really don't think so!big-all said:
I have not the first clue what the b*ggery b*llocks you're talking about here.big-all said:
If you could frame your posts using the following generally accepted terms, it would be immensely helpful:
force
magnitude
direction
momentum
velocity
rubber wheels on rubber conveyor would give you probably about 90% of the weight as avalable grip for acceleration and deceleration
Actually, I've got a better explanation than my previous tethers.....
Imagine a plane, on the runway, with a line going to a winch directly in front of it.
Start up the runway, the plane stays still.
Turn the winch, the plane goes forward. Wheels and conveyor keep spinning below, but the plane goes forward. The conveyor matches the wheel speed, not the plane speed. In fact, given a good contact between the two, they can't do anything but match each other.
Replace winch with thrust, and QED !!!
Imagine a plane, on the runway, with a line going to a winch directly in front of it.
Start up the runway, the plane stays still.
Turn the winch, the plane goes forward. Wheels and conveyor keep spinning below, but the plane goes forward. The conveyor matches the wheel speed, not the plane speed. In fact, given a good contact between the two, they can't do anything but match each other.
Replace winch with thrust, and QED !!!
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This is diy ... Let's get simple.
Pretty much like riding a bicycle on level ground, overcome the inertia or resistance to initial movement thereafter less energy being required to maintain a steady velocity.
Unladen aircraft is at idle, brakes on, in still air, running indestructable tyres and wheels. ... currently no velocity therefore conveyor is stationary.
Aircraft winds engines up to required thrust, releases brakes and begins to roll, too late the conveyor reacts, (inertia has been overcome ) aircraft is rolling at 10 m/s relative to the earth or terra firma... the belt passes tyre at 10 m/s in opposite direction .. net result? The 'freewheeling' wheel revolves at twice the normal rpm required for 10 m/sec aircraft forward speed .... Aircraft now has to overcome the additional friction of the ever faster rotating wheels at any given forward speed ... wheels, axles and bearings are designed to be as near friction free as is possible whilst retaining design strength (a good surplus being required for landings etc)... The (unladen) aircraft having surplus power is well placed to overcome the added friction.. it accelerates through the air, relative to terra firma up to take off velocity then leaves the runway/ belt, with wheels spinning very quickly.
And if you do not rate that then I propose a Harrier !
Pretty much like riding a bicycle on level ground, overcome the inertia or resistance to initial movement thereafter less energy being required to maintain a steady velocity.
Unladen aircraft is at idle, brakes on, in still air, running indestructable tyres and wheels. ... currently no velocity therefore conveyor is stationary.
Aircraft winds engines up to required thrust, releases brakes and begins to roll, too late the conveyor reacts, (inertia has been overcome
) aircraft is rolling at 10 m/s relative to the earth or terra firma... the belt passes tyre at 10 m/s in opposite direction .. net result? The 'freewheeling' wheel revolves at twice the normal rpm required for 10 m/sec aircraft forward speed .... Aircraft now has to overcome the additional friction of the ever faster rotating wheels at any given forward speed ... wheels, axles and bearings are designed to be as near friction free as is possible whilst retaining design strength (a good surplus being required for landings etc)... The (unladen) aircraft having surplus power is well placed to overcome the added friction.. it accelerates through the air, relative to terra firma up to take off velocity then leaves the runway/ belt, with wheels spinning very quickly.And if you do not rate that then I propose a Harrier !
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