plane on a conveyor belt?

[WhiteHawk]

Originally posted by eskimo2
Depends on the mass of the plane.  A light enough plane will skid and accelerate.  If the skidding drag isn’t too much it might even take off.  It has been stated; any plane capable of taking off with its brakes locked (negating tire blow out leading to prop hitting the ground, etc.) will also take on the conveyor where the conveyor matches the wheel speed (also negating tire blow out leading to prop hitting the ground, etc.).

So the WS=0; CS=0; with the brakes on, but let the brakes out and we get forward some activity.  although most of the forward movement is absorbed by the traction or friction or drag of the wheels on the conveyor?  The lighter the plane the lesser the effect on PS (Plane speed?)?  Ok i am with you.

PS wife is home so I may have to bolt unexpectadly.

[john9001]

Originally posted by WhiteHawk
although most of the forward movement is absorbed by the traction or friction or drag of the wheels on the conveyor?    

if wheel drag is that high, how do real planes take off from real runways?


opps sorry, i forgot it is hypothectical, we must use assumptions that prove our result.

[eskimo2]

Originally posted by WhiteHawk
So the WS=0; CS=0; with the brakes on, but let the brakes out and we get forward some activity.  although most of the forward movement is absorbed by the traction or friction or drag of the wheels on the conveyor?  The lighter the plane the lesser the effect on PS (Plane speed?)?  Ok i am with you.

PS wife is home so I may have to bolt unexpectadly.

Not absorbed, transfered.

[eskimo2]

Originally posted by WhiteHawk
So the WS=0; CS=0; with the brakes on, but let the brakes out and we get forward some activity.  although most of the forward movement is absorbed by the traction or friction or drag of the wheels on the conveyor?  The lighter the plane the lesser the effect on PS (Plane speed?)?  Ok i am with you.

PS wife is home so I may have to bolt unexpectadly.

Think about this:

The plane on the treadmill has big electric generators driven by the wheels. The generators can turned on or off.
Plane sits on treadmill. Treadmill is off. Plane engine is off. Generator switch is off.

Treadmill is turned on to 50 mph: plane basically sits still, wheels freewheeling. Plane engine is still off.

Treadmill is still on at 50 mph. Generator switch is turned on: generators make electricity. The plane starts moving back on the conveyor. Plane engine is still off.

Right?

[WhiteHawk]

Originally posted by eskimo2
Not absorbed, transfered.

To what?  The conveyor?

[WhiteHawk]

Originally posted by eskimo2
Think about this:

The plane on the treadmill has big electric generators driven by the wheels. The generators can turned on or off.
Plane sits on treadmill. Treadmill is off. Plane engine is off. Generator switch is off.

Treadmill is turned on to 50 mph: plane basically sits still, wheels freewheeling. Plane engine is still off.

Treadmill is still on at 50 mph. Generator switch is turned on: generators make electricity. The plane starts moving back on the conveyor. Plane engine is still off.

Right?

Right, More friction required to turn the generators. The plane would start to move back at a speed proportional to the additional energy required to fire the generators.

[eskimo2]

Originally posted by WhiteHawk
Right.

Good,

Plane engine is now turned on enough to keep it from moving back.  Plane stops scooting back and now sits still, engines on, treadmill on, making electricity (about the same amount of energy as the plane’s engines produce).

Right?

[WhiteHawk]

Originally posted by eskimo2
Good,

Plane engine is now turned on enough to keep it from moving back.  Plane stops scooting back and now sits still, engines on, treadmill on, making electricity (about the same amount of energy as the plane’s engines produce).

Right?

Right.

[eskimo2]

Originally posted by WhiteHawk
Right.

Great,

We have all of this electricity being made by the plane; what to do with it…
Onboard the plane is an electric motor connected to a fantastic transmission that turns a huge flywheel.  Every second the plane makes electricity, the flywheel goes faster and faster.  The flywheel is now absorbing about the same amount of energy as the plane’s engines produce.

Right?

[2bighorn]

Originally posted by eskimo2
Steve & 2bighorn,
You interpret this question that a plane that normally takes off at 50 mph will take off at 50, and the conveyor will be going…
50?  Is this right?  Why 50?  Why not 40?  Why not 60?

Speed of the conveyor doesn't really matter. I'll try to explain.

Plane on the normal runway has to overcome amount of forces in order to take off, like drag, gravity, rolling resistance etc.

In your little story about wheels with different mass all planes would take off at the same speed after rolling the same length on the runway.

Why? Because the total sum of the forces remains the same. Only thing that will change is amount of kinetic energy stored in the wheels which is result of different wheel mass.


Now put plane on the conveyor which matches the speed of the plane but travels in opposite direction. Plane takes off normally.
(In real world it would actually take off slightly earlier, since part of the rolling resistance force is out of plane's equation and taken over by conveyor)

Case 3, plane is on conveyor which travels opposite to plane's direction, and is accelerating to the speeds well above that of the the plane.
Same result, plane would take off, the only difference is the net amount of stored kinetic energy (wheels angular moment).

Your assumption is that somehow the angular moment of the wheels when of appropriate size will negate the plane's thrust. You believe when energy stored in the wheels will match that of the thrust the equation result of net forces is 0 or even negative and the plane wouldn't take off.

For the sake of the argument, let's say there's no drag, let's say there's no friction in the ball bearings and the wheels freely rotate. In that case there can be no energy transfer from the wheels to the plane. Energy remains stored and wheels continue to rotate after take-off almost indefinitely.

If you have to account for the friction and such, then the total amount of energy transfered from wheels to the plane is equal to that of rolling resistance of the ball bearings. The rest of the energy is stored and the wheels continue rotating after take off until all the energy dissipate due to friction forces (drag and rolling resistance).

Conclusion:
Your hypothesis is fundamentally flawed because:
a) you assumed there's linkage between the wheels and plane which can transfer almost any force from conveyor to the plane
b) you assumed that forces transfered from the conveyor to the wheel have to have immediate effect on the plane.

[eskimo2]

2bighorn,

Read the story, answer the questions:

Here’s a story that illustrates my idea:  (Note that the term wheels in this story refers to wheels and tires)

Identical triplets Al, Bob and Chuck buy three identical bush planes.  Since they live in Alaska, all three brothers buy and install large balloon “tundra tires” and wheels.  The wheels, planes and brothers are identical.  All three planes will take off from a normal runway in exactly 100 feet and at exactly 50 mph.  The brothers fly their planes to an air show in Wisconsin.  At the air show Bob finds and buys a set of fantastic wheels.  These wheels are exactly like the wheels he has on his plane in every way except they have half the mass.  Their mass is distributed in the same proportion as the wheels that he plans on replacing.  Al thinks Bob is silly and is content with his old wheels.  Bob thinks that Al will eventually want a set, so he buys a second set to give to Al on their birthday.

Bob finds a buyer for his old heavy wheels and installs a set of his new lightweight ones.  He loads the second set into his plane so that it is balanced just as it was before.  Bob’s plane now weighs exactly the same as Al’s and Chuck’s, but its wheels have half the mass.

Meanwhile, Chuck runs into a magician who sells him a set of magic wheels.  These wheels are exactly like the wheels he has on his plane in every way except they have no mass.  Chuck installs his magic wheels.  He loads the second set into his plane so that it is balanced just as it was before.  Chuck’s plane now weighs exactly the same as Al’s and Bob’s, but its wheels have no mass.

When the brothers leave the air show they request a formation take off.  They line up wing tip to wing tip and apply power at exactly the same time.  All three planes weigh exactly the same and must hit 50 mph to lift off.  When Chuck’s plane lifts off his wheels stop spinning instantly since they have no mass.  Since they have no mass, they also have no rotational inertia.  When Al’s plane lifts off his heavy wheels are spinning at 50 mph and have considerable rotational inertia.  When Bob’s plane lifts off his half-weight wheels are spinning at 50 mph and have exactly half the rotational inertia as Al’s wheels.  

Where did the rotational inertia and energy in Bob’s and Al’s wheels come from?
How did the rotational inertia and energy now stored in Bob’s and Al’s wheels affect the take off distance of their planes?
We know that Al’s plane will still take off in exactly 100 feet; where will Bob’s and Chuck’s planes take off?

[hitech]

Why? Because the total sum of the forces remains the same. Only thing that will change is amount of kinetic energy stored in the wheels which is result of different wheel mass.

This is incorrect. Where did the wheel energy come from? If you have an increase in energy (which you agree) then some work had to be done.

Work = Force * Time. Therefore there has to be more force.

The energy stored in the wheel came from the wheel being turned by the runway (or conveyor). This force is opposite  the thrust of the plane (I.E. less net thrust), hence they would not all accelerate at the same rate.

HiTech

[2bighorn]

Originally posted by eskimo2
Where did the rotational inertia and energy in Bob’s and Al’s wheels come from?

You need to understand the difference between rotational inertia and angular momentum and how they are related. Both play the roll.
Anyways, thrust has to overcome the rotational inertia. Wheel with more mass has bigger rotational inertia but also bigger angular momentum. More force is needed to overcome inertia, but less later, since heavier wheels stores more energy therefore later on less is needed to keep it rolling.
As long as both planes total mass remains the same (plane + wheels) same amount of thrust is needed.

Originally posted by eskimo2
How did the rotational inertia and energy now stored in Bob’s and Al’s wheels affect the take off distance of their planes?

The take off distance does not change since it is depended on total weight (plane + wheels). In your story the reduced mass of the wheels is offset by ballast therefore total mass sum is unchanged and equal thrust is needed.  

Originally posted by eskimo2
We know that Al’s plane will still take off in exactly 100 feet; where will Bob’s and Chuck’s planes take off?

As already said, they all take off at the same speed and after same take-off distance.

[2bighorn]

Originally posted by hitech
This is incorrect. Where did the wheel energy come from? If you have an increase in energy (which you agree) then some work had to be done.

Wheel energy comes from the conveyor not from the thrust.
Increase is translated into work which is wheel rotation.

Originally posted by hitech
Work = Force * Time. Therefore there has to be more force.

Why? it works same way if we increase time (flywheel effect)

Originally posted by hitech
The energy stored in the wheel came from the wheel being turned by the runway (or conveyor). This force is opposite  the thrust of the plane (I.E. less net thrust), hence they would not all accelerate at the same rate.

By that account no plane would ever take-off westward because of the angular moment of the Earth.

There's no way that conveyor force opposing the plane thrust can be greater than rolling resistance. All the excess force (total conveyor force acting on wheels minus rolling resistance) is stored (angular moment of the wheel).

[hitech]

More force is needed to overcome inertia, but less later, since heavier wheels stores more energy therefore later on less is needed to keep it rolling.

Say what?

If wheel is beeing accelerated a bigger Moment always requires more force for the same rpm change.

If it is remaining a constant speed the Moments have nothing to do with the forces involved. Then we are just talking friction.

rotational inertia and angular momentum

Some how I am missing your point here, In my view those 2 terms are 100% the same.