Author Topic: plane on a conveyor belt? (Read 21995 times)

moot · « **Reply #660 on:** February 04, 2007, 03:05:21 PM »

By now, the best reply to this thread will include a working model of the problem, either physical or digital.. for everyone to see with their own eyes, rather than misundertandingly theoretize about.

eskimo2 · « **Reply #661 on:** February 04, 2007, 03:16:06 PM »

Quote

Originally posted by Holden McGroin
Magic wheels?

Lightweight wheels allow for a quicker accelleration. This is why TDF bicycles have $5000 carbon titanium rims. The rotational equivalent of F=Ma is Torque = (WK^2) angular accelleration. Yes, there is some energy you are storing in the flywheel effect of the spinning wheel / tire set. I can add a relatively small amount of energy I put into spinning my wheels (which is what this thread has been doing since the first post) I could build a machine that probably could have a wheel set of a 747 spinning to an equivalent of 100 mph on 30 seconds or so with a 5 hp motor.

So, I accelerate, but I don't, because I can only apply enough power to overcome the rolling friction due to this rediculous conveyor on which I find myself. I CANNOT APPLY FULL THROTTLE BECAUSE THRUST AND ROLLING FRICTION* MUST BE BALANCED IN ORDER TO STAY WITHIN THE CONTRAINTS OF THE PROBLEM regardless of the magical qualities of the wheels.

*+ 1/2*k*M*R^2*(ratational speed Rad/sec)^2 This is the flywheel energy stored in the spinning wheels... you would need to diffrerentiate with respect to time to get the force at any one specfic time.

Either the aircraft is horribly underpowered or the original question is flawed.

Holden McGroin,

You could hypothetically build a machine that probably could have a wheel set of a 747 spinning to an equivalent of 100 mph on 30 seconds or so with a 5 hp motor? Good! Now use the same principle to build a hypothetical machine and use a 60,000 hp motor to get those wheels up to 1,200,000 mph in 30 seconds. Do you think that would give the 747 motors a challenge to work against?

The holding force/ traction requirement/ essential coefficient of friction, or whatever you want to call it, at the contact patch between the moving conveyor and spinning wheels would be the same traction requirement as a 747 with its wheels locked on a runway and engines on full power.

Like I’ve said many times, any plane capable of taking off on a normal runway with its wheels locked may also take off on the super conveyor, but how many planes can do that?

cpxxx · « **Reply #662 on:** February 04, 2007, 04:17:58 PM »

14 pages, 14 pages and I still can't get my head round this problem. I'm sure one of you is right. This is toppling my brain:eek:

Holden McGroin · « **Reply #663 on:** February 04, 2007, 04:39:53 PM »

Quote

Originally posted by eskimo2
Now use the same principle to build a hypothetical machine and use a 60,000 hp motor to get those wheels up to 1,200,000 mph in 30 seconds. Do you think that would give the 747 motors a challenge to work against?

That flywheels need energy to accelerate is not at issue.

The fundamental premise of this thread is flawed. Airplanes generally have enough power to overcome rolling resistance, and because they accelerate, enough to overcome the flywheel energy of the wheels spinning up. The flawed premise of the thread is that the thrust must exactly match the rolling/flywheel resistance in order for the conveyor relationship to be preserved.

End this thread!

eskimo2 · « **Reply #664 on:** February 04, 2007, 05:11:09 PM »

There’s no question we really could not build a conveyor that needs to match the wheel speed of a normal plane. There’s another question out there where the conveyor matched the plane’s speed. That question would require a conveyor the full size of a runway… Who the heck has ever dreamed of that except to answer that question? It’s not quite as far fetched as the super conveyor but clearly both questions are hypothetical.

There is no question that an airplane could take off on a conveyor that matched its (plane) speed. But, that’s not our question. Why does our question have the conveyor control match the wheel speed? Is it a typo? Or, does our question turn “hypothetical” up a notch? It really takes more understanding to comprehend the difference between the two questions.

In our question where the conveyor matches the wheel speed: it accelerates at such a rate to keep the plane in place because it must! The question says it does; therefore if there is an explainable way and/or speed(s) that will hold the plane still, it has to be the answer! Our question forces this answer. Why was our question phrased this way? Because it is a much harder and much more interesting question! The answer is not obvious; at first we all thought it was an impossible question. I honestly thought that rabbidrabbit asked an honest question that showed a poor understanding of physics and was poorly worded. Once someone pointed out that there was a thread on a physics board that was 458 pages long, I Googled the question and found that the plane speed question has been all over the Internet for a couple of years. Our question, however, is new (I think). At least in my brief searching I have not found it as an original question starting a discussion. Someone either put a very clever twist on it or unknowingly added a word that changes everything! This question also is all about identifying forces and physics, it just has another level. Most people have no comprehension of the effects of rotational inertia.

If we would have had the question where the conveyor matches the plane speed, this thread may have ended on page 3 or 4. But, like it or not, we got the hard one.

Holden McGroin · « **Reply #665 on:** February 04, 2007, 05:50:45 PM »

The question of rotational inertia was answered for me when I took several classes in dynamics when I studied for my engineering degree. It is much more simple than this thread.

F = ma is the linear equivalent of

Torque = Moment of Inetria * angular accelleration

That's it.

Now lets illustrate this fundamentally simple principle by imagining an impossibly underpowered airplane on a impossible conveyor, and control the relation between the two with an impossible control system....

wait... let's not.

eskimo2 · « **Reply #666 on:** February 04, 2007, 06:17:59 PM »

Why does the plane have to be impossibly underpowered? Any wheeled plane that can’t override its brakes with its engine thrust theoretically be held in place by the super conveyor.

What makes the conveyor control system impossible?
Consider the question where the conveyor control must match the plane’s speed. Why don’t we have a problem with how exactly or accurately the conveyor matches the plane’s speed? Isn’t the conveyor control issue really the same? The plane must move first! Not only must the conveyor match the initial acceleration of the plane, it must constantly lower the acceleration rate as the airplane has less power to devote to acceleration due to the ever increasing air resistance, in a way that is more complex! For some reason, no one has a problem accepting that its control works. The answer is that it does not matter how the control works, what is important is to understand that the plane can take off because the aircraft motor grabs air, not the ground (or treadmill). The fact that the wheels consume a bit more energy is irrelevant. This question is all about identifying forces and physics.

This is a theoretical question. In my mind it helps people understand, explore, explain, discuss and theorize ideas. What’s so wrong with that?

If you hate this thread so much, why do you keep coming back? Are you forced to read every response and thread? When most folks get tired of a thread they stop reading and posting in it.

Since you are posting in this thread, please explain why a treadmill of unlimited speed and strength could not keep a normal plane (with wheels that have unlimited strength) from moving.

moot · « **Reply #667 on:** February 04, 2007, 06:37:43 PM »

Why doesn't someone just build a scale model?

Holden McGroin · « **Reply #668 on:** February 04, 2007, 06:48:26 PM »

Quote

Originally posted by eskimo2
If you hate this thread so much, why do you keep coming back? Are you forced to read every response and thread? When most folks get tired of a thread they stop reading and posting in it.

I was trying to free you from your prison. If you wish to live there, so be it.

Quote

Since you are posting in this thread, please explain why a treadmill of unlimited speed and strength could not keep a normal plane (with wheels that have unlimited strength) from moving.

So I am to answer what will win, an ummovable object or an irresistable force?

That is philosophy, not physics.

eskimo2 · « **Reply #669 on:** February 04, 2007, 06:56:20 PM »

Holden,

This is a great responce, do you agree with what JCLerch has to say?

Quote

Originally posted by JCLerch
This is the crux of the problem. Your question, and the original thought experiment, mix "real world" physical items with intellectual constructs. (Non-existent hardware).

For instance, You put a "real Cessna 172" on a intellectually constructed, artificially constrained, treadmill. Aka a "Super Treadmill" Then to answer your question to the finish, we have to add some additional constraints.
Are the Cessna wheel assemblies indestructible? Yes/No
Are the Cessna wheel bearings and axles indestructible? Yes/no
Is the super tread mill indestructible? yes/no
Is the super tread mill capable of continually accelerating at 910ft/sec/sec? Yes/no
shall we ignore the effects of Relativistic Quantum Field theory? Yes/no
IF you answer yes to all the above, then the airplane will not move forward (with respect to the ground the treadmill is attached to) while producing thrust. Of course, this will be an interesting experiment to an outside observer, as the surface of the treadmill will go from 0 to 700mph in about 1 second. 11 seconds later (12 seconds after opening the throttle on the airplane engine) the treadmill surface speed will be traveling at Mach 10 and still accelerating. In a little over 2 years, the linear surface speed of the super tread mill will exceed the speed light. As I said, answering YES to all the thought constraints will be very interesting to observe from a distance!

IF you decide to answer yes to everything except the last question, and include Einesten's theory of relativity, then the end result is sufficient energy in the system to be equivalent to enough mass to create a Black Hole.

IF you answer yes to everything except the last two questions, then the treadmill will eventually stop accelerating, and the airplane WILL take off.

IF you answer no to ANY of the first three questions, you will roll the Cessna up into a little metal ball.

IF we constrain the experiment to things that CAN BE FABRICATED, then I predict it is impossible to build a treadmill assembly that can accelerate at 910ft/sec/sec for anything longer than a few fractions of a second. Once the treadmill stops accelerating, the airplane will move forward relative to the ground the treadmill is attached to, eventually gain enough airspeed and takeoff. How unusual the takeoff appears depends on the rate of acceleration and the top speed the treadmill is capable of. [/B]

Holden McGroin · « **Reply #670 on:** February 04, 2007, 07:32:33 PM »

The construction of the aircraft or the conveyor does not matter, it is the construction of the problem that is fundamentally flawed, at least from a physical if not philosophical POV.

In order for the premise of the problem to remain true, the aircraft cannot move as any movement by the aircraft is counteracted by conveyor speed.

If thrust is counteracted by wheel drag, (the only counteracting force available) the thrust and drag vectors cancel so that no net force is available to accelerate the plane.

No net force means there is no acceleration, the conveyor is at a constant speed. A constant speed, the rotational inertia force (which is only required to accelerate) is zero, so we are back to just rolling resistance.

Even if you accept that acceleartion can occur, it is only acceleration of the wheels, and that is perfectly balanced by engine thrust..... somehow

Once again, we are back to immovable vs. irresistible and round and round we go.

eskimo2 · « **Reply #671 on:** February 04, 2007, 07:38:17 PM »

Quote

Originally posted by Holden McGroin
The construction of the aircraft or the conveyor does not matter, it is the construction of the problem that is fundamentally flawed, at least from a physical if not philosophical POV.

In order for the premise of the problem to remain true, the aircraft cannot move as any movement by the aircraft is counteracted by conveyor speed.

If thrust is counteracted by wheel drag, (the only counteracting force available) the thrust and drag vectors cancel so that no net force is available to accelerate the plane.

No net force means there is no acceleration, the conveyor is at a constant speed. A constant speed, the rotational inertia force (which is only required to accelerate) is zero, so we are back to just rolling resistance.

Even if you accept that acceleartion can occur, it is only acceleration of the wheels, and that is perfectly balanced by engine thrust..... somehow

Once again, we are back to immovable vs. irresistible and round and round we go.

Who said the conveyor remains at a constant speed? The conveyor must be at a constant acceleration rate to hold the plane in place (assuming the plane’s engine’s are at a steady power setting). A constant acceleration rate requires constant power.

eskimo2 · « **Reply #672 on:** February 08, 2007, 08:16:27 PM »

Mark Luper · « **Reply #673 on:** February 08, 2007, 08:48:29 PM »

WOW!

I thought this horse died last week!

Mark

mietla · « **Reply #674 on:** February 05, 2008, 05:30:59 PM »

just because the horse is dead does not mean you can't kick him some more.

http://forums.hitechcreations.com/forums/showthread.php?s=&threadid=225421

some never give up