Airplane physics question

There you have it. Blind conviction trumps proof and science EVERY time. With practice, you should be able to stomp your feet for more emphasis as well. That will really prove your point.

 

If I do choose to model it, you'll see what science says. Otherwise, keep on keeping on man, blissful, no doubt.

 

Yup, it can be modeled in software as well. But will you believe what you see on the screen and on the results table? Or will you just say that it assumes something instead of proving it?

 

By the way, I figured out how you could pretty much simulate the whole thing on a treadmill. Once you have the first constant speed experiment done, you can switch the fan to a higher speed. This will result in more thrust. Measure the skateboard's new, higher speed from the additional thrust on level, smooth ground again. If you can, you can simulate the plane firing up its engines, trying to take off

See if you can get your treadmill to go at exactly this higher speed as well. If you can, you can do the experiment. To mathc the question perfectly, you would have to get them to accelerate at the same rate, but since you can't do that easily, you will just have to hope that the two objects (treadmill and skateboard) reach their new higher speeds in reasonably similar times.

Now start with the first treadmill and fan speed and get everything to stay still without anything having to be held in place (steady state, just what you like). Flip the switch on the fan to the higher speed and turn up the speed on the treadmill at teh same time. The skateboard WILL move because the acceleration will be different between the treadmill and the fan. If the treadmill accelerates faster, (and if I am right), the skateboard will move back a bit untill the new speeds match. Or, if the treadmill accelerates slower (and again, if I am right), the skateboard will move up the treadmill. After a while however, if I am right, it will sit there quite happily again. In fact, if you can figure out the skateboard's acceleration and the treadmill's acceleration, you can predict how far up or down the treadmill the skateboard will move. It will not continue on to the end of the front or the back of the treadmill and fall off. If you do this right, you will see that the wheels are now spinning at a higher speed even though they are freerolling, and yet the skateboard and fan are not moving. If it was a plane, it wouldn't move either. No movement, no airflow, no lift, no takeoff. You know how that song goes...

You may need to do this a couple of times, starting the skateboard at the front, and then the back of the treadmill, in order to give it enough room to reach the new equilibrium whether it moves up or down the treadmill. But it should get to a point where it sits there happily, wheels turning faster, but not going anywhere. A long tread lenght and slower speeds will help. You can't make a treadmill longer but you can get lower thrust force on the fan by blocking off portions of its face, so that it draws in and pushes out less air.

 

Guys, we collectively need to let this go.

Did any of you genuises stop to think or read about the fine print?? If the wheels are moving in the OPPOSITE direction of the conveyor... The opposite direction!!?? Every single post and argument has been made around the idea that the wheels are moving in the SAME direction as the conveyor. 8 pages of useless argument about a gimmick that is ludicrous to begin with.

Wheels in opposite direction to the conveyor at the exact speed?? That would mean zero, which would mean the wheels were not spinning at all, and in fact you were simply trying to take off with the equivalent of locked brakes. Which in most aircraft would not be successful...

Art- you need a life!! (You're goofy wrong BTW!!)

Cheers,
dec

 

Sorry, you are wrong. If the treadmill was moving in the opposite direction, it would not result in stationary wheels. What are you smoking? What you are saying is absurd. Absolutely, completely wrong. I don't think anyone in either camp agrees with you. We are all agreed that the wheels are moving at X in relation to the conveyor belt going at -X, and at 2X with respect to the ground. Where do you get a wheelspeed of zero from? Get off the sauce!

Why put in your two cents' worth and then tell everyone else to drop it? You want to drop it, fine, drop it. Don't post. I don't feel like dropping it. I want to be either proven right or proven wrong. No one disagreeing with me has posted any proof. I have a particle physics ph.d and two engineers agreeing with me, as well as Newton's laws proving what I am saying. On the other side, there's a lot of junk science, and a refusal to do the experiment that proves my point, as well as a refusal to ask those who would know, ie. physics professors.

I know I am right. I know the plane will sit there, wheels spinning. I have proven it. I have told shiggins (and anyone else who cares to try it) how to replicate it and prove it to themselves. As long as they don't agree, I will be happy to keep repeating my points and proving why it will happen the way I am saying it will.

 

Alert the presses please, I'm going to agree with Art, partially. You're wrong. The wheels will spin. Thanks for coming out.

 

Art, I was searching around on the myth busters website for the odd chance that they did this experiment. They haven't, but there is a thread about it on their forums and much like ours, it's way to long. Someone posted this link, it explains the right answer decently.
http://www.avweb.com/news/columns/191034-1.html

 

Now, I'm going to disagree with you You haven't proven anything, and you don't have Newton on your side. Though I know you'd like to think so. You also have someone with a PhD disagreeing with you.

 

I also wanted to pose this question.
You have an airplane coming in for a landing. it's doing 100mph groundspeed on a calm day. The conveyer knows this and so starts spinning at 100mph in the opposite direction of the planes motion. The airplane touches it's wheels to the conveyor belt.....what happens?

By your arguement, the conveyor should immediately counteract the planes forward motion, and the planes groundspeed will be 0. Makes me wonder why they don't use convyors on runways.

 

I didn't notice anyone with a PhD disagreeing with me. If that is the case, it's really sad. Why don't you do the experiment and prove a PhD wrong? That would be a nice feather in your cap. I don't think you will prove reiserkaiser wrong.

Also, you said you could set this up in software? What software? I would not mind trying it if you could point me to the software that could simulate this. I am not sure I would be able to learn to use it, but I am willing to try.

 

I will likely use ADAMS. http://www.mscsoftware.com/

 

They don't use conveyors because they are expensive and pointless, when a long runway will do. It also allows for runoff, different speeds, etc.

They do use the concept though. When they can, seaplanes land against the current. Don't say it's not the same thing. It is.

To be specific about your example, the wheels are at zero speed when the plane is in the air. As soon as they touch down on the ground, their speed goes up to 100mph. On a conveyor going in the other direction at 100mph, their speed would go to 200mph. If they could do it instantly, the plane would slow down dramatically. But it would not stop. You are thinking in terms of speed only. Think in terms of kinetic energy. Read on.

If something could take away all the kinetic energy of the plane, it would stop moving forward. The kinetic energy of the plane is given by half mass times velocity squared. The wheels, since they are rotating, have to have their kinetic energy expressed in terms of angular motion. That is half mass (of all wheels) times velocity squared divided by radius (of the wheels). To keep it simple, just assume it has one wheel and can balance perfectly. For the wheels to take away the kinetic energy of the plane, they would have to accelerate up to thousands of miles an hour. I already solved this equation for you by assuming one wheel and running some numbers. Give me the numbers of the variables here or solve for it yourself, and you will figure out the speed that the conveyor belt needs to be rotating at in order to accelerate the wheel enough to stop the plane's forward motion. Of course, in real life, things would break, but in our hypothetical situation, the plane WOULD stop.

 

Do you know how to use it? How long would it take you to set this situation up in the software?

 

Serious question: Wouldn't the conveyor itself be adding energy into the system though? It's forcing the additional wheel rotations.

 

Yes. It gets complicated really quickly. We are trying to keep it simple. But yes, you are absolutely right.

 

Aiieeee!!! 5 more pages in half a day!!! I think we already have a software model right here. The post count rate on this thread is accelerating and it's still getting nowhere. Pretty soon the post count rate will reach infinity and the servers will go into melt down!

P.S. I gave up posting on this somewhere before page 10 after I got wrapped around the axle and started making dumb mistakes. So you're not going to suck me in again.

 

See, I told you it accelerates in place without going anywhere. Just like this thread!!!!

 

The conveyor adding energy to the system nullified your point...

 

Depends how much energy it adds to the system. It can never add more than it has. If it could, then the sum of the plane's energy and the conveyor's energy would increase the entire model's energy and my point would be nullified. But from first principles we know energy is conserved.

The overall energy does not increase. Different amounts go into different parts of the model at different times (but rather quickly--that's why you need to stand well back!).

 

Obviously, but because the wheels are seperate from the plane, as they are in reality, then the rotational energy that doesn't come from the intial landing is added in by the conveyor belt...

 

But the initial problem stated defines a system which as infinite energy....because wheel speed would go to infinity...

 

They are separate from the plane, but connected to the energized system that is the plane. The plane's momentum/kinetic energy is present in every molecule of the plane/landing gear/wheels.

 

Suggestion: Read through the myriads of useless posts and find my first post- I think you'll find it a little more enlightening- and perhaps I didn't just come out- I was using discretion- which seems to be in short supply here.... (Spinning wheels... Take a pencil, put it on a piece of paper, and see what the hell I'm talking about... In order to have zeroed out the differential the paper needs to be moving with the same vector as the contact vector of the wheels. Otherwise you have resistance and.....)

I have a Ph.D in Astrophysics from MIT- and Art is wrong- you are right. You belittle me because you do not know me, and because I presume that you have not read my post umpteen pages ago. Art is arguing nonsense and you are arguing basic science- assuming that the wheels move in the same direction as your treadmill!! (Which the original post states to the opposite...)

Newtons Third Law: Equal and opposite: How exactly do the hypothetical wheels dissipate the energy generated by the engines? They are passive devices, again- read my earlier post. The wheels don't transfer the energy, they can't, they haven't, and they never will. Please please please let this thread die the peaceful death it deserves... (Or read my previous post and find the flaws... ID notwithstanding I revel in the scientific method...)

Cheers,
dce

 

IF the wheel speed goes to infinity. It doesn't.

It builds speed to the point at which the incremental thrust of the plane's engine equals the incremental deceleration experienced by the wheels due to air turbulence, rolling resistance, friction at various points, etc. The sum of the opposing forces will create a resultant thrust vector of zero and it will achieve a steady state. Not because of no forces (which always cause acceleration), but because of two equal and opposite forces (causing equal and opposite accelerations).

Zero friction would get you to infinite speeds. But you have to be careful about where you are eliminiating friction. At the wheel bearings? At the conveyor belt surface? Zero friction everywhere would mean that the wheels slide forward. This is not what the question is trying to make you think about. In real life, it would accelerate twice the speed of a plane moving horizontally on the ground in a near-vacuum.