crazymjb

Newtons 3rd law states: Every action has an equal an opposite reaction. I don't know exactly how you say I am violating it. If you meant what I think you mean than the force of the engines has to do something... And that would be countering the backward force applied by the treadmill through the resistance in the wheel bearings...

Can you explain how the first law of Thermodynamics should apply?

Mike

wstevens

Okay, here's a quote about wheel bearings in aircraft design from Aircraft Design: A Conceptual Approach by Daniel P. Raymer (this was the book from my undergrad aircraft design course):

"For most aircraft gear systems, the torque required to put the bearings into motion from rest is higher than that required to keep the bearing running once wheel rolling starts. However, both starting and rolling bearing friction is negligible compared to skin friction, and thus can be neglected in determining minimum required take-ff thrust."

OK, so let's say by Newton's 3rd, when the engines start up, the rising thrust by the engines is matched (in an equal and opposite way) by the friction in the bearings. Based on the above information. the friction in the bearings will decrease as soon as the wheels start to roll on the treadmill (which will happen almost instantaneously by the way).

So the bearing friction decreases, but the engine thrust doesn't decrease. So what is counteracting the thrust by Newton's 3rd? Well, the engines are creating a force on the air outside the plane, and the air outside the place is creating a force on the plane in return. The engines force the air backwards and the air forces the plane forward, which moves the plane.

The first law of thermo is basically the conservation of energy. You had said that the friction of the bearings would keep increasing until they wheels seized (which is wrong according to Raymer's book). But if the wheels seized and nothing moved, then how would the energy of the system be converted or dissipated?

crazymjb

Ok... I think Raymer is trying to apply Newtons First Law to wheel bearings.

Anywho... according to how I am thinking about this(the treadmill instantly matching wheel speed, which makes it theoretically impossible for the aircraft to move forward) the bearings would be going infinitly fast, resulting in their failure in siezure. The bearings would eventually seize and fail because even if the resistance was constant, the higher speed would multiply that friction. THink about it, you know if you spin an axle within bearings faster and faster it will get hotter and hotter.

Also, the resistance applied by bearings though a constant is still resistance. So if you multiply that constant times a super high speed, you get a super high amount of total resistance which the engines can not overcome. I again want to make it clear that I agree that if the conveyer speed matches the actual aircraft speed the aircraft will take off.

As for conservation of energy, the energy from the engines would to to the heat output by the bearings spinning at some rediculously high speed, or whatever else fails, rubs, and creates heat.

Mike

wstevens

??? Explain this

(a) The resistance of the bearings goes down, it doesn't remain constant once the wheels start to spin. (b) solid-to-solid friction is nearly independent of velocity. What is your reference for your statement that it depends on velocity? Lastly, the reason why something heats up under friction is because it can't dissipate heat fast enough, not necessarily because the friction is increasing. In this case, the friction is certainly not increasing.


Wrong. And still ignoring what I wrote before. A very Water-S type statement.

Again, solid-to-solid friction is isn't going to vary much with speed.

wstevens

OK, I'll use crazymjb's logic to answer this question.

The plane will take off because:

(a) treadmills or conveyor belts use bearings too, and those bearings are definitely not aerospace grade, so they will fail and the conveyor will seize up before the plane's wheels will.

(b) even if the treadmill or conveyor belt was built using aerospace bearings, a super-long conveyor belt is going to have a ton of rollers underneath it, and consequently will have many, many more bearings than are in the wheels of the plane. Hence, the probability of a bearing in the conveyor belt system failing is much, much higher than that inside the plane wheel. So the conveyor would seize before the airplane wheels even if they used the same grade bearings.

Thus, in either case, the conveyor will seize before the aircraft wheels will. Once this the conveyor belt stops, it's just like a normal runway and the plane will take off.

Q.E.D.

wstevens

And crazymjb...if you do ever become an engineer, can you please write "crzymjb" on anything you design? That way I'll know not to ride in it or use it.

crazymjb

If you spin bearings excessivly faster, whatever the reason, whether it be heat dissapation or not, they WILL heat up and as a result will reach a failure point.

The treadmill is already make believe, logic need not be applied.

Its a theoretical impossibility for it take off if the conveyer belt is isntantly matching wheel speed, which is what I am arguing. As a result of trying the wheel speed would simply just increase and increase and increase until the bearings failed, whether it be from them overheating(which can and does happen) or from the Centrifugal force going up so high the metal can no longer sustain its shape. Again, if it matches vehicle speed, the aircraft will take off.

Mike

wstevens

If the treadmill is make believe and can have infinitely strong bearings, then so can the plane. And the only reason you think the plane can't take off is because it's bearings will seize.


Again, I think you're missing the point of this problem. But, oh well.

I like your enthusiasm crazymjb. I do have to say though, that through 10 years of engineering school and several years out in the workplace, I've found that people with your mindset are the worst students and professional engineers. There's nothing worse than an engineer who thinks he or she knows something without actually learning it first. These people tend to argue with the ultimate quest of "winning" some debate instead of learning and trying to find the best answer. Most times, thhey don't have any references to back up their opinions because they've never actually taken the time to learn anything.

wstevens

And, by the way, the reason I think your missing the point of the question is because I think the question is a brainteaser that is meant to test
(a) the application of Newton's laws
(b) the concept of relative motion
(c) the ability to distinguish between one's every day experience with cars (in wich thrust is transmited through the ground-to-tire interface) and airplanes (in which thrust is transmitted through the air-to-airplane interface).

I don't think this was meant to be a question that hinges on the strength of the bearings in airplane wheels.

crazymjb

I could say a lot about professional engineers(in terms of quirks, my great grandfather chewed 100 times before swallowing, etc, though he lived to 90 so maybe it worked)

Lets say the bearings are invincable. If the conveyer is instantly matched to the wheel speed, the plane will still not move.

Now... As I will say yet again. If the conveyer is simply matching the aircrafts forward speed, assuming the bearings and tires cand stand up to that the plane will take off.

Now the whole other debate... If the bearings were built perfectly and were ivulnerable, and also didn't expand when heated than yes, the treadmill could go at any speed and the aircraft could take off with the normal negligable bearing resistance. This does not change the fact that if it was an instant speed match, the aircraft would not move forward as it's a paradox.

Mike

fdl

wstevens

Ok, so here you;ve said that
(a) the bearing resistance is negligable. I agree
(b) If the bearings don't fail the plane will take off. I agree with this too.


You're last statement doesn't seem the jibe with the rest of the paragraph.

The plane will move horizontally if the force of the of the engines on the plane is greater than any other horizontal force. The only other one you've identified is bearing friction, which you've stated is negligable.

According to Newton:

F = (m) X (a)

The net horizontal force on the plane = (the mass of the plane) X (the plane's horizontal acceleration)

The net horizontal force = the thrust - the friction of the bearings.

If the the thrust is greater than the friction of the bearings, then by Newton's law (equation above), the plane will have a horizontal aceleration in the direction of the thrust.

No where in the above equation does the speed of the conveyor come in. If you can write a simple equation that alter's Newton's Law to include the speed of the conveyer, please do. That's the only way you can answer this point. ANything else you type is not engineering.

GIBSON6594

thread

wstevens

Yeah, after estimating how much time I've spent discussing this with Water-S Jr., I decided that I'm the real asshat here.

crazymjb

I don't mean to be a complete prick, but I'd like to be in NO WAY associated with water S. I would like to think my arguments, thoughts, and possesions themselves have a little more substance to them. For example... I have engineered and built some stuff before(if you read my threads...) and I provide pics and video clips if applicable.

Dubya, I am saying I agree with you as long as the conveyer belt is matching vehicle speed not wheel speed, and the wheel speed instantly at that(as again that is an impossible situation as it IS a paradox).

Now... If the conveyer speed was trying to match the wheel speed, accelerating at a rapid rate yet remianing behind the speed of the wheels, than assuming the bearings didn't fail yes the plane could still take off take off, HOWEVER, the bearings would fail.

Mike

bigman

i never thought it would turn into this, damn.

wstevens

Your thoughts are better laid out than Water-S's, so I tak eback the Water-S Jr. reference.

Okie Dokie. Good enough - I'm done with this thread.

crazymjb

And FTR, you were 16 once, you know where always right

Mike

Bdog

Petition to get on Mythbusters!

http://www.petitiononline.com/plntdml/petition.html

subinf

Done

srika

sounds like a job for myth-busters..

crazymjb

I already sent them an E-mail... They said they read it all but don't respond. anyway... Mythbusters@m5industries.com, maybe if we all send requests...

Mike

JT Money

I figured it was in here somewhere. It just started in another forum and I can't believe what some people are saying.

NiteQwill

Holy back from the dead... I think I just getting used to the sand here when I started reading this thread.

Never had a chance to follow it, what was the answer?

slayer202

http://forums.anandtech.com/

post it on this forum. They love this stuff there

(Joking, they will probably ban you if you do it)

srika

hahhaha, ass. go play some Resistance: Fall of Man. thx

slayer202

yeah, im confused. But I was literally just playing resistance though

JT Money

The plane will take off.

suXor

Not without air flowing over the wings to create lift. If the plane is not moving due tot he runway keeping it in place, it will not generate lift.

svtmike

Smalls

here we go again....

it will take off

JT Money

http://www.straightdope.com/columns/060203.html

is300eater

what about a plane landing onto a conveyer? will it just come to a halt? I don't think so

Pure Adrenaline

iTrader: (2)

How is it possible for people to believe that the plane would take off?

Yes, it is creating thrust (whether propeller or jet powered) forward in order to stop moving backward with the conveyor belt. But there is no lift being generated. You need thrust/propulsion to move forward, and lift to take off. You don't have lift in this case. You can increase the power to the max and sit there all day long spinning the tires on the conveyor belt but you are never going to move vertical because there is no air moving around the wings.

is300eater

you need this vvv



if the plane was sitting still, it dosent matter how fast the wheels are spinning, there will be no airflow to left it off the ground er'... I mean conveyer

DanL

iTrader: (2)

Whether the conveyor belt is stationary or moving has absolutely nothing to do with the forward speed of the plane. All it affects is what speed the wheels spin. The plane WILL move foward and WILL take off. Look, dust off your old treadmill, find a hot wheel or matchbox car, and try an experiment yourself. Turn on the treadmill, hold the car on there, and then move it forward with your hand. You have no problem moving the car forward, right? In fact, it doesn't matter how fast the treadmill is running, does it? All that happens is that the wheels spin faster. It's the same with an airplane. It doesn't rely on the wheels and friction with the ground (or belt) for propulsion. The engine is pushing against the air. So the plane will move forward even though the conveyor belt and wheels will be spinning like crazy.

CUNextTuesday

How are people still saying that this won't take off?

It's been dicussed over and over - the answer is YES. The plane WILL take off....

Whiskers

Wait...What was the question again?

trancemission

I work on airplanes and I can honestly say "who really gives a shit about this?"

Ill worry about it when one of our jets trys to take off from a treadmill...

Whiskers