7

11/20/2007 10:58:46 PM

no offense, cause i think persian people are the fucking shit, but i dont appreciate that last post


totally makes me feel unimportant....why dont you pm me the right answer...hell i might even troll some acting like the wrong answer is the right one...

11/20/2007 11:04:04 PM

Quote :

"^ That's all well and good, but ask yourself this: does it really make physical sense that the airplane, under power of its own engines, could be traveling at takeoff speed relative to the conveyor belt and NOT have any forward movement? The problem says nothing about headwind, I see no reason to introduce variables it doesn't call into question. I believe it's pretty sensible to use Occam's Razor and default the headwind to zero."

I don't think it makes physical sense that a conveyor belt could be constructed to carry out this problem, but assuming it could be done, it makes perfect sense that an airplane under power of it's own engines could travel at takeoff speed (or any other speed << light) relative to the conveyor belt and have zero forward velocity relative to earth if the conveyor belt is moving with an equal and opposite speed, but relative to the earth. The trick is that the pilot would have to control the throttle so that the plane only counters frictional forces between the plane and belt (i.e., sum of forces = 0).

The plane could easily take off by going full throttle because it would quickly overcome any wheel/rolling friction (sum of forces = m*a). But if the plane is constrained to only go the same speed relative to the belt that the belt is going in reverse relative to the ground, then it doesn't matter if the plane is doing Mach 1 or 1 mph relative to the belt; it will still have zero velocity relative to the ground.

11/20/2007 11:07:13 PM

words

11/20/2007 11:07:51 PM

yeah

i dont even remember what the question was anymore

11/20/2007 11:09:33 PM

/thread? lock? no one else cares until the mythbusters ep hits youtube

11/20/2007 11:10:42 PM

dec. 12th people.

11/20/2007 11:13:50 PM

Quote :

" Lift is generated by the movement of air over the airfoils...if there is no forward movement of the plane then the air is not being moved over the airfoil and lift cannot be generated. End of Story. ... haha...yeah... but don't make me break out my Lift equation "

you mean the P + 1/2*rho*V^2 = Const one? That doesn't have anything to do with this problem. The plane takes off.
or maybe the Sum[Gamma] = 0 or whatever form it takes one? Still doesn't have anything to do with the question. Plane still takes off.

Quote :

"and if you just want to bet the money, I'll do that too provided we meet the aforementioned conditions. Maybe a poll of 5-10 mechanical/aerospace engineering professors?"

I asked my MS advisor this question just for fun a couple of years ago when I was working on my AE Thesis. (Dr Charles Hall, not exactly my favorite professor in the world, but no one can accuse him of being unintelligent.) He didn't even blink before he responded with, "of course it takes off. The wheels don't have anything to do with takeoff at all."

[Edited on November 21, 2007 at 12:42 AM. Reason : ]

11/21/2007 12:41:14 AM

im not an ME or AE, but god damn, this question is simple: the plane can not take off. if it could, then every 747, 767 will have VTOL capability merely if the engines can be ramped up to full throttle.

and as far as the WAT...

Quote :

"At first, I was wondering what in the hell that conversation had to do with anything. Then I lolled all over myself."

11/21/2007 2:29:10 AM

Quote :

"Please tell me how you are more qualified than a guy who has been repairing aircraft for 10 years."

I know you weren't talking to me, but since I generally disagree with your buddy:

I've been building model aircraft for almost 20 years

and I have a degree in mechanical engineering

and I've done a decent bit of civilian flying

and I'm a military aviator


and this isn't to discredit those who work on our aircraft in the least--a lot of them are EXTREMELY good at what they do...but being a mechanic and being an engineer are not the same thing (and that goes both ways...I've met engineers who've never changed oil in a car or welded anything)




There is simply no way that an airplane's landing gear are designed with such a low safety factor...ESPECIALLY the wheel bearings. The life expectance of a bearing is indeed strongly influenced by how fast you're spinning it, but there is absolutely no way any engineer would use a wheel bearing that would fail just by making a single takeoff run with the wheels rotating twice as fast as normal. The weight and cost penalty for using a more capable bearing would be pretty minimal, and if you underdesigned it so badly that it would fail in this scenario, you would have to replace it CONSTANTLY even under normal use. Furthermore, this would be a totally stupid risk to take, as it's a relatively critical component.

On top of all that, the stresses that everything in the landing gear withstands in even an average landing far exceed that of a takeoff. For that matter, the temperatures endured in landing are in another league, too (since you're on the brakes on landing rollout).


I'm pretty sure that if anything failed, it would be the tire. The chance of a wheel bearing becoming an issue is somewhere much less than "remote". I'm very, very confident that the tire would be fine, though.

Also, tires failing on aircraft is far from routine, but it isn't THAT rare of an occurance...but every single tire failure I've ever heard of occured on landing, and most were the result of heavy braking.

[Edited on November 21, 2007 at 2:59 AM. Reason : asdf]

[Edited on November 21, 2007 at 3:01 AM. Reason : asdf]

11/21/2007 2:57:23 AM

Quote :

"im not an ME or AE, but god damn, this question is simple: the plane can not take off. if it could, then every 747, 767 will have VTOL capability merely if the engines can be ramped up to full throttle. "

Are you kidding?

have you not read a single post in this thread?

11/21/2007 3:45:51 AM

the only thing that could stop this from tahinkn off is friction. i think this was said all ove the place arluyier. im drunk i anr i realize this. there is no way the friction from the treadmill and landing gear will ever overcome aonything other than a rubber band airplane. the plane will still move in respect to the ground, and therefore take off.

11/21/2007 4:44:56 AM

Quote :

"there is no way the friction from the treadmill and landing gear will ever overcome aonything other than a rubber band airplane."

lol

11/21/2007 4:49:29 AM

Quote :

" From : humandrive To : drunknloaded Received : Tuesday November 20, 2007 at 11:05 PM Subject : Plane taking off. Really how is the plane going to takeoff if the treadmill is holding it back. The correct answer is that the plane won't takeoff"

i'm guessing this n00b told me the wrong answer...

11/21/2007 4:51:07 AM

that dude is tryin to hold u back.

11/21/2007 4:52:43 AM

rtc seems like my kinda man no homo

11/21/2007 4:55:16 AM

u mean like drunk at 5 am?

11/21/2007 4:57:05 AM

Quote :

"There is simply no way that an airplane's landing gear are designed with such a low safety factor...ESPECIALLY the wheel bearings. The life expectance of a bearing is indeed strongly influenced by how fast you're spinning it, but there is absolutely no way any engineer would use a wheel bearing that would fail just by making a single takeoff run with the wheels rotating twice as fast as normal. The weight and cost penalty for using a more capable bearing would be pretty minimal, and if you underdesigned it so badly that it would fail in this scenario, you would have to replace it CONSTANTLY even under normal use. Furthermore, this would be a totally stupid risk to take, as it's a relatively critical component."

(pulling some stuff out of my ass and assuming Boeing's engineers would do the same thing I would do: )
The typical engineering limit method involves taking the worst-case scenario and adding 50% to it.
The worst case for landing or take-off would be landing at damn near full speed. And then add 50% to it. So for a 747, that would be 500mph + 250mph = 750mph, more or less. The takeoff speed of those things is significantly less than that - probably around 150-200mph. Double the speed of the wheels and give them a gradual buildup to that speed, and you're probably looking at no problem.

[Edited on November 21, 2007 at 11:10 AM. Reason : ]

11/21/2007 11:10:44 AM

11/21/2007 11:14:13 AM

Quote :

"If a plane is traveling at takeoff speed on a conveyor belt, and the belt is matching that speed in the opposite direction, can the plane take off?"

I really do not see as to how the landing gear has anything to do with the myth. It will take off. The tires will not fail. The bearings will not fail.

Hey theDuke866, is there a typical ratio of takeoff speed vs. landing speed or is the landing speed dictated by other factors?

What I am getting at is the fact that doubling the wheel speed at takeoff has little or no impact on the wheel bearings. Now, if the plane was trying to LAND on a treadmill, there might be some problems due to the treadmill speed because of the braking (kind of like landing with an extreme tailwind).

11/21/2007 11:21:36 AM

with no air going over the wings it isn't going to just leap up off the treadmill

if the treadmill stopped, of course the plane would have more than enough power to take off like a bat out of hell, since it would hit a ground speed high enough in short order to sent enough air over the wings for it to get lift.

UNLESS the power to weight ratio was high enough that it would act like a rocket does

11/21/2007 11:24:35 AM

Quote :

"UNLESS the power to weight ratio was high enough that it would act like a rocket does"

Then imagine we're dealing with an F-22. Would that change your opinion?

11/21/2007 11:27:51 AM

how did some of you people get into college?

11/21/2007 11:30:15 AM

The plane WILL have air traveling over its wings because it will be moving just like normal.

As it has been said several times in this thread, the plane propels itself with thrust against the air around it, not by its wheels.

Here is another myth for you:

If one person is traveling at the muzzel velocity of a certain rifle and shoots that said rifle in the opposite direction of motion, does the bullet leave the end of the gun barrel at double the normal speed? (this scenario should have roughly the same though process as the treadmill)

11/21/2007 11:37:53 AM

Quote :

" Also, tires failing on aircraft is far from routine, but it isn't THAT rare of an occurance...but every single tire failure I've ever heard of occured on landing, and most were the result of heavy braking"

You disappointment me Duke.

http://www.livinginperu.com/news-4976-transportation-four-tires-burst-as-airplane-attempts-take-off-pucallpa-peru
http://abclocal.go.com/kabc/story?section=local&id=3742817
http://query.nytimes.com/gst/fullpage.html?res=9507E6DF153DF93AA15754C0A9669C8B63
http://www.aircrash.org/burnelli/aw910722.htm
http://www.aircrash.org/burnelli/nyt_mex2.htm
http://www.aircrash.org/burnelli/ht951004.htm
http://www.aircrash.org/burnelli/lst183.htm

Tire blowouts on takeoff are known causes of many Aircraft crashes, including the Concorde.

[Edited on November 21, 2007 at 12:07 PM. Reason : btw, my friend is also a civilian pilot]

11/21/2007 12:03:40 PM

Quote :

"cause i think persian people are the fucking shit"

Persian people eh? Are you scared of saying Iranian?


PS - I'm glad someone asked Dr. Hall about this, he may be pretty strict and he may be very hard on his students, but knows his stuff.

11/21/2007 12:36:05 PM

Quote :

"If one person is traveling at the muzzel velocity of a certain rifle and shoots that said rifle in the opposite direction of motion, does the bullet leave the end of the gun barrel at double the normal speed? (this scenario should have roughly the same though process as the treadmill)"

its like throwing a tennis ball from the back of the truck.

11/21/2007 12:47:29 PM

Quote :

" The typical engineering limit method involves taking the worst-case scenario and adding 50% to it. The worst case for landing or take-off would be landing at damn near full speed. And then add 50% to it. So for a 747, that would be 500mph + 250mph = 750mph, more or less."

750 mph = transonic speed
transonic speed at sea level = bad
bad = air craft pretty much being destroyed

Who would design landing gear that would survive while the plane was bring ripped to shreds? Hell, the plane would prolly catch fire and explode within a minute.

11/21/2007 12:49:58 PM

There was actually a video kinda like that that someone posted a while back. They had some kind of pressurized air gun and they modified it to shoot a ball out at like 50 mph exactly. They mounted it to the back of a truck and drove it at 50 mph exactly, and when they shot the ball, sure enough it fell straight down.

11/21/2007 12:51:05 PM

mythbusters did that i believe.

i remember seeing something about that.

11/21/2007 12:53:25 PM

The video I saw wasn't Mythbusters, it was a group of Japanese (I think) guys. If not Japanese, I'm pretty sure it was Asian.

11/21/2007 1:02:32 PM

At least to me the problem seems to be in frame of reference

if the plane is stationary in space (meaning that the engine is producing enough thrust to keep the plane stationary) in responce to the force of the treadmill pulling the plane backwards

so you have a force <--------thrust and a force -------> landing gear on treadmill

then the plane is stationary in relation to the ground (not the treadmill) it wont takeoff, because there is no airflow over the wings (unless its created by the engine, but lets assume the engine is a pusher located at the back of the plane)

Now the problem here is this


the vectors probably look like <----------------------------- Thrust and -----> rolling resistence

so eventually the plane will move in relation to the ground therefore producing lift

I dont see any way you could keep a plane still on a treadmill, except if it had weak engines.

I think this would be hard to illistrate with model planes because from my limited experience some of the planes engines produce enough power to let them have extreamly short takeoff runs.

I think i know a simple way to test this experiment

Make a paper airplane, and hold it in yoru hand.

Start running on a treadmill, and let go of the plane, i think you will find it drops straight down and hits the treadmill directly under where you released it. The reason for this is it has no forward momentum.

[Edited on November 21, 2007 at 1:59 PM. Reason : dd]

11/21/2007 1:57:09 PM

i just tried that and the paper airplane glided forward about 4 feet.

11/21/2007 1:59:30 PM

yea, im sure, do it with a ball of paper, or use your airplane again standing still on solid ground (it should glide 4 feet again)

you or in the planes case the wheels are isloating the plane from movement

[Edited on November 21, 2007 at 2:00 PM. Reason : dd]

[Edited on November 21, 2007 at 2:01 PM. Reason : dd]

11/21/2007 2:00:11 PM

well a big ball of metal wouldnt take off even without the treadmill. where would you even put the engines?

and i still find it hard to believe that the bearings in the landing gear could make so much friction. theyre designed to make very little.

[Edited on November 21, 2007 at 2:03 PM. Reason : .]

11/21/2007 2:01:23 PM

think of it this way. you're running on a treadmill with a gun. You shoot the gun. Say... hypothetically that the speed of the treadmill is the same speed as the velocity of the bullet once fired. Does the Bullet Still accelerate forward ?



Of course it does. Apply this to the logic of the plane. Its almost the same logic. Think about.


The wheels and the acceleration of the plane have nothing to do with each other. It TAKES OFF.

Im not even a mechanical or aerospace engineer and I cant believe that some of you ME's here are saying that it wont take off. Jesus, you people.

11/21/2007 2:08:15 PM

Quote :

"At least to me the problem seems to be in frame of reference if the plane is stationary in space (meaning that the engine is producing enough thrust to keep the plane stationary) in responce to the force of the treadmill pulling the plane backwards "

Your assumptions of the premises is completely wrong.

The treadmill WILL NOT keep the plane from moving forward at all. So you can't say "the force of the treadmill pulling the airplane back" because the treadmill is exerting exactly 0 force* on the airplane. The frictional force of the treadmill is applying a force on the wheels, causing the wheels to turn, and the force of the engines on the plane is also causing the wheels to turn, but these forces on the wheel are acting in the same direction. So the plane moves forward as it would were it not on a treadmill, but the wheels, having 2x the force applied in the same angular direction, is just spinning 2x as fast.

[Edited on November 21, 2007 at 2:18 PM. Reason : * neglecting friction in the bearings]

11/21/2007 2:18:17 PM

http://www.youtube.com/watch?v=LwThQL-VunA

hilarious!

11/21/2007 2:24:32 PM

^THE PHYSICS DOESN'T CARE!

11/21/2007 2:50:36 PM

^a ton

you can't put the concord's tire failure down as an example of speed or friction. you take any tire and run over sharp metal, it'll fail.

11/21/2007 3:29:09 PM

Some of the rampant stupidity in this thread made me wonder if dogmatic idiots tend to collect on TWW, or if there actually was some hope for humanity elsewhere.

So I went to youtube to see if someone had finally posted a visual explanation of this problem that everyone could understand, and to my chagrin, I found more ignorance than you'd find at a WWF match in West Virginia during Black History month. Such as that kid posted above...when he went into his assertion that "the physics doesn't care where the force comes from," a little part of me died.

It completely astounds me that, even though this problem has been spelled out so simply in so many different ways, in ways that a pre-schooler could understand as well as with more advanced physical concepts, that people STILL think the plane won't take off.

This is my official "I GIVE THE FUCK UP" post. This thread and all like it depress me more than reading the comments on myspace videos. I can't say I'll never check this again, it's like a goddamn trainwreck, but I'm done trying to sway the minds of the hopelessly thickheaded.

and dear god I hope the mythbusters don't fuck this up and give the "it wont take off" people any more fodder.

11/21/2007 4:03:48 PM

Quote :

"you can't put the concord's tire failure down as an example of speed or friction. you take any tire and run over sharp metal, it'll fail."

I was using it as an example of an aircraft that crashed due to a failure in landing gear at takeoff, off which theDuke866 said he never heard of. Also, the Concorde is a damn good example about how catastrophic a tire blowout can be during take off.

11/21/2007 4:04:31 PM

oh now i understand why im angry at people who say it wont take off. its just particle physics vs general planar motion. so in that case, i should only be mad at the engineers who say it wont take off and just give everyone else the benefit of the doubt.

11/21/2007 4:24:22 PM

MORE LIKE PLANER MOTION AMIRITE

11/21/2007 4:26:44 PM

durrrrr bernoulli and um stuff

11/21/2007 4:34:19 PM

^^^^ i never said it doesn't happen

i just said i've never personally known it to happen, and that all of the incidents I'm personally aware of were on landing, often due to overzealous braking. also, it's pretty unusual for this kind of stuff to happen.

and if you're so concerned about the landing gear, do it with a fixed-gear airplane...they are typically stronger than the retractable variety.

Quote :

"transonic speed at sea level = bad bad = air craft pretty much being destroyed"

[NO]

plenty of stuff can go supersonic at sea level with no problem...you know:

bullets
missiles
land speed record cars
fighter jets

[Edited on November 21, 2007 at 4:41 PM. Reason : sadf]

11/21/2007 4:38:37 PM

He was talking about a 747 going transonic at landing....that shit is impossible.


Give me some credit Duke, I know shit can travel at supersonic speeds at sea level, but a 747...that'd be funny as hell

[Edited on November 21, 2007 at 4:47 PM. Reason : a]

11/21/2007 4:45:55 PM

oh, well yeah, i'd put money against that.

although i've heard of people taking the Prowler supersonic, and it's probably damn close to a 747 in terms of official top speed...

but the way they did it is to take it up to 40,000' or so, then put it in a near-vertical dive, passing Mach 1.0 by 20k' or so, from what I hear. The Prowler probably has a somewhat stronger airframe than the 747, though, and prob a somewhat lower Cd.

11/21/2007 4:50:27 PM

Are you stuck with the Prowler btw? Or can you eventually retrain into another craft? The Growler seems like it be a much cooler plane to be in.

11/21/2007 4:58:06 PM

Moron

My reading of the original myth is that the plane stays static in real space and begins to fly, not that it wont roll down a treadmill.

ie they are arguing that if a plan has to be moving 30 fps to takeoff, and you roll the treadmill 30fps the opposite way, and hold the plane still using only thrust from its engines, it will begin to fly.

I understand that you need air moving at 30fps over the wing surfaces to make it takeoff.

if the plane had physics wheels, meaning no rolling resistence, the treadmill woudnt matter.


As i said, the thrust from the eingines would far outdo what a treadmill would exert, however in the case above, where you treat the speed of the treadmill as the speed over the ground the plane wont takeoff.

[Edited on November 21, 2007 at 5:02 PM. Reason : dd]

11/21/2007 4:59:00 PM