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Melonie
12-10-2005, 10:23 AM
've seen these little "sail-cars". They've basically got 4 wheels, and a sail. Now, lets say I put one of those on your conveyor, with a wind maching behind it. So, I start the windmachine, which would blow the sail-car forward, right? Right. OK, so if I start the conveyor at the same time, and can keep the conveyor moving at the same speed as the wind machine, the sail-car won't go anywhere, since it's forward movement (caused by the wind-machine) is cancelled out by the conveyor. Does this make any sense

Unless the sail-car's wheels have very bad bearings, no this doesn't make any sense. When you start the wind machine, the wind blows the sail forward ... with the sail being rigidly connected to the sail-car body carries the sail-car forward as well. The wheels of the sail-car can be standing still, or going 100mph in either the forward or reverse direction, and the wheel motion will not affect the motion of the sail-car body since there is no rigid connection between the wheels and body - just free-spinning wheel bearings.

The fallacy in first impressions comes into play because we're used to thinking about cars ... which DO have a rigid connection between the car body and the wheels (as long as the transmission is in gear anyhow). Airplanes and sail-cars do not have any connection between the wheels and the body (other than friction drag of the wheel bearings, which is theoretically zero) . If the wind machine is running, or if the airplane engine is running, you've got all the force you need to push the body forward regardless of whether the wheels are spinning forward, stopped, or spinning backward. In the case of the airplane, when the engine pushes the body and therfore wings forward with sufficient velocity it will take off, even if the wheels are spinning 100mph in the reverse direction.

kitana
12-10-2005, 10:27 AM
No, it will not take off because it will not have lift from forward motion. It takes wind passing over the wings at a certain speed to create lift.

Not exactly true Sitri, since the flaps can be controlled manually, and therefor create and updraft essentially, which would cause the plane to take off.



And Gynger, please don't forget some of the newest military planes, which have jets, that move from the traditional position to facing downwards, which causes them to be able to take off like a chopper, sans runway.

The harrier was one of the first planes of that style. But they worked better for decent, and not as much on takeoff.

wakko0129
12-10-2005, 10:33 AM
If I only had a treadmill...

DancerWealth
12-10-2005, 10:33 AM
I am stubborn as well, and I do know what lift, thrust etc. is.. I have studied airplanes, and

I also am quite aware of the wheels not having ANYTHING to do with it, I am NOT stuck on that,

LOL...but see, you ARE stuck on this. This is the single fatal flaw in your confusion here. You seem to think that the plane won't have forward motion once the engines turn on because your brain is stuck on the fact that, "But the wheels are moving in the oposite direction". So, again, let me repeat myself. THE WHEELS HAVE NOTHING TO DO WITH IT. There would be no hopping, no hesitation, no anything from the airplane itself. Nothing is any different here.

Let's say you were a spectator of this entire conveyor-belt situation off on the sidelines, looking at the plane on the ground. You would see NO DIFFERENCE in how this plane would take off on this conveyor belt versus how a plane takes off from runway 3 at LAX or JFK. Obviously we need to continue to quote Newtons 3rd Law over and over again until you get it because THIS is what is causing the plane to lift off via propulsion of the jet engines, not the wheels in relationship to the ground. I'm done arguing this. When you figure out 1+1=2 and not 435, give me a buzz. :)

kitana
12-10-2005, 10:37 AM
But DW; 1+1=0 remember?! LOL

Oh and Happy Hanukkah!!!!

And no ducks!!! (wink, wink!)

Melonie
12-10-2005, 10:43 AM
f I only had a treadmill...

Perfect counter-example. You've got a treadmill which is angled upward at say 15 degrees, and the treadmill belt is running uphill. You take a child's toy airplane or toy sailcar and set it on the running treadmill belt. Does the toy airplane / sailcar go uphill with the treadmill belt ? Unless the wheel bearings are very bad, the toy airplane / sailcar is going to roll downhill no matter what the treadmill belt is doing ! The only difference with this example is that the motive force is gravity instead of air thrust.

Deogol
12-10-2005, 10:52 AM
Maybe we should debate something easier like... abortion!

wakko0129
12-10-2005, 11:07 AM
Perfect counter-example. You've got a treadmill which is angled upward at say 15 degrees, and the treadmill belt is running uphill. You take a child's toy airplane or toy sailcar and set it on the running treadmill belt. Does the toy airplane / sailcar go uphill with the treadmill belt ? Unless the wheel bearings are very bad, the toy airplane / sailcar is going to roll downhill no matter what the treadmill belt is doing ! The only difference with this example is that the motive force is gravity instead of air thrust.

Now see, that was sort of my intention... but why is the treadmill belt running in the wrong direction? Did you modify yours so you could run backwards (which is really good for you quads, by the way)?

It's starting to make sense, but I still can't see, which is why I always sucked @ physics. I could do upper level calculus in my head, but make me apply it to something IRL, and I couldn't do it, unless I could see it.

Here's what I'm picturing. I've built myself this little sail car, and set it on a treadmill. (ex1)If I just turn the treadmill on, the car will shoot off the back of the treadmill. Why? Nothing holding the car in place. (ex2)Now, if I put a fan at the end of the treadmill, and turn it on, with the treadmill of, the car shoots out the front. OK, step 3. I turn on the treadmill and fan at the same time. The treadmill attempts to shoot the car out the back of the treadmill (as in ex1), and the fan attempts to push the car forward (as in ex2)... thus ex1 and ex2 will cancel each other out, and the car will not move, or at least that's what's happening in the little video in my imagination.


Oh, and if anyone was wondering about Newton's 3rd law (like I was), since it's been referred to, but never actually stated.

Newton's 3rd law

For those that are unsure, here it is: "Forces always occur in pairs. If object A exerts a force F on object B, then object B exerts an equal and opposite force -F on object A"

or

"Every action has an equal and opposite reaction"

dlabtot
12-10-2005, 11:29 AM
LOL...but see, you ARE stuck on this.

Actually, as a disinterested observer, it looks to me like you are stuck on insisting she's saying something other than what she's actually saying.

Sitri
12-10-2005, 12:48 PM
Not exactly true Sitri, since the flaps can be controlled manually, and therefor create and updraft essentially, which would cause the plane to take off.



And Gynger, please don't forget some of the newest military planes, which have jets, that move from the traditional position to facing downwards, which causes them to be able to take off like a chopper, sans runway.

The harrier was one of the first planes of that style. But they worked better for decent, and not as much on takeoff.

Flapping up and down??? You can't change the rules and start creating helicopters or birds...
For a helicopter, it is still windspeed over the helicopter blades that create lift.
Put it another way. If a plane is on the ground and the engine is not even running. And, you have a 100 mile gust of wind hit it from the front, will it fly? The answer is yes. Lift is created.

Biz
12-10-2005, 01:06 PM
Actually, as a disinterested observer, it looks to me like you are stuck on insisting she's saying something other than what she's actually saying.

What Dancerwealth meant was that she was stuck on the wheels because she's assuming they can provide enough force to prevent the jet engines from pushing the plane forward. Typically, people who say no to this question come back to that reasoning, even unconsciously. So, yes, she's still stuck on the wheels....for very good reasons, but....at its heart, the plane question is essentially a trick question in a lot of ways. It focuses attention on the conveyor belt and the wheels intentionally.

If the conveyor belt goes --> 100 mph, and the plane goes <-- 100 mph, all that happens is that the wheels go <--- 200 mph.

Not exact numbers due to the very small friction that would be involved, but just an example.

wakko0129
12-10-2005, 01:49 PM
If the conveyor belt goes --> 100 mph, and the plane goes <-- 100 mph, all that happens is that the wheels go <--- 200 mph.

Not exact numbers due to the very small friction that would be involved, but just an example.


Now see... if you had something like that earlier, I might have gotten it. I think I understand now. Just don't ask me to try and explain it to someone else. ;D

Sitri
12-10-2005, 06:10 PM
Actually,after reviewing the question, the plane will fly. If the treadmill /runnway accelerates at the same speed as the plane this means that the tire will in fact not rotate. The plane however will accelerate just as it would for any take off. Therefore, it will have windspeed and lift.

Edited to say it doesn't matter what the speed or the treadmill is i.e. if it is 0, (the ground) it flys because the plane still moves forward. The difference between the ground and the plane speed is handled by the wheel speed. If you make the treadmill move forward, the plane will still move forward but the wheels will move more slowly. If the treadmill moves backward, the plane's wheels will move faster. In all cases however the planes acceleration is constant and it will fly.

DancerWealth
12-10-2005, 07:12 PM
Here's what I'm picturing. I've built myself this little sail car, and set it on a treadmill. (ex1)If I just turn the treadmill on, the car will shoot off the back of the treadmill. Why? Nothing holding the car in place. (ex2)Now, if I put a fan at the end of the treadmill, and turn it on, with the treadmill of, the car shoots out the front. OK, step 3. I turn on the treadmill and fan at the same time. The treadmill attempts to shoot the car out the back of the treadmill (as in ex1), and the fan attempts to push the car forward (as in ex2)... thus ex1 and ex2 will cancel each other out, and the car will not move, or at least that's what's happening in the little video in my imagination.


OKAY...You're so close to seeing the light on this issue you have no idea! You're about 9/10th right on this. The 1/10 you're missing is the crucial part of this whole equasion. Let's take your example with the sail car and the fan. What you need to picture though is that your fan has three settings, Low, Medium, and High. Now, to keep the car steady on the treamill, you have to have thrust equal to the downard force of the car on the treadmill caused by gravity. By equaling out the weight of the car plus the value of drag plus the forward thrust of the wind, makes the car stay in one place. Let's say the "Low" setting on the fan will do this.

So you take your fan and turn it from "Low" to "High". What will happen is you have now increased the level of thrust to overpower the drag and weight of the vehicle on the conveyor belt and the fan will now push the sail car off the front of the treadmill. By adding more thrust, you overpowered the other two factors keeping the car "running in place" if you will. The wheels will be spinning away like mad, but that doesn't matter because your forward thrust is overpowering the motion of friction of the treadmill plus gravity pulling the car down.

Think of it in one more example. Let's say your sail car is a toy sail car. It's small...say 1 foot wide. It can even be a regular toy car...doesn't matter. You hold on to the car with your hand and place it on a treadmill at your gym. You'll look silly, but just follow me here. Now, you are standing off to the side, holding the car, and putting it on a moving treadmill. What happens? The wheels start spinning on the car, right? Of course they do. Remember now, the car isn't power itself, your hand it holding it in place with the little wheels spinning like crazy. Now, you take your had and just move the car up the conveyor belt, right? Of course. It's common sense that you can do this!!!! According to the nay-sayers on here though, they would tell you that's impossible, but use your common sense...of COURSE you can move it up the treadmill. Well, the plane example is 100% identical in every single way. The only difference is that there is no giant hand moving the plane up the treamill. In place of a giant hand, the plane has jet engines which cause the same result. It is an external force not connected to the wheels which causes the forward motion of the plane and eventual takeoff.

dlabtot
12-10-2005, 07:18 PM
What Dancerwealth meant was that she was stuck on the wheels because she's assuming they can provide enough force to prevent the jet engines from pushing the plane forward..

Except she didn't say that, those are words you are putting in her mouth.

AinNY
12-10-2005, 07:21 PM
oh no...not here too...

the answer is yes...please dont try to argue it.

DancerWealth
12-10-2005, 07:37 PM
She didn't say that, those are words you are putting in her mouth.

She actually is saying it but not in those exact words. She's dancing around it over and over again but she actually is still making the connection that the wheels are relevant in one fashion or another. I can say that the sky isn't blue, it's cyan, but that's dancing around the issue much the same way she is doing it, but she just doesn't realize she's doing it.

toomuchinfo
12-10-2005, 07:42 PM
You are all wrong (except perhaps Kitana).

The question/problem is unanswerable as written.

An airplane...ANY airplane has two relevant metrics of speed: air speed (measured in knots) and ground speed (measured in mph). Since the problem never defines which definition applies, it is impossible to answer.

If the airspeed reached by the plane (regardless of the groundspeed) is sufficient for lift, voila...we have take off.

As an example, if the conveyer were mounted on top of some other (perhaps moving in air or on land...or sea) object (defining its "runway"), the answer would be equally skewed.

Recall how the initial version of the Space Shuttle was launched...from the top of a 747. Virually all gliders are lauched the same way.

So my answer is....

Not enough info.

Gynger
12-10-2005, 07:44 PM
Last time I checked, I spoke fucking english and I think I'm quite aware of what I meant and what I did not.

Thankyouverymuch.

And, I did say that it would, although, I brought up the question in MY OPINION is a trick question..

So DW and Biz.. you can both kiss my ass for putting words in MY mouth and failing to read what I was getting at.

DancerWealth
12-10-2005, 07:54 PM
LOL...Okay Gynger. You're right. Sorry for bringing science and common sense into helping you find the right answer. How silly of me.

Gynger
12-10-2005, 08:04 PM
Actually DW silly you for claiming that I SAID something that I DID NOT...

and common sense and science, yes, know lots about both and if you go back and re read my posts, perhaps you might GET what my point was... but, perhaps you can't because you don't believe in looking past yourself????

And since you OBVIOUSLY know so much about what I wanted to say, why don't you go right ahead and explain what I meant??? Yeah, do that..

ANd, when you have decided what it was that I wanted to say, give ME a buzz.. ;D Since you know so much about my thoughts because obviously I'm missing something that I meant to say???

You pissed me off. Don't bother even trying to explain, anyone who puts words in my mouth is not worth the time or effort in my book. You picked the wrong girl to tangle with DW... I have no respect for those who "claim" to know what someone else is thinking, feeling or saying...

This is all I have to say about that.

Biz
12-10-2005, 08:41 PM
Last time I checked, I spoke fucking english and I think I'm quite aware of what I meant and what I did not.

Thankyouverymuch.

And, I did say that it would, although, I brought up the question in MY OPINION is a trick question..

So DW and Biz.. you can both kiss my ass for putting words in MY mouth and failing to read what I was getting at.

Whoa, calm down ;). Like you said, it's basically a trick question to get people to overanalyze it in a scientific way. The reason DW and I thought you were still using the wheels as a focus for it not flying was this..


Of course this just my theory... I could be completely wrong. After all, who would fly a plane off a conveyor belt? LOL But I wonder....it seems really fucked up... Since the wheels aren't the driving force I would think the thrust would push the plane forward. But once it starts moving, the belt would quickly match the speed...and since the wheels aren't motorized, they would stop moving...and the belt would stop. Of course the thrust of the engines would start moving the plane again...turning the wheels...and moving the conveyor belt...and the process continues...

I re-read what you said after that, and yeah you did finally step away from the wheels, but you were just laughing at how it was a trick question (which it is). That is why DW and I thought you were still thinking about the wheels.

Everyone hug :)

Gynger
12-10-2005, 08:51 PM
Ice cold Biz, Just like always and always Calm.


No room for being told what I have said, what I feel or think.. my opinion is just that and had you or DW taken the time to TRULY read what my debate was, perhaps you would have gotten it.

And, I'm far from stupid. So don't even try anymore or tell me to calm down.

I have a mouth and a very stubborn head, and I'm VERY analytical.

What's done is done and its the prinicple of the matter. As stupid as this is, I find it very sad when someone fails to READ and COMPREHEND. Debate is just that. I knew it was a trick question from the beginning, but yet you had to go back and read what I said in order to realize that. That in itself tells me, that you failed to see what my point was.

I don't hug. I walk away.

Biz
12-10-2005, 09:07 PM
You're right, acting hardcore on a messageboard is so much more adult. My bad for trying to make amends ;).

Gynger
12-10-2005, 09:13 PM
Amends? You dont' even KNOW me... why the amends? Of course a whole new debate could emerge here..

Not hardcore. Blunt. There is a difference. And adult? Putting words in someone else's mouth? Last time I checked, adults read, comprehended, and oh yes, listened before stating what someone felt, said or thought? But what the hell do I know right?

Amends.. on a message board..

Now, if you were someone who I know well here, it would be different of course.. but failing to understand that stating something that I did not state is what my issue is, not whether an airplane can fly.

:)

Biz
12-10-2005, 09:19 PM
Why would I need to know you to say "Yeah, I missed what you said, my bad." I don't. I just realized what I missed and was acknowledging that you did move away from the wheels mattering. Yes, sometimes people do realize when they missed something and admit it.

Your post made it sound like we were calling you stupid, which was not the intent. I doubt DW meant that, either. After all, if DW and I thought you meant something else, that's not putting words in your mouth. Look at what I bolded that you said. We were simply thinking you were still pushing that line.

You typed it and later backed off it, we just missed the backing off it.

Again, that's not putting words into your mouth.

polecat
12-10-2005, 09:35 PM
The question/problem is unanswerable as written.

An airplane...ANY airplane has two relevant metrics of speed: air speed (measured in knots) and ground speed (measured in mph).
Bingo.. This is the correct answer. Albeit Melonie's original answer is also correct as it stipulates the wheel bearings as the missing link for ground speed/air speed.

There isn't enough information to determine if the airspeed velocity needed for enough lift to take off would be present given the conveyor belt and wheel bearing friction. If enough maximum thrust by the airplane is possible to reach the airspeed needed vs. friction of the wheel bearings being opposed by the conveyor, then the plane would indeed take off.

People don't seem to understand that the wheels on airplanes are simply free-rolling. A 747 could sit at 0 mph on a conveyor belt spinning 120mph in reverse if the wheel bearings had no friction and you slowly brought the conveyor belt up to speed. It's the prop/jets and thrust that actually propels the plane forward. This is also why float planes have no wheels- but instead two pantoons from which to glide over the water to take off. You just have to compensate for any water current with more thrust.. so a float plane could get enough airspeed to take off even in 30-40mph currents in the opposite direction... it just needs enough thrust power to overcompensate for the reversed friction of the pantoons on the water.

Whatever part of a plane is touching the ground is simply "friction" in the equation.. which is why planes even HAVE wheels- to reduce the friction and thus need less thrust to reach enough airspeed to take off. In this conveyor equation- the conveyor is simply increasing the ground friction based on the wheel bearings spinning in reverse so it's a question if the plane at hand has enough thrust to overrule this friction.

AinNY
12-10-2005, 09:47 PM
are there any engineers here?

to solve problems...you make assumptions...do research...use your brain and figure it out.

The plane takes off based on the info.

im not going to say im an engineer...but i did stay at a holiday in express last night.

that line never gets old :great:

wakko0129
12-10-2005, 11:18 PM
I get it. Took some outside discussion with some other friends to finally determine what piece I was missing to make it make sense in my head.

ricosuave312
12-10-2005, 11:25 PM
Ummmm.... did I hear you say...


.... Blunt. ...
:)

Well what ya waitin for girl? Spark it up and we'll be HIGH before these dorks have a clue whats up.

Get higher baby
Get higher baby
Get higher baby....
Don't ever come down.

--- Grand Master Flash

DancerWealth
12-11-2005, 02:05 AM
Look Gynger. I read through all your post and I truly do appologize if my intent came across as putting words in your mouth. That was not the purpose of my posts at all and it's my bad if that is how it was interpreted so forgive me if it was. I certainly never said, implied, or otherwise suggested (and certainly don't think) that you are stupid. Far from it actually. It kind of reminds me of an exercise a friend of mine does in one of his seminars.

He poses a similar type of riddle to the group of people and almost every time during the debate, one person usually winds up getting the right answer almost immediately and then every other person in the group tries to talk him out of it. It's amazing to watch and almost the same result happens every single time. It's just the way different people think and analyze information, that's all.

Lizette
12-11-2005, 02:29 AM
Gynger is correct. She explained it more than well enough.

I suspect the OP is not worded correctly if the plane is to fly. Air needs to flow above and beneath the wings in order to get off the ground. The plane essentially is standing still with respect to the earth, according to the OP, and the air is not moving. Sorry, no flight in this case. Unless it is a special plane.

Jay Zeno
12-11-2005, 11:07 AM
Debate goes to Gynger. Why?

She gets hit with Bernoulli's principle, then throws it back, fully detailed and explained, and her posting is unrefuted.

Wheels don't matter. Fine. Take an airplane and block it up or suspend it by wing tips at wheel height over a treadmill. Fire up the engines and run that treadmill at 200 miles an hour. The airplane will not lift unless there's enough airspeed to lift it.




And yes, you guys were telling her that she said something she didn't say, and it did come off to be as trying to be belittling at worse and patronizing at best, particularly after she provided the formulas.

And to say "the answer was given, and still people argue" doesn't mean squat if the answer given was incorrect.

DancerWealth
12-11-2005, 11:56 PM
Debate goes to Gynger. Why?

She gets hit with Bernoulli's principle, then throws it back, fully detailed and explained, and her posting is unrefuted.

Wheels don't matter. Fine. Take an airplane and block it up or suspend it by wing tips at wheel height over a treadmill. Fire up the engines and run that treadmill at 200 miles an hour. The airplane will not lift unless there's enough airspeed to lift it.


This is also not true...well, mostly. The ground is just acting as a means to suspend the aircraft off the ground until it gains speed and eventually the lift causes it to go in the air. Now, you are correct that if you were to suspend it over the treadmill and fire up the engines, the plane would drop like a rock. Why? Because gravity at that point is a stronger force than the airplane. The mass of the airplane is stronger than the force of the engines IN THIS PARTICULAR EXAMPLE. However, this would be totally untrue if the thrust of the engines was more powerful than the mass of the object. A great case for this is a bottle rocket.

Let's say you like a bottle rocket and drop it in mid-air. The fuse catches and off the bottle rocket goes. It doesn't need to have contact with the ground first...it can take-off in mid air while falling to the ground. The reason is because the thrust from the rocket is stronger than the mass of the rocket. An airplane is different because it has so much mass, the engines need a slower pace to accellerate the plane in order to create lift and force the plane in the air. So, in your example, yes, the plane would drop straight down and hit the treadmill. Then guess what would happen? It would now have something to support it underneath and the plane would just cruise down the treadmill runway and take off just like it would on a stationary concrete runway.

Jay Zeno
12-12-2005, 07:17 AM
Thanks, DW. I appreciate the response.

Two things. Groundspeed doesn't matter. Airspeed does. The thrust generated for an airplane is not solely to overcome gravity - it is to build airspeed for the wings. (Thrust is generated to overcome gravity for a rocket, since rockets don't depend on aerodynamic lift. Essentially, rockets are self-propelled big bullets.)

So the ground doesn't matter. If you steer, say, a small private prop plane into a headwind, standing stock still on the runway (decent low temperature, decent air density, light load) and get hit with a 100-mph headwind gust, you can lift and fly into it. In fact, you might anyway without wanting to - it's certainly happened enough times in real-life situations. So it'll look like a copter or a VTOL, but it can happen. Groundspeed=0, but airspeed=100. Flight happens.

That's why planes take off and land into a headwind. They don't have to go as fast with the added lift. When you can engage the ground at a lower speed, it makes it safer for the inhabitants and less stressful on the plane.

The second thing (really, the same, just a different aspect of the explanation) is that although thrust and gravity are being correctly identified in this treadmill example, there are four components to aerodynamics: Thrust, gravity (weight), drag, and lift. You need thrust and lift to overcome gravity and drag. In this case, an explanation of the lift is missing to get the airplane to fly.

(I'm getting the impression that the arguments are more a relativity argument - that the plane is fast relative to the treadmill; therefore, it'll fly, just like if you run on top of a moving train - your running speed is not relative to the ground, but relative to the train. However, the relativity that matters to flight ability is not the ground, but the air.)

Just get the proper knots of airspeed under the wing while the treadmill is going, and that plane will fly. Just explain to me how the air hits the wings fast enough over the treadmill to provide lift, and I'll believe it.

But once it rises out of whatever airstream you're applying to it, there better be enough ambient airspeed to keep it flying. (Interesting - kind of a ground effect in reverse.) If there is quiet air 25 feet above the treadmill, and the plane lifts straight up into that quiet air, it'll lose flight profile and fly like a brick when it hits that still air.

Bunny
12-15-2005, 06:46 PM
I asked a guy today who has testified as an expert witness in many aviation cases. In fact I think it's what he used to do for a living. Anyway, he said the plane won't take off because there is no airflow over the wings.

DancerWealth
12-16-2005, 08:55 AM
First of all, all the nay-sayers here are still equating force with speed. They are two completely different things. Your Friend, Bunny, is answering by a point of air speed and he is assuming there will be none to cause the lift of the aircraft. The challenge is, there will be just as much airspeed as there would be if the plane was taking off from a solid, concrete runway. The airspeed will occur becuase (as has been mentioned numerous times in this thread) the PHYSICS involved here is the issue with the wheels. Everyone is hung-up on the fact that because the wheels are spinning like crazy because of the treadmill, the plane would have no forward thrust from the engines. On that premise, they are wrong. The thrust occurs off the frame of the plane, not the propulsion of the wheels like on a car. Hence. the flaw in thinking that the plane will have no forward motion, no airspeed, and no takeoff. I gave an analogy of a toy on a treadmil earlier (which it seems as though nobody read or countered yet) and so I'll repost it again. This time, not in my words, but in the words of two users who got this question (and answered correctly might I add) on a pilot's forum. One of them even (independently) gave my exact toy analogy...

Alright....I'm standing next to the end of a treadmil with a toy airplane in my hand. I take the airplane and set it on the treadmil. The treadmill is off, and I push the airplane a foot forward. I turn the treadmill on and it'll slide back because the friction of the wheels acts as drag on the airplane and with my hand removed, there is no forward thrust to counter it. Now....the plane reaches the beginning of the treadmill again and I put my hand behind it and stop it from moving backward any more. Now, the drag from the wheels is being countered by the thrust from my hand. My hand is producing a force on the airplane that is exactly equal and opposite to the drag force due to the wheels. Now...while holding the airplane like that....the treadmill is turned up to full speed. There will be a negliable increase in the drag from the wheels, matched perfectly by an increase in the force from my hand. The airplane is still stationary. Now....I increase the force applied by my hand (basically I move my hand forward). Is there any doubt that when I move my hand forward relative to the beginning of the treadmill that I will push the airplane forward as well? Let me be perfectly clear, I moved my hand with the airplane in front of it, from the beginning of the treadmill to some distance down the length of the machine. Regardless of how fast the treadmill is moving, I could still push the airplane down the length of the treadmill. The airplane and my hand will have motion relative to the base of the treadmill, and faster motion relative to the conveyor belt part. The treadmill cannot produce a speed that can counter the force of my hand moving forward because the movement of the conveyor has no bearing at all on my hand. In the real airplane example, my hand is replaced by the thrust produced by the engine, which again is not affected by the movement of the conveyor/runway.

---------

I am going to try and make some observations that will help us visualize what is happening.

First example
1. First rule, the ground is stationary. 0 mph, it will not move.
2. The wind is calm. There is no wind.
3. Thrust from the prop is pulling the aircraft forward at 80 mph relative to the ground.
--a. The wing has a relative wind of 80 mph.
--b. The tires are spinning at 80 mph.

*In this example, the aircraft will fly.

Second example
1. First rule, the ground is stationary. 0 mph, it will not move.
2. The wind is calm. There is no wind.
3. Thrust from the prop is pulling the aircraft forward at 80 mph relative to the ground.
4. Put a conveyer belt going the opposite direction at 80 mph relative to the ground.
--a. The aircraft is still moving forward at 80 mph relative to the ground.
--b. The conveyer is moving the opposite direction at 80 mph relative to the ground.
--c. So aircraft speed and conveyer belt speed are equal but in the opposite direction.
--d. The aircraft and the conveyer are moving at 160 mph relative to each other
--e. This means the tires are spinning at 160 mph.


◄← Aircraft moving at 80 mph through the air.
● Tires at 160 mph on the conveyer.
▬ Ground is stationary at 0 mph.
►→ Conveyer moving at 80 mph relative to the ground and opposite the aircraft.

*The ground is stationary.
*The wind is calm.
*The aircraft is moving forward through the air at 80 mph.
*There is 80 mph airflow over the wings.
*The conveyer is moving the opposite direction of the aircraft at 80 mph.
*The conveyer is moving at 80 mph relative to the ground.
*The aircraft is moving at 160 mph relative to the conveyer.
*The conveyer is moving at 160 mph relative to the aircraft.
*The aircraft tires are spinning at 160 mph on the conveyer.
*The aircraft is still moving forward through the air at 80 mph.
*The airflow over the wings is still 80 mph.

We have satisfied the requirement that the conveyer belt is moving at the same speed of the aircraft but in the opposite direction.

Conclusion
[/b]The aircraft will fly.[/b]

DancerWealth
12-16-2005, 09:11 AM
Here's another few quotes I pulled off of some other aircraft forums:

The conveyor belt speed is set to cancel the speed of the wheels turning, not the forward movement of the aircraft. The thrust of the engines will push the aircraft forward, and the conveyor belt will rotate the wheels in the opposite direction. The turning of the wheels will not excert any force on the aircraft, so it will continue to move forward.

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Ahh, yes. Physics. I was actually a physics major for 3 years. There is no opposite force being exerted on the aircraft.

There is no force counteracting the thrust of the aircraft, therefore, the thrust would propel the aircraft forward.

At first I thought of it your way Richard, but after thinking about it for a while and the physics side of things it will be possible for the aircraft to build speed.

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I don't think you're seeing the full picture. You have to look at the overall scheme of things.

There is no force being exerted backwards on the plane. None at all. The wheels are not physically connected to the engine, so the speed they move at is irrelevent to the speed of the actual aircraft itself.

Because the wind is calm, we are assuming, the thrust from the engines will work against it, therefore propelling the plane forward in relation to the air. The wheel speed and conveyor speed can totally be neglected. As long as that air is calm, the thrust will propel the plane forward, which will produce airflow over the wings.

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Ok, after all my arguing AGAINST the aircraft taking off, Im going to swallow my pride and change my stance Smile

The aircraft will take off.

Why?

Start the conveyor moving without the engines on. In a perfect world (since we dont know anything other than the question, we have to assume perfection), the plane will sit still because the wheels will freewheel backward.

The engines add an additional thrust component to the equation, which is not affected by the conveyor. Thus forward motion can be generated and thus also lift.

Apologies

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The convayor belt isnt moving the air! Its not even moving the aircraft, its simply spinning the wheels.

It doesnt matter how fast the wheels spin, the aircraft will still move forward as the engines push against the air and generate thrust.

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Surely the conveyor belt wouldn't really make a difference. The aircraft is moved forward by the engines, not the wheels as in the case of a car. Therefore, as the wheels are free spinning, the thrust of the engines should still move the aircraft forwards, even though the wheels themselves may be spinning at twice the speed the aircraft is physically moving.

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I agree entirely, the spinning of the wheels is irrelevent, it does not excert any physical force on the aircraft, they mearly spin. The thrust of the engines will produce forward motion which will not be apposed, so the aircraft will move forward and take off.

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Jay Zeno
12-16-2005, 09:16 AM
Everyone is hung-up on the fact that because the wheels are spinning like crazy because of the treadmill, the plane would have no forward thrust from the engines. DW, bud, I think you're the one who's hung up on the wheels and contact with the treadmill. I specifically said in my example, fine, let's not use the wheels. So ..... let's use frictionless wheels. Or use rotating cement blocks. Or tracked ball bearings. Or magnetic levitation.

I don't care a bit if wheels are spinning or not. Yes, if you match forward speed and backward speed, and then add more forward speed, the object will go forward at that speed. That's movement relative to the ground. But if you don't have proper speed relative to the air, the plane won't fly.

If we don't want Bernoulli's principles to apply, fine. If we don't want to talk about the aerodynamic components of thrust-drag-weight-lift, fine.

Just tell me how being on a treadmill generates 80 (or 100 or 200) knots of airspeed on the wing. That's all. Then it'll fly, whether the treadmill is at 1 mph or 1,000 mph, with magic frictionless wheels or locked brakes. Just get me that airspeed hitting the wing, and I'm a believer. Without that airspeed, the plane's a brick.