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#617100 - 28/12/06 07:33 AM Re: Know that if you do not post in this thread today the human race may cease to exist
TravelingFool Offline
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Registered: 17/10/00
Posts: 6013
Loc: Prior Lake, MN
Quote:
Originally posted by RiNkY:
The engines may indeed push the plane, but if the plane is on a conveyor belt moving in the opposite direction at the same speed that the engines are pushing the plane, it's still not gonna go anywhere. Unless it's a harrier jump jet - British Engineering can overcome this scenario smile

Me and my friend at work have been arguing about one of these kinda things for a few weeks now.

He says that if you fire a bullet from a gun perfectly level with the ground, then drop a bullet from the same height at the same time, both will hit the ground at the same time, as the Gravitational pull on both bullets is the same.

I call Bullshit as the aerodynamic flow over the bullet and the velocity of the bullet has to play into it somewhere.
:rolleyes:

You're one of those trailer park dudes at the racetrack who wants to know why the jet powered drag racers don't do burn-outs, huh!?
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kjw &
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#617101 - 28/12/06 07:40 AM Re: Know that if you do not post in this thread today the human race may cease to exist
TJ Offline
Member
*****

Registered: 08/03/01
Posts: 7756
Loc: Lawrenceville, NJ, USA
Quote:
Originally posted by NY Madman:
[QB]The plane would not fly.

Some of the others are correct when stating that a plane in that situation could not attain the lift required to fly.

The lift is from air speed, which has zero to do with ground speed or tire speed.

If I left the landing gear up, so the plane is resting on its belly...and applied enough thrust, it would push the plane along the ground skittering and grinding, etc...until it had enough air speed to fly.

So its moving relative to the air, creating lift as the the air moves over the wings...the tires just hold it up.

Remember the old Road Runner Cartoons where he'd strap his ACME rocket pack on, stand on roller skates, fire it up, and chase the Road Runner?

Imagine he's on a tread mill...the rocket pack will push him forward throught the air EXACTLY the same way, even if the tread mill is making the roller skate WHEELS go faster...

...but, the roller skates themselves, and the attached Wiley Coyote, will be propelled foward, through the air, off the cliff, just like always.

laugh

Its the air speed that creates the lift...the wheel speed is not relevent.

_______

Oh, and as for the ballistics question...its a fairly well known principle that the time for the bullet, or cannon shell, etc...to hit the ground, is the same for one fired horizontally or dropped from the same starting point...to the same end elevation.

Obviously, a more powerful blast will make the END POINT further away..but, it will be going faster, and need less time to cover the distance as a less powerful blast, etc.

laugh
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- TJ

2001 Xterra '03 VG33, SE 5 spd, 305/70/16's, Revolvers, UBSkidderz, Doubled AAL's, 3"SL/2"BL, winch/bumpers, skids, sliders, OBA, Snorkel, pine stripes....

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#617102 - 28/12/06 07:49 AM Re: Know that if you do not post in this thread today the human race may cease to exist
NY Madman Offline
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Registered: 09/05/02
Posts: 5232
Loc: Florida
Quote:
Originally posted by TJ:

The lift is from air speed, which has zero to do with ground speed or tire speed.
You're wrong.

In order to create lift, the plane requires air to move over the wing. The wing deflects the air downward and creates lift.

In the hypothetical situation posted here, the plane virtually stands still and there is no air moving over the wing to create the sufficient force for lift.

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#617103 - 28/12/06 07:50 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


Blah blah blah. wink
Nowhere in that post does it say the 'engine' is running, just wheels spinning....

MGJ

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#617104 - 28/12/06 08:08 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


I agree with the ballistics thing if both bullets are fired / dropped in an airless vacuum, but only then would I believe that the friction of the bullet against the air wouldn't make one of the bullets land a fraction of a second sooner than the other.

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#617105 - 28/12/06 08:11 AM Re: Know that if you do not post in this thread today the human race may cease to exist
TravelingFool Offline
Member

Registered: 17/10/00
Posts: 6013
Loc: Prior Lake, MN
Quote:
Originally posted by NY Madman:
Quote:
Originally posted by TJ:

[b]The lift is from air speed, which has zero to do with ground speed or tire speed.
You're wrong.

In order to create lift, the plane requires air to move over the wing. The wing deflects the air downward and creates lift.

In the hypothetical situation posted here, the plane virtually stands still and there is no air moving over the wing to create the sufficient force for lift.[/b]
The plane gets its forward momentum from the engine thrust, NOT from the wheel spin. The plane could be sitting on a pure piece of glacial ice and it would easily take off.
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kjw &
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#617106 - 28/12/06 08:15 AM Re: Know that if you do not post in this thread today the human race may cease to exist
TravelingFool Offline
Member

Registered: 17/10/00
Posts: 6013
Loc: Prior Lake, MN
From "The Straight Dope.com"

The implicit assumption is that if the conveyor belt's speed backward exactly counteracts the airplane's "speed" (whatever that means) forward, the plane remains stationary relative to the earth and, more importantly, to the air. (We assume the winds are calm.) With no wind moving past its wings, the plane generates no lift and can't take off.

But the assumption is false. While the conveyor does exert some modest backward force on the plane, that force is easily overcome by the thrust of the engines pulling the plane ahead. The plane moves forward at roughly its usual speed relative to the ground and air, generates lift, and takes off. Many people have a hard time grasping this (although it can be easily demonstrated in the lab), but eventually they do, smack their foreheads, and move on. We'll call this Basic Realization #1.

Message-board discussions of this question tend to feature a lot of posters who haven't yet arrived at BR #1 talking right past those who have, insisting more and more loudly that the plane won't take off. Then there's a whole other breed of disputants who, whether or not they've cracked the riddle as originally posed, prefer to reframe it by proposing progressively more esoteric assumptions, refinements, analogies, etc. Often they arrive at a separate question entirely: Is there a way to set up the conveyor so that it overcomes the thrust of the engines and the plane remains stationary and doesn't take off?

The answer is yes. Understanding why is Basic Realization #2.

The conveyor doesn't exert much backward force on the plane, but it does exert some. Everyone intuitively understands this. To return to the analogy in my original column, if you're standing on a treadmill wearing rollerblades while holding a rope attached to the wall in front of you, and the treadmill is switched on, your feet will initially be tugged backwards. Partly this is due to friction in the rollerblade wheel bearings, but partly--this is key--it's because the treadmill is accelerating the rollerblade wheels and in the process imparting some angular (rotary) but some linear (backward) momentum to them. You experience the latter as backward force. Eventually the treadmill reaches a constant speed and the rollerblade wheels cease to accelerate. At this point you can easily haul in the rope and pull yourself forward.
_________________________
kjw &
the PNUTMNM

The liver is evil, and must be punished...

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#617107 - 28/12/06 08:47 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


lol....OK....First off, the bullet dropped from a height will hit the ground FAR before one shot out of a gun from the same height because of the extra force exerted on the bullet being shot.

Secondly, the 3rd child's name is Timmy.

Thirdly:
Quote:
First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.
laugh [Wave]

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#617108 - 28/12/06 08:54 AM Re: Know that if you do not post in this thread today the human race may cease to exist
TravelingFool Offline
Member

Registered: 17/10/00
Posts: 6013
Loc: Prior Lake, MN
Quote:
Originally posted by Hawk:
lol....OK....First off, the bullet dropped from a height will hit the ground FAR before one shot out of a gun from the same height because of the extra force exerted on the bullet being shot.

Gravity works the same on all objects. IF the gun is fired while the barrel is parallel to the ground, both the bullet fired from the gun AND the bullet dropped from the same height WILL ABSOLUTELY hit the ground at the same time.

Without little "bullet wings," the fired bullet will NOT rise or gain lift. its forward thrust will propell it FAR FURTHER from the gun, but it will hit the ground when the dropped bullet does. This is one of the first things they teach you in a ballistics course and its simple 7th grade physics.
_________________________
kjw &
the PNUTMNM

The liver is evil, and must be punished...

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#617109 - 28/12/06 08:58 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


Quote:
Originally posted by NY Madman:
Quote:
Originally posted by TJ:

[b]The lift is from air speed, which has zero to do with ground speed or tire speed.
You're wrong.

In order to create lift, the plane requires air to move over the wing. The wing deflects the air downward and creates lift.

In the hypothetical situation posted here, the plane virtually stands still and there is no air moving over the wing to create the sufficient force for lift.[/b]
but as the jets push it down the runway, you adjust the leading and trailing egde flaps to obtain the lift. the air doesnt get udner the wing and blow upwards into the wing to create the lift, its the airstream over and more so under the wing. or it could be like osprey(sp?) with the vertical take off capability. take a paper air plane, trim some flaps at the rear of the wings. tilt the tabs up and throw it. and then shove it up your ass [Finger] (ALR material....no harm ment)

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#617110 - 28/12/06 09:35 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


Quote:
Originally posted by zuzuman03:
Quote:
Originally posted by NY Madman:
[b]
Quote:
Originally posted by TJ:

[b]The lift is from air speed, which has zero to do with ground speed or tire speed.
You're wrong.

In order to create lift, the plane requires air to move over the wing. The wing deflects the air downward and creates lift.

In the hypothetical situation posted here, the plane virtually stands still and there is no air moving over the wing to create the sufficient force for lift.[/b]
but as the jets push it down the runway, you adjust the leading and trailing egde flaps to obtain the lift. the air doesnt get udner the wing and blow upwards into the wing to create the lift, its the airstream over and more so under the wing. or it could be like osprey(sp?) with the vertical take off capability. take a paper air plane, trim some flaps at the rear of the wings. tilt the tabs up and throw it. and then shove it up your ass [Finger] (ALR material....no harm ment)[/b]
Who gives a damn about flaps. There's no fucking airflow going over the flaps (ALR), flaps only work when there is air traveling over them. Unless there happens to be a 200mph wind or somethin - the plane ain't going anywhere. The treadmill is pushing it backwards at the same speed the engines are pushing it forwards.

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#617111 - 28/12/06 09:42 AM Re: Know that if you do not post in this thread today the human race may cease to exist
NY Madman Offline
Member
*

Registered: 09/05/02
Posts: 5232
Loc: Florida
Quote:
Originally posted by zuzuman03:

but as the jets push it down the runway, you adjust the leading and trailing egde flaps to obtain the lift. the air doesnt get udner the wing and blow upwards into the wing to create the lift, its the airstream over and more so under the wing. or it could be like osprey(sp?) with the vertical take off capability. take a paper air plane, trim some flaps at the rear of the wings.
You are forgetting one major thing.... the plane in this hypothetical situation IS NOT moving down any runway. To an outside observer it is staying in the same place, like a stationary object.

The argument seems to be do the engines at maximum thrust have enough effect on the air behind the plane to achieve lift.

It just might lift off the ground, but it definitely isn't any plane I would want to be sitting on. [Freak]

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#617112 - 28/12/06 09:45 AM Re: Know that if you do not post in this thread today the human race may cease to exist
XPLORx4 Offline
Member

Registered: 23/03/01
Posts: 1906
Loc: San Jose, CA
Quote:
Originally posted by RiNkY:
Unless there happens to be a 200mph wind or somethin - the plane ain't going anywhere. The treadmill is pushing it backwards at the same speed the engines are pushing it forwards.
That 200mph of wind is being created behind the engines, pushing against the air, which passes over/under the wings and creates lift, so the plane takes off. It doesn't matter how fast the wheels are rolling.
_________________________
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#617113 - 28/12/06 09:54 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


Quote:
A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.
Now, you on the rollerblades is the airplane, the treadmill is the conveyor belt, and you pulling the rope are the turbines. You will still move forward on the treadmill regardless of how fast the treadmill is going because of the external force moving you forward.

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#617114 - 28/12/06 09:57 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


what about with a space shuttle? it sits stationary until they light those thrusters. good conditions for flight is no wind correct? have you ever stood or walked behind(at a safe distance) a prop plane when they had the engines running? i have. they would sit out there and feather them in front of the hanger to test the systems. when you go to walk out of one hanger to the other, you have to walk right through the air stream. they sit still, but they sure do push alot of air. and with a twin turbo prop for example, the engines are attatched in front of the wings.
so when the props turn, the airstream travels back over the airfoils and could potentionaly create enough lift(not sitting still;rolling) to take off. when you stand 100yds or so behind one with the props turning full speed, you can really feel it. and even more so with a jet engine. what about an f-16 taking off an aircraft carrier? they start off standing still and they dont have that much room to get up. same idea, except the with conveyor/tread mill bullshit.

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#617115 - 28/12/06 10:07 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


OK. I don't get it... It's pretty easy to see that the plane would fly regardless.

Whoever says that the plane WOULD NOT fly is thinking of a vehicles because the speed depends on how fast the tires can spin and the tires accelerate the vehicle by coming in contact with the ground.

This is not the case with the plane. It all depends on air speed.

Think about it this way:

You take a bottle rocket. Attach some wheels to it and let it ride on the treadmill. It's attached with a rope to an end of the treadmill so it wouldn't fall of. This way it keeps riding down the belt while it's steady in proportion to the surroundings. You lite that bottle rocket and what's gonna happen? I'll fly forward no matter how fast the treadmill spins.

The engines on the plane push off against the air and not against the ground. The tires are simply supporting the weight and reducing the rolling resistance.

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#617116 - 28/12/06 10:16 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


exactly.

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#617117 - 28/12/06 11:25 AM Re: Know that if you do not post in this thread today the human race may cease to exist
GrayHam Offline
Member

Registered: 17/04/01
Posts: 8849
Quote:
Originally posted by NY Madman:
You're wrong.

In order to create lift, the plane requires air to move over the wing. The wing deflects the air downward and creates lift.
Lift is not created by air being directed downward towards the ground.

An airfoil, or wing, has a greater surface area on the top side of the wing as opposed to the underside.

As air rushes over the airfoil, it must move faster over the wing, as it has more distance to cover, versus the underside of the wing.

This increased velocity of airflow over the top of the wing results in a decrease in pressure on the top of the wing.
This decreased pressure generates the lift.

Greater angles of attack can therefore create greater distance differences on the top versus bottom surfaces of a wing, resulting in greater lift.
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#617118 - 28/12/06 11:34 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Smith Offline
Member

Registered: 19/07/01
Posts: 2032
Loc: Winston-Salem, NC
On a sidenote,ear wax does not taste even remotely like caramel.

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#617119 - 28/12/06 11:43 AM Re: Know that if you do not post in this thread today the human race may cease to exist
MidnightX Offline
Member
*****

Registered: 08/03/01
Posts: 3745
Loc: Jacksonville, Florida
Quote:
Originally posted by Smith:
On a sidenote,ear wax does not taste even remotely like caramel.
Hmm. Interesting!

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#617120 - 28/12/06 11:45 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


well, assuming you ate some, what DOES it taste like? and no im not gona taste it.

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#617121 - 28/12/06 11:48 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


The plane would fly. Assuming near zero friction between the plane and the wheel bearings would leave a force acting on the plane of zero. The thrust force would therefor overcome the near zero friction force, moving the plane forward to take off speed and then up and away. As someone already stated the effect on the plane would be the same if it tried to take off on a sheet of perfect ice.

The two bullets will hit the ground at the same moment. Some factors for curvature of the earth and imperfect bullet design would keep it from hitting exactly, but all things considered it would be very, very close. Wind resitance would have no effect since the force of the friction from the wind would be oriented horizontally, not vertically; this leaves only the force of gravity acting vertically, which would be exactly the same for both bullets.

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#617122 - 28/12/06 11:56 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


You dumbasses who think a plane that is perfectly stationary to the airflow because all of the forward thrust is being canceled out by a rearward treadmill need to wake up.

This is nothing to do with wheel bearings or glaciers. Read the original question.

The plane is thrusting forward - Yes, if it weren't for the fact that the treadmill is canceling out any actual flow against the air it would take off. Planes with air flowing over their wings take off. But planes that are perfectly stationary to the airflow don't.

Are you trying to tell me that when you work out on a treadmill you actually go somewhere? Because it has nothing to do with your feet actually touching the treadmill!!!!!! eek

Stick a forward thrusting object on something that equally matches its forward thrust with rearward motion / thrust and it won't go anywhere.

Here's another example for you dumbasses who think it will fly to think about - which actually involved movement of air.

Stick the same plane on a regular runway with a constant 200mph tail wind. Accelerate the plane so it is going 200mph (Ground Speed). Will the plane fly? No - because in essence it's air speed is 0mph. The 200mph tail wind cancels out the 200mph forward speed. Airflow over the all important wings is 0mph.

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#617123 - 28/12/06 11:57 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Anonymous
Unregistered


Maybe here is a more real world explanation:

Imagine Plane on a river, traveling upstream, the engines have to overcome the speed of the relative resistance of the pontoons to the river flow. The only thing to stop it from taking off is the friction of the water acting on the pontoons. The plane would still be able to take off as it's airspeed increases the wings will create more lift, thereby reducing drag and friction until the plane has taken off completely.

Timmah!

And in a vacuum, without influence of air or objects the bullet fired from a gun with a trajectory parallel to the ground and one dropped in the same instance would hit the ground at the same time. Newton's law of Universal Gravitation. Both bullets will still be falling at the measured gravitational acceleration at the Earth's surface, about 980 cm/second/second. Linky

Interesting reading, kinda forgot all that stuff.

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#617124 - 28/12/06 11:58 AM Re: Know that if you do not post in this thread today the human race may cease to exist
Smith Offline
Member

Registered: 19/07/01
Posts: 2032
Loc: Winston-Salem, NC
Quote:
Originally posted by RiNkY:


Stick a forward thrusting object on something that equally matches its forward thrust with rearward motion / thrust and it won't go anywhere.

My wife would disagree.

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