Can the Plane Take-Off?

[greencactus3]

PS... although everyone is repeating the same things over and over, see how this can expand to an almost 1000 post thread? less than 24 hours and we've reached 36. and the other forum has flamers and postwhores and idiots, and well. almost 40k people

[sciguy125]

Originally Posted by greencactus3

correct. which is why you can say either side is correct. if you ignore conveyorbelt limits

I don't understand your basis for that. Let's go back to Elgin's equations that I've been working with:

Fy = Fl + (-Fg) - Fl is lift force, Fg is force of gravity
Fx = Ft + (-Fc) - Ft is force of thrust (engines), Fc is force of conveyor

and some I'm adding:
F = ma <- solve for a
a = F/m <- integrate with respect to t
V = (F*t)/m
Vx = [(Ft - Fc)*t]/m
Fc = umg = uFg - u is coefficient of friction

Vx is a function of Fx. And we know Fy is a function of Vx. So, the only thing we really care about is Vx. If Vx > 0, we'll have Fl > 0. If Fl > Fg, Fy > 0.

So, how do we get Vx > 0? Well, Fx has to be >0. This is true as long as Ft > Fc. We know that u << 1. With this, we'd only have to provide some Ft that is some small fraction of Fg.

As you can see, the speed of the conveyor never comes into play.

[vivek16]

Originally Posted by KenWittlief

Ok, I gotta fess up here and admit that when I took physics 101 in college I had to first unlearn 'roadrunner physics'

our thinking gets so ingrained to the things we experience personally, like driving a car or running on a treadmill, that we have a hard time doing thought experiments on things we are not familiar with (like flying a jet plane).

The question here makes you think the plane will not move at all, because our minds jump to something familiar - like a car on a dyno, or a person running on a treadmill.

Go back to the initial conditions: the plane is at rest, the runway is not moving.

The pilot hits the throttle and the plane has 30,000 pounds of thrust on its airframe. How on earth is the runway going to apply 30,000 pounds of thrust in the opposite direction to STOP the plane from moving? Through 3 wheels with greased roller bearings?

Lets say the plane gets up to 10mph like I said before. The runway is only allowed to move backwards at 10mph - to match the forward speed of the plane. There is no way the wheel bearings are going to have 30,000 pounds of friction while spinning at 20mph net speed! the wheels will have a few pounds of friction at the most, and the plane will continue to accelerate

and take off, pretty much like normal.

Picture the tablecloth trick, you yank the tablecloth out from under the dishes, and there is not enough force to pull them off the table.

'yanking the runway' out from under the plane is not going to have enough friction to overcome 30,000 pounds of thrust from the jet engines.

ok, before I read ken's post, I assumed that if they went canceling speeds then (like running on a treadmill) then the plane would stand still and Bernoulli's principle would have no effect. Now I see that the run way moving would have no effect whatsoever on the plane except making its wheels spin faster.

Cool, this would make for a very fun team debate

thanks, Vivek

[greencactus3]

Originally Posted by sanddrag

Wow, that other forum must not be full of really bright people. Easy question. As mentioned previously, a plane's flight is based on it's relation to the air, not to the ground. Lift comes from engines pushing on the air, not from wheels pushing on the runway.

true. but some are quite smart. and well...most are the normal "ur stupid" flaming.... but you gotta have an open mind to understand why both opinions are valid. everyone agrees that the plane must be moving forwards relative to the air to fly... but the plane isnt moving forwards relative to the air.. because the wheels are mounted to bearings which are on the axles... and to my knowledge there are no true frictionless bearings... and because of that friction, the conveyorbelt can keep the airplane from moving forwards relative to the air completely. ok. now shake your brains around and give them a nice stretch... and think slowly... at first the plane is sitting still. now the pilot turns on the enignes. afterburners are fine too. just no vtol technology allowed. so the plane should move forwards... but no, the plane CANT move forwards because the conveyorbelt tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). ... so the plane does not move. because if the plane moves 1mm forwards that would mean the conveyorbelt is not turning the belt at EXACTLY the same speed as the plane.(but in the opposite direction)..
so simply put, the plane DOES NOT MOVE. but in which case where does the air being pushed by the plane's engines go? backwards. but since air molecules have inertia too, the plane should move forwards now. but it doesnt.. so in this problem, because of an improbable conveyorbelt control system, some parts of physics must be ignored... or looked into detail... depending on how you look at it.

[KenWittlief]

Originally Posted by greencactus3

...so the plane should move forwards... but no, the plane CANT move forwards because the conveyorbelt tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). ...

ok, now you got me laughing :^)

if the plane cannot move then the conveyor belt cannot move either. When the planes 'speed' is zero the conveyor belt speed is also zero !

[greencactus3]

Originally Posted by KenWittlief

ok, now you got me laughing :^)

if the plane cannot move then the conveyor belt cannot move either. When the planes 'speed' is zero the conveyor belt speed is also zero !

exactly... which is why this problem is so confusing.. and fun!

[Elgin Clock]

Originally Posted by KenWittlief

ok, now you got me laughing :^)

if the plane cannot move then the conveyor belt cannot move either. When the planes 'speed' is zero the conveyor belt speed is also zero !

Define "move".

The wheels may counteract and spin in the opposite axial direction and thus they will "move" but my thoughts are since they cancel each others linear "move"ment out, then technically this is a trick question, cause the plane is not "moving" as most people think of as moving.

When I think of a car "moving" it's displacement is constantly changing. Down the highway. But who's to say that same car on my dyno is not "moving". It's not moving linearly, but the wheels are moving axially.

Originally Posted by greencactus3

static and kinetic friction

Thanks. I should of know that. lol



Oh, and does anyone foresee an episode of mythbusters coming on about this????

[KenWittlief]

Originally Posted by Elgin Clock

Define "move".

the engines on all types of planes have no connection or effect on the wheels. The engines do not rotate the wheels. Therefore, on an aircraft, the only thing that can 'move' is the aircraft relative to the air

in fact, when you are flying it is impossible to measure your ground speed, unless you know the wind speed and direction. Aircraft do not have speedometers attached to their wheels :^)

[Ryan F.]

Originally Posted by KenWittlief

in fact, when you are flying it is impossible to measure your ground speed, unless you know the wind speed and direction. Aircraft do not have speedometers attached to their wheels :^)

In jumps a beautiful aircraft device we like to call theDME .

Watches your change in distance from a VOR point, and gives you your airspeed. It only works when the pilot is flying directly too or from the VOR (though that is fairly common).

[KenWittlief]

I guess I should have been more specific. In an aircraft its impossible to measure your ground speed directly. GPS also reports ground speed, but that is measured relative to satellites in orbit, with known positions and velocities - its not measured relative to the ground. If the satellites fail (or if your VOR points on the ground fail) then the aircraft cannot know its ground speed.

There is no instrument you can put in a aircraft that can directly measure the speed vector of the groud under the plane by itself.

By contrast, a car measure ground speed directly, by the rotation of its wheels.

[dlavery]

Originally Posted by KenWittlief

There is no instrument you can put in a aircraft that can directly measure the speed vector of the groud under the plane by itself.

By contrast, a car measure ground speed directly, by the rotation of its wheels.

Not to make too fine a point of it, but that is incorrect. A car measures the rotational rate of the wheels and from that determines the surface velocity of the tires. It does not measure ground speed, ever. We rely unpon the presumption that as long as the tires are firmly in contact with the ground, the tire surface velocity and the true ground speed of the vehicle will usualy be the same. For most cases, this is a safe presumption, but it is not always true. E.g. drive slowly on a slick road and hit the gas hard. The wheels spin, the speedometer will slam the needle around to the right, but your actual ground speed will remain unchanged.

The distinction becomes important if we try to put details on the original problem statement. Depnding on how the "plane speed" is determined (e.g. speed relative to the ground as determined by a ground-based observer, speed relative to the belt as determine by a belt-based observer, speed relative to the belt as determined by an airplane-based observer, speed relative to the ground as determined by an airplane-based observer, etc etc etc), you are going to come up with a different answer to the question.

-dave

[KenWittlief]

Originally Posted by dlavery

...
The distinction becomes important if we try to put details on the original problem statement. Depnding on how the "plane speed" is determined (e.g. speed relative to the ground as determined by a ground-based observer, speed relative to the belt as determine by a belt-based observer, speed relative to the belt as determined by an airplane-based observer, speed relative to the ground as determined by an airplane-based observer, etc etc etc), you are going to come up with a different answer to the question.

-dave

but there is no equipment on any aircraft that measures ground speed through the tires - they dont use the same speedometers that a car uses.

so if you ADD one to an aircraft for the purpose of this experiment, then you are altering the way that aircraft measure their ground speed, and you are rigging the test to get the answer you want.

[EricH]

Apologies for resurrecting this ancient thread, but THIS JUST IN:

The Mythbusters will be going for this one next week at 10 E/P!

[Jay H 237]

Originally Posted by Elgin Clock

Oh, and does anyone foresee an episode of mythbusters coming on about this????

It has been discussed on the Mythbusters website forums since early November. So far it looks like they're not into testing it.
Now if I could only convince them.............too bad I'm not dating Kari.

[sciguy125]

Ah, I now see the error in my thinking..

Let's say we drop a missile from an airplane. Better yet, from a helicopter that is hovering in place. The missile will still go forward even if it has no contact with the ground. So, the ground has nothing to do with the airplane's ability to take off.