fdl

We're not arguing about speed, we are arguing about force. Obviously the plane needs to exceed the speed of the water....it does need to move forward..noone is arguing it doesn't. But The plane will easily be able to exceed the speed of the water, as the force required to do so is not proportional to the speed of the water coming the other way.

curls

That is a different effect on the plane than a moving conveyor and free-spinning wheels. Bearings in the wheel hubs absorb any backward motion the conveyor imparts on the wheel, thus the "free spinning". However, in the case of pontoon planes on water, there isn't a near-perfect isolation from the rushing water because a pontoon has a lot of drag.

Billiam

For purposes of this conversation, downforce on a race car should not be compared to lift on plane. Downforce is an enhancement on a reace car. It is not a requirement for it to accelerate. Lift OTOH is absolutely required for a plane to do its primary function of getting off the ground.

NetEditor

What you're referring to is inertia. If the water is going 10 mph in one direction and the plane is going 10 mph in the opposite direction, it doesn't mean the plane is standing still.

spooky3ce

You are 100% correct... And i'll be using your calculation...

Hypotheticly speaking:
Plane requires to go 50 MPH on a runway to lift off... Thats 50 MPH wind to create enough lift for the plane to take off... (assuming wind speed on that day is 0 MPH)

Put the plane on the treadmill going 50 mph... The wheels will be spinning at 50mph the plane will be going 50 mph and the wind will be at 0 mph... So that the plane can take off, it will need to create enough lift (from the air) around the wings to take off... so the plane would need to be going 100 mph exceeding the speed of the treadmill to move forward on a 50 mph treadmill to create 50 mph wind...

Still wind is to planes as the road is to cars...

Now get 50 mph winds and push it agains a plane going 0mph like a kite... the plane will take off if it drives against the wind... ever see those tunnels that are created to test planes???

The plane would still need a treadmill long enough to exceed the treadmills speed to create enough air to push agains the wings to creat lift...

fdl

lol. you're still missing the point. If the treadmill is moving at 50mph backwards, and the plane wheels are traveling at 50 mph forwards, the plane will be actually traveling at 0 mph, not 50 as you state. It wont be moving, it will just be sitting there.

Again, noone is saying the plane will take off from a standing position. Of course not! Only that it will easily be able to move forward on the treadmill in order to achieve lift. The fact that the treadmill is there will have little consequence. It can easily thrust forward and take off.

curls

Perfect... let's continue

ERROR. Plane will be going 50mph relative to the air around it. conveyor will be going 50mph in the other direction. However, since the conveyor does NOT affect wind speed, the plane will take off. The conveyor ONLY affects WHEEL speed, and WHEEL SPEED <> AIR SPEED.

No. The plane will need 50mph worth of thrust from its jets/propellor, plus whatever miniscule amount is required to overcome the very slight increase in rolling resistance from the free-wheels on the ground. Probably something quivalent to 1mph... so the plane will require approx. 51mph worth of thrust.

Yes. But the conveyor has NO effect on the wind/air around the plane!! Just on the ground (wheels). Since the wheels are irrelevant to the wing speed, the conveyor is irrelevant to the wing!

No argument there... but please explain how the conveyor affects this at all?

No. The plane would require a conveyor/treadmill exactly as long as a typical runway, because again, the conveyor has NO effect on the windspeed or wingspeed, so it's irrelevant!

Whiskers

You guys are so thick....

NetEditor

I think you're confusing the point that the plane needs to overcome the effects of the treadmill. Yes, if you stick something on a treadmill and turn it on, it will move backward. But that's because of inertia caused by the weight of the object and the resistance due to rolling friction.

For instance, if you put a toy car on a treadmill and used your finger to keep it in place, the force required to keep it in place wouldn't increase in portion to the speed of the treadmill. Also, the force to move the toy car forward would be more or less the same than if the treadmill weren't on. (The force applied with your finger would be equivalent to the engines on a plane since the propulsion of a plane isn't transferred via the wheels.)

fdl

Exactly.

And no one is debating the need for the plane to move forward relative to the earth, and the need for the associated airflow to achieve lift. This is scientific fact. The argument is that the force to do this will be independent of how fast the treadmill is going in the opposite direction.

Billiam

Exactly. Now light a small model rocket engine you've taped to the car and let go. It's the best example I can think of to illustrate the forces (or lack thereof) involved in the plane scenario.

curls

Copied and pasted... This should pretty much /thread:




"This is somewhat of a "trick" question. Not because it is phrased in a
deliberately tricky way, but because people tend to have trouble
thinking about the operation of other vehicles apart from cars which
they know so well.

The heart of the confusion is simply these two important facts:
* cars propel themselves by pushing against the ground via friction
* airplanes propel themselves by pushing against the air
If you can let go of how cars operate and think about what an airplane
does, you'll be able to see the problem clearly.

One good way of tackling this problem is to find a good analogy. But
the analogy must be a valid one else you'll just get more confused.
For example, someone posted the analogy of running on a treadmill. Why
is that a bad analogy? Because one runs by pushing against the ground
via friction between their shoe and the ground. This is how a car
propels itself! It is not how an airplane propels itself, by pushing
against the air. Bad analogy.

Let's use this analogy. Instead of looking at the airplane, let's back
up and go into the airport. Suppose you're walking down to your gate
and pulling your carry-on bag behind you. It's a nice new bag with low
friction wheels. No problem! Up ahead you see one of those moving
walkways. You don't see anyone coming, so you decide to do a little
experiment. You go over to the walkway that is moving TOWARDS you and
place your bag on it. Meanwhile, you step off to the side of the
walkway, and still holding on to the handle of your bag, you continue
to walk along. In fact, you intentionally walk along at the same speed
that the moving walkway is going, just in the opposite direction.
Question: does the bag move or does it remain stationary as you keep
walking? Obviously it moves with you. So why does your bag move
forward when you are walking at the same speed of the conveyor going
in the opposite direction?

The answer to that question is also the answer to the
airplane-conveyor question. To complete the analogy, the pull of your
arm is analogous to the force of the airplane engines. The bag's
wheels are analogous to the airplane tires. Do the nice low-friction
wheels on your bag on the conveyor pull against you anymore than they
do when you're just pulling your bag along normally? No, they don't.
They are free-wheeling, after all. Meanwhile, you're pulling the bag
with the same force in both cases. So in both cases, the bag keeps
moving forward. Likewise with the airplane, the pull of the engines
doesn't change nor does the force on the airplane imparted by the
tires change no matter what the ground is doing underneath the tires.
You have the same force imbalance in either case, and since Force =
mass x accceleration, you have the same acceleration. Remember, we are
talking airplane engines which push against the AIR, not the ground.
The acceleration is with respect to the AIR, thus the airplane
develops a speed relative to the air and can eventually take off.

That's a long winded analogy. Here's a quicker solution. Engineers
learn to draw Free Body Diagrams to understand such problems. A FBD is
just a block diagram which illustrates the forces acting upon an
object. The net force can be calculated from all the contributing
forces. If that net force is not zero, the object accelerates in the
direction of that force. Let's draw a FBD now. Represent the airplane
by a simple rectangle (the shape doesn't matter). Indicate the force
of the engines pulling on the plane with a large arrow, labeled "F_e"
with e for engines. What force are the tires imparting to the plane?
Remember, they spin freely except for bearing friction and rolling
friction, which produces forces that are quite tiny compared to the
engines. Represent those forces with a small arrow, labeled F_t where
t means tire. What other forces are operating? There is a drag force
too. But remember this drag force is also small compared to the
propulsion of the engines at least at takeoff speeds. Label that F_d.

FREE BODY DIAGRAM

F_d |----------| F_e
<---| Plane |------------->
|----------|
<-- F_t

So what do we have here? We haven't put numerical quantities on these
forces, but instead have just been talking in terms of large and
small. That's okay for the purposes of this illustration. It's enough
to know that F_e is going to be much greater than F_d + F_t pulling in
the opposite direction. If that's the case, we have an unbalanced
force on the plane. Therefore, it accelerates. And it accelerates with
respect to the AIR, since F_e is produces by the engines pushing
against the air.

To sum up: Yes, the airplane takes off. The motion of the surface
underneath the freely-spinning tires is irrelevant to the acceleration
of the aircraft since the tires cannot impart any force to the
aircraft (aside from the aforementioned very small rolling and bearing
friction).

I hope this discussion has helped somewhat and not muddied the waters any further.

John Strong
Ph.D., biochemical engineering
M.S., chemical engineering
B.S., mechanical engineering"

Copied and pasted...

cibs

ah finally some sense in this thread... great explanation curls! it's much more thorough than mine...

dom

As you were.

srika

thanks

Whiskers

You must have me confused for another Whiskers....

fdl

/THREAD

Mizouse

i do miss Wsklar

Whiskers

He liked you....

fdl

They're both fat so they clicked right away.

Whiskers

wsklar still had more sex though....

Smalls

ah I remember. It was like a lightswitch.


......[whiskers] ah ok, you're right. [/whiskers]

srika

Here are the facts:

Let's use a 737-400 to keep it simple - not sure if all of these values would be needed:

Takeoff speed: 150 mph
Takeoff run: 8,483 ft
Takeoff weight: 100,000 lbs
Takeoff thrust: 22,000 lbf
Runway length: 13,000 feet (longest runway at O'Hare, for example)

Wouldn't it be possible to figure this out mathematically? I would specifically be interested in the speed the wheels / treadmill are moving at the time of takeoff. I think it would be ridiculous!

Math and Physics people!! GO!!!

edit: added Takeoff Run length

tdog2313

threads on this question always to turn into crap, lol.

basically what you need to realize is that on a plane, the wheels just spin freely, they dont do anything. you cant think of it like a car.

srika

can you elaborate on that?

cibs

shabaaz

Ok, here is my theory.

Lets say there is a massive treadmill a few miles long and the belt is moving at 300mph. Anyone who has taken fluid mechanics/dynamics knows that a moving solid surface will have an effect on the air directly above it. Air is a fluid and the air directly touching the moving surface of the treadmill will move at the same speed of the treadmill (300mph). As you move away from the surface vertically the velocity decreases to zero in the form a parabolic function. This is called the boundary layer effect because there is friction between the air and the surface of the treadmill.



The picture (from wikipedia) shows air moving over a stationary plate but the same phenomenon occurs if the plate is moving and the air is stationary.

At the front end of the treadmill the boundary layer will be tiny, maybe only few thousandths of an inch. Since the thickness increases the further you move along the treadmill towards the back, there will be a larger layer of moving air above the treadmill. If the air flow is turbulent, the boundary layer increases in thickness faster as you move further from the leading edge of the treadmill.

So imagine if you cranked up the speed of the treadmill to a ridiculous amount so that a huge amount of air is pushed by the treadmill and a large boundary layer is created. The airplane could be placed near the back end of the treadmill where the boundary layer is the thickest. The engines are run so that the thrust overcomes the forces of friction from the rotating wheels and the drag caused by all the moving air. Now the plane is stationary with respect to the ground but if the air is blowing past it fast enough the wings will generate the lift needed to overcome the weight of the plane and it should take off and float in the air.

Also remember that the boundary layer is not infinitely thick so if the plane pulled up and out of that layer then the air velocity would be zero. If there is no air moving past the wings the plane will simply lose lift and fall.

Does this make sense to anyone? Obviously testing this theory with a real plane would be near impossible. I imagine you could do small scale testing of this scenario but replace air with a viscous fluid. Using a higher viscosity and denser fluid will make the boundary layer larger. I'm gonna wait and see what the Mythbusters can come up with but can already guess that it will leave me dissatisfied.

I should mention that I have not done any research and I just thought this up now so I not saying I'm right. Also, it has been a couple of years since I've done fluid mechanics. I just wanted to share my thoughts, thats all.

is300eater

<-------- very

Billiam

Yes, however there are two things that I believe may be wrong with applying your idea to the plane discussion. First, even at the scale of our giant treadmill I don't think the boundary layer is going to be anywhere near thick enough to reach the height of the plane's wings. Second, even if the boundary layer did reach the wings, I think the type of turbulence found within it would likely have a detrimental effect to the amount of lift the wings produce. Some turbulence can be a good thing but I believe it's usually applied in a very controlled manner. I could be wrong but I think typical commercial airplane wings more or less like laminar (smooth) airflow.

Whiskers

Dom threatened to ban me if I didn't get it...

juniorbean

iTrader: (5)

Well, since this thread was about Mythbusters, then went back to the whole "will the plane take off" discussion, I just merged the two together. No need to have a new topic dedicated to the same thing as an existing 23 or so page topic that was already beat to death...

srika

read through some of the rest of this thread.. this thread = ridiculous.

basically you got the people who are saying the jet will take off (headed by cibs), the people saying it won't (headed by JJ4short), and the people asking if the jet could take off, why don't airports have shorter runways..

btw I did some checking - originally, this question did not include the condition that the speed of the belt would change to match the speed of the plane...

http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

http://www.airliners.net/discussions...in/136068/1/#1

it has been beaten to death all over the net and I can't believe we talked about it AGAIN.

fdl

Fixed

Whiskers

fixed again...

is300eater

anyone know how they are going to prove it on Mythbusters? Model airplane? A small (real) plane with a very large treadmill?

Yumcha

FuriousGeorge83

Why is this thread still alive? Why wont it die?

Bearcat94

I think there was a link a few pages back. Some how using an Ultralight. Don't know about the treadmill.

BTW: Love >>> Electric (IMHO. )

juniorbean

iTrader: (5)

Bdog

I just turned in, I forgot it was on tonight... what happened?