Can inexperienced 'MythBusters' really fly commercial aircraft?

Originally posted by Zaphod58
It doesn't matter how fast the conveyor goes. If it was a CAR it would matter, but since the thrust comes from the engines, all the energy is being imparted into the AIR, not onto the conveyor/ground. The plane still is going to move forward and have air moving over the wings, creating lift.

Let's try a thought experiment here:


Instead of a conveyor belt, lets lock the plane's wheels, chock the tires and chain the landing struts to the tarmac.

Now, fire up the engine and give it full throttle.

"...since the thrust comes from the engines, all the energy is being imparted into the AIR, not onto the conveyor/ground."

So, by your reasoning, the airplane Should be able to take-off like a scalded bat-out-of-Hell.

But I think we both know what will actually happen....Nothing.

Except, maybe a burnt-out engine.

Why?

Because, even with all that "energy being imparted into the AIR", the plane was being prevented from moving THROUGH the AIR.

You obviously understand the principles of aerodynamics well enough to comprehend the importance of lift in relation to an airplane's ability to fly. However, I think you might be a bit unclear on the source of that lift.

Wheels, skis, or pontoons act to reduce the friction resulting from the surface upon which the plane rests. Reducing this friction enables the plane's propulsion system, whatever it may be, to more easily push (or pull) the craft through the air-mass until a speed sufficient to allow the wings to generate sufficient lift gets the plane air-born.

Unless the thrust of the engine is, in and of itself, sufficient to offset the mass of the aircraft (as would be the case with a rocket), the thrust of the plane's engines do not themselves generate the lift the plane needs to fly.

If an aircraft's engines cannot move the plane forward, relative to the spot upon which the plane rests, either because the surface upon which the plane rests is too "sticky" (the plane is stuck to the pavement), or because the surface upon which the plane rests negates all forward motion (our conveyor belt, for example) the wings cannot generate lift and the plane cannot fly.

Oh so is THAT why the plane never took off when we were doing engine runs with chocks in place! I always wondered about that!!! Except in THIS example we're talking about a plane that is NOT chained down and NOT locked in place. Of course if we lock it in place it's not going to take off, that goes without saying.

I have a complete understanding of where lift comes from, especially considering that I used to be a sailplane pilot. If you don't know where lift comes from in one of those you're in a world of trouble fast.

How did you get the idea that I don't understand lift from that post? Talk about a stretch. I very clearly said that the engines provide thrust and the air moving over the wings provides the lift.

Originally posted by Bhadhidar
The purpose of an airplane's propulsion system, be it propeller or jet thrust, is to move the aircraft's wings through the air at sufficient speed to generate enough lift, via the bernoulli(sp?) effect, to loft the craft into the air.

If, for whatever reason, there is not sufficient airflow over the wings (say, no forward motion, for example) the plane will not fly.

Right, what you don't seem to grasp is that the wheels are FREE ROLLING. The conveyor belt going in the opposite direction has no effect on the airplanes ability to move forward and thus take off. The only effect it has is on the wheels, which will spin two times as fast and therefore under normal circumstances probably rupture due to friction.

reply to post by SilentGem


"...move foreward..."


In relation to What, exactly would the plane be moving forward to?

We agree that for the plane to fly, it must move forward, through an air-mass; allowing the wings to generate lift.

That forward movement, at take-off, normally, is in relation to the ground upon which the plane rests, right?

We also agree that the plane's wheels, "free-wheeling" as they are, do not in any way contribute to, or detract from, the ability of the wings to generate lift. The wheels (skis, or pontoons, etc.) on a plane merely make it easier for the plane's propulsion system to push (or pull) the plane across the take-off surface, at a speed sufficient to provide enough airflow over the wings to generate enough lift to fly.


If, however, the "ground" upon which the plane rests at take-off impedes the forward movement of the plane, to the extent that the forward motion of the plane is insufficient to generate enough lift, the plane will not be able to fly.

As I attempted to point out earlier, in our scenario with the conveyor belt, as the plane's engine powers up, its thrust attempts to pull (or push) the aircraft forward through the air-mass; forward that is, in relation to its starting point.

But because, in this experiment, the plane's starting point is not a fixed, unmoving point, like a runway, but rather an un-fixed, and counteracting point on an endlessly rotating surface, the plane will not be able to advance "forward" in relation to the surrounding air-mass. Hence no lift, no flight.

It doesn't matter if it's on a conveyor belt. The engines will STILL push the plane forward and generate lift. Because the wheels aren't pushing against the surface to propel the plane, like they do in a car, the plane will move forward. I stand by what I said earlier. Since the engines are pushing air backwards, the opposite reaction is going to be for it to move forward, conveyor belt or not. The only thing that would keep it from taking off would be a failure of the wheels, because they're going twice as fast as the plane.

[edit on 12/9/2007 by Zaphod58]

Originally posted by Bhadhidar

"...move foreward..."


In relation to What, exactly would the plane be moving forward to?

We agree that for the plane to fly, it must move forward, through an air-mass; allowing the wings to generate lift.

That forward movement, at take-off, normally, is in relation to the ground upon which the plane rests, right?

We also agree that the plane's wheels, "free-wheeling" as they are, do not in any way contribute to, or detract from, the ability of the wings to generate lift. The wheels (skis, or pontoons, etc.) on a plane merely make it easier for the plane's propulsion system to push (or pull) the plane across the take-off surface, at a speed sufficient to provide enough airflow over the wings to generate enough lift to fly.


If, however, the "ground" upon which the plane rests at take-off impedes the forward movement of the plane, to the extent that the forward motion of the plane is insufficient to generate enough lift, the plane will not be able to fly.

As I attempted to point out earlier, in our scenario with the conveyor belt, as the plane's engine powers up, its thrust attempts to pull (or push) the aircraft forward through the air-mass; forward that is, in relation to its starting point.

But because, in this experiment, the plane's starting point is not a fixed, unmoving point, like a runway, but rather an un-fixed, and counteracting point on an endlessly rotating surface, the plane will not be able to advance "forward" in relation to the surrounding air-mass. Hence no lift, no flight.

You are missing the entire point, sir :-)

I know this might be difficult to wrap your mind around, because it might seem counter-intuitive. The whole point I'm trying to make is that the airplane will move forward in relation to it's starting position. It will advance forward on the conveyor belt. It does not matter how fast the conveyor belt moves in the opposite direction (if you exclude the problems of friction) the airplane will always move forward along the conveyor belt until it reaches take of speed and then it will be able to take off. This is because the thrust of the airplane is not directly linked with the wheels, the wheels are free rolling.

If you put a car on a conveyor belt with 4 free rolling wheels, and take out friction of the equation ENTIERLY. Then start the conveyor belt in either direction, what will happen? The car will remain in the exact same spot it started in, except the wheels will rotate to match the conveyor belt.

Do you understand now?

Originally posted by Bhadhidar
In relation to What, exactly would the plane be moving forward to?

We agree that for the plane to fly, it must move forward, through an air-mass; allowing the wings to generate lift.

That forward movement, at take-off, normally, is in relation to the ground upon which the plane rests, right?

The forward motion is relative to the airspeed. If they wanted to, they could get some HUGE fans on a runway, throttle up the engines of the plane and the fans, and get the plane to take off vertically with no ground velocity, as if it were in a wind tunnel. It would be a difficult thing to do but you could do it. the formula for lift is quite simply L = 0.5 x CL(coefficient of lift) x V^2 x S (area of wing) x density of air. The V is the airspeed. It's why planes typically take off into the wind, so they need less ground speed to take off.

As to the idea of can inexperienced people land a plane, probably not. If they were really good at concentrating, knew how to ignore the panic of it being entirely down to them to land a plane with 400 people on, maybe possible.

reply to post by Bhadhidar


Alright, lets try another example. How does a plane take off from an aircraft carrier then? Let's even hop into the wayback machine and go back before they used the catapult to assist jet aircraft in obtaining enough velocity for take off. Lets go back to WWII tech and prop jobs.

Do they stop the carrier so the planes can take off? No, they don't. So the plane is, essentially, sitting on a moving surface, correct? (Yes, I know the deck of the carrier is static, but the whole carrier is moving, so it negates the "fixed point'.) By your example, the plane should not be able to take off because the carriers movement is going to negate the thrust of the prop. And yet, you can see actual film footage in those old WWII documentaries that the plane is able to generate enough thrust, gain wind speed, and launch.

Or, here's an easier thing to try. Go put a skateboard on a treadmill. Yes, you will have to provide a little force to keep the skateboard from flying off the treadmill when you start it up since we can't overcome friction, but the principle is still the same. The wheels on the skateboard do not provide any power to propel it forward, or for that matter, anything for braking. With a small amount of force to hold it in place, you will see that the wheels just spin. Now, to provide some forward movement, use your hand to generate some "thrust". You will see that you will need apply little force to move the skateboard forward, since the force you are applying is not dependent on contact with the treadmill nor a fixed point.

Originally posted by SilentGem
The whole point I'm trying to make is that the airplane will move forward in relation to it's starting position.

If you put a car on a conveyor belt with 4 free rolling wheels, and take out friction of the equation ENTIERLY. Then start the conveyor belt in either direction, what will happen? The car will remain in the exact same spot it started in, except the wheels will rotate to match the conveyor belt.

Do you understand now?

Please consider the following image, which I previously posted on page 2 of this thread.


www.conceptcarz.com..." target='_blank' class='tabOff'/>


As you can see, this automobile derived its propulsion from a forward mounted aircraft-type propeller. There is no connection between the car's motor and its wheels. The wheels are in fact "free-wheeling", as in your example.

Now, because this vehicle would be deriving all of its forward motion not from the action of its wheels against the surface of the belt in question, but from the thrust generated by its prop-wash; are you saying that this old heap should be able to pull itself off a counter-acting conveyor belt?

Bearing in mind that this old Helicron is, in essence a "plane without wings", the comparison is, for our purposes apt, I think.


You claim that the plane would be able to move forward in relation to its starting point/position.

However, in our example, that starting position is a point on a surface which itself rotates about a fixed axis. Any object rolling on that surface, will itself be stable, that is to say, unmoving, in relation to the fixed axis of the surface's rotation.

That object therefore is Not moving.

Which is to say that, although the wheels will spin, as they would normally do, as the forward motion of the plane, imparted by the thrust of the motor, is transmitted through them (the wheels) to the take-off surface (normally the tarmac of the runway, but in our example, the belt surface of the conveyor), the struts to which the wheels (and the rest of the plane) are attched will remain fixed in position, relative to the axis of the roating surface

The rotating surface of the conveyor cancels all forward movement relative to the fixed point, which is the axis of the conveyor, therefore the struts, to which the wheels (and the rest of the plane) are attached do not move relative to the fixed position of the axis of rotation.

I'm sorry but you're still completely wrong. The wheels are freewheeling, so all that is going to happen is they will have to move twice as fast as the conveyor belt, but the plane will still take off. I don't know how to say it any clearer. In the case of a car, it gets forward motion by imparting power to the wheels, which push forward against the ground. In THAT case it wouldn't take off. But the ENGINES of a plane push back against the AIR, so as long as the wheels are turning, it will move forward, creating lift and take off. You are right in that the wheels are simply there to allow the plane to move, but they are not doing ANYTHING to create the movement, as they do in a car. They are responding to the AIRFRAME being propelled by the engines pushing against the air.

reply to post by sheetrockerr


As in the example with the aircraft carrier, you, yourself concede that the deck of the carrier remains static relative to the the plane on the deck.

This is the crucial point; and the point which negates the example in its entirety.

The fact that the deck is unmoving relative to the airplane allows the plane to move forward along the deck, into the air mass which provides the lift the plane needs to fly.

The fact that the deck of the carrier is moving relative to the sea around it is irrelavent, save for one important consideration:

By heading Into the wind as the plane attempts take-off, the motion of the carrier may actually assist in the plane's launch. Theoretically, if you could get the carrier going fast enough, you wouldn't even need a catapult, or a runway for that matter, to launch the planes; they'd take off like kites in a stiff breeze.

Consider this: how often have you seen an attempt made to launch a plane off the rear of an aircraft carrier at flank speed?

Or this; How often do they launch planes off of carriers while the carrier is going full reverse?

As to the skateboard example;

This is a much better example of the scenario presented. However, here again, if the additional force your finger provides is negated by the rotation of the surface on which the board sits, the board cannot move.

The force you supply to the board is transmitted to the wheels of the board by the axles. The axles convert the vectored force you supply into rotational force. In our example, the conveyor has the ability to exactly counter this rotational force; hence no supercedence of the board's inertia.

A body at rest tends to remain at rest until and unless another force acts upon it. However, if that other force is itself countered, inertia remains.

Except that the engines are moving the airframe, and the wheels are only turning in reaction to that. So it's the AIRFRAME that matters in this case. As long as the airframe moves fast enough to generate lift it will take off. The conveyor belt has nothing to do with the airframe movement, only the wheels. If the wheels were responsible for moving the airframe, then you would be right, but they don't.

I had to chase my tail around the room a few times to get this one past my mental inhibotors. It made me think of another challenging riddle.

If you're up to it, please visit Perception Challenge: How to Make a Dollar Disappear!

By the way, "Zaphod58" is right.

Hmmm.......

Which way is the conveyor belt running???? Is it running in the direction of flight, or in the exact opposite direction? If the opposite direction, it will NOT fly.

Regarding Zeta115 post on navigation - you're mostly wrong. 767s use IRS. In 2000 GPS was a very expensive after-market retro-fit, which most airlines didn't take up on their existing aircraft.

Secondly, the terrorists hijacked the aircraft when they were around 200 miles away from their targets (~ 30 mins flight time) and in the furthest case (Flight 93) they were approximately 400 miles away (50 minutes flight time). Even at high altitude, you couldn't see the WTC from there, and VFR flight would be harder than you'd think, considering the nearest ground you could comfortably see from up there would actually be around 10-20 miles away horizontally, maybe more depending on where you were sat.

I strongly suspect the hijackers knew how to use the FMC. It actually isn't as complex as it seems, although you'd have to be aware that the radial to fix is actually 180° to what you'd enter in the autopilot to fly to it, as it gives the radial you are ON, not the heading to fly to it. If you're on the 285° radial, you actually want to fly a heading of 105° (assuming no wind).

As for knowing quite where to point the aircraft - that is something else entirely. a few degrees out at 300 miles could put you a long way out when you get to the coast. As Caustic Logic showed quite nicely on his blog, Flight 77 made a rather tidy direct routing to the Pentagon, all but a couple of miles - that kind of accuracy takes a bit more than luck. It is something that gets consistently overlooked. I'm still interested in the ghost DME reading in the FDR data allegedly taken from Flight 77 - that alone shows it couldn't have been at the co-ordinates it was reportly at as there is no DME station at the range recorded, even allowing for the speed of the jet and the recording rate of the FDR.

I recommend Googling how DME works - there are some good resources out there on it.

The conveyor belt is going in the opposite direction of the plane, but yes, the plane WILL take off still. No matter how fast the conveyor belt is running, for precisely the reasons stated in this thread.

If this is true then i guess all them Carriers out in the Gulf and whatnot wont have to use slingshots anymore for their planes to take off without much runway. We can just put em on a big conveyor belt and let them take off.

lmao give me a break.


it is in my honest opinion and thoughts, that mythbusters will FAIL at this attempt.

if it does work....why didnt we think of it back in the day when we came up with slingshots as a method of launching aircraft from carriers.


ugh.

They would still need to have catapults, because it has nothing to do with what we're talking about. They don't have the DISTANCE required to take off without them.

So what are you going to say after the show when they prove it'll take off? Admit you were wrong or tell us all how they hoaxed us and the show is fake?

naw, unfortunately sorry to burst ur bubble, i dont spread about disinformation and i dont go around saying things are hoaxes.

i think you'' find those kind of ppl in a diff section of this forum.


all im thinking is the bernoulli effect.

and we all dont even know how long this conveyor belt is gonna be to make EXACT predictions about the outcome of this.

im not saying for a fact that its NOT gonna work, i even said if u would actually take the time and patience to read my frakkin post. i said in my honest opinion and THOUGHTS.

if it works, illtip my hat to all of you that say it will work. if it doesnt....well im not gonna rub it in your face but what-can-i-say. This is the first time its been tried no?

who's to say it will work or not. we'll have to wait and see. It just doesnt seem very likely to me that it will work.

reply to post by mirageofdeceit

I strongly suspect the hijackers knew how to use the FMC.

I would guess that you are correct.

NTSB Flight 77 and Flight 93 autopilot activity (PDF)

Originally posted by BhadhidarNow, because this vehicle would be deriving all of its forward motion not from the action of its wheels against the surface of the belt in question, but from the thrust generated by its prop-wash; are you saying that this old heap should be able to pull itself off a counter-acting conveyor belt?

Yup :-) As long as this prop generates enough thrust to overcome the effects of friction, then yes, it will be able to pull itself off a counter-acting conveyor belt, no matter how fast it is moving in the opposite direction.

Friction is the force that makes the big difference here. When you are standing on a conveyor belt in an airport for instance, the reason your body moves along with the conveyor belt is due to the friction generated between the conveyor belt and your shoes/sandals/feet! If there was no friction, the conveyor belt would not take you anywhere.

When the airplane begins to thrust its self through the air on a backwards moving conveyor belt, the only effect this conveyor belt is having on the airplane is on the wheels, they will move twice as fast (assuming the conveyor belt matches the speed of the aircraft). The airplane still moves forward through air, thus generating lift, as normal.

I might be horrible at explaining this, but do you understand now? :-)