RPG already explained this, but since I had written this, long, long time ago...
Now, this is going to sound a strange question, but where is this conveyor belt setup? Inside or outside?
You mean in a building or outside in the wild? Well, I suppose that if we are talking about a real, life-sized aircraft, it would have to be outside. As not only the conveyor has to be slightly longer than the minimum take off run the plane would need normally, attempt to fly a real aircraft inside a building would most likely face some challenges - namely walls and ceiling.
Other than those things, it really doesn't matter. Or, we can assume that it's a R/C model plane - small and slow enough one to be flown inside a large building. Then it can be inside as well.
The way I see it, the plane only can take off with the air moving around it, right.
True.
Now the only way it can get enough lift, is by going fast AGAINST the air current.
True.
If its on a conveyor belt, it is not going anywhere, therefore surely there is little air resistance being created, therefore no lift, therefore no airbourne....ness....
But it is going somewhere, that is the whole point of the question.
The belt has no way of stopping it from going somewhere - at least with the current rules of the question.
RPG's example about rollerblades, treadmill and a car is very good, though. This is the very same thing; plane is using method of propulsion that doesn't give rat's ass about the conveyor and the wheels underneath it take care of any effect the belt might otherwise have, so if that example is easier to grasp than planes and conveyors....
The wheels ARE important, because they allow the aircraft to ROLL and accelerate forward.
I'm glad that we agree on this.
Why do you think runways are so long? Why do you think aircraft carriers have catapault launchers? If a conveyor system worked like you think, it would not be a lot cheaper (not to mention SMALLER) to use, than a 1/2 mile long deck with 3 x 50 ton, steam-driven catapault?
Like RPG said, I don't think that you understand the question. If the conveyor system worked like I think, it would work almost exactly like a regular runway.
Certainly, you could reverse the belt's direction,
engage wheelbrakes of the aircraft, and use the belt as a catapult. But that is not what the question is about.
But I don't really see how you can think of three or four mile long conveyor belt capable of running at 200 kts as a cheaper or smaller than a catapult
(or even physically possible, for that matter)? Because the belt is not any shorter than a regular runway, actually it should be bit longer to counter for the energy losses in rolling resistance of the wheels.
Aircraft do not move by PUSHING AIR. It is the SHAPE of the aicraft moving THROUGH the air that produces lift. The engines burn fuel with air to produce thrust, put they're not simply 'moving air'.
I said nothing about what produces lift.
If you don't want to call it 'moving air', sure. That's semantics, but it makes no difference; the point is that a jet engine's
(or propeller's) frame of reference is the air surrounding it - unlike for example car's, which frame of reference is the road underneath it. Or to be specific, in car's case the road is the reference point for the wheels, since they do the job of actually propelling the car forwards - just like engines do for aircraft.
What is important here that the aircraft's engines do not care about the conveyor. They are propelling the airframe forwards with a force that is not dependent on what the belt is doing.
However, if you take away the forward speed (via the conveyor belt), then the nessesary airflow from the airframe accelerating through the air never happens.
But the whole point is that the belt is not capable of that. Not unless you engage the wheelbrakes on the aircraft... and why would you do that, if you are trying to take off?
If the wheels are able to rotate freely, only thing the belt can do is make them spin faster.
Again, this is why aircraft carriers are designed the way they are. The aircraft are connected to the catapault. The aircraft throttles up his engines. Then the catapault pulls the aircraft forward (at a vastly accelerated rate) so it is at the proper airspeed/airflow at the end to maintain flight (0-175 MPH in 2 seconds).
And usually the carrier is also sailing headwind, at full speed to give the plane additional airspeed - some 30 knots plus the wind, actually. See, some of us know quite a bit about planes and aviation, some of us might even be interested in them.
That paragraph of yours is not really related to the question at hand in any way, though.
All the wheels do is allow the aircraft to move (and accelerate) down the runway.
I'm glad that we agree on this.
But I have to ask, why do you claim otherwise in the very same paragraph?
You say that the conveyor has the power to stop the plane from moving. Okay, how does it do this?
- Wheels are the only part of aircraft that touch the belt. Do we agree on this?
- Thus the wheels are also the only part of the aircraft the belt can affect directly. Rest of the aircraft is affected through the wheels, so to speak. Do we agree on this?
- So, when the belt is trying to keep the aircraft from moving, like you seem to think, it has to somehow do this through the wheels. Do we agree on this?
- Those wheels turn freely, allowing the aircraft to move and thus accelerate on a runway. Like you said. Do we agree on this?
- Which means that when the belt is trying to keep the aircraft still by running to the opposite direction, only thing it actually achieves is to make the wheels spin faster. Do we agree on this? If not, please provide an answer about how the belt can stop the plane, by merely affecting its wheels.
The engines are NOT pushing the air. They transfer kinetic energy to the AIRFRAME, in the opposite direction of the "thrust" caused by the turbine engines. Turbine engines are NOT designed to simply move air. They draw in air, which is mixed with fuel, compressed, then ignited. They use turbine engines in a lot of different things, including large generators and the M1 Abrams Main Battle Tank.
Now you are confusing things. While you can indeed have a gas turbine outputting power through a shaft, like in M1, jet engines are different. They are somewhat simpler, and yes, they infact do propel the aircraft by the thrust of their exhaust gas. Or in layman's terms; by pushing hot air out of their ass.
You should seek a certain Top Gear
(I'm fairly sure that it was a Top Gear) episode on Google video. They demonstrate the power of 747's jet engine by keeping the plane still on the runway with wheelbrakes, while having *one* engine running at full throttle for a short period of time
(can't have more than one, or for long periods of time, as the brakes, wheels and landing struts can't take the stress). Then they drive a car across the runway, some distance behind the 747. Regular family car swerves and tilts rather badly when the airflow hits it. However, good old Citroen CV2 - being both light and having somewhat highish center of gravity - actually flips over in the wind.
So, yes, jet engines are very much pushing air.
And while we are at it; while internal combustion engine only makes a shaft turn, a propeller attached to that shaft is... you guessed it, pushing air.
That's on a prop aircraft, of course.
Not that this would really matter, since we mean by "pushing air" the very same thing you mean by "transfer kinetic energy to the AIRFRAME, in the opposite direction of the "thrust" caused by the turbine engines". It's just semantics.
The wheels allow the airframe to move in the opposite direction of the "thrust" from the engines. The wheels are NOT supplying any motive force, they simply allow the aircraft to roll over the surface.
Indeed.
HOWEVER, some kinetic force is transfered to the ground, which lessens somewhat as it picks up speed (until the airflow "lift" cancels it out entirely). This is because GRAVITY is trying to pull/hold the mass of the aircraft down to the ground. The friction between the wheels/ground is irrelevent.
Indeed. This is called rolling resistance, or sometimes rather curiously rolling friction, in layman's terms. And trust me, compared to aerodynamic drag, this is a very small force indeed, especially at higher speeds. Me and Otokoshi have been mentioning this since our very first posts, we are aware of it - it's just not nearly enough sufficient to slow the aircraft considerably, much less stop it.
Volkswagen actually had a nice demonstration; they were showing off their Touran SUV with a V10 diesel engine. They towed a B747 with it.
So, the rolling resistance isn't that massive. Granted, the Touran was somewhat modified - for one it was carrying a great deal of extra weight, but it does show that a powerful SUV is capable of overcoming the rolling friction of 747's tires. And even the static friction, which is - as I'm sure you know - greater than the rolling friction.
Because the wheels are free-wheeling and we have assumed zero friction at the hub, it follows that the conveyor belt, no matter how fast it is moving, CANNOT EXERT ANY FORCE on the aircraft with respect to forward motion! There is no force in our experiment that can oppose the thrust vector of the aircraft.
If the conveyor belt cannot exert any relevant force on the aircraft, you can completely ignore it. Ergo, the aircraft takes off as if nothing unusual is happening.
Here, somebody went wrong big time. The conveyor belt, by moving in the opposite direction to the "thrust vector" of the aircraft, CANCELS OUT the forward motion of the aircraft, in relation to the surrounding air. It's NO DIFFERENT than a person, bike, or car "keeping pace" on a treadmill. The method of force tranferance doesn't matter, the results are still the same. ZERO relative movement to the surrounding air.
OMG. Of course the method matters. Especially when in this case the belt has NO method at all to transfer any force directly to the plane. Except through the wheels, and for the umpteenth time; the rolling resistance is a very small force compared to aerodynamic drag.
And yes, it is very much different from your examples, because of that very reason.
It isn't the conveyor or treadmill exerting force on you.. it is YOU (or the object) exerting force on the conveyor. That's an important distinction, because the conveyor (when not turned on) is no different from any other stationary surface. It's only when it's in motion, that it adds or subtracts to our relative speed, depending on our direction in relation to its movement. The airframe is only trying to move relative to the ground (because of gravity and inertia). This gives it forward speed, and in addition, airflow over the airframe. But because it is on a conveyor which is running in the exact opposite direction, and same 'apparent' speed, this would in fact cancel out the forward speed of the aircraft.
No. Again and again, no.
The engines are pushing the airframe in reference to air. And while there indeed is rolling resistance - like me and Otokoshi haven been saying since the beginning - between the wheels of aircraft and the belt, it's not nearly significant enough to prevent the plane from taking off.
The reason Ooine makes the assumption that there's no friction at the wheelhubs - or rolling resistance for that matter - is because it's not significant enough to be an issue. And thus the whole point of the question is easier to understand if you ignore it.
EDIT: Removed one unnecessarily repeated sentence from one of the quotes.
EDIT2: It's actually Touareg, not Touran. I blame Volkswagen and their stupidly similar names.
Anyway, couple of links about it:
1,
2.
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