[JungleDNB]

Last night, my roommate, my brother and I got into a debate over some question my roommate proposed to us.
It was concerning if a plane could take off if it was on a conveyor belt which was moving backwards at the same speed the plane is trying to move forwards at (say 100mph just for an example). Below is the mail I got from my roommate this morning concerning his argument vs. my brothers'(Stu) side.
Any opinions are appreciated.


Please read completely. My friend is sending me the video. But read below and with an open mind you will see that an airplane in fact can and will lift off. Chris, you will be the outside judge and give us your third party opinion. If you can provide me anything that disputes the information please send it to me.

1. If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots, will the plane be able to take off, or will it just sit stationary relative to the ground, with the backwards speed of the conveyor belt counteracting the forward thrust of the plane?
Thrust acts accordingly to Newtons Third Law of Motion - every action has an equal and opposite reaction. In the case of an aircraft, the reaction of the engines is that of forward motion, against whatever medium it is stationary. But the ground the aircraft is sitting on in this case is NOT stationary, it’s providing an exactly CANCELLING force pushing the aircraft back.
The problem here, of course, is that people cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel it self forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.

2. Now here is a comment sorta taking Stuart’s view: I respectfully disagree. A plane does not move itself by moving air. It moves itself by moving through the air. Its forward speed causes air to flow around the wings at different velocities (faster above, slower below), and that in turn creates lift. If the plane doesn’t move forward while it’s on the ground, air is not moved over the wings with sufficient speed to create lift — so the plane doesn’t fly. Based on your premise, we shouldn’t need landing gear or runways as planes should be able to lift off directly from the gate without any ground-relative motion whatsoever. We know it doesn’t work that way, so speed relative to the ground is in fact critical for flight. The plane must move forward relative to the medium upon which it stands, and it must move with sufficient speed to create lift. Until a plane is in the air, it is bound by the same principles that control how cars move. Remember, planes don’t fly on the ground — they are driven on wheels and steered by the same. Think of aircraft carriers for a real-world example of this riddle (planes must reach a certain speed relative to the carrier before they’re able to take flight, hence the catapult and high engine thrust).

3. Now here is the response: If the plane doesn’t move forward while it’s on the ground, air is not moved over the wings with sufficient speed to create lift — so the plane doesn’t fly.
You are assuming that the plane will not move forward, but have not explained why you think that. The plane is generating forward thrust. What force acting on the plane (note: NOT the plane’s free rolling wheels) is going to counteract that force and keep the plane from moving forward?
Based on your premise, we shouldn’t need landing gear or runways as planes should be able to lift off directly from the gate without any ground-relative motion whatsoever.
The landing gear are merely a way of eliminating (most of the) friction between the airplane and the runway. How fast the wheels are turning is meaningless. For instance, to take an example from real life, say the runway is coated with ice. The wheels will not be turning as fast as they would on a dry runway. In fact, on a perfectly frictionless surface, the wheel would not turn at all! The only thing that matters if how fast the airplane is moving through the air. The thrust generated by the plane moves the plane forward. The conveyor belt moving backwards spins the wheels backwards at twice their normal rate for that speed on a dry runway. So now the plane is moving forward at 100 knots and the wheels are spinning forward at 200 knots and the runway is moving backwards at 100 knots. So the wheels’ relative speed to the ground is 100 knots, the 100 knots generated by the airplane.
Until a plane is in the air, it is bound by the same principles that control how cars move.
False. Cars move forward by turning their wheels. Planes move forward by generating thrust through the air. The wheels spin as a result. The wheels are free rolling, which means that they are not connected to the engine, and do not work to counteract it.
Think of aircraft carriers for a real-world example of this riddle (planes must reach a certain speed relative to the carrier before they’re able to take flight, hence the catapult and high engine thrust).
Actually, the plane must reach a certain speed relative to the air. This is why aircraft carriers point into the wind for takeoffs, as it increases the amount of air moving over the wings, and hence the amount of lift generated by the wings. If an airplane needs to be moving at 200 knots through stationary air in order to take off, and the aircraft carrier is pointing into 200 knot wind (extreme, yes)… the airplane could take off without moving forward on the runway at all… it would just lift straight up.

4. Here is another argument piece siding with Stuart and another response: In this experiment the conveyer belt was rigged to sense the airplane’s speed from the thrust it was exibiting and turn the same speed in the opposite direction. In other words if the plane’s thrust was typical of an airliner traveling (or taking off) at 150 mph then the conveyer belt would roll at a rate of 150 mph in the other direction. This being said, the craft could not possibly take off. Whether it’s a jet plane, a prop plane a car a scooter or a go cart it would sit seemingly idle because any hint of speed it could generate would be counteracted by the belt. An airplane can’t do anything until it generates lift and lift is created by airspeed. What I’ve just discribed is a plane which can’t generate any. The end result is a perfectly good airliner with burnt up wheel bearings sitting at the back end of a conveyer belt in a smoking heep. BTW, this experiment has been done at model airplane airports nationwide with the same results.
Let’s look at this whole thing from a different perspective. Let’s say that the jet engines, or props, are shut down on the plane and the conveyer belt is at a dead stop. Then suddenly the conveyer belt instantaneously begins it’s 100 mph backward motion. Remember, the airplane’s engines are off, what happens? Well, the plane’s wheels begin to spin forward at 100 mph and the plane sits perfectly still in respect to the earth. Got it so far? Remember, we agreed that the plane’s bearings are almost drag free and that’s the only thing between the conveyer belt and the airplane. Now, the pilot fires up the plane and cranks the engines to a point that would normally propel the craft at about 100 mph. Remember the plane that was sitting still on the conveyer belt with it’s wheels spinning? You’ve just entered on more thing into the equation…thrust. Thrust is independent of the ground, it pushes against the air behind the plane, the action creates a reaction and the reaction is forward motion. Motion equals lift and lift gets this whole thing in the air. The conveyer belt is just a silly part of this equation that makes the wheels turn faster than they’re supposed to. The end result is a plane that would zoom down the runway pretty much like there was no conveyer belt and takeoff in a normal manner. That is, if the plane’s design would allow it to take off at 100 mph.

5. Example: Hurrincane force winds can lift a stationary aircraft because it provides lift. Now if 4 747 turbine engines provides 47,000 lbs of thrust each (3x the strength of a hurricane) all fire at the same time pushing air (thrust) to the wings, you are going to tell me that the plane will not lift?

[Scrotumlock]

That was painful to read; I'll admit I didn't get through every word. The answer to the question depends on the plane's position relative to the atmosphere around it before the engines are powered up... IOW is the plane stationary on the belt and moving backwards through the air at 150MPH, or it stationary in the air with it's wheels spinning on the belt whipping by underneath at 150MPH?

The plane will not take off in the first case but will in the second given the same minimal amount of thrust. However both cases describe a plane on a conveyor moving backward at the plane's forward take-off speed, so the question needs clarification.

[Big_One]

No.

[ventrue]

I think this thread was already done in the War Room some time ago. Do a search and you might get your answer there. I remember reading all the War Room "scientists" and "engineers" handing out their answers to this question. If I wasn't so lazy, I would give you a link to that thread. Good luck.

[NothingChanges]

if the plane is moving a 100 mph and the convueyer in reverse the same speed then the plane is still stationary so if the conveyer belt was to suddenly stop is the plane going to still be going a 100 mph I think it is because it will still be forceing itself unlike a car which relies on the traction I dont think that it would be able to take off though...

[Ultrashogun]

On that conveyer belt the airplane is stationary, it can only lift off if it moves at a certain speed in respect to the air.

[Ultrashogun]

If your on a running machine indoors, do you feel wind on your face? No, and neither do the planes wings in this case so it stays on the ground.

[Hog-train]

Dude just go look up Einstein's relativity theories. Even though the conveyor belt is moving backward, the plane itself is also moving with it backward. Thus, in relation to the conveyor belt, the plane is stationary and can take off when it moves forward. To take it even farther, from the perspective of the belt and plane, it is the Earth, ground and everything surrounding, that is actually moving and not the plane/belt area. If you think about it movement is all really relative and if for exapmle the objects surrounding the plane/belt was moving, the exact same physical laws would apply.

If you really want to get a more in depth understanding, I recommend:

Brian Greene- "Fabric of the Cosmos"

and he goes more deeply as well as exploring string theory in "The Elegant Universe"

[Ultrashogun]

Dude just go look up Einstein's relativity theories. Even though the conveyor belt is moving backward, the plane itself is also moving with it backward. Thus, in relation to the conveyor belt, the plane is stationary and can take off when it moves forward.

The plane would however be moving backwards relative to the air. The meccanism of flight doesnt depend on the conveyor, but on the velocity of the wings relative to the air.

[MoxyNoxy]

Originally Posted by Ultrashogun

On that conveyer belt the airplane is stationary, it can only lift off if it moves at a certain speed [through] the air.

Yes to Ultrashogun. No to the moron asking the question. (FYI: I'm a pilot)