Yes, right before my post an hour or so ago. What I keep hearing is that the plane won't fly because of the conveyer affecting the wheel speed of the plane. I'm just trying to clarify why you guys say it won't fly. Because of the way the question is worded or because the conveyor prevents it from flying? GT
Ugghh... If the plane is on the Earth, and the engines push, the wheels are held to the belt by gravity and start to turn in exact proportion to the push. Since the belt is moving at the same speed, the plane itself gains no forward momentum. Just like driving your car or running on the same belt would. With out air flow over the wings, the plane will not gain lift. Thus, not fly.
Your example is flawed. You are comparing a vehicle which is driven from the wheels as opposed to a plane which is driven by thrust that in no way related to the wheels. GT
If the plane is at full thrust, how does a conveyor's direction or speed have anything to do with the speed of the plane relative to the ground (and thus, the air)? GT
Example is not flawed because BOTH the car and plane are NOT moving forward - regardless of what is creating the thrust to do so. In the plane example, movement on the conveyor belt is NOT movement relative to the fixed ground. The air mass the plane is sitting in is NOT moving (it is a no-wind day). Therefore, the air mass is sitting still over the fixed ground (which surrounds the conveyor belt). Because the plane is NOT moving relative to the fixed ground, it is, therefore, NOT moving relative to the air mass it is within. And, if it is NOT moving in the air mass, then the wings feel no relative wind, and there is no lift and so the plane cannot take-off. .
oooo... I have another example. Let's say your Jeep is stuck in the mud. Give it full throttle and the wheels just spin. Of course it goes no where. Now, let's say an airplane is in the same mud. Is it possible that the plane could have enough thrust to move itself out of the mud? Of course. GT
Are you thinking the "engine at full thrust" creates its own wind? I already covered the fact that the engine's wind is NOT sufficient for the wings to fly. This is what happens every time the plane does a run-up. The prop wash only passes over the roots of the wing (the part near the fuselage). The outer 3/4 length of each wing has no prop wash going over it. So, in the hypothetical example, you could walk up to the plane running full speed on the conveyor belt and place your hand on the end of the wing and not feel any wind on the outer part of the wing. .
Okay, let's modify your example then. Let's say that Jeep is on the conveyor belt and it is doing 30 mph. If you put your hand out the window, how much wind will your hand feel? The answer is 0 mph. Because the conveyor belt is doing 30 mph to offset the Jeep's wheel speed. .
Are you saying that the conveyor is preventing the plane from moving forward? The plane's thrust is acting on the air that it produces. How does a conveyor that touches the wheels have an affect on that? GT
Agreed. But you keep using examples where the thrust of the vehicle is accomplished through the wheels. In the case of an airplane, this is not the case. GT
Yes, I am saying the plane is NOT moving forward relative to the fixed ground. It IS MOVING FORWARD relative to the surface of the conveyor belt - but that belt is moving at that same speed in the opposite direction.
I understand that the wheel speed of an airplane has NOTHING to do with it's forward speed. If you put KY jelly on the runway and locked the brakes, would the plane move forward when it was under thrust? GT
But, the very definition given in the hypothetical question is that the conveyor belt speed is SUFFICIENT to keep the plane from moving forward. It is in the wording that the "speed of the wheels will be matched by the conveyor belt". In a low friction scenario, the conveyor belt speed might have to be 10x the thrust speed of the plane's engine in order to keep the plane from moving forward. But, by definition, the conveyor belt would speed up to whatever speed was necessary to keep the plane from moving forward. If you make the assumption the plane is NOT moving forward relative to the fixed ground, then you agree it will not take-off, true? .
But it does. Force is one thing acting upon another. Tires against pavement (car) vs. thrust against air. GT Edit: In the case of this problem, where the force is applied is paramount.
Oh, I completely agree that if that plane isn't moving relative to the ground that it won't take off. I still don't see how the movement of the wheels has anything to do with the plane moving forward? See my funny KY post for reference. GT
The hypothetical question says "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." Since the speed of the wheels is exactly matched, then that, by definition, says that the plane CANNOT move forward relative to the surrounding ground. Now do you see "how the movement of the wheels has anything to do with the plane moving forward"? By definition, the only way the plane can move forward is if the speed of the wheels is slightly higher than the speed of the conveyor belt. .
Gravity of course, which is a constant in all scenarios. But, what is holding it in it's fixed position on the ground? The plane won't move without force and I think we agree on that. In order for force to exist, one thing must act against another. In the case of the car, the force of the tires act against the pavement. If the pavement moves in the opposite direction (like a conveyor) the relative speed of the jeep will be zero (as measured from the ground). In the case of the airplane, the force is not relative to the ground or anything happening on the ground so it will move forward and eventually take off. GT Note: I still stand by my answer a while back; the question does not provide for an answer. But, I wanted to start by at least showing that the conveyor doesn't have anything to do with the plane's ability to move.
I'm not sure that I have any more ammo in the mag. Oh wait... here's one: Why doesn't the Navy use big ass conveyors going the same direction to further propel the aircraft off of the flight deck? GT
I understand that logic but all it takes is even a tiny amount of rolling resistance in the wheels and the hypothetical question's conveyor belt could go fast enough to keep the plane from moving forward. Granted, it might have to be a 6000 mph conveyor belt, but there is a level of belt speed that would keep that plane from moving forward (even though the engine's thrust is in no way connected to the ground). .
