Originally posted by: JonTom
More to the point - can a floatplane take off from a moving river?
Originally posted by: JonTom
why would the flow speeding up change anything?
Originally posted by: Horus
Ok.
Let's say the maximum speed of the plane is 300mph. You place the aircraft on the belt, and set it in reverse for 300mph. With the engines off, the plane will fall off the back of the belt rather quickly.
Turn on the engines, put them to full power. The speed of the plane FORWARD is 300mph, but that's being counteracted by the speed of the belt BACKWARDS of 300mph. Include the friction of the wheels, and you see that there is NO WAY FOR LIFT TO FORM UNDER THE WINGS! Therefore it cannot take off! Simple physics.
Originally posted by: JonTom
More to the point - can a floatplane take off from a moving river?
Originally posted by: beansbaxter
Alright, this is NOT a homework question. In another thread on the Anandtech forums, I saw this mentioned deep within a thread but it was never addressed. I am still curious so I ask it here:
You have an airplane on a conveyor belt - when the airplane moves forward, the conveyor matches its speed in reverse. Can the airplane take off?
Originally posted by: Horus
Ok.
Let's say the maximum speed of the plane is 300mph. You place the aircraft on the belt, and set it in reverse for 300mph. With the engines off, the plane will fall off the back of the belt rather quickly.
Turn on the engines, put them to full power. The speed of the plane FORWARD is 300mph, but that's being counteracted by the speed of the belt BACKWARDS of 300mph. Include the friction of the wheels, and you see that there is NO WAY FOR LIFT TO FORM UNDER THE WINGS! Therefore it cannot take off! Simple physics.
Originally posted by: JonTom
More to the point - can a floatplane take off from a moving river?
Originally posted by: Aflac
Originally posted by: dkozloski
The conveyor belt is of little consequence. It's just noise in the problem. The only way it would come in to play is if it's too short for the aircraft to reach flying speed. The velocity of the aircraft relative to the ground is of no consequence. The only thing that matters is the speed of the aircraft relative to the air. The wheels and/or tires may be oversped and fly apart by contact with the belt. This is no different from an aircraft being launched from an aircraft carrier except for the relative speed of the belt. There really is no serious question here.
Excellent answer.
Think of this situation in another form: you're standing on a treadmill, wearing roller skates, while holding onto a rope attached to a far wall in front of you. Ignoring friction, while you're standing on the treadmill, the wheels counteract the pull of the treadmill itself, so that you stay in place. But when you start pulling on the rope, you begin moving forward. This is akin to the airplane's engines "grabbing" ahold of the air and pulling the plane forward. The conveyor belt has no way to counteract this force and thus the plane will move forward, allowing it to take off.
It is erroneous to assume that the plane does not move just because there is a conveyor belt in the equation.
Originally posted by: Howard
I forgot that the wheels are free-spinning...
Originally posted by: kpb
Originally posted by: Shaftatplanetquake
Originally posted by: kpb
The problem with a plane is that the movement is generated by the jet engine or propeller and is not dependant on the wheels at all.
I disagree. Unless the plane doesn't have wheels (one of those crafts that is stored on a body of water) then the plane is completely dependent on the wheels for moving forward if it is grounded. If you take the wheels off the plane either doesn't move at all or moves completely unpredictably.
No it's accurate to say that they don't depend on the wheels for movement. The fact that you can use skys, or floats or what ever just proves that. The wheels on a plane are like the wheels on a roller skate. They are not hooked up to any motor and can not move the plane forward on thier own. They do allow it to easily roll but it's the jet engine that actually causes the motion to occur.
Originally posted by: TheoPetro
I used to respect HT...now with so many of you saying it doesnt take off im kinda losing that
Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.
Originally posted by: smack Down
Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.
That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.
Originally posted by: Born2bwire
Originally posted by: smack Down
Originally posted by: dkozloski
It makes no difference what the conveyor belt does. It is irrelevant. The belt can go forward, it can go backward, it can start and stop, it can alternate back and forth, and it makes no difference other than being an annoyance.
That is simpley wrong. By definition if the treadmill is going backwards at the same speed the wheels are turning then plane can't go anywhere.
Why? The plane's only contact with it's wheels are free rolling bearings.
Originally posted by: jagec
Originally posted by: dxkj
Correct, a small amount of energy is used to rotate the mass of the wheels. but wouldn't that energy be generated by the super fast super powered tread mill, and not by the plane? As long as the friction is zero on the axles the wheels could move infinitely fast with the tread mill, and the plane wouldnt move at all even under 0 power.
edit: IE, the wheels have a frictioned connection to the conveyer belt with the rubber touching rubber, but the ball bearing connection to the plane is frictionless, thus the energy and angular momentum provided to the wheels comes completely from the surface the wheels are touching.
If you draw the FBD, you find that the conveyor exerts a linear force on the base of the wheel, and the engines exert a linear force in the opposite direction through the axis of rotation. This creates a torque. If the force of the engines is not balanced perfectly by the force of the conveyor, you'll get linear as well as angular acceleration. This would be the case in any real-world scenario, since it's nearly impossible to exert the same force on the wheels with the conveyor, as you can on the plane with the engines. F=ma; when you're working against the large mass of the plane, that's fine. When you're working against the small rotational inertia of the wheels, the acceleration term becomes exceedingly large.
But in magical physics land, where we can accelerate the conveyor as fast as we bloody well please, we're able to equalize the forces. Thus, there is no net lateral force, just a huge torque that's spinning the wheels faster and faster, until the plane runs out of fuel.
Even with frictionless bearings, the wheels are able to exert a lateral force on the plane through the angular momentum term. That's the secret. It's easier to think of a massively weighty wheel on a treadmill...if we exert no additional forces on it, the wheel will begin to slowly turn and slowly move backwards at the same time when we turn the treadmill on. If we add a force through the axis of rotation, we're able to keep the wheel in one place, but it builds angular momentum that much faster.