Well, the runway is the conveyor belt.
I really meant exactly what I said:
The friction could be anywhere in the equation, either:
Of course, if the friction in #2 is great enough, it’s the same as #3.
I was waiting for someone to say that an ultra light airplane engine may not have as much pulling force on the atmosphere as an adult man pulling on a rope. And it may not.
The real point that I wanted to make is that pulling something with free spinning wheels across a moving level surface will require very little more force than pulling the same thing across a non-moving level surface.
Put the ultra light airplane on the 75 mph backward moving belt with the engines off with only its wheels free spinning. It too will back up against the wall. Now, attach a rope to the front of it. You could put a loop in the end of the rope you are pulling on and just pull it off using your little finger. Real easy.
Put an empty little red wagon on the 75 mph backward moving belt and you could pull it off of the front with a kite string.
There will still be people that don’t understand this, but to me it is real simple. There is not a moving belt that can be built that can hold back an airplane of any type.
The problem w/ you solution is that it depends on a power source external to the plane/treadmill. Think of a boat sailing up river against a 10 knot current w/ a 10 knot following wind, it gains no headway, now compare that w/ a boat on a canal w/ mules pulling it. The air moving though our planes propeller is, like the river, fluid, not fixed. Even if there was a current in the canal, the mules, being independent ot the water, could move the boat forward, after overcoming the friction of the water.
While the sailboat could continually “tack” across the river to gain momentum, if the plane tried that the belt would speed up to impede forward movement.
No, look at it this way. Imagine that you are on calm waters in your motorboat with a 10 knot headwind. The maximum speed your boat can go is 10 knots. Are you saying that the motorboat would be still against the 10 knot headwind? The headwind would be acting like the belt. Only it would actually have a little better ratio of force compared to the belt. But the motor for the boat is acting on the water, so the wind would not be able to completely cancel out the force of the propeller in the water. Just like the belt has minimal effect at canceling out the force of the airplane engine.
It is the same thing as the car on the belt. If you want the power source to be on the vehicle, then suppose that the rope is attached to a tree in the front of the belt. The car has no windshield and someone is sitting in the middle of the front seat. Do you think that they could not pull the car forward on the belt? Think about it. It is the same thing. If you don’t understand that, then I guess explaining this to you is hopeless.
What? Of course it could given a strong enough headwind. In fact, given a strong enough wind it could sail against the current. How else do you think sailing ships navigated rivers?
I will consider your analogy, just let me point out that I’m here to debate the question
at hand, right or wrong. Your last sentence was insulting, arrogant and completely unnecessary.
That is why I said only 10 knots. Even 20 knots probably could not hold it back.
The reason I say it is hopeless is because it seems there are so many people that cannot make the disconnect between the free spinning wheels and how an airplane moves itself.
Those who believe that the airplane will not move forward and fly just need to learn how forces are transmitted. It is a simple thing.
Too many people are just trapped into a mental illusion and just do not get it.
No insult was intended. But I guess that I am done here.
The only insult you have undertaken here is against yourself. The speed of the air compared to that of a river is largely irrelevent. All that matters is if I can generate enough force from the air to counteract the force from the river. The only difference between a 1 knot wind and a 10 know wind is the size of a sail that I need.
The airplane will or will not fly depending on your interpetation of the problem. If its the linear speed of the center of mass of the wheels then the plane takes off with almost no difference. If its the speed at the edge of the tire then that plane goes absolutely nowhere. The force opposing the engines is transmitted from the treadmill, through the axle to the body of the plane.
How about a small RC plane? How about a rubber-band powered child’s toy? Would that make the problem different? (I note the original question said nothing about the scale of the plane.) Be careful of absolutes.
Here’s the issue: If a plane can generate eough power to only go 75 mph, most (almost all) of that power is used up combating aerodynamic drag. So the 75mph is 75mph air speed, not speed relative to the ground. In your scenario, the plane would be moving at 75mph relative to the belt and still have plenty of horsepower left over to accelerate.
Somebody mentioned a sea plane on a moving river. So let me rephrase the question. Replace the treadmill with a flowing river. Picture one plane on a lake and another on the river. Will the flowing river keep the seaplane from moving fwd. No, and neither will the treadmill keep the wheeled plane stationary. The two seaplanes would both take off the same way with no regard how the water beneath them is behaving.
Anyone who thinks the plane would be stationary needs to think again. It would not take a lot more thrust to overcome the effect of the treadmill on a wheeled plane.
Oh? What if the plane were, for example, a scale RC model?
What? Certainly a strong enough current could keep a sea plane motionless in reference with the ground.
Well, like I said. It appears to be hopeless for some people to understand this. That is not an insult, that is just a statement of fact.
The bottom line is that Cecil is correct.
But if Cecil cannot explain it, what good can I do?
Some of you that do not yet understand may eventually get it if you can release your mind to think correctly about it. But it has been my experience here and at other places that if you do not understand it after a little bit of explaining, it seems like you can never understand it.
I will state it one more time just because I want to. In the problem as usually stated, the belt is not even trying to hold the plane still. The plane will move forward and fly. It really is very simple.
I know completely how you feel. Its even worse when you are right and the other person is spouting incorrect information.
Eh. You’ve pretty much ignored everything I had to say, so I don’t feel particularly insulted.
And very, very few people disagree. Except perhaps for the “as usually stated” part.
Christi7df, answer me this. We have a plane with engines that produce a combined force (Fe) and a total tire mass m. Assume for the sake of the problem that the engines are completely horizontal at all times. Now, at the same instant you flip on the engines I turn on the treadmill with a constant acceleration of 2*Fe/m. Where does the plane go?
A. R. Cane, you seem to be one of the few people on here who really doesn’t get it.
There are two interpretations of the scenario. One is that the belt moves backwards to match the aircraft’s forward speed, this is the interpretation you seem to have taken. The answer to the problem interpreted this way is very simple. The aircraft’s engine applies a force to the air which is not connected to the belt. The belt applies minimal force to the aircraft. Therefore the aircraft moves forwards and takes off as usual.
There are two faults in your scenario. First, it is airspeed that is limiting to an aircraft not groundspeed, moving the belt backwards does not limit the aircraft from moving forwards. Second, although an aircraft would move with a conveyor belt if you moved the belt gently, it doesn’t take much force to get the aircraft moving, and indeed, if you tried moving the conveyor belt suddenly, the aircraft would not move with it.
In your scenario with an ultra-light capable of moving at 75mph. The belt will wind up to its max speed of 75 mph. The ultra light applies enough power to stay stationary. At this time, the aircraft is stationary relative to the ground, stationary relative to the air, and moving at 75 mph relative to the belt. You are saying that 75 mph is as fast as the plane can go so it won’t take-off. However, the plane is limited by airspeed not ground speed. At this time the airspeed is 0, therefore there is plenty of excess power available to move the aircraft forwards up to 40 mph airspeed at which time it takes off. It also happens to be moving at 115 mph relative to the belt but that is not relevant.
The other interpretation basically allows the belt to move as fast as necessary to prevent the aircraft from moving. It would seem that a constantly accelerating conveyor belt does apply a force to the aircraft, and that there is an acceleration value that could keep the aircraft stationary.
Evil Captor, twin engine propellor driven aircraft gain no significant lift from the prop slipstream over their wings. If they did, then they wouldn’t need much runway to take-off, and running the engines up to full power with the brake set would make the aircraft light on the ground. It doesn’t, to any significant degree.
It is transference of force that matters. Very little can be transferred through the wheels to the airplane unless the brakes are on.
The belt would have to be moving at a speed many orders of magnitude greater than the take off speed of the airplane to hold the airplane still. That kind of belt is impossible to build.
Go back to my car on the belt. How much force would you need to apply to the belt to prevent someone from pulling the car off of the belt? And it really does not matter that the force is being applied from outside the car. Force is force.
If you want to apply a force to an airplane that can hold it still attach a cable to the tail of the airplane and then attach the cable to a large tree. Assuming the cable is strong enough and the tree is strong enough, that will hold the airplane still.
And remember, the belt is supposed to be moving at exactly the same speed as the airplane is moving forward. Forward movement for an airplane is forward movement through the atmosphere. So, it does not matter how much force the belt is producing it is not even trying to hold the airplane still, so it will move forward.
Now if a moving treadmill could hold back an airplane, why not replace aircraft carrier landing decks with treadmills. Get rid of those tail hooks and cables. All that they would have to do is have the belt moving in reverse of the jet speed at a speed a little slower than the jet landing speed. After all we wouldn’t want the belt moving at exactly the landing speed or that would probably kill the pilot when the jet comes to a dead stop as soon as it touches down on the belt. Right?
Why don’t you just answer my question?