Airplane physics question

Now my friend won't speak to me, as he's all black and blue from falling off thr treadmill.

By the way you're still wrong, because you don't set the treadmill to replicate this. The motion of the treadmill matches the rotating wheels. You are not going to move.

 

Because it does not matter that the legs are powered by a jet engine or by a body attached to it. How the legs or the wheels are powered, or how they are attached to the engine that is providing the power, does not matter. All that matters is that it is providing a force. The force tries to accelerate the body it is attached to.

The first thing to accelerate is the wheel. The treadmill counters it by accelerating in the opposite direction. Wheels spin up, but there is no motion for the rest fo the body. The wheel is allowed to accelerate as much as it wants, in angular acceleration. All the force is expended this way. Because of the large amount of thrust from airplane engines, the wheels will accelerate up to very high speeds, but, if it's sitting on a treadmill that is matching the wheel's acceleration in the opposite direction, the rest of the plane will not move. How can it? You would end up separating the wheels from the plane.

It will take less force to accelerate the wheels yet again than to break the landing gear. So the wheels will accelerate and the landing gear will stay intact. The force does not build up to the level needed to break it, as it gets expended in accelerating the compliant wheels first.

 

Actually, it does, very much so.

 

It's not enough to just say soemthing and then keep repeating it. That won't make it a fact. You have to actually prove it. I have already posted the experiments you can try to prove my statements.

 

It's the same system, the series of events are just carried out backwards. If you had tried it, you would see, that you are wrong. Just for a second, think about your freind on the treadmill. You set the treadmill to 5mph, and hold him in place. The wheels on his rollerblades are moving as if he has a forward velocity of 5mph, but he is stationary, it's the ground that is moving at 5mph. Now if you move backwards at 5mph, we have the very system we are discussing, and your freind getting his groin smashed into the cup holder at the top of the treadmill.

 

I can't prove it to you. You don't listen, nor does anyone saying it will not fly. I have said numerous times that the wheels are FREEWHEELS. They produce no meaniful force in the direction oposite of motion. NONE. They will simply spin twice as fast.

I will try one last example. A hover craft is sitting on the same treadmill. I'm driving and you're sitting on the side with a stick and a wheel. milstanselnino is sitting on the other side, with a similar stick and wheel, and tony danza is at the front with the same stick and wheel. I fire it up, hover it over the conveyer. You, milstanselnino, and Tony, reach your sticks over the edge, and place the wheels on the conveyer belt. I throttle up the engines and begin to move forward. How could your FREEWHEEL on the treadmill which is providing and equal and opposite speed, possibly stop the hovercraft?

 

Art (if I may):
You see, there's the thing: the wheels and legs are not truly powered at all. I too thought it would not fly, for the same reasons you cite. But, I cannot see how my hypo fails. I do admit that I truly do NOT understand the physics of this, but I know from experience what a toy plane will do when suspended from a string. It will "fly." Assume a real plane will do the same (unless it can be shown why it will not).
The string is a substitute for the treadmill. The tread mill and string do one thing only. That is to prevent friction to the point of securing the plane in place. Just as the plane on the string "magically" flies so too will the plane on the treadmill.
Sam

 

Very close. What you are missing is that the treadmill needs to accelerate to 10mph. At that point, you will feel a stronger force on your arms. Your freind will stay put, but his upper body might bend if you pull him by his neck. Keep increasing the speed of the treadmill and keep pulling harder...Something's gonna give...

 

Let's assume you all ate your spinach, wheaties, etc that day. And you could hold on. Would the force ever be greater than the 100,000 lbf that the engines could produce in the opposite direction?

What I am asking, could a well designed bearing ever under normal operation, produce 33,000 lbf of rolling resistance?

 

A plane hanging from a stirng seems to fly even when its engines are not running. The string is an entirely different situation. You can't really use it to illustrate what will happen with the treadmill.

What would be a similar analogy would be if it were hanging from a loop that is rotating backwards, and threaded through a wheel on top of the plane. With no power, the plane will hang from the bottom of the loop. Fire the plane's engines, and the plane will slide forward and up along the loop to a different equilibrium point. With enough power, it will do loops all day long. Rotate the loop backwards, at the same speed as the plane going forward, and the plane won't move in relation to you, even though the loop is whizzing through it and the engines are screaming full blast.

Also, a plane hanging from a string is not flying. It is simply hanging, and being stopped from falling to the ground by the string. Now, if you fire upt he engines, they move it forward. As the plane goes forward, the string pulls it at an angle, so it goes off in a new direction. Again, the string pulls it in a different angle. This is angular acceleration. It results int he plane going round and round in a circle. The engines are providing thrust, not lift. There is no take off. The string keeps it up.

 

It's a theoretical question. In real life, planes are not used this way. Why debate this? The question is simply about what would happen if all the other things were possible, not whetehr its feasible or desirable.

 

It's a theoretical question, but it's totally plausible. It's just a 'why the hell would anyone do it' type of questions. The fact is, a free rolling wheel, could not overcome the thrust of the aircraft. It simply could not.

Lets look at a rear wheel drive car, on a road. And, let's isolate the front wheels for our observation, as they are free rolling. Someone in the car applies the throttle, would the car move forward? of course, because the forward force of the rear wheels overcomes the rolling resistance of the front wheels. Now, put a similar conveyer belt under the front wheels. It can match the wheel speed at any given time. With the application of throttle to the rear wheels, which are still on pavement, would the car still move forward? or would it suddenly be stuck?

 

No, put both sets of wheels, and a hundred other sets, all attached to the car, on the conveyor belt. Accelerate the driven wheels, and accelerate teh belt in the other direction, at the same rate. Does the car move forward? Of course it doesn't. Now, remove the engine from the car, and put on a jet engine. It is not connected directly to any wheel. Fire up the jet engine, and see if the wheels stay at a constant speed as the car changes position. They can't. Tehy accelerate. Now accelerate the belt backwards at the same rate. Do the axles change their position or do they stay in place while the wheels accelerate around them, in place?

 

Yes, but what is happening? The free-rolling wheels are going forward faster than the treadmill under them is going backwards. That is why they are moving forwards. If you sped up the treadmill, then the front wheels would speed up from the "push" in the back, but would not change their position on the treadmill. Their angular speed changes, but their position on the treadmill does not. The force results in a higher wheel speed, and if you keep it up either the wheels would keep spinning up while huge forces build up in the chassis, or the chassis would break.

 

Art, if you can tell me, how a freerolling wheel can overcome the force of an airplanes engine, I'll concede. That is all you have to do.

So, for your arguments sake, we'll have an airplane, with an engine capable of producing 150,000 lbs of thrust, and weighing 300,000lbs. On a normal runway, it'll take off no problem. You just have to tell me, how a conveyer belt can increase the rolling friction in a free rolling wheel so dramatically as to stop the airplane.

 

Art,

You want proof, the proof is out there! I can tell you still haven't watched this video below. It is a crude experiment, but it it is proof the plane will accelerate and fly.

http://videos.streetfire.net/Player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B

Rick

 

It doesn't overcome the force. It uses the force for angular acceleration of the wheels, resulting in circular motion of the wheels. A jet engine produces a lot of force, so the acceleration and resulting speeds will be very high. But that is what will happen.

It doesn't "stop" the plane. It stops the plane from rolling forward. Key difference. It stops the plane from rolling forward by negating any forward acceleration with an equal and opposite acceleration in the conveyor belt. This results in the wheel's rotation speeding up, but no movement of the airplane. You are right about the forces being so large, and that is what makes this problem/solution hard to believe.

If you exert a large force on a small mass, it results in a large acceleration (f=ma). The plane's wheel, in relation to the large amounts of thrust, is a small mass. So it will accelerate hard and spin up to a very high speed. In the real world, this is virtually impossible, and it is even more impossible to think that the conveyor belt can realistically match it at each moment in time. But in a hypothetical problem, this is what is happening. The wheels are spinning up and going faster and faster, dissipating the thrust energy. If they were on static ground, the plane would roll forward, as it does on a runway. But on the conveyor belt, which exactly matches each movement of the wheels in the opposite direction, the plane's position does not change. So there is no airflow on the wings and it does not take off.

Yes, it can all be proven with a skateboard, variable speed fan, and variable speed treadmill.

Believe me now?

 

I saw the video. Nowhere in is the speed of the skateboard wheels matched by the person pulling the carpeting backwards. For some small period, the skateboard's forward acceleration is decreased as the surface is pulled backwards underneath. But that is a laughable example overall.

However, as I said in my previous post, you can make it work on an exercise treadmill.

 

No, I don't believe you, it's not a question of belief, it's a question of reality. You're not describing it. You don't understand the problem, nor physics, and it doesn't appear you're making an attempt to. It's that simple.

So, here you go. The first solution step in a physics problem, a diagram of the forces. If, as you say, the plane does not move, the forces must balance. As an example, we have G, the force of gravity or the weight of the aircraft, and N1 and N2, the normal forces, G = N1 + N2. So, the plane sits on the ground. But, then we have thrust, T. If the plane is not to move, some force, or combination of forces, must produce and equal and opposite force. Balance the forces, show me what forces will balance the force of thrust.
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Here, I looked it up. From force = ((m)*(v^2))/r, we can get
v^2 = (F * r)/ m.

My reference is metric, so I will need to do some conversions.

150,000 lbs = 666,666 Newtons (metric unit of force)

The force acting on the wheels is large. The wheels' mass is small, let's say 50 kg (about 100 lbs) just to make the math easy. Radius, let's say it's 0.5m.

So force = 50 * (v^2) / 0.5 = 10 * v^2 = 666,666 N
v^2 = 66,666 meters per second.
v = 816.5 meters per second.

That's 29,393.87 km/h or 17,636 mph. Pretty fast. This is why it's a hypothetical question. But, if you get a spinning conveyor belt that can match the acceleration and do at least this much top speed, and a plane with wheels and bearings that can do this speed without the landing gear overheating and seizing up, you could do this experiment in real life. I wouldn't stand too close though.

 

I think before you play in the playgorund with 2000yellow360 you need a little english lesson:

No. I don't believe you. It is not a question of belief, it's a question of reality. You're not describing the solution. You don't understand the problem, physics or appear you're even making an attempt. It's that simple.

Proper sentence structure and following the basic double negative rules are very important.

 

That is completely false, and doesn't answer my question. The wheels would not spin that fast. I have read all your posts, some of them twice, and I can't for the life of me figure out why you think the thrust is directly accelerating the wheels. It's simply not the case. It's not.

A typical jumbo jet has a take off speed of 150mph. If a conveyer belt is moving in the opposite direction at the same speed as the aircraft, it would be going 150 mph backwards. So how could could the wheels be rotating at 18,000 miles per hour. It's not physically possible.

The engines thrust does NOT drive the aircrafts wheels directly.

 

Way to go. You corrected my frustrated banter. Good for you. It solves nothing, and makes you look petty and unable to explain arts words.

 

Art,

I actually expected you would attack the video, given it contradicts everything you beleive in this thread. I agree it is a crude experiment, but it does account for the forces involved in the puzzle. Last time, there is no way for the wheel bearings to somehow stop an airplane, since, as the experiment shows, they can't even hold back a household fan mounted on a skateboard! The onus is on you now to show us an experiment that supports your theories!

Rick

 

You are on the right track. Unfortunately, I cannot draw too well in real life, let alone onthe computer. That is why I have described what is happening with the forces about 10 times now, without accusing you of not wanting to hear or understand.
All the forces balance except the thrust.
The thrust pushes the airplane body.
The airplane body pushes the landing gear.
The landing gear pushes the wheel.

The wheel experiences friction at the treadmill surface. If it did not, it would slide. But since it does, it tries to tip forward. It tips forward about the axle, since that acts as a pivot point. This initiates the roll of the wheel, which rolls because it is circular. Now, force creates acceleration in a mass. The mass is the mass that is moving, or accelerating (from rest to moving = accelerating). The moving mass in this case is the wheel. It accelerates. Because of the way it is mounted, with an axle in the middle that is pushing with the airplane's thrust, it does not slide. It rolls.

Ordinarily, on a runway, this would result in a horizontal acceleration of the airplane. On a treadmill accelerating equally in the opposite direction, it results in the wheel and the treadmill surface rolling against each other equally, resulting in a net displacement of zero.

So, what's all that thrust to do? It keeps getting transmitted to those objects that "give". That is the airplane wheel in this case. It keeps accelerating, with the treadmill doing a correspondingly equal acceleration in the opposite direction. The airplane's acceleration with respect to the air around it is still zero. So, no airflow over wing, no lift forces that cause the airplane to rise, no takeoff.

If the wheel bearings were to overheat, expand and sieze, the force would be transmitted to the next thing that "gives"...this would probably be a shear in the landing gear. That's why I don't recommend standing too close.