Airplane physics question

I am not saying they are spinning that fast when a jumbo takes off at 150 mph. I am saying that's how fast they would be spinning at max thrust wiht a conveyor belt spinning under them at the same speed, in the opposite direction, if the engines were at maximum thrust.

When an airplane takes off, the thrust is powering all that weight against the wind, as well as rotating the wheels at 150mph.

 

I said it was fine, but not scientific. You have two different speeds, you will get movement relative to each other. To make it more scientific, put it on a variable speed treadmill and put on a fan with variable speed (which will allow you to control the thrust). You will see that you can make the skateboard seem to stand still at different speeds even though the wheels are spinning. Standing still means no airflow. No airflow means no lift. No lift means no take off.

 

It's doing the exact same thing here. The only difference, at a 150mph aircraft groundspeed, you would have a conveyer moving at 150mph in the opposite direction. Why would the addition of a conveyer suddenly cause the wheel speed to jump 2 orders of magnitude?

 

That's what I was trying to say. The original question was butcherd by someone. Conveyers moving the speed of light, and bearings blowing up was not what the question was intended to make you think about.

 

Oh man, I don't believe you guys have sucked me back into this thread!

You guys are arguing two different questions:

That is NOT what the original question asks. Go back and read the opening post:

That is a completely different question. Now play nice, and don't make me come in here again!

 

Because the airplane's mass is not being accelerated. Only the wheel's mass is, and it is two orders of magnitude less than an entire airplane. f=ma. If you decrease mass by orders of magnitude, and apply the same force, guess what happens to the acceleration...

 

Right. And the question as originally asked is a conundrum. An impossible situation that can't be answered, thus it has no answer.

Art, you have over-analyzed. We are not going to allow you to charge us your standard hourly rate for the time you have spent on this problem.

 

No. No. No.
(If the plane doesn't move, as you have stated, the wheels don't turn. And you can't say the wheels just barely start to move, so that then the conveyor starts to move. Not even a little, teeny, tiny bit. Because you have then violated your statement that the plane does NOT move. To your thinking, if you were to jack up the plane front and rear so that the wheels were off the ground, and then applied engine power, the wheels would turn)

 

Thanks, brilliant. I simply told everyone what the question was supposed to ask. The original question is garbage.

 

That's not right Art. Not at all. You're assuming the airplane is not accelerated, not proving it. You have to prove it, not just say it isn't, so here's what would happen to the wheels. Which, incidentally, is not what would happen to them.

 

Maybe you are right, but your version of the question is so simple it's just boring.

So the question becomes, what "gives" on the system described? And I stand by my answer from 'way back on page 6....

 

If the conveyor is moving as the same speed as the plane's wheels, the plane cannot be moving. If the plane isn't moving, it isn't getting any air over the wings. If it doesn't get any air over the wings, it isn't going to fly. Doesn't get much simpler than that.

Art

 

Yes, it most certainly can. But, it's clear someone is going to have to do an experiment to prove this to some.

 

Would you be able to do the skateboard, fan and treadmill experiment?

 

I think I know what you are saying here Art, you are equating this situation to a car sitting on a dyno with the conveyor (dyno rollers) matching the speed of the wheels. The car body (plane) does not move.
The disconnect between this and the plane problem lays in the fact that the wheels are not driven by the plane, they simply follow it down the runway. Therefore the wheels cannot spin UNLESS the plane moves forward, which invalidates the original question.

 

Nope, it's not invalid. If the plane could accelerate, and yet have its wheel speed matched by the treadmill moving in the opposite direction, it would be an invalid question. It would be invalid because that situation calls for the wheels to have two speeds relative to the belt:
1. The same speed as the treadmill because the question states it as a given.
2. A higher speed because it is moving up along the treadmill which is moving in the opposite direction, according to all the people who claim it will accelerate up the treadmill.
We know this to be impossible. In one frame of reference, it can only have one speed. How can it have two speeds relative to the treadmill?
Since it is a given that the wheel speed matches the treadmill speed, the plane will sit there happily, while increasing amounts of thrust energy finds its way to the wheels and moves them faster and faster. The conveyor, according to the given in the question, always matches the wheel speed in the opposite direction. So the plane does not go anywhere. And the whole situation can be simulated with a fan, skateboard, and exercise treadmill. So this is not what makes the question invalid.

What does make the question invalid is that in real life, no wheel could spin up to 18,000 miles per hour and have a conveyor match it at every moment, in the opposite direction. Something would give...in a very big and nasty way.

In the experiment I am asking you (anyone) to do, the situation created is different to a car on a dyno. Since you all seem to think that the wheels being driven directly somehow make a big difference from wheels being driven indirectly, I have shown you how to set it up so that the wheels are not directly driven. Rather, something (the fan) is creating a thrust force on the skateboard just like a propellor or jet creates a thrust force on a plane with free rolling wheels. It is all sitting on a treadmill which is moving in the opposite direction. I bet you it can be made to stand still while the plane/skateboard's engine (the fan) provides thrust, and then provides a higher level of thrust as you increase the fan speed. If it sits there in one spot, that means there is no airflow over the wings.

Mathching opposite speeds of belt and wheel mean zero linear speed for plane.
Zero linear speed means no movement of the airplane relative to the air.
No movement relative to the air means no airflow.
No airflow means no lift.
No lift means no take off.

Simple as that.

 

Also, I have seen biking athletes train on rollers with both bicycle wheels rotating freely. Would they sell these if the bike accelerated and jumped off the front of the rollers? As the bicycle rider powers up and increases his speed, the rollers increase their speed in the opposite direction. The wheels speed up but the bike does not move forward. This is the same situation.

I know that in this situation the bike's rear wheel is directly driven by the chain powered by the human engine, but it absolutely does not matter. Even if there was a jet strapped to the rider's back, or to the bicycle's seat, the thrust would be translated into rotational motion on the wheels and the biker would not move.

Every time I have seen this, I have also seen a fan pointed at the rider for cooling. You think he would need that if he was moving against the air?

So, you can also do the experiment with a bicycle or tricycle (should be easier to balance) on a set of rollers, with a fan or small jet strapped to it. I just think the exercise treadmill and fan and skateboard will be far easier to access for most people. To get decent results, you will need a fan with a continuous speed control, not a notched one that brings the power up in "steps" unless you can increase the treadmill's speed in the same steps.
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Art,

You (plane) get on the bike in an upright sitting postition. I will put my hand (jet engine) on the middle of your back and push. I guarantee you will go forward right off the rollers.

 

Nope, the wheels can spin without the plane moving. Just as your rollerblade wheels can spin while you stand still on a conveyor belt. They are not directly connected to the engine (you) except through the landing gear (roller blade bottom assembly) as well. Exact same situation. Let's see what is really happening:
Your thrust pushes the wheels on your roller blades. If you had cross country skis on, they would slide. But you have wheels on, pinned at the center by the axle. If they were in the air, they would move forward without rotating, as it is easy to push past air. But if their bottom is touching the ground, frictional forces cause the bottom of the wheel to "stick" or grip. There is a great thrust force acting on them with little to no opposing force, so they tip forward. This happens continuously because they are circular, so it causes them to rotate around the axle.

On level ground, this results in you moving forwards. On a treadmill moving in the other direction, you stand still in relation to the air. If you push harder, you move up the treadmill, because you have increased the wheel speed to greater than the treadmill speed. If the treadmill were to speed up as well, you would not move up the treadmill, and you would stay standing still relative to the air around you. Simple dynamic equilibrium.

 

Only if the treadmill does not accelerate in the opposite direction. You push and someone speeds up the treadmill an equal amount in the opposite direction, and I will stay sitting happily in the middle.

You simply cannot push me off the front of the treadmill without having my wheels go faster than the treadmill going in the other direction.The question says that the wheel speed and treadmill speed are always the same, but in opposite directions. If the speeds are always the same, there is no linear acceleration. No linear acceleration means I do not move along the treadmill.

 

Think of it this way. The force of the pedal-driven chain could cause the rear of the bike to squish up the bike's frame. But it doesn't, because the wheel "gives" about its axle first and the force is expended in rotational acceleration of the wheel first. If you exert an additional independent force on the same rigid body (the bicycle) it will also be transmitted to the weakest component that can "give" under the additional strain of the new force. This is the bike's wheels again. So they accelerate more.

Now, on the other side, the rollers were rolling backwards to match the wheel speeds. As a result, the bike seems to stand still in relation to the ground and air. If the wheels accelerate, and the rollers accelerate an equal amount in the opposite direction, the same situation will still hold.

 

Alright.............. somebody needs to buy/steal/somehow aquire a bunch of treadmills and a radio controll airplane so we can solve this thing once and for all.

I'm also gonna call Maranelloman to find out what his position on the subject is

 

Can I have some of what you're smokin?

 

Go do the experiment on the treadmill. If the skateboard can be made to stay still under additional thrust, you all owe me a good lunch.

I will use my plane to fly out to each of you for the lunch, but I will take off from a regular runway, since my plane works on real physics principles, not junk science.