airplane riddle (taken from another board)

[Chris_Ford]

Originally Posted by SteveC

examples

In your first example, I'm going to disagree, however. Your foot lifts off the treadmill and moves forward, this covers the distance that the belt moves your foot back...

And in your rollerblade example... let's insert some concrete numbers...
You start at 0 m/s (in true physics notation)... You accelerate to 10 m/s as the belt also accelerates at the same rate (and at the same time) to the same speed. Net result: 0 m/s. Basically, this is elementary relativity... If we look at the system as a whole, however, there's no net velocity. Which means there's no wind, thus no lift on our hypothetical wing, yes?

Not saying you're wrong or anything, just trying to get an explanation... I might have to find some rollerblades and go on the treadmill tomorrow

[SteveC]

Originally Posted by Chris_Ford

Also, since I mentioned how I don't believe in physics, please explain how a spring gets heavier when you stretch it. That's absolute BS.

I have no idea what you are talking about here. Well, in the first part ("I don't believe in physics") I think I understand what you mean (I have some issues with Thermodynamics, myself). It's the "spring gets heavier" part I've never heard of before.

Okay, I'm going to be the idiot that bites (note: I'm also the idiot that doesn't believe in physics... but that's another post)...

Oh and by the way, those are your words, not mine. I certainly wouldn't be so callous as to call you an idiot on an open internet forum.

[drl5555]

Hopefully you guys will have better luck convincing Chris than I did, we just spent an hour arguing about it in the kitchen. Good luck.

[Chris_Ford]

Originally Posted by SteveC

I have no idea what you are talking about here. Well, in the first part ("I don't believe in physics") I think I understand what you mean (I have some issues with Thermodynamics, myself). It's the "spring gets heavier" part I've never heard of before.

Spring gets heavier: relativity. E=mc^2 E=1/2 kx^2, thus mc^2 = 1/2kx^2. From a physics professor, not me. On that question, my answer was, "No, that's ridiculous. If the answer of (delta)kg isn't 0, I don't believe in physics.

As for the treadmill. The treadmill picks up speed as the airplane is rolling on the ground. This is where the logic hole lies. As thrust from the jet (I'm assuming it's a jet, daggommit) increases, it has to "push" something. The wheels have to push against the belt of the treadmill (because there *is* friction, and I'm assuming there's normal friction, thus the ice example is a slight bit different) So once the airplane starts rolling, the tires *have* to spin, because the treadmill is resisting... Thus, as the tires speed up, as does the treadmill and so on and so on... Someone needs to make a video

[SteveC]

Originally Posted by Chris_Ford

In your first example, I'm going to disagree, however. Your foot lifts off the treadmill and moves forward, this covers the distance that the belt moves your foot back...

Ah, but your body does not move in relation to the ground around you. Yes, you expend energy and you move in relation to the treadmill surface, but you won't feel any wind in your face from moving relative to the earth's surface.

And in your rollerblade example... let's insert some concrete numbers...
You start at 0 m/s (in true physics notation)... You accelerate to 10 m/s as the belt also accelerates at the same rate (and at the same time) to the same speed. Net result: 0 m/s.

But your numbers are wrong. The treadmill does not cancel you from moving forward.

In my roller blade example you are pulling on the handrails, moving yourself forward. The treadmill moving underneath you does nothing except make your wheels spin. If the treadmill moves backwards at the same speed (-10 m/s) that you are pulling yourself forward (10 m/s), your body will continue to move 10 m/s forward relative to the earth around you. Your wheels will be spinning at 20 m/s because of the treadmill moving the opposite direction that you are.

The act of pulling yourself forward by the handrails is the same thing that an aircraft does by pulling itself forward through the air.

Basically, this is elementary relativity... If we look at the system as a whole, however, there's no net velocity. Which means there's no wind, thus no lift on our hypothetical wing, yes?

Incorrect. If you look at the system as a whole you will find a mass of air being accelerated *backward* and an airplane accelerating *forward*. It will move in relation to the ground and in relation to the airmass, there will be a relative wind, and the airplane will fly.

.. I might have to find some rollerblades and go on the treadmill tomorrow

Good idea! It would then become very apparent to you.

Here's the way to do it. Put your rollerblades on, and stand on the stationary treadmill. Hold your feet still, facing straight forward. Trial #1 is to pull yourself forward on the treadmill using your hands on the handrail. Notice how much (little, acutally) force is needed to move you forward. Now, remember that your arms holding and pulling on the handrails is the same as the aircraft's propeller pulling against the air. Turn the treadmill on, and hold yourself still by hanging on to the treadmill. Turn the treadmill speed as high as it will go, then start pulling yourself forward by pulling on the handrails. Notice how the force required to pull yourself forward is basically the same as when the treadmill was shut off? The reason is twofold: 1. you are not pushing against the treadmill surface to propell yourself, and 2. the wheels isolate you from the movement of the treadmill because they can turn freely.

Those same two factors are what allow the aircraft to freely pull itself through the air, completely independent of what the treadmill is doing underneath it.

[Chris_Ford]

Originally Posted by SteveC

Put your rollerblades on, and stand on the stationary treadmill. Hold your feet still, facing straight forward. Trial #1 is to pull yourself forward on the treadmill using your hands on the handrail. Notice how much (little, acutally) force is needed to move you forward. Now, remember that your arms holding and pulling on the handrails is the same as the aircraft's propeller pulling against the air. Turn the treadmill on, and hold yourself still by hanging on to the treadmill. Turn the treadmill speed as high as it will go, then start pulling yourself forward by pulling on the handrails. Notice how the force required to pull yourself forward is basically the same as when the treadmill was shut off? The reason is twofold: 1. you are not pushing against the treadmill surface to propell yourself, and 2. the wheels isolate you from the movement of the treadmill because they can turn freely.

Yes, but in our hypothetical example, the treadmill would speed up as I pulled forward, because the wheels would be going faster.

What I think you guys are assuming is that there is excess thrust.... but there isn't any to be found...

[SteveC]

Originally Posted by Chris_Ford

Spring gets heavier: relativity. E=mc^2 E=1/2 kx^2, thus mc^2 = 1/2kx^2. From a physics professor, not me. On that question, my answer was, "No, that's ridiculous. If the answer of (delta)kg isn't 0, I don't believe in physics.

I don't recognize the second equation. My college education ended over 25 years ago.

As for the treadmill. The treadmill picks up speed as the airplane is rolling on the ground. This is where the logic hole lies. As thrust from the jet (I'm assuming it's a jet, daggommit) increases, it has to "push" something. The wheels have to push against the belt of the treadmill (because there *is* friction, and I'm assuming there's normal friction, thus the ice example is a slight bit different)

Sorry, that is an incorrect assumption. The tires do not push against the belt. If an aircraft required it's wheels to drive against the ground to get it to move, how could an amphibian aircraft ever take off without something to *push against* the water? How would a plane on skis ever get moving?

The jet engine, or propeller (doesn't matter which) reacts against the airmass to move itself. It does not push against the ground. Shoot, if the jet had to push against the ground to get it to move, how in the world does it ever stay in the air?

[Chris_Ford]

Originally Posted by SteveC

The jet engine, or propeller (doesn't matter which) reacts against the airmass to move itself. It does not push against the ground. Shoot, if the jet had to push against the ground to get it to move, how in the world does it ever stay in the air?

Right, I agree 100%. But what I'm saying is that as a byproduct of this thrust being generated, since the airplane is on the treadmill and there is friction, the wheels will turn, just like they turn when you takeoff on a regular runway. As these wheels speed up, so will the treadmill. This keeps the airplane in the same location as it is in the start. If we had an icy treadmill, I'd agree

[therookie]

Originally Posted by Chris_Ford

If we had an icy treadmill, I'd agree

Instead of an icy treadmill assume the frictionless point (or nearly frictionless) is the wheel bearing. Same result.

[SteveC]

Originally Posted by Chris_Ford

Yes, but in our hypothetical example, the treadmill would speed up as I pulled forward, because the wheels would be going faster.

What I think you guys are assuming is that there is excess thrust.... but there isn't any to be found...

There is plenty of excess thrust.

Here is a different way to look at the same problem.

You will agree that a person can move a Cessna 170 over flat ground by pulling it by hand? The force to do so is probably something like 40 or 50 pounds. That force is required to overcome the rolling friction in the wheel bearings. (It takes a higher force to get it moving initially which is overcoming the static friction which is higher, but no longer is a factor once the wheels start turning.) That being true, it would also be possible for a person to be standing out front, holding a rope tied to our airplane-on-a-treadmill and hold it in place while the treadmill moves backward at some slow speed (walking speed for example), yes?

The thing to remember is that rolling friction (such as in a wheel bearing) does not go up as the speed increases. This is the key! Our person standing in front of the airplane, holding a rope with about 40 or 50 pounds of pull against it, can hold the airplane from moving backwards whether the treadmill is moving at 1 mile per hour, or 100 miles per hour!

Given that a C-170 generates some hundreds of pound of thrust, there is plenty of excess thrust and the plane will move forward (in relation to a bystander) no matter how fast the treadmill runs in reverse. The treadmill cannot exert enough force on the airplane to keep it from moving forward. There is not enough friction in the wheels to overcome the power of the engine!

Think about it. If there were enough friction in the wheel bearings to keep the plane from moving forward, a plane could not take off from a non-moving runway.

Quote:
Originally Posted by Chris_Ford
If we had an icy treadmill, I'd agree


Instead of an icy treadmill assume the frictionless point (or nearly frictionless) is the wheel bearing. Same result.

Exactly. The wheel bearing's whole purpose is to reduce the friction between the ground and the airframe. And it does so whether the ground is stationary or moving, with almost no change in the amount of drag transmitted to the aircraft.

[Philip]

Lol.

[Philip]

BTW think of it as if you had a tow truck hooked to the plane, and was just on the ground ahead of it. Same thing.

next week: does putting the gear down really affect Vmc...

[Chris_Ford]

Originally Posted by SteveC

The thing to remember is that rolling friction (such as in a wheel bearing) does not go up as the speed increases. This is the key! Our person standing in front of the airplane, holding a rope with about 40 or 50 pounds of pull against it, can hold the airplane from moving backwards whether the treadmill is moving at 1 mile per hour, or 100 miles per hour!

Agree that mu does not change, the thing is that as the airplane goes faster and faster, so does the treadmill. Regardless of how much thrust is actually generated. This keeps the plane in one location no matter how hard the engines are pushing! If it was able to take off of a treadmill, why don't Japanese airports have big treadmills to launch planes? (Japan is my example because I'd imagine it'd be cheaper to build a large scale treadmill than a whole new island for an airport...

Honestly, I can see how you can argue both ways and as far as I'm concerned, this is why physics is pure and utter crap. It's the religion of sciences

[Doug Taylor]

Probably another way of explaining it, is that the engines pull/push the aircraft thru the air.

Otherwise, if you're taking off into a 50 knot headwind, you'd require a 100 knot TAS in order to get the 55 knots of GS to rotate in a 150!

[SteveC]

Originally Posted by Chris_Ford

Agree that mu does not change, the thing is that as the airplane goes faster and faster, so does the treadmill. Regardless of how much thrust is actually generated. This keeps the plane in one location no matter how hard the engines are pushing!

Sorry. The treadmill can go as fast as it wants and it cannot stop the airplane from moving forward. It can go 100 times as fast as the airplane and the plane will still move forward in relationship to the air (and the ground and our intrepid Mr. Taylor sitting alongside the treadrunmillway in a lounge chair, drinking Margaritas).

The airplane is pulling itself through the air, the wheels spin freely underneath it, and the treadmill can go forward, backwards, or stop and the airplane will still take off. The treadmill needs to be 2000 feet long, or whatever length is needed for a regular take-off, but it cannot stop the airplane from moving.

If it was able to take off of a treadmill, why don't Japanese airports have big treadmills to launch planes? (Japan is my example because I'd imagine it'd be cheaper to build a large scale treadmill than a whole new island for an airport...

Because a 10,000 foot runway on an island is cheaper than a 10,000 foot long treadmill on an island!

Just as the treadmill cannot stop a plane from taking off if it is running backwards, it does not help the plane in taking off if running forward.

Honestly, I can see how you can argue both ways...

But only one way is correct. The other is wrong (your way, in case you weren't paying attention).

... and as far as I'm concerned, this is why physics is pure and utter crap. It's the religion of sciences

I don't work on correcting mental illness. You're on your own with that problem.


Can't help you with that delusion.

[Chris_Ford]

Okay, after consulting with my roommate, we figured out where the difference lied. I was assuming the tire speed was being matched by the treadmill, not the airplane's speed.

Can you confirm that if the tire speed was being matched by the treadmill, the airplane would not take off, yes?

[Philip]

it wouldn't matter. Tire speed and aircraft speed are in no way related.

[Chris_Ford]

Originally Posted by Philip

it wouldn't matter. Tire speed and aircraft speed are in no way related.

Exactly, if the tires are spinning 200 MPH forwards and the treadmill is spinning 200 MPH backwards, the plane is stationary. That's what I got hung up on, I was looking at the tire speed being matched, not the airplane speed. But it makes sense that if the airplane is moving 10 MPH forward and the treadmill moving 10 MPH backwards, the tires are moving 20 MPH, I agree. And that's how the riddle is worded. But I'm asking a separate question.

If the tire speed is matched by the treadmill, the airplane remains stationary and cannot takeoff... I'm fairly certain

BTW Steve, thanks for the patience (although I saw you were losing it towards the end) and remember that I was just trying to get clarification, not arguing over your physics knowledge. Now get cracking on how a spring gets heavier when stretched

[Philip]

Originally Posted by Chris_Ford

Exactly, if the tires are spinning 200 MPH forwards and the treadmill is spinning 200 MPH backwards, the plane is stationary.

No, not if there is thrust being applied to overcome the friction of the tires spinning.. the body moves on... You can spin the tires backwards, forwards, hell, sideways. it's not going to make a whole lot of difference.

[Philip]

BTW this thread went on for 205 pages and is still going where I "borrowed" it.

[JaceTheAce]

Here's the answer and if someone thinks I am wrong, please prove it to me...

While there are the 4 forces of flight which act upon an airplane, the thrust is what is needed to propel the aircraft into relative wind. Without relative wind, there is no possibility of the aircraft to stay aloft. An exception to this is an R/C airplane which with a powerful enough engine can remain inverted while hovering for an extended amount of time. Granted, the propellar on a full scale airplane is a small contribution to the relative wind in order to create lift, it's way too small to make enough wind to cover all of the wing's surface area to make Burnoulli's principle start workin'.

The thrust is there but the treadmill prevents the most important force involved: LIFT. Now, if there is a strong enough headwind that is at or above Vs or whatever speed you choose to stay aloft (without stalling the aircraft) then the aircraft will take off. That's the ONLY way it would.

Without relative wind, air will not rush over and under the wing surface area and not generate lift, PERIOD (period-------->.<-------period)

[Philip]

Originally Posted by JaceTheAce

Here's the answer and if someone thinks I am wrong, please prove it to me...

While there are the 4 forces of flight which act upon an airplane, the thrust is what is needed to propel the aircraft into relative wind. Without relative wind, there is no possibility of the aircraft to stay aloft. An exception to this is an R/C airplane which with a powerful enough engine can remain inverted while hovering for an extended amount of time. Granted, the propellar on a full scale airplane is a small contribution to the relative wind in order to create lift, it's way too small to make enough wind to cover all of the wing's surface area to make Burnoulli's principle start workin'.

The thrust is there but the treadmill prevents the most important force involved: LIFT. Now, if there is a strong enough headwind that is at or above Vs or whatever speed you choose to stay aloft (without stalling the aircraft) then the aircraft will take off. That's the ONLY way it would.

Without relative wind, air will not rush over and under the wing surface area and not generate lift, PERIOD (period-------->.<-------period)

lots of talking, none of it really related to the problem at hand

[JaceTheAce]

Originally Posted by Philip

BTW this thread went on for 205 pages and is still going where I "borrowed" it.

Give us the link LOL

[JaceTheAce]

Originally Posted by Philip

lots of talking, none of it really related to the problem at hand

Please PM me with the answer... would love to hear it.

[skywestseth]

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

Okay it looks like after two pages of this going on the main question was sort of lost....it says in the question quoted right above here ...that the "plane" or aircraft speed would be identically matched by the treadmill...

so the answer is no it will not take off or accelerate....


by jacetheace

Here's the answer and if someone thinks I am wrong, please prove it to me...

While there are the 4 forces of flight which act upon an airplane, the thrust is what is needed to propel the aircraft into relative wind. Without relative wind, there is no possibility of the aircraft to stay aloft. An exception to this is an R/C airplane which with a powerful enough engine can remain inverted while hovering for an extended amount of time. Granted, the propellar on a full scale airplane is a small contribution to the relative wind in order to create lift, it's way too small to make enough wind to cover all of the wing's surface area to make Burnoulli's principle start workin'.

The thrust is there but the treadmill prevents the most important force involved: LIFT. Now, if there is a strong enough headwind that is at or above Vs or whatever speed you choose to stay aloft (without stalling the aircraft) then the aircraft will take off. That's the ONLY way it would.

Without relative wind, air will not rush over and under the wing surface area and not generate lift, PERIOD (period-------->.<-------period)

In my opinion Jacetheace spelled it out right here....