Physics Question for you.....

[cabinetman]

Here's My Answer To The Question:

When I was a little kid, and when I say little, maybe I should have said young. I wasn't little by the physical definition of little, as in small. Anyway, I was always impressed by demonstrations, and some had a lasting impression.

So, where was I. When I was young, I saw a demonstration on TV. It was a black and white TV, very prevalent among households that even had a TV. I think in my neighborhood, there were only three or four homes with a TV. As I never really took a neighborhood poll about how many TV's there were, that figure could have varied.

The demonstration I watched was a guy in a tuxedo at a dining room table. It was a fairly large table, maybe five or six feet long, and maybe three or four and a half feet wide. I'm sure it was wood, because I could see some of the apron and legs, and it was dark in color. Now as an adult, and an avid woodworker I have some knowledge about species of woods, and as I think about it as I'm writing this, it was likely Walnut or maybe Mahogany, but I'm not positive, because it was a long time ago.

That said, it had a very smooth table cloth covering the top, that looked like a linen or cotton type in pure white. Now, I say that about the fabric, because I'm not sure if permanent press was invented back then, and with all physical properties of the table cloth taken into consideration, it may or may not have a bearing on the outcome of the demonstration.

On top of the table cloth there were what were called "place settings". I say "place settings", because as a young child, that was the terminology my mother used for the dishes, silverware, glasses, and cups and saucers set up in a nifty arrangement for each person who may be sitting at the table in the hopes of getting a meal. There were four of these "place settings", along with some bowls and serving dishes, salt and pepper dispensers. All were matching pieces in a type of porcelain or ceramic (I always get those two confused), except for the glasses, which appeared to be crystal, but in reality, maybe not.

So, this guy in the tuxedo walked up to the table, and stood at one end, and facing the table, grabbed the table cloth at each corner, but now that I think more about where he actually grabbed the table cloth, it could have been shy of the corner something like six or eight inches. I can't remember...it was a long time ago. But, as for the demonstration, it could make a physical difference in the outcome, maybe not.

He slowly lifted the edge of the table cloth to what appeared to be table top height or slightly higher. Then after a moment of what appeared to be extreme concentration, snapped the table cloth to his rear, and abba kadabra, removed the entire table cloth from the table top without upsetting the place settings. They didn't budge one iota. To say the least, I was very impressed.

Sometime after that revelation, our family had company over for dinner. We had a nice wood dining table, pretty much like the one I saw in the demonstration on the black and white TV. It was probably Walnut or Mahogany, I can't really remember, but I can tell ya that it was a heavy one. My mother had prepared the table with a white table cloth, could have been linen or cotton. I don't think it was permanent press, because as I previously said, I doubt that permanent press had been invented. On top of the table cloth, my mother set up "place settings", just like the demonstration on TV.

Here's where it gets very interesting. As the company and my parents sat down at the table, I had a brainstorm. As I looked at the table, I thought to myself that it looked exactly like the demonstration I saw on TV. Now, I didn't want to spoil the surprise to everyone, because I didn't know if any of them had ever seen such a demonstration. Announcing my intentions would have definitely removed the element of amazement to the outcome of the demonstration.

So, after patiently waiting for the best opportunity, which was my mother leaving the end of the table to go to the kitchen to get some salad. I'm pretty sure that was what she was after. We usually started our meals off with a salad. Now, the only problem I could forsee was the fact that I wasn't wearing a tuxedo. I didn't know if that detail would have a bearing on the outcome of the demonstration. I remember thinking any discussion about not wearing a tuxedo would have been an instant clue that the surprise of their life was about to take place.

I walked over to the end of the table, just like the guy did on the black and white TV. I stood for a moment at the end, and verrrrry inconspicuously grabbed the table cloth at about the same place as the guy did on TV. I took a very brief moment to employ the same extreme concentration as the guy did on TV. Then, I snapped the table cloth to my rear as fast as I could.

Believe it or not, something in my demonstration went wrong. It didn't happen like it did on TV. Instead of the table cloth disappearing and leaving all the place settings as they were, a unique phenomenon occurred. All the dishes and glasses went flying. I was very disappointed.

I did receive an education on physical demonstrations, and one heck of a whooping.
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[Anna]

Sorry, can't help it. Have to respond.

Lets assume the wheels are the famous massless frictionless bearing surface (these come from the same catalog as massless inextensible strings, frictionless blocks, pulleys, and surfaces, etc.)

In our thought experiment the airplane is sitting quietly on the conveyer with nothing moving. Now without turning on the engines, start the treadmill. The wheels will start spinning, but plane STILL won't be moving because you have not exerted any lateral force on it F=MA, with no force, you have no acceleration.

If the system is truly frictionless, i.e. the treadmill surface and the wheel's bearings, then the wheels will not move at all. Why? The wheel turns because of static friction. When a car is driving on the road, the speed at the point of contact between the wheel and the road is zero. Or more accurately, the speed of the wheel at the point in contact with the road has the same speed as the road. When the surface is frictionless, then you have the same situation as when you are skidding on an icy road. That is, the wheels are not turning at all.

In a world (like ours) with friction, the plane will in fact start moving back due to friction on the wheels, but not at the same speed as the treadmill, and the smoother the bearings, the less movement there will be. So, there would be a small force vector pointing toward the back of the plane (a small amount of force in the negative direction).

Well, in the real world, if the static friction force is large enough, the plane will in fact move as fast as, and with, the treadmill. That is, the relative velocity between the plane and the treadmill will be zero. When the plane does move with respect to the surface, it's due to inertia, usually when the treadmill suddenly starts (cf object on truckbed, truck suddenly accelerating or decelerating).

Also, the plane starts moving with respect to the treadmill when the horizontal force exerted on the plane exceeds the static friction force. As long as the static friction force is greater than the thrust (which is still zero above), then the plane has to move with the treadmill. Maximum static friction force, by the way, is pretty much the weight of the plane times the static friction coefficient. So with a big plane, this force is huge. The treadmill will have to go really really fast to overcome this force.

Now lets look at the props or the jet engine. It's forcing mass (air) toward the back of the plane. Thanks to conservation of momentum and our buddy Newton's laws of motion we know that an equal and opposite force will be pushing the plane forward at whatever amount of thrust the engine generates. So now we have a large amount of thrust (the normal takeoff thrust) in the positive direction.

F=MA if the sum of the small amount of negative force and the large amount of positive force is positive then we have a net positive force and MUST have the airplane accelerating.

I think that "moving forward" in this hypothetical means moving with respect to the surface that the plane is standing on, which itself is moving backwards. So relative to the ground, the plane is still not moving.

It's true that if there is a net force, then the plane will start accelerating. But the treadmill is so smart that it will also accelerate in the opposite direction. The accelerations cancel each other (they're vectors after all), and the plane still ends up being stationary when observed by someone standing on the tarmac. Not moving, no air flow, no lift, no takeoff.

Actually, this should be sort of easy to test in the gym. We know that when we go running, at fast enough speeds, we can feel the air past our faces in a nice breeze. Has anyone tried to run really really fast on a treadmill and generate the same breeze? Better yet, go running with a paper plane in your hand. Run fast enough, let go, and the plane "flies." Now do the same thing on a treadmill. I kinda think the plane will drop like a rock.

One counter example is a wind tunnel. Plane is not moving, but it is flying.

[Kristofor]

The answer is still the same as it was on 12/9/2006.

Guys.... Think back to your basic high school physics/mechanics and draw up a force diagram.

Force = Mass x Acceleration (F=MA)

Lets assume the wheels are the famous massless frictionless bearing surface (these come from the same catalog as massless inextensible strings, frictionless blocks, pulleys, and surfaces, etc.)

In our thought experiment the airplane is sitting quietly on the conveyer with nothing moving. Now without turning on the engines, start the treadmill. The wheels will start spinning, but plane STILL won't be moving because you have not exerted any lateral force on it F=MA, with no force, you have no acceleration.

In a world (like ours) with friction, the plane will in fact start moving back due to friction on the wheels, but not at the same speed as the treadmill, and the smoother the bearings, the less movement there will be. So, there would be a small force vector pointing toward the back of the plane (a small amount of force in the negative direction).

Now lets look at the props or the jet engine. It's forcing mass (air) toward the back of the plane. Thanks to conservation of momentum and our buddy Newton's laws of motion we know that an equal and opposite force will be pushing the plane forward at whatever amount of thrust the engine generates. So now we have a large amount of thrust (the normal takeoff thrust) in the positive direction.

F=MA if the sum of the small amount of negative force and the large amount of positive force is positive then we have a net positive force and MUST have the airplane accelerating.

Since velocity = acceleration * time, and we've just shown that the plane is accelerating its velocity is increasing when it reaches takeoff speed, the plane will in fact take off.

The reason these aren't used on aircraft carriers is because as you just saw above it takes just as long (time and distance) to reach that velocity, the treadmill only serves to put extra mileage on the landing gear.

Adding in questions about ambient wind speed doesn't change the above. It can change the relative ground speed values and thus distances needed to obtain flight, but otherwise it's irrelevant.

Likewise whether the design of the wing leads to lift is generated by the pressure differences from the Bernoulli principle or conservation of momentum by air being deflected downward by the lifting surface is also irrelevant. In either case the lifting surface must be moving relative to the air and it clearly will be.

Kristofor.

I watched the Mythbusters show and it did appear to me that the plane was moving forward, ie, that the "conveyor belt" wasn't quite matching the plane's speed. Not sure about that though.

A lot of the "experiments" that they do make good shows but in many, many cases are far from the reality they are "testing," with many variables left out or poor models used.

[Anna]

I'm jumping in late in this discussion, too, and sorry if this has been mentioned before.

For the plane to take off, there has to be a relative velocity between the plane and the air around it. You can have a dead plane (engine not running) "taking off" (although erratically and unstably) just because the wind speeds are high enough (see Santa Ana Winds).

You can also have a "moving" plane not taking off, e.g. when the plane tries to take off in the same direction as the wind blows. In the latter case, if the plane runs at 100 mph and the wind is blowing at 80 mph in the same direction, the airspeed is only 20 mph, which is probably not enough to let the plane take off. (Planes tend to take off against a headwind for this reason.)

If the plane is held stationary by a treadmill, then no matter how much thrust or propeller speed you put out, as long as the threadmill matches the speed of the plane's wheels, there will be no lift, i.e. there is no movement of the air against the wings. You'll pretty much get the same lift if you run a box fan on the ground, or tying the plane down (zero relative velocity to the ground).

With the MythBusters video, there were too many uncontrolled variables. For one thing, what were they really proving? Were they trying to match the speeds of the plane and the "treadmill"? There were no measurements of the speeds, and on a windless day, there's a chance that the plane could have taken off because the "treadmill" wasn't matching the speed of the plane, allowing it to achieve critical velocity anyway.

Since wind velocity (speed and direction) is a critical factor, did they measure it for that day? Was it a headwind or a tailwind? If it's a headwind, the plane needed less ground velocity because the headwind would have taken care of much of the lift. So even if the treadmill did match the plane's ground speed, a strong headwind could still have helped to let it take off.

As a real-world scientific experiment, the MythBusters exercise was very poorly controlled and, therefore, does not really prove anything conclusively.

(Oh, and sorry for being pedantic, but the V in PV=nRT stands for "volume" not velocity.)

[Alex Franke]

I'm unfortunately late to this conversation, too, but in the posts that I did read, there seems to be a lot of mention of this air over/under thing and that the air travels faster over the wing due to its shape, and that in effect sucks the wing and the plane up. I don't think that this is where the majority of the lift comes from, though. I think it's the angle of the wing mainly -- as the plane accelerates, the air meets the bottoms of the wings. The wings push in a more downward direction against this air, and so the air pushes back against the wings in a more upward direction. If it's only the shape of the wing that provides the lift, then how could planes fly upside down?

The conveyor belt experiment is interesting, though. There's not a lot of friction involved with rolling a wheel as opposed to sliding without a wheel, so a conveyor belt would have to go *insanely* fast in order to counteract the thrust of the plane and keep it still to the outside observer. But if it could, the plane would remain essentially in the doldrums with no wind to help it lift or fly. Take the wheels off the plane and you have a much better chance of accomplishing this, though. Put one of those water planes aiming upstream on a rushing river, for example.

I didn't see the Mythbusters experiment, but I imagine that it appeared that the plane was moving forward just as any plane would at take off. If so, then the experiment only proves that planes can also take off on ice, or in a strong headwind, or in any case where the thrust of the engines can overcome the friction between it and the ground.

So sorry if all this has been said before... Just wanted to throw in my opinion on it.

[LinuxRandal]

Thanks for that link. However that is not what the question that was asked.

Could a plane take off IF the rolling runway matched the forward speed of the plane. In that video it did NOT.

MYTHBUSTERS DID NOT DISPROVE THE MYTH! The Plane on the video went faster than the rolling runway!

I feel vindicated!

Maybe I am an idiot here (ok yes), but the myth I heard, NEVER involved the engine running. It was based solely on the old air over/under the wing statement. The engine helps give it push or pull and was supposed to be out of the equation (I guess more like a glider of old, but heavier).

[Wood_workur]

I don't think the wheels will be doing 200 mph. If it were a car and being driven by the wheels. If the wheels didn't spin (0 mph) the plane would move 100 mph in reverse -- but since the plane is stationary they'll be moving at 100mph. If the plane moves forward at 1 mph then the wheels would be spiing at 101 mph..

the plane is moving at 100 mph of a surface going 100 mpg against it. 100+100=200. the plane will take off normally, just with super fast spinning tires.

[leehljp]

PLANE ON THE TREADMILL

Mythbusters did the airplane on the treadmill bit last night. The airplane took off with a normal takeoff roll.

http://boortz.com/more/video/mythbus...eyor_belt.html

Thanks for that link. However that is not what the question that was asked.

Could a plane take off IF the rolling runway matched the forward speed of the plane. In that video it did NOT.

MYTHBUSTERS DID NOT DISPROVE THE MYTH! The Plane on the video went faster than the rolling runway!

I feel vindicated!

[leehljp]

Jim,

Actually the question is a "loaded" question, IMO. According to Myth Busters, it can and did. I AGREE with you 100% in reality.

Those that say yes it can, and along with Myth busters, - "flying" versus "Take Off" are two different things.

I used to fly control line model planes in the late 50's and through the early 70s. As you probably know, those kind of engines produced enough power to take off straight up. I made one of my own design with the wing area to include 1/4 of the area as flaps. That plane never rolled, It jumped of the ground every time if I had the flaps down at all.

The point of that and the "take off" is that IF the plane has a powerful enough engine to suck and push its way upward off the ground regardless of the wind speed over the wings, it will do it.

The reality is that today's normal planes and jets cannot. Even fighter jets can't do that with all of their power. They have to have a catapult to get enough speed to let it fly in a short time because the jet engines cannot generate enough power to make it fly from a standstill. It takes forward wind speed INTO the engines to help generate enough power to make them fly, even with afterburners. ONCE enough forward speed is achieved, engine power can make some jets go straight up, but not from a static start. AND a rolling tarmac/runway is a static start.

(Rocket assist take offs are a different story.)

Back to the experiment - It will take off on power alone if the power is enough to keep it moving in its appointed direction. This is NOT flying.

What most people don't realize is that the rolling surface and wheel speed is a "miss-direction" in that people focus on this aspect of it, as I did. The backward rolling surface and subsequent forward motion produced by the prop or jet are irrelevant, IMO. It is the power of the plane or jet to pull itself up and keep it on that trajectory until controlled flying speeds are met. IF there is enough power, (power to weight ratio) it will, but not because of a backward rolling tarmac and generated forward rolling speed.

You would never do this with a 747 without some kind of warp drive.

[JimD]

I cannot tell what mythbusters did but the only thing that matters is the velocity of the air across the wing. I did not look at all the posts but I did not see anybody say this correctly. The reason there is lift is that the product of the pressure and velocity of ideal gasses is constant. The ideal gas law is normally stated at PV=nRT but what that means is that if the moles of material and it's temperature stays the same, PV stays constant. Air is not an ideal gas but is close enough that this is still true for most purposes. Because the air moves further to go over the wing than it does going under it, the pressure under the wing is higher and there is lift generated. As has been stated, the lift is a function of the square of the velocity (regardless of what units the velocity is stated in, all that changes is the units of the pressure change).

If you believe the airplane moves relative to the air, then lift will be generated and if the velocity of the air relative to the wing is high enough, the plane will take off. If the airplane is stuck in one place due to the conveyor, it will not fly.

The function of jet or propeller engine(s) is to force gas in the opposite direction of what the desired plane direction is. The result is forward motion of the airplane. If the engine also moves air over the wing, it could increase lift but that is not the primary purpose of the engine and it will not make a stationary plane lift off.

Hover craft can lift off just from thrust because they direct the engines at the ground. Regular aircarft direct the thrust the direction the plant wants to move away from and will not lift the plane other than by moving it thorugh the air (increasing it's velocity).

I am a mechanical engineer is anybody cares. I can dig out the books and give you all the equations but what I've just said will not change.

Jim

[annunaki]

Plane will Fly

PLANE ON THE TREADMILL

Mythbusters did the airplane on the treadmill bit last night. The airplane took off with a normal takeoff roll.

http://boortz.com/more/video/mythbus...eyor_belt.html

[Wizzett]

Harold Pendell PE, Design Engineer

The plane will move and will be able to take off.



Reason for this is because the planes engines work by applying thrust against the planes mass and do not apply physical drive to the wheels that are in direct contact with the runway.



This means that Newton's Third Law Of Motion applies which states that for any action, there is an equal and opposite reaction. (i.e. thrust from the engines exhaust in the aft (backward) direction will cause a proportional reaction in the forward direction, thus moving the plane forward).



Because the plane is reliant on airspeed to take-off (approx 150 knots for a commercial type passenger aircraft) the plane will move forward regardless of what speed or indeed in what direction the conveyor moves.



If the same was done to a car which applies physical drive to the road wheels, the car would remain static regardless of how fast the wheels are traveling.

[crokett]

The plane will fly.. period...

Yep. Coming late to the discussion but I was formerly confused and thought it would not fly. It is real simple. The plane's props/jet exhaust is pushing against the AIR NOT THE conveyor. Therefore, the plane will move. Therefore, there is airspeed over the wings and when it gets fast enough the plane will fly.

[Ed62]

Obviously, whether or not the plane takes off will depend on whether or not the chicken crossed the runway. And if he did, we want his name.

Ed