Physics Question for you.....

Has anyone ever done a Google search on this problem? I just did. Hundreds of sites have debated this (one has close to 1,000 posts?) including a physics site, an airliner site, and others. Guess what? Their conclusions are the same as here:

1. Yes, it takes off
2. No, it doesn't

Oh well. I guess I'll go with the Mythbusters since that is apparently the only "test" done on this problem using a real plane.

Lots of folks posted their own videos with scale models;

http://www.youtube.com/watch?v=IZGdU...eature=related

And I still like cecil's explanation;

http://www.straightdope.com/columns/060203.html

[cabinetman]

I've been reading this post since it started. All I can say now is ....


I HAVE A HEADACHE!!!

Paul

How about: Dee-Dee-Dee.
.

I've been reading this post since it started. All I can say now is ....


I HAVE A HEADACHE!!!

[williamr]

Are we forgetting about gliders? When the tow cable is released, gliders don't drop straight to the ground.

Some of you Canadians will remember the commerical jet about 15 years ago in which there was a mix up in liters/gallons for the American pilot. He ordered gallons/ they put in liters. The plane was somewhere near 30,000 feet or so and the engines quit - fuel starvation. No Fuel. They glided 50 or 60 kilometers and did a dead stick landing safely at an abandoned airfield where there was a lot of Sat/Sun afternoon crowd, jsut missing them.

Gimli Glider...

http://en.wikipedia.org/wiki/Gimli_Glider

Interesting... but

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Will

[williamr]

The seaplane on a river is not really a good experiment. The effec tof the current on the pontoons would create much more drag than the wheels a regular airplane experiences.


I'll wager that both would show the plane moving at 9-10 mph. Meaning the conveyor speed has little effect on the planes relative speed.

Gotcha!

Correctly formed pontoons are planing hulls. A planing hull has low drag -- otherwise speed boats would not exist. :-)

Going upriver would mean high initial drag, but the plane would become "on-plane" sooner.

A smooth lake is another issue... :-) But rivers almost always have some chop...

It's just a sub-case of your original problem. And the friction is only nominally higher than wheels or skis -- once the plane is "on-plane".

You have to first solve the equation on the fluid medium of air. Then look at other drag sources -- and you know that. :-)

Are you sorry you asked the original puzzle? lol

It is a lot of fun tho...


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Will

[Russianwolf]

The seaplane on a river is not really a good experiment. The effec of the current on the pontoons would create much more drag than the wheels a regular airplane experiences.


A skiplane would be a better experiment since the wheels would be totally removed from the equation, butagain, they produce more friction/drag.

But the mythbusters experiment could be effect if not perfect. If they measure the distance the plane needs to take off at 100% throttle, then compare it to that needed to takeoff at 100% with the conveyor moving (speed of the conveyor is actually irrelevent). If the needed distance is substantially greater, then the doubt of a matched speed takeoff would increase. If the distance didn't substantially increase hen the match speed takeoff would be proven likely.

Another way to do the same thing.
1)Start the plane at taxiing speed 10mph let's say. Then start the conveyor. What speed is the plane doing relative to the ground now?
2) start the conveyor, then start the planes engines to the rpm needed to taxi at 10mph. What id the speed of the plane relative to the ground now?

I'll wager that both would show the plane moving at 9-10 mph. Meaning the conveyor speed has little effect on the planes relative speed.

[williamr]

How about a seaplane on a river with a really strong current going in the opposite direction. How easy will it be for the seaplane to take off? How much more power does the plane need to overcome the river current? Any seaplane pilots out there to give a definitive answer and put this whole matter to rest?



With the MythBuster guys, the "conveyor belt" did not match the speed of the plane. What they should have done, to approximate the hypothetical, is to have the "conveyor belt" - which looked like just a very long piece of cloth - hooked up to a similar plane with the same engine as the test plane, but going in the opposite direction. Then, with the equal thrust, one can probably get similar accelerations. That would have been far more convincing to me than using a truck and a plane.

In New Westminster BC -- on The Fraser River-- there is a seaport for light planes. They normally take off in the centre of the river and are considered a motor vessel while in the water. :-) IOW they have to follow the same rules I do in the water when I cruise past --- under sail, canoing or stink-potting. They are not required to clean my drawers when they pass with in a foot of the cabin. Not even when the fiberglass whip vibrates... ROW depends on who has the most maneuverability...

Like any other pilots, they prefer to take off into the wind. They prefer to be airborne rather than waterborne.

Yes there is more drag on the pontoons than on the wheels of an airplane.

Yes they have to overcome the drag of the river -- upstream or down. Planes with pontoons on during the summer season are slower than when they fly with skis in the winter -- or wheels if they are "down south" near the Great Lakes. Ideally you want to take off downstream -- into the wind -- and with prevailing westerlies they almost always have their wish. The pontoons create a lot of drag in the fluid called air.

Correctly designed pontoons form a planing hull. Thus defying the "dreaded Bernoulli's law". Sailboat hulls remain in the water and are captive to the dreaded law. Speed of an "in-water" hull is clearly definable, and the speed is thus a function of the length -- the longer the hull, the faster the boat could be -- IOW bigger engines won't help beyond this point.

Speed boats are "planing hulls" -- water wings if you will...

So, the faster the current, the sooner the plane will get the pontoons "on plane" -- defying the dreaded Bernoulli and the sooner the plane can reach flying speed.

So yes the water is a "big conveyor" and yes they do overcome it. And with all the extra drag.

When you look at a problem like this it is helpful to think of the "problem space". In this case you are dealing with an airplane in a fluid (air). The question is can the airplane archive flight speed in air -- despite any other "outside" influences.

You have to "draw the box" around the correct system. You can of course draw boxes around sub problems (sub-systems) of the complete problem space. The contact with ground or water is a sub-space of the entire problem space.

Rockets lift off easiest near the equator... Higher rotational velocity of the earth... There is a lot of related stuff you can think of. (Pardon me -- of which you can think.)

Hope that helps Anna. The rest is just small stuff... :-)

I will leave it to mpc to provide any math. His post is correct -- regardless of what we would like to think. This is the best I can do without a few boring equations -- and It's Sunday -- my day of rest. No math permitted on Sundays.

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Will

[niki]

Lets make an experiment.
Take your car, remove the engine and the gear so the wheels can rotate free forward or back.

Put your car on the conveyer. Put some barrier behind the car just not to let it go backward, the barrier will be anchored to the ground, not to the conveyer.

Turn the conveyer to 500 MPH.
Because the wheels can move free, they will rotate at 500 MPH but the force backward will be only the tire drag + bearing friction, I think that you can stop the car from moving backward only with your hand (lets say that you need 100 lbs to stop the car moving back)

Now, on the roof of you car, install an engine with propeller or jet engine or a rocket than can produce 10,000 lbs of thrust (it will not have any connection to the car wheels that still can move free).

Go back on the conveyer, turn it to 500 MPH, get into the car and start the engine/jet/rocket to maximum power.

To overcome the tire and bearing friction you need only 100 lbs, the rest 9,900 lbs are pushing the car forward off the barrier. Of course the wheels speed will increase but you are pushed and accelerated forward.

Another experiment.....
Take a matchbox car, tie a string at the front and tie a fish-scale to the string.
Put the car on your belt sander, switch it on, and measure the force required to hold the car in place, let's say that you measured 2 lbs, this force is the product of the tire and bearing friction.
now, increase the belt speed, you shall still read 2 lbs (maybe a little more) but the tires are turning faster.

Now, install on top of the car roof some propeller or jet engine that can produce 3 lbs or more. 2 lbs of thrust will be required to overcome the tire/bearing friction (without any connection to the tire speed) and the rest of the thrust will produce forward motion relative to the ground, or more correct, to the space.

The wheels, are only medium between the plane and the belt, they are not tieing the plane to the belt and are free to rotate. They produce some drag force, but ones we overcome this drag force, the plane is like in the free air.

niki

[Anna]

Three forces:
1. Down (GRAVITY, weight of the plane).
2. Forward (Engine thrust).
3. Backward (Conveyer).

Is the forward motion cancelled by the backward motion?

Don't forget normal forces (perpendicular to surface).

[Anna]

Just think how a sea plane takes off. It relies entirely on the propeller to pull it through the air to gain enough speed to take off. There are no little outboard motors strapped to the pontoons to give it thrust. A regular airplane needs wheels only to roll and not to drive. It also relies only on the propeller or jet engine to move forward. It does not matter what the conveyor belt is doing as long as the wheels are free to rotate.

How about a seaplane on a river with a really strong current going in the opposite direction. How easy will it be for the seaplane to take off? How much more power does the plane need to overcome the river current? Any seaplane pilots out there to give a definitive answer and put this whole matter to rest?

It's true that the plane on a tarmac needs some way to thrust it forward. Whether with a propeller or jet engine, it doesn't really matter. The thrust moves the plane on a regular tarmac. It can roll, like on wheels, or it can slide/skid, like on pontoons. Either way, in normal circumstances, the plane moves forward.

As it moves forward, with constant thrust, we get constant acceleration so that we go from zero speed to a critical speed that allows the plane to take off.

We all agree on that, I think. So on to the controversy with the conveyor belt.

Now let's say we have a thrust that normally allows for an acceleration of 1 m/s^2. Starting from rest, in 1 second, the plane would have traveled 0.5 meters (that's from x = x0 + v0t + (1/2)at^2). The conveyor belt, in that time, also travels 0.5 meters in the opposite direction (say to the right).

Result: To someone standing on the tarmac, the plane is in the same position it started with and has not moved. To someone standing on the conveyor moving to the right, the plane just moved 0.5 meters to the left.

After the next second, the plane would have normally moved a total of 2 meters. But so did the conveyor. Again, to an observer on the tarmac, the plane still has not moved; to an observer on the conveyor, the plane moved 2 meters to the left, with an instantaneous speed of 2 m/s. (That's from v = v0 + at.)

So, depending on who's doing the observing, the plane is either moving or it's not.

The key then is the air flow around the plane. On a still day, an observer on the tarmac will see a wind speed of zero. To a guy on the conveyor belt moving to the right, the wind speed (after 2 seconds) will be 2 m/s to the left.

What about the plane? 2 seconds after we started the exercise:

To an observer on the tarmac, the plane is not moving. The wind is also not moving. So the plane sees no motion of the wind.

To an observer on the conveyor belt, the plane is moving to the left at 2 m/s. The wind is also moving to the left 2 m/s. Since the plane and the wind are moving with the same speed, in the same direction, then an observer on the plane sees no motion of the wind.

We can keep incrementing time, we can change the values of the acceleration, and we'll still get the same result. There is no relative motion between the plane and the wind.

If there is no wind, as far as the plane is concerned, there is no lift. No lift, no takeoff.

With the MythBuster guys, the "conveyor belt" did not match the speed of the plane. What they should have done, to approximate the hypothetical, is to have the "conveyor belt" - which looked like just a very long piece of cloth - hooked up to a similar plane with the same engine as the test plane, but going in the opposite direction. Then, with the equal thrust, one can probably get similar accelerations. That would have been far more convincing to me than using a truck and a plane.

[tjmac44]

Three forces:
1. Down (GRAVITY, weight of the plane).
2. Forward (Engine thrust).
3. Backward (Conveyer).

Is the forward motion cancelled by the backward motion?

[williamr]

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.

I have watched many of their shows -- seen them get the model wrong -- then correct it later. Sounds like good science to me.

Sometimes they get corrected by their viewers and re-look at and re-think a problem. Again -- good science.

Mostly they get the model right. That impresses me! They display a good research model and a good ethic.

Wish I had worked with more people of their attitude.

$0.02 worth only.

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Will

[williamr]

A plane is standing on 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?

For anyone still having trouble -- air is a fluid. Just like water -- yes -- a touch thinner it is true.

The plane and its engines -- with all their thrust -- work inside the fluid media. The conveyor (and wheels) are a red herring -- other than a wee touch more drag.

For those of you arguing with mpc -- relax! -- if he's wrong he won't have a job Monday. :-)

(Come to think of it -- neither will I now. :-) )

15 pages?


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Will

[billwmeyer]

Ok, The gist is that the wheels have no bearing on the thrust of the plane. I agree with that. Therefore the conveyer belt is only responding to the wheels. The power plant of the plane, actually pushes the plane off the conveyer belt and thus onto land and then to the air, because this is the only way the plane gets lift.

I guess my question would be, how fast are the wheels on the conveyor belt turning when the plane hits dirt. Would the wheels be going so much faster than the plane at that point that the force of the wheels hitting dirt would in turn force the nose of the plane into the dirt? If that happens the plane doesn't fly. Either way, I will not be on the plane.

Bill

[DaveSur]

Just think how a sea plane takes off. It relies entirely on the propeller to pull it through the air to gain enough speed to take off. There are no little outboard motors strapped to the pontoons to give it thrust. A regular airplane needs wheels only to roll and not to drive. It also relies only on the propeller or jet engine to move forward. It does not matter what the conveyor belt is doing as long as the wheels are free to rotate.