Where are the engines fastened? To the airplane! The propellers push the air, the air pushes back. The propeller wants to go forward, ergo there is a force acting on the propeller. Equal and opposite reaction. That force is transferred through the engine to the plane, thus there is a force acting on the plane. Moving it forward. The friction between the wheels and the ground act the opposite way, but since that force is tiiiiiiiny - the plane moves forward.

The "wind" over the wings at the back of the propellers does not create enough lift to make the plane take off. There needs to be a certain degree of wind over most of the wing surface for that to happen.

Flight simulators is great fun, but you cannot use them to test physics. Of course, a headwind of about 140 knots (+/- 20 knots), a 737 could be lifted off the ground while standing still (depending on weight and model). EVEN if it is placed on a conveyor belt moving the same direction as the wind at the same speed, and the plane was kept from moving backwards without stopping movement in any other directions).

I really, really, really, really cannot for the LIFE of me understand how this concept eludes people - even those who claim to be physicists! Newtons laws, folks! Equal and opposite forces, the sum of all forces and so on.

It does not matter where you put the engine(s) - as long as they are connected to the plane, they WILL move the plane regardless of how fast the wheels are turning!

And I am not a physicist, nor a pilot. However, I got an A on my physics exam when I studied engineering. And I know a lot about aircrafts (especially the boeing 737 series).

This one made me laugh:

"Change the propeller for a jet engine in which you also, don't have any airflow through the wings (place the engines outside the wings). If the engines allows you to reach a ground speed at which the airplane should take off, I bet the airplane will not move... not even an inch above the ground."

Dude, a propeller or a jet engine does the same thing - they create a force acting on the plane!

Besides, that quote is completely stupid. ALL jet powered planes has the engines "outside the wings". And still, hundreds of thousands of passengers get to their destination every single day! It's a conspiracy!

And when the air moves over an airplane's wings at the takeoff speed for that aircraft, the aircraft WILL take off.

There is, nor will ever be, a situation where a plane won't take off from a conveyor belt. Ever. Get a treadmill and try it yourself! Model aircrafts works on the exact same principles as their big siblings.

There will be no airflow on the wings, then the airplane will not get off the ground.

There would be airflow over the wings.

You seem to lack a fundamental understanding of the purpose of the propellers. It's not to throw air over the wings. It's to pull the plane forward through the air (which is how the air gets "over" the wings).

Go to any flight sim and put a wind aligned with the airplane, the same speed as the take off speed and see what happens. You can do this without the engine working and brakes ON. Will it fly? Why is it flying? ground speed? engine? wheels? Nope, only airflow through the wings.

Change the propeller for a jet engine in which you also, don't have any airflow through the wings (place the engines outside the wings). If the engines allows you to reach a ground speed at which the airplane should take off, I bet the airplane will not move... not even an inch above the ground.

What happens on the video is that the propeller is pushing air though the small portion of the wings and tail surface control parts that allows the plane to take off with the lowest speed it can have (on the air) to take off.

I'm no pilot, I like planes and I'm an experimental physicist. No equations just facts. I would like to see it happening with the other engine configuration.

Best regards,
Gzo

Forces involved:
-thrust from the propeller, Tp
-friction between the wheels and the conv.belt, Fw
-gravitational force (plane's mass times gravity constant) G
-ground force pushing on the plane, N
-lift generated by the wings, Lw

Vertical speed = (N + Lw) - G

Horizontal speed:
Tp - Fw

Fw would be neglible since wheels are made to roll as easy as possible; that means with as little friction as possible.

Here's the key: if the speed of the conv.belt is the same as the plane's speed through the air, the friction will be ca. Doubled. That's the ONLY impact a conv.belt would have. And since the friction is neglible in the beginning, if it is doubled it's still neglible. The plane will use a slightly longer distance before takeoff, but it can't be accurately measured due to variables such as wind, weight, temperature, uncertainty in the measurements and so on.

Do not over-complicate this. It is physics, not math. Physics tells you that it is impossible to cut a donut so that 360 people get an equal piece each. Math says it is okay - one degree of donut per person.
All powered planes depends on thrust from a propeller or a fan (ignore rocketpowered ones, they are not "normal" aircrafts) to move forward, be it through the air or over the ground (two sides of the same coin, really). Thus, if an airplane was placed on a super-long conveyor, the brakes full on and the belt speeded up to, say 200 mph - the plane would go bacwards with 200 mph. As soon as the brakes are released, the plane would SLOW DOWN because of drag. Sooner or later, the plane would travel backwards at a constant speed. Then start the engine and hit full power. The plane would slow down, stop, then move forward and eventually take off.
The only significant occurrence will be that the wheels and the bearings would be warmer than normal since the wheels would turn twice as fast as normal.

The thrust from the propeller determines the plane's forward movement. The resistance from the wheels is insignificant and can never stop the plane from moving forward. As a matter of fact, on a lightly loaded Boeing 737 you need to give a small boost with the engines to start rolling, after that you need to brake every now and then in order to taxi at a safe speed.

A plane cannot take off vertically unless it tries to take off in a hurricane. Theoretically, you could do it even with a 747, but it would not be a good idea...
It needs air to move past the wings at a certain speed, and that can only happen if the plane moves forward.

About the catapults on carriers: the use them because the carriers aren't long enough to house the necessary length of runway. The use wires as brakes too, or the planes cannot land.

The original question was: if a plane is placed on a conveyor belt that has the same speed as the plane - but in opposite direction - will the plane take off?

The answer will allways be: yes.
At least if we keep the experiment within possible limits. Light speed on a conveyor belt is impossible, therefore a moo point. (reference to Friends)

And thisnis the physics of this conundrum. Nobody can conspire against physics and win.

Still, a "PhD in theoretical physics" argued against this. He or she should ask for his/her student loans back.....

And I do not have a degree in physics, nor am I a pilot. But i know a lot about planes, and I got an A on my physics exam when I studied engineering (dropped out because i lost interest in the field I studied). Therefore I was puzzled when I saw the show the first time. Still am puzzled by all the conspiracy theories out there. People are ignorant, and in a LOT of cases: stone cold stupid. Cudos to the blogger for not being PC about it. A PC world is insane; let us just say it in stead of polishing a turd in case someone is offended. If you are offended, I say my observation was actually spot on!

Ps: to you PhD's out there overcomplicating this: this is NOT theoretical physics. It is PRACTICAL physics. The frame of reference can only be between the plane and the earth. No matter what you cram in between. What's in between will only determine a small factor in a bigger picture.

Waiting for the flame war to commence...

… one could not believe a plane (a small one no less) could have a liftoff with what appears to be such little speed.

I believe take-off velocity was only 25 MPH. I too was unaware that planes could take off at such low speeds, but like you said, it was all perfectly normal. Except that it made me want to buy and learn to fly a small little kit plane that can take off at 25 MPH!

On second thought, remote-controlled planes take off at reasonable speeds. There's no reason, I guess, to suspect that a scaled model (same lift to weight ratio) wouldn't take off at the same speed.

After seeing the episode myself i too was confused,especially about the plane's liftoff.

But the Mythbusters could have explained this test a little better themselves because from seeing this test one could not believe a plane (a small one no less) could have a liftoff with what appears to be such little speed.Apparently it was all just very normal.

It looked like all forces of nature were applied from the conveyor belt onto the airplane untill the airplane took off.
Atleast that was what i was thinking or wanted to believe.
Think of a runway where a Piper, F-16 or jumbo jet would throttle after a few meters smoothly on a speeding treadmill before take-off;
Maybe for some blue smoke coming from accelerating tires everything would look the same for each individual airplane.

The purpose of the experiment was not to keep the airplane stationary on the treadmilll and then see if it takes off. The purpose of the experiment was to determine if the moving treadmill would be able to keep the airplane from taking off. At no time did Mythbusters ever mention any intention to determine if a stationary airplane can takeoff. Anyone who objects to the Mythbusters experiment on these grounds is just making stuff up. This is called a Straw Man argument.

Clearly a stationary airplane (relative to the surrounding air) cannot takeoff. Mythbusters proved that the moving treadmill was incapable of keeping the airplane from moving relative to the surrounding air. This was its only intention.

I've had to do much the same on FaceBook (but that is because I discuss politics on FB only).

Erik -- thanks for keeping this thread alive as it has provided a consistent spot of entertainment every few months.

Sad understanding of basic physics ........ the wheels will spin at EXACT speed .... either on the conveyor belt or on the runway.

Wrong. If the plane's moving 25 MPH in one direction and the ground - the conveyor belt - is moving 25 MPH in the opposite direction, the wheels will be spinning, at the moment of takeoff, at twice their normal RPM on a normal liftoff from a standard runway.

sad said on February 18, 2012:

The definitive proof of how stupid you really are. You are an idiot.

Wrong. The "P" is a force, but pound-force per square inch is not a measure of force. It's a measure of force over an area. Big difference.

Martin said on February 18, 2012:

You're blocking people who disagree with you?

Yep. After awhile, arguing with idiots who think 1+1=3 gets old.