Can inexperienced 'MythBusters' really fly commercial aircraft?

reply to post by sheetrockerr


An airplane needs whatever it takes to get forward movement to provide lift. In the case of the LC-130 it's skis and wheels. In the case of the seaplane it's floats. You CAN land an LC-130 in Antarctica with just wheels, but you're not going to have as much control over it as with skis. They have landed C-5s and C-17s down there with just wheels.

Lift is generated by the wings movement through an air mass it has nothing to do with the wheels on the aircraft. Planes have lifted out of their chalks when parked at the gate, and the wind has been high enough to generate enough lift to overcome the weight of the aircraft. However, If you put the plane on a conveyor belt that turns at a high enough speed causing insufficient forward thrust to generate air movement under the wing, then not enough lift is generated and the weight of the aircraft will not be overcome. Thus it will not take off.

Originally posted by defcon5
However, If you put the plane on a conveyor belt that turns at a high enough speed causing insufficient forward thrust...

There's your problem. Unlike a car, the thrust of a plane is not generated/transfered at the wheels. The wheels on a plane are free rolling, and have no relation/connection to the thrust.

A conveyor belt has almost no affect on the surrounding air; so a spinning prop will still pull a plane forward. Thus generating airflow around the wings, and lift.

Think of it like this. If you put a 30' conveyor belt in front of a tree, then tie a rope around that tree, and hop on the conveyor belt with a skateboard underneath you. You can pull yourself forward, using the rope, even if the conveyor speeds up in the opposite direction. Why, because your thrust is not being generated/transfered by the wheels. Your thrust is entirely independent of the conveyor, and your wheels are merely "free spinning". Likewise, a plane would still move forward, generate lift, and take off.

reply to post by redmage


It’s going to end up being a technical problem for them to run the test. If the conveyor belt is too slow, the plane is going to generate enough thrust to take off anyway. They are going to have a very difficult time balancing the turning of the wheels and the movement of the conveyor belt. If they use an RC plane, its going to be even worse as RC aircraft have such a low weight that they have incredibly high acceleration which the belt is not going to achieve. I think the plane is going to beat the belt every time based on acceleration.

Originally posted by defcon5
They are going to have a very difficult time balancing the turning of the wheels and the movement of the conveyor belt...

There's a bit of a "catch 22" there. A plane's speed is not measured by its wheel speed. It's measured by the air speed around the plane. So, if a plane is not moving forward, its "speed" is virtually "null" (barring any natural light-breeze, or wind), and the conveyor would basically be standing still.

[edit on 12/8/07 by redmage]

reply to post by redmage


Right, so if you had a perfect balance the plane would not take off, unless you had an engine powerful enough to takeoff with a zero groundspeed. Some of your fighter planes have engines powerful enough to stay aloft in a complete stall.

reply to post by Zaphod58


Zaphod, I guess I should not have trimmed my original post down.

The point I'm making is that the wheels do not provide any thrust for take off. The same situation for the seaplane. The floats are only a platform to allow it to float on the water. They do not provide power for the movement of the craft. Same thing with the ski's on the C130. Again, they are just a platform to support the aircraft.

People argue about the conveyor belt under the fallacy that the wheels are providing power to get the plane into the air, which they are not. They are a platform to support the aircraft while on the ground.

Originally posted by defcon5
Right, so if you had a perfect balance the plane would not take off

The problem is that a conveyor belt, physically, can't "balance". The thrust is input from an entirely independent system, and the wheels are free rolling; so the conveyer cannot slow/hold the plane at all.

reply to post by sheetrockerr


Then I guess we're on the same page, because that's the point I was going for too.

Originally posted by SilentGem

That would be accurate if the aircraft used its wheels to produce its thrust. Alas, an airplane is propelled through the air via a propeller or jet engine which is not directly connected to the wheels. The wheels are free rolling, which means if the conveyor belt is moving in at the exact speed of the aircraft’s forward motion in the opposite direction, all that will happen is that the wheels will rotate twice as fast (and probably rupture due to friction), however the aircraft would take off as normal.

I.e. if you put a car on the same conveyor belt, the car will in fact, remain in one spot because the cars propulsion is directly linked with the wheels that are in contact with the conveyor belt. This is not true for an airplane.

Understand?

[edit on 6-12-2007 by SilentGem]

So, are you therefore saying that this vehicle would fly off the belt?


www.conceptcarz.com..." target='_blank' class='tabOff'/>


The purpose of an airplane's propulsion system, be it propeller or jet thrust, is to move the aircraft's wings through the air at sufficient speed to generate enough lift, via the bernoulli(sp?) effect, to loft the craft into the air.

If, for whatever reason, there is not sufficient airflow over the wings (say, no forward motion, for example) the plane will not fly.

reply to post by Zaphod58


My apologies, I misread your post.

Please forgive this one line post.

It doesn't help that I have a head cold this weekend and am all fuzzy headed. I THINK I'm making myself clear most of the time, but sometimes what I'm thinking and what I'm typing don't exactly match.

The only way a conventional plane would be able to take off from a conveyor belt moving in the opposite direction would be like this:

Get one huge conveyor belt.

Now let's say the plane requires a takeoff speed of 150mph.

Set the conveyor belt to 50mph.

Apply enough thrust so that the wheel speed reached 200mph.

Actual airspeed would still only be 150mph.

Barring any limitations on the equipment used, a conveyor belt could counter all forward-thrust by increasing speed extinguishing airflow over the wing surfaces which is required to provide lift.

reply to post by jackinthebox


Nah.
I think red mage nailed it. Because the movement is based on the engines thrust acting upon the air, if you throttled up to an airspeed of 150 knots, and had the belt moving backwards at 150 knots, the wheels would be turning at airspeed + conveyor belt speed or 300 knots (or whatever that translates into RPM), and the plane would take off like normal. Therefore, in your example, the wheels would be turning at an RPM equivalent to 200 mph.

reply to post by defcon5


If your belt is moving backward at 150mph and your wheels are moving at 150mph, you have no forward motion regardless of thrust and therefore no airspeed.

It doesn't matter how fast the conveyor goes. If it was a CAR it would matter, but since the thrust comes from the engines, all the energy is being imparted into the AIR, not onto the conveyor/ground. The plane still is going to move forward and have air moving over the wings, creating lift.

reply to post by Zaphod58


But the thrust would have to counter the effects of the conveyor belt to create forward motion. O wait! I get it now.

Conveyor belt in reverse at 150mph lets say. Plane applies enough thrust to reach 150mph on normal runway, and would still achieve that airspeed on a huge conveyor belt even though the wheel-speed would measure 300mph.

Exactly. Since the wheels are free spinning, unlike in a car, the conveyor speed doesn't matter any. The wheels will just spin faster and faster until they fail, but the plane will still take off.

Right, the only factor is going to be the failure of the wheels due to heat from friction at high speed.

reply to post by Boone 870


After playing around a flight simulator, I'd say even harder than landing (which I'd say they stand no chance on if they have no experience) would be navigating.

Jesus that got complex.