it very simple actually.if u have a miniature plane made of foam,tied one end of a string to the front of it and another to a rigid stationary object.switch on a table fan in front of it.u will see the plane flying rite?more or less.what happens is the and pushes the air through the wings of the plane to create lift.imagine the fan is swapped with propellers attached to the plane.it will do the same thing.so yes plane could take off on conveyor belt but only for very light propeller plane.air pushed by the propeller must be strong enough to create lift on the wings.i think u can visualize this, while u're at it try jet planes being put in similar condition,it wont take off

sometimes mythbusters do overlook some variables in an experiment.maybe we should call them missbusters?