But they go straight down when they're launched, like bombs, not missiles. (Well, not quite straight accounting for gravity) until the engine builds enough thrust to guide them forward.
the wheels are turning with a rate respective of the speed of the plane and the speed of the conveyor belt added together since both effect the wheels but in oposite directions thus their rates are cummulative.
However, the wheel speed does not matter since it has nothing to do with the plane taking off. The only question you have to ask is "When is this thread going to die? " and realize the only thing that matters is the thrust (push) of the engines making the plane move forward so that the relative air speed over the wings becomes great enough to cause lift sufficient enough to allow the plane to take off.