Wow, this is too hard to stay away from. New scenario. Same conveyor.
Instead of airplane, i hold in my hand a toy car with freespinning
wheels. If you can agree that thrust from the prop/turbine would be
akin to my hand pulling on the toy car then this will work. It is a
force acting outside of the conveyor. I start to pull the toy car, the
conveyor starts moving backwards at an equivilant speed. so what? my
hand is still pulling the car forward at say one MPH, the belt moves
back wards at one MPH... the belt is only putting a slight amount of
resistance on the car through the friction of the wheels, but nothing
that my hand(or prop etc) cant over come.... i keep accelerating the
car and the belt does too, the wheels see 2 X my forward speed, i can
increase this accelration until the wheel fall off, or untill the car
grows wings or whatever. if the car can accelrate with my hand, so can
the plane using thrust against the air. there it is. one of the amazing
things about this is that some very intelligent people have completely
different views of this. Have yall checked out the boards at straight
dope?

another example, say im on a skate board facing bakwards with a fan....
same thing, the fan pushes against the air and forth do go I.
weeeeeeeeee!

what force exactly does the conveyor hold you back with? the only force
i see is the friction in the wheels, and a negligible amount of power
from even a O-320 will overcome that. really... that and wind
resistance is the only force keeping you in one spot. If your thrust is
greater than that, newton prevails....

it just took me a while to sort it all out... have fun, stay up too
late!
Jester