Originally posted by redmage
The best example is the wagon/handrail example.
If you're in a car on a treadmill, your wheels are your driving force so a treadmill can cancel your momentum out because the wheels "pull"
against the ground/treadmill to move you forward.
However, if you are in a wagon(plane), and put the treadmill in a hallway with a handrail, your wheels are not your driving force, aka the wheels spin
freely like on a plane.
Then your arms are the "prop" and the handrail is the air that you pull on to move you forward.
Your wheels can spin like crazy, but, it's your arms/prop and the handrail/air that pull you forward, regardless of the speed (or direction) that
your wheels spin.
[edit on 2/15/06 by redmage]
I think your analogy is a bit flawed.
We seem to agree that lift can only be generated if air flows over the airfoil of the wing, right?
We also all seem to agree that on a plane, the wheels serve basically two purposes; to support the weight of the plane while on the ground, and to
facilitate the plane's ability to taxi efficiently.
I think we can also agree that that neither function of the plane's wheels is related to the propulsion of the plane while it taxis or once it is
airborn. And neither function is especially crucial to the generation of lift, which allows the plane to become airborn.
Your analogy does a fine job of pointing out the relative irrelevance of the actions of the wheels insofar as not enhancing the forward progress of
the plane through the air mass in the given scenario, while at the same time underscoring the necessity of the plane to move relative to a fixed point
on the ground, as would be required to generate the needed airflow over the wing, and thus lift the plane off the runway.
However, you may
have overlooked one significant factor: The handrails, which in your scenario are analogous to the air mass the plane is to
fly through, are solidly fixed to the walls of the hall housing the treadmill/runway. These walls are themselves solidly fixed to a point on the
This detail, rather than proving that the plane can indeed take-off, only serves to move the moment of inertia from the non-stationary
point between the plane's tires and the "conveyor belt runway" to a Stationary
contact point between your hand and the Fixed
to say a point fixed relative to a point on the unmoving ground) contact point on the handrail.
The analogy would only prove your point IF
you could show that air is both solid and
, more importantly, fixed to a specific point on
Obviously, air is not solid, nor is the mass of air above a specific point on the ground fixed to that one spot. Therefore, I propose that your
analogy might be more germain to the scenario if instead of grasping the handrails and pulling yourself along, you instead "grasp the air" and
attempt to propel yourself forward by making pulling motions in the air, as it were.
Another point relating to the action of an aircraft's propulsion. This may be little more than a question of semantics, but nevertheless...
I believe that it has been stated that the purpose of an aircraft engine is to "pull/push against the air" thus driving plane forward.
As I understand it, however, the function of the enigine is actually to push/pull the mass of the plane through the air
velocity to allow the wings enough lift to counter-act the weight of the aircraft, vis-a-vis the pull of gravity. Heavier than Air flight is thus made
So it might be more precise to say that, in keeping with Newtonian physics, the force generated by the plane's propulsion (whatever form it may take)
is employed to push/pull against the airplane itself
to counter its inertia and lend it suffcient velocity to fly.
Again, it may be just a matter of semantics.