Quoting flyboyseven (Thread starter):
*For the minimum size, I would figure that one would reach a size where the relative size of the wing to the air molecules would be too small. You would reach a point where a molecule of air would simply smack into the wing, and just sorta bounce off.* |

This was actually Newton's original explanation for how lift worked...it's completely wrong in all but a rarified gas, but for very very tiny wings (or very large ones in very sparse gases) it still works, and you do get lift. Just not nearly as much.

Once you're smaller than a few mean free molecular paths, you'll have almost no control over the direction of the lift though (Brownian motion will overtake your lift unless you're going supersonic).

Quoting flyboyseven (Thread starter):
*would you reach a size where the wing is so large that the airflow cannot remain laminar long enough to create the required lift?* |

You can reach a size where the airflow cannot remain laminar; you can't reach a size where you can't create lift (assuming infinite amounts of space, air, materials, etc.).

Pretty much.

Quoting flyboyseven (Reply 2):
*I know that the airflow is not laminar even nearly all the way, depending on the wing, but i was thinking that you might reach a size where the wing would become just too big for the air to wrap around it in a way that creates lift.* |

As you get larger, viscous effects (including laminar flow) become less important. For extremely large wings, they disappear to zero and you end up with flow that very nicely approximates the inviscid assumptions that your aero prof told you never happen in real life.

Quoting ANITIX87 (Reply 3):
*1) For a given airfoil section, is there a maximum size at which is becomes too inefficient or even ineffective?* |

No.

Quoting ANITIX87 (Reply 3):
*2) For ANY airfoil, is there a maximum size allowed?* |

No.

Quoting vikkyvik (Reply 4):
*That's not quite correct as you have stated it. Reynolds Number, as applied to wings, has the chord of the wing as one of its factors.* |

It doesn't have to be the chord, it just has to be be some charachteristic length. Chord is logical one to use for wings, but it's not required. However, whatever length you use, the Reynolds number will go up with size. Since Reynolds numbers are basically the ratio of intertial to viscous forces, high Reynolds number equates to flows where viscosity is mostly irrelevant, which causes them to become size-insensitive.

Quoting vikkyvik (Reply 4):
*So to maintain similar flow conditions, for a wing that has twice the chord, you need flow that is half as fast.* |

Yes (or change viscosity or density, but that's annoying).

Quoting vikkyvik (Reply 4):
*With that in mind, there actually will be a difference in the flow as the wing gets larger and larger, if you keep the flow velocity the same. Eventually, as the adverse pressure gradient got larger, the flow might separate.* |

The adverse pressure *gradient* gets smaller as the wing gets larger. The total pressure differential is size insensitive (as long as we're talking real-world sizes and speeds), and the distance is getting larger as your wing gets bigger, so the gradient gets shallower.

Aerodynamics, except at very small sizes, assumes continuous fluid. The larger you get, the faster you go, and the thinner your fluid, the closer you get to approximating perfectly continuous inviscid flow. As a result, there's no upper size bound, since all the size effects drop away as you get bigger.

At the small end (low Reynolds numbers) the physics change a lot, viscosity and laminar flow start becoming very important, and things get weird. This is part of the explanation for why insect wings don't look like they should work.

Tom.