Depends what we mean by grade school. For young kids (and honestly most adults) I don't think you need much more than this: "A wing, or anything that sends air moving past it down toward the ground will cause some lift (a push toward the sky), but also some drag (a push on your front toward your back). How much of each depends on the shape of the wing and how it's moving through the air. Really good wings cause a lot of lift without a lot of drag, which is good for not using a lot of fuel to get where you're going or for going really fast."
From my perspective, a much superior explanation would be something like: "A wing causes an aircraft to fly because its shape, and the angle at which it moves through the air, creates regions of higher air pressure under the wing, and lower air pressure above the wing. This causes an upwards force on the wing, and a corresponding reaction force downwards on the air itself."
The turbulence caused by a sharp leading edge of something like a flat board causes momentum transfer to the top of the wing. The problem isn't in explaining how a conventional airfoil works, Newton's law works well enough for that. The problem is in explaining what happens when things go wrong. Turbulence has been a problem for physicists for a long time...
The Newton's third-law explanation is "air bangs into a bottom of airfoil and pushes it up". Without having the concepts of boundaries layers, laminar and turbulent flow, flow separation and (more generally) the entire Navier-Stokes toolbox you don't have the tools for explaining why turbulent flow is a problem, for example.
The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards.
The Navier-Stokes equations merely model fluid flows. Understanding them provides no understanding of the behaviour of such flows. That behaviour is emergent from the interaction of a great many particles.
>The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards.
But that doesn't have any explanatory power at all. If we assume Newton's laws hold, then obviously if there's a force upward on the airfoil then there's a reaction force downward on the air.
It'd be like explaining the combustion engine by saying "the drive shaft from the engine rotates this way, and the reaction force - because the engine is more-or-less rigidly mounted to the frame - is resisted through the suspension by the wheels being in contact with the ground". OK, sure, but I still don't know how the engine actually works.
I dunno. If I look at even a very simple diagram of the flow of air around a wing I see air deflected downward on the bottom and air accelerated around a curve on the top. Both would be expected to produce a downward reaction force.
Added: Or more Newtonish (no action at a distance), there is more upward vertical force contributed by the particles in both cases than downward force.
Yes, if someone tells you how the air flows around a wing you can immediately deduce that it's producing lift, since the air is deflected downwards. The real task is explaining why the air flows the way it does.
If I stick out a flat board from a moving car window, and hold it at an angle, it will "lift up". So indeed, airfoil shape does not matter. Angle of attack matters more, because that dictates the path of least resistance.
Planes fly by slicing through a lattice of air, with blades (wings) that only slice easily in directions that lie on a single plane. Orthogonal tail fins means that the vehicle doesn't go from side to side as easily, so it mostly keeps flying on a line. Take a `+`-shape and elongate it so you get a "dentastix" like shape, then hold that out the car window. It will go in whichever direction you point it.
Same idea of a boat rudder. And yet with boat rudders, we don't say "force of lift". The angle of attack changes, which means it now cuts through the water in a different direction (and the rudder piece wants to go straight in the direction it is pointing, since that way has comparatively little resistance in the water), which changes the way the rear of the ship moves which ultimately steers the ship.
... what do you mean by that? If you mean "you can demonstrate the aerodynamic force using a flat plate", then yes you can do that. If you mean "a flat plate is a good tool to explain the aerodynamic force", then that's much less true. If you mean "in the real world, airfoil shape is irrelevant to aerodynamics" that's obviously false.
I was being a bit facetious, sorry. It matters, but I think what I was getting at is often simply overlooked in favor of airfoil shape and the pressure difference explanation. The situation of gravity no longer being a factor such as with vertical rudders seems often missed. Then, it's suddenly called "rudder force". Even though it's the same thing as "lift". It seems the field of physics has trouble with isolating this concept/phenomenon and coming up with an apt name for it.
Rudders are symmetric, i.e. don't have camber to create high/low pressure on one specific side all the time, and yet they work in redirecting (the relative) flow and thereby through Newtons 3rd redirecting the vessel!
The pressure difference explanation is the basic explanation, though. The name of the force is "the aerodynamic force", and there's not really any confusion on that point.
The difference of shape between hydrofoils and airfoils is determined by the properties of the masses in which they move, explained by the same theories of fluid dynamics, rather than any fundamental difference.
That's just because when you angle the sheet, more molecules of air hit one side imparting part of their kinetic energy, and fewer molecules on the other side to counteract this. I do realize I'm explaining pressure on a molecular level here, but to me it's still "slicing through" and "pushing against" a lattice of molecules.
It’s still Newton’s third law; Push air down and minimize pushing air sideways or in swirling vortices because pushing air the wrong way wastes energy as per newtons law.
That’s what the aerofoil does. It pushes air down but mimimizes wasting energy on drag. It’s still newtons law.
For just a Newton's third law analysis, you have to have the air moving downward behind the wing. Doesn't the shape of the upper surface matter a lot in order to get the air moving downward?
There is no single explanation for why airfoils generate lift that works at a grade school level.