Comment by ColinWright | Hacker News Reader

Comment by ColinWright | Hacker News Reader

Parent Full thread

> With the pressure argument, you have not explained why the air is faster over the top surface of the wing, you have just stated that it is.

That's true. However ...

> ... the circulation generated by the wings ...

You haven't explained why there is "circulation generated by the wings", you have just stated that there is.

Note, if you take the full flow over the wings and subtract the vector field that is the undisturbed average flow then you are left with a circulation. The two are effectively equivalent, so "explaining" by saying that there is "a circulation" is basically saying that the air flows faster over the top and slower underneath.

It's a non-explanation.

I do have an explanation as to why the air flows faster over the top, but this discussion is too small to contain it.

PS: I agree that all the modelling and equations work very well, and from that point of view its "well understood". However, the discussion here shows that for many people, even those who are technically capable, it is not "well understood".

I think we have turned this into a point scoring contest rather than clarifying the issue.

To summarise: Any statement to the effect of "aircraft lift generation is complex" is probably wrong. It is wrong because a high-level picture given by Newton's third law (momentum conservation) is correct and adequately explains lift generation. You do not need to go into the details of velocity or pressure distribution on the wing. You can step back and look the overall airflow and reason about why lift was generated.

> You haven't explained why there is "circulation generated by the wings", you have just stated that there is.

I only mention circulation, to state that both the arguments are similar, just from a different perspective. In my initial post, I reference only Newton's third law (the conservation of momentum). Naturally the source of the circulation is the same as the source of the velocity/pressure distribution. As stated earlier, you do not need these details to explain lift. You do need them to explain phenomenon like stall or drag.

I suspect we are really not far from each other, but we might have different ideas of what is "intuitive", or "obvious", or "complete", or "well understood".

I've spoken with several aerodynamicists over many years, and they all say that the "conservation of momentum" idea, and the "downwash" explanation are only part of the story, and when you run the numbers the lift you get is not completely explained by those alone.

But at this point I need to go back to work, and I'll come back when I can to see how the overall discussion is going. It does feel unlikely that I'll have anything useful to add.

Ok.

To clarify the physics, the "conservation of momentum" idea explains all of the lift, not just some of it.

If you draw a bounding box around the wing and measure the momentum at either end, the lost momentum will be because of the wing, some of that will be lift generation (there is also drag + other turbulent losses).

If it's a 3D wing with finite span, then you're well into vortex-shedding and momentum-carrying plumes of gas.

The usual way to avoid this is to make the wingspan be infinite, with no wingtips. But this is dishonest, because it transforms the problem into a "venturi effect," where the airfoil is producing an instant-force against the ground. Then, the Newtonian force-pair exists between wing and ground. (Yet real, non-infinite wings don't need any ground surface to react against. Their force-pair is between the wing and the vortices being launched downwards.)

To simplify: first explain a hovering helicopter. Wings work the same, acting as air-pumps, pulling in air from all directions, then creating a momentum-carrying plume launched downwards. (Helicopters and wings, both are examples of fluid propulsion, where Bernoulli doesn't apply.)

Reply on news.ycombinator.com