So full disclosure I personally understand Bernoulli’s principle and understand how a wing works due to it.
Here’ is what I need. My 5 year old son loves planes, and loves watching birds fly. As such he is asking questions of how a bird’s wing works vs how an airplane’s wing works. I can’t seem to simplify down Bernoulli’s to the extent that he can understand how a wing works. So I’m coming to you all for advice.
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A wing’s basic purpose is to push air down with much greater force than the relative airflow pushes the wing backwards. This ratio is called “lift to drag.”
Only a small proportion of lift occurs due to Bernoulli’s principle.
What actually happens is that air divides as it passes a wing. If the wing points up a little in relation to airflow (called “angle of attack” or “attack angle”) then it pushes the air beneath down. Easy enough to understand.
However for the air that goes over the top, the wing surface underneath gradually recedes from the air passing over it. Being a gas that exerts pressure, atmosphere tries to fill in the void left by the wing’s receding top-surface. Importantly, atmospheric pressure acts to fill this void from all directions: above, below, ahead, and behind.
By the time it finishes it’s trip over the wing, the air still moves down meaning it’s momentum changed, having been acted upon by air from above, which only did so because air below the wing was restrained from filling in the partial vacuum.
So the wing rides this constant wedge of atmospheric pressure trying to fill in the partial vacuum above it.
If attack angle increases enough, then atmospheric pressure is insufficient to hold airflow against the wing’s surface and it separates, causing the wing to “stall.”
Nature abhors a vacuum, so tendrils of gas split away from the airflow, spill around the trailing edge, and even sneak in from just below the leading edge (which airplane stall indicators detect to warn of a stall) to fill the void beneath the separated airflow with turbulent atmosphere.
This turbulence drastically increases drag. As well, the separated upper airflow no gains downward velocity (thus momentum) which means that the wing loses around half it’s lift, or even more depending on the wing design.
In any case, the magic *lift to drag ratio* suddenly drops well below what is needed to sustain efficient flight, so the aircraft is unable to fly normally. Depending on the airplane and its speed, altitude, and orientation, a stall might result in a gentle descent, a spin, or even total loss of control.
Bird wings operate on precisely the same principle. But the fact that they can instinctively control their wing shape and profile almost instantly with muscles means birds often eke out more performance from their wings than a human designed airplane wing of similar proportion or size.
Of course birds need this extra efficiency because their wings also provide propulsion. Thinking of a bird wing as your arm and hand, during each wing flap the bird closes its hand and thrusts its arms forward, then quickly opens its hands, spreads its featherd fingers, grabs as much air as it can and pushes it back and down.
Between flaps, the bird holds its wing in an efficient shape to maximize lift.
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