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 bird (and it’s wings) fly by using its muscles to push down air, which push it (the bird) back up. Point out how when jumping (on the ground), you push down on the ground with your muscles, and that moves you up (makes you jump).
For airplanes, the shape of the wing makes the air push the wing up from the bottom, and pull it from the top. This pulls the wings upwards, which (because they’re attached), pulls the plane up as well.
Bernoulli isn’t actually a very good way to understand wings. It’s one of those explanations that sounds good but isn’t wildly physically accurate, or at least incomplete, so if anyone thinks about it for too long (like a budding 5 year old airplane enthusiast) they’ll get confused really fast.
Lift happens by pushing air down. Air goes down, wing gets pushed up, just like throwing a baseball on a skateboard. *Anything* that pushes air down generates lift…a brick will fly just fine with a good enough engine. There are flying lawnmowers (happy Youtubing).
A *wing* is specifically designed to push air down *while minimizing drag*. That means you can push enough air down to lift you up without needing a stupidly large engine.
Airplanes push airdown with a wing, and get thrust from their engine.
Birds combined their wings with their engines…they get lift from the wing the same way an airplane does, but they *also* get thrust by flapping. This is why they can glide just fine with their wings still but need to flap to takeoff or climb or stay level.
Bernoulli’s principle for an airfoil shows that the airflow at the under section of the airfoil is slower and thus has higher pressure than the above section of the airfoil, so the air below the wing is essentially pushing up more than the air on top of the wing is pushing down.
For an actual 5 year old, I guess you could just describe it as some sort of tug of war between the air below the wing and the air above the wing? And the shape of the wing makes it so that one side is stronger than the other and pushes the plane upwards
When Bernoulli’s principle is applied to an aerofoil it creates a pressure difference because the air is travelling at different speeds under and above the wing. The pressure on the bottom of the wing is lower than on the top. Knowing that pressure times area is force, Bernoulli’s principle creates a lift force due to this pressure imbalance.
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.
The easiest way to understand the basic concept of a planes wing is to stick your hand out of a window of a car. Form a “knife hand” as they called it in the corps (flat hand together as much as possible) to simulate a wing. Then change the angle of your hand and feel the wind pressure push your hand upward or downward depending on how you angle it.
It’s not exactly right for a wing, but it’s pretty close since the top of your hand is more rounded than the palm side.
So you can get lift with symmetrical wings. You can even get lift where the path length on the lower part of the wing is larger than the upper part. Also BP would actually predict lower lift than what is actually seen
Run and slide on file/linoleum floor in your socks.
Now, if you had an umbrella over your head and you ran and slid you could use the umbrella to either stop (point it forward) or maybe even lift you (if you angled it back behind you just the right way). The umbrella would catch the air and lift you off your feet.
Now, an umbrella is actually the wrong shape, what we need is a wing. A wing is just the right shape to lift you off your feet (if you can run as fast as a car). People actually do this with hang-gliders and parasailing (show them a video).
So a wing works because it’s the right shape to push the air underneath it down and lift itself (and whatever is attached to it) up.
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You can use a piece of paper to demonstrate the concept, hold a piece of paper just below your lip and blow over the top of it and the far end will rise up and flap about a bit because the faster moving air from your breath exerts less static pressure on the top of the paper than the slower moving air below it.
If nothing else it will give the kid something to do even if they don’t understand the details.
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