Comment by globular-toast
2 years ago
This looks incredible as usual. What puzzles me, though, is why some people find flying puzzling. At least the kind that we do, ie. helicopters and fixed-wing aircraft. It's easy to accept a fan works: just put your hand there and feel the draft. A wing is just like a linear fan pushing air down. It's completely intuitive to understand for me. The difficulties are just in making it practical and controllable. Conversely, many people don't seem concerned at all with bird or insect flight, which I find a lot harder to understand.
I think many of us were taught in school that airfoil shape was somehow magical -- that the fact that it was bowed more on the top was responsible for the fact that it worked.
This is only partially true, though; a totally flat wing can also support flight. The shaped nature of the wing contributes to its efficiency (and other factors) but do not make other wing shapes incapable of supporting flight.
The reality is that the Wright brothers' innovation was not the airfoil shape or even the lightweight motor. It was the control surfaces, to allow the operator to adjust the plane's attitude on the three axes of rotation, allowing actively stabilized flight.
Paper airplanes and kites demonstrate all the same principles of heavier-than-air flight (the Wright brothers even had a kite version of their airframe they used for testing), despite the fact that they generally do not exhibit shaped airfoils.
The Wrights did use a rudder and "horizontal rudder" on the 1903 Flyer, but they were for some time determined to achieve roll control by warping the wings rather than using control surfaces, and were only forced to adopt ailerons as other pioneers began demonstrating how superior a paradigm that was. So they don't deserve too much credit on that score!
"Control surfaces" was more specific than I intended; what I meant was that their plane allowed them to control all three axes of rotation, and that was the innovation - that they could control pitch, yaw, and roll independently and that allowed them to have active stable flight.
Without those controls, flight is basically impossible, and with them, you could use nearly any airfoil shape (modulo engine power, drag, and stall speed considerations) and achieve heavier-than-air flight.
Ailerons were really only invented when they were (and named in French) because the Wrights were extremely litigious, they sued Curtiss for using ailerons and basically destroyed American aviation for a decade allowing the French a temporary lead. This had an interesting cultural effect of lots of things becoming named in French across aviation (including things like the weather code for mist being "br" for brume to this day).
Because the explanation in school misses something like 90% of the detail replacing it with zero-explanation magical thinking.
For example, yes, the air above the wing moves faster than the air below the wing, and it's related to shape of the airfoil.
However, it has nothing to do with magical "air has longer to travel".
It starts with how combining flows at the trailing edge of the airflow create a vortex which induces an opposite vortex around the wing, which is a bit counter-intuitive (but it has nothing on why swept wings work, which can be summarised for practical aircraft design purposes of "because if we calculate at an angle we get better values and reality is crying in the corner")
> It starts with how combining flows at the trailing edge of the airflow create a vortex which induces an opposite vortex around the wing,
Wait, I was under the impression this Cutta circulation was a computational simplification and the "real" reason were the pressure differences as explained in this submission. What am I missing?
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The whole air has longer to travel thing is obviously hand waving a lot of different properties that are all combining to get better efficiencies. For example, don't forget the coanda effect and its contributions to the shape of a wing. Luckily we can always just return to the navier-stokes equations to help us out.
Growing up I got the "air has longer to travel on the top of the wing than the bottom" explanation, and it always smelled like BS. This is the first explanation of flight aerodynamics that really made sense to me — incredible article as always from this author.
Without their wind tunnel optimized airfoils, the wright flyer wouldn't have flown. Without the controls, it wouldn't have flown. Without the high power to weight ratio motor, it wouldn't have flown. Which was the most critical?
Surely it's a less impressive result that something powered by mains electricity can move the air in a draft than that a multi-hundred-ton aircraft can fly over the highest mountains.
It's the size of the aerodynamic forces and the complexity of the physical mechanisms that create them that many people have trouble with. In particular: intuitions can be pretty wrong, most simplified explanations are wrong under simple experiments, and the problems exhibit scale variance that is unfamiliar (e.g. Reynolds number).
One time I was working on air data computer for a transonic aircraft that could fly up to about M0.95 - during flight test, an air data probe mounted on a nose boom was used to supply impact and static pressures, angle of attack and sideslip etc. for various air data calculations like airspeed and altitude.
I was fascinated that there was a term in the calculation that related to the aircraft flap position - what's happening way out on the trailing edge of the wing actually has a meaningful effect of pressures measured on a boom out the front of the nose during certain regimes of flight.
It's just a matter of scale. What's impressive to me with the big aircraft is that we can organise thousands(?) of people to build something that big. But when it comes to the principle of flying it's just a bigger version of the fan. If you were to say they used the same amount of energy as a fan then that would be impressive. But they don't, they burn tons of fossil fuels. Geese can fly over the highest mountains too and all they eat is grass.
>it's just a bigger version of the fan
I mean, actually, it isn't - that's the whole point about scale variance and Reynolds number and why wings that work for insects are not the wings that work for jumbo jets.
This is an incredible (over)simplification of flying.
We still don't have a very good understanding of turbulence.
Navier Stokes equations still make Aerospace engineers drink.
Yes its stupid simple if you care about the simplest of analogies, but if you try to understand it, there are reasons why 80% of people drop out.
I think you mean incredible oversimplification
Because an airplane doesn't move its wings like a bug or helicopter, and it's wings aren't shaped like fan blades. One might look at a plane and conclude that since the wings and engines are parallel to the ground, it must only move laterally.
Of course it moves its wings. That's what the runway is for.
Rectilinear fan blades are shaped very similarly to aircraft wings. And it does only move laterally until the ailerons are moved away from being parallel with the ground.
Who are these mysterious people you're interacting with who are concerned with (but don't understand) the physics of flight, but who are also not concerned with bird or insect flight?
I guess this is somewhat counter intuitive:
https://www.youtube.com/watch?v=NBsvzMi9-f8
So yeah, fans are puzzling too.
If you watch it very slowly, the paper initially folds under the mouth and then it blows out straight.
I'm guessing the initial puff creates a high pressure area on top of the paper, rolling it downward and back. Them after the puff has pushed the air away, there is now a low pressure zone on top of the paper which lifts it up as the air below is rushing upwards around the sides of the paper.
That seems like the Coanda effect more than anything.