Comment by andrewla
2 years ago
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?
Essentially the work in the article shows the harder to grok, but still half of the whole equation, with only one small mention of an effect that points to the wider environment. Essentially, this is a more close-in view of the airfoil without consideration of the wider flow around.
One comment already mentioned how position of flaps could have visible effect on pressure sensors in front of the plane, and this is slightly mentioned in how the pressure created by front of the air foil has an impact on air "at a distance" from the airfoil.
The vortices created around the airfoil result in significant change of flows, which especially at low speeds provides big chunk of the pressure changes necessary for the creation of lift, with the effect IIRC getting lower as you go faster, with transsonic regime breaking it - because that's when the resulting speeds go beyond speed of sound at given pressure in the air, which in very simplified way means that air can't move towards front of aircraft anymore in those areas, breaking all sorts of flows you depend on at lower speeds.
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?