10 ms·

The physics of airplane flight

165 points by luu 2y ago

nvy 2y ago
>A common misconception about wings is that they need to have the classic airfoil shape to work. In reality, just about any surface can create lift and function as a wing Left unsaid is why aircraft wings have airfoil-shaped cross sections with cambered (concave-down) shapes: they produce more lift for a given wing loading and angle of attack. This is why aircraft have flaps, as well. They increase the camber of the wing so that the pilot can fly slower without pitching the nose up, which is important for example when maintaining sight of the runaway on landing approach.
- exabrial 2y ago
  Great explanation! It also decreases the stall speed too; which allows a plane to switch to ‘low speed mode’
  - tonyarkles 2y ago
    It's a really fun set of tradeoffs to be honest. You get more lift at a given speed/AoA, which lets you go slower without stalling. But you get even more drag, which means the engines have to work harder to keep you flying at that slower speed.
    - lutorm 2y ago
      Increasing drag is actually useful when landing. too. Trying to land an airplane with very little drag at a given spot is difficult because if you're just a little fast they'll float a long time.
      - tonyarkles 2y ago
        Definitely! Flaps are one of the few “almost free lunch” things in aviation where almost everything else has harsh tradeoffs to consider.
- hyperpape 2y ago
  > You will notice that past a certain angle, the lift starts to decrease, and is replaced by a lot of drag, a force trying to slow down the wing. This is called a stall, and it limits how much lift a wing can create at any given speed. A proper airfoil geometry can generate more lift before stalling, and creates less drag for a given amount of lift, which is why most airplanes use them. I'm a bit of a novice, so it sounds like this is the same explanation you're giving. What is the article leaving out?
  - Nition 2y ago
    Speed. You'll always stall if you go too slow to get enough lift. Putting the flaps down increases lift and drag and lowers the stall speed. As for why the article mentions decreased lift whereas flaps down increases lift, that's maybe a bit more complicated. Lift vs. Angle Of Attack is a curve that tops out at about 15°.[0] Flaps often go to steeper angles than that, but I'm not sure if lift actually starts to decrease again in that scenario. Certainly adding a curve to the back of the wing (flaps) isn't quite the same as changing the angle of the entire wing. [0] https://commons.wikimedia.org/wiki/File:Lift_curve.svg https://commons.wikimedia.org/wiki/File:Lift_curve.svg
    - roeles 2y ago
      > You'll always stall if you go too slow to get enough lift. The amount of lift needed is determined by the load factor. In a turn, or when a glider launches via a winch, the load factor is higher than in level flight. This means that the stall speed varies with load factor. On the other side, you can still not be stalled below the straight and level stall speed, if the load factor is low enough. In parabolic flight you can see this, and with a load factor of 0 you cannot stall.
  - nvy 2y ago
    .
    - carabiner 2y ago
      No, flaps increase CL. https://www.researchgate.net/figure/Figure-1-4-Effect-of-flap-and-slat-on-lift-coefficient-13_fig1_331718321 https://www.researchgate.net/figure/Figure-1-4-Effect-of-fla... You might be thinking of total lift on a 3D wing, which is something you alluded to re: wing loading. But 3d wings vs 2d airfoils are really different concepts that you seem to be mixing up. Airfoils are advantageous regardless of wing loading (which is an inherently 3d concept) because of better L/D and stall characteristics.
- mannykannot 2y ago
  Except in dire circumstances, airplanes are not struggling to produce sufficient lift. What matters is to produce it with reasonably low drag, up to all the other constraints that go into making a practical airplane for its intended purpose. There are some airplanes that can be in a situation of flying, but unable to climb or even maintain altitude, with flaps fully lowered. In such a situation, only carefully raising the flaps will allow it to climb away.
rootusrootus 2y ago
Given how straightforward the physics are, what is the limiting factor that kept us from developing planes sooner? Insufficient propulsion power to overcome the drag of an inefficient wing?
- bumby 2y ago
  My understanding is that it wasn’t power-to-drag but power -to-weight. Although the Wright brothers deservedly get a lot of credit, I think they made progress once they enlisted the help of Curtis due to his knowledge of lightweight internal combustion engines. The book Bird Men is a god read on this subject.
  - rootusrootus 2y ago
    > The book Bird Men is a god read on this subject. Thanks for that recommendation, I'll add that to my book reading list!
  - Stratoscope 2y ago
    For anyone who has trouble finding this book like I did, the actual title is Birdmen (no space): https://www.amazon.com/dp/B00GL3MGTI https://www.amazon.com/dp/B00GL3MGTI
- foobar1962 2y ago
  Weight. A typical steam train has enough propulsion power to overcome an enormous amount drag, but its power-to-weight ratio is low.
- imoverclocked 2y ago
  The basics are straightforward while having the hindsight of all the development we have enjoyed in the last hundred years. Stable flight was a hard nugget to crack. Better mechanical design, understanding of fluid dynamics, lighter materials, engines with higher power-to-weight ratios, etc all brought us to where we are today once the Wright Flyer was demonstrated.
- blt 2y ago
deleted 2y ago
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deleted 2y ago
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schiffern 2y ago
>Additionally, a horizontal stabilizer in the back needs to be pitched down relative to the wings, creating downwards lift, pitching the plane up. Naturally, this is a fundamental source of inefficiency. This is something I appreciate about the Lilium aircraft: they use canards to avoid this problem. Their latest design places the rear wing slightly above the canard[1], minimizing the downwash disadvantages[2] inherent in many canard configurations. [1] https://www.youtube.com/watch?v=qZ73PftBfFg&t=273 https://www.youtube.com/watch?v=qZ73PftBfFg&t=273 [2] https://aviation.stackexchange.com/questions/83584/are-canards-actually-more-efficient https://aviation.stackexchange.com/questions/83584/are-canar...
- cjbgkagh 2y ago
  The Lilium Jet does not appear realistic to me, most VTOLs barely fly in the best of circumstances. It seems more like a scam to me. Not my field so I could be wrong but the added efficiency of a canard is small change compared to the other challenges involved.
  - carabiner 2y ago
    All of these electric VTOL urban air mobility things are hopeless. Various tech companies including google have taken a stab at them over the past two decades and none are anywhere close to entering service. Google shut theirs down. I really want them to work, but nothing gets past the physics/economics of batteries having piss poor energy density relative to fossil fuels and the massive inefficiency of small rotors. I think Boom has a better chance of flying than Wisk. Meanwhile, Uber Copter is up and running in NYC with regular helicopters.
  - schiffern 2y ago
    Perhaps VTOL is entirely impractical and has no sizeable niche, but it appears Lilium is the least wrong out of all the existing VTOL physical layouts. - No additional draggy/heavy structures for the VTOL components, they reuse the existing wings - No large exposed props, which have noise and hazard concerns - Propulsors are synergistically combined with the wing upper surface, enhancing lift during cruise - VTOL mode actuators are synergistically combined with ailerons/elevators, reducing part count - They have wings for efficient long-distance flight (surprisingly some don't!) - Contingency ability for runway landing if the battery is too depleted - The aforementioned canard advantage If any of the VTOL schemes are workable (which is admittedly an open question!) it will be Lilium.
carabiner 2y ago
> Additionally, when plane speeds up, more drag is produced, slowing it down. Wait, is it speeding up here or slowing down? Slowing down means deceleration, speeding up is acceleration, and it can't be doing both at the same time. > When the plane slows down, it produces less drag, allowing to to pick up more speed. Same deal? I think what he's getting at is that drag increases with the square of speed, but it's a very confusing way of explaining it.
- mansoor_ 2y ago
  A common equation you will find in aerodynamics texts is: Drag = 1/2 * fluid density * velocity^2 * C_d * Ref. Area It approximates the drag experienced by objects as they move within a fluid (atmosphere). You can see that drag is proportional to the square of velocity, so going twice as fast induces 4 times the drag. Ergo, when you speed up, you produce a lot more drag. This will slow you down until you reach an equilibrium between thrust and drag (unless you apply more thrust).
  - carabiner 2y ago
    So you're just agreeing with what I said: > I think what he's getting at is that drag increases with the square of speed
calmbonsai 2y ago
Probably the best practical flight explanation website: https://www.av8n.com/how/ https://www.av8n.com/how/
iamgopal 2y ago
Side note, but are there any international aircraft racing held ? For efficiency ?
- lutorm 2y ago
  There are efficiency races for general aviation aircraft, yes. Check out https://en.wikipedia.org/wiki/CAFE_Foundation https://en.wikipedia.org/wiki/CAFE_Foundation
- upofadown 2y ago
  I suppose the Open category of the World Gliding Championships could be considered such a contest.
dguest 2y ago
Do we still teach kids that planes fly because of Bernoulli's principle? I remember learning about it and wondering why newton's 3rd law wouldn't suffice. It's pretty obvious that the wings push air down and it's not that difficult to understand (even as a kid) that newton's 3rd law works. The essence of the Bernoulli argument is that the top of the wing is longer -> air has to move further -> faster air has lower pressure "because Bernoulli" -> pressure imbalance means lift. Ok, cool, but the "Bernoulli principle" I got as a kid was "faster air is lower pressure", which is both empirically wrong (the air in a compressor hose is obviously moving faster than the air in the workshop) and logically inconsistent (speed is relative, after all). You add in a half dozen qualifiers and it becomes true, but I wonder if this is more complicated than "the wings push air down, the air pushes the wing up".
- cm2187 2y ago
  You can experience faster air is lower pressure when you are trying to breath in strong wing (like sky diving or by an open window on a car on the motorway). It makes you usually gasp for air. But yeah I was taught planes fly that way in the 90s.
- elygre 2y ago
  My mental model is that you can push hard on a wall while standing still, but the faster you run along the wall, the less you are able to push it.
- lupusreal 2y ago
  The Newtonian explaination of lift is partially but not completely correct. It only explains some of the lift which is empirically observed. Particularly the "push air down" model; the tops of wings also pull air down along themselves (assuming there isn't flow separation, e.g. a stall) and direct it down. To really explain that flow you need fluid dynamics.
- krisoft 2y ago
  > Do we still teach kids that planes fly because of Bernoulli's principle? Not just for kids, but it is also in pilot training materials. I distinctly remember that it was how lift was explained in my PPL book.
quibono 2y ago
> With a simple rectangular wing, the center of pressure is 1/4 of the way along the wing from the leading edge Is there a nice way to derive this? I find it interesting it's not the exact center though I guess it makes sense given the angle of the surface.
- kwertzzz 2y ago
  I was asking myself the same question. I would love to see a derivation from e.g. the Navier Stokes equation for this. I think, intuitively, when you draw the streamlines under the rectangular wings, the applied force should be related to the curvature of the streamlines (which is larger at the beginning of the wing). I made some simple 2D Navier-Stokes solver here where you can use the mouse to draw a section of a wing: https://alexander-barth.github.io/FluidSimDemo-WebAssembly/ https://alexander-barth.github.io/FluidSimDemo-WebAssembly/ The color represents the pressure (I should add a proper color bar!)
- JumpCrisscross 2y ago
  > Is there a nice way to derive this? “It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center (ac) of the airfoil” [1]. As you go faster, it goes from quarter chord to half. For a rectangle, the chord length is equal to the airfoil length. Why quarter? It comes from thin-airfoil theory [2]. (Unintuitively, it holds across atmospheres.) [1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/ac.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a... [2] http://aero-comlab.stanford.edu/aa200b/lect_notes/thinairfoil.pdf http://aero-comlab.stanford.edu/aa200b/lect_notes/thinairfoi...
deleted 2y ago
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thecryptopapi 2y ago
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taegge 2y ago
https://m.youtube.com/watch?v=edLnZgF9mUg https://m.youtube.com/watch?v=edLnZgF9mUg I recommend this MIT opencourseware video to anyone interested in this topic
jcynix 2y ago
Airplanes can do rolls and barrel rolls, i.e. they can fly upside down. Good luck with simple explanations ... https://www.scientificamerican.com/video/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/video/no-one-can-explain-...
- nothacking_ 2y ago
  Easy, the natural angle of attack is determined by the elevator position. Flying upside down is totally possible with the right inputs.
bobtheborg 2y ago
Related, very deep, and amazing work: https://ciechanow.ski/airfoil/ https://ciechanow.ski/airfoil/ https://news.ycombinator.com/item?id=39526057 https://news.ycombinator.com/item?id=39526057
vain 2y ago
I remember being taught that Bernoulli's principle causes lift. I was skeptical—how does the air on top know to reach the other end at the same time as the air at the bottom? I think I did ask, and I was just told this is how it works, and that's the correct answer for the exam. This was before the internet, and I couldn't just look up the correct explanation. I parked it in my brain as something I didn't really understand and forgot about it. This was until not so many years ago when I found a satisfactory answer on YouTube. It was criminal to have been raised in an era without the internet.
EncomLab 2y ago
I've flown hundreds of hours using single-surface hang-gliders that effectively have little to no "flat plate" effect, and they make huge amounts of very draggy, slow speed lift. I've flown hundreds of hours using double-surface hang-gliders that make much less slow speed lift, but far less draggy lift at moderate to high speeds. As in all things with aerodynamics - you optimize the design for the performance you want.
alexb23 2y ago
For a more in-depth resource that is still very approachable at a high school level, I highly recommend John S. Denker’s book, See How It Flies, full text online https://www.av8n.com/how/ https://www.av8n.com/how/ Edit: added book title

165 points by luu 2y ago

nvy 2y ago

>A common misconception about wings is that they need to have the classic airfoil shape to work. In reality, just about any surface can create lift and function as a wing Left unsaid is why aircraft wings have airfoil-shaped cross sections with cambered (concave-down) shapes: they produce more lift for a given wing loading and angle of attack. This is why aircraft have flaps, as well. They increase the camber of the wing so that the pilot can fly slower without pitching the nose up, which is important for example when maintaining sight of the runaway on landing approach.

exabrial 2y ago
Great explanation! It also decreases the stall speed too; which allows a plane to switch to ‘low speed mode’
- tonyarkles 2y ago
  It's a really fun set of tradeoffs to be honest. You get more lift at a given speed/AoA, which lets you go slower without stalling. But you get even more drag, which means the engines have to work harder to keep you flying at that slower speed.
  - lutorm 2y ago
    Increasing drag is actually useful when landing. too. Trying to land an airplane with very little drag at a given spot is difficult because if you're just a little fast they'll float a long time.
    - tonyarkles 2y ago
      Definitely! Flaps are one of the few “almost free lunch” things in aviation where almost everything else has harsh tradeoffs to consider.
hyperpape 2y ago
> You will notice that past a certain angle, the lift starts to decrease, and is replaced by a lot of drag, a force trying to slow down the wing. This is called a stall, and it limits how much lift a wing can create at any given speed. A proper airfoil geometry can generate more lift before stalling, and creates less drag for a given amount of lift, which is why most airplanes use them. I'm a bit of a novice, so it sounds like this is the same explanation you're giving. What is the article leaving out?
- Nition 2y ago
  Speed. You'll always stall if you go too slow to get enough lift. Putting the flaps down increases lift and drag and lowers the stall speed. As for why the article mentions decreased lift whereas flaps down increases lift, that's maybe a bit more complicated. Lift vs. Angle Of Attack is a curve that tops out at about 15°.[0] Flaps often go to steeper angles than that, but I'm not sure if lift actually starts to decrease again in that scenario. Certainly adding a curve to the back of the wing (flaps) isn't quite the same as changing the angle of the entire wing. [0] https://commons.wikimedia.org/wiki/File:Lift_curve.svg https://commons.wikimedia.org/wiki/File:Lift_curve.svg
  - roeles 2y ago
    > You'll always stall if you go too slow to get enough lift. The amount of lift needed is determined by the load factor. In a turn, or when a glider launches via a winch, the load factor is higher than in level flight. This means that the stall speed varies with load factor. On the other side, you can still not be stalled below the straight and level stall speed, if the load factor is low enough. In parabolic flight you can see this, and with a load factor of 0 you cannot stall.
- nvy 2y ago
  .
  - carabiner 2y ago
    No, flaps increase CL. https://www.researchgate.net/figure/Figure-1-4-Effect-of-flap-and-slat-on-lift-coefficient-13_fig1_331718321 https://www.researchgate.net/figure/Figure-1-4-Effect-of-fla... You might be thinking of total lift on a 3D wing, which is something you alluded to re: wing loading. But 3d wings vs 2d airfoils are really different concepts that you seem to be mixing up. Airfoils are advantageous regardless of wing loading (which is an inherently 3d concept) because of better L/D and stall characteristics.
mannykannot 2y ago
Except in dire circumstances, airplanes are not struggling to produce sufficient lift. What matters is to produce it with reasonably low drag, up to all the other constraints that go into making a practical airplane for its intended purpose. There are some airplanes that can be in a situation of flying, but unable to climb or even maintain altitude, with flaps fully lowered. In such a situation, only carefully raising the flaps will allow it to climb away.

rootusrootus 2y ago

Given how straightforward the physics are, what is the limiting factor that kept us from developing planes sooner? Insufficient propulsion power to overcome the drag of an inefficient wing?

bumby 2y ago
My understanding is that it wasn’t power-to-drag but power -to-weight. Although the Wright brothers deservedly get a lot of credit, I think they made progress once they enlisted the help of Curtis due to his knowledge of lightweight internal combustion engines. The book Bird Men is a god read on this subject.
- rootusrootus 2y ago
  > The book Bird Men is a god read on this subject. Thanks for that recommendation, I'll add that to my book reading list!
- Stratoscope 2y ago
  For anyone who has trouble finding this book like I did, the actual title is Birdmen (no space): https://www.amazon.com/dp/B00GL3MGTI https://www.amazon.com/dp/B00GL3MGTI
foobar1962 2y ago
Weight. A typical steam train has enough propulsion power to overcome an enormous amount drag, but its power-to-weight ratio is low.
imoverclocked 2y ago
The basics are straightforward while having the hindsight of all the development we have enjoyed in the last hundred years. Stable flight was a hard nugget to crack. Better mechanical design, understanding of fluid dynamics, lighter materials, engines with higher power-to-weight ratios, etc all brought us to where we are today once the Wright Flyer was demonstrated.
blt 2y ago

deleted 2y ago

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deleted 2y ago

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schiffern 2y ago

>Additionally, a horizontal stabilizer in the back needs to be pitched down relative to the wings, creating downwards lift, pitching the plane up. Naturally, this is a fundamental source of inefficiency. This is something I appreciate about the Lilium aircraft: they use canards to avoid this problem. Their latest design places the rear wing slightly above the canard[1], minimizing the downwash disadvantages[2] inherent in many canard configurations. [1] https://www.youtube.com/watch?v=qZ73PftBfFg&t=273 https://www.youtube.com/watch?v=qZ73PftBfFg&t=273 [2] https://aviation.stackexchange.com/questions/83584/are-canards-actually-more-efficient https://aviation.stackexchange.com/questions/83584/are-canar...

cjbgkagh 2y ago
The Lilium Jet does not appear realistic to me, most VTOLs barely fly in the best of circumstances. It seems more like a scam to me. Not my field so I could be wrong but the added efficiency of a canard is small change compared to the other challenges involved.
- carabiner 2y ago
  All of these electric VTOL urban air mobility things are hopeless. Various tech companies including google have taken a stab at them over the past two decades and none are anywhere close to entering service. Google shut theirs down. I really want them to work, but nothing gets past the physics/economics of batteries having piss poor energy density relative to fossil fuels and the massive inefficiency of small rotors. I think Boom has a better chance of flying than Wisk. Meanwhile, Uber Copter is up and running in NYC with regular helicopters.
- schiffern 2y ago
  Perhaps VTOL is entirely impractical and has no sizeable niche, but it appears Lilium is the least wrong out of all the existing VTOL physical layouts. - No additional draggy/heavy structures for the VTOL components, they reuse the existing wings - No large exposed props, which have noise and hazard concerns - Propulsors are synergistically combined with the wing upper surface, enhancing lift during cruise - VTOL mode actuators are synergistically combined with ailerons/elevators, reducing part count - They have wings for efficient long-distance flight (surprisingly some don't!) - Contingency ability for runway landing if the battery is too depleted - The aforementioned canard advantage If any of the VTOL schemes are workable (which is admittedly an open question!) it will be Lilium.

carabiner 2y ago

> Additionally, when plane speeds up, more drag is produced, slowing it down. Wait, is it speeding up here or slowing down? Slowing down means deceleration, speeding up is acceleration, and it can't be doing both at the same time. > When the plane slows down, it produces less drag, allowing to to pick up more speed. Same deal? I think what he's getting at is that drag increases with the square of speed, but it's a very confusing way of explaining it.

mansoor_ 2y ago
A common equation you will find in aerodynamics texts is: Drag = 1/2 * fluid density * velocity^2 * C_d * Ref. Area It approximates the drag experienced by objects as they move within a fluid (atmosphere). You can see that drag is proportional to the square of velocity, so going twice as fast induces 4 times the drag. Ergo, when you speed up, you produce a lot more drag. This will slow you down until you reach an equilibrium between thrust and drag (unless you apply more thrust).
- carabiner 2y ago
  So you're just agreeing with what I said: > I think what he's getting at is that drag increases with the square of speed

calmbonsai 2y ago

Probably the best practical flight explanation website: https://www.av8n.com/how/ https://www.av8n.com/how/

iamgopal 2y ago

Side note, but are there any international aircraft racing held ? For efficiency ?

lutorm 2y ago
There are efficiency races for general aviation aircraft, yes. Check out https://en.wikipedia.org/wiki/CAFE_Foundation https://en.wikipedia.org/wiki/CAFE_Foundation
upofadown 2y ago
I suppose the Open category of the World Gliding Championships could be considered such a contest.

dguest 2y ago

Do we still teach kids that planes fly because of Bernoulli's principle? I remember learning about it and wondering why newton's 3rd law wouldn't suffice. It's pretty obvious that the wings push air down and it's not that difficult to understand (even as a kid) that newton's 3rd law works. The essence of the Bernoulli argument is that the top of the wing is longer -> air has to move further -> faster air has lower pressure "because Bernoulli" -> pressure imbalance means lift. Ok, cool, but the "Bernoulli principle" I got as a kid was "faster air is lower pressure", which is both empirically wrong (the air in a compressor hose is obviously moving faster than the air in the workshop) and logically inconsistent (speed is relative, after all). You add in a half dozen qualifiers and it becomes true, but I wonder if this is more complicated than "the wings push air down, the air pushes the wing up".

cm2187 2y ago
You can experience faster air is lower pressure when you are trying to breath in strong wing (like sky diving or by an open window on a car on the motorway). It makes you usually gasp for air. But yeah I was taught planes fly that way in the 90s.
elygre 2y ago
My mental model is that you can push hard on a wall while standing still, but the faster you run along the wall, the less you are able to push it.
lupusreal 2y ago
The Newtonian explaination of lift is partially but not completely correct. It only explains some of the lift which is empirically observed. Particularly the "push air down" model; the tops of wings also pull air down along themselves (assuming there isn't flow separation, e.g. a stall) and direct it down. To really explain that flow you need fluid dynamics.
krisoft 2y ago
> Do we still teach kids that planes fly because of Bernoulli's principle? Not just for kids, but it is also in pilot training materials. I distinctly remember that it was how lift was explained in my PPL book.

quibono 2y ago

> With a simple rectangular wing, the center of pressure is 1/4 of the way along the wing from the leading edge Is there a nice way to derive this? I find it interesting it's not the exact center though I guess it makes sense given the angle of the surface.

kwertzzz 2y ago
I was asking myself the same question. I would love to see a derivation from e.g. the Navier Stokes equation for this. I think, intuitively, when you draw the streamlines under the rectangular wings, the applied force should be related to the curvature of the streamlines (which is larger at the beginning of the wing). I made some simple 2D Navier-Stokes solver here where you can use the mouse to draw a section of a wing: https://alexander-barth.github.io/FluidSimDemo-WebAssembly/ https://alexander-barth.github.io/FluidSimDemo-WebAssembly/ The color represents the pressure (I should add a proper color bar!)
JumpCrisscross 2y ago
> Is there a nice way to derive this? “It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center (ac) of the airfoil” [1]. As you go faster, it goes from quarter chord to half. For a rectangle, the chord length is equal to the airfoil length. Why quarter? It comes from thin-airfoil theory [2]. (Unintuitively, it holds across atmospheres.) [1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/ac.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a... [2] http://aero-comlab.stanford.edu/aa200b/lect_notes/thinairfoil.pdf http://aero-comlab.stanford.edu/aa200b/lect_notes/thinairfoi...

deleted 2y ago

[deleted]

thecryptopapi 2y ago

[dead]

taegge 2y ago

https://m.youtube.com/watch?v=edLnZgF9mUg https://m.youtube.com/watch?v=edLnZgF9mUg I recommend this MIT opencourseware video to anyone interested in this topic

jcynix 2y ago

Airplanes can do rolls and barrel rolls, i.e. they can fly upside down. Good luck with simple explanations ... https://www.scientificamerican.com/video/no-one-can-explain-why-planes-stay-in-the-air/ https://www.scientificamerican.com/video/no-one-can-explain-...

nothacking_ 2y ago
Easy, the natural angle of attack is determined by the elevator position. Flying upside down is totally possible with the right inputs.

bobtheborg 2y ago

Related, very deep, and amazing work: https://ciechanow.ski/airfoil/ https://ciechanow.ski/airfoil/ https://news.ycombinator.com/item?id=39526057 https://news.ycombinator.com/item?id=39526057

vain 2y ago

I remember being taught that Bernoulli's principle causes lift. I was skeptical—how does the air on top know to reach the other end at the same time as the air at the bottom? I think I did ask, and I was just told this is how it works, and that's the correct answer for the exam. This was before the internet, and I couldn't just look up the correct explanation. I parked it in my brain as something I didn't really understand and forgot about it. This was until not so many years ago when I found a satisfactory answer on YouTube. It was criminal to have been raised in an era without the internet.

EncomLab 2y ago

I've flown hundreds of hours using single-surface hang-gliders that effectively have little to no "flat plate" effect, and they make huge amounts of very draggy, slow speed lift. I've flown hundreds of hours using double-surface hang-gliders that make much less slow speed lift, but far less draggy lift at moderate to high speeds. As in all things with aerodynamics - you optimize the design for the performance you want.

alexb23 2y ago

For a more in-depth resource that is still very approachable at a high school level, I highly recommend John S. Denker’s book, See How It Flies, full text online https://www.av8n.com/how/ https://www.av8n.com/how/ Edit: added book title