The physics of airplane flight

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".

  >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

[2] https://aviation.stackexchange.com/questions/83584/are-canar...

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

> 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.

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.

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

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/

Edit: added book title

>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.

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?

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.

> 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.

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

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

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-...