No One Can Explain Why Planes Stay in the Air (2020) • Hacker News Board

Re: No One Can Explain Why Planes Stay in the Air (2020)

#11

Post by injb » Sat, Jan 28, 2023, 3:43 AM UTC

rawgabbit wrote: post #10

morpheos137 wrote: post #9

Earlier quoted context omitted.

Ok. Why? See if this is an educational discussion it makes sense to support one's argument, not just say no, you're wrong...I supported my argument with a plausible explanation. If planes are not supported by the air beneath them pushing up why don't they fall out of the sky?

There are simple experiments you can perform to prove the correctness of Bernoulli’s principle. If the math cannot accurately describe completely what is happening, then it is our math that is lacking. Technically the air is less dense on top which generates lift. And yes planes do fall from the sky when they stall due large angles of attack which disrupt the laminar flow.

Bernoullis principle isn't necessary though because there are planes with symmetric wing sections that fly just fine, not to mention the fact that planes with conventional wing sections can fly upside down.

The article should probably be called "Half of people can explain why planes stay in the air, but the other half doesn't know they're wrong".

Re: No One Can Explain Why Planes Stay in the Air (2020)

#12

Post by wayeq » Sat, Jan 28, 2023, 3:49 AM UTC

entropicgravity wrote: post #8

Just as no one can explain how bicycles stay up once they get moving. Or why hot air rises but it's always cold in the mountains when they should be hot from all that rising hot air.

And why all these windmills don't keep us cool

Re: No One Can Explain Why Planes Stay in the Air (2020)

#13

Post by injb » Sat, Jan 28, 2023, 3:54 AM UTC

There's a very famous book called Stick and Rudder that does a good job of explaining this. A plane stays up because of Newtonian reaction. The Bernloulli principle and its associated airfoil shape should be thought of as an efficient way of displacing air downwards without the appearance of the wing being angled upwards.

IOW a classic airfoil that appears horizontal is really best thought of as a flat surface that's angled upwards. When a plane with a wing like that inverts, it really does "lift" downwards, and it takes considerable elevator correction to keep level.

If the wing had a symmetric airfoil section, then it needs to be visibly angled upwards in order to fly; but on the other hand when upside down it doesn't take as much elevator input to stay up as the classic asymmetric version.

Re: No One Can Explain Why Planes Stay in the Air (2020)

#14

Post by morpheos137 » Sat, Jan 28, 2023, 3:54 AM UTC

rawgabbit wrote: post #10

morpheos137 wrote: post #9

Earlier quoted context omitted.

Ok. Why? See if this is an educational discussion it makes sense to support one's argument, not just say no, you're wrong...I supported my argument with a plausible explanation. If planes are not supported by the air beneath them pushing up why don't they fall out of the sky?

There are simple experiments you can perform to prove the correctness of Bernoulli’s principle. If the math cannot accurately describe completely what is happening, then it is our math that is lacking. Technically the air is less dense on top which generates lift. And yes planes do fall from the sky when they stall due large angles of attack which disrupt the laminar flow.

Less dense is the same as lower pressure above the wing than below. It seems we are arguing about semantics.

Re: No One Can Explain Why Planes Stay in the Air (2020)

#15

Post by rawgabbit » Sat, Jan 28, 2023, 4:30 AM UTC

morpheos137 wrote: post #14

rawgabbit wrote: post #10

Earlier quoted context omitted.

There are simple experiments you can perform to prove the correctness of Bernoulli’s principle. If the math cannot accurately describe completely what is happening, then it is our math that is lacking. Technically the air is less dense on top which generates lift. And yes planes do fall from the sky when they stall due large angles of attack which disrupt the laminar flow.

Less dense is the same as lower pressure above the wing than below. It seems we are arguing about semantics.

There is a slight semantic difference. The poster suggested the plane compressed the air underneath. I believe he was trying to describe lift. Technically the plane is too small to compress the air. The plane’s wing does not compress the air it generates lift.

Re: No One Can Explain Why Planes Stay in the Air (2020)

#16

Post by rawgabbit » Sat, Jan 28, 2023, 4:35 AM UTC

injb wrote: post #11

rawgabbit wrote: post #10

Earlier quoted context omitted.

There are simple experiments you can perform to prove the correctness of Bernoulli’s principle. If the math cannot accurately describe completely what is happening, then it is our math that is lacking. Technically the air is less dense on top which generates lift. And yes planes do fall from the sky when they stall due large angles of attack which disrupt the laminar flow.

Bernoullis principle isn't necessary though because there are planes with symmetric wing sections that fly just fine, not to mention the fact that planes with conventional wing sections can fly upside down. The article should probably be called "Half of people can explain why planes stay in the air, but the other half doesn't know they're wrong".

Can you tell me the name of a commercial plane with a symmetric wing cross section?

Re: No One Can Explain Why Planes Stay in the Air (2020)

#17

Post by musicale » Sat, Jan 28, 2023, 9:47 AM UTC

We know why paper airplanes stay in the air. They are lifted up by hot air from the teachers they are thrown at.

Re: No One Can Explain Why Planes Stay in the Air (2020)

#18

Post by morpheos137 » Sat, Jan 28, 2023, 1:07 PM UTC

rawgabbit wrote: post #15

morpheos137 wrote: post #14

Earlier quoted context omitted.

Less dense is the same as lower pressure above the wing than below. It seems we are arguing about semantics.

There is a slight semantic difference. The poster suggested the plane compressed the air underneath. I believe he was trying to describe lift. Technically the plane is too small to compress the air. The plane’s wing does not compress the air it generates lift.

I am sorry but I disagree. Air as a gas is a compressible fluid. You can compress air with your hand. I don't understand why this is hard to grasp.

Basically it comes down to what is "lift."

It is known winged planes can not fly in a vacuum.

Rather planes fly by some sort of reaction force between air molecules and the wing.

You seem to suggest the wing is pulled up from the top. How does that work? Do air molecules have little hooks that attach to the upper wing surface?

Obviously no.

So lift is just another name for the ordinary newtonian reaction force. It pushes from the bottom.

How is that possible? It is possible because there are more air molecules colliding with the bottom of the wing than with the top. I.e. pressure is higher beneath than above.

These collisions transfer kinetic energy from the compressed air to the wing.

Re: No One Can Explain Why Planes Stay in the Air (2020)

#19

Post by rawgabbit » Sat, Jan 28, 2023, 7:52 PM UTC

morpheos137 wrote: post #18

rawgabbit wrote: post #15

Earlier quoted context omitted.

There is a slight semantic difference. The poster suggested the plane compressed the air underneath. I believe he was trying to describe lift. Technically the plane is too small to compress the air. The plane’s wing does not compress the air it generates lift.

I am sorry but I disagree. Air as a gas is a compressible fluid. You can compress air with your hand. I don't understand why this is hard to grasp. Basically it comes down to what is "lift." It is known winged planes can not fly in a vacuum. Rather planes fly by some sort of reaction force between air molecules and the wing. You seem to suggest the wing is pulled up from the top. How does that work? Do air molecules…

You cannot compress air with your hand. What you are doing is displacing it, moving it somewhere else.

What you described It is possible because there are more air molecules colliding with the bottom of the wing than with the top. I.e. pressure is higher beneath than above. These collisions transfer kinetic energy from the compressed air to the wing. -- when I was in school a long long time ago -- we called it LIFT.

We called it lift because at subsonic speeds, airplanes relied on Bernoulli's principle. At supersonic speeds, the shock wave created at the bow of the wing prevented laminar airflow meaning no LIFT. At supersonic speeds, the airplane flew due to the newtonian breakdown of airflow against the wing's undersurface. At supersonic speeds, airplanes are inherently unstable which meant they relied on computers to constantly correct their trajectory.

Re: No One Can Explain Why Planes Stay in the Air (2020)

#20

Post by p_l » Sun, Jan 29, 2023, 12:47 PM UTC

rawgabbit wrote: post #16

injb wrote: post #11

Earlier quoted context omitted.

Bernoullis principle isn't necessary though because there are planes with symmetric wing sections that fly just fine, not to mention the fact that planes with conventional wing sections can fly upside down. The article should probably be called "Half of people can explain why planes stay in the air, but the other half doesn't know they're wrong".

Can you tell me the name of a commercial plane with a symmetric wing cross section?

Extra 300 would be one example. Generally you use symmetric profiles for acrobatic planes - as all profiles have their own pros and cons, and symmetric ones increase capabilities in inverse flight (and acrobatics) while sacrificing low speed and straight flight performance.