No One Can Explain Why Planes Stay in the Air (2020)
scientificamerican.comThere'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.
The title is complete clickbait. It should say “no one’s short intuitive explanations of why planes stay in the air are fully correct”
Exactly. Like many things in physics there are simple answers, there are correct answers, and there's a sliding scale between the two.
Not 'fully correct' there are also simple/correct answers and complex/incorrect answers.
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
We know why paper airplanes stay in the air. They are lifted up by hot air from the teachers they are thrown at.
I'd say reaction force is a sufficient explanation. The wing compresses the air column below it. The pressurised air generates buoyancy sufficient to dynamically support the air craft while moving at a high enough speed.
And unless you cross over to supersonic regime it's completely wrong (^_-)
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?
Sorry for a bit flippant answer, but I was on phone and the long answer is, well, long. Going to try to make a somewhat summary of it.
EDIT: Wanted to add a link to NASA's page about it, but unfortunately the links changed and I can't find the old (ca. 2000) site material.
You have a bunch of responses with Bernoulli's principle, but they are incomplete.
Essentially, the lift is higher pressure of air on the underneath of the wing, creating a force pushing the wing away. Now, how do you get said increased pressure?
Bernoulli's law shows that air moving with higher speed will give you effective lower pressure compared to air moving slower, and this is main source of lift in subsonic regime.
Now the secret is "why is the air under the wing moving slower than above it?" and the answer is much more complex than high school level physics textbooks try to say.
When airflow leaves the airfoil and mixes again, it creates a vortex rotating in the same axis as wing. If you were looking at a plane flying to the left, the vortex would be rotating counterclockwise. This is called sometimes a "bootstrap vortex", because it's rotation induces a vortex around the wing (in the hypothetical viewing angle, it would rotate clockwise). When you combine the induced vortex movement with forward movement of the plane, you end up with air underneath the wing moving slower compared to aircraft, and the air above the wing moving faster, giving you the conditions for bernoulli's law to provide you with pressure differential necessary for generation of lift.
Differences in Angle of Attack both impact parameters of the bootstrap vortex, impact pressures on the wing (but less than people would think, at least in the range where you get usable lift instead of stalling), and most importantly decide the rotation direction of the bootstrap vortex - which is why symmetrical wings work so long as you have positive angle of attack - and why flat plane doesn't work (because it doesn't create a good bootstrap vortex, just random turbulence).
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?
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.
Symmetrical wings still use Bernoulli's principle.
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.
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.