No One Can Explain Why Planes Stay in the Air (2020)(scientificamerican.com) |
No One Can Explain Why Planes Stay in the Air (2020)(scientificamerican.com) |
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.
If planes are not supported by the air beneath them pushing up why don't they fall out of the sky?
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).
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".
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.
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.