You can see why planes stay in the air by sticking your hand out a car window.
If you know better contradict me, but:
An inclined flat surface will push air down, creating pressure below, and create an empty area above, creating a partial vacuum pulling up.
Airfoils are used instead of flat surfaces because they eliminate turbulence by creating laminar airflow.
Regular planes are not built for flying upside down and have airfoils optimized for regular flight — the airfoil section we're all familiar with.
At an airshow once, a pilot showed me that on a plane built for acrobatics, the wing section is vertically symmetrical.
tmtvl · 4h ago
Helicopters, on the other hand, we know perfectly well how they fly. They fly by being so ugly gravity rejects them.
bastawhiz · 3h ago
In thinking about it, this implies that by spinning the blades faster, it makes the helicopter more ugly. Finally, we have an objective unit of measure for relative ugliness: RPM!
My headcanon is that it’s an interaction between cavitation from the object dragging through the fluid and recoil from the particles flinging off of the surface. If you move fast enough then the momentum imparted by the ram surface onto the molecules are large enough to overcome the external pressure trying to fill the cavitation, so you get boundary layer separation.
zabzonk · 4h ago
When I was a kid I used to make small balsa-wood gliders who's wings were completely flat, and with no angle of attack. They flew OK, which has always made me wonder about both Bernoulli and Newton in this regard.
euroderf · 3h ago
Did they fly basically the same when chucked upside-down ?
There's that stabilizer to complicate things.
zabzonk · 3h ago
> Did they fly basically the same when chucked upside-down ?
sadly, never occurred to me to try.
> There's that stabilizer to complicate things.
True, getting the tailplane right is kind of key to a successful aircraft. but these would have been basically the same as the wings. so they may have provided some lift, and so some down-pitch?
nostrademons · 2h ago
I tried, and yes, an inverted balsa-wood glider will still fly. Actually just tried right now with a styrofoam glider, upside-down, and it's fine. Marginally less range than when flying right-side-up, but it's a very small difference.
I suspect that what's going on is that the center-of-gravity interacts with the center-of-lift to create a slight angle of attack regardless of what orientation the plane itself has. Then there is some unknown feedback loop that keeps that angle of attack from getting too large and stalling. It's not unlimited - if you make a paper airplane whose wings are too far forward or center of gravity is too far back, it will still stall - but it keeps most reasonable planes moving forward rather than down.
zabzonk · 1h ago
Yeah, I agree it must have something to do with the forces on the model.
I loved playing with these things way back when (1960s) - the most frightening one was a model attached to a jetex rocket engine https://en.wikipedia.org/wiki/Jetex which after taking off immediately immolated itself, which I suppose is par for rocket powered aircraft, of any size.
lolc · 3h ago
Easy, you stay in the air by accelerating air downwards. Physically, you make sure to collide more with air molecules from below than from above. It's called angle of attack and you can experiment with it by sticking your flat hand out of a car window.
In helicopters it's easy to see, in planes people get all magical about it.
jvanderbot · 3h ago
That's not true entirely, in fact the air is forced downward more by the curvature of the wings.
nrds · 1h ago
Which explains the well-known phenomenon that planes cannot fly upside down.
jvanderbot · 31m ago
Well they can't, not by the same mechanism. They can in the same way a wingless missile would, which doesn't generate lift it just turns itself through air.
tekla · 3h ago
This explanation works for a 8 year old as its true enough for their purposes. It's useless at "actually describe whats happening" level.
We get magical about lift because we don't actually understand it and we know we don't understand it.
dleary · 1h ago
I think it's far from useless... Because it helps to focus on what could possibly be happening. It's not "true enough for an 8 year old's purposes", it is the actual truth.
The air is being accelerated downwards by the airplane. Newton's third law. To stay aloft, the airplane has to manufacture a counteracting force to gravity. And the only way to do that is by accelerating something downwards.
So, we can discuss whether or not the curvature of the airfoil matters, or the angle of attack matters, but the simple fact is that a lot of air has to be accelerated downwards somehow for the plane to stay up.
And the more magical descriptions of lift, including the broken Bernoulli airfoil model that was accepted for so long, are easy to discount once you focus on this important fact.
jvanderbot · 33m ago
The wing is curved to force air downward without the clumsy ruddering effect described in top level comment. That downward force is the newtons law lift. If you go too fast you lose the smooth flow over the wing and you lose lift. Too slow and you don't force enough down to generate enough lift.
danielschreber · 49m ago
What about stalls?
AStonesThrow · 4h ago
I stopped reading when they accused Nature of being "mindless" and "random". The sheer hubris involved in scientists sniffing their own farts and declaring one another to be the king of knowledge. Disgusting arrogance.
I can conceive of more than a few mindless and random pursuits within Science that would save us a lot of money and heartache, if they were abandoned.
If you know better contradict me, but:
An inclined flat surface will push air down, creating pressure below, and create an empty area above, creating a partial vacuum pulling up.
Airfoils are used instead of flat surfaces because they eliminate turbulence by creating laminar airflow.
Regular planes are not built for flying upside down and have airfoils optimized for regular flight — the airfoil section we're all familiar with.
At an airshow once, a pilot showed me that on a plane built for acrobatics, the wing section is vertically symmetrical.
There's that stabilizer to complicate things.
sadly, never occurred to me to try.
> There's that stabilizer to complicate things.
True, getting the tailplane right is kind of key to a successful aircraft. but these would have been basically the same as the wings. so they may have provided some lift, and so some down-pitch?
I suspect that what's going on is that the center-of-gravity interacts with the center-of-lift to create a slight angle of attack regardless of what orientation the plane itself has. Then there is some unknown feedback loop that keeps that angle of attack from getting too large and stalling. It's not unlimited - if you make a paper airplane whose wings are too far forward or center of gravity is too far back, it will still stall - but it keeps most reasonable planes moving forward rather than down.
I loved playing with these things way back when (1960s) - the most frightening one was a model attached to a jetex rocket engine https://en.wikipedia.org/wiki/Jetex which after taking off immediately immolated itself, which I suppose is par for rocket powered aircraft, of any size.
In helicopters it's easy to see, in planes people get all magical about it.
We get magical about lift because we don't actually understand it and we know we don't understand it.
The air is being accelerated downwards by the airplane. Newton's third law. To stay aloft, the airplane has to manufacture a counteracting force to gravity. And the only way to do that is by accelerating something downwards.
So, we can discuss whether or not the curvature of the airfoil matters, or the angle of attack matters, but the simple fact is that a lot of air has to be accelerated downwards somehow for the plane to stay up.
And the more magical descriptions of lift, including the broken Bernoulli airfoil model that was accepted for so long, are easy to discount once you focus on this important fact.
I can conceive of more than a few mindless and random pursuits within Science that would save us a lot of money and heartache, if they were abandoned.