I think it's quite easy to understand how planes fly. I figured out this as a kid when I pushed my hand out of a car window and tilted it at different angles. This way you can feel the pressure differential and the how the air pushes the hand upwards or downwards.
The mysteries of aerodynamic lift
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Re: The mysteries of aerodynamic lift
#62A wing is a device that pumps air downward, which in turn pushes the wing upward, by newton's third law. For a large plane, the wing will be pumping many tons of air per second. Start with a cube of still air, with zero mean velocity. Fly a plane through it, and that cube will have a mean downward velocity. http://www.aviation-history.com/theory/lift.htm
True...and if people thought about it they would see this is true. Air must flow along the top of a wing and then be sent downward. The bottom of the wing of course "rides" along air like a boat on water. These two things cause air to go down and the plane to go up. Every pilot knows if you hit the relative air at a critical angle the air on top of the wing will stop "sticking" and seperate from the wing...this is called a stall...and you won't have flight anymore. Also...frost on the top side of the wing will cause the air not to stick to the wing...and you can't fly in that case either. So we see easily it is not some magical pressure differential that is "sucking" the plane up.
Re: The mysteries of aerodynamic lift
#63This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…
What you're getting at is the Kutta condition: https://physics.stackexchange.com/questions/135707/what-is-a.... This only arises in viscous flow and ensures that the upper and lower flows leave the trailing edge smoothly. Without this, the flow would not be turned.
Re: The mysteries of aerodynamic lift
#64I think it's quite easy to understand how planes fly. I figured out this as a kid when I pushed my hand out of a car window and tilted it at different angles. This way you can feel the pressure differential and the how the air pushes the hand upwards or downwards.
A scientific explanation though would explain why those regions of high and low pressure get created.
Re: The mysteries of aerodynamic lift
#65I suspect if you made a wing out of a flat piece of material, tilted at the appropriate angle of attack, it would be sufficient to fly a plane. It just wouldn't be optimized at all. Really, you need full Navier-Stokes behavior to explain all the forces acting on the wing. Bernoulli doesn't generalize to a full vector field, it's a simplified version of Navier-Stokes. Calling in the big guns doesn't make for an easy d…
Certainly not optimized but also I think the very definition of a stall.
Ground school was ages ago for me but I do recall a bit of it. I thought the laminar flow on the top was vital for lift even a slight ripple was bad. A flat wing top and bottom would not create lift or poor lift. Well at least for aircraft with flaps and aelerons that need consistent airflow to maintain control.
The difference in the bottom flat part of an aircraft wing compared to the curved top part is what creates the difference in velocity; faster bottom, slower top which creating lift. But again maybe not necessary if you are not in an aircraft that needs to be controlled.
The ramblings of a former private pilot student, but an eternal fan of physics.
Re: The mysteries of aerodynamic lift
#66This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…
> can also sail faster than the wind at times. Really? Can you elaborate?
my analogy: if you have ever launched a pumpkin or watermelon seed by squeezing it between your fingertips just so then you can understand how the craft can move faster than whatever provides the impulse.
\\ wing is to wind as seed is to fingers \\
Re: The mysteries of aerodynamic lift
#67Earlier quoted context omitted.
Yeah. There are two kinds of sailboats. There's the big square sailed ones which always have to move downwind. Then there are the triangular sailed ones, which can move in any direction except for right into the wind. (Maybe a 30 degree on each side dead zone). The square sailed boats can't move faster than the wind, as they just rely on the air pushing on the sail to move with the wind. That's also how the triangula…
> There's the big square sailed ones which always have to move downwind. Actually, that's not true. Square rigged "pirate-style" ships are more efficient when sailing downwind compared to fore-and-aft style sailboats, but they can still sail just fine close-hauled (into the wind) or on a reach (perpendicular). There are some efficiency pros and cons to both styles, but they all have the same aerodynamic capabilities.…
Well said, and I just want to add some detail about how square-rigged ships are able to sail close-hauled:
They typically (or maybe always?) have "staysails", which run fore-and-aft between the masts, which works mostly like the main triangular sail you think of with a simple sailboat (with the exception that, as far as I know, these staysails aren't on a boom that can change its angle relative to the ship). These are able to take the wind at an angle but still generate forward thrust.
The "yards", the sections that hold the square sails perpendicular to the ship, are also able to rotate between perpendicular and nearly-parallel to the ship, which lets them take advantage of a wide range of wind directions.
(I'm also not much of a sailor, I've just sailed some tiny single-sail boats, but I've learned a lot while reading through the Aubrey-Maturin series and from playing the game Naval Action)
Re: The mysteries of aerodynamic lift
#68Doug McLean (Boeing Technical Fellow) has a quite decent talk about the question of intuitive explanations for aerodynamic lift: https://www.youtube.com/watch?v=QKCK4lJLQHU Also Philippe Spalart's quote is right on the money: "It's easy to explain how a rocket works, but explaining how a wing works takes a rocket scientist".
We have trouble explaining it at a lay level, but we can solve the entire time-varying (for unsteady cases) 3d flow field around a wing to very high accuracy. Lift seems like it should be intuitive because we've experienced the force of wind and see birds flying, but therein lies the rub. Straightforward results arise from a series of complex, interacting phenomena.
Another example might be explaining why sticky tape is sticky.
Re: The mysteries of aerodynamic lift
#69I think it's quite easy to understand how planes fly. I figured out this as a kid when I pushed my hand out of a car window and tilted it at different angles. This way you can feel the pressure differential and the how the air pushes the hand upwards or downwards.
A scientific explanation though would explain why those regions of high and low pressure get created.
Depends. Newton’s third law is scientific enough to explain what’s going on. Bernoulli and Navier-Stokes and all the hard equations are needed when you need to optimize the system or understand the micro scale, but they’ll simplify into conservation of energy or momentum in the macro limit.
Re: The mysteries of aerodynamic lift
#70this article is so confused and unscientific i have a hard time forming a coherent response. > although bernoulli's theorem is largely correct ... the theorem alone does not explain why this is so or why the higher velocity atop the wing brings lower pressure along with it this blurb is accompanied by an upside down plane with the caption "doesnt explain why planes can fly inverted". this is a "tide goes in, tide goe…
I find the article perfectly reasonable. We can model flight quite well, and predict the behaviour of wings quite well, yet it is hard to give an accurate account of it at a layman's level. The article goes on to discuss two accounts that have been given historically, and outlines why they are insufficient. A plane flying upside down is a perfectly fine refutation to the naive Bernoulli "the wing is curved on the ups…
Perhaps this:
>In inverted flight, the curved wing surface becomes the bottom surface, and according to Bernoulli’s theorem, it then generates reduced pressure below the wing. That lower pressure, added to the force of gravity, should have the overall effect of pulling the plane downward rather than holding it up.
This is not what Bournoulli's theorem states. Bournoulli's theorem doesn't say anything as to why fluid flows the way it flows. It just says for a fluid in a steady state flow, increases in speed are associated in decreased pressure. Hence the principle suggests, if you find a plane is able to fly steady upside down, then expect to find the air flowing faster over the wing on the opposite side of gravity under those conditions - just likewise when a plane is normally flying right side up.
Again why fluid flows the way it does around airfoils is separate question -and really the crux of the mystery here. Answers to this is probably hidden in the solutions to the Navier–Stokes existence and smoothness problem: https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_existenc...