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Wed 27 Oct, 2004 04:16 pm
what causes an aeroplane to stay and fly in sky,and what causes it to turn?please reply me
it's very simple.
The air is a gas, which means it is composed of molecules that float around.
Airplanes that stay in the sky work the same way as boats floating in water.
With a boat, they make the boat less dense than the water by having the boat filled with air...and air is less dense than water.
In the sky, they make the air below the wing denser and the air above the wing less dense...
This is done by the shape of the wing, which has a flat bottom and a rounded top. This means that when the plane is flying, the air molecules above the wing have a longer distance to travel, so they get spread out more...aka less dense.
As you can see, it will only be this way when the plane is moving.
Helicopters work the same way except that the blades are shaped this way instead of the wings, so only the blades need be moving as opposed to the entire craft.
VERY simply.
The top of the wing is curved and the bottom of the wing is straight. This means that the air travelling over the top of the wing has to speed up. The difference in wind speed over the top and bottom makes a pressure differential, low pressure on top, high pressure below. That pressure difference creates lift, the wing wants to move upwards into the lower pressure zone.
The wing is almost always angled into the airflow. The leading edge of the wing is tilted up higher than the trailing edge. This creates more lift by deflecting some of the air that passes over the wing downwards. Air goes down, reaction from wing is to go up.
Steering is a bit tricky. The vertical tail is used to turn the plane left and right while keeping it level. The horizontal tail is used to raise and lower the nose. Most turns are done by rolling the plane over and then raising the nose. There are flaps on the wings used to do this. To roll right you use a flap on the left wing to deflect air downwards, this pushes the wing up on that side and the plane rolls to the right. Then you use the tail to raise the nose. This turns the plane.
Oh yes, I completely forgot about the steering. And on rolling...once the plane has turned 90 degrees, the same force that was once keeping it airborne instead is used to turn it sideways, which is why a jet can make such sharp turns by rolling. Commercial jets don't roll at very steep angles though, because they don't want to have to replace all those little baggies too often.
Adrian wrote:The top of the wing is curved and the bottom of the wing is straight. This means that the air travelling over the top of the wing has to speed up. The difference in wind speed over the top and bottom makes a pressure differential, low pressure on top, high pressure below. That pressure difference creates lift, the wing wants to move upwards into the lower pressure zone.
This is incorrect. (Edit: Er, I am incorrect, actually.)
Adrian wrote:The wing is almost always angled into the airflow. The leading edge of the wing is tilted up higher than the trailing edge. This creates more lift by deflecting some of the air that passes over the wing downwards. Air goes down, reaction from wing is to go up.
This is the correct explanation. Airplanes fly because of Newton's 3rd Law (action/reaction). The wing pushes air down, so it in turn is pushed up.
Airfoil Lifting Force Misconception Widespread in K-6 Textbooks
Oh, how embarrasing. Thank you Merlin. Interesting article also.
Merlin.
I was trying to be as simple as I could.
Anyway, when you get down to it Bernoulli's and Newton's equations both work in there own ways. Neither of them is a complete answer though. In a proper 3d environment they both have holes.
If the Bernoulli explanation were correct then planes could not fly upside down.
First, quite a few planes can't fly upside for any great length of time.
Second, the site you linked to has the explanation you are looking for.
Look for "area of lowest pressure".
I re-read the NASA article as well as your original post, and I see that you are, indeed, correct! You did not make the argument that the airflows meet at the trailing edge.
Quote:Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface.
Quote:For a gas, we have to simultaneously conserve the mass, momentum, and energy in the flow. Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct.
NASA: Bernoulli and Newton
Yeah, the whole "air must meet at the trailing adge" thing is the biggest problem with Bernoulli. I can see how he thought that way, I mean he didn't have any way to really model what he was working on. The technology wasn't there. Once you get past that misconception though, his ideas work... in there own way.
He was certainly ahead of his time when it came to fluid dynamics.
Flying upside down is possible because the underside of a wing is curved, not flat, but to a lesser degree than the top. The same forces are at work to produce lift, though now of course it's in the opposite direction.
The reason you don't often get aircraft flying upside down for any length of time is nothing to do with aerodynamics. It's because the fuel system is designed for "right way up", with just a small amount of fuel in a "negative g trap" to allow the engine to run inverted for around 15 seconds typically. Back in 1936 the Germans flew their Me 109s with fuel-injected engines upside down for an hour or more until they got bored.
In pilot training I was taught the Bernoulli explanation as "Theory of Flight". Many years later I found it still being taught, but since nobody had advanced another theory it was now "Principles of Flight"!
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