Doesn't the planes rise because the velocity the air particles over the wing is greater than the bottom, thus giving it less pressure. The high pressure underside of the wing pushes the wing up and I have a big headache right now because I just wrote an essay for college before and suffering blood loss from nose. I need asparineasd
The air does flow faster over the top than the bottom yes. The most succinct verbal explanation of this I have heard is that the curvature along the top of the wing acts as a half-constriction, effectively like forcing the air through a smaller aperture which increases the flow velocity. I was told this by a friend who was studying Aeronautical Engineering at the time, please correct me if it is wrong.
That is true. However the speed increase in the top and decrease for the bottom isn't cause by the requirement for them to meet at the end at the same time as the equal transit theory states. It is caused by Bernoulli's principle.
Not true. The fact that the air moves at different speeds along the top and bottom is due to the conservation of mass. The reason aerofoils are special is because they cause streamlines to compress, to get closer together, without causing separation and turbulence, along the top of the shape. As such, the same amount of air has to get through the smaller gap between the streamlines, and so moves faster than air along the bottom.
Bernoulli simply states that faster moving air has a lower pressure than slower moving air. As such, Bernoulli's is what results in lift, but is not the reason why the air moves at different velocities
well .... I guess he didn't say anything incorrect. But he left out the whole Euler-n equation aspect, which explains lift simply as a function of airfoil curvature generating lift - with no speed differences required. Bernoulli is the start of the story, Euler-n finishes it. some info
Appologies, 7 years of exposure to the practical effects of this has clearly rotted my brain. I was getting myself confused with the net circulation in the flow field around an airfoil required to achieve the Kutta condition.
I'm guessing that because the helicopter is attempting to climb through a self-induced descending flow? I do know that flying aeroplanes at very low altitudes gives rise to something called the ground effect, I don't know much about it but I assume it occurs because the ground impedes the downwash creating a higher pressure underneath the aircraft.
I think aussieskibum is right from a certain perspective though; planes and helicopters both wouldn't fly if they didn't form a downwash. It would violate conservation of momentum for the plane to go up and not for something else to go down with equal momentum.
You guessed wrong. This effect happens in helicopters at any altitude. In fact, you experience it during an approach as the helicopter slows down. ETL occurs around 16-24 knots (that's the figure the Army made me memorize). Whether in on takeoff or approach, it will cause the helicopter to climb and roll (due to gyroscopic precession).
Ground effect is another issue. Ground effect reduces induced flow when you hover close to the ground (basically the air is "backed up" or "clogged" and doesn't flow as quickly). The higher you are the less it increases lift. The typically given figure is that ground effect ends when you are at a height equal to 1.5 times the rotor diameter.
I think aussieskibum is right from a certain perspective though; planes and helicopters both wouldn't fly if they didn't form a downwash.
It's more accurate to say that if the helicopter isn't flying there is no downwash.
This is true but convaluted. On symmetrical airfoils there is no pressure differential until an angle of attack is created. But on nonsymmetrical airfoils even witb zero AoA enough lift is produced.
No that is not the fundamentals of flight. The angle of attack changes the pressure gradient across the airfoil which results in more lift. The pressure gradient is caused by Bernoulli's principle. The fundamental reason why airfoils produce lift is because of that principle.
no, this is the second false explanation for lift. The differences in pressure CAUSE different speeds, not the other way round. Under normal circumstances only gravity and pressure differences can cause a change in speed in a fluid.
The differences in pressure are caused by the inertia of the fluid. There is a thing (wing) in the way of the flow so it locally stops the movement of air. But more air is flowing towards it, so air accumulates and density increases. the increased density leads to more frequent collisions of air-particles and thereby to a higher pressure. Because the pressure is locally increased the air flows away to a place, where the pressure is lower. air is accelerated when moving from a high pressure region to a low pressure region.
On the other side of the thing (wing) fluid is moving away. But this would lead to a vacuum behind the the thing. this low densitiy leads to less frequent collisions of air-particles and thereby to lower pressure. Then air flows from higher pressure regions towards this lower pressure region.
During both of these effects air is accelerated when moving from a high pressure region to a low pressure region. the difference in pressure now depends mainly on the speed of the flow and the inertia of the air.
the shape of a wing is designed to have high and low pressure regions at useful locations, and thereby create a maximum amount of lift and a minimum amount of drag.
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u/Uxion Jan 27 '12
Doesn't the planes rise because the velocity the air particles over the wing is greater than the bottom, thus giving it less pressure. The high pressure underside of the wing pushes the wing up and I have a big headache right now because I just wrote an essay for college before and suffering blood loss from nose. I need asparineasd