Let's try this again: How is lift ACTUALLY created?

Question

Let's try this again: How is lift ACTUALLY created?

Answer 1

As the air flows over the upper surface of an airfoil, its speed or velocity increases and its pressure decreases. An area of low pressure is thus formed. There is an area of greater pressure on the lower surface of the airfoil, and this greater pressure tends to move the wing upward. This difference in pressure between the upper and lower surfaces of the wing is called lift. Three-fourths of the total lift of an airfoil is the result of the decrease in pressure over the upper surface. The impact of air on the under surface of an airfoil produces the other one-fourth of the total lift.



An aircraft in flight is acted upon by four forces:

(1) Gravity, or weight, the force that pulls the aircraft toward the earth.

(2) Lift, the force that pushes the aircraft upward.

(3) Thrust, the force that moves the aircraft forward.

(4) Drag, the force that exerts a braking action.



Newton's Laws of Motion

The fundamental laws governing the action of air about a wing are Newton's laws of motion. Newton's first law is normally referred to as the law of inertia. It simply means that a body at rest will not move unless force is applied to it. if it is moving at uniform speed in a straight line, force must be applied to increase or decrease that speed.



Since air has mass, it is a "body" in the meaning of the law. When an aircraft is on the ground with its engines stopped, inertia keeps the aircraft at rest. An aircraft is moved from its state of rest by the thrust force created by the propeller, by the expanding exhaust gases, or both. When it is flying at uniform speed in a straight line, inertia tends to keep the aircraft moving. Some external force is required to change the aircraft from its path of flight.



Newton's second law, that of force, also applies to objects. This law states that if a body moving with uniform speed is acted upon by an external force, the change of motion will be proportional to the amount of the force, and motion will take place in the direction in which the force acts. thìs law may be stated mathematically as follows:

Force = mass X acceleration (F = ma).



If an aircraft is flying against a headwind, it is slowed down. If the wind is coming from either side of the aircraft's heading, the aircraft is pushed off course unless the pilot takes corrective action against the wind direction.



Newton's third law is the law of action and reaction. This law states that for every action (force) there is an equal and opposite reaction (force). This law is well illustrated by the action of a swimmer's hands. He pushes the water aft and thereby propels himself forward, since the water resists the action of his hands. When the force of lift on an aircraft's wing equals the force of gravity, the aircraft maintains level flight.

The three laws of motion which have been discussed are closely related and apply to the theory of flight. In many cases, all three laws may be operating on an aircraft at the same time.



Which brings us to airfoils. An airfoil is a surface designed to obtain a desirable reaction from the air through which it moves. Thus, we can say that any part of the aircraft which converts air resistance into a force useful for flight is an airfoil. The blades of a propeller are so designed that when they rotate, their shape and position cause a higher pressure to be built up behind them than in front of them so that they wìll pull the aircraft forward. The profile of a conventional wing, is an excellent example of an airfoil. Notice that the top surface of the wing profile has greater curvature than the lower surface.



The difference in curvature of the upper and lower surfaces of the wing builds up the lift force. Air flowing over the top surface of the wing must reach the trailing edge of the wing in the same amount of time as the air flowing under the wing. To do this, the air passing over the top surface moves at a greater velocity than the air passing below the wing because of the greater distance it must travel along the top surface. This increased velocity, according to Bernoulli's principle, means a corresponding decrease in pressure on the surface. Thus, a pressure differential is created between the upper and lower surfaces of the wing, forcing the wing upward in the direction of the lower pressure.



The theoretical amount of lift of the airfoil at a velocity of 100 m.p.h. can be determined by sampling the pressure above and below the airfoil at the point of greatest air velocity., Tis pressure is 14.54 p.s.i. above the airfoil. Subtracting this pressure from the pressure below the airfoil, 14.67, gives a difference in pressure of 0.13 p.s.i. Multiplying 0.13 by 144 (number of square inches in a square foot) shows that each square foot of this wing will lift 18.72 pounds. Thus, it can be seen that a small pressure differential across an airfoil section can produce a large lifting force. Within limits, lift can be increased by increasing the angle of attack, the wing area, the freestream velocity, or the density of the air, or by changing the shape of the airfoil.

Answer 2

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Answer 3

Very simply put:

Low pressure air on top of a wing and a relatively higher air pressure on the underside of a wing.

Now think of a gas cylinder with pressurised gas inside. Take the lid off and what happens? It escapes.

With an aerofoil the pressure isnt held in, so it tries to esacpe. The air around it is at the same pressure so the only other place for it to go is to push against the lower pressure area which happens to be the other side of the wing. Its as if its trying to find the easiest route. If the air pressure is high enough to overcome the weight of the aircraft and the air pressure above the wing then lift will occur.

This is why Newton and Bernoulli understandings help.

Answer 4

Bernoulli pretty much explains it. It's based on the air having to go faster over the top of the wing, raise the airspeed, lower the pressure, hence you get lift. This works with an airfoil that has camber, the mean line of the airfoil is cambered towards the top surface. An aerobatic aircraft that uses a symmetrical airfoil, one with zero camber, has to fly with a positive angle of attack. A positive AOA has the air stagnation point down under the nose of the airfoil, therefore this gives the effect of positive camber; further distance over the top of the wing, higher speed, lower pressure. A cambered airfoil will make lift with a zero AOA, some with high camber will stil make lift with a negative AOA.

Answer 5

Here, try this...



http://www.allstar.fiu.edu/aero/airflylvl3.htm

Answer 6

the key : difference in air pressure, less on top and more on bottom of an object (not only wings), this difference is creates lift, BUT lift force must be more than the object's weight in order to push the object into the area where pressure is less. The object can already be in the air (Lift=Weight) like an airplane in cruising/flying, or heli in hovering, when the airplane pitch up (more lift in the wing) or heli's main rotor rotates faster (or main rotor blade pitches up) then the aircraft gets more lift, the aircraft moves up.



so... this is why F-117 without any curvature on its wings, can fly



also, wingless aircraft can fly, like X-24 and so on



if you can make an object (whatever it is), that can have difference in air pressure, and create lift force greater than its weight, you got yourself an airplane/rocket or ufo :)

Answer 7

There's a really good Wikipedia article on lift at http://en.wikipedia.org/wiki/Lift_(force)



It explains both the Newtonian and Bernoulli principles and the misconceptions surrounding lift.



The most important thing is that it works. If you're going to design wings for a living you may need to know the principles, but even then they reduce to a series of equations you can get from a book. If you're going to fly airplanes then you just check to make both wings are attached to the plane, then fire it up and go.

Answer 8

Simply put, forward movement of the plane creates a higher air pressure under the wing than above it,so it rises.

Answer 9

Heres a simple answer...put your hand out the window of a car when your on the highway...exparement with differant angles and feel what the air is doing to your hand.

Answer 10

Look up Bernoulli's Principle in Yahoo. When air rushes through a venturi, an area of low pressure is created at the throat. If you slice a venturi in half, you will see that the curved surface (throat) is shaped like the upper surface of an airfoil. The upper surface of a wing creates a lower pressure that the flat lower side. Lift is created by the pull of the low pressure side and the push from the high pressure side. Propellers, fan blades are airfoils that rotate.

Answer 11

Lift is the force that holds an aircraft in the air. How is lift generated? There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments for many years.





Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.



NO FLUID, NO LIFT



Lift is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is not generated by a force field, in the sense of a gravitational field,or an electromagnetic field, where one object can affect another object without being in physical contact. For lift to be generated, the solid body must be in contact with the fluid: no fluid, no lift. The Space Shuttle does not stay in space because of lift from its wings but because of orbital mechanics related to its speed. Space is nearly a vacuum. Without air, there is no lift generated by the wings.



NO MOTION, NO LIFT



Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion. Drag acts in the direction opposed to the motion.





The proponents of the arguments usually fall into two camps: (1) those who support the "Bernoulli" position that lift is generated by a pressure difference across the wing, and (2) those who support the "Newton" position that lift is the reaction force on a body caused by deflecting a flow of gas. Notice that we place the names in quotation marks because neither Newton nor Bernoulli ever attempted to explain the aerodynamic lift of an object. The names of these scientists are just labels for two camps.



Looking at the lives of Bernoulli and Newton we find more similarities than differences. On the figure at the top of this page we show portraits of Daniel Bernoulli, on the left, and Sir Isaac Newton, on the right. Newton worked in many areas of mathematics and physics. He developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in 1686, he presented his three laws of motion in the Principia Mathematica Philosophiae Naturalis . He and Gottfried Leibnitz are also credited with the development of the mathematics of Calculus. Bernoulli also worked in many areas of mathematics and physics and had a degree in medicine. In 1724, at age 24, he had published a mathematical work in which he investigated a problem begun by Newton concerning the flow of water from a container and several other problems involving differential equations. In 1738, his work Hydrodynamica was published. In this work, he applied the conservation of energy to fluid mechanics problems.



Which camp is correct? How is lift generated?



When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.



What is the argument?



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. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time. If we know the correct velocity distribution, we can use Bernoulli's equation to get the pressure, then use the pressure to determine the force. But the equal transit velocity is not the correct velocity. Another incorrect theory uses a Venturi flow to try to determine the velocity. But this also gives the wrong answer since a wing section isn't really half a Venturi nozzle. There is also an incorrect theory which uses Newton's third law applied to the bottom surface of a wing. This theory equates aerodynamic lift to a stone skipping across the water. It neglects the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas.



The real details of how an object generates lift are very complex and do not lend themselves to simplification. 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. The conservation of mass introduces a lot of complexity into the analysis and understanding of aerodynamic problems. For example, from the conservation of mass, a change in the velocity of a gas in one direction results in a change in the velocity of the gas in a direction perpendicular to the original change. This is very different from the motion of solids, on which we base most of our experiences in physics. The simultaneous conservation of mass, momentum, and energy of a fluid (while neglecting the effects of air viscosity) are called the Euler Equations after Leonard Euler. Euler was a student of Johann Bernoulli, Daniel's father, and for a time had worked with Daniel Bernoulli in St. Petersburg. If we include the effects of viscosity, we have the Navier-Stokes Equations which are named after two independent researchers in France and in England. To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.

Answer 12

Let's see if we can work through this.

In the beginning Bernoulli discovered a Principle that states in an ideal fluid (low speed air is a good approximation), with no work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or a change in the fluid's gravitational potential energy (thank you Wikipedia)

When aviation was in its infancy, aircraft engines were not powerful enough to generate the thrust needed to overcome weight and drag alone The Wright Brothers aeroplane only had a four horse power motor! So aircraft designs relied on the lift created using a curved upper wing surface to produce lift to cause an aircraft to fly. True to Bernoulli's principal, as an airfoil passes through fluid air, pressure is lower on the curved or upper surface of a wing while higher pressure remains on the lower surface of an airfoil or wing and the airplane flies.

We now know that given enough thrust, anything can fly. Take a brick and throw (thrust) it and it will fly through the air, uncontrolled but it will fly. Modern jet fighters have thrust in abundance. Most all other aircraft rely on lift generated from their wings or rotor blades to get off the ground. Thanks to the lift generated from the wings, aircraft engines do not have to work at 100% peak efficiency for an aircraft to take through the skies.



Aircraft that fly upside down are those with a high thrust to weight ratio. Take a look at those aircraft with a marginal thrust to weight ratio that fly upside down. If you look closely at them while they are upside down, you will notice that the angle of the aircraft is not a true straight and level. The aft end of the aircraft is signifigantly lower than the front portion. This allows the wings to generate lift by causing the angle of attack of the wing to be such that an upside down airplane is generating lift opposite of what it would normaly generate.

Of course aircraft like the Thunderbirds or Blue Angels have the thrust to overcome lift issues from flying upside down.



Even in modern aviation, Bernoulli's principal is alive and well

Answer 13

Personally I think that the lift is produced by the "clash" between the wing and the air,(Newtonian exp.) because there's an angle between the wing and the horizontal line, when the plane moves forward, simply the air "hits" the wing from below which produces force upward. Try to do this with a paper by moving it forward while it is oblique you'll find that the paper goes up.

Answer 14

Lift is a function of BOTH Bernoulli and Newtonian laws. A kite has "lift", due to the (simple?) action of the wind pressure on it's surface. If you design it as a wing, it will require less "force" of wind pressure to lift it from underneath, due to design. Like someone else mentioned, some acrobatic aircraft can fly upside down...what it means is they have enough power to overcome the Bernoulli design and force the wing to surf through the air regardless of orientation. But as a lift AID, it's the design which allows air to flow over the top of the wing at a higher speed which causes a pressure drop, and if you have the ANGLE of ATTACK, and the GROUND SPEED, then you will generate LIFT to get airborne. A wing design is only required to make it easier to get lift, not because it's absolutely necessary, if you had enough power and could make quick enough adjustments, you could make a 4x8 sheet of plywood fly. The wing is only a stabilizing function of the lift equation. Of course we always want stability, so we need correct form and design to get maximum efficiency out of our flying objects.

So, nutshell - lift is created by ANGLE OF ATTACK and DESIGN parameters. Both are necessary for optimum performance.

Answer 15

For the most part, highschool physics will do it for you.



Bernouli's principle; accelerating the volume of a gas decreases it's pressure.



On MOST aircraft, that's the way things work. The lower surface of the wing is flat, or close to, and the upper surface is curved. As the wing moves through the air, it forces the air passing over the upper surface to reduce pressure. The higher pressure underneath the wing pushes the aircraft into the air, explaining the definition you'll find of aircraft wing loading in square feet.



Hold a piece of paper in your hands so that it folds over your middle finger and hangs down away from your body. Blow across the top of the curve in the paper formed by how you're holding it, and the far end of the piece of paper will rise. You've created higher pressure in your lungs to exhale the air, but as the accelerated gas leaves your body, it loses pressure and the paper rises.



That's Bernouli's principle. It's the same way that a carb sucks fuel through a jet out of the float bowl to provide fuel/air mixture to an engine.



Angle of attack is related more to for Newtonian theory that for each action, there is an equal and opposite reaction. While there aren't many, some aircraft fly on this alone, but it applies to all aircraft.



As the angle of attack, nose up attitude of the airplane attached to the airfoil, increases, so will lift. Lift is the force that lifts an airplane.



You did it as a kid, do it now. Stick your hand out the window of a car on an interstate at 60 mph, elbow stiff, hand held flat. As you rotate your hand so that your thumb is higher than your pinky, you'll experience an upward force on your hand. Twist it the other way and you'll experience a downward force on your hand.



The other thing you'll notice is that with your palm tilted in the direction of travel, there's more pressure pushing your arm to the rear of the car. That's drag. As you tilt your hand forward and slide into the wind, this will abate. Somewhere in the middle, you'll find a point of stasis where your hand will stay at the same height, and the only force you have to exert is forward.



If you've got big enough engines strapped to it, a flat bottomed boulder will fly. Not efficiently, and certainly not controllably, but it'll fly.



Obviously, using Bernouli's principle to generate lift decreases the amount of thrust required to overcome the drag of Newton flight designs, allows greater speed and overall efficiency.



For most aircraft, equal parts of Bernoulian and Newtonian theory apply. Increase the pitch of the wings, and you increase the amount of air going over the top of the wing as well as slightly compressing the air underneath the wing. Try to increase lift on an airfame and you increase the drag on the airframe and lower the airspeed.



Obviously, you need to climb, and you need to be able to slow down to a speed where landing on hockey pucks at the end of stilts is controllable. Nose up, slow down, nose down, speed up. Throttle up, climb, throttle down descend.



Nose up and more power equals climb, nose down and power off equals descent.



Let's not get into the operation of turbines.

Answer 16

thrust from the propeller and the wings create lift. The propeller pulls the aircraft through the air and the forward momentum created puts air under the wings. the air under the wings is what creates lift.