Flight

[AtomSplitter]

How do Aeroplanes actually stay in the sky? does anyone know ?Exactly how?

[Pantaz]

Why not start by telling us how much you know about the process?

[AtomSplitter]

not a great deal, but i read a small article that no-one is sure just why it stays in the air, i just wanted to ask if anyone here knew how?

[the tree]

Okay well more or less the lift is achieved by ensuring that there is a lot more air pressure under the wing than above. The high air pressure below the wings pushes the aeroplane up in the same sense that blowing underneath a sheet of paper will lift the sheet of paper up.

The difference in air pressure is created by the shape of the wing (an airfoil) directs the air travelling towards it (since the plane is moving forward) underneath. The faster the forward motion of the plane, the higher the difference in air pressure.

[AtomSplitter]

why exactly does the shape create a higher air pressure?

[the tree]

That's a hell of a difficult question.

 

If a particle is heading from point A to point B then it will take the path of least resistance which can explain the way that water moves from a spring at the top of the hill towards the foot of the hill when it's pulled down by gravity.

 

If you were to have an airfoil with the front facing upwards and then poured water directly on top of it you'd note that nearly all of the water is directed to the underside of the airfoil, it does exactly the same thing to air that is heading towards it (or that it is heading towards, depending on your frame of reference).

 

Does that help at all?

 

(water and air are both Newtonian Fluids and so they behave in a similar way, water is just a lot denser)

[swansont]

why exactly does the shape create a higher air pressure?

 

Bernoulli's principle is one way of looking at it: when you make air flow faster the pressure drops — each has an energy associated with it, and energy is conserved. The air that isn't speeded up (as much) — the bottom of the wing — has a higher pressure.

 

Another way is through Newton's laws (also referred to as angle-of-attack). The wing deflects air and the recoil force pushes on it, generating lift.

 

Depending on the geometry and circumstances, one explanation may be easier to visualize than the other.

[the tree]

Was my understanding of what an airfoil does wrong, then?

[AtomSplitter]

thank you very much, this has been a great help, both of you, if any one has anything else that may help, it would be greatly welcomed. or if you want to add anything else please do

[Pantaz]

NASA has some fantastic resources online. Their free "FoilSim" software graphically simulates different airfoils (wings) and the resulting air flow & pressures.

 

Beginner's Guide to Aerodynamics (NASA)

[swansont]

Was my understanding of what an airfoil does wrong, then?

 

I don't think so. From what I can tell it's consistent with Bernoulli's principle.

[Klaynos]

I don't think so. From what I can tell it's consistent with Bernoulli's principle.

 

This is also my understanding.

[dirtyamerica]

+1 on Bernoulli's Principle

[Mr Skeptic]

My understanding was that a small part of the lift is generated via Bernoulli's Principle and the shape of the wing, and the major portion is generated by deflecting the air downward due to the angle of attack.

[insane_alien]

mr skeptic, it is the same thing. bernoullis principle generates the pressure difference on the wings which gives lift and newtons third law means air also gets chucked down.

[D H]

Bernoulli's Principle does not explain hypersonic flight, period, and it does not fully explain subsonic flight. Even for subsonic flight, one also needs to account for the Coanda effect, vorticity, angle of attack. Bernoulli's Principle alone does not explain flight.

 

Ultimately, if something does not result in a downward deflection of the airflow there will be no lift. Newton's third law.

 

Excerpts from some references (click on the hyperlink for the full article):

 

http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm

The conventional or standard explanation of aerodynamic lift states the higher streaming velocity at the upper side of the airfoil as cause of the lower pressure, due to Bernoulli’s law. But a higher streaming velocity is the effect of a lower pressure and never its cause. The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil - which depends on the angle of attack and its velocity.

 

 

http://www.aviation-history.com/theory/lift.htm

Many ask the simple question "what makes an airplane fly"? The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. We are going to show you that lift is easier to understand if one starts with Newton rather than Bernoulli. We will also show you that the popular explanation that most of us were taught is misleading at best and that lift is due to the wing diverting air down.

 

 

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

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.

[swansont]

Many ask the simple question "what makes an airplane fly"? The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. We are going to show you that lift is easier to understand if one starts with Newton rather than Bernoulli. We will also show you that the popular explanation that most of us were taught is misleading at best and that lift is due to the wing diverting air down.

 

"Easier to understand" is not the same thing as right vs wrong

 

Bernoulli is a statement of conservation of energy. Unless one is going to propose that energy isn't conserved, Bernoulli is just fine. Newton is a statement of conservation of momentum. Likewise, this is fine as well. In my observation, it usually boils down to which example one gives — there are situations where it's easier to see Bernoulli (the horizontally oriented asymmetric wing that's flat on the bottom) and others where it's easier to see Newton (symmetric wing at an angle).

[D H]

Explain inverted flight with the Bernoulli principle.

[insane_alien]

easy, the angle of attack makes the air over bottom of the wing flow faster than the air below(top being the surface facing the sky). it is not as efficient as normal flight(unless you have a symmetric wing) but is sufficient to provide lift.

 

as has already been said, both models hold. just one is an energy balance which must be true if the laws of physics are true and the other is a momentum balance which must be true if the laws of physics are true.

 

they are two ways of saying the same thing.

[D H]

easy, the angle of attack makes the air over bottom of the wing flow faster than the air below(top being the surface facing the sky).

That's begs the question. You haven't explained why angle of attack makes that happen. You have just said that it does; a hand-wave.

 

The problem with Bernoulli's Principle to explain lift is that elementary attempts to explain why the air moves faster over the upper surface fail. The most commonly used explanation, the equal transit theory, is a falsified theory. To get to some explanation of the pressure differential that does not rely on false premises you have to first arrive at the fact that an airfoil turns an air flow. So, why bother? The turning of the air flow fully explains lift.

[insane_alien]

That's begs the question. You haven't explained why angle of attack makes that happen. You have just said that it does; a hand-wave.

 

The problem with Bernoulli's Principle to explain lift is that elementary attempts to explain why the air moves faster over the upper surface fail. The most commonly used explanation, the equal transit theory, is a falsified theory. To get to some explanation of the pressure differential that does not rely on false premises you have to first arrive at the fact that an airfoil turns an air flow. So, why bother? The turning of the air flow fully explains lift.

 

i was not arguing equal transit, i know its a bunch of baloney.

 

consider a venturi meter, in the restriction the flow is faster now, to make it more like a wing, make it so the restriction only comes off the bottom side, the flow is still faster, remove the top completely and measure the pressure from the surface of the restriction and you will still find the pressure lowered and the velocity faster.

 

the increased angle of attack in effect makes the restriction larger leading to a faster flow over the top but it also slows down the flow under the wing relative to the wing. this results in an increased pressure difference. ie, more lift.

[D H]

consider a venturi meter, in the restriction the flow is faster now, to make it more like a wing, make it so the restriction only comes off the bottom side, the flow is still faster, remove the top completely and measure the pressure from the surface of the restriction and you will still find the pressure lowered and the velocity faster.

That is yet another erroneous application of Bernoulli's Principle. NASA's Glenn Research Center (the NASA center that researches aerodynamic flight) goes so far as to label this "Incorrect Theory #3".

 

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

Let's use the information we've just learned to evaluate the various parts of the "Venturi" Theory.

• The theory is based on an analysis of a Venturi nozzle. But an airfoil is not a Venturi nozzle. There is no phantom surface to produce the other half of the nozzle. In our experiments we've noted that the velocity gradually decreases as you move away from the airfoil eventually approaching the free stream velocity. This is not the velocity found along the centerline of a nozzle which is typically higher than the velocity along the wall.
  
• The Venturi analysis cannot predict the lift generated by a flat plate. The leading edge of a flat plate presents no constriction to the flow so there is really no "nozzle" formed. One could argue that a "nozzle" occurs when the angle of the flat plate is negative. But as we have seen in Experiment #2, this produces a negative lift. The velocity actually slows down on the upper surface at a negative angle of attack; it does not speed up as expected from the nozzle model.
  
• This theory deals with only the pressure and velocity along the upper surface of the airfoil. It neglects the shape of the lower surface. If this theory were correct, we could have any shape we want for the lower surface, and the lift would be the same. This obviously is not the way it works - the lower surface does contribute to the lift generated by an airfoil. (In fact, one of the other incorrect theories proposed that only the lower surface produces lift!)
  
• The part of the theory about Bernoulli's equation and a difference in pressure existing across the airfoil is correct. In fact, this theory is very appealing because there are parts of the theory that are correct. In our discussions on pressure-area integration to determine the force on a body immersed in a fluid, we mentioned that if we knew the velocity, we could obtain the pressure and determine the force. The problem with the "Venturi" theory is that it attempts to provide us with the velocity based on an incorrect assumption (the constriction of the flow produces the velocity field). We can calculate a velocity based on this assumption, and use Bernoulli's equation to compute the pressure, and perform the pressure-area calculation and the answer we get does not agree with the lift that we measure for a given airfoil.
  

[swansont]

Explain inverted flight with the Bernoulli principle.

 

Again, "easier/harder to explain" should not be construed with right/wrong.

 

Does conservation of energy fail with inverted flight? If not, Bernoulli's principle must hold, even if it not convenient to apply it and use it as an explanation.

[D H]

If not, Bernoulli's principle must hold, even if it not convenient to apply it and use it as an explanation.

Bernoulli's principle is not a law of physics like conservation of energy. Bernoulli's principle derives from conservation of energy assuming an inviscid fluid undergoing laminar flow at low Mach numbers.

 

The flow around an inverted wing is anything but inviscid or laminar. Conservation of energy does of course apply, but Bernoulli's principle is an improper expression of conservation of energy for inverted flight. In inverted flight, it is the bottom of the wing that does the bulk of work in diverting the airflow downward. Bernoulli's principle is approximately correct for normal subsonic flight when it is the upper surface of the wing that does most of the work in diverting the airflow downward.

 

Even for normal flight, Bernoulli's principle is to me but a hand wave: It begs the question as to why airflow is faster along the upper surface of the wing. By the time you get to a valid explanation for this effect (e.g., via the Coanda effect, the Kutta condition, onset of turbulence), you have already arrived at the fact that a wing diverts airflow downward.

[swansont]

I had no idea the flow assumptions failed for inverted flight — every time this had come up before, that part was never mentioned. Learned something new!