Aerodynamics

[msmspilot]

So just to clear up what you are saying:

1. Both the upper and lower surfaces of a wing have a lower pressure than the undisturbed air. However, because the upper surface essentially disturbs the air more (due to its camber) the pressure on the upper surface of the wing is only at a lower pressure relative to the lower side/ Or to look at it from another POV- the lower surface is a higher pressure because it has less camber than the upper surface, meaning it will displace less air COMPARED to the upper surface.

So if camber is why a wing produces lift, how does a flat piece of plywood produce lift?

EDIT: It won't let me upload my "stirring the pot" emoticon

 

[FDX8891]

So if camber is why a wing produces lift, how does a flat piece of plywood produce lift?

EDIT: It won't let me upload my "stirring the pot" emoticon

It's not just the camber of the wing that is responsible for producing lift. A flat piece of plywood can produce lift because of Newton's law of equal and opposite reaction. In the case of the flat piece of plywood creating lift, air (relative wind) strikes the lower portion of the plywood, and the air is deflected downward. Because the air is deflected downward, there is an equal and opposite reaction of the plywood that acts upward. This reaction of the plywood is commonly referred to as "barn door" or "impact" lift.

It is never just one "thing" that is responsible for creating lift. It is always bunch of variables (airspeed, angle of attack, wing size, wing shape, air density for example) working together.

 

[tgrayson]

In the case of the flat piece of plywood creating lift, air (relative wind) strikes the lower portion of the plywood, and the air is deflected downward. Because the air is deflected downward, there is an equal and opposite reaction of the plywood that acts upward. This reaction of the plywood is commonly referred to as "barn door" or "impact" lift.

Here is an image of a flat plate generating lift. As you can see, the air mostly doesn't "strike" the bottom of the plate, but flows parallel to it, just like a regular airfoil. The only place being struck is the leading edge, just like a regular airfoil.

Clearly, the lift on a flat plate is generated by the same mechanism as a regular airfoil, which is via the pressure differences between the top and bottom, generated in turn via circulation. The only difference between a regular airfoil and a flat plate is that the flat plate lacks the smooth transitions from the leading edge to the top and bottom surfaces of the plate and so the plate will stall at a low AoA.

 

[msmspilot]

Here is an image of a flat plate generating lift. As you can see, the air mostly doesn't "strike" the bottom of the plate, but flows parallel to it, just like a regular airfoil. The only place being struck is the leading edge, just like a regular airfoil.

Clearly, the lift on a flat plate is generated by the same mechanism as a regular airfoil, which is via the pressure differences between the top and bottom, generated in turn via circulation. The only difference between a regular airfoil and a flat plate is that the flat plate lacks the smooth transitions from the leading edge to the top and bottom surfaces of the plate and so the plate will stall at a low AoA.

tgrayson,
Am I correct that the main difference between a flat plate and a cambered wing is that the cambered wing has a "nicer" stall?

 

[tgrayson]

tgrayson,
Am I correct that the main difference between a flat plate and a cambered wing is that the cambered wing has a "nicer" stall?

The flat plate probably has a leading edge stall, but it also can generate a higher AoA before the stall, and hence ligher lift coefficient. There are also probably lift coefficients where the drag exceeds that of more properly shaped airfoils due to partial flow separation.

 

[msmspilot]

The flat plate probably has a leading edge stall, but it also can generate a higher AoA before the stall, and hence ligher lift coefficient. There are also probably lift coefficients where the drag exceeds that of more properly shaped airfoils due to partial flow separation.

But the disadvantage is no or very little warning of the stall?

 

[FDX8891]

Here is an image of a flat plate generating lift. As you can see, the air mostly doesn't "strike" the bottom of the plate, but flows parallel to it, just like a regular airfoil. The only place being struck is the leading edge, just like a regular airfoil.

Clearly, the lift on a flat plate is generated by the same mechanism as a regular airfoil, which is via the pressure differences between the top and bottom, generated in turn via circulation. The only difference between a regular airfoil and a flat plate is that the flat plate lacks the smooth transitions from the leading edge to the top and bottom surfaces of the plate and so the plate will stall at a low AoA.

So, lets say we have a piece of paper and stick it right in front of a blowing fan. Air strikes completely perpendicular to the piece of paper. Since the air hits one side of the paper, there is an equal and opposite reaction of the paper to blow backward.

Yes, as you bend the piece of paper so that it meets the wind at less than a 90 degree angle, pressure differences do take place a la Monsieur Bernoulli's Law, but I really don't see how you can tell me that impact lift is not responsible for any component of lift at all in regard to the "plywood" scenario. Maybe I spoke on too absolute of terms. It's simple, air hits the bottom portion of the wing, and there is an equal and opposite reaction that pushes the wing upward. While it is not solely responsible for all the lift that is created, it surely is a component.

 

[tgrayson]

But the disadvantage is no or very little warning of the stall?

From what I recall of the lift slope diagram, it's "peaky", meaning that lift drops off abruptly at the stall, rather than gradually, which is a characteristic of airfoils with good stalling behavior. But I'll have to verify the shape of the curve when I get home.

The better stalling behavior is a trailing edge stall, because the portion of the wing that's stalled starts off small, then grows as the AoA increases. The initial separation at the trailing edge doesn't affect lift that much, but the turbulent flow hits the horizontal stabilizer and provides good warning.

 

[sundog]

So, lets say we have a piece of paper and stick it right in front of a blowing fan. Air strikes completely perpendicular to the piece of paper. Since the air hits one side of the paper, there is an equal and opposite reaction of the paper to blow backward.

the back side of the paper has stagnent air, static pressure only.

the fan side of the paper has static pressure plus the dynamic pressure from the moving air. the fan itself increased the pressure of the air on that side.

the paper moves because the pressure on the side of the paper with dynamic+static pressure is greater than the static pressure, with no additional energy added, of the air behind the paper. so there is a pressure difference over an area and that results in force.

it's the same thing as a sail on a boat.

the only forces acting on a wing are pressure differences over an area. and weight, thrust, and drag of course.

 

[shdw]

Air strikes completely perpendicular to the piece of paper.

This was the interesting lesson for me in this quest for impact lift understanding, which I now know doesn't exist. The air doesn't "strike" the surface. Instead, it diverts around the surface while, at the same time, changing the relative pressure on the side you think is being struck.

 

[tgrayson]

So, lets say we have a piece of paper and stick it right in front of a blowing fan. Air strikes completely perpendicular to the piece of paper.

Actually, the airflow bends away as it nears the perpendicular plate, just like it does for the wing. There is a small area of zero velocity air (highest pressure) at one point, which is the stagnation point, but the streamlines of the surrounding air bend away from that area of high pressure.

Here:

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The airflow applies a force to the plate even when it is flowing parallel to the plate, because that airflow has a higher than ambient pressure. That's why the Newton vs. Bernoulli is a false choice, because they're the same thing, since the Bernoulli effect is, in the end, due to Newton's third law. Pressure is simply the force applied via molecules of a fluid bouncing off another object.

The important difference between the airfoil and the above plate is that the airfoil creates a greater pressure reduction on top of the airfoil, which increases the lift compared with what you would calculate with just the air bouncing off the bottom. If you consider the above plate an airfoil, it's in the stalled condition.

 

[tgrayson]

From what I recall of the lift slope diagram, it's "peaky", meaning that lift drops off abruptly at the stall, rather than gradually, which is a characteristic of airfoils with good stalling behavior. But I'll have to verify the shape of the curve when I get home.

Actually, it turns out that a flat plate has a very rounded peak on the lift slope curve, meaning that lift declines gradually with increasing AoA. This is contrast to some thin airfoils which show the leading edge stall that I described.

The reason that a flat plate has more gentle stalling characteristics than some other thin airfoils is that the airflow detaches then reattaches a small distance away, creating a separation bubble. The bubble grows larger and larger as the AoA is increased, with a gradual decay of lift. I wasn't able to determine whether or not it provided the same turbulent airflow to provide warning of the stall that a rear-stalling airfoil does. Still, the gradual loss of lift should proavide some indication, compared to the leading edge stall which gives up the ghost all at once.

 

[Fly_Unity]

In my college class in Aerodynamics, they really stressed that Newtons law wasnt the primary source of lift. Bernoulli law applies to every principle of lift. Some flying books will incorrectly show a picture of someone water skiing on water and compare it to lift, But its not the same principle UNLESS the water ski's are IN the water (Not on top of it).

A flat piece of plywood can create lift by having an Angle of Attack, which creates a so called camber. The camber is responsible for the lift, Not the air striking the underside.

 

[msmspilot]

Actually, it turns out that a flat plate has a very rounded peak on the lift slope curve, meaning that lift declines gradually with increasing AoA. This is contrast to some thin airfoils which show the leading edge stall that I described.

The reason that a flat plate has more gentle stalling characteristics than some other thin airfoils is that the airflow detaches then reattaches a small distance away, creating a separation bubble. The bubble grows larger and larger as the AoA is increased, with a gradual decay of lift. I wasn't able to determine whether or not it provided the same turbulent airflow to provide warning of the stall that a rear-stalling airfoil does. Still, the gradual loss of lift should proavide some indication, compared to the leading edge stall which gives up the ghost all at once.

Interesting. I would like to see those diagrams. I guess I'm going to have to get some "real" aerodynamics texts.

 

[sundog]

In my college class in Aerodynamics, they really stressed that Newtons law wasnt the primary source of lift. Bernoulli law applies to every principle of lift.

neither one causes lift. lift is a phenomenon that both theories describe and explain. in other words, lift happens. bernoulli is one way of partially explaining and describing the thing that happens, and newton is another way to partially explain what happens, and likewise magus, circulation, etc.

water-skiing actually is lift, but since it happens at the boundary of two very different fluids, and because the Reynolds numbers are so very different from what we deal with for aircraft, it makes a horrible analogy.

 

[tgrayson]

Interesting. I would like to see those diagrams. I guess I'm going to have to get some "real" aerodynamics texts.

 

[sundog]

Interesting. I would like to see those diagrams. I guess I'm going to have to get some "real" aerodynamics texts.

one i found quickly is fig 4.31 on page 297 of Anderson's "Fundamentals of Aerodynamics" ISBN 0-07-001679-8

 

[msmspilot]

Thanks!