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Revolution Models . CarbonXtreme . Midland Helicopters

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Main Discussion > Which lift effect is greater?
 
 
cessna151
Veteran
Location: In transit... Originally Indiana

The low pressure area above the airfoil of a blade or the high pressure under the airfoil?

--Eagles may soar high, but weasels don't get sucked into jet engines!--
12-03-2007 06:04 PM
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TerranAce007
Veteran
Location: Texas

Lift is the result of the difference in pressure above and below the airfoil. They are part of a single system, not independent forces.

USE LINUX!
12-03-2007 06:10 PM
 
 
Droid
Elite Veteran
Location: Deep down in the Southwest- UK

Good question actually, for a symetrical section aerofoil the distance travelled by the passing airflow is the same therefore the pressure difference seen for a standard aerofoil is different??

Let the boffins join in......
12-03-2007 06:23 PM
 
 
AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

Both matter and change with airfoil shape and angle of attack.
12-03-2007 06:24 PM
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cessna151
Veteran
Location: In transit... Originally Indiana

Refering to airwolf's diagram, it would appear that the low pressure is stronger than the high pressure. I never would have thought that to be the case.

--Eagles may soar high, but weasels don't get sucked into jet engines!--
12-03-2007 06:30 PM
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Droid
Elite Veteran
Location: Deep down in the Southwest- UK

I suppose the difference is that symetrical gives no lift at a zero degree angle of attack, whereas semi-symetrical will. Answered my own thoughts.....
12-03-2007 06:31 PM
 
 
cessna151
Veteran
Location: In transit... Originally Indiana

Quote 
I suppose the difference is that symetrical gives no lift at a zero degree angle of attack, whereas semi-symetrical will.

Good point, lets assume for the moment a symetrical airfoil with a positive angle of attack.

--Eagles may soar high, but weasels don't get sucked into jet engines!--
12-03-2007 06:37 PM
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dannyh5
Senior Heliman
Location: England

it's not stronger above the wing. THE AREAS ARE EQUAL. You are refering to a localised high spot. Think about it, the system must remain equal.
12-03-2007 10:25 PM
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Bad Karma
Veteran
Location: UK

Plus that wing section is non-symetrical, symetricals would have a lower pressure spike on the top of the wing (flattened shape), like in the above picture, the pressure gradients get better (more lift) when it is non-symetrical.

You have to be careful when using those types of pictures though, they only show pressure magnitude and direction, and by the looks of it, thats not a typical plot judging by the pressure distribution near the leading edge, or has been simplified, some of that should be creating drag mainly rather than lift, but it doesnt look the right shape.

There should be a distinct lobe at the leading edge, that isnt blended into the area depicting the pressure on the bottom surface
12-03-2007 10:38 PM
 
 
Topher
Veteran
Location: Grosse Ile, Michigan

Ace had it right before any of you posted anything. There is no such thing as a "low pressure force" as you are referring to it. The only force creating lift is the one thats underneath the wing. Now, which effect contributes more to that force on a semi-sym wing, Bernoulli or Newton. Thats a subject worth discussing.

Get a life? How the hell do you fly a life?
12-04-2007 12:24 AM
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CK_
Senior Heliman
Location: Redondo Beach, CA

If you look at that pressure plot, it's easy to visualize in your mind adding up this distributed force to get a net lift. And you can mathematically integrate (add up) that pressure function to get the net lift and pitching moment. Drag is another story... However, this is a difficult and cumbersome way to find the answer.

There is a much simpler and more elegant solution to the problem. It's called the circulation theory of lift. I won't get too far into it because you can use Google but this is it in a nutshell. Air has viscosity and all airfoils have sharp trailing edges. When the viscous air flows back together again at the trailing edge of an airfoil, the viscosity forces the flow to match velocity, kind of like how two lanes of merging traffic will have to travel the same speed. This flow similarity is called the Kutta condition. This matching of velocities is what causes air to flow faster over the top of the airfoil and slower over the bottom of the airfoil. The net effect of this is a horizontal vortex wrapped around the wing. This is called circulation. Lift is directly proportional to circulation strength. No circulation, no lift. Notice I have not yet mentioned airfoil shape. In thin airfoil theory, as long as the airfoil is not stalled, every airfoil produces the identical amount of lift for the same angle above the zero lift angle of attack regardless of airfoil shape. In the real world, this holds true plus or minus a few percent. A sheet of plywood at 2° angle of attack would produce the identical lift as the best computer optimized symmetric airfoil at the same angle of attack. This is why people can fly their helis with wood rulers for blades. Airfoil shape affects drag, stall characteristics, and pitching moment but not lift assuming the airfoil is not stalled.

This works for 2 dimensional airfoils and 3 dimensional wings. On a 3D wing the lift varies across the span. Pick an arbitrary point along the span. If the strength of the lift distribution to your immediate right is different than that to your immediate left then you have a spanwise difference in vorticity. This difference between the two vorticies is another vortex that trails from the wing and gets blown downstream. The more rapidly the lift distribution changes, the more vorticity gets shed downstream. The lift distribution changes most quickly at the wingtips where the lift distribution rapidly drops to zero and this is where the strongest vorticity is seen in the wake as wingtip vortices. Induced drag is caused by the energy lost to all the swirling vortices trailed downstream from the wing.

This took a while to type so I hope some of you had the light bulb go off in your head.

12-04-2007 02:42 AM
 
 
oldboldpilot
Key Veteran
Location: Southern California

Wow, here's where I come in.

OK, you guys were looking at a pressure distribution over a symmetrical airfoil, which was correct, but some of you didn't buy it? Why not?

Consider that there is something special about an airfoil shape. There is, and there is a lot of math to it, but there is, indeed.

What is special is that an airfoil shape causes circulation[U].

For simplicity, picture a constant-chord airfoil and any reasonable airfoil section flying right to left. Circulation causes a clock-wise whirlpool centered on the 1/4 chord point. That's from the math..

This whirlpool makes the air move faster over the top of the airfoil than the bottom.

Bernouilli then says the pressure is lower if the air is moving faster, which is over the top, thus you get a net force up - lift.

The standard explanation about air having to move further over the top than the bottom is garbage. If this explanation were true, inverted flight would be impossible. Think about it..

Helis are Man's Defiance of the Laws of Nature - OCHC
12-04-2007 04:03 AM
 
 
"Cam"
Veteran
Location: Asia

The above expanation is what I was taught in school. (Never liked it). I like the idea that an aerofoil is shaped to allow the flow to move like natural stream-lines, simply reducing drag.

ALL shapes can generate lift, just some make less drag.

A heli blade has to be thick enough so it has resistance to bending, thin spars (or thin composite sandwitches)are too flexible. Otherwise blades may be almost flat.

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12-04-2007 04:49 AM
 
 
Bad Karma
Veteran
Location: UK

oldboldpilot, there is a lot of garbage there, having studied aerospace engineering, I can say for myself that I really dont subscribe to the circulation model.

Cam UK, I agree with you, I dont like the circulation explaination either, seems just wrong.

Infact there is no reason why the air above the wing has to meet up with the air below the wing, its not like the molecules have a link between them so that they know where each other is, also the fact that in the case of most wings below M1.0 a good portion of the trailing 50% is in turbulent flow, the laminar free stream above that isnt going to join back up until quite a bit behind the wings trailing edge.

Even without the circulation model, inverted flight is very easy, even a non-symetrical wing will produce negative lift, just not at as low of an AoA as for upright lift, it goes from relativly efficient right way up, to fairly inefficient inverted but still working.

Topher, the reason for the questions about the pressure plot is that its clearly been simplified, as I said there should be a lobe of drag pressure near the leading edge that is distinctly not connected to the other pressure gradients.

Also to say that all the lift comes from the underside high pressure is wrong, it comes from the difference in pressure between the top and bottom sides of the wing, if the top side didnt reduce in pressure, the lift generated would be significantly less, its not a case of either/or, its a case of a bit of both.
12-04-2007 09:28 AM
 
 
CK_
Senior Heliman
Location: Redondo Beach, CA

Quote 
Refering to airwolf's diagram, it would appear that the low pressure is stronger than the high pressure. I never would have thought that to be the case.
If you could understand my circulation explanation, then here is what is going on. The circulation causes the pressure above the airfoil to decrease and the pressure below the airfoil to increase exactly in equal proportion. This pressure difference is the component directly responsible for lift. In addition to this, the air must also flow around the thickness of the airfoil. To do this, the air speeds up and the pressure drops on both sides as it flows around the airfoil. This component of the flow has no impact on lift. So the upper surface will see low pressure from the circulation (lift) plus low pressure from the thickness. The lower surface will see high pressure from the circulation plus low pressure from the thickness. The net result is that the upper surface decreases pressure more than the lower surface increases pressure as you have noticed. Hope that helped.

Quote 
Infact there is no reason why the air above the wing has to meet up with the air below the wing, its not like the molecules have a link between them so that they know where each other is
It is impossible for a viscous fluid to support an instantaneous velocity change across it. If you tried to shear two layers of a viscous fluid across each other, an internal shear force would be created. That internal force would lead to internal pressures that would accelerate the flow to smooth out the instantaneous velocity change to a more gradual one. So yeah, molecules in a viscous fluid do know what their neighbors are doing.

All the different theories of lift give the same answer in the end. It's just a matter of how you get there. The circulation theory is the most useful computationally but some people have a hard time understanding it.
12-04-2007 12:57 PM
 
 
Bad Karma
Veteran
Location: UK

Yeah but they stop being neibours (molecular scale) like when the air splits to flow over the top of the wing and the bottom of the wing.

Personally I dont really like it, it seems just odd, and its only really useful in professional uses for convienience.
12-04-2007 01:07 PM
 
 
nnonname
Heliman
Location: NewSanfraKota

Cessna151,

The theories surrounding how the airfoil works seem arguable. Simply, it seems on a flat bottom airfoil, lift is generated by the vacuum created on the top side because air is void there due to the leading edge deflecting air away from the trailing surface. On a symetrical airfoil, it seems, at nuetral attack, the lift or lowering effects are equal. (This is why fixed winged planes with symetrical airfoils are sometimes referred to as "point and shoot"- they go and continue to go where they're pointed until you "fly them" So, when using symetrical blades on a heli, the angle of attack is adjustable (ie: pitch) and this pitch actually is the method of introducing a sort of artificial flat bottom, or high pressure to the bottom of the blade and the resulting low pressure is again created on the top of the blade causing lift again. That's my theory, be it right or wrong. However, I am curious of your orginal question "which lift is greater?" seems the lift on the high pressure side must be greater or there would be no lift. Does the question really relate at all to inverted flight?
12-04-2007 02:19 PM
 
 
cessna151
Veteran
Location: In transit... Originally Indiana

It seems most people understand my original question but it is confusing, i will try to clarify...

First assume a symmetrical airfoil with a positive or negative angle of attack. If you have 3 barometers, place one in static air then place one each side of the airfoil. Would the low pressure or high pressure barometer show a bigger difference from the static air barometer?

I would think this would vary with angle of attack and also with the shape of the airfoil so an explanation of any variation is welcome. A graph showing the relationship of both would help.

--Eagles may soar high, but weasels don't get sucked into jet engines!--
12-04-2007 02:38 PM
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skydude
Heliman
Location: Gainesville, Florida, USA

Everything you said about AOA. The lower pressure and the higher pressure would have different relativities to the ambient pressure depending on everything about the wing. A sheet of plywood can be an airfoil.

If it were a simple problem to work out the best arrangement for a given task the Wright brothers would have had the perfect airplane. Or at least Howard Hughs.

There is a lot of misunderstanding about lift/wind force. I teach continuing education courses on the topic of hurricanes and the effect on structures, for design professionals.

Many people think suction is a force. It is not. The extreme of suction would be a vacuum. Apply a vacuum (complete lack of molecules) to a surface and what happens. Nothing. It is only an unbalanced situation where there is something on the other side of that surface pushing on it that dynamic things start taking place, or at least that something needs to be resisted.

A vacuum cleaner does not 'suck' up the dirt. The dirt is pushed into the machine by atmospheric air pressure.

--

Watch out all you moles!!! (Vae, puto deus fio)
12-04-2007 03:07 PM
 
 
GyroFreak
Key Veteran
Location: Florida ... 28° 50' N 81° 16' W

CK

Thanks, your gallery has a very interesting spread sheet on rotors. Something else to cause me to spend too much time on the computer and not flying.

Support bacteria. They're the only culture some people have. !
12-04-2007 03:53 PM
 
 
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