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ohh I can't handle the pressure :)


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Ok, really... what is the relationship with Bernoulli's Principle and Magnus effect...or difference?

BP: airfoil, low pressure produces lift

MG: Cylinder, pressure difference, produces lift..

 

I know I am missing the boat on this...not putting it together or seeing the connection. Any comments (other then, calling me a dummy)

 

Thank you as always

:huh:

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Magnus effect has nothing to do with an airfoil producing lift. I believe the books author is trying to illustrate the fact that a difference in airspeeds produces a difference in pressure. This is in preparation for the introduction of Bernoulli's principle.

 

For all practical purposes (at least as a pilot) Magnus effect can be disregarded.

 

Do a search as 67november suggests. I, and others, have written more extensively about this and other confusing topics.

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We are to disregard what we don't understand? There is a circulation pattern around an airfoil. It contributes to some of the pressure difference accounting for lift. Magnus discovered it so he gets credit, as do Bernoulli and Newton for their respective parts. Its in the RFH for a reason, and its first btw. Remember that theoretical airflow lines used in airfoil diagrams are simplified for the purpose of illustration and aerodynamic lesson introduction. Review wake turbulence films for more reality. The one question an instructor cannot ask: "Are there any questions?". The one answer you cannot give: "Diregard...".

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We are to disregard what we don't understand? There is a circulation pattern around an airfoil. It contributes to some of the pressure difference accounting for lift. Magnus discovered it so he gets credit, as do Bernoulli and Newton for their respective parts. Its in the RFH for a reason, and its first btw. Remember that theoretical airflow lines used in airfoil diagrams are simplified for the purpose of illustration and aerodynamic lesson introduction. Review wake turbulence films for more reality. The one question an instructor cannot ask: "Are there any questions?". The one answer you cannot give: "Diregard...".

 

No one suggested disregarding it because of a lack of understanding. And if you read the forums, you will find there is quite a bit of understanding. I simply suggest that it can be disregarded because it is not critical to the understanding of the production of lift as it relates to helicopters.

 

Furthermore, Magnus Effect (ME) specifically refers to the lift produced by a spinning object (ball, cylinder, etc). It does not in any way describe the method by which an airfoil produces lift, except the fact that differential pressure is a major factor in that lift. As far as I am aware there are no helicopters that use spinning cylinders in lieu of rotor blades. The presentation of ME is used as a method to introduce a student to the concept of differential pressure producing lift. It could also be said that it is an introduction to the illustration of simplified flow patterns that are used extensively throughout the aerodynamics portions of the RFH.

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Lift part 1: "..an upward force known as the ME. This mechanically induced circulation illustrates the relationship between circulation and lift. An airfoil with a positive AOA develops air circulation..." Part 2: Air flowing over the top surface accelerates. The airfoil is NOW subjected to BP or venturi effect." (RFH 2-3) If it had no part, an author need not explain apples to prove oranges. Daniel discovered his part watching the sewer flows, and most people have seen leaves speed up through a narrowed area, even off their own driveway. Even those that don't golf or throw 'barrels' get that. All airfoils have circulation. Part 3 is Newton's Third (reaction force). Rotorcraft-flying-handbook: in the PTS. VR is not.

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Lets take a closer look at that section, shall we.

 

LIFT

MAGNUS EFFECT

The explanation of lift can best be explained by looking at a cylinder rotating in an airstream. The local velocity near the cylinder is composed of the airstream velocity and the cylinder’s rotational velocity, which decreases with distance from the cylinder. On a cylinder, which is rotating in such a way that the top surface area is rotating in the same direction as the airflow, the local velocity at the surface is high on top and low on the bottom.

 

As shown in figure 2-7, at point “A,” a stagnation point exists where the airstream line that impinges on the surface splits; some air goes over and some under. Another stagnation point exists at “B,” where the two air streams rejoin and resume at identical velocities. We now have upwash ahead of the rotating cylinder and downwash at the rear.

 

The difference in surface velocity accounts for a difference in pressure, with the pressure being lower on the top than the bottom. This low pressure area produces an upward force known as the “Magnus Effect.” This mechanically induced circulation illustrates the relationship between circulation and lift.

 

An airfoil with a positive angle of attack develops air circulation as its sharp trailing edge forces the rear stagnation point to be aft of the trailing edge, while the front stagnation point is below the leading edge. [Figure 2-8]

 

Figure 2-7. Magnus Effect is a lifting force produced when a

rotating cylinder produces a pressure differential. This is the

same effect that makes a baseball curve or a golf ball slice.

 

I am going to keep this incredibly simple. Magnus Effect specifically relates to the lift produced by a rotating object, not with how an airfoil produces lift.

 

As I said, the author is trying to relate a more easily understood concept (ME) to a more difficult to understand concept (lift production through mechanical circulation around an airfoil) to aid in a students understanding. The lift produced by circulation around an airfoil IS NOT and CANNOT be ME. Even the RFH specifically states that ME occurs around a spinning cylinder.

 

Magnus effect

From Wikipedia, the free encyclopedia

 

The Magnus effect is the phenomenon whereby a spinning object flying in a fluid creates a whirlpool of fluid around itself, and experiences a force perpendicular to the line of motion. The overall behaviour is similar to that around an aerofoil (see lift force) with a circulation which is generated by the mechanical rotation, rather than by aerofoil action.[1] In many ball sports, the Magnus effect is responsible for the curved motion of a spinning ball. The effect also affects spinning missiles, and is used in some flying machines.

 

German physicist Heinrich Magnus first described the effect in 1853,[2] but according to James Gleick, [3] Isaac Newton described it and correctly theorised the cause 180 years earlier, after observing tennis players in his Cambridge college.

 

Note the emphasis I have placed on the second sentence.

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The standard explanation for ME is that it has nothing to do with lift produced by an airfoil, but that it describes differential pressures or something, blah blah blah, spinning cylinders, etc, etc, etc. Forget curve balls and rotating cylinders. This is the key sentence:

 

The production of lift is based upon the airfoil creating circulation in the airstream (Magnus Effect) and creating differential pressure on the airfoil (Bernoulli’s Principle).

 

The "spinning cylinder" is the airflow around the airfoil. In RFM's figure 2-7, stick a wing in the middle of their blank sphere and you're getting closer to what they're talking about. Magnus Effect generates the differential velocities above and below the wing that then lead you to the effect from B's Principle.

 

Another piece of the puzzle is this sentence...

 

An airfoil with a positive angle of attack develops air circulation as its sharp trailing edge forces the rear stagnation point to be aft of the trailing edge,

 

And, when they go on to talk about induced drag...

 

This causes a spiral, or vortex, which trails behind each blade whenever lift is being produced. These vortices deflect the airstream downward in the vicinity of the blade....

 

That rear stagnation point/downwash aft of the airfoil causes a trailing vortex to develop behind the wing, and that vortex in turn generates circulation around the airfoil. The trailing vortex acts like a gear turning the air around the airfoil...on top of the airfoil, air is circulated down, over the wing, and in the direction of travel. This accelerates the airflow on top of the wing, developing a low pressure region. Below the airfoil, the trailing vortex works the airflow forward, slowing airflow under the wing and generating the high pressure region there. This is the best I can do for a figure.

 

 

 

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That rear stagnation point/downwash aft of the airfoil causes a trailing vortex to develop behind the wing, and that vortex in turn generates circulation around the airfoil. The trailing vortex acts like a gear turning the air around the airfoil...on top of the airfoil, air is circulated down, over the wing, and in the direction of travel. This accelerates the airflow on top of the wing, developing a low pressure region. Below the airfoil, the trailing vortex works the airflow forward, slowing airflow under the wing and generating the high pressure region there. This is the best I can do for a figure.

 

 

 

 

Magnus is about rotating cylinders and the airfoil doesn't rotate, nor does the air flow backward over the bottom of the airfoil as the pic suggests.

 

Differing pressures are caused by the venturi effect = Bernoulli.

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Magnus is about rotating cylinders and the airfoil doesn't rotate, nor does the air flow backward over the bottom of the airfoil as the pic suggests.

 

Differing pressures are caused by the venturi effect = Bernoulli.

 

I didn't say the airfoil rotated. That would be silly. I didn't say that air flows backward over the bottom of the airfoil. That would also be silly. The figure depicts the generation of the trailing and bound vortices, and the resultant circulation pattern around the airfoil.

 

To get lift via Bernoulli, you need accelerated air over the upper surface, and relatively slower air over the lower surface. The camber of the wing is usually invoked to explain the acceleration: the longer path over the top means that air has to accelerate to rejoin the air passing over the bottom...if parcels of air like to stay together. In reality, the air flowing over the upper surface doesn't feel any compulsion to rejoin the air flowing over the lower surface [

], but it is still accelerated. This is an age-old problem with the simple Bernoulli's Principle explanation, and also fails to explain other observations (eg, that an inverted or symmetric airfoil can still produce lift). This leaves the Bernoulli-Principle explanation kinda lacking, since there is no way to explain how airflow is accelerated over the upper surface. Enter Magnus. But, you can accept the B's P explanation and write off Magnus Effect as some eccentricity thrown in for no apparent reason by the FAA. You will still get through all your check rides just fine.
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Kodoz’s picture looks like it suggests circulation theory of lift, which is related to the Kutta-Joukowsky theorem. I could be wrong.

 

The simplest explanation for lift is that it derives from the pressure distribution and shear stress (friction) on an object as the result of a fluid flowing over the surface of the object. Therefore, aerodynamic force on an object is the integrated distribution of pressure and shear stress exerted on the exposed surface. Stick your hand outside your car window and it is going to experience aerodynamic force. A flat plate will produce lift; however, at small angles of attack it is plagued by large amounts of pressure drag due to flow separation at the leading edge. Hence, why flat plates are not used on airplanes and helicopters. It just happens in nature that the shape of an airfoil is the most efficient at producing the forces necessary for flight by accelerating the fluid element over the top of the surface creating an imbalance of pressure. Remember that Bernoulli’s equation is only valid for inviscid incompressible flows. For example, Bernoulli’s principle would not apply to some helicopters where the rotor blades operate in low subsonic speeds at the root to transonic speeds at the tips. In real life air is viscid and compressible but at low speeds where V < 100 m/s incompressible can be assumed.

 

Aerodynamic force is a very complex subject where whole books have been written. Furthermore, all you would have to do is visit the American Institute of Aeronautics and Astronautics and wade through the journal articles to appreciate its complexities. For pilot training the pressure on the top surface is lower than the bottom surface of the wing - Man. However, a cylinder does appear in airfoil nomenclature and geometry as the leading edge circle or nose radius.

Edited by Tom22
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No one has ever taught me this, but I just kinda always thought that since the pitch of the blade is changing as collective is raised and as pitch is slightly adjusted accordingly as it spins in the plane of rotation that, as it does change one of the things the D spar/leading edge does in fact do is rotate backward just a bit and that is where I just kinda thought the magnus effect came into a bit of play and why it was part of the lessons, but I never really cared very much anyway, I just learned the answer so I could pass my stage checks and check rides. :P

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Kudos to kodoz; I think you get it :). To rodrop (OP): During my training I became tired of, and frankly insulted by, the typical "accepted" non-answer, or worse exclusion of discussion on this topic. No, no, its taboo; don't even say the name (Magnus) or you'll turn into a pillar of salt. This even from nice/smart guys/gals, repeating what they received and perpetuating this problem. Aviation students deserve better than I and others got, and not only during ground sessions. I was hence determined to fulfill the considerable responsibility of an AGI/CFI, especially regarding primacy. Being a Veteran and admiring the KISS principle, I was delighted to discover the truth, right under my nose on those (less than) two pages of "plain" English.

 

Do you get how a good test score for supply type questions (multiple choice), relies both on your subject knowledge and the ability to decipher the language used by the author (question, answer, and dsitractors)? That's how I became 'cured'. Notice how, throughout our "Bible" the RFH, each chapter has a titile, an introduction, and sections, whose presence is made known with a header using an extra large first letter. The parts of the sections are labeled with smaller type, no larger first letter, but still bold and caps. The Lift (extra large L!) section has three parts, each relating their relationship to both airfoils and lift (using those words). These parts are lift sources (next to last paragraph), and essentially placed in event order.

 

The "accepted" but unsubstantiated 70/30% values, ignores the stated existence of circulation, it as being the source of BP's required acceleration, and does not jive with "a comparatively small percentage of the total lift" attributable to Newton's Third.

 

A golf ball uses circulation, a carburetor venturi effect, and a ceiling fan reaction force.

An airfoil must use all three to get our butts in the air up there; nothing complex about that.

RFH 2-4 "In summary, the production of lift is based upon the airfoil creating circulation in the airstream (ME) and creating differential pressure on the airfoil (BP)."

 

It is actually all there, in black and white. I got past the messenger, and found the message (symbol vs. symbolized; AIH/FOI). I hope you do as well. Shiny up all...and KISS(ES).

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Be cautious when using the Venturi effect when explaining lift.

See the link below.

 

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

 

A golf ball uses circulation in flight and aerodynamicist call the dimples tripping the boundary layer. The dimples artificially create a turbulent boundary layer at the front face at low values of Reynolds number to decrease pressure drag. A laminar boundary layer separates more easily than a turbulent boundary layer. Just a bit of trivia.

Edited by Tom22
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Well I guess we can all agree to disagree. I still like venturi. ;)

 

My point is simply to keep it simple. The majority of my students would never want to go this in depth. They just want to fly helos! The ones that do want to study these areas further I encourage to do so on their own time. I've got to get them through a course in a reasonable amount of time and there is a lot to learn!

 

I did find that NASA site interesting. Their explanations for their theories of lift are here.

http://www.grc.nasa.gov/WWW/K-12/airplane/right1.html

http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

 

What they do say about magnus is here. Check out the ship at the bottom of the page!! :lol:

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

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