Nortonious

All I know is "LIFT" is the name of our new coffee.

If you put two bags in, you get a cup of "LIFT" and "THRUST"

shdw

Not really sure what you are talking about with this statement. See below, where I correct one of my previous statements.

Effective angle of attack is useless term IMO, that was introduced by pilots to avoid having to describe what is really happening. What happens isn't a change in some arbitrary idea "effective angle of attack," but instead each angle of attack has an increase in its efficiency or an increased lift for any given AOA. This is due to the increase in lift coefficient discussed throughout this forum, not some imaginary term.



Correcting a previous claim:

Earlier I claimed forms of lift include: impact, downwash or deflected air, and bernoulli.

This was a false claim which I later found out from another forum member and confirmed through "Flight Theory for Pilots" by Charles Dole and "Introduction to Aircraft Flight Dynamics" by Louis Schmidt.

The difference in air pressure around the wing is what causes lift, some of this from Bernoulli's principle and other more complex factors. Point is, lift is measured from one idea, pressure difference above versus pressure difference bellow the wing. All pressures above the wing is calculated and then all pressures below is calculated, their difference is lift.

ryan1234

NASA seems to use both terms: Effective Angle of Attack and downwash

Inclination Effects on Lift and Drag

Effective AoA is generally measured from the orientation where the wing has zero lift. Zero effective AoA is often (always?) lower than zero geometric AoA. This term is seen in various fluid dynamics and aerodynamics books.

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shdw

I stand corrected on this being an imaginary term, thanks. Though I believe it is still introduced by pilots to sound smart and avoid having to describe any of the basic concepts. The primary concept being difference of pressure which is lift, these terms, from what I am reading on your source and others, only act to effect the pressure in some way.

Here: "In practice, airfoils tend to shed vortices from the wingtip, which creates a downwash and deflects the local airflow in the vicinity of the wing downward by an angle of αi. This is the induced angle of attack. The airfoil section itself is then responding to an effective angle of attack equal to the geometric angle of attack minus the induced angle of attack: αeff = α - αi. This is related to finite wing theory." Source: angle of attack@Everything2.com

To me, this is saying that the downwash air is effecting local flow which, logical application here (or so I hope), would leave me to think that this action does something to change the pressure of the air around the wing. This fundamental change in pressure would be what actually is changing lift.

Your source appears, by my understanding, to further support this, "Near the tips of a wing the flow spills from the under side to the top side because of the difference in pressure. This creates a downwash which changes the effective angle of attack of the flow over a portion of the wing and affects the magnitude of the lift and drag."

The areas where "flow spills" from one side to the other would change the pressure at that given area of the wing. While it is still downwash, it isn't downwash causing the lift change but instead causing a change in pressure. This change in pressure would in turn cause a change in lift.

Basically these appear to me as factors that go into the larger idea, pressure differences around the wing. Am I misunderstanding this?

Also would you mind making some attempt to explain this: Thin Airfoil Theory Derivation

From what I get out of it, it is proving that pressure difference is how we properly calculate lift. Not by using Cl, "a measure of this doesn't make too much sense when dealing with hard facts," I believe you said, to calculate actual lift. However, the math is far above my left of understanding (Calculus, 4 years ago) and I am getting tied up in the nomenclature.

Cubdriver

I will, but I do not want to break down the math because it's too involved. Instead I will stick to words and offer a commentary. The math is not required for the purpose at hand anyway. Most of it is college Trig with some Calc III thrown in.

Well, kind of. It is a solution for finding the velocities around a flat plate in an inviscid (non-viscous) medium but with a bunch of caveats. 1) There must be enough viscosity to allow circulation to occur, but no more than that or you can't get very good results from the math; 2) the flow from the bottom (or top if it is flying at negative angle of attack) must not work its way back to the other side around the trailing edge. This is the famous Kutta Condition, which says a flow must leave the trailing edge of a wing or plate cleanly.

It took me a while to get back to speed on it too and I had the whole 6 years worth of courses. The math is downright tricky even for those often doing it, and it requires lots of practice. You begin to see how smart the great theorists were when you actually go through solutions like this and work them out.

Ok, back to the weblink. I do not recommend this weblink because it does not include most of the best reasons for having a Thin Airfoil Theory in the first place; not that this derivation is wrong of course, just that I know of a few better ones (my opinion), better in how they tie together the reasons for having the derivation with its uses and practical applications.

My recommendation is to put aside the current weblink for a bit and get a book like John J. Bertin, Aerodynamics for Engineers, Fourth Edition, Prentice Hall, 2002 (used $83). However, since we have the web link I will first offer a few general comments, go to bed, then come back for specifics related to contents of the point by point analysis.

The purpose of Thin Airfoil Theory is to quantify and show the relationships between circulation and velocity around a very simple airfoil, actually just a flat plate. To do this simple task, you have to make a bunch of assumptions some of which are listed above. Some more are-

1) no thick boundary layer
2) only small angles of attack
3) only small amounts of camber
4) thin airfoil (preferably flat)
5) incompressible flow (this means slow, for air)

Also, because there is no third dimension in play and very little viscosity, there is no drag to be found.

The name of the game in classical Thin Airfoil Theory is simply to show what velocity field would satisfy all these preconditions and develop an expression mathematically that covers it. Such an equation will be a Governing Equation from which other solutions can be obtained. This equation must satisfy the boundary conditions-

1) the vortex sheet we place on the airfoil to represent circulation must operate as a streamline of the flow, and there cannot be any air moving perpendicular to the airfoil surface

2) the circulation at the trailing edge must be zero. This is in fact what happens on a real wing or airfoil.

Later on we may kick the results around in order to extract useful information from it like coefficients of moment, lift, center of pressure, lift curve slope, etc.

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shdw

Thanks bud, I am still trying to take that all in, a bit tired now but I will get back to it in the AM. I don't have that book all I have is flight theory for pilots and aero for naval aviators. I have one other book Introduction to aircraft flight dynamics by louis schmidt which I am having trouble understanding.

Is there more to come, you said go to bed and come back for specifics...?

bubi352

trivia question: imagine yourself flying towards a sharp cliff and pitching suddenly straight up to fly parallel within half a span from this cliff. Are you experiencing ground effect?

Cubdriver

Yes, I'm working on it. Just got back from San Diego on work detail but I'll get to it asap. Sorry about the mod edit on your last post by the way, I hit the wrong button.

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Planespotta

. . . yes

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Da Magic

like i've said before, its PFM. nuff said.

I've been out of the aerodynamics classes for years. I couldn't even explain the physics behind it.

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