experienced AVL Vortex Lattice users?

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Tom Kyler

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I know this is "homebuiltairplanes.com" and not "homedesignedairplanes.com"; however, this section is here :) I'm wondering if there are any experienced (and patient) AVL (Athena Vortex Lattice) users here? Disclosure. I'm a ME designing/engineering my own aircraft and fumbling my way through AE with lots of gaps in my knowledge I'm trying to fill, so I do not quite have the same aero foundation an AE would, but I do speak a bit of AE gibberish...and unforuntaely, I'm too old and don't have the time to go back for an AE education.

Anyhow, I'm working through my wing planform/layout and wanting to analyze, with AVL, a few variations of the 3D wing/flaps etc. With regards to AVL.....when looking at a simple, AVL sample file...unless I'm just not seeing the obvious, I see no input regarding any type of initial section Cl....and my mind is struggling to comprehend how the VL method could calculate the CL of the entire 3D surface with "no knowledge" of a 2D section Cl?; HOWEVER...and perhaps this is my real question. The AVL manual gives an example using the phrase: "(setting a CL constraint of 0.7).....will make alpha be implicitly constrained by the condition CL = 0.7". So does the reverse hold true? such that if a Alpha of 0.0 (default) is given, some Cl is assumed and indeed required for the calculation? I guess I'm not seeing this "implicit relationship".... Anyhow..trying to get out of the fog one step at a time. Anybody with radar, a radio, insight and some time is appreciated.
 

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ragflyer

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Quick point...keep in mind all airfoils have the same lift slope- they only vary in terms of zero lift AOA, max lift (stall) and drag. I will follow up with a more detailed post when I have a few spare mins later today.
 

ragflyer

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to continue ... max lift and drag are not considered in VLM as they require viscous effects to be taken into account. All that is required is lift slope and zero lift AOA. VLM assumes an airfoil that has no thickness (thin airfoil) and the lift slope given by simple airfoil theory to be 2PI per radian (0.11 per deg AOA) which is very close to actual slope of real world airfoils. Zero lift can be derived from the camber line.

In summary, VLM is very useful in knowing the lift distribution given a certain wing platform and consequently the induced drag. It also is useful in getting NP for stability calculations. All of this does not require section cl and is independent of airfoil geometry. On the other hand VLM cannot do max lift or profile drag of the wing as this would require viscous modeling and is dependent on airfoil section geometry.

Hope this helps in understanding why the actual section is irrelevant to VLM.
 

Tom Kyler

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Jul 30, 2019
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Location
San Antonio Texas
not quite yet, but that's on me, I'll take this and keep digesting till it does. Thank you for the response! Getting to the neutral point is, of course, a big part of this process also as I'm starting to shoehorn mass into the design . Thank you again!
 

ragflyer

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Many folks assume that a surface lifts because it is airfoil shaped. The reality is a flat plate or a barn will lift the same amount per degree of AOA. In fact as long as the flow does not separate all shapes will lift about the same per degree of AOA.

So why are wings airfoil shaped? In simple terms, an airfoil shape helps with two things. First it postpones flow separation for as long as possible so you get a higher max lift cl (postpones stall). Second it reduces profile drag for a given AOA.

Tools such as VLM only model behavior when the flow is not separated (small AOA), In this regime all airfoils are close to identical.
 

ragflyer

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One point I have not mentioned is that cm can also be well modeled by some versions of VLM. For this you would need to specify the camber of the airfoil which in a sense is describing the geometry of the airfoil. The camber line determines the loading of the section with respect to chord and hence determines cm.
 

Tom Kyler

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Jul 30, 2019
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Location
San Antonio Texas
..another point that is slightly confusing is that the AVL manual talks about VLM being well suited to surfaces.."at a small angle of attack", which begs the question, how do results typically trend if you start specifying larger angles of attack (or Cl assuming fixed lift slope) etc. FWIW, the whole driving force behind this inquiry of mine is probably moreso about stability than anything, certainly in the cruise config . I'm in a point in my design where I would like to start locking in some of the longitudinal placements as is found in cruise, etc.. Not sure if that narrows the discussion any assuming small AoA, Sorry for the ramblings...the early stages of learning concepts are tough.
 
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