Introduction to Aeroelasticity in Nastran (NX Nastran with Femap)

so my name is Ben names I'm a senior aerospace analyst at a stretch analyst at SDA and today we're gonna I'm just going to give a brief overview and a bit of an introduction on conducting air elastic analysis using an X and a strand with C map now just as an overall kind of disclaimer to put out there at the front air elasticity and everything that it can encompass is a pretty broad field as specialized as it ends up being so I won't be able to get into too many specific details you know this isn't an air lasticity class in general hopefully this will give you a really good flavor as to some of the things that Nash tran is capable of running with fie map and I'll give you a really good kind of idea as to when when this type of analysis is useful and when you can you know really leverage it don't worry by the way my contact info is gonna be at the very end so if you want to get in contact with me about some question you might have had about the webinar that'll be there as well and this will be recorded so if anyone is worried about missing a step or something like that again you can email me or you could just go ahead and watch the recorded webinar all right let's get started so today for those of you who don't know I'm just gonna talk a little bit about who we are SDA is a company in what we do then I'm going to just give a little bit of a brief overview at best you could say about air elasticity I'll give an example a kind of conceptual example of an air elastic effect just to give you guys an idea what some of that coupling can how it can manifest and then I'm going to go over three examples static or elasticity just how to set up a simple static air elastic problem then I'll talk briefly about air lastik tailoring and just really give us a good hint or a flavor as to what you can achieve by using that or taking advantage of those kind of coupling effects when you start talking about composites and then finally I'm going to go over how to set up a simple flutter example by the way for those who might be interested this gift right here is a picture of the it's a body freedom flutter model that the Air Force Research Lab was made so interesting behavior yes you can't actually model body freedom flier and a strand so I'm not going to talk about that today but again that's that's a good example of some of the capabilities so SDA is a structural analysis aerospace company were based a little bit north of Dulles Airport about five miles north or so and we specialize in aircraft the design analysis of aircraft and spacecraft structures we specialize in composites and we have a whole range of different engineers from holding bachelor degrees up to PhDs this is just to give you a little bit of a an idea of some of the project we've worked on again we handle everything from spacecraft like the composite crew module in the Orion crew module to aircraft like the eros on and Shatter that you can see there we've handled a whole bunch of different other ones but this just gives you all a good ideas to some of the typical projects that we work on here on a day to day basis alright now in terms I you know I lived this to allude to this at the beginning I'm not going to be able to touch on everything so in terms of these air elastic solutions that nasara is capable of running I'm only going to touch on two today static air elastic response and aerodynamic flutter there is again the dynamic air elastic response that you can model as well as you can do a sensitivity optimization using Aero elastic models in this case not going to get into them today the other thing too that's worth mentioning today I'm only going to be using the doublet lattice method for the static or elastic case for those of you don't know this is going to boil down to a vortex lattice method so anyone's who's familiar with like X flyer or ABL something like that it's very simple kind of potential flow models that that will end up being used it is subsonic compressible but again mastering can handle any range of aerodynamic regimes as well just as a heads-up remember when we're talking about potential flow this does mean no drag so it'll be really good from a structural analysis standpoint an air elasticity standpoint unless you aren't too interested in the aerodynamic characteristics this is not going to be able to replace you know your aerodynamics team for example but it'll give really accurate lifting loads for for what you're the behavior that you're interested in and if you want to find out a little bit more about how the double lattice works double lattice method works you can check out my thesis if I go to my antenna there's a link right there so okay air elasticity what is it well fundamentally elasticity is a coupling between several different forces we get aerodynamic forces on earth and elastic forces now this sounds really basic so let's break it down and talk about some concrete more concrete examples for those of you who are familiar with free vibration or normal modes analysis where you're looking for the natural frequencies in a structure that's where you're looking at the interaction or coupling you might say between the structural mechanics the you know the elastic behavior of a structure and the inertial forces that that mass within a structure right so you have your mass and stiffness matrix that's ends up what what goes into that analysis the two types of other air elastic phenomena we'll be talking about today our static air elasticity static air lasticity which is the coupling of the elastic nature of the structure and our air dynamic forces and then we'll also be talking about flutter which includes all three of those guys so as a really simple example let's just take some weighing let's say we where we put a wing in a wind tunnel an elastic wing we give it you know there's some angle of attack and we end up generating lift about our quarter chord but we know from kind of simple beam theory applications that really our structure is going to respond at the year Center not at the aerodynamic center right and so when we apply lift at the aerodynamic center we actually get an additional torsion on the wing right well that torsion is going to produce some twists along the wing which locally depending on where you're at on the wing will give you an additional angle of attack at that station right okay so we just produced a little bit more of an angle of attack that means more lift right and again that means more torsion so we're twisting the weight again and we get an even higher angle of attack which means higher lift right so you can see how these forces are kind of playing this runaway kind of game where you're getting this feedback and hopefully you will converge to some kind of equilibrium solution the case where you don't where you just keep producing more and more and more lift is known as divergence right so you do end up producing greater forces with your wing but really what's important to keep in mind is that the aerodynamics end up softening the structure and actually reducing the effective stiffness all right so let's get into a more concrete example we're gonna start with let's just say you're part of a team where you're analyzing a wing in a wind tunnel it's gonna be fixed at the root down here okay you can see it's pretty high aspect ratio there are all the parameters that you might want in order to model this sans the structural information we're gonna assume the stem is provided for us ahead of time okay and with a further ado let's go ahead and jump on into that so I have the model already up here let me go ahead and just show you all so this is where we're constraining this model this is like the the wall of the wind tunnel and then the rest of it is you know again just going to be immersed in the flow alright so the first thing we need to do is actually just an artifact of how Nastran works we need to make a property card just an arrow panel property card it really doesn't matter any information on here since we're not doing a body so I'm just gonna put wing one hit okay and we're done that's it again this is really just an artist acted on a strand works now first most important stuff is we're gonna make an arrow panel okay so this is our aerodynamic model aerodynamic surface that we're going to be modeling some it again just put wing one for the name just really matter let's go ahead I'm going to turn filled edges on just that we can get a good idea as to where the leading edge of the wing actually is so let's zoom on in there let's try that again a hang on sometimes with these large aspect ratio wings rotating can be a little dangerous alright so we're going to select a point at the leading edge and we're also going to select a point at the leading edge of the tip of the wing right so that was the root this is the tip now so that's what that point one-in-four would typically correspond to and now we want to provide the cord distance from points one to two so generally this is one point one here again this is 2 this is 3 and this is 4 right ok now whenever your rustic analysis I've found that it's really helpful to have tables of these typical values again we'll get into the sweater stuff later for now we're going to focus on the static or elastic parameters so I'm gonna go ahead and copy our cord length which I noted ahead of time notice there is no sweep in this case of course you can't add sweet taper whole nine yards so that's that's something absolutely you can do generally for a good starting point I like to use about 10 boxes for the in the cord length for arrow panel you can use any any number that you want and then but obviously you know higher is going to be more accurate and then usually you want to try and keep the aspect ratio of your boxes to about an aspect on anything above 3 the accuracy starts to become rather suspect so keep that in mind usually this shouldn't end up being too computationally intensive all right don't worry about this error this is just because we haven't set up an error logic analysis yet they're asking for a some coordinate system but it's ok we'll resolve that later in the meantime we're done actually we set up our aerodynamic model right so we have a beautiful structural model nice and detailed let's actually turn off the ledges so we can see that a little bit nicer so we have our structural model we have our aerodynamic model now the only problem is we need some way to transfer information from the one model to the other right you saw that in our simple case with our airfoil section as we deformed right as our airfoil deflected and rotated really we ended up generating more lift right so as our structure deformed that information had to be passed to our aerodynamic model and then with that aerodynamic model we generate new lifts and then we have to apply those forces back to the structure so we need to use a spline to do that that's all that's all a spline really is you might here use you know reference that's really all a spline is it's that bridge that transfers information from your aerodynamic model to your structure model and then back okay now to set up the spline you'll see we'll need information about the aerodynamic model here in this case I'm going to just select all boxes on the aerodynamic model nothing too fancy about that and then in this case I went ahead of time it'll show these nodes in a second and I selected a series of nodes that I want to model on the structure okay all right so we're gonna hit enter now you can see so all these kind of teal points these are all the nodes that I've selected along the structure in order to transfer how the if the structural model moves how that end up ends up translating today aerodynamic model all right okay now with that we're all set for the model the model is done we don't have to touch that anymore at this place if we wanted to run a static your elastic and dynamic flutter analysis whatever you want the model is done so at this point we just have to set up an analysis oh let's go ahead and do that you can see how the normal modes set up here from when I was doing stuff before just testing it out all right static air elasticity let's go ahead and do static this city I'm gonna just put some information about our regime so if you remember from the from the slides we're gonna do at a free stream airspeed is 74 miles per hour and then the other important piece of information is going to be one the angle of attack equals one okay this is just no keeping it doesn't mean anything it's just a title but I find it's useful just if we run it later we'll know what's going on all right so we're gonna skip a few of those preliminary things now the first most important thing that I kind of I hadn't talked about this yet is we have this aerodynamic coordinate system right here move this one box so you see we have this aerodynamic coordinate system right here this is really important okay in order for the aerodynamics to be calculated correctly when you set up your model you need to make sure that it references this coordinate system such that X is always in the downstream direction okay Y is out the direction of one of the wings it really doesn't matter which direction except for then you have to make sure that Z is in the opposite plunging motion right so that's that's the way that list is going right lift is vertical so that's that's Direction going to be going so we're gonna set both our aerodynamic and reference coordinate system to that air elastic coordinate system there okay and now again these are some of those parameters that I said we would need to write in typically so there's our chord length again I still have that copied now this is a little bit tricky so stay with me if you have any questions I'd be happy to answer them later okay so don't get lost when we're referencing the span for our model we need to actually use the full span of the aircraft in this case right now we're only doing the we're only looking at a wing right but if this were on an aircraft the span would be from tip to tip okay so in this case we actually needed to put that value so I'm using double the span here in this case or double what we have geometrically if we were to measure it right so let's go ahead and put that in and now what's tricky is the wing area is the cord length times the span that we see in our physical model okay so that's actually times half of this value okay and again I've pre computed this right here so again put that in now finally we did say this this model for example is in a wind tunnel right so we're gonna make sure we want to consider symmetry about this XZ plane all right okay with that we have one last step we're going to enable trim on the air lesyk tremor a Matar typically whenever we see that when we're doing air lastik analysis Mach number is just an indication of how important compressibility effects 74 miles an hour is pretty slow so we can actually neglect that and just say we have a Mach number of zero and then we're going to go ahead and input our dynamic pressure you can see a little bit of the math that we that went into this now as a note if you'll notice the density that I'm using for this is in flinch per inch cubed okay or pound foot second squared per inch to the fourth this is because the baseline units from my model are in inches seconds and power and and that's lynches okay depending on what kind of convention unit convention you're using this can get tricky so this is where you really need to pay attention on that density what your units are in the model all right so let's go ahead and put that in there all right and the last last but not least we're gonna go ahead and put in some angle of attack we're gonna set that as a fixed parameter and we're going to put in one angle of attack but this has to be in radians not degrees C so 3.9 over 180 so that's one Radian go ahead and add that and then we have our typical kind of boundary conditions in this case we're just having a sixth constraint and that's it and we're done so at this point we set up our air elastic analysis we could write it no problem I'm gonna just go and go ahead and go over to the final results if we look at this model so same model right this looks relatively like a reasonable kind of distribution this looks like something we would expect if we wanted to we could go ahead and look at our max failure index nothing too revolutionary about that one thing I want you to know we're gonna look at it from the front you can actually imagine you have the option to run a rigid trim or a just normal air elastic analysis rigid trim analysis means that we're not taking to account the flexibility of our aircraft right now notice if we consider the full air elastic analysis if you pay close attention the deflections are slightly different okay now depending on how flexible your structure is compared with the whatever your dynamic pressure is this might not you know may or may not be a big deal one thing that's really important in this just a few things you take away from this this is one of those things for whatever reason I've found and I'm this this appears to be a bug that I'm working with people at Siemens actually on when writing your trim cards so hopefully some of these values we have our density here we have our angle attack here see map auto writes this one value and what happens is this parameter says that if it's one run a full air elastic normal analysis right consider the flexibility of structure if it's zero pretend the structure is rigid and generator dynamics that way now I found that no matter what this value is it tends to be running always nastran will only run a rigid air lastic analysis so whatever you do if you're ever for now until this update comes through the seam at people to remove this value because it is just an optional parameter whenever you're running a static air elastic analysis make sure to remove this value okay all right and then so yeah you would just go ahead delete that and then just hey they'll analyze from there all right so that that should give you a good flavor now one of the things that's awesome about this not once did I have to map any pressures not once did I have to talk to an aerodynamics team and yet I have 3d very you know relatively high fidelity aerodynamics for a pretty low computation time and it also considers the aerodynamics of the structure notice if I even if I use CFD I'm just going to be at best doing this rigid trim analysis right I'm just going to be applying those rigid aerodynamic loads again for this structure didn't be appear to be too big of a deal but for our next example you see those air lastic effects can really play a role in how the structure behaves in the overall list actually it's generated out of the structure if we were going to want to go ahead and check our our solution against some analytic results if we go ahead and use strip theory right so we assume the lift curve slope of 2 pi constant constant loading over the wing we predict our wing to produce about 30 pounds of lift and using mass transit lift analysis we get about 26 pounds if we consider the actual air elastic you know elastic structure our elastic lift we actually get about 3/4 of a pound so again in this case it didn't it wasn't significant but you'll see an example next where it was very significant so let's jump in the presentation again all right so let's talk about air elastic tailoring one of the biggest things that can come up if you have a flexible stretcher or for example there have been a couple of more famous forward swept wings your lift distribution is not going to be no matter how you design it it's not going to end up being some nice elliptic list distribution the aerodynamics team you know designed what's going to dup happening is the additional twist and deflection of your structure will end up resulting in some some other lift distribution right now what does that mean well in this case for this picture here we're putting more force out towards the tip and it's going to lead to a higher bending moment at the root right so definitely bad for us structure guys you can definitely play with some of these values for example you can either mess with your composite couplings you know your aunt isotropic couplings due to the fiber orientation stuff like that to tailor this lift curve slope so behaved a little bit more like an elliptic lift curve that you want but another thing that's really important to consider is that it can allow you to passively reduce the effects of for example of Guus let's say you get some big gust over the wing you can tailor your wings so that at that higher dynamic pressure you lash reduce the amount of lift that's generated or maybe keep it more constant as opposed to increasing and really spiking okay now today again we're going to mess with some bending torsion coupling but you can mess with any kind of composite couplings for your wing so the cross-section of our wing is going to look exactly the same we're going to have some skin here and here we're going to have two vertical shear webs but what's interesting is this top and bottom laminate are going to have their fiber orientations changed now parametrically those so they'll change at the same time the same values but in manipulating them that way when we get this bending we'll also introduce some twists right so now that twist could be positive it could reduce the amount of lift that we get or it could be negative it could really increase and blow up right and reduce the stiffness of our structure so without further ado let's go ahead and jump on into that now it's essentially the same model so I'm not going to go over the setup because all that procedure is very similar I'm really just going to talk about some of the difference in results from a conceptual perspective at least at first alright so you can see again we're plotting our maxilla index you know that's probably that might be characteristic you're looking at and in this case our wing looks like it's deflecting normally so this is if we consider only rigid only those rigid air loads for example at when we have a fiber angle orientation of minus 20 degrees now I just want everyone to pay very close attention okay watch what happens when we consider the elastic behavior of the structure under these air loads all right so two things our deflection is crazy high now we have max failure indexes way above one and you can definitely tell that the structure the stiffness of the structure is degrading very quickly right so in this case that that minus theta orientation that's going to decrease our stiffness okay if you notice when we applied just minus 10 degrees all of a sudden that that stiffness is much closer to what we want it is definitely generating more lifting you'll see that in a second this is for the flexible wing case now if you notice again we as we go positive we're actually getting less deflection now this is this a little bit special there's a little bit more deflection for the theta 28 case but that's actually because as we're we're trading off between our torsional stiffness in some ways and our bending stiffness as well so that's why we see more deflection although if we went in you would be able to see there's actually less twist okay so let's get a little bit to the proof of that let's talk about loading all right so I went in I use the rigid body tools of phim and I was able to get the span-wise loading of the wing okay so you can see here's here's that rigid span wise loading of our wing if we consider the air elastic effects we would jump up to this yellow curve okay and you'll see how that kind of how that propagates throughout an analysis and you can see that as we increase data right as we go from minus 10 theta 2 to 22 we're reducing the effective angle of attack along our span and reducing the amount of lift that we're actually generating okay all right so here this is again this is just our shear force again similar pattern as we increase that that parametric angle of attack of our earth sorry the parametric angle of our fiber orientations were increasing on angle of attack sorry we're increasing our shear are a shear force and then again same pattern as we increase as we increase theta in this case we're actually decreasing our bending moment and if you'll notice just as a side note for when you think this might be important or not there's actually an 8% difference in the bending root moment between the rigid and elastic analysis okay so if 8% is a big deal for a fairly rigid wing this kind of gives you an idea as to how a small tiny difference for example fairly small difference in our span wise loading but 8% in bending moment can be a huge difference in in the structural integrity at the root all right okay so I breezed through that I'm going to talk about setting up a flutter analysis okay now one of the things that stuff about flutter it does again it takes into account those three sources instead of just two sources of forces right it's the inertial aerodynamic and the elastic forces so let's take flutter we're going to boil it down and actually take it to where the airspeed is zero so what does that mean okay well if we take our flutter equation and we reduce it to an airspeed of zero it's actually going to reduce back to our normal free vibration analysis okay so what this means is that when you're conducting a flutter analysis your zero airspeed frequency should be very close to your free vibration frequencies if they don't that might be something you want to take a look at that may be a units issue all right I'm going to go ahead and turn off our spline for now just because it's just visuals it's a little distracting and again we don't have to mess with the model really at all I'm just going to set up a flutter analysis so let's do flutter and all right breathe through a couple of these now the first important thing to keep in mind when we get to the modal analysis flutter analyses are almost always imaginary use imaginary numbers so you'll want to switch your solution your modal solution to a complex solution method so we're going to switch it to complex launches and for a simple wing just you know a single structure usually six modes is enough you can do more you can do less but I find usually six modes encompasses at least the first the first or second free vibration modes from most of the possible degrees of freedom of our wing alright now again we're going to switch it to our air elastic coordinate system our velocity actually doesn't matter for a flutter analysis it will matter for a dynamic air elastic analysis all right so let's put in a reference length is again just our cord length here so let's go ahead and enter that guy in we have a reference density again this is in flinches per inches cubed the for you all you guys who use SI units this will be a little bit easier to handle i'm again we have the symmetry now when we generate our aerodynamics we're going to do that for a combination of reduced frequencies and mach numbers okay so i have this copy data table right here in excel i'm gonna paste that in a clipboard and we're all set to go that's all we need to do there notice the units by the way it's mach number versus they say frequency although it again it's um reduced frequency all right we can continue on we get to the flutter parameter so we're going to enable flutter in my opinion typically the the solution i prefer to go with most often is a peak ANL method it has all the robustness of the peak a method and an accuracy but it's a little bit more efficient than the peak a method all right now for density ratios this is if we wonder for example consider different altitude effects i'm always going to just consider that we're at sea level so we'll maintain that all right our mach number again we're probably going to consider this to be incompressible if we need to we can always add compressibility and consider that in an iterative process so that's you know on the user side so that's not too difficult if if your flutter speed ends up being high enough to consider that and now when we're using the PK method we enter velocities okay and again our units in this model are in inches so we have to input our velocity in inches per second if you'll notice leaping from about 30 miles an hour 30 miles per hour to about 255 okay let's go ahead and paste that in and again we have our own boundary conditions and we're done actually now let's go ahead I'm gonna switch over to the finished model I could run it but it'll just take a little bit of time again what we should see let me scale these values down so this is a normal modes results okay so we should see at low air speeds our let's see sorry at low air speeds this is what these are what our modes should end up resulting as okay so you can see a second bending mode but here's our first torsion mode so that's probably gonna be important mode for keep an eye out for that in the future and we get two things out of this okay if we were to go into the fo6 we can get not only can we get our actual air elastic mode shape so I'll go over these in a second here's one example of them let me try this again alright so here's one example of our one of our mode shapes okay and we can keep going over that but we get so the first thing we get is our frequency okay so this is important again at that zero air speed we should see the frequencies at very close if not almost exactly the same as our free vibration modes but then if you notice as we increase air speed stuff happens to the frequencies okay most notable is mode one you can see that we're going from some frequency eventually down to zero this indicates that mode one is going from kind of a dynamic response that we give it a perturbation to a more quasi static response as we give it a perturbation and what this really means is likely this mode is going to diverge at some point so you'll want to look at the damping of this mode in the future now remember how I said keep an eye on that fourth mode torsion mode okay well we have modes for and modes three here you notice they're growing closer together this is really indicative of flutter and what's actually happening is it's an indication that we can transfer energy between the fourth and the third mode okay so what does that end up looking like on the damping side all right now we always want negative if we give a perturbation we have negative damping it's going to reduce our amplitude to zero as we go at a time equals infinity right so we like it when our damping is lessons here here we have our zero mark see we can see mode four again this mode four it definitely goes unstable roughly at about two hundred and thirty miles an hour then we have also this this orange line might be going unstable okay so if we take a little bit of a closer look we find that yeah that mode that mode four is definitely going unstable but our mode two actually goes unstable a little bit sooner and again this didn't this is not that that that divergence mode that was mode one right so if you have some structural damping anywhere between one and five percent this might not be a big deal right you might still see that first fourth mode slider first but that's not always the case and now I'm getting towards the end one last thing I want to show that's a really important thing okay it's not always self-explanatory how we get these these output results if you run a flutter analysis really all you're going to get is an F oh six that looks roughly let's see if I can pull this up quickly so you're gonna get some fo6 and it's going to look roughly so you can see this is tabulated results where we get our velocity we get our damping and we get our frequency okay so that's all you would get in order to actually get your mode shapes in tune a strain what you have to do all right oh and as a side now let's take a look so this was at fourth mode let's see let's try this again just hmm for some reason it's not showing the total translation all right I'm going to come back to that in just a second then so if we go into our flutter analysis in order to get those those eigenvectors really are just that those modes post-process in tune a strand again you're going to want a preview input edit that preview and then if you go down this is that the FL is where we input our velocities just go ahead let's say we want to figure out we want to know what the the mode shapes are there put a negative sign in front of that velocity that'll ensure that you'll get the the mode shape which shows the damping and the frequency at that airspeed okay all right I'm not quite sure why that's not showing if it were to show alright again this is this is actually that mode that I was showing you so you can see not only are we getting some bending but we're also getting some torsional behavior about the wing as well all right so listen that about wraps everything up hopefully I've just given you all a good idea as to what what you can do with Nastran in terms of air elastic analysis I really in some ways only brushed the surface so there's definitely a lot more that you can do so if I didn't say something that that that you're interested in check with us it's likely that Nash Tran can do it and I just didn't have time to talk about it how we are again that resellers of these Siemens products as well as hyper sizer if you're just learning fee map alright if you're new to see map our very own Eric Gustafson actually wrote a book on how to learn see map this doesn't have anything with flutter in it or air elastic analysis but again just if if you're new to see map and you're unfamiliar with it this is a this is a really good learning tool and again finally my name is Ben names if you have any questions about what I went over today feel free to email me or get in contact with me somehow and if you have any questions about sales you can contact Marty and I think with that we'll we'll go ahead and take some questions see not sure I'd having trouble looking at these Jim you may want to is it possible for you to read off any of these questions if we get them in oh yeah well I can see every question that's been asked but none so far okay all right hopefully you weren't over everybody's head so they're not sure what to ask now hopefully you got one what's the difference between MK arrow one and MK arrow two cards good question um you can look that up let's see i-i'll try and blaze through this if we get any questions further I might jump to one of those you can look it up in the documentation and let's see if I can just I'm gonna try and find these on the fly let's see how fast I can find it ah okay yeah great question I think the way CMAP writes these cards it writes it as an MK air or two cards the only difference is how you actually present the information when you do an MK arrow one card you list out all of your Mach numbers and then all of your reduced frequencies if you use an MK arrow to card you list your Mach number and reduce frequencies in pairs okay so I think fee map just does this probably for robustness although I'm not I can't really speak to why they made that decision but they're they're equivalent it's just how you want to write the cards is really all that matters okay your next question is could these methods be applicable for structures like buildings or bridges yeah absolutely I don't see why not so I didn't bring this up because it's always mentioned but there's that famous Tacoma Narrows Bridge example this would be perfect for modeling a structure like that in that case it was a it was a bridge that exhibited slaughter and actually ended up taking all the way to failure again with it with a building I think that'd be a little bit less conventional but you know you can model slender bodies and stuff like that you might have to go ahead and modify your pressures with something a little bit more than a potential flow method because when you're talking about buildings depending on the the way it's a line right your viscous effects may no longer be negligible right you might have some separation and flow and stuff like that but but yeah again as a simple rough answer I don't see why you couldn't do that yeah okay and the final question I see is can you go over what you did or they span an area and static air elasticity and why you did that absolutely that's a great question okay so the reason why is gonna be a little bit unsatisfying okay let me um let's say I'm gonna back out of this I'm gonna just go to that excel sheet that I have for whatever reason when you're using symmetry in a model within masked ran the numbers that it takes in is that it needs to consider double the span right so if we were talking about the air the performance of an aircraft okay when we talk about that Oswald efficiency factor right that's always when you go from tip to tip of the of the aircraft wing right that's not when you go from tip to route right and so this is trying to it's again I can't speak to why that was done but that's just a convention that's taken and it likely has something to do with the fact that you're trying to be consistent with how stuff was either normalized or or that efficiency thing right so all you have to do is if you're using if you're running your elastic analysis on a model with symmetry okay and you have to reference the span which I think it only came up in static or elastic example use double the span you can see here's here's the actual span that I have in the model right and I just doubled it now when you're calculus in the area you actually want just a physical area that's in the model because when you're reading those elastic parameters your air lastik influence coefficients you need to read dimensionalize it by you're actually area of your dynamic model so you can see I did the cord times the span right that double span that I have and then I just divided it by two you know it's it's something hopefully you'll learn once and you just keep it in the back of your mind really nothing fancy if we were doing a full aircraft you wouldn't have to worry about that right but since we were considering symmetry that's really when that comes into play again I know it can be tricky but but that's just for whatever reason that's a convention that Nash trained ended up going alright Ben that's all the questions if anybody else has any more questions feel free to email Ben or myself this is Jim jeans and we will post a recording of this to GoToWebinar probably tomorrow if we can get it done that fast thanks for coming and we'll talk to you all later

Channel: Structural Design and Analysis, Inc.

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Length: 41min 31sec (2491 seconds)

Published: Mon Feb 20 2017