Parametric spline modification (rotation/translation)

[giulianoj.zennaro]

Hello,

After multiple searches on the forum, it appears to me that there is no way to parametrically edit a spline. The scale tool is the only one that is usable, whereas there is no way to move the spline in a rigid manner. Constraining one of the points of the spline to something which is parametrically defined in space also does not work as the spline is not updated properly when the parameter is changed. 

This issue seems to affect few people, however it has been the case for years with already several people mentioning it and no viable workaround. Here is an example of a post clearly describing the issue with images : https://forums.autodesk.com/t5/fusion-design-validate-document/how-to-fix-the-relative-shape-of-a-sp...

In my specific case, I need to design a wing parametrically since I want to export multiple step files with slight geometric variations for CFD simulations. One of the parameters I need to modify is twist and so the profile (spline) at one end of the wing has to be rotated parametrically. Another parameter I have involves the translation of a profile. Obviously, being an airfoil, the splines are made from far too many points to manually constrain them all (I use airfoil DAT to spline).

Have there been any updates on this necessary feature, new viable workarounds or a separate possible workflow for my specific use case ? 

Thank you in advance for your time and suggestions !

PS : for the twist variation, I have a possible workflow which I still haven’t tested. I could draw one profile at each end of the wing and multiple construction lines representing the chord along the wingspan. The construction lines can easily have their angle changed from parameters and I can connect them all in 3D with lines or splines which then can be used as rails for a loft. For the second profile, I can then manually rotate it each time, export the model and then rotate back to zero before choosing the new desired twist. My issue here however is I’m not sure I can select a curve which is composed of multiple splines or other curves, even if they’re coincident and tangent (another major issue I have run into), as a rail.

[giulianoj.zennaro]

 The twist should be f(height) as you go higher ideally. This would give more freedom to test actual racing conditions.

The change in cord length is used to change the spanwise lift distribution, but that is fixed once the sail is built. Twist is then needed to maintain a constant angle of attack of the profile due to the wind being twisted and also to have a dynamically adjustable parameter (refer to previous post's link to North sails).

I am working on a way to achieve that and will send it once I am done. Perhaps, I can already ask what you think about splines. My issue here is I am intending to use a fit point spline attached to each rib. That would mean a lot of control points which I assume gives bad curvature when looking at it with the comb analysis tool. I could also only define 1 or 2 ribs along the span instead of 11 (+ the 2 extremities) like I have been doing. Do you think that is the better way to work ? I could also instead use straight lines instead of a spline. Quick note, the twisting needed is not only of a few degrees, it can go up to 30 for example.

I have attached here the leading edge of the sail. As you can see, the result looks good even thought the comb analysis tool isn't happy. What do you think ?

[giulianoj.zennaro]

Here is a quick example of what I mean. As you can see in the file, twist is controllable. What I am unsure about is whether the end shape corresponds to my needs and if the same concept can be applied to 13 ribs.

With this method, I can just calculate the twist I want for each rib and then modify the corresponding parameter. Maybe I can do fewer ribs, not sure which would give the most precise outcome...

[davebYYPCU]

For a curve comb to cross the curve like that I would not be trusting the chord length data, if no typos.

As a person with zero knowledge of the CFD stuff, I can't advise on that.

For Fusion, you can get an excellent 2 point spine curve to match the majority of those chord lines.  Least amount of spline points the better the result.

See mine.

Might help...

[giulianoj.zennaro]

That's what I was thinking. I'll have a think wether to go for the spline + few ribs or line

 + lots of ribs option. Considering how the actual sail is built, perhaps straight lines are more reliable (at least I know exactly how they behave even when deforming the sail a lot).

I hadn't realised initially that for a sweep, several lines can be used as a rail contrary to the loft tool. Thanks for that information!

[davebYYPCU]

Generally, you have done what is described as required, 

In the previous file, the chord length data was not as accurate as Fusion can work to.

If it were me, I would not need the trailing edge curve, I would be lofting the chord lines and let Fusion apply the curve for the Sweep guide rail.  (on average Body edges are higher quality over sketches)

Surface Loft (3 chord lines and no rails) comb is better than sketch curve comb, less work.

Might help....

[giulianoj.zennaro]

That's an interesting detail I have never quite understood from other discussions. How come the surface loft comb gives better results ? Is it in the way it is mathematically defined or just a numerical accuracy difference ?

[davebYYPCU]

Don’t know why, @TrippyLighting knows why, will let him explain it, I just know from his lead.

Might help…..

[TrippyLighting]

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@davebYYPCU wrote:

Don’t know why, @TrippyLighting knows why, will let him explain it, I just know from his lead.

Might help…..

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I don't know implementation specifics, but I an say that the code that generates:

• Sketch Intersection curves
• Projections from curved  edges or curves into sketch planes that are not co-planar to the original curve/edge
• Splines offset with the sketch offset tool

 creates sketch curves with curvature problems. 

Those same problems do not exist when the edges of trimmed surfaces are used for modeling.

Generally I move away from sketches as soon as possible in any CAD software.

[giulianoj.zennaro]

So I have made the wing, and I ended up simply accepting the fact that the spline has a terrible curvature. For some reason, I cannot do a sweep where both rails are made up of separate lines. Maybe I could sweep several times (I don't mind the fact it then has edges)...

The issue I have now is creating the winglet. I don't really understand why when I use offset face, the new face is not flat (parallel to the ground). This has been creating issues trying to create the guide rails. After some messing about, I finally got some sketches done and although in sketch 17 there is coincidence constraint, the tool says that the rails do not touch the surface. And actually, if you zoom in really close, you will see that the coincidence constraint seems to not work (even in the sketch where it was applied). Not sure what happened there...

Bottom line, what have I done wrong here ? And mainly, is there a way so that the sweeping tool keeps the surface flat (parallel to the ground) ? 

I am happy with the sail other than I think I will redo it with multiple sweeps rather than using the spline. Will that maybe solve my winglet issues ? And for information, c0w is the chord at the bottom of the winglet (dependent value). c1w is at the end of the bend and c2w is at the tip of the winglet. All parameters needed are already in the file.

[laughingcreek]

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@giulianoj.zennaro wrote:... and although in sketch 17 there is coincidence constraint, the tool says that the rails do not touch the surface. And actually, if you zoom in really close, you will see that the coincidence constraint seems to not work (even in the sketch where it was applied). Not sure what happened there...

...

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that particular problem isn't with sketch 17.  the rail causing the problem is sketch 18, which you use as the guide rail for the sweep.  it doesn't originate on the surface your using for the sweep profile. which means at the other end of the sweep, the guide rail won't be on the surface either.  then you use the endpoint of the sweep guide rail to attach the end of the loft guide rail.   thus the "rails don't intersect all profiles" error.

...For some reason, I cannot do a sweep where both rails are made up of separate lines....

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haven't looked at your model further, but this can generally be done if the path is valid.

FYI-you can't :zoom way in" to check if things are actually coincident. past a certain point of zoom, you'll get graphic artifacts, and somethings that are actually coincident will appear to be separated.  to double check things like this, yo can zoom in a bit, and then use the measure tool to see if there is any distance between the points. (ref pic above)

[laughingcreek]

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@giulianoj.zennaro wrote:... I don't really understand why when I use offset face, the new face is not flat (parallel to the ground). This has been creating issues trying to create the guide rails. ....

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if you mean the face at the top of sweep1, it's because the face your offsetting isn't parallel to the origin to begin with. (again, use the measure tool to verify things like this).  it's not parallel b/c the path you used for sweep1 is a fit point spline. is there a reason the path in sketch 14 can't be a single straight line? then you could make the top of the sweep flat to the origin.

[giulianoj.zennaro]

I have to admit I still can't exactly see why the coincidence didn't work. I might remake the part and be extra careful what points I select. 

The path in sketch 14 unfortunately can't be a straight line as the true geometry is a wing hinged at the line above the origin. Then the ribs are rotated like I have done which twists the wing. If you just draw a straight line instead, you see that it does not connect the points. In this case it is close, but with more twist (up to 30 degrees for example) this fact would be pretty clear.

I had noticed the top surface was not flat (with the tool). My issue here is, is there a way to simply have a 2d surface, being kept horizontal, be swept up to make the body ? Instead of having it be perpendicular to the path ? The two extremities of the profile obviously have to move to connect to each point.

[laughingcreek]

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@giulianoj.zennaro wrote:

I have to admit I still can't exactly see why the coincidence didn't work. I might remake the part and be extra careful what points I select. ...

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the coincident works.  the point your coincidenting to isn't on the profile!

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@giulianoj.zennaro wrote:...I had noticed the top surface was not flat (with the tool). My issue here is, is there a way to simply have a 2d surface, being kept horizontal, be swept up to make the body ? Instead of having it be perpendicular to the path ? The two extremities of the profile obviously have to move to connect to each point.

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not when using a guide rail.  (when you're just using a path, there is an option to keep the profile oriented in the same direction).  you might experiment with making the tangent handles at the beginning and end horizontal, so the profile starts and stops perpendicular. i tried, and that works for keeping the face flat to the origin, but IDK what that does for your design.

[giulianoj.zennaro]

I finally managed to find the issue ! It was in the scaling step, the point chosen wasn't correct...

Thank you so much for your help ! It steered me in the right direction.

Here is what I have achieved so far. I'll still have to mirror the winglet, see if all the parameters update properly and try to use straight guide rails rather than a spline. Other than that, I am really happy with what we have achieved. Thank you to everyone that helped !

[davebYYPCU]

Was working on fixing your loft problem, but seeing your happy with the latest file, 

I wonder if the fluid testing program will appreciate your curve combs.

I see that you are not fixing the winglet trailing edge as pictured in message 26.

Might help....

[giulianoj.zennaro]

Honestly I don't know what to expect for the CFD part if it will be an issue or not. But I'll just have to try and see. If I manage to get straight guide rails instead, I'll change to that.

I think for the time being, the simple geometry of the winglet is sufficient. I don't mind the change in curvature on the trailing edge. 

[wersy]

I wouldn't attach too much importance to the winglet (or more precisely, the wingtip fence).
Many properties that are important in aircraft construction are not found in sails.
There is certainly a reason why this has not yet been used in sailboats. The effort involved in joining a rigid wing tip with a flexible part is disproportionate to the expected effect.

Twisting also has a completely different purpose in aircraft construction than in sails.
In an airplane, it is primarily intended to delay the stall. In a sail, it is about different flow speeds at the same time. The geometry is more comparable to a propeller.

[giulianoj.zennaro]

My intuition is that it does not work mainly because of the structural point of view (high loads and weight at the top of the sail). For this exercise however I'm simply interested in the aerodynamic point of view. I want to figure out what are the parameters that most negatively affect the winglet on a sailboat. Obviously heeling does not exist on a plane and I suspect that it has a negative impact. The wide range of sailing conditions as well such as wind speed, camber, twist probably make the use of a winglet impossible. 

Again, this is mainly an exercise that I'm doing, it does not have any practical implications... 

[davebYYPCU]

it does not have any practical implications

I wonder if Ben Lexan was thinking that in 1980”s. (winged keel on Australia II). 

Now the whole boat runs on half of his (developed) design.

[giulianoj.zennaro]

Well if I have any positive results, obviously I'd be extremely delighted and I'll do a proper study ! But many people did point out that they don't expect it to work and I think they have good reason to think so. Apparently winglets have already been tried on sailboats without success. That's why for now I'm treating it more like an interesting experience rather than simulating any random structure.