hi,

i have tried and tried but i can't seem to blend the shape i'm going for. i'm trying to see if its possible to connect a circle to an airfoil using MoI. this is for a swept propeller blade (but it would be useful for straight propeller blades too). for the swept propeller blade, the circle and airfoil are continuously bending along a curve. the leading edge of the airfoil is an ellipse and the trailing edge is an arc. you can use the script bemfarmer created about a month ago to get a general swept or straight propeller blade. the airfoil curve is in four segments.

the closest i can get is to break the circle up into four segments. sweep it. then do the blend on each curve. the suction and pressure side of the airfoil are close then i have to do a network curve to get the leading and trailing edges in as the blend doesn't work there. the problem is the resulting geometry is not tangent everywhere. i'm not an expert with the terminology but i'm thinking i need g3 or g4 continuity based on reading over the forum here. i only get g1 and g2 options when doing the blend. the g3 option only seems to appear when working with curves. MoI won't let me do a blend or network with all eight curve segments selected. I can do a loft. The problem there is it is only connecting the curves with straight lines and that is not helpful, a smooth transition is needed in all directions.

i tried working with curves as well but i don't get anything that makes sense that way. the g3 option shows up but i can't seem to get anything i can understand or use out of it. so surface edges are what i have been messing with.

trying to figure out the curves and doing a network command is all i can think of. but that would be really difficult if not impossible. which makes me wonder if that is why i can't get it to work in MoI. maybe what i'm trying to do is not possible right now with MoI.

i know this is hard to explain without pictures and a model. but if anyone can try this even on a straight propeller i would be curious to see if its even possible. the circle is centered at the 1/4 chord point and ends at the leading edge. so its diameter is much smaller than the chord of the airfoil. this leaves a lot of curvature trying to connect the trailing edge to the circle. if you have ever seen a typical traditional aircraft propeller blade, that is what i am trying to experiment with.

thanks,

anthony

edit; here are a few pics with the loft

Hi anthony, just g1 gives you tangent continuity, g2 is continuity of curvature and g3 is continuity of the rate of change of curvature.

But if it is tangency that you are shooting for, then no you don't need g2 or g3 type options in order to attain that - g1 is the one for tangent continuity.

It's really hard for me to visualize what you are describing - I would really need at least a screenshot and more ideally a 3DM model to have much of a chance at figuring out what you are trying to do.

- Michael

Hi Anthony - I see you've got a picture there now.

You're going to have some problems with that due to the fairly extreme change in shape that you're trying to do all in one single step there.

You'll probably have to construct more of the in between portion yourself for a case like that, otherwise you'll likely have the various blend pieces end up sticking through each other as they sort of swoop around and try to maintain continuity to the much differently shaped sides.

Also when you use blend usually you want to delete the end caps so that the edges that you are blending between are not connected between 2 different surfaces, because when there are 2 possible surfaces to be smooth with the wrong one may be targeted. See this previous thread for some information on that:

http://moi3d.com/forum/index.php?webtag=MOI&msg=4895.1

But probably I'd recommend constructing some in between shape with something like sweep or network and don't initially try to make it smooth at the connection spots, make it initially connect with a sharp edge and then put in a fillet there.

- Michael

thanks michael,

i updated my post to include some pictures with the loft. i am just using the no save eval of 2.0 so i can't provide a model. sorry. the blend example in your help and in the forums definitely seems close to what i need. i'm thinking i'm probably envisioning g3. basically if you imagine an aluminum forging and someone with a grinder transitioning from a cylinder to an airfoil. it would be tangent in two directions and also have a smooth rate of change in the third direction. i'm sorry i don't know the right cad jargon for this. blend with surfaces is only letting me choose between g1 and g2. the g3 option only seems to appear when you have curves selected. when i try that i get a curve that is nonsensical. the blend between surfaces almost works but not quite. i mean works for what i'm trying to do, not that it does not work.

the problem is complex because the base coordinate system is constantly changing and you are trying to hold tangent to two coordinates along a changing coordinate system. then also have a smooth rate of change i believe in the remaining axis. its hard for me because i can see it in my head but can't express it well for you.

i'm just curious if some expert moi users can achieve something like this. i have tried everything i can think of but can't figure it out.

this is typically how a propeller blade is connected to the hub. i have been using a novel geometry of my own design. one thing as mentioned way back when is i'm having to increase the trailing edge radius by 2.5x what i really need or MoI will often malform it. you had mentioned that it had to do with how small the feature was to the rest of the geometry. is this something that might get better in 3.0?

thanks,

anthony

Hi anthony, or possibly you may get better results building the whole entire thing out of one long sweep instead of trying to construct things in different separate sections like you are doing currently.

It is easier for one large sweep to be smooth throughout the interior parts of the sweep.

It's also easier for a large sweep to have a more gradual change from one shape to another, you've got too distinctly separate shapes and it will be hard to mask that by just trying to do a separate in between region.

- Michael

Hi Anthony - well g3 is not a magic cure for masking that there is a large section of your current objects that adhere to one distinct shape.

The best way to make a large smooth shape is to have some more gradual changes between things as you move along, if you have one distinct shape and then only a small area where you try to make things change rapidly and then go to another separate distinct shape, it is just not very practical to make it look like a natural transition, whether it's g2 g3 g4 or whatever g you want.

The different levels of g blending only really refer to the properties of how 2 curves or surfaces relate right at the juncture point. It's totally possible to have a g4 shape that does not look good because even though it is technically g4 at the juncture spot it has a bunched up shape or not enough space for it to give a relaxed form or stuff like that.

- Michael

Hi anthony,

> you had mentioned that it had to do with how small the
> feature was to the rest of the geometry. is this something
> that might get better in 3.0?

Hard to say, probably not as it's just an overall difficult area to make really tiny features to merge will with much larger ones within a single shape. I'd really recommend avoiding that type of situation.

- Michael

thanks again michael,

yes i saw that thread before i posted and i tried it out. but moi was already working fine without having to delete the ends. that is to say when i tried it i got the same results. so it seems to be picking up the right curvature to go with. i thought maybe that applied to an earlier version of moi. either way i am still stuck.

i tried giving more distance between the circle and airfoil and also breaking the circle up all different ways.

i guess one thing i am unsure of is that i can't get around the airfoil being in four segments. it seems like that forces me to have to have the circle in four segments as well. i don't know exactly how this works, but just my experience with trying it over and over. i wondering if i could somehow have the airfoil in one segment if that would change things. its easy to have the circle in one segment. but i don't see of anyway to get the airfoil in one. join doesn't do it and union doesn't work in this case either. so just some noob confusions there.

the geo is really complex and that could be basically the problem. so this is what i'm hoping someone might know. i did also try putting in shapes in between as you suggested. i have also tried using an ellipse instead of a circle but no luck. i have tried this at least 20 different ways. i am definitely stuck.

edit; here are some pictures using a combination of blend and network commands. its really close to what i'm going for but it is off sligthly. i'm thinking a higher level of continuity is the missing link.

Hi anthony, so I am planning though in v3 to improve the Blend tools, specifically to allow you to pick a longer chain of edges for each side of the blend instead of the current situation where you can only pick one single edge on each side.

So that would probably help out a lot in your particular situation here.

Right now since MoI's blend only works between 1 edge on each side you may have to split edges up in order to get them down to a 1-to-1 matching. You can split an edge into multiple smaller edge pieces by selecting the edge and running the Trim command and using the "Add trim point" button which then lets you pick a point on the edge for where to split it.

But instead of doing all that, right now I'd recommend taking your object into Rhino and using Rhino as a helper tool along with MoI for this model - Rhino's blend is able to do a multiple chain of edges so you will probably save a lot of frustration by doing the blend for this situation in Rhino instead of MoI.

That is an area that I do expect to work on improving for v3 though.

- Michael

Hi anthony,

> edit; here are some pictures using a combination of blend and
> network commands. its really close to what i'm going for but
> it is off sligthly. i'm thinking a higher level of continuity is the
> missing link.

If that's close to what you want, then you can probably use Rhino to help you get the final step, which would be by using the MatchSrf command in Rhino to edit the surfaces to make them have a particular continuity to their neighbors.

That is also another area that I want to improve in Moi as well - those continuity tools are not really a mature area of MoI right now, MoI is much more focused on making a lot of the more simple stuff like just generating stuff from 2D curves more fluid and quick to use. It's not as sophisticated in the areas of advanced surface modeling with continuity yet.

So to do that kind of stuff right now you will likely need to use another program in combination with MoI instead of only MoI totally all by itself. Rhino makes a good companion for that kind of stuff.

- Michael

thanks michael,

i think you nailed it. i believe that is exactly the issue. its almost perfect but the continuity is off some because i can't select the whole chain of curves.

here are a few more picks. it kind of develops some peaks along the way. the suction and pressure sides i can do with blend at .7 the le and te i have to use network. so it develops the problem areas. but its close. the whole blade is then unioned into a solid.

i used to use rhino until i discovered moi. i would never go back to rhino. i love moi. i am only evaluating it however for users of PROP_DESIGN. i'm retired now so other than evaluating moi for use with PROP_DESIGN, I don't do any other modeling. moi is the best choice as far as i'm concerned. this little problem area doesn't worry me too much because users of PROP_DESIGN would have to move on to much more expensive tools like FEA. moi is a great transition between PROP_DESIGN and FEA. You get the base blade geo and that is all that is really needed at this step. i was just experimenting some and I guess I ran into a small limit here. no worries.

your planned changes should fix the issue i'm having. thanks for the explanation. i won't keep beating on this now that i know i'm not doing something wrong.

i do hope that something like 64-bit precision can be added someday though. that should help with my trailing edge radius problem. but again that can be cleaned up in a downstream program. just another nicety.

give 'em an inch and they want a mile :-)

Hi anthony - MoI actually does already use double-precision floating point numbers which are 64-bit values. The issue about your little segment combined to a much larger profile is not a problem due to non-64-bitness, it's a problem of finding a good tolerance value for fitting operations that does not fit things with too dense or too loose results. Making a mix of a much smaller sized segment along with a larger one for the same surface operation makes it difficult to use one tolerance value for the whole operation. And stuff like using multiple tolerance values for different parts of a calculation can quickly lead to very complex UIs where you do things like tag a bunch of things with esoteric tolerance values for every step that you try to do things...

Anyway - re: continuity and this particular shape you're building - really I would recommend doing things that you want to be smooth like this all as one big long surface rather than trying to do it in chunks like you are doing now.

If you did it as one long sweep or network instead of one bit at a time, it would avoid any need to mess with continuity related stuff at all...

Usually for stuff that you want to be broadly smooth and look like one continuous shape, building it as a larger piece all in one surfacing operation is the best method.

- Michael

hi michael,

thanks for the clarification. i believe i understand some more about the trailing edge being a tolerancing issue.

as far as the sweep i may have confused you. the blade is made by a sweep. then i cut away the hub portion using a Boolean op. the circular part of the blade is also made the same way. the transition is the part that i need moi for. in the old days we used to just put a note on the drawing that said blend. it was left to the guys grinding the blades out or making the dies to figure the blend out (depending on construction methods). so only that one piece is where there is a loft/blend/network type op. everything else is made with sweep.

Hi anthony re: sweep - what I mean is instead of building it in 2 separate sections that then necessitates putting in a blend connection piece, you might instead try to build the entire length of the whole final result just as one long sweep with several cross sections in it.

Right now you're building it out of different separate pieces and that then leaves you with the problem of connecting them together smoothing with a transition piece.

If you were instead to build the entire object out of one longer sweep then you could get a shape that was all smooth throughout it without ever having to worry about the continuity of separate surfaces, because you'd have just one long surface instead of several little adjacent ones.

Often times if you want something to have a smooth continuous quality to its shape, it tends to be best to build the whole thing that you want to be smooth out of one big surface instead of building one smaller section of it separately.

- Michael

One thing you might try is to rebuild the aerofoil curve before you do the sweep. That way the curve will be one piece and will blend with a circle.

hi steve,

is that possible. that is something i wondered about but i don't know how to get it to be one curve. it is created with four different curves and all i can seem to do with it is join it. is there something else that can make it one curve?

Hi anthony, if the pieces are smooth where they touch then join them together so that they are one connected curve and then run the Rebuild command on it:
http://moi3d.com/2.0/docs/moi_command_reference10.htm#rebuild

That can reconstruct a curve that is made up of multiple smooth sub segments into one single piece.

- Michael

Thanks guys,

I have been trying various sweep stuff and I tried the rebuild command. I got something almost perfect with sweep. The rebuild didn't work out. It caused a lot of problems. Once I get this just right, I'll post how I did it and include some pics. I do think the upgrade to blend and network would be good. Allowing you to select multiple curves and hopefully do a g3 or higher surface. But the sweep method looks like it is going to work.

Here are some pics of a model that is really close. The only problem is there are problems with some surfaces that I'm trying to work out. All sorts of small slivers are being created in certain areas. I am using a mixture of sweep, network, rebuild, and other commands. Basically the sweep works but its a bit jagged in some places. So I'm trying to extract the curves and do a network. That is almost working. I'll keep working on it. I think it can be done it is just pretty difficult. Also I attached some pics of the small trailing edge radius issue. Right now I am increasing by about 6 times what is needed. The reduces the chord by about 1 percent.

I need to perfect the process to avoid the surface sliver elements but I can tell at this point the final model will look like the pics attached. The rebuild command is useful. I'm learning when and when not to use it. The multi-profile sweep is useful too. I wasn't familiar with it before.

Ideally however I am looking forward to an upgraded blend and network command.

Hi anthony, it's looking good! I was not able to see any of the slivery stuff that you were talking about in those particular screenshots. If it looks just a little triangular, it is possible that you're just seeing some display artifacts because the display mesh just to not have generated a lot of triangles in that particular area. That kind of display issue does not necessarily mean that there is a problem with the actual geometry.

The display system is generally oriented towards trying to do things fast so that you don't have to sit around for too long after doing every operation, so sometimes with this emphasis on speed of the display there is a chance for there to be some imperfections in it as well.

If it looks more messed up than just slightly jaggedy then it could be a problem though. It's hard for me to make a more accurate guess without seeing an example.

- Michael