Hi Anthony,

> spoke too soon. when i tried the method burrman used
> i found the surfaces at the tip near the trailing edge were
> malformed.

It looks like most likely your actual surface are ok there - that's just a display artifact.

In order to make the display MoI has to convert your surfaces into triangles, because your video card only knows how to display triangles. If some areas do not get enough triangles created for them it can end up looking like what you show there.

Really the best thing to do for a situation like that is to just ignore that display glitch.


> Changing the mesh angle option to .1 seems to fix this issue.

That's ok for just checking it out, but I strongly do not recommend leaving it set at such a tight angle, because a tight setting there will cause a display mesh of enormous density to be created in other areas of the model. That will tend to cause an excessive amount of memory consumption once you construct a model that has more pieces in it.

So once you are satisfied that it was just a display glitch, you should change the mesh angle setting back to the default 10 degrees for regular use.

- Michael

Hi Anthony,

> Right now the MoI model is scaled up 1,000,000 times its true
> size with default settings, so that the trailing edge radius comes
> out right.

This is kind of a sign that your project is just not going to work very well - why is it that you need to create such an extremely small detail in your object?

If you are modeling the object for rendering, that is so extremely small that you won't be able to see it.

If you are modeling for manufacturing, that is so extremely small that there isn't any manufacturing process that will be able to generate such a high level of detail.

Why is it that you're trying to create a model that has such an incredibly tiny little arc as part of it?

You would really be much better off not having things like that extremely tiny arc in there in the first place - just have the sides come to a sharp point instead of putting such a tiny arc at the top and your entire project will probably be much easier to handle.

If you do need to create a model with such a huge variation in scale within the same profile, you probably will need to use some kind of specialized CAD program to do it and not MoI - from MoI's perspective that is a kind of strange and not well suited shape to try and build in MoI.

- Michael

Hi Anthony, I loaded your MOI_EXAMPLE.zip attachment from above.

So now you have the opposite problem - your model is now too extremely huge numerically and that will cause other kinds of problems.

When I select your objects, I can see that the bounding box around them is of this size:



A bounding box width of 4330590 units is extremely huge - that's so far out of scale from the regular fitting tolerance of 0.001 units that it will cause an enormous amount of detail in various calculations, like very heavy control point counts and probably hitting recursion limits in various algorithms.

You should generally target an object size of something like less than 5000 units in overall size with individual features no smaller than 0.05 units, or something similar to that.

If you have features of an extremely small micro size approaching or lower than 0.001 units that will cause a variety of problems, and if you have objects of an exceedingly gargantuan numerical size like more than 6 orders of magnitude above 0.001 units, that will also cause a variety of other kinds of problems.

If you have a project that is unable to work within those limits, you will probably need to use some kind of specialized CAD software to do it and not MoI.

If you want to complete your current project in MoI, my best recommendation is to scale your model to have an overall size of about 10 units across and eliminate the little teeny tiny micro arcs that you have in your profile shapes and instead make your profiles just come to a sharp corner instead of a micro arc curved tip. If you do that you should likely be able to finish your model without issue.

You will be spinning your wheels and running into no end of problems if you try to use either a huge scale or micro sized features as you've been doing so far.

- Michael

thanks for the help. ill have to stick with rhino.

Here's the model back with a fillet at the join, in case anyone else was following.. I scaled the model down by 100 times, twice, then put a .005 m fillet on the join, then scaled it back up to ludicrous! I also put a blend at the tip for grins. (Though I ended up with that last little non-continuous patch, for obvious reasons).

The OP should really just use a point as the edge up there. If a machine cut the edge sharp, I could probably apply the radius by licking it!!

Here it is with a 5 meter G1 fillet on it:

Here you can review some airfoil information:

http://www.ae.illinois.edu/m-selig/ads.html

Note in the format section, he descibes this:

""""""""""Some of the airfoils listed do not close at the trailing edge, i.e. the trailing edge has a finite thickness as designed""""""""

Looking at airfoil generators, you will find the option for the open end on the trailing edge...

I think the tiny number is a calculated exacting figure for the trailing adge, but not really used in anything... Like Michael had pointed out, "What will you do with THAT"...

Thanks burrman,

I think that would make the final scale that you worked in with you applied the .005m radius 10,000:1. The reason I had the crazy 1,000,000:1 scale was to get the trailing edge radius to display right. I may start from scratch and see how it goes again. How did you actually do the radius and the G1 blend. I didn't understand how that worked because the intersection line wasn't coming out at the actual intersection.

The radius is just something specified in the definition of the NACA 65A009, it's not like I personally have something to do with it. The radius will get larger with a larger chord blade. However, the chord has to be a certain dimension for performance reasons. In real life you could have a straight line there I suppose. But the purpose of this model would be for FEA to do a fatigue analysis at the trailing edge radius. So that little point at the base of the hub is an important area for analysis. The idea is to try and model it as it actually would be. Small radius are nothing new to aerospace. It can easily be done with a CNC machine. I don't know what dimensions are in there right now as its been scaled around a lot. But the point files are the real dimensions in meters. Whatever that radius comes out to be, is what is needed. Does that make it any clearer?

If you import the points and model in meters that is the real dimensions. The problem is, from what I am gathering here is that the tolerances and mesh settings aren't exposed like in rhino. So burrman's recommendation of scaling up is what I was trying to do. It sounds like you have to go up to about 10,000:1 in order for MoI to capture it. I will try it out again. The only thing is I don't know how you got to do the base fillet. But since I see it can be done, I will pound on it until I either give up or get it right.

Hi Prop,
In the file with the .005 fillat, I just worked it and did the trimming... Remember Michael describing the viewport being meshed by your video card, and turning everything into triangles instead of curves.. That part would mean at some type of zoom level, you wont be seeing whats really there.. "Maybe that instersection was actually right!!""

But this is just only going to get worse, as Michael had described.

The G1 fillet was easy. I just removed the little radius on the trailing edge and then ran the fillet tool..

Please note that everywhere you look, it will be "removed" for actual work.. It's just there as part of a proper mathematical representation... But like mentioned here in a wiki for airfoils, you'll note that they mention changing some numbers to use 0 as the little radius, when when you need to do computational work with the airfoil:

http://en.wikipedia.org/wiki/NACA_airfoil

Equation for a symmetrical 4-digit NACA airfoil

The formula for the shape of a NACA 00xx foil, with "xx" being replaced by the percentage of thickness to chord, is:



where:

c is the chord length,
x is the position along the chord from 0 to c,
y is the half thickness at a given value of x (centerline to surface), and
t is the maximum thickness as a fraction of the chord (so 100 t gives the last two digits in the NACA 4-digit denomination).

Note that in this equation, at (x/c) = 1 (the trailing edge of the airfoil), the thickness is not quite zero. If a zero-thickness trailing edge is required, for example for computational work, one of the coefficients should be modified such that they sum to zero. Modifying the last coefficient (i.e. to -0.1036) will result in the smallest change to the overall shape of the airfoil. The leading edge approximates a cylinder with a radius of:



Now the coordinates (xU,yU) of the upper airfoil surface, and (xL,yL) of the lower airfoil surface are:

Hi Anthony, it likely is possible to do it, but it's probably going to require some fairly difficult steps like maybe working part of the time at one scale and doing other operations at a different scale.

Just fundamentally the model that you're trying to build is not a particularly good fit for CAD technology in general - a mixture of extreme different scaled features within the same design will lead to many problems.

Once again, my best advice is to not try and create a model that has that kind of problematic structure in it - maybe put a line in there or make the end just come to a simple sharp point.

It is still not very clear to me why you think that little tiny radius is necessary - what kind of CNC are you expecting to build with that will be able to cut such tiny features?

- Michael

If you actually get it built, and then actually get it "Meshed" for FEA, hopfully you have access to an SGI Octane III at a minimum.

When the tool cuts the metal there will be a radius. It won't come out as a straight line. That is what I was trying to say. Also, from a stress and fatigue point of view you can not have a straight line or sharp features of any kind or the part will crack and fail. There will and should always be some sort of fillet at that boundary. The downstream analysis for the CAD model would be FEA, CFD, aeroelastic etc. You want the correct geometry for a lot of reasons. My program outputs the information to build the le and te based on the naca 65a009 definition. The radius will vary with each design but the part bothering you will never change. The te will always be very very small compared to the rest of the model. Making it a point or line is not only not a real geometry but if it was made it will fail instantly. If you look at turbine or compressor blades they have small fillets at the base. Those will be way smaller than in this model.

Yeah it takes a lot of elements, a lot of memory, etc. About a 10,000 workstation at a minimum and ansys costs over 40,000. Its not cheap but done everyday. I used to do it for years.

Well, it's kindof a conundrum there prop.. Yes, stuff like this is done everyday. But the guys doing it would just design it the way it needed to be...

Can you show me the cnc machine that is being used everyday that works at a .000006 tolerance and the tooling that it uses to make a fillet of that radius in a corner area???

Hi Anthony, if that's really what you need then MoI is probably not the right tool for your particular job - you may need a more specialized kind of program to deal with unusual (unusual in the sense of what MoI is oriented towards working with) criteria like that.

MoI is just not specifically designed to handle this particular task that you need to do.

It may be something that I can adjust in the future, but it's a difficult task to try and expose a whole bunch of adjustable tolerance values in the UI for every kind of action that you want to do - that would probably help with what you're trying to do in this particular case but would also result in a much greater avalanche of technical info being presented to the user all the time, making for a much more complex and more difficult to use program.

It's a major goal for MoI to be easy to use and to not try to require a math degree to operate it...

- Michael

burrman

the ter is .000067 meters roughly. i attached some info that may help. the moi model here is in the actual dimensions. probably through all the scaling some confusion arose. the issue bugging michael is that the largest dimension / smallest dimension (radius / ter) is approx. 47619. for a compressor or turbine blade that would be a lot smaller. But even the chord / ter is still 4761.9. Somewhere in there lies the rub I believe. Burrman got it to work so I wanna see if I can too. I like MoI better than rhino because it is simpler to use. I think you have a great program here. I wouldn't change it.

Hi Anthony,

> the ter is .0067 meters roughly.

But that's not the case in the A400M Propeller Example.3dm file that you posted previously.

In that file the little rounded tip here:



Seems to be 0.0000666 meters as you can see... That's 2 orders of magnitude smaller than what you wrote here - is one of these incorrect?

Then of course to actually cut an arc shape on your CNC you will need a precision of some fraction of that, maybe a precision of around 1/1000 of a millimeter or so to cut that accurately...


EDIT: I see you updated with the correct figures above, I didn't see that when I first posted this one.


- Michael

Yeah its really small and if the chord gets smaller like the first points that started this thread it would get even smaller. That's why I switched to this example. The large blend that burrman did at the base is what i was going for. But it would still have to blend into the ter. I used to work for McCauley propeller and Hamilton Standard. We made these types of blades all the time. Even guys with hand grinders could do it. The CNC was just an example. I was trying to say that machines don't make lines or points. There will be some radius based on the drill bit or what not. At P&W we drilled with lasers. Believe me there are a lot of ways to do all sorts of things, that's all I was trying to say. Unfortunately the ratio (radius/ter) that I believe is bugging you I can't change. It comes from the fact that the blade has to have a certain chord. The airfoil spec does the rest. Making the ter larger is an option, but not ideal. I was trying to say it still needs to be a radius for a real perspective and from a stress perspective. So if it becomes too small to manufacture as burrman is saying it would have to be enlarged. I was saying my code just uses the naca spec. Its not like I drew it in there for fun or to bug you guys. I'm ok with making it bigger if need be at this point. But what I sent is the actual definition.