wing tips, propeller tips

[John_H555]

I am drawing a propeller from a very old diagram showing angles of attack, airfoils, etc.  The problem is similar to drawing a wing.

Most of the blade can be lofted easily, using the airfoils.  But surfacing the tip is a problem.  I have tried loft, network of curves, 1-rail, 2-rail and even patch, with the most bizarre looking results. Any thoughts on how best to do this?

The tip drawing is basically an airfoil, with a simple loop-around from the leading edge to the trailing edge of the blade.

Thank you for your insights on this.

[Ivan]

well, as you already said, you've tried them all. But there might be something wrong with your edge maybe or something.
If you like, you can attach the 3dm file here, or send it to me by email, so I can check it out.

[John_H555]

Hello Ivan,

Here are a two screen grabs -- the 3dm is too large to upload.  The first shot shows the propeller blade as lofted (the tip is not surfaced).  The second shot shows the ribs, with the problem area at the tip highlighted.

Many thanks for your thoughts on how to surface the tip.

[Juan Santocono]

HI, If you model often this kind of smooth things (propeller, fuselage, etc.) you should try T-Splines plugin. A free 30 day demo is avaliable at www.tsplines.com
Bye
JuanS

[Juan Santocono]

You can do stuff like this.

JuanS

[Ivan]

Juan is right, although I really like challenges so I came up with somewhat acceptable solution:

First of all, you have some things that need tweaking. First of all some of your sections (3) are weirdly closed curves. Check the first attached image.

Secondly, on the other side of your sections the curve and line intersections aren't on the path curve. Check the second attached image.

So after fixing those, I've made an InterpCrv connecting your top end points of each section (image 3)

Using sweep2 command I made the surface you usually made with loft.
Check the image4, you can see your section curves are not scaled down proportionally. You need to work on that a little more.

Then I mirrored the last section curve over section line. (img5)

Next I made InterpCrv from the Mid of upper section curve, connecting the Mid of your (purple) cap curve, and ending it on the Mid of mirrored section curve. (img6)
[I would extract an isocurve, and match that InterpCrv with extracted one, and get the right curvature, but that would take some tweaking of your cap curve, and your section curves]

Next, split all the curves. The upper section curve (annot. dots 1&4), the connecting InterpCrv we made (dot 2), and your cap curve (3&5) ->(img7)

Now, using NetworkSrf, start with one half of your cap. Click on 3, 1 and 2 (that order). That is one half, and for the other, click on 3,4,5. Next, using matchsrf match those two halves (img8 for matchsrf settings)

And in the end using mergesrf merge the surfaces...
p.s.
I haven't made the "flat" bottom surface, I think you'll be able to do that yourself.
HTH

[John_H555]

Wow!  Thank you, Ivan.  I am often astonished at what Rhino can do, when it is expertly driven. 

I started with Rhino in January. I am doing the propeller as a learning exercise, and there are (to me) several useful-looking new commands here.  InterpCrv is certainly one of them. Let me study this and try to reproduce what you accomplished so deftly -- some more questions will probably come up along the way, which I will post.  Thank you thank you!

[Ivan]

no problem. I am here to help out.
Just keep in mind, surfaces are made out of curves, so if your curves are bad, then your surfaces will end up worse!

I always like to simplify my curves and make them perfect. At least I try to. Don't forget to use Osnap a LOT, and since you are a beginner, I recommend going through all tools from the left toolbar. And note, there is a little triangle in the right bottom corner of the button if there are more commands inside that toolbar.
And lastly, InterpCrv is for me better solution for drawing curves because it goes through the control points you make. And when you activate any of the commands, you can press F1 key and get help for that command.

[John_H555]

I cleaned up the ribs, rescaled one to a better size, and successfully made the 2-rail surface. 

Interpolate curve turns out to be an old friend -- I have been using it from the icon, and just did not recognize the written command, InterpCrv.

The outermost rib would not mirror, but I was able to rotate it.  I was also able to draw the interpolated curve from from the Mid of the upper rib to the Mid of the cap to the Mid of the lower rib -- using "elevator" a lot.

However, the curve circles the cap without quite touching it.  I cannot seem to get it unite with or snap to the cap.  In your image 6, there seems to be the same tiny separation between the new curve and the cap.  This is a good example of curves that "almost but not quite" intersect I asked about in my other post. 

At this point I am stalled, because I cannot split the curves without an intersection.  I have redrawn the interpolated curve many times now, but each time it just misses the cap curve.  Snaps are on, and report a Mid -- but when I click, the lines do not unite.  Is there a way to edit or force an intersection?  Would Join do this?

Thank you again for your thoughts on this,

Michael

[Ivan]

hm, thats interesting problem, but I am 100% sure you're doing something wrong. If you are making an interpolated curve, then you are defining the control points where you click. That means that your curve MUST go through the control point you define, and that means that your curve must intersect with cap curve.
Attach the three curves in a 3dm file.

[John_H555]

By drawing the interpolated curve over and over, I finally achieved a valid snap to the Mid of the cap. I think the problem was coming from an improper use or understanding of the "elevator mode."  It is a question of when to hit the Ctrl key, when to drag, when to do both.  I am still not sure about it but what seemed to work is the following sequence.

1) hover the cursor over the point where you hope to bring the Interpolated line. In my case this is a point well above the construction plane, but two things happen.  A flag appears signalling a snap (in this case, a Mid). And on the Construction Plane, a white dot appears, as though you had dropped a plumb bob down to Construction Plane the from the spot where you want the snap.

2) Press the Ctrl key. Press the Left mouse button. Slide down the plumb bob line to the CP. Still holding the Left mouse, still holding Ctrl -- slight right back up the plumb bob line to the elevated point where you want to snap.  At the point (still holding Ctrl), make a left click to achieve the snap. 

Is this right?  It seems to work or anyway, it worked on the wingtip problem. Somehow I think there must be a simpler method.

-----
Two more questions:

Having achieved the snap by luck or by gosh, I was then able to create the upper and lower surfaces of the propellor tip.  It is shown in the image.  As you can see, the surface of the propeller tip is offset at the leading edge from the (2-railed) surface of the rest of the propeller. I understand I am able to fix this using Match, but the Match command asks to know what curve it is supposed to match. What curve should I click? 

The highlighted rib in the prop blade is too short.  I scaled it up before running the 2-rail, but this is guesswork and it still came up slightly short.  Isn't there a way to edit this part of the blade in such a way that both the leading edge and the rib are edited?  I tried this on the wireframe, but the Leading Edge came unstuck from some of the adjacent ribs. 

I will email the 3dm. 

I am learning a lot from this exercise, and I hope that these are basic problems that lots of beginners also encounter and can learn from.  Thank you again, Ivan. 

[Ivan]

Ok, there are some things you didn't quite follow.

First, I didn't use closed ribs for sections in sweep 2 rail, I used only the curves, not the lines. Check the image 1 below.

And for making your mirroring easier, you've got 4 different viewports (you can add more if you like), and you don't have to mirror from perspective -> that usually won't even work. But, instead, mirror that curve from Right viewport, and mirror over your line. check image 2

Next, I don't know why are you complicating your life, but when making that curve all you have to do is click three times. So, start with Interpolated curve, and hover over Mid (this time it is End as your curves are now split) and when the caption says End then click. Then move your mouse over to the cap curve, and do the same. Hover over, and click when it snaps to End. And in the end, do that for the mirrored curve too. Hover over Mid, snap, click check the video:

http://www.rhino3dhelp.com/video/propeler-video/

[Ivan]

one more thing. you didn't follow the order of selecting the curves when making Network surface.

[John_H555]

"Next, I don't know why are you complicating your life, but when making that curve all you have to do is click three times."

Let's see if we can analyze this problem.  Precisely what is the difference between what you are doing and what I am doing?

You are able to click three times in the perspective viewport, apparently without using the elevator mode, and successfully achieve snaps.  It is a swell video, demonstrating this.

Here are two screen captures to show what I get, following the same procedure (this time, without complicating my life by using the elevator mode).

Each snap delivers the line to the CP. Three snaps at three points in 3-space, automatically projected to the CP plane, achieve a nice little parabola on the surface of the construction plane.

You and I are operating on the same file in the same context.  We get different results, therefore we must be  following a different command procedure.  What is the difference?

In other words, there is an underlying problem here that we are not identifying.  Doubtless with a super simple solution.  But what is it?
 

[Ivan]

the problem is I don't quite understand what are you doing to be honest, I don't know what elevator mode is