Lofting two similar curves along a guideline


Hi all,

Is there a way to loft the top and bottom curves in the screenshot attached, and have the surface follow the “front” and “back” edges while maintaining their proportions?
This could for instance be a boat keel.



(Willem Derks) #2

Hi Ramon,

Does the Sweep2 command do what you want?



No, or at least I couldn’t get it to do what I needed.

(Willem Derks) #4

Hi Ramon,

Then I suspect you want the cross sections to remain horizontal right?
If so I have no solution I know of right now, maybe someone else can chime in.



Hi Willem,

Correct. The cross sections would remain normal to the straight, right side guideline, but scale to the distance set out by the curve opposite.



(Menno Deij - van Rijswijk) #6

In the Sweep2 command window you can say “Add Slash”. Then connect the first and second rail perpendicularly to the straight rail to get the desired cross-sections.

In the attached document use ‘first rail’ and ‘second rail’ to sweep both NACA sections. Then use the guide lines (not in screenshot, but in document) to create the slashes.

Sweep2.3dm(378.1 KB)


How about splitting the two profile curves. Straight loft two of them. Then match that surface’s leading edge to the guide curve with refine match. Mirroring that surface and then run match surface with tangent average both surfaces. It’s not perfect but it’s close and produces reasonably light surfaces.


Ok I think I have the solution. Run surface from edge curves. Mirror and tangent match both the surfaces at the leading edge.


Hi menno,

That certainly looks like what I’m after. Not a bad guess on the type of foil either!

I wasn’t able to follow along - if it’s not too much trouble, do you think you could explain in greater detail?

What does ‘add slash’ do?




Hi Stratosfear,

That worked as well. I’ll run a couple of templates and check deviation to make sure it’s tolerable.
Did you simply match the leading edge curves to one another to invoke tangency, or were you able to maintain tangency some other way?

Appreciate your help.




I just made the foil curves with simple degree 3, 4 point curves. As you can see the curves are drawn with implied tangency by having the leading edge control points at 90 degrees. The surfaces need to be matched because surface from edge curves does not maintain those control point relations for the cross section control points. Something I forgot to mention was when you match the two surfaces at the leading edge for tangency. Check Average both and this is really important check Match target isocurve direction. Just leave Preserve the other end as Position.


I ran a few test sections and the sections from arbitrary positions along the leading edge and all yielded thinner sections than a correctly scaled foil for that particular chord location.
Are you able to test your section to see if maintains proportions?




Hi menno,

I was ended up obtaining a good result using this method. Thanks. I’ll try a few more arbitrary points to make sure I have it worked out.



(Menno Deij - van Rijswijk) #14

Add slash is accessible from the options panel of the sweep2 command. You need to select one point on each rail and then the foil is added at that position on the rails. I have drawn the guide lines in the document such that they are perpendicular to the straight rail. If you select the intersection of the guide line with the rail, using for instance smart track, the slash is added at those points and the rudder will have its control points perpendicular as well.

(Pascal Golay) #15

Hi Ramon, so the idea is that the cross section that you’ve drawn at each end needs to stay perpendicular to the airflow (or whatever) throughout, and only scale but not rotate, correct?



Hi Menno,

Got it. I did a test foil yesterday and it worked well. I’ll try some more later today.

Pascal - correct, I need the foil to scale to it’s local chord and not twist or rotate.
Is there a rule of thumb for the amount of slashes that are required so that the foil will maintain it’s proportions?




I’ve created numerous fin keels using the sweep2 command with the root chord and tip chord each being a single curve sweeping the leading and trailing edges. This method has always given me the result I’m looking for. To assure as good a polygon surface as possible make sure that all your curves have the minimum number of control points that still give you the curve you want. This is good practice when surfacing with NURBS in any case. Also, I’ve found that that if you desire the leading edge to “radus” into the tip chord; you might want to try the following: 1) construct a simple straight line from the bottom tip of the trailing edge to the bottom tip of the leading edge. 2) fillet the leading edge curve to the trailing edge curve with the desired radius. 3) delete the remaining straight line. 3) re-scale the tip chord such that it comes directly in contact with the aft end of the new radius curve (as well as the trailing edge). 4) join the radius curve to the leading edge curve. 5) sweep2 using the new leading edge and the trailing edge as your sweep curves and of course the root and newly scaled tip chord as your shape curves. The result will be a nicely shaped keel with good looking leading edge radiused into the tip chord.