How do you draw a tilted arc?

I am trying to draw a tilted arc on a 13 degree angle. The first radius is 74mm. However, when I draw a quarter arc the second radius is shorter. Should they both be the same?

Not sure what you want, can you post a sketch?


There are two ways to do it:

  1. Draw the arc on the construction plane before tilting it with the gumball, or Rotate command, or Rotate3D command, or MPlane command.
  2. Use CPlane command to make tilted construction plane before drawing the arc on the tilted construction plane.

I had to create a plane and rotate it 13 degrees. Then I used the Project command to project the arc to the surface.

Either I do not understand you, or you do not understand my post. The construction plane is not ordinary plane (made, for example, by Plane command). If you project an arc on the tilted plane, the projection will distort your arc. (The projected arc will be elliptical, not circular.) Do you want to make distorted arc? Do you understand the basics of Rhino program?


Here’s a file the shows projected cures on two planes.

ImpellerAndShaft.3dm (6.3 MB)

Setting the CPlane to the surface works. But, project command is faster.

Arcs.3dm (50.5 KB)

Now I understand what you are trying to make (helical impeller). Your first model (ImpellerAndShaft.3dm) is very rough approximation of helical solid. (It is shown in the Default layer). The easiest way to make real helical solid is making two helixes (inner and outer), making (helical) surface between the helixes, and extruding the surface into the solid. The helixes can be made either by Helix command or by Spiral command. The surface can be made by Sweep2 command. The solid can be made by ExtrudeSrf command.

Your second model (Arcs.3dm) proves what I wrote in my previous post. Your rotated and tilted arcs are real/circular arcs, while your projected arc is part of an ellipse. You cannot use the projected arc because real impellers have circular cross-sections (circular, not elliptical). You can verify the curvature of your arcs with Curvature command and CurvatureGraph command.

By the way, when you make your custom construction planes, it is good idea to name them either with NamedCPlane command or with MPlane command. If you do not name them, you will probably lose them.

You can make variety of propellers with the help of this grasshopper script: Blade for propeller - Grasshopper

Thanks Andrew.

I think I got it. I had to adjust the height of the helix to match the slope of the 18 degree curve. The Grasshopper script didn’t work. It’s missing files.

Helix.3dm (34.0 KB)

Everything looks good.

If you are serious about hydrodynamics, you will use variable pitch helixes rather than ordinary helixes because the impellers resemble airplane wings. Real airplane wings have curved cross-sections for good aerodynamic reasons. The ordinary helixes resemble flat airplane wings. Variable pitch helixes resemble real airplane wings. The following grasshopper script makes variable pitch helix. You can change the helix by moving the sliders and control points of the 2D curve. (Left-double-click the sliders to type precise numbers. Left-double-click the 2D curve box to edit the control points.) If you right-click the far-right box (Nurbs) and click Bake in the menu, the script will make the helix.

variable pitch (8.7 KB)