Circle on a curving surface best way?

I have a fuselage side curving notably top to bottom, as they do !..and maybe a tad fore/aft.
I have to plot a 120mm dia panel on this, I have circle in ortho view as it would be on a plan (plans drawn as if aircraft 2D on datum, )

Using project is wrong as it becomes an elipse top to bott.

I then thought of projecting the centre cross to surface then creating a sphere dia 120mm on that cross and using intersect.

Is there an even better way of getting a 120mm dia panel on this surface ?

Lets assume the panel is allowed to curve, as would be normally.


Is it possible for you to post an example file? That’s the best way to get an accurate answer.

example attached.

projected the 120mm circle on the engineeering drawing (raster not in mockup, just the circle is there marked 120mm) that should feature on the aircraft fuselage skin becomes more, for obvious reasons.

I need a 120mm dia circle on the skin.

is drawing a sphere 120mm dia having projected the centre cross the best way ?

CirclesOnSurfaces.3dm (183.0 KB)


The intersection of a sphere and the surface will not give you exactly 120 mm circle.

How about this: make a CPlane that is more or less in plane with the projected circle (orange), then draw a circle on that construction plane and project it to the surface. This way, the projection distance is mostly perpendicular to the surface, rather than under an angle. With some trial and error I could get the projected circle’s radius down to 121 mm or so.

Alternatively you could see it as an optimisation problem: which initial radius gives me 120 mm after projection? Probably start with 118 mm or so, that should get you in the right ballpark.

If I draw a line perp to surface at the centre cross, then create a cylinder around that of dia 120mm, that would be same as the Cplane but more accurate. use intersect etc.

I am surprised there is no command to draw native shapes on a surface.


But, how should it look, since it cannot possibly be a circle?


Perhaps CreateUVCrv and ApplyCrv would be useful here?

Expanding on Menno’s method, this gets you pretty close:

Extend cross centers out to 120 guide circle
CPlane to Stbd
Poroject cross hairs to Surf
3-pt Cplane using Fore/Aft Cross at surface for pt 1 and 2 and vertical cross at top for 3rd pt to get a good average for the CPlane
Move the CPlane to the cross centre on surf
Draw 60r circle from the new CPlane 0,0,0
Project circle onto surf

It is a pretty wavy surface so if you wanted a better result, you could fill the circle (2nd last step) with a planar surface and rotate it and the circle around until it looks best, then redo the CPlane to that new position before projecting

I think the splop command might be one of the fastest ways to do this if you are doing a lot of these.
make your 120mm circle anywhere. Splop. pick centerpoint of circle, then a quad of the circle. Then pick your surface, use the copy=yes rigid=no options. Click on surf for first ref point, then type 60mm for second ref point.
very quick. no cplane needed.

Can you elaborate on the requirements? Seems to me that this is a geometric impossibility.


All good interesting methods.

The enginering 2D drawing, which are drawn as if aircraft is projected to Cplane, shows a circle, knowing the subject matter and how things were done many yrs ago, the panel would be circular on the skin with centra about the centre cross. I think its fair to project cross to skin, then create a circle on a Cplane, the Cplane having its Z axis on a line perp to surface at the circle centre. My method of creating a line perp to surface at cross then creating cylinder about this line and intersect with surface seems logical.

Yes its impossible to have a circle on a surface where the surface is not planar. The circular panles on aircraft follow curvature so geometricaly are not exact circles.