ive got this coordinates from naca profile plotter and exported them to generate hydrofoil. making the surfacing is not so easy and many control points result in a multi-span curve… is there any way i could correctly surface this example (image below) like class a speaking?
The usual approach to modeling hydrofoils, airfoils and similar is to first create section curves nwhich accurately model the desired shape and then loft or sweep between the sections.
Accurate modeling of airofoil/hydrofoil sections almost always requires multispans surfaces. Multispan surfaces by themselves are not inherently evil.
Using higher degree curves can result in oscillations when creating a NURBS curve by interpolation through a set of points; for example using the Rhino commands InterpCrv or CurveThrougPt. The oscillation tenedency is inherent in using higher degree polynomials for interpolation. It is not specific to Rhino. Lower degree polynomials are less smooth however. Degree 3 is typically a good compromise between smoothness and lack of oscillations.
Why does the surface need to be “Class A”? Is that a requirement of the customer? Or is it because you have been reading the threads here with claims that “good” surfaces are “Class A” with only single span surfaces?
What will the model be used for?
It is possible to create a single span surface which looks like an airfoil/hydrofoil, and such a model may be good enough for illustrations or animations. But such a model will not be close enough to actual airfoil/hydrofoil sections for technical purposes.
The tricky part is the nose - these are just like any other body that SORTA looks like a body of revolution, but isn’t:
ETA - Also for “quick and dirty” modeling here, I’d just use CageEdit on a body of revolution. I’d use that to get my “close enough” shape, and then if I wanted to, make an explicit patch model from that quick and dirty model.
I find that NACA foils can be quite a challenge depending on what is needed.
I am completely with David on this when trying to build a smooth curve from a set of plotted points:
and as should always be critically looked at:
Some things to think about that help:
Most NACA foils are defined by plotted points in space. The process of rebuilding that into a NURBS curve is important. Many times I split the top cord, bottom cord, training and leading edge apart from each other because the requirements for each are so different.
The trailing edge of the profile shows as a sharp point, but rarely is in reality. Leave a small trailing edge allows the upper and lower curves to get to the back edge without interfering with each other.
The front edge of NACA foils are specified by a radius. Of course if it si a radius at the front, it will result in a G1 condition. If G2 is wanted, then it cannot be a single radius. Many times splitting off the front radius and treating the top and bottom cord separately can help.
Working this profile with Rhino tools can get you to the continuity you would like. It just takes time and effort when starting with a set of plotted points along a length and not a lot of information between is where it gets interesting.
What is the customer’s requirements?
What is the input you have when starting with the NACA?
If you look at his original picture, it’s not at all a revolved body - the entire thing seems to have an “arc” to it in side view. You can take a body of revolution that’s close to that overall shape and then CageEdit to deform it, but you may run into pinching issues at the nose.
To be clear - if something IS a body of revolution, no need to overthink it! Revolve that puppy! But, there’s all these weird cases (aircraft noses, low drag radomes etc) that look/feel/smell like a body of revolution but actually are not. I’ve done enough radomes and low drag antenna enclosures at this point to know how one when I see one lol.
The nose “radius” specificed for many airfoil sections is what the curvature is at a single point, not that a portion of the nose is a circular arc. If both the upper and lower curves have the same curvature where they meet at the nose then by definition they are G2 continuous.
The tangency direction at the nose can also be frequetly inferred from the specification of the airfoil. For a NACA airfoil specified as a thickness distribution and a camber curve the upper and lower surface curves at the nose will be tangent to a line perpendicular to the nose end of the camber cove.
Yes!!! So many people think that an airfoil (theoretically) has a constant radius nose - I think a lot of this has to do with going back to old paper drafting, where you’d draw circles along your camber line, and then folks would connect the circles with a french curve (see, my childhood making model airplanes from scratch). In reality, there is an infinite number of circles and so as you approach the leading edge, the portion of the leading edge described by an actual arc is a limit that goes to zero.
No matter if hydrofoil or keel bomb - a trivial mathematical NACA profiles may look similar to the upper curve in the attached image. If you calculate the y values by constant steps on the x-axis an interpolated curve may be wavy when the profile goes perpendicular to the x-axis.
The lower curve is “Class A” - means this is the design intention with good surface quality.
Note: The tool geometry may look very different to get close to the design intention, no matter if it is fabricated by punch drawing sheet metal or injection molding