Drawing a foil cross section in SketchUp

I am trying to draw a foil shape like an airplane wing with specific geometry. It would seem easy to connect the dots with a spline, but I can’t figure it out. I tried Fredo spline using a variety of the tools in Fredo and can’t get it to come out even close. I can export to Autocad, draw the spline and then bring it back, but seems I should be able to do this in Sketchup. I measured the foil that is intact in stations and made a skeleton. I guessed at the tangents to the leading and trailing edges. Once I get the base profile drawn I think I can push pull and create the actual foil shape. The application is a sail boat keel that was damaged. I’m going to 3d print a mold and recast the shape. Sorry for the selfie, but couldn’t be helped.

What do you have for the data for the foil cross section? Can you share the data?

that was quick. I’m trying to figure out how to include the attachment

keel mold 2018.skp (8.0 MB)

OK. So you don’t really have the NACA data for the foil profile.

With what you have, a Catmull Spline from Bezier Spline might be the best option. It would help if you add a few more stations, though.

What is the NACA foil number?

It’s a sailboat keel, so not a NACA airfoil. I’m impressed you are familiar with the NACA foils. My grandfather was an airplane designer in the days when the foils were established. I have a book of the NACA airfoils dated in the 20’s. The stations are a bit difficult to measure due to the damage and, but I can split the stations and get some more. I will add them tomorrow. It’s easy in autocad and I’d like to do it in Sketchup. I will check out catmull spline.

Ah… That sort of explains the odd image. I guess I know about a lot of weird old stuff that most people today would think of as useless. Did you know that the wings on the Globe and Temco Switfts (1946 to early 50s) were designed by the same people that did the wings for the P-40 Warhawk? And the slot near the leading edge in front of the ailerons was added to direct air over the ailerons at low airspeeds and high AOA to maintain aileron authority? Cool technology at the time.

What is the boat? Maybe you can find the profile that the keel should really have. How was the keel damaged and how badly?

1 Like

Cool details on the airfoils. I didn’t know the details of the foils. Orville and Wilber did some remarkable research with a home made wind tunnel made from orange crates, particularly on propellers. The compared each section to a flat plate. Those guys were incredible and it wasn’t that long ago.

The boat is a Catalina Capri 25 and I have asked the users group if anyone has the fairing template dimensions for the keel, but no response so far. I’m not looking for something that precise, just something close so I can make a mold, inject filled epoxy than sand to match the existing profile. If I try to lay it up with fiberglass cloth it will take many layers.

The boat was grounded on a rock coastline. It’s not much damage relatively speaking and the hull to keel joint is sound. The tracing photos in sketchup show the damage. I have access to another boat I can measure, but I’ll have to pick it up with a crane to measure the keel profile.

There are various NACA foil calculators online that could be useful for something.

I wonder if Butler ever published the keel profile for the 25. I would be inclined to wrap a wood batten around the keep and try to work out the shape from that. a long piece of screen door stop would make a good batten. If you were able to do that and then use a ticking stick and a piece of plywood or carboard you could lift the shape from the batten.

very cool

lovely boat… dad had one in Sydney … was his favourite cruising yacht

Thanks for all the input. I have a plan for the repair and I’m comfortable with the profile dimenstions, it looks right. I’m struggling how to model the curve in sketchup. It shouldn’t be difficult. I can draw it in autocad, import to sketchup. I’d like to learn how to draw a spline in sketchup from points or coordinates. I can make a table of the geometry if that helps.

A table of offsets would be useful. The more stations you have, the smoother the curve will be. As when lofting a boat hull, though, the curve may need to be faired a little. The offsets are usually not exactly right. If you were laying out the curve on the loft floor from the table of offsets, you would stick nails in the floor on the stations at the half-breadths. Then bend a batten over the nails and look for places where the batten has a kink of misses a nail and move them as appropriate to fair the curve. A Catmull Spline goes through the control points so that’s generally my go to for drawing those kinds of curves. The curve can be faired after it’s drawn.

You might also look at connecting the points with a series of straight lines and then use Fredo6’s Curvizard to smooth contours.

I’ll grab more points today. Where do i find the catmull spline? I looked last night. I have Fredo’s spline and have tried several of the tools and I can’t figure out how to make them follow the points. The tools want to distort the line by dragging points that are not on the geometry. I’m guessing I just don’t know how to use it. I have watched Fredo’s videos without success.

As I wrote last night:

Do you have Fredo6’s Bezier Spline? It’s in that set.
Screenshot - 12_2_2021 , 8_42_35 AM

FWIW, the keel profile probably is a NACA 00xx profile. The first digit in the NACA profile number is the camber. Zero means there’s no camber and the foil is symmetrical.

FWIW, here’s a symmetrical foil shape using a curve drawn using a Catmull Spline. I used an online NACA foil generator to get the points which I saved as a CSV file. Then used a point cloud importer to get the guidepoints. You can create the points as you did in your previous model and still use the Catmull Spline to connect them.

NACA 0012

1 Like

Thanks for all the help. Fredo calls the catmutll spline a Global Fit Spline now. I’m pretty happy with the results. I’m cutting cardboard templates to see if the shape needs any tweaking. Then I will make a break apart 3d printed mold to recast the proper shape.
keel mold1.skp (8.5 MB)

1 Like