How to identify a stabilizer airfoil?

[mcg]

Attached are a couple of images of airfoils used in two different versions of the Henry Struck free flight aircraft, the New Ruler. The 1940 original is curvey. The flat bottom airfoil appears in a scaled down, 1970 version of the New Ruler.

Can anyone identify these airfoils just by good-eye?

If not, where could I find a procedure for measuring them, and then translating these measurements into a name or number?

If I can identify the airfoils, I can download formally correct versions and load them into Rhino 3D.

I can just trace, fair and fit them, but it doesn't create a really precise outline. I have been trying this without getting very good results. So I need to translate these images into exact names and numbers.

Another approach would be some sort of airfoil recognition software or site, where you upload a sketch and download a best guess. But I have not seen anything like this.

Many thanks for your insights.

Michael

[MCarlton]

I would hazard a guess that it doesn't HAVE a name, and was just a "that looks about right" shape drawn by the designer.

I would think that if you scanned the image of the centre section rib and scaled it for the remaining ribs, it would be perfectly good enough for the purpose.

[JetPlaneFlyer]

As MCarlton says...
Plus on a tissue covered wing or tail the shape that the air 'sees' only resembles the rib at the point of the rib itself. Between the ribs to tissue sags so 95% of the wing is only a loose aproximation of the desired airfoil. Fortunately at model airplane Re numbers the air does not appear to care very much.

Bottom line.. Just make something 'that looks about right'.. which is most likely the same approach as the original designer took.

[mcg]

Here is a "Reverse Look-up" for names and NACA numbers on a site I found referenced on Michael Selig's site.

http://www.ae.uiuc.edu/m-selig/ads/aircraft.html

Might be useful to scale modelers with the same problem I am having, since it names the airfoils for a large and wonderful selection of aircraft.

I have already built this stabilizer in balsa. As you point out, it is not a big problem.

Modeling it in Rhino3D is a different game. You need precision curves. You can't reach through the screen and do a little sanding. Much as it would help.

Michael

[Montag DP]

Here is a "Reverse Look-up" for names and NACA numbers on a site I found referenced on Michael Selig's site.

http://www.ae.uiuc.edu/m-selig/ads/aircraft.html

Might be useful to scale modelers with the same problem I am having, since it names the airfoils for a large and wonderful selection of aircraft.

I have already built this stabilizer in balsa. As you point out, it is not a big problem.

Modeling it in Rhino3D is a different game. You need precision curves. You can't reach through the screen and do a little sanding. Much as it would help.

MichaelMy suggestion would be to roughly measure the max thickness and camber and pick an airfoil with similar numbers to use in the design in Rhino. It would probably actually be helpful to trace it into Rhino first and use CAD to do those measurements.

[BMatthews]

On a lot of his models the wings were the NACA 6409. The New Ruler and the Record Hound being two of those. But you likely already know this. However it suggests that at least some of his choices for the tail MAY have also been NACA airfoils. The ones in the second attachment look suspiciously like a thin and lower cambered NACA style. If you can measure the thickness and then do a few mid point camber line spots to find the highest point of the camber and determine what it is in % of the chord and where on the chord it is then you can try running up a NACA number airfoil and see if it matches pretty closely. For example a NACA 4402 or 5402 or 4403 or 5403 would not be far off the shape of the second one as a guess. But some simple measurements and That first one with the flat bottom airfoil is likely just a home brew or it MAY have been one of the simplex arcs attached to a flat bottom. Profili has the Simplex arcs in its library.

[mcg]

You are a miracle, Bruce. Now I know how to tackle this. Many thanks! Michael

[mcg]

This is my best guess. The New Ruler stabilizer appears to be a using a NACA 2409.

A Rhino tracing from the 1940 Air Trails construction article for the New Ruler is overlaid on the 2409 in the attached jpg. Not a perfect match on the lower surface.

The traced airfoil has, however, been rebuilt in Rhino, to clean up the curves so this could be a source of the departure. Another possiblity is that it isn't a 2409. Or my constructions and calculations (henscratchings, below) may have gone wide.

I notice some of these airfoils are coded with a suffix, e.g., 2409-34. No idea what this means, but maybe the suffix suggests how to modify the curve.

The NACA 2408 was also a pretty good match, but that airfoil has a curious teardrop at the trailing edge.

Wonder why Henry Struck was making these design choices in 1939, that is, why did he favor 240n airfoils?

Anyway. Computer driven archaeology.

Michael

[Montag DP]

Wonder why Henry Struck was making these design choices in 1939, that is, why did he favor 240n airfoils?

MichaelHonestly, I think it was probably TLAR (That Looks About Right) method. In any case, you're not going to notice the slight difference between the tail airfoil section and the 2409. You're picking nits.

[mcg]

Here is the port side of the stabilizer, lofted in Rhino3D using a downloaded NACA 2408 data set from Michael Selig's database at the University of Illinois. The UI database does not include the NACA 2409, so that's out, and their NACA 2408 does not include the bizarre little teardrop on the TE, so that's good.

The point of the whole exercise is to get a clean surface in Rhino. You can do this by tracing the airfoil, and then rebuilding it and refining the curve by editing control points, etc., but it is a long, drawn out struggle. Given the airfoil's numbers from the database, you are starting with a good curve, and the good surface follows almost immediately. But to get the numbers, you have to know the name of the airfoil.

Thanks to Montag DP for the idea of using CAD to quickly extract the airfoil's dimensions from the original plans, and to Bruce for the detail on Henry Struck's design choices.

Next step is to salami slice the lofted surface into individual ribs.

[Montag DP]

Here is the port side of the stabilizer, lofted in Rhino3D using a downloaded NACA 2408 data set from Michael Selig's database at the University of Illinois. The UI database does not include the NACA 2409, so that's out, and their NACA 2408 does not include the bizarre little teardrop on the TE, so that's good.

The point of the whole exercise is to get a clean surface in Rhino. You can do this by tracing the airfoil, and then rebuilding it and refining the curve by editing control points, etc., but it is a long, drawn out struggle. Given the airfoil's numbers from the database, you are starting with a good curve, and the good surface follows almost immediately. But to get the numbers, you have to know the name of the airfoil.

Thanks to Montag DP for the idea of using CAD to quickly extract the airfoil's dimensions from the original plans, and to Bruce for the detail on Henry Struck's design choices.

Next step is to salami slice the lofted surface into individual ribs.Oh, I didn't realize you were drawing the airfoil yourself...I could have given you a .dxf file of the airfoil drawn by Profili. All's well that ends well, I guess.

[mcg]

For Rhino3D users, here is a link to the free airfoil script.

http://en.wiki.mcneel.com/default.aspx/McNeel/RsAirfoil

Basically you download the zipped script -- the UI airfoil database comes down with it. Unzip and, from within Rhino, run the script. (If you have not downloaded or used Rhino scripts before, read the Help for the command "Edit Script.")

When the script prompts for the name of the airfoil, select one from the long UI list and click. The chosen airfoil will be drawn at the origin. You can then use the "Orient" command to position it on a spar or wherever you need it.

Five minutes, the whole thing.

mcg