Plotting Aerofoils for Ribs

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Midniteoyl

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You have a pattern?

or, take what you do have and run it over to a printer and have them printed at full size..
 

Norman

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What airfoil section, skin thickness and chord do you want? I don't have any specialized airfoil plotting software handy but I can make you a basic template with skin thickness offsets and chord stations. This would work for a hershey bar wing, any more than that would get into actual work. I could send you a DXF file that most reprogrographics shops can print out on long paper for a reasonable fee (sorry, I don't have a plotter). If you're doing a tapered wing and/or using more than one airfoil section you need a plotting program that can do the intermediate shapes automagically. A local hobby shop can get you in touch with a modeler with Compufoil or something similar. The old version I have on a DOS box (which is currently a door stop) has a limit of 8' chord length but it can do tapered and elliptical planforms with different root and tip. Newer versions can do a lot more. Check with local hobby shops and the local EAA chapter. It's always better to be able to see what your drafter is doing and actually TALK to him.
 

orion

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Plotting an airfoil is relatively straight forward. The first question is whether your ordinates are in the standardized format or whether they're already scaled to the rib size you need. Usually ordintes are listed from a chord position of 0 to 1.0, or from 0 to 100. Either way, you're going to have to scale the numbers so that the chord length is that which you need for your airplane. Make sure to multiply not only the chord positions but also all the ordinates.

Then lay out your chord line (just a plain straight line) and mark your zero point. Then for each chord ordinate (x), mark the upper and lower surface ordinate (+y and -y). At the end, you'll have the chord line and a series of points, top and bottom, that will represent your rib shape. Now you have to connect them all together.

Assuming you laid all these out on a piece of good wood (MDF is probably the best), place a finishing nail in each point, then take a flexible strip of material such as a thin strip of wood or plastic, and clamp it to the nails so you get a smooth curve interpolating all the ordinates. Then just draw a line connecting all the points.

Around the leading edge, you may have to do this in several steps or use something like a "French Curve" to plot the line.

Once you have the lines drawn and are happy with the shape, you can then cut out the rib and use it as your master pattern for all your jigs and fixtures.

The easiest way to do all this though is with a computer and a simple airfoil plotting program or a drafting program. Once you draw all the lines and create the drawing file, you can then take it to someplace like Kinko's, and have them plot the lines to any scale you wnat.
 

Johnny luvs Biplanes

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Of interest the aerofoil is a USA 35B with upper chord of 39" and lower chord of 36".
Orion, what you describe is about what is called for in my plans, could you just explain which actual figures to multiply together?
Thnaks all.
John :)
 

CNCRouterman

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Ribs - n - stuff.

As Bill said, a computer can be your friend. If you can get access to a CAD program, and plot out the coordinates as point, or even as short line segments, then do so. Convert to a spline with the open end being at the trailing edge. Splines do not conform to sharp angle very well. The object of the exercise is to create a vector drawing of your airfoil. Now, you need to decide if that is the outer profile (the actual outer surface of the wing), or the inner skin profile which I presume would be the same as the rib profile.

Now, you need to scale the drawing to your appropriate size.

Ok, now for the shameless plug for my company’s services.

Once you are happy with your CAD drawing, you can either print it on a plotter for your use, or send the CAD drawing to a CNC Router job shop like mine and have them (me) machine out the pattern. You can use light duty material such as 1/8" or 1/4" hardboard if you like, or for a more distortion free (less subject to movement due to changes in humidity) version, cut out of 1/4" aluminum.

For a 5' long cord, 1' tall profile out of 1/4" aluminum, just one part, I would estimate a cost of about $75.00 including the 1/4" 5052-H32 aluminum. That is assuming you provide a clean CAD drawing in DXF or DWG format.

Your cost per part gets better with volume, but aluminum is just plain spendy right now.

I have 1/4" alum on hand for a few patterns, and some 0.080" but shorter lengths. Hardboard is a lot cheaper and machines faster and of course would be cheaper to ship. You just have to decide what makes sense. Aluminum is great as a template, durable, immune to moisture swelling, and so on. Hardboard works well, and is way cheaper. I could easily machine many parts out of a full 4 x 8 sheet for less than just the cost of the aluminum would be for your project. Maybe getting several "disposable" patterns makes more sense than one "indestructible" expensive pattern?

Anyway, best wishes.
 

orion

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Assuming that you have the ordinates in a normallized format (0 to 1 or 0 to 100), the numbers represent the fractional ratios, all based on the chord length. In other words, the chord is assumed to be 1 (or 100) inch long and all the "y" values are based in proportion to the chord measurement. Therefore, as you plot the top and bottom of your wing, you must proportion each set of data to the chord you are trying to achieve.

On a computer this is easy since you just plot out the spline to whatever coordinates you have, then just scale the whole thing to get the chord length you need.

Otherwise, you'll need to do all the multiplication twice, to get the two chord lengths for each of the two wings.
 

Norman

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OK, here is a DXF with the USA 35-B drawn twice with 25%C and the LE radius marked and scaled to the lengths you asked for. In good CAD programs you have several options for curved lines. I used a polyarc instead of either of the B-splines. The cubic B-spline is way off the nominal profile, especially with these antique airfoils that used a fairly small set of ordinates. The newer NURBS spline is identical to the poly arc except for a slightly smaller LE radius that, I think, results from too few ordinates on the lower surface.

As drawn the leading edge radius is 2.002%

You can use the free demo of QCAD to do your detail work. It doesn't do a good job of working with splines but it's good'nuf for most other 2D stuff. Download QCAD here:
http://www.ribbonsoft.com/qcad.html

Oops, not a valid file extention for posting to this list. :wail: Use the <pm> button at the bottom of the message window to end me your e-mail address and I'll e-mail it to you off line
 
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First multiply all numbers in the table by your cord ( in your case 39 inches) and make new table with these numbers and these numbers are now in inches rounded off to 1/100 th of an inch,
Make horizontal line mark the starting point and start laying out the points row by row (three in a row) , lay the points of the first three numbers “X” is the distance on your horizontal line from the starting point, “Y upper” puts the point directly above the “X” point , “Y” lower puts the point directly below the “X” point , move to the next line of three and so on, connect the resulting dots and viola.
Usually the coordinates are written that the airfoil comes up upside down ( hint - turn the paper around when finished)
George
 

CNCRouterman

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John,
For such a case, I would want to start with the designed finished proflile, ie: airfoil, and draw successive offsets for the skin and any other buildup between the "finish" profile and whatever I am asked to machine. My software will easily handle parallel offsets of entities and geometries.

Here is a screen shot of an imported WIG airfoil. I scaled it to 48 inch cord, and drew two offsets, one at 0.10" and one at 0.250" inside the original geometry.

Adding cutouts or tabs or whatever for spanwise features would not be hard.
 

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CNCRouterman

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dxf files.

Norm,
I imported your DXF file. Is this drawing representing two distinct entities or is one supposed to be an offset from the other?

I tried using the center of the circle for overlaying the patterns as well as the intersections of the two lines. In neither case do I recognize a constant offset.

Anyway, I think that dxf files are better than my attempt at a screen shot.
 

Norman

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Re: dxf files.

Originally posted by CNCRouterman
Is this drawing representing two distinct entities or is one supposed to be an offset from the other?

He asked for two usa35b of differing chord so that's what I made. I drew in the LErad, chord line and a reference line at .25C before I scaled them to the requested lengths (should have rotated them so the chord is horizontal, oops :whistle: ). I would have included a skin offset but he didn't say what thickness he wants to use. Now all he has to do is find a print shop with a plotter (I take my drawings to a surveying supply store) and have them print it in inches.
 

Johnny luvs Biplanes

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Two chords are required as I'm building a biplane with different top and bottom chords!
The skin is not practical to show as there is a ply LE forming the "D" box and the lot covered in Polyfiber. Any outline purely needs to represent the finished rib profile at the chords mentioned. Also there seems to be at least 2 slight variations of the USA35B aerofoil, can't remember the names off hand though differences are minimal.
Cheers,
John
 

CNCRouterman

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Ribs

John,
Norm's drawings are pretty good except for one minor issue. The Chord line terminates at the trailing edge, and the bottom of the airfoil terminates in space approximately 0.109" below the vertex formed by the chord line and the upper plot for the airfoil.

By pretty good, I mean that I could machine from them with out having to spend any time cleaning up errant points or extraneous bits of geometry floating around. You would be surprised how sloppy some "professional" customer supplied drawings can be! Norm's is clean, just one bit of adjusting required.

I suspect that when Norm was trimming the trailing edge, he picked the cord + upper plot line in stead of the lower plot + upper plot.

Either that or my importing corrupted the file.

Eric
 

orion

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The trailing edge offset is not a big deal and actually may be there on purpose. In building a wing, unless the trailing edge is of two thin sheets of aluminum being brought together, there will most likely be some finite gauge as a result of the structural material underneath. In the case of the biplane, the trailing edge is most likely wood. This has to have a thick enough edge so that minor contact does not cause damage. Bringing it to a fine point is generally not practical.

There is also a theory that suggests that a thicker trailing edge cutoff may have a hair less drag and lower control surface moments.
 

CNCRouterman

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Closing the trailing edge.

Ok Bill,
That makes a lot of sense. I guess that I am so used to being anal about working with closed geometries, it did not occur to me that it might be on purpose.

My apologies to Norm.

For my purposes, should I be elected to perform any machining, I would have to close out that geometry. It can be a truncation, or an arc, or whatever, as long as there is a feature that closes the trailing edge, otherwise, there will be a tail of scrap material left behind that will have to be dealt with by somebody.
 

Norman

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The ref I have is from a test at M.I.T. in 1923. Many airfoils from that period show an offset at the TE. I agree with Orion, it's most likely because of the materials and construction techniques. At that time some planes were still being built with wire trailing edges e.g.
the scalloped TE on many W.W.I planes.

The book I have shows two versions of the USA35. The A is much thicker than the B and not nearly a efficient or stabile. Unfortunately early '20s wind tunnel data isn't all that accurate but at least we know the USA35B works ok on Piper Cubs.
 
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