CafeenMan

This is an article I've been trying to get around to for a while. I've received a handful of e-mails asking me how to do this. I finally went through it step by step and got it posted on my site.

The article describes how to manually plot an airfoil. I know there are a lot of computer programs (some are freeware) out there that can do it, but I still manually plot my foils because it's fast and simple.

If you design planes and plan to use a computer, then you still might want to read this article just so you're familiar with how it works.

As always, constructive feedback is welcome. If anything confuses you let me know so I can reword things or add more detail where needed.

- Paul

How to Plot an Airfoil

Tall Paul

That looks like the way I used to do it, 40 some years ago. Back when you could describe an airfoil with a few points. Nowadays, with 5-digit precision and 100+ points, it's easier to let the program do the plotting. I use Profili V2 for this.

CafeenMan

I had a lot of problems with that program. Especially when trying to import into autocad. If my printer would work more than once a quarter, I would probably use a program to generate my foils though.

Tall Paul

I haven't tried importing a Profili .dxf file into AutoCAD. That's on a different computer, and is release 12, DOS only. Way out of date.
The Profili. dxf's go into TurboCad 9.0 without any problem at all.

SeditiousCanary

AutoCAD is funny. It works great, or you don't know what you are doing. Plotting airfoils is really easy though. Here's how.

1: Get the Coord Data for the airfoil you want (in this case, a RG15). I recommend an excellent website run by Michael Selig of the [link=http://www.aae.uiuc.edu/m-selig/ads/coord_database.html]University of Illinois at Urbana-Champaign's Department of Aeronautical and Astronautical Engineering[/link].
2: Once the Coord Data is open in Microsoft Internet Explorer, highlight the listed Coord Data, right-click the Coord Data, then click Copy.
3: Open Notepad.
4: Click File, then click Paste.
5: Delete any Spaces in front of the columns of Coord Data.
6: Replace any Spaces between the columns of Coord Data with a Comma.
7: Type OSNAP NONE and PLINE at the beginning of the Coord Data. These must each be on thier own line.
8: Type C at the end of the Coord Data. The Coord Data should be formated like this once you are done:

osnap none
pline
1,0
0.99671,0.00054
0.98726,0.00229
0.97237,0.00514
0.95248,0.00865
0.92764,0.01254
0.89810,0.01685
0.86427,0.02152
0.82660,0.02644
0.78557,0.03149
0.74165,0.03654
0.69537,0.04146
0.64723,0.04612
0.59778,0.05039
0.54753,0.05414
0.49702,0.05727
0.44676,0.05966
0.39727,0.06123
0.34902,0.06190
0.30248,0.06162
0.25809,0.06036
0.21624,0.05810
0.17730,0.05486
0.14161,0.05068
0.10945,0.04564
0.08108,0.03985
0.05673,0.03343
0.03658,0.02654
0.02076,0.01935
0.00932,0.01214
0.00235,0.00526
0,0
0.00002,-0.00048
0.00336,-0.00534
0.01247,-0.01006
0.02670,-0.01436
0.04596,-0.01811
0.07010,-0.02123
0.09896,-0.02372
0.13224,-0.02559
0.16963,-0.02688
0.21073,-0.02762
0.25509,-0.02785
0.30221,-0.02762
0.35156,-0.02696
0.40257,-0.02590
0.45463,-0.02446
0.50713,-0.02262
0.55944,-0.02025
0.61128,-0.01717
0.66244,-0.01366
0.71237,-0.01015
0.76037,-0.00691
0.80575,-0.00413
0.84779,-0.00192
0.88583,-0.00034
0.91925,0.00062
0.94748,0.00101
0.97003,0.00097
0.98652,0.00064
0.99660,0.00021
1,0
c

9: Click File, then click Save as.
10: In the Save in box, choose Desktop.
11: In the File name box, type a Name for the airfoil, then click Save.
12: On the Desktop, right-click the Airfoil text file, then click Rename.
13: Change the File Extension to .scr, then press Enter on the keyboard.
14: Open AutoCAD.
15: Click Tools, then click Run Script.
16: In the Look in box, choose Desktop, then double-click the Airfoil.scr file.

As a side note, if you want to plot the airfoil for a certain size, multiply each coord data point by the desired chord. I like using Excel to do it. You can enter a formula into one cell, then drag it into the rest of them to quickly calculate all the data at once. Get used to importing data with comma-separated values though.

Tall Paul

2. Make that into a script file.
3. Tell autocad to load it..
4. Watch it plot. <G>

SeditiousCanary

Yeah, assuming you know how to.

I think I covered it for just about any version of AutoCAD. I'm running AutoCAD 2004, but the directions are the same going back to AutoCAD11, probably even further.

Tall Paul

Be absolutely certain there's NO spaces in any of the data pairs. AutoCad treats a space as a <enter>, and will fail to load the .scr file.
I had a program that would compute NACA airfoils and output the data in AutoCad and Generic Cadd formats.
Profili and Compufoil etc. do so much more now...

CafeenMan

SeditiousCanary - By the time I do all all that, I've got the wing built.

Seriously, I've been asked enough times how to do it that I thought the how-to would be helpful even for people who use their computers to generate airfoils just so they know how it works. The problem I had with profili was that it would import into AutoCad with wing chords of 17 miles and I could never get it to the exact size I wanted. I didn't spend much time messing with it because I needed it done, so I just manually drew it.

Sometime when I have nothing to do for a few months I'll sit down and learn all this stuff again. Last time I used AutoCad, my company had just bought the fastest computer of all time - a 386 with a huge 5 Mb external Bournelli box.

SeditiousCanary

Actually, I export it and have Excel do all the hard work. Comma seperated values are your friend if you know how to use them and the bain of your existance if you don't. I certinaly don't type all that though.

CafeenMan

SeditiousCanary - Actually... LOL... I scanned the table from the book and then pasted it in excel, told it to multiply everything by 9 and it was done.

SeditiousCanary

Yeah, but you have a jigsaw and I don't. You have a great shop setup.
My shop is much more geared for bicycles and mechanical things requiring bolts and not much in the way of balsa forming. I doubt my worm drive circular saw would be of any help to me for model building. I am thinking about making a router table got my wife's moto tool to make sanding former radii easier and for dadoing wing spars and such. I'm also thinking about designing a few simple metal blade holders for the same applications with micrometer style adjusters for depth and runout. Machining classes are bad, they make you want to make tools...

CafeenMan

Umm... how did we go from plotting airfoils and exporting from excel into AutoCad to bicycle shops??? Is this a Wright Brothers thing?

BTW - a decent scroll saw is about $100. I worked on bikes for a long time, and the tools I use don't cost as much as bike tools do. Especially when you consider how often they change the standards of bikes and you have to go out and buy all new tools every couple years.

Tall Paul

The 1000 data point airfoil tables create a close affinity with spreadsheet use... RcCad displays the airfoil using normal coordinates... but while beginning at the trailing edge, it goes along the lower surface to the leading edge, then over the top to the trailing edge.
Use the current method of plotting the upper surface front to rear and then the lower surface, you get the shape, but it's "from the inside" for the color. The bright yellow wing is the RcCad process, the dark yellow from using an unmodified data set..
The normal data set has to be split into upper and lower segments, and then each sequence of points from leading to trailing edge sorted backwards and recombined. Can't even be done longhand!

CafeenMan

Tall Paul - I think that's the standard used by xfoil or some other old DOS program. I downloaded a ton of ordinates many years ago and that's how they worked.

The problem I have is that I don't do enough CAD work to make it worth it to go through the hassle of learning it. It has a steep learning curve. If I had a better grasp on it, then I would certainly use it rather than hand plot.

However, there is still an accuracy issue. How accurately can the average modeler make a part? Maybe to 1/128"? I doubt even that close, so 5-digit precision is a lot more than necessary. 2-digit is close enough.

Also, you don't have to plot all 1,000 points if you're doing it manually. Pick a couple dozen points evenly distributed and you'll make a reasonably accurate airfoil.

I actually have AutoCad 2002 and several designs that I need to draft. Every time I sit down and mess with AutoCad, I end up going back to my drafting table and doing it in ink. For the life of me I can't get AutoCad to draw a curve the way I want. If all I had to do was draw straight lines, I'd have no problem.

SeditiousCanary

Rather easily. I was tyring to point out that your ability to build the wing that fast was due to you having the right tools, where as mine aren't bad, just not suited for building wings. A Craftsman 15mm wrench is a piss poor cutting tool on balsa.
Oh, and that you have more building experience then I do may be a factor too.

CafeenMan

Gotcha

Hey, I just read your long post above again and I have a tip for you. Instead of pasting in Notepad, paste in Word. Then press CTRL + H and replace all spaces with nothing. Works great for stuff like that.

Tall Paul

Building accurancy? That's my middle name!

Here's 4 foils, 3 of which are guaran-darned-teed to be better than the Clark-Y, always... With supernatural effects attributed to the 3 look-alikes which seperate each of them from that old "flat bottomed thing" and each other, building any of them to a precision that results in THAT profile and not something that's just close, or really resembles a Clark-Y (horrors) takes too much time.
A female CNC molded wing set of each of the three -might- be testable and it's characteristics determined to be different than any of the others, but I doubt it.

Tall Paul

I find AutoCad easy to use, even after a few years layoff from it, the old commands come out of the murk.
Using TurboCad instead irritates me though. Knowing what can be done in AutoCad and finding it not possible in TC.... but that's the difference between a $4000 program and a $70 program.
But my version of AutoCad is Windows hostile (objects to Windows even existing on the same computer), and I have to pass the modern stuff thru TC to get it to the 'net.

SeditiousCanary

I do that. It depends on the coord data though. Some sites put spaces in front of the data, some don't. I suggested Notepad because everyone has it. Word had too many veribles for me to be comfortable giving directions.

I should mention that I am a technical writer and write step by step technical support directions for the ISP I work for. I didn't feel like writing OS and client specific instructions because I'm a slacker. Besides, the TV was on.

CafeenMan

Tall Paul - I was a professional draftsman for about 5 years. Got laid off and couldn't find work in the field after that and went on to other things. About a year before the layoff, we got autocad. I learned a great deal about it at the time for what we did, but basically our work consisted of boxes and lines and the occassional circle. They kept ribbing me for using @ to draw the next line instead of the mouse, but it was a lot faster and more accurate for me to do it that way.

After work I attempted several times to do plans in Acad but just couldn't get the curves right. I'm still having that same problem. Plus, it's difficult for me to see if the design is what I want on screen. I visualize a lot better with a full drawing in front of me. I'd like to learn to use the program, but I have so many irons in the fire now that I don't have a ton of time to devote to it.

wrongwaywayne

I am looking for a good 3D airfoil. Any ideas?

CafeenMan

The status quo is about a 60% thick airfoil with the thickest point about 1% from the leading edge.

Actually, none of the 3D airfoils are "real" airfoils as far as I can tell. I think they're just something the designers drew around their shoe and then everyone more or less copied.

Woodpile

So that's why my aircraft flies like a boot! I just knew it couldn't be me.

Just to throw my 2 cents in, I've been using CompuFoil for a while and it works pretty good. It will export a .dwg file, so AutoCad should have no problems.

You can get a free trial at:

http://www.compufoil.com/index.shtml

It's not a bad program and it gives you plenty of flexibility...

wrongwaywayne

I just downloaded the compufoil. There is my answer - at least partly. It has a NACA generator. I have been told that everything from NACA00166 to NACA0020 works. By the way where would the best place for the top of the airfoil be from the leading edge? 20% 25% or 30%????