Airfoils in CAD programs (Alibre)

[Boo]

Hi,

I want to use Alibre to make a CAD model of a model I'm designing and I wondered what is the best way to go about importing airfoil sections ? Alibre has a file import function that will take a bunch of points from a file and put a spline through them. I want to know if splines will generate the "correct" airfoil section given a bunch of top and bottom coordinate pairs ? The wing will be coinstructed from balsa ribs and covered in film so I don't suppose 3 figure accuracy is required 'cos I can't build that well anyway.

I'd prefer to avoid having to use an outside program to generate the rib sections because that seems to remove a large part of the utility of using a CAD program in the first place.

Thanks,

Boo

[uriah]

Hi, Boo.

I have never messed with Alibre. I know Rhino and 3d max. Let's say you have 30 points that make up your choosen airfoil. If you place 30 random points (or, as known in Max, verticies) and assign them the coordinate points of the airfoil they will make the perfect shape, but not connected. This I do by connecting all those verticies into a spline (a series of connected lines defined by the coordinates. This area can now be filled with a polygon or what ever "solid surface" your program uses. This is all with my knowledge of max.

Another way I thought of would be to make a 3d rectangle with the exact number of coordinates around it's perimeter. Then you could align a bottem corner so that that verticie would be at X=0, Y=0, Z=0. You would then align each following coordinate by doing some math. ;P

Unless you can build plines verticie at a time.

Any thats my 2 cents... sorry I don't have a nickle.

What kind of plane are you building???

good luck.

[capncrunch]

Hi,

I want to use Alibre to make a CAD model of a model I'm designing and I wondered what is the best way to go about importing airfoil sections ? Alibre has a file import function that will take a bunch of points from a file and put a spline through them.

Autodesk products have the same function. That's how I do it.



I want to know if splines will generate the "correct" airfoil section given a bunch of top and bottom coordinate pairs ?

they sure will. The accuracy will depend on the math that drives the spline and the number of data points in the spline. When you get an airfoil's polars from a site like NASG, you get 140 data points. this is too many, even if you are just connecting them with line segments. I trim it down to the 30 or so necessary ones in excel, then format it like inventor likes it and import them. once you have the points in a sketch, connect them with a spline, and remember to make it an open spline, rather than a closed one, or the TE will give you problems.

-barrett

[Boo]

Hi Chaps,

Thanks for the replies. I did find a points import function in Alibre, it needs the .dat files editing so as to have a third (zero) co-ordinate - the 2-d points in the airfoil data files don't work as is. Alibre doesn't seem to offer the distinction between open and closed splines but my TE seems OK anyway so it must be doing the right thing.

The plane I'm thinking of building is a biplane based on the Honeypot which was a free plan in one of the mags. I'm going to scale it up 150% to 48" wingspan and put a YS 63 in it. See my thread over on RCU with "Honeypot" in the title if you're interested in seeing a picture of the original smaller version.

Cheers,

Boo

[capncrunch]

Hi Chaps,

Thanks for the replies. I did find a points import function in Alibre, it needs the .dat files editing so as to have a third (zero) co-ordinate - the 2-d points in the airfoil data files don't work as is. Alibre doesn't seem to offer the distinction between open and closed splines but my TE seems OK anyway so it must be doing the right thing.

You can edit the .dat files, I trust.

an open spline is just one that does not make a closed loop. if you close the spline around an airfoil, you'll get a funny shape at the TE, which you want to be sharp.

-b

[rebell]

Boo, can your CAD program read DXF or DWG files. If you mail me the coordinates of the airfoil you want in text or Excel files, I will send a true polyline to you in DXF/DWG format.

[coosbaylumber]

I dunno, will Alibre take and understand a LISP command file? There are a couple airfoil programs to which you can download for free at CADALOG. Just place into directory in which the main .EXE file for Alibre is located then LOAD it.

And for those who did not read the manual, using a SPLINE is incorrect use on an airfoil. Proper function when working with polylines is the FIT CURVE option. It will look about the same but is not a curvy, goes exactly through the coordinates, and you can add straight line segments to them. This option has been within Autocad since the 1980's, but everyone gets on to using the other option, which gives wrong type of interpolation.

Wm.

[kf2qd]

I use autocadLT for my drawings and I find that most airfoils work just fine using the spline function. The more points described the better. I rework the airfoil file I have so that I start at the back edge, go around the leading edge and back to the trailing edge. Some airfoil files list the percent of chord and the top surface then the bottom surface in one line so they have to be editted.

I wrote a program to clean the airfoil file up, put into a format that autocad likes and the just copy it to the command line of autocad, and out comes an airfoil. Have done several hundred that way.

Can you read DXF files? I could zip some for you and email them, or maybe upload them here. they are drwan out to a standard 1" long so it is easy to scale them to any size.

Pete

[Todd_C]

Does AutoCAD or Alibre offer free 3D CAD software that would allow me to design airplanes and print the plans? Are there any databases of airfoils either of these applications can import?
...I find alibre Express....will it do the job for me?

[mikeperik]

I don't believe AutoCad can be had for free. Alibre has a free version that has some limitations on the # of parts you can have in an assembly. You can find Alibre at http://www.alibre.com/xpress/

Mike