Well, I finally dove into cutting my own cores. The results were far from perfect, but I am encouraged. I learned how very important a SMOOTH template is. That wire will hang up on ANYTHING.

My question is this:

When cutting a tapered wing, (in my case a slightly tapered leading edge and a more pronounced trailing edge), how do you make up the difference in the chord width? I assumed you just went at a faster speed on the root side, but this didn't work out too well. The wire just seemed to want to cut at one speed (this was blue foam) and if I "stalled" on the tip side, it gave me those famous ridges in the foam.

Should I simply cut the core until the wire exits the tip and hold it there on the template and then "sweep" the root side? I know an even speed is crucial, but how do you do that when the tip is half the chord of the root?

thanks for any help
splatman

If you want nice cores you need a gravity fed pulley system. See Charles River RC for details.

AB

Next, after you think you are getting good cores, you need to start doing things people are telling you not to do. Like using .018 SS fishing leader or finer for your cutting wire, wet sanding your Cores before bagging or not at all, using 7 inches of mercury vacume tops or going all out to 18 inches. And then after a couple of tries, you are making wings that are the envy of your club...

The wire has to be moving continuously on both the inner (root) section and the outer (tip) section. So there is a 'ratio' of movement that must be calculated. Let us say the root is 9 inches long and the tip is 6 inches long. The wire most move from the leading edge to the trailing edge on BOTH lengths (9 inches and 6 inches) in the => same amount of time <= (that is the tricky part). So the wire must move over the tip section slower than over the root section, yet the movement of the wire cannot stop on either template. Some people mark off distances on each template in terms of percentages (not inches). So they mark the template in 5% distances. In this case when the wire reaches the 50% point on the root template the wire should also be reaching the 50% point on the tip template, etc. Eventually someone designed a mechanical setup using adjustable pulleys and strings attached to a single lever that allows that speed ratio to be set and thus be 'automatically' produced. This clever setup allows the pull ratio for each template to be 'set' and the operator simply allows the 'level' to be pulled down by weights (gravity fed lever movement) to produce the perfect ratio of movement and continuous movement of the wire across both templates. The result is that the wire starts at the leading edge of both templates at the same time, travels over the templates continuously and reaches the trailing edges of the templates at the same time. And this is repeatable when cutting more than one core.

I have had good results from using the zero point pivot method, were you have a fixed point at one end of your cutting board that the wire is tied to. The blank core is located by lining the wire up with the edges of the blank. The root template is fixed to the root end of the blank (no need for a tip template); the wire is pulled tight and drawn over the template and through the foam. I give the cutting board a light misting of 3M77 to stop the foam from moving while cutting and also hold the foam down with some weights. It is a very simple way of cutting tapered wings by yourself.
Glenn

Thanks for help guys.

Hey Zipper,
Thanks for the description. I was able to make a Formica template with Profil amd used a 10.5 " chord of NASA-LANGLEY RC08-64C .for use with 1" pink foam. I used your method and made a half dozen fair cores that turned excellent with a bit of sanding. I found that my best cores were cut with a warm wire that melted the foam without causing the deep lines that a hotter radiant wire caused. It was a case of show being faster! I was just doing a light sanding to remove the hot melt strings and noticed that my fine lines disapeared and I was left with really smooth wings. It was smiling time!!!

Good to hear you had some success.
Glenn

We have a foam cutting video on our club site at www.utahflyers.org. For written instructions go to "Lee's Blog" at the same site. We have cut foam for years and have received many questions. We did the video to help give some answers.

Hope this helps.

Your comments are welcome.

Lee

Splattman
Cutting a highly tapered wing core does present problems but there are ways around them. My current project has such a wing and after a couple of unsuccessful attempts I made three templates and cut the wing in sections. With a root, tip, and mid template I made each wing half in two parts. By using an intermediate template the ratio of root chord to mid chord was not as severe so the problem of over burning at the smaller template was minimized. A friend suggested placing a fan to move air over the wire at the small template end in an effort to cool that end of the wire. He's an expert and says it helps with this problem but I haven't tried it yet.
Keep trying you'll get the hang of it eventually.
Paul

This is a great site with lots of pics on how to set up a drop arm cutter for tapered wings: http://aeropic.free.fr/pages/modelisme/decoupe/

Enjoy!

Brad

Good thread for a very simple foam cutter

http://www.rcgroups.com/forums/showthread.php?t=191128

Finishing the wings

http://www.rcgroups.com/forums/showt...7&page=4&pp=15

Just a tip for you foam cutters.... To help keep the foam in position ( my table is formica covered) I use a couple of strips of double sided tape!

K

Have a look here http://www.sea-gull.demon.co.uk/oldaero.htm for info on a drop arm cutter and also a program which calculates the spacing of the pull cables on the arm

http://www.sea-gull.demon.co.uk/File...r/FOAMCALC.ZIP