Sendhendrix

My Feedback: (3)

Okay, here goes.

I made a power supply. It works great. I have a foam cutting bow. It works great. I made 2 templates for a constant-chord, zero-washout wing. They are great. I have a (getting smaller) huge slab of pink foam. It's great stuff.

I'm trying to cut 28" wing panels with 10" chords by myself. So far all 5 of my cores have turned out like crap... I think it's because I'm trying to do it myself; I can't keep track of what's going on at both ends at the same time. What can be done about this? Do I need to get the help of a second pair of hands, or is there a technique for using a foam bow solo while still getting decent cores?

I guess I thought I'd be able to pull the wire from behind the core while going by "feel," checking side to to side as I went, making sure the wire was riding the templates. Does this just take a lot of practise, or should I give up until I have myself on one end of the bow, and someone else on the other?

What is the experience of those that cut a lot of foam?

Thanks guys.

Sendhendrix
The meaning of life is that those of us who are alive get to discuss the meaning of life while everyone else lies around dead all the time.

linclogs

Many years ago (in about 1970), while stationed in the US Air Force in Germany, I cut some foam wing cores. This was all a fairly new process back then and there was no commercially available hobbyist equipment for making the cores. Here's how I did it.

I made a homemade bow (from broom sticks), "borrowed" a Variac power suppy from my shop on base and used .020 (if I remember correctly) stainless steel safety wire (also "borrowed" from my shop). I made templates of the airfoil shape out of 1/16" plywood and pinned these to the ends of my foam blocks (adjusting the templates on the ends to provide for washout, when wanted). I marked 1/2" increments along the top and bottom of the templates and put a number at each increment. To find the proper setting for the variac, I sighted along the wire while increasing the voltage, until the wire just started to sag. I then backed the power off just enough until the wire came back straight. This turned out to be exactly the right "heat" for cutting the foam. So it didn't matter what size wire was used, this seemed to be a good method to determine what voltage was right for whatever wire was used.

I "enlisted" (good military word!) the help of my wife when cutting the cores, for exactly the reasons you mentioned - it was too difficult to try to control the cutting at both ends of the block by yourself. By having the numbered increments on each template, I could call out where I was at on my end and she could slow down or "catch up" as needed.

It seemed to cut cleaner by also holding a slight amount of pressure "downwards" on the bow - so the wire actually was a little lower than the template, outside the template. In other words, push down a little while cutting rather than letting the hot wire "just barely" touch the template.

When cutting a tapered wing, I used the same numbered increments on each template, but obviously they were closer together on the wing-tip end. But the "calling the numbers out" method still worked and I can honestly say, every core I tried came out great.

blvdbuzzard

Is this the first 5 you have cut?? If so it may take a few to get the hang of it. What did you use for templete material?? What is wrong with the cores?? Are they rough and channeled like furrows in a field??? The smoother the templetes the easyer the wire will go through the foam. Have you tried to increase the temp of the wire?? If the heat is not up enough it makes it hard to cut a good core. If it is to high it will melt to much foam.

Dru.

rajul

My Feedback: (58)

Jon, nice and very informative writeup.

Flypaper 2

Linclogs:
I started cutting cores about the same time you did. Used the same # system too. To cut a tapered core, say 10 in. chord at the root and 5 in. at the tip. Put a mark with a Magic Marker at 1/2 the root chord do the same with the tip. Put another mark halfway between the LE and the centre mark. Do the same at the trailing edge. Now mark a line between each of those marks. Keep splitting the marks till they are about 1/2 in. apart. Number them from LE to TE . do the same thing at the tip with the same # of marks. the tip ones should be 1/4 in. apart. Then cut the core by the #s. This works with any chord width from root to tip if the root marks are roughly 1/2 in. apart. I cut a wing for a FW 190 wing 106 in. span, 21 in. root chord a couple of yrs. ago, this way. Hope this helps.

JBenson

My Feedback: (2)

Hey guys,
I was in the same boat as you until about 3 years ago. I have probably assisted in the "hand" cutting of 100+ cores. I don't want to rain on your parade, but it is almost impossible to get a "good" quality core. Until recently the only other option was to call a company that had "perfected" the art of the making of templates and cutting them out of foam. Even these "high quality" cores were not perfect and required prep work. The biggest problem with this was the cost. Because they had to make templates, the "setup" fees made the price for one set of cores too high.
Now for the solution. There is a company called Flyingfoam.com. A member of my club owns the company. Do yourself a huge favor and give him a call. He is a very knowledgable person and his prices can't be beat. He designed and built a full blown CNC machine that cuts the cores with no templates. The advantage to this is that there is no "wire chatter" that is generated by the use of templates. His cores come out absolutly perfect every time (as long as there is no imperfection in the foam!). He also stock all the major types of foam used for wings. Now don't quote me on this, but I beleive a set of cores the size you are talking about will run you about $30-$40 dollars max.
I hope this information helps you. Bellow is the contact info for Flyingoam. All you need to do is give him a call or shoot him an email with the dimensions you need and he will get you a quote. If you like the quote, contact him with you CC info and he will send your cores out in a week to 2 weeks depending on how busy he is at the time.


Flyingfoam info:
http://www.flyingfoam.com
(909)324-8205
Owner: Bob Mellen

Flypaper 2

A bit of candle wax rubbed on the edge of the template stops the wire chatter.

Volture

My Feedback: (1)

Build a deadweight, pulley-based cutter. I built one for about 20 bucks and it works great.

http://www.public.iastate.edu/~orman...r/hotwire.html

Kenny R

Sendhendrix, here is a couple things i thought that might help you, i hope so anyway.

First off your templates need to be very smooth. Take your fingernail and run it down the part of the template the hot wire rides on, if you feel any catches in the template while doing this, it is not smooth enough. Formica scraps make good templates.

Your bow wire must be tight. You need a couple of small handles on the wire, to pull by the wire, not the bow. I generally grab the small handles on the wire with my thumbs and 2 fingers, palms facing up and lay the bow back on my forearms. Slide the handles in close to the width of your foam block.

Cut the bottom side first starting at the trailing edge. Now cut the top starting at the trailing edge. It may be helpful to lay another block of foam under the block your cutting to raise it up. Put some weight evenly on top of the cutting block to hold it in place.

I guess the main thing to cutting a good wing is make sure you templates are placed on the foam right. Make sure to get the wire to enter the foam evenly. Use the number system eveyone is talking about to stay even. When the wire exits the foam block it needs to be evenly.

The temperature of the wire is important. I like my temp to where i cut slow and steady and see a few hair like strings of foam stick to the wire as it comes out. I have been told that if you see the little strings then your temp is about right.

It does take some practice but you will get the hang of it.

rajul

My Feedback: (58)

What type of foam is best for wing cores ? I am assuming there are many types. Thx

Kenny R

In my opinion 1lb density white foam. Its lighter than the pink and blue foams ive seen.

rajul

My Feedback: (58)

When you say 1lb, is it per yard cube ?

Kenny R

rajul, im not positive but i think its one pound per cubic foot. Dont get any reground foam use virgin foam. The regrinds are hard spots and will hang your hotwire.

rajul

My Feedback: (58)

Kenny, that sounds kinda heavy

Al Stein

Actually the white expanded bead polystyrene foam is the lightest stuff we use, and the weight is pounds pre cubic foot. For stronger wings, people use extruded foams instead and various amounts of glass or other fiber cloth reinforcement, or sheeting. The extruded foams start near a pound and a half per cubic foot and go up from there.

There was an article published years ago comparing weights of foam wings to those of built-ups and it found foam to be OK until you get to spans of about 2 meters... from there built-up gets lighter than foam pretty quick. (Of course if you add sheeting, that changes things a lot!)

I made a machine similar to the Del Brengman one that Volture recommends and I can tell you, It's fantastic for reliably cutting outstanding wings. With skill and practice, you could do good wings without the machine, but not reliably. Every time a group of us get the machine out we end up cutting several extra planes just for the pleasure of watching how well the thing works. You will like it.

Deadstik

My Feedback: (8)

Rajul,

I have to go with the white expanded polystyrene expanded bead in the 1# per cubic foot weight. One other thing that hasn't been touched on. This foam is also made in two types. Closed cell foam which is stronger and a little lighter (more expensive)and open cell foam which is not quite as strong (less expensive). I find that the closed cell cuts easier and is definitely stronger than the open cell. I use the closed cell foam in the pattern kits I make because I feel it is worth the extra cost and produces a higher quality wing.

Look around and you can find a local distributor. I found two plus an OEM within an hour of my house. Prices vary but you can get closed cell white foam for about $21 in a size of 36" W x 72" L x 4" thick. I use a seperate "block cutter" to get the blocks squared off and to the dimensions I need for my wings. I have templates to "square off" the foam to.

I use what is called an AMAP foam cutter. (not a clue as to what it stands for)... but.. It cuts via dead weights but has automatic parallax correction built into the cutting arm and so all I need do is adjust for sweep and the wing cuts itself. I use two piece templates (bottom/top) made of formica.

Good luck...if you need any further help, just ask.


Dan Hines

Carolina Custom Aircraft
[8D]

rajul

My Feedback: (58)

Thanks for the info guys. This is all new to me. Is there a way to visually differentiate between open cell and closed cell polystyrene ?

patternwannabee

My Feedback: (1)

I'll second the Del Brengman swing arm type foam cutter recommended by Volture and Al Stein. Once set up properly, you turn on the switch and it will do everything else. You don't even have to hold your breath. It will also cut tapered wings reliably.

If you plan on cutting more wings, it's worth building one.

If not, I have a suggestion which may improve your cutting of non-tapered wings:

First, suspend the bow from the ceiling by attaching a string to the center of the bow but allowing the arms flop down on the templates. Adjust the height of the bow so that the arms of the bow are parallel to the work surface when the cutting wire is in contact with the templates.

Then at each end of the cutting wire, attach a pulling string to the cutting wire (alligator clips work well here). Attach the two strings to each other making sure the lengths are equal.

When performing the cut, pull the pulling strings where they join directly back. The longer the pulling string, the easier it will be to see whether you're veering off to one side. This should help keep the cutting wire parallel to the LE and TE of your foam.

Since a picture is worth a thousand words, here is the top view.

Deadstik

My Feedback: (8)

Rajul,

To tell between Open and Closed cell foam..look at a side cut. Closed cell foam will appear to be individual cells like a honeycomb. Open cell foam will appear as a solid white mass with no distinction between individual cells.


Dan Hines

Carolina Custom Aircraft

rajul

My Feedback: (58)

Thanks Dan, you've been helpful

Al Stein

Oooh -- Patternwannabee makes excellent point about suspending the bow. I was alos taught to do that and I think it's one of the things that can make a big difference. Set it up so there's some weight on the wire to pull it toward the pattern, but not a huge amount.

bjmaloney

Well I am new to this to but have had some success, I started by following the tips at rebelflying club web site for building a zugly and it flys ok. in the last month I have built a further 4 slope gliders all fly, I have only ruined one set of cores. My advice is use laminex or formicaca for the templates, I tried thin ply but my wire burnt into it, I print out the pattern then glue it to the laminex then use a bench mounted belt sander to shape. I have found tapered wings the easiest to cut, I set up on my bench and mark the the spot where the leading and trailing edges would meet if the wing tapered to a point, then I bang a nail in there, I then clamp an extension to my bow with a bit of sheet metal on the end with a hole in it, this slips over the nail as a pivot point. Now when you cut the core the numbers on your templates automatically line up. Sounds complicated but its not. I also use an occy strap (bungee cord? rubber thing with hooks each end) to keep the tension on my bow. Good luck

ini

go to www.tekoa.com and look at Feather/Cut

ini

MinnFlyer

My Feedback: (4)

A big ditto to Kenny's use of "Handles". They give much better control.

Something else that I find important, is to have "tabs" at either end of your template to give the wire something to rest on before, and after the cut

DMyer

My Feedback: (2)

"I guess the main thing to cutting a good wing is make sure you templates are placed on the foam right."

The best way to make sure the templates are correctly aligned is to use separate templates for the top and bottom cuts. I build and race Pylon racers... thus the need to make lots of good wings. If you are not familiar with these types of template... there are two separate templates for the top and bottom airfoils or four total for one wing. The bottom is flat and sits on the table. You cut your block to shape/size needed(templates are good for this also if you are making production runs of the same core). I have a stop at the front of each template to locate the front edge.... I then sit it in place flat in the table(all the bottoms are flat) and pin it in several places into the core ends. I usually cut the bottom first, switch templates and cut the top. The whole setup process through completion takes about 10 minutes per set of cores and they are all 100% dead accurate. One of the best features of this type of template is the ability to easily make a long fully supported leadin/runout regardless of how thin the leading/trailing edge is. In my Q-500 cores I cut full length cores with no balsa trailing edge(the trailing edge/ailerons are composite of foam/balsa/glass/carbon) which calls for a trailing edge cut tapered to nothing. Good luck and happy cutting!