Lead Sled Alley - December 2002

You may have noticed the name of this column changed from "Crunchie Corner" to "Lead Sled Alley". This is because the word "crunchie" encompasses all non-foam airplanes, but "lead sleds" are, for the most part, what will be covered in this column. This month however, the column will cover something a little different.

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You may have noticed that I changed the name of my column from "Crunchie Corner" to "Lead Sled Alley". This is because the word "crunchie" encompasses all non-foam airplanes. Lead sleds are my specialty and for the most part, that is what I will be covering in my column. This month however, I decided to do something a little different.
I have seen many posts in the forums from guys who want to start building their own planes, but do not know where to start when it comes to cutting their own wing cores. The biggest problem is usually that they do not know what to use for a power supply. I have heard a variety of ideas, some of which could be a little dangerous, such as using 110VAC straight out of the wall, with nothing but a dimmer switch to control the current. Therefore, I thought it would be worthwhile to dedicate this month's column to the subject, keeping three things in mind.
  • Simple - It should be something that anyone can throw together with basic tools
  • Inexpensive - Not everyone has hundreds of dollars to spend on an off-the-shelf system
  • Safe - Safety is by far the most important thing
There are many ways to cut wing cores. In this article, I will show how to build a fixed-point cutter. This is simply a cutter with the wire tied to a fixed point at one end. This type of cutter may not be as elegant as the off-the-shelf systems, but it is very simple, very affordable, and best of all, it will get you up and running on that new plane that you have been just dying to build. Just think; you could be cutting wings by this weekend!

Power Supply

As luck would have it, any run-of-the-mill 12-Volt battery charger will put out just about exactly the right amount of power to cut foam. For this project, I bought a relatively inexpensive charger for about $30. It has selections for 12V/2Amp, 12V/6Amp and 6V/6Amp. I used the 12V/2Amp setting. The only drawback to battery chargers is that they generally do not have an On/Off switch. For this application, we need a switch. What I decided to do was to wire up the output of a light switch to the input of an outlet. I mounted both of them in a junction box and used a power tool replacement cord to plug the whole thing into the wall. It was quite easy to build. Refer to the schematic below.
Run the black wire to one side of the switch. Run the other side of the switch to one side of the outlet. Run the white wire to the other side of the outlet. Connect the green wire to the ground screws on the outlet and the switch.
Now the battery charger can be plugged into the outlet on the junction box and switched on and off at the box. The nice thing about doing it this way is that you do not have to cut the power cord of the charger or open the charger up. The charger remains in tact and can still be use to charge batteries if needed. The charger is also UL approved, and it is best to keep it that way. Here is the whole setup.
Plug the charger into the outlet on the junction box and use the switch to turn the power on and off. Clamp the outputs of the charger onto one end of the speaker wire. Solder alligator clips onto the other end of the speaker wire. Use a piece of wood or some other means of keeping the clamps from touching.

Building The Cutter

Now that we have our power supply, lets move on to building the cutter. As mentioned above, this is a fixed-point cutter. There are advantages and disadvantages to this type of cutter.
  • Simple to build
  • Inexpensive
  • Easy for one person to use
  • Only the root airfoil template is required (This may not seem like such an important point, but building airfoil templates is tedious, and anything that reduces tedium is a good thing in my book!)
  • Can be used to cut cores for elliptical wings
  • Can only cut tapered wings
  • Cannot cut wings with different airfoils at the root and tip
  • Not as easy to cut wings with washout
  • Not as fast as some of the off-the-shelf systems that cut the top and bottom simultaneously

Let's Look At The Setup

The Cutting Board
I used a 2'x4'x3/4" piece particleboard.
The Wire
Tie the wire to an eyebolt at one end at a height of 1" above the cutting board. Tie the other end to a simple handle. I find that a good length for the wire is around 60". There are a couple of reasons for this. First, you may want to cut some wings with less taper, which would require moving the foam further out away from the fixed end. Second, your cutter can double as a foam chopper, easily enabling you to block out the foam before cutting the airfoil. If you buy foam in 4'x8' sheets, the wire will need to be long enough to reach all the way across the sheet.
Using the right type of wire is very important. The first wire I tried was stainless steel fishing leader wire. Even at its highest setting, the battery charger was not able to heat a long enough section of this wire, so I ordered some new wire from Tekoa.com, the makers of the Feather cut System. When I tried the cutter with this wire, I was able to heat a five foot section with no problem, using the 12V/2Amp setting.
Foam Guide
The guide helps to easily align the foam with the wire on both sides. Align the guide with the wire so that when the foam is placed against it, the wire will automatically be aligned with the foam on one side. Then move the foam along the guide until the wire aligns with the foam on the other side. The large, flat piece of wood on the cutter is a piece of 1/2" material that I had lying around. Its purpose is to raise the foam up so that the base of the airfoil template will slide underneath it.
Connection Wire
Some speaker wire with alligator clips on one end works well for connecting the charger to the cutting wire. It helps to solder the alligator clips to the speaker wire so that you get a better connection.
Vertical Guide
A vertical guide makes it easy to block out the plan form before cutting the airfoil.
Airfoil Templates
You will probably want to get some sort of airfoil utility to allow you to print out the airfoils you want to use. One such program is Profili. Profili is a shareware utility that provides a fair amount of functionality. It works great for plotting airfoils and will work just fine for our purposes here. If you do not have an airfoil program, you can download Profili at the following link. (http://utenti.lycos.it/profili2/ENG/default.htm)
I prefer to make a separate template for the top and bottom of the airfoil. Using this method, the templates can be clamped solidly to the cutting board, and they will not move during cutting. The most important thing is that the guiding surface of the template should be smooth so that the wire will not get stuck and burn a groove into the foam. The starting and stopping points (the centerline of the airfoil) on the airfoil templates are also set at 1" above the cutting board.
I use 1/8" plywood for my templates (the five ply, not the lightweight ply), but Formica is also a good template material.
Template Clamp
You will need some sort of clamp to keep the airfoil templates from moving while cutting. I used a small piece of 1/4" plywood. You will need to be able to rotate the clamp to align the template with the wing root when you turn the foam over to cut the other wing. A single screw in the middle will hold the template and will allow the clamp to rotate.
That's all there is to it. Now let's cut some foam!

Blocking Out The Foam

The first thing to do is to block out the foam in the shape of the plan form. Mark the plan form on the foam with a thin marker. You will need to put the foam up on spacers so that the wire will come all the way out the bottom.
I have the wire mounted at a height of 1", so I used some 2x3 material to get the foam up above the wire.

Position the foam so that as you bring the wire down the guide, it comes down right along the mark.
Once the foam is in position, turn on the power and pull the wire straight down through the foam. Repeat this process for the remaining three sides of the plan form.

Cutting Cores

Now that the plan form is blocked out, you are ready to cut the airfoil. First, align the foam with the wire by sliding it along the foam guide until the wire lines up with the other side. It is best to cut the bottom of the airfoil first because the foam will drop down by the thickness of the grove cut through the foam by the wire. This is called wire curf.
You will need a spacer of some kind to raise the foam up so that the base of the airfoil template will slide underneath it.

When setting up to cut the airfoil, it is very important to make sure that the speaker wire and alligator clips will not catch on anything. You do not want the clip to be pulled off the wire in the middle of the cut. It is a good idea to put some marks on the foam indicating which edge is the leading edge. Also, put some reference marks on the ends of the foam so that the cores can be aligned to the saddles later when sheeting the wing. Clamp the airfoil template in place so that it will not move while cutting.

When you are ready to make the first cut, pull the wire tight, then turn on the power and guide the wire steadily across the template. After making the first cut, flip the foam over, turn the airfoil template around, and cut the bottom of the other wing. Then change out the template and cut the top of the airfoil.

Notice that I have moved the connection wire and charger over to the other side. I like to pull the connection wire along behind the cutting wire so that it cannot get tangled during cutting.

Here is the finished wing.


I noticed that on this particular wing, the tips burned a little because the wire moved too slowly during cutting. With the more taper the wing has, the more of a problem this will be. Using smaller diameter wire will help speed up the cut, which will help to alleviate burning. If you do not mind spending the money, a more flexible power supply will also help in getting the heat on the wire just right. The Tekoa "Thermal/Generator Plus 60" is listed at $134.50, and has settings for 24", 40", and 50" wire lengths. You could also buy an adjustable 12V power supply. You will need to find one that has at least a 3 to 5 Amp output. An adjustable power supply will cost between $300 and $500.
This is why a battery charger is a nice option for basic cutting. The total cost of this project was in the neighborhood of $50. My goal was to demonstrate how to build a simple wing cutter to help get you started, and to show you how to do it safely without breaking the bank. Now that you know how to cut foam, you can get more creative and build bows, wafer cutters, and all sorts of other neat foam cutting jigs.
Now let's build some airplanes!
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