Perfection to the core!

Frequently Asked Questions










What is CNC?

CNC stands for Computer Numerical Control. Simply put, it is a tool controlled by a computer. In this case, the tool is a hotwire foam cutter.

How is this done? Software on the computer translates digital templates into X and Y movement signals for each of the 2 separate ends of a hotwire. Each end of the hotwire is independently moved by motors. The computer is able to control both the position and speed of the wire to great precision. In theory, my machine will position each end of the wire to within 1/40,000 of an inch (0.000025")! Now, there are other things that limit the precision possible when cutting foam using any kind of hotwire saw but, suffice it to say that a CNC hotwire is much more accurate than a builder and their helper using a handheld hotwire bow made of 2x4s, safety wire, and a battery charger.

Another thing that is possible with a CNC hotwire is that the computer can also precisely control the wire heat. It can turn it on and off as well as adjust it anywhere in between during the cut if necessary.

Combine the abilities to precisely control the speed, position, and temperature of both ends of a hotwire and it is possible to cut foam without physical templates to extremely tight dimensional tolerances and virtually eliminate problems such as wire lag! Also, there is no "angel hair" so the parts come out ready for applying glass.

No templates required!!!!


Why should I buy Eureka CNC foam cores?

Haven't hundreds of these airplanes already been built by using a hand-held bow to cut the cores?

Absolutely! It can certainly be done well enough by hand to give completely adequate results. I just want to give you a few points to ponder first.

1. EZs and Cozys are very labor intensive projects. Unfortunately, a great number of them are never completed due to the huge amount of labor involved. They sit in garages all over the world collecting dust.

Buying pre-cut cores is simply a way to reduce the amount of overall labor involved in the building process and make it that much more likely to eventually get off the ground.

How much labor?

If you cut cores by hand, here are a few things you will have to do:

    A. Either cut up your plans templates or make copies of them. Making copies presents its own challenges. These days it is fairly easy to find a place that can copy or print out large format drawings but it isn't easy to find a place that can do it with any kind of precision. How much error is too much? Personally, I like to keep error to a minimum, especially on the pattern.

    B. Glue your paper templates to some sort of stiff template material. Make sure you use the right kind of glue or the paper will possibly shrink or expand causing more error.

    C. Cut out your stiff templates being very careful not to mess them up. Make sure they are very smooth or your wire will catch. Oops, cut over the line a little? There's a little more error.

    D. Build a hotwire bow and power unit. Not too hard to do but more work and expense nonetheless.

    E. Get some foam and a partner and do some test cuts. This may cost a block of foam depending on how much practice you need. The more you get the less likely you will be to mess up the cuts that count.

    F. Cut your foam blocks to size using straight templates. I use my CNC hotwire to do this and there's still a lot of work and time involved. I'm sure it's even more time consuming when you have to nail two straightedges on the foam for every cut.

    G. Glue your foam blocks together using popsicle sticks

    H. Precisely position your templates on the foam blocks and nail them in place. The position of the templates on the block is critical to get the sweep and twist correct. Don't mess it up!

    I. Get your partner and go to work cutting the parts. Make sure you get the speed and temp just right. If you're too fast you'll get wire lag and a misshapen part. Go too slow and you'll get an undersized and cratered part. Don't sneeze or allow the wire to catch or you'll get ruts in your part.

    J. Go back and add more templates to cut your conduits and aileron cutouts.

    K. Lightly sand off the angel hair.

    L. Use filler to fix any ruts and depressions.

    M. Viola! You are finally to the same point you would have been if you had ordered the cores from Eureka in the first place. If you made any big mistakes along the way and had to order more foam you may have spent the same amount or more money as well.

If, like some builders, you decide to cut oversized cores and sand down to final shape with a long sanding block or try the hard shelling technique, add a whole bunch more labor to this.

2. CNC cutting allows for extremely tight dimensional tolerances and ensures the left wing is the same as the right and the same as was originally intended.

If you want to endeavor to cut your own cores then, by all means, go right ahead. I'm just offering an alternative.

How long will it take to receive my order?

I try to keep most of the smaller and more popular items such as Cozy IV and Long-EZ canards in stock so that they will ship as soon as the order is received and payment has cleared. Even if I'm out of stock of these items, I can cut them fairly quickly so it usually doesn't take more than a week to ship them. For the larger items such as wing cores, it depends on a number of factors such as how many orders are in the que and how much time I have available to get them done. Still, I'm usually able to ship the wing cores for my supported aircraft within 2-3 weeks and it is often less. For custom orders it will depend on the above factors plus how much time it takes to get the CNC files ready.

Do you use the right kind of foam?

I have always used the exact same kind of foam sold by the "Authorized Suppliers" in the plans of the VariEze, Long-EZ and Cozy. In fact, I usually buy my foam from an "Authorized Supplier". "Authorized Suppliers" are the suppliers that have been chosen by the designer to provide the right kinds of materials for their designs. The other designs I support are derivatives of these designs and call for the same foam. The Berkut was a kit and the foam for the wings, winglets, and canard were simply provided in the kit. In order to make sure it also uses the same foam, I obtained some wing core parts of an original Berkut kit for comparison. I found that it also uses the same foam.

How are the cores packaged?

The cores are cut from foam blocks. The rest of the blocks they are cut from makes excellent packing material and it provides the builder with scrap that can have a variety of uses. I pack the cores inside the blocks they are cut from. These blocks are then either put inside boxes or packed inside a wooden crate depending on the shipping method.

Why is there no angel hair?

If using the .032" stainless steel safety wire as called for in most plans, you will get angel hair (very fine strings of foam) on the finished part. This is caused by the molten foam adhering to the wire and getting pulled into these strings. I use a much smaller inconel wire that makes a better cut with a smaller kerf. It also has the advantage of not causing angel hair which means the builder doesn't have to sand it off.


I compared my Eureka cores to the hotwire templates from the plans and they were slightly smaller. Why is this?

When developing my hotwire machine I did lots of experimenting with different sized wires, cutting speeds, wire temperatures, etc. I found that the wire kerf is actually slightly larger than the wire since the foam melts back away from the wire a little. I did experiments with the same type of wire called for in the plans and measured the “burn-back”. I also found a reference to this in either the plans or newsletters and I found that the plans templates had been adjusted to compensate for this. The result would be that a foam core would end up slightly smaller than the template used to cut it. I adjusted my digital templates to compensate for this so that my finished product would be the same size you would get if you were using the plans methods and tooling for cutting.

For the canard, when I originally digitized the templates I noticed some discrepancies in the plans. Templates A and B are the two templates that define the airfoil shape for the canard. When I compared them to one another, I found that they were not exactly the same. Template B was ~1/32” thicker than A. I have several sets of original plans and found this to be true on all of them. This is probably due to the lack of CAD when they were originally designed, reproduction errors, etc. This is one reason the plans tell you to stack the templates together and sand them to match. If you were to do this, all the templates should match the smallest template so I used the smallest template to make my digital template.

One thing that is included in the Long-EZ Roncz canard plans is a drawing of the cross section of the canard that says “THIS IS AN ACCURATE, FULL SCALE DRAWING.” It is almost exactly the same drawing that is included on page M-11 in the Cozy IV plans. Dimensionally, they are the same. This drawing is unique in that it shows the skin on the foam core so you can see both the finished cross section of the canard and the slightly smaller shape of the foam core. I compared this to the hotwire templates and found that it was the same as the smallest hotwire template when adjusted for burn-back.

I made my digital templates to give the most accurate shape possible taking all the above into account. They will be slightly smaller than the plans hotwire templates but they should be the correct size to match the contour templates for that part if there are any. You may still see some small differences depending on your particular copy of the plans, the humidity and temperature when you compare, and how much you squeegee the epoxy out of the skin.

When comparing one of my elevators to the other, I found that one of them is wider than the other. Is this a mistake?

The elevators are made a little extra wide in order to allow more foam to lay the bottom skin up on to. Before laying up the top skin, you should trim the width down by at least 0.3" as shown in the plans (see elevator trailing edge drawing in photo above). This allows for the glass to glass bond on the trailing edge. The trailing edges on the foam cores for the elevators are very thin. When trying to hotwire such a very thin piece, the foam will melt back to different degrees sometimes. If one of your elevators is slightly wider than the other, it will only mean that you will trim slightly more foam off of one than the other. Other than a slight difference in width, the shape and size of both elevators should be identical.

Why are there so many pieces to the wings?

The wings are cut from the same sized blocks and an almost identical layout as called for in the plans. Eureka wing cores have the same number of pieces as you would have if you were to cut them using the plans method. Using a CNC hotwire process doesn't make the blocks any larger.

There are larger blocks available but they are not big enough to eliminate all joints. Using the block sizes called for in the plans allows for the most efficient use of foam. Larger blocks might eliminate a few joints but they would increase the cost substantially since there would be a lot more wasted foam. You would be buying a lot more foam that would end up in the trash can. What's more would be the added shipping costs incurred both on my end and yours. I chose the block sizes and layout in order to give the maximum value to my customers.

I already have foam. Can I send it to you to cut for me?

Yes, however, due to the high costs of  shipping these days, unless you live nearby it probably won't save you much money.

Can I come pick up my parts so I don't have to pay shipping?

Yes. If you live outside the states of Texas and New Mexico, you will have to pay sales tax. It probably won't save you any money unless you are coming through this area anyway.

Do you do custom work?

Yes, let me know the basic details of what you want and I can give you an estimate. I will need enough information to estimate how much foam will be necessary and how much work it will take to build the CNC files.

I am building a kit or plans built airplane you don't yet support. What does it take to get you to cut parts for it?

I will look at this on a case-by-case basis but if I don't already support the design, it's probably because there isn't much demand for it. If there isn't much demand for it I can still cut the cores for you but it will be more like a custom project.

Does the scrap foam for the winglets come with the winglet cores so I can use it as a jig?

No. Due to the way the parts are laid out in the foam, the upper winglets have to be cut inverted from the way the plans call for. This means that the scrap left over is for the outboard side of the winglet instead of the inboard side. I have had very few complaints about this so I don't believe it is much of an inconvenience. I will offer wooden jigs for winglet in the near future.

How big are the boxes?

Long-EZ, Cozy III or IV, Berkut, and E-Racer wings:

4 boxes     57" x 15" x 15" weighing approx. 16 lbs apiece
2 boxes     40" x 15" x 15" weighing approx. 13 lbs apiece
1 box         60" x 18" x 8" weighing approx. 14 lbs

Canard for all above aircraft is 1 box measuring 57" x 13" x 10" and weighing approx. 12 lbs