Choosing the Right Foam
Here is a list of the foams that we stock, with their density, compression rating and relative cost level:
|EPS (Expanded Polystyrene)||1#||10@10%||Low|
|EPP (Expanded Polypropylene)||1.3#||11@25%||Med|
|Dow Surfboard (Extruded Polystyrene)||2.2#||60+@10%||High+|
Note: EPP foam compression ratings are measured at a deformation of 25%. All other foams are measured at 10%.
What type of foam should I use for my cores?
We cut cores from several types of foams and densities, and if you are new to using foam wings, foam selection can be confusing.
Foam wings became popular in the 1960's when modelers quickly figured out that they made the job of building a tapered wing a lot easier. The first type of foam wings were constructed of 1# EPS foam that was covered with 1/16" Balsa sheeting applied with Epoxy glue or contact adhesive. This type of construction was popular for .40 and .60 sized glow engine R/C aircraft which ruled the sky of most R/C fields at the time. The finish was typically MonoCote or one of many heat shrink covering materials. Not surprisingly, this type of foam wing construction works well for many types of aircraft and is still very popular today.
Foam type relates to overall wing strength, but not necessarily the way you might think. In most cases, foam type is the least important of many factors as you will see as we get into the details.
Basic wing strength is all about tension and compression. When a wing is placed under load, the lift that the wing produces tries to push the wing up, and the weight/mass of the airplane tries to bring the airplane down. This results in increasing compression on the top surface of the wing, and increased tension on the bottom surface of the wing. In a typical wing that is built from spars and ribs, the tension strength comes from the wing covering, and the compression strength is provided by the spars. Primary failure occurs when the ribs that keep the spars in place crush, and the wing "kinks" like a piece of copper tubing that is bent too sharply. The use of shear webs between the spars helps to prevent this. The most popular type of shear web is thin balsa pieces applied between the spars with the grain running perpendicular or normal to the spars.
The primary strength of a foam wing is provided by the sheeting and covering. The foam gives the wing its shape, and keeps the sheeting in place, but most of the structural strength comes from the sheeting and covering. When a foam wing is stressed to the point of failure, it will also "kink" when the balsa sheeting caves in from compression. The failure is usually not a tension failure because film covering has an amazing amount of tension strength if it is intact and properly adhered to the balsa sheeting.
Another factor that controls the overall strength of a wing is its thickness. Thicker wings are stronger because the sheeting and covering that provides the basic wing strength are further apart. This is true for foam wings as well as built up wings.
As wing thickness decreases, so does its inherent strength, as the forces that try to compress the sheeting have more leverage on a thin wing. Higher density foam helps to keep the sheeting in place and helps prevent failure of the sheeting on the compression side of the wing.
"All right, I'll always use a high density foam," you say. Let's not forget that high density foam adds weight to a wing, and the thicker the wing is, the more of a problem this is going to be. If weight is critical, and it is on most things that fly through the air, a light weight wing is important.
Put a spar in it!! A popular solution is to add a tubular carbon spar in the center of the wing to increase it's strength. This is a quick and simple way to add strength, but it also adds weight. The weight increases not only by the weight of the carbon tube, but also from the glue used to adhere the spar to the foam.
A better solution is to use 2 smaller spars on the top and bottom of the wing. Spars that are placed close to the sheeting have the most resistance to compression, and inherently greater strength.
The best solution is a full-depth shear web spar, sometimes known as a "D-Tube." This consists of a vertical grain shear web spar with some carbon tow, or thin carbon strips on the top and bottom of the shear web. The carbon tow/strips keep the shear web from breaking through the sheeting when the wing is under heavy stress by spreading out the loads on the top and bottom of the shear web.
Most foams need some sort of covering as it provides the large portion of the wings strength and is a major consideration for foam selection.
Coverings include heat shrink plastics, shipping tape, balsa, plywood, obechi wood sheeting ,and various composites such as fiberglass, carbon fiber and aramid cloth. Combinations of all these coverings have been used.
Heat shrink plastic coverings such as Monocote, Ultracote and many others have been around for years, and were originally intended to be used as a replacement for silk and dope! Low heat versions of these coverings can be applied directly to most foams if the foam is covered with a light coat of spray adhesive. These coverings provide a tremendous amount of tensile strength, but no compression strength and all but the lightest and smallest airplanes usually use carbon tube spars to provide the necessary compression strength. This is a typical construction method that works well for Slope combat flying wings, but it does not scale up well for larger models.
Colored shipping tape is a cheap replacement for heat shrink coverings and will also provide high tensile strength if properly applied.
Wood coverings such as balsa, plywood, and obechi wood sheeting are common for many large and small models and provide high compression and some tension strength. These coverings are usually finished with Monocote-type plastic coverings or light layers of fiberglass or composite coverings and paint. This results in extremely strong wings with unlimited design and color choices.
Composite coverings of fiberglass, carbon fiber and Kevlar cloth and resin are common on high performance sailplanes, gliders and commercial UAVs. They are also used on high-end scale and aerobatic aircraft. These coverings are used in a variety of ways, and are some of the most expensive material choices, but they provide the ultimate combination of strength and light weight.
All of the foams we stock and hot wire cut are compatible with Epoxy resins or Polyurethane glues such as Gorilla Glue or Elmer's ProBond.
Polyester resins will attack and degrade Polystyrene foam and should be avoided.
How do I choose the right foam?
Foam selection is dependent on the size of the model, type of covering, intended use and construction methods. Foam is produced in many densities, colors and types. Only a few types and densities are used in the modeling world.
Density is defined as the weight of 1 cubic foot of the material. In other words, a 12" x 12" x 12" cube of 1# density foam will weigh 1 pound. Higher density foams are harder to the touch and more rigid, but they weigh more.
The higher density foams are typically used in high aspect ratio wings that are relatively thin, like high performance thermal gliders. Powered pylon racers and even giant scale racers may also use 2# foam. Large models with thicker airfoil cross sections will work fine with 1# foam. The 1# foam is also used on most powered sport airplanes, trainers, and any airplane or glider where weight is a concern.