Foam Types:

Selection:

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, 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.

Types of Foam:

We stock a number of types of foam, and some types are available in different densities:

  - 1.0# EPS

  - 1.5# EPS

  - 2.0# EPS

  - Owens Corning Pink Foam (1.8# density)

  - Surfboard Foam (AKA Spyderfoam, 2.3# density)

  - 1.3# EPP

  - 1.9# EPP

 

EPS - Expanded Polystyrene Foam:

The EPS foam that we use is a white beaded foam that is commonly used as  insulation material or in architectural applications.  White foam is available in any thickness up to 40 inches in 1#, 1.5# or 2# density.  We purchase EPS foam in large billets, and slab cut it into sheets.  Slab cutting in house produces a foam sheet that is flat, has a better surface finish, and a more uniform thickness than is typically available from insulation suppliers.

Blue / Pink Foam:

Blue and Pink foam is readily available in home improvement stores in many parts of the U.S., but not in Texas.  These foams are a higher density material that is typically used as cold weather insulation on basement walls and footers.  In Texas, we have few basements and little cold  weather, so Blue and Pink foams are not as readily available here. 

We stock Owens Corning Pink Foamular in 25 psi compression.  This is a  closed cell extruded foam that has a finer grain than 2# white beaded foam, but the mechanical properties of the two are similar.

Surfboard Foam (AKA Spyderfoam):

Surfboard foam is manufactured by Dow, and is a polystyrene foam similar in feel and texture to blue foam.  The big difference is that surfboard foam has grain structure that runs vertically through the sheet.  Because of the grain structure, it has a compression rating greater than 55 PSI, which is higher than other types of blue or pink foam of similar density.  Surfboard foam density is 2.2 # per cubic foot and Blue in color.

Surfboard foam is known in the Modeling world as Spyderfoam.  The term Spyderfoam came about years ago, when some modelers began purchasing foam from Spyder Surfboards, a company no longer in business.  The name stuck, and modelers have known this material as Spyderfoam (or Spiderfoam) ever since.  If anyone has a better explaination of this story, we would would love to hear from you!

EPP - Expanded Polypropylene Foam:

EPP is flexible foam that is used as a packing material for delicate electronic parts.  It is also used in the bumpers of some automobiles.  It is a rubbery material that is light in weight, yet very durable. 

EPP is available in 1.3# and 1.9# densities.  EPP is used in the modeling world for combat gliders that need a high degree of durability.  Since EPP is not a rigid foam, most all EPP gliders use fiberglass, carbon fiber, or spruce spars for stiffness.  Because of it's lack of rigidity and higher cost, EPP foam  is not generally used in models that are covered with balsa, plywood, or obechi sheeting.  EPP wings are typically covered with colored shipping tape, or low temperature heat shrink coverings like UltraCote.  EPP is about 3 1/2 times more expensive than white beaded foam.

Bob HATES BLUE FOAM!

Blue foam has been around for years, and  is readily available in Big Box home improvement stores in many parts of the country.  It has been a common choice for gliders and other high stress applications.

So, why do I hate the stuff?

1. Stress

Blue foam is produced by an extrusion process, and has a slick surface on the sheets.  This slick surface makes it easier to handle the sheets, and allows adhesives to stick well.  Both are desirable properties for building contractors where blue foam is used as an insulation material.

The problem with the slick surface is that it causes a lot of stress in the material.  When wing cores are cut from the stuff, the beds will warp like a potato chip.  The stress can also cause the foam blocks to move while the material is  being hot wire cut, causing other problems.

2. Flatness

The sheets are not flat, which aggravates the stress problems.

3. Beading

When any foam is hot wire cut, the wire melts the foam out of it's way.  This path of the wire is called the "Kerf".  That melted material has to go somewhere, usually into the surface of the core, but not Blue foam!

The melted blue foam beads up on the wire and falls off in the form of droplets.  These droplets form dimples in the cores surface, and in many cases, makes the cores unusable.

4. Dirt

Blue foam typically has a lot of fine dirt particles embedded in the material that causes wire marks when cut.  This problem alone can cause scrap rates as high as 50%.  That means we have to cut 2 or 3 cores to get 1 good one in many cases.

Otherwise, I like blue foam a lot!!!

As a result of these problems, we quit purchasing blue foam some time ago, and have very little remaining in stock.

So what are the options?

Pink foam and Spyderfoam are excellent replacements.  Pink foam is manufactured by Owens Corning, and it's manufacturing specifications are the same as Blue foam. 

Pink foam also has a slick surface, but we have equipment that will peel away the outer surface of the sheets leaving a flat and stress free sheet.  Pink foam does not have as severe of a beading problem, and is relatively dirt free.

We recommend using Pink foam for applications that call for Dow Square Edge or other varieties of 25 PSI Blue foam.

Spyderfoam and Blue foam are both products of  Dow Chemical, however, Spyderfoam is manufactured using a different process than blue foam, and has none of the problems mentioned above.

Spyderfoam is 2.2# density, Dow Hi-Load 40 is 2.1# density and Hi-Load 60 is 2.3# density. 

We recommend using Spyderfoam for applications that call for Hi-Load 40 or Hi-Load 60 Blue foam.

Bob Mellen

FlyingFoam.com

Custom Foam Cutting

What type of foam should I use for my cores?

Answer:

We cut cores from several types of foams and densities, and if you are new to using foam wings, foam selection can be confusing.

The Basics:

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.  Tapered wings are inherently more efficient than constant chord wings because the wing tips are smaller, and efficiency losses at the tip are smaller if the tip is smaller.  Well, in theory at least.

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 if foam wing construction works well for many types of aircraft and is still very popular today.

Wing strength:

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 comes from the wing covering, and the compression 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, and 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 it's 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 MonoCote 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 it's 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. 

Thin wings:

As wing thickness decreases, so does it's 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.

"Alright, 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 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.

Here is a list of the foams that we stock, with their density, compression rating and relative cost level:

PSI Rating  Density Cost  Material

----------  ------- ----  --------------------------

10.0 @ 10%   1.0#   Low-  EPS (Expanded Polystyrene) 

15.0 @ 10%   1.5#   Low   EPS (Expanded Polystyrene) 

25.0 @ 10%   2.0#   Med   EPS (Expanded Polystyrene) 

11.0 @ 25%   1.3#   Med   EPP (Expanded Polypropylene)

23.5 @ 25%   1.9#   High  EPP (Expanded Polypropylene)

25.0 @ 10%   1.8#   Med   Owens Foamular (Pink Extruded Polystyrene)

25.0 @ 10%   1.8#   Med   Dow Styrofoam (Blue Extruded Polystyrene)

40.0 @ 10%   2.3#   High  Dow Hi-Load 40 (Blue Extruded Polystyrene)

60.0+ @ 10%  2.2#   High+ Spyderfoam (Blue Surfboard Polystyrene)

Note: EPP foam compression ratings are measured at a deformation of 25%.  All other foams are measured at 10%. 

As you can see, EPP is a poor construction material due to it's very  low compression ratings.  It does little to keep a wing from "kinking" under heavy stress.  EPP works will on small, light weight airplanes and gliders where the weight and expected stress levels are low, but is a poor choice for large wings unless they are designed with other structure that provides the necessary strength.

In almost all cases, Polystyrene foam is a better choice in terms of structural strength and cost over EPP. 

Coverings:

Most all foams need some sort of covering to prevent them from becoming discolored and dirty, if nothing else, however, the covering provides the major portion of the wings strength, and is a major consideration for foam selection.

Coverings include heat shrink plastics, and shipping tape; balsa, plywood,and obechi wood sheeting; and various composites such as fiberglass, carbon fiber and Kevlar cloth and resin combinations.  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 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 UAV's.  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.