April 2001 Newsletter

I have been building foam core wings since the mid-1970's. The main reason at that time (as it is today) was the low cost. But, there are many other good reasons for using foam in model construction. Balsa, the most common material used in kits, is strongest in tension. High speed impact with the ground or another object usually results in severe damage from compression. A standard wing with balsa ribs, balsa leading and trailing edges, and balsa sheeting will fracture and collapse. Foam on the other hand is strongest in compression. By combining the best features of balsa and foam, you can build a composite structure that is much stronger and more durable than either alone. Fiberglass reinforcement at stress points, such as the center section joint, will make the wing very strong, but light.

High performance radio controlled planes such as pylon racers and sailplanes are commonly constructed with foam core wings. These wings usually have balsa, obechi (wood veneer) or fiberglass skins. You have to watch a pylon racer turning at high speed, or watch the 120 inch span wings of a glider flex as it being launched on a powerful electric winch to fully appreciate the stresses this construction can handle.

You might have heard that foam core wings are too heavy. In my experience, there is little if any weight penalty. Even electric indoor and park fliers are constructed of foam. A 60 inch span foam core wing sheeted with 1/16 balsa requires no spars or wing joiners, items usually made of plywood, spruce or other heavy materials.

Other advantages of foam core wings include accurate airfoils over the whole wing (no sagging covering between ribs), torsional stiffness, quick construction, and crash survivability. I recently crashed a 60 inch stick-type plane with a balsa sheeted foam core wing. The light ply fuselage and the balsa tail surfaces were completely destroyed, but the wing had only minor damage. I can repair the wing with little effort, build a new fuselage and tail surfaces, and the wing will fly again.

So, what do you need to build a foam core wing? Some simple tools, wing sheeting material, and of course you need some foam.

The white beaded foam (commonly called bead board) as used for insulation in building construction works very well for most foam core wings. This type of foam is usually about one pound per cubic foot. The balsa most often used in building R/C models weighs about six pounds per cubic foot. This type of foam can be carved or sanded to shape, but the bead structure makes it less suitable for this shaping method than the extruded foams.

The pink or blue extruded foam, which is easily carved with common tools, is most often used in construction of cowlings or other components that have compound curves. Some builders use blue foam exclusively in construction of scale R/C model wings, cowlings and fuselages.

EPP (expanded polypropylene) foam is being used more and more for trainers and for slope soaring planes. It is much more durable than the other types of foam since it is very resistant to crushing and tearing. It is heavier and much more expensive than other foams. It can be cut with a hot wire or shaped with hand tools. Commercial EPP foam kits are most often made up of machine molded components. EPP components are usually covered with clear or colored plastic packaging tape.

White beaded foam in a four foot by eight foot by two inch thick sheet will cost about ten dollars (compare that with one 1/16 x 3 x 36 inch sheet of balsa at the local hobby shop). Pink or blue foam in the same size sheet as the white beaded foam costs about twenty dollars. You should be able to make at least six 60 inch wings from one 4 x 8 foot sheet of foam.

The basic tools for cutting foam wing cores are a hot wire (usually mounted on a simple bow), airfoil templates, and a source of electricity to heat the wire.

The foam cutting bow that I use is made of a four foot piece of 1 x 2 inch fir. Eight inch lengths of heavy music wire (5/32 inch or larger) is fitted into holes drilled near each end of the fir strip. The cutting wire is attached to the ends of the music wire so that the music wire maintains tension, keeping the wire straight during cutting. Other methods include using a spring one one end of the cutting wire to maintain tension.

The cutting wire I use is single strand stainless steel fishing leader that I found at White Elephant in Spokane. Twenty five feet of .013 steel leader costs about a dollar. The steel leader is available in several diameters. The smaller diameter leader will require less current, but will break more easily. Nichrome wire is often used as cutting wire, but it is much more expensive, and requires higher current to reach the proper cutting temperature.

I use the four foot bow for cutting most wing cores, but a smaller bow might be useful for cutting basic shapes for cowlings.

My airfoil templates are usually made of scrap Formica or other laminate material. It stiff and strong, and cuts easily with a band saw. The resulting cut edges can be sanded to a very smooth finish (this prevents drag or catching the cutting wire). Thin aluminum sheet or ply will also work.

I use a variable transformer (called a variac) as my electricity source. It provides zero to 140 volts from a 110 volt input. This allows very fine control over the heating of the cutting wire. The output from the variac is fed into a surplus signal transformer to step down the voltage and provide some safety from electrical shock. For many years I used an old Lionel train transformer as my source of electricity. A dimmer switch using 110 volts, or a 12 volt battery charger can also be used, though I haven't experimented with these.

Automated foam core cutting machines are available. These are either mechanical or computer controlled. If you are going to make LOTS of foam cores, it might be something you would want invest in. If , like me, you might cute a few wings a year, it probably isn't cost effective. If you've got lots of cash and a insatiable appetite for technology, go for it!

Foam core wings can be cut to either rectangular or tapered shapes. Most warbirds, scale aerobatic planes and gliders use tapered wing panels, so this is a good area to develop proficiency. It is also easy to cut cores with washout, a twist in the wing panel that makes the wing tip fly at a lower angle of attack than the root. This is especially useful in scale warbirds, which are notorious for having high wing loading and for being susceptible to tip stall.

Wing cores can also be cut with different airfoil sections at the root and tip. This allows you to tailor the wing to specific needs. The "Chipmunk", a full scale British military trainer, has a thick semi-symmetrical airfoil section at the root and a thinner, flat-bottomed section at the tip, with washout. This helps prevent tip stall at slow speeds, an important feature in trainers.

Something that I've had problems with are the plastic cowlings and other fuselage parts that come in kits. Cowlings can often be replaced with fiberglass components purchased from aftermarket suppliers. These cowlings usually cost in the $30-$50 range. An alternative is to make a foam core form that is covered with fiberglass to make a single cowling, or it can be used to make molds for creating as many identical cowlings as you need. These techniques are especially useful when you are building a plane from your own design, or from purchased plans.

Other plastic components found in kits include turtle decks on fuselages. These are difficult to mount and require painting, something I've not had much luck with. These components can be replaced with foam cores sheeted with balsa. These are light and strong, and are easily mounted on the fuselage. They can be sanded to make joints invisible, and can be covered with plastic film. Foam cores for turtle decks with simple shapes can be cut using templates, or can be carved and sanded to shape if they have compound curves.

Al Culver builds 1/12 scale combat planes that are primarily constructed with white foam cores for wings and fuselages. These components are sheeted with balsa. These combat planes must be built quickly, cheaply and lightly. These planes rarely survive more than one or two combat sorties when as many as a dozen planes at a time are trying to cut their opponent's streamers.

I plan to go into more detail in future articles and to do foam construction demonstrations at club meetings.

We discussed the Mall Show, the type of club events to have this year, and whether we should rent a port-a-potti for Sawyer's field, and the club meeting location.

Mall Show:
Having our Mall Show and the Spokane Swap Meet on the same weekend meant that the Mall Show was understaffed on Saturday, the busiest day. Minimum staffing should be two people, but we had only one person at a time for most of the day. We decided that next year, the Mall Show would be held the second weekend of March. There was a mixup in getting a TV to the Mall Show on Saturday, so we were unable to show videos. Next year we will make sure a VCR and TV are there. Mike Underwood pledged new signs for the Mall Show. The ones we have used for the last couple of years were not in good enough shape to use this year. We would also like to see more planes at the show, so a suggestion was made to invite other clubs in the area to participate.

Club Events:
April 21 - Flying at Mountain View Park in Moscow. A model flying demonstration by various types of planes and buddy-box flying for the public were suggested. We will talk about this in more detail at the April club meeting.

June 17 - Club Fun Fly Contest and Picnic. Low stress fun fly events.

July 14-15 Orrin Crooks Memorial Glider Meet in Moscow. Thermal precision duration and precision landing for R/C gliders. Location to be determined.

September 16 - Fall Fly In and Picnic. A day of flying and BBQ. No competition. This date is tentative depending on whether there is a home football game.

January 2002 - Super Bowl Snow Fly at the Nelson Aerodrome, near Troy, ID. Flying with skis or floats (if there's enough snow).

Other events suggested and yet to be scheduled: a float fly at Bob Boettcher's place on Lake Coeur D'Alene; bomb drop contest; slope soaring (depends on weather); indoor rubber powered free flight (warm winter flying fun).

A suggestion was made that we hold the club meetings at the flying field during the summer.

Port-a-Potti:
Ted Frisbee is looking into the cost of renting a port-a-potti for Sawyer's field on a monthly basis. This is contingent on John Sawyer's acceptance.

Club Meeting Location:
The noise from the gaming room and other activities at Hobby Town prompted a discussion about moving to another location for future club meetings. I contacted Eric's Cafe and reserved the banquet room the first Tuesday of each month at 7pm (Eric's Cafe closes earlier than Hobby Town).