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Bob Hunt’s Lost-Foam Wing Building System

Construction Manual

Congratulations, you have purchased the most accurate

and logical model airplane wing building system in the

world. The Lost-Foam Wing Building System is the

only method of building a wing that keys on the outside

shape of the wing. Accuracy is assured, providing you follow the instructions completely and carefully.

The Lost-Foam Wing Building System kit

Your Lost-Foam Wing Building System kit includes

foam fixtures on which to build your wing, a set of ex-

tremely accurate 2-pound foam wing rib templates and a

set of 2-pound foam Leading Edge Mold Bucks. The

entire plan for your wing has been drawn out on the

foam wing building fixture in advance and the foam

wing rib templates have the necessary information to

achieve perfect spar locations.

There are several cautions to be observed and a cou-ple of operations you must complete before beginning

assembly of your Lost-Foam built-up wing.

The actual building fixture is the lower foam cradle

halves from which the wing rib template cores were cut.

It is made from sturdy 2-pound expanded polystyrene

foam. This type of foam cannot tolerate contact by any

solvent based adhesives. It is also intolerant to most CA

adhesives. You can use some of the foam-safe CA adhe-

sives, but we do not advise using them in this system.

You will need to use regular CA adhesives to glue many

of the parts together, but you must be very careful to not let the CA glue contact the foam parts.

You can coat the foam cradles with a water-based

Polycrylic coating before beginning construction and

that will protect the surface of the foam cradles to a de-

gree. The best thing you can do is to follow the instruc-

tions closely and apply adhesives as suggested. If some

CA does get on the cradle and melt into it, don’t panic;

the system will still work fine. Let’s get started.

Stuff you will need

There are a number of things you will need to gather

and have ready at hand for building your wing. The first thing you will need is a sheet of Luan or MDF (Medium

Density Fiberboard) to use as back stiffener material for

the Leading Edge Mold Bucks. We highly recommend

the MDF as it is more stable than plywood and generally

comes in sheets that are very straight and warp free. This

inexpensive material can be found at Home Depot or

Lowes.

If you have decided to protect your foam building

fixture, you will also need to purchase a can of Min-

Wax water-based Polycrylic varnish.

You will need to study the plans for the design wing

you are building and have on hand the necessary

wood in the required lengths and thicknesses. For

example, most smaller wings use 1/16-inch leading

sheeting, trailing edge pieces, center section sheet-

ing and cap strips. Most larger wings will be built using either .077 or 3/32-inch wood in these areas.

Only you can make that decision and collect the

necessary wood. You will need to have the appro-

priate balsa wing spar stock, some 3/32 to 1/8-inch

by 3/4 inch wide thick basswood strips, and a sheet

of 1/8-inch thick Lite-Ply (Poplar Plywood) as well.

You will need to have all the necessary adhesives

on hand. These include Aliphatic Resin Glue

(yellow woodworker’s glue), 30 or 45 minute epoxy

glue and both thin and medium viscosity CA glues

(we use and prefer ZAP CA glue).

It is a good idea to also have a small polypropyl-ene glue bottle fitted with a thin aluminum or brass

tube applicator (We get these from Tom Morris, 327

Pueblo Pass, Anniston, AL 36206). You will also

need mixing cups and mixing sticks, microballoons,

waxed paper, straight pins (Dressmaker’s pins), and

a Dressmaker’s rule (Helpful but not absolutely

necessary).

Normal modeling tools will be required and these

include a #11 blade knife and lots of sharp replace-

ment blades, a Razor Saw, a long straightedge, and

a variety of freshly surfaced sanding blocks in vari-ous grits from #220 grit to #80 grit.

You will also need a variety of building weights.

You will see in these instructions the “Foam

Bricks” that we use and offer. These are regular

building bricks that are fitted into a custom-cut

foam sleeve so that the rough surface of the brick

will not scratch or mar the surface of the soft balsa

parts.

Other items that will be needed are 3/4-inch wide

masking tape and a variety of spring clamps. We

use and recommend the spring clamps that are

available from Sears. They have swiveling jaws that will clamp parts securely without sliding one part

against the other when they are applied.

It’s time to get started building a Lost-Foam

wing. If you are not sure of an operation, re-read the

instructions carefully. If you are still not sure of an

operation, please feel free to call Bob Hunt at

Robin’s View Productions between 8:00 A.M. and

5:00 P.M. Bob is usually in the RVP shop most

evenings as well and will take the time to help you

thorough any spots you are unsure of. Help with

your projects is a big part of our service!

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Preparing Leading Edge Mold Bucks

It is a good idea to mold the leading edges first so

that they have adequate time to dry while you are

building the wing frames.

The first step in this process is to make and install backing pieces on each of the two Leading Edge Mold

Bucks. Place the back edge of each of the foam Lead-

ing Edge Mold Bucks onto the MDF board and trace

around the base of the mold bucks with a ball point

pen.

Cut these pieces to shape using a band saw and

sand the edges smooth.

Attach the MDF backing pieces to the rear of the

foam bucks using Aliphatic resin glue and secure

them in place by placing several weights across the

two bucks. Let the glue dry completely.

Be sure to carefully align the edges of the mold

bucks to the edges of the MDF pieces before you

leave them to dry. The edges of the MDF board

should not extend above or below the surface of the

Leading Edge Mold Buck sections.

Molding Leading Edges

Using your dressmaker’s rule, measure around the

root section of one of the Leading Edge Mold Bucks.

Measure from approximately 1/4-inch behind the rear

spar line location on the top of the Mold Buck to 1/4-

inch behind the spar line location on the bottom of the Mold Buck. Record this number on a pad of paper.

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In this case the resulting measurement would be 6 3/4

inches.

Repeat this process at the tip end of the Mold Buck

and record that number as well.

The next step is to choose the wood from which

you will mold the Leading Edge Shells. This wood

should be A or AB-grain material. A or AB-grain

balsa is the type that has long, straight grain proper-

ties. This is the only type of balsa wood that will bend

properly around a mold buck when wet. You can test the wood you have chosen to use to see if it will mold

properly by bending it slightly as shown. If the wood

resists bending, it is not a good choice for this opera-

tion. Avoid C-grain wood here as it will certainly

crack if you try to mold it. C-grain wood is the type

that looks like it has “fish scales” on the surface.

Normal CL model design leading edges will require

three pieces of 3-inch wide wood per shell (Larger CL

designs and large RC designs may require wider

pieces or more pieces of wood here). These pieces must be joined to make up a shell blank that will even-

tually wrap around the Leading Edge Mold Buck.

Choose pieces of wood for the center of each of the

shell blank sets that test bend most easily. These are

the pieces of wood that will be going around the tight-

est radius at the leading edge of the shell. The two

additional pieces of wood in the shell blank set that

will be molded over the shallow bends aft of the ac-

tual leading edge do not have to be as bendable as the

center piece. Trim the edges of each of the pieces of wood that

will make up the shell blanks. Use a #11 blade knife

and a long straightedge for this operation.

Next, sand the edges of each of the pieces of wood

so that they are absolutely square to the surface of the

wood. For this operation we use a 22-inch long Per-

magrit sanding bar turned on its side. We position the

Permagrit bar next to the edge of a piece of flake

board. We use a 12-inch long Great Planes sanding

bar fitted with #80 grit sandpaper as a “pusher” to

allow us to slide the balsa stock against the fine-grit

side of the Permagrit bar. The result is perfect edges

on the balsa blank pieces that will glue properly later

on.

Lay the center piece you have chosen for one shell

blank on your workbench and put a tick on it at the

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center at both ends using a ball point pen. You will

eventually connect these tick marks and draw a faint

center line down the length of the center balsa piece.

Measure the longest of the two Leading Edge Mold

Bucks (In CL the inboard wing is usually a bit longer than the outboard wing.) and then then cut the center

balsa blank piece to be at least an inch longer than the

measured dimension at either end. For wing designs

which have a significant leading edge sweep, you may

need to leave the center piece of balsa a bit longer to

allow for sufficient material when the balsa blank is

molded over the buck.

Lay the length trimmed center piece on a flat bench

and then position the outer pieces against it on either

side.

Check the dimensions you took earlier of the dis-

tance around the mold buck at the root end and meas-

ure out half this distance from the centerline and make

a mark on each of the outer balsa pieces.

Repeat this process to find the proper blank width at

the tip end of the blank.

Cut the excess wood from the outer balsa pieces

and then trim the outer sheets to the pen tick marks.

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Put a piece of waxed paper on your flat workbench

and position the three L.E. mold blank pieces as

shown in the photo below. The side with the tick

marks should be facing towards the waxed paper.

Position building weights along one edge of the

center balsa piece. These weights should be back

about 1/16-inch from the edge of the balsa.

Put a piece of thin Teflon tubing in the spout of a

bottle of medium viscosity CA glue, and then run a

liberal bead of glue along the edge of the weighted piece of balsa. Be certain that no glue gets on the sur-

face of the balsa pieces.

Position the outer piece of the blank against the

glued edge of the center piece and then, working

quickly, turn the weights across the seam to hold the

two pieces of balsa together with the surfaces flush.

Pull the seam together tightly using strips of mask-

ing tape and then repeat this entire process to glue the

other outer piece to the blank assembly.

After the assembly has dried sufficiently, remove

the weights and turn the blank over so that the side

with the tick marks at the center is showing.

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Using a straightedge and ball point pen, connect the

tick marks and draw a center line down the length of

the bottom, or “inside” of the L.E. mold blank. Note

that any excess CA glue will be on this side of the

blank. The upper surface should be clean and free of glue.

Block sand the upper surface of the mold blank. We

use #220 sandpaper attached to a Great Planes alumi-

num sanding bar for this operation. Sand the surface

until the joints between the pieces are smooth and no

seams can be seen or felt.

Repeat this process to make the other leading edge

shell blank.

For the next step you will need a tub in which to

soak the blanks. For this operation you can use the tub

in your bathroom, or you can purchase a large plastic

storage bin which can serve as a dedicated soaking

tub. Heat a gallon of water to near boiling and carefully

pour it into the tub. Place the balsa leading edge

blanks into the tub and let them soak for at least 20

minutes.

The lid that comes with the plastic storage bin helps

keep the water hot while the blanks are soaking. It helps to rock the tub a

bit during the soaking

process. Harder wood

will require a longer

soaking time in the

tub. Soft wood will

saturate more quickly

and requires less time.

Remove one of the blanks from the water and posi-

tion it as shown below on its corresponding buck.

Note that the center line of the blank is being aligned with the center line on the inside face of the blank.

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Check both ends of the blank to be certain that the

center lines on the blank and the mold buck are

aligned, and then fold the blank around the buck.

Once you are satisfied with the alignment of the

blank on the buck, secure the blank to the buck with a

strip of masking tape at both ends of the buck.

At this point you will need a helper to wrap the blank down snuggly against the buck along its entire

length. Use an ACE Bandage for this operation.

Overlap each wrap by a third or so and when the

entire blank has been wrapped down snuggly, use a

piece of masking tape to secure the bandage.

Repeat the process to mold the other blank into a

leading edge shell and then let both of them dry thor-

oughly. We recommend letting them dry for at least

two days. While you are waiting for them to dry you

can make your ribs and build your Lost-Foam wing.