Balsa and Foam Board Composite R/C Glider

This project will guide a new radio controlled pilot through the steps to build a lightweight and inexpensive aircraft that is durable, easy and inexpensive to build. It will introduce model aircraft building techniques that have become popular among enthusiasts - using foam board as a base of construction, and we will also incorporate traditional balsa construction.

Introduction

Since foam board aircraft have only become very popular in recent years, it might help explaining why. Foam board is relatively inexpensive compared to balsa sheets of the same size. It is lightweight, durable and easy to work with; it can be shaped, sanded, bent, folded, and cut with simple tools and a variety of adhesives can be used to bond it to itself and other materials. For this project, we will be using balsa as a means of increasing the structural integrity of the mostly foam airframe and wings.

Safety

The use of knives, scissors, hot-glue guns, and adhesives in general are potentially dangerous. Use caution and be aware of your surroundings when using any tool or machine.

Bill of Materials

balsa sheets: 3 each

- 3/32"x3"x36" - three sheets

- 1/16"x3"x30"- two sheets

- 3/16"x3"x30" - 1 sheet

balsa stock

- 3/16" square

- 1/4" square

- 1/8" square

- 3/16"x1/4"

two 30"x30"x3/16" foam boards

hot glue gun

hot-glue sticks

cyanoacrylate glue (CA glue)

2-part epoxy(fast and slow setting)

clear packing tape roll

hobby knife (x-acto)

utility knife

Assorted rulers, tape measure, etc

wax paper

Gorilla Glue

320 grit sandpaper

sanding block

cups for mixing epoxy

small paintbrush for spreading epoxy

Choose a plane

We will be building the Papillon, a lightweight motor glider with a simple design aesthetic and easy construction that was marketed by Kyosho in the 70's and 80's. Because of its relatively large wingspan and it being classified as a glider, it should provide a great base for the beginning R/C pilot. Gliders tend to have very easy flight characteristics due to their low wing loading; this design is meant to be as light as possible and with modern radio and electronic equipment, it will be even lighter than it was originally intended. The Papillon was meant to have only 2-channel control (rudder and elevator); for power, an unthrottled .049 nitro engine was mounted in the nose. It would simply pull the plane to altitude and once it ran out of fuel it could continue flying by "catching" thermals. The firewall section could be easily modified to accommodate an electric motor to provide easy throttling and longer run times than the glow counterpart. I will be building per the plans, however, and installing a Cox .049 nitro engine. I found the plans here, along with hundreds of other free plans:

http://outerzone.co.uk/plan_details.asp?ID=5109

Electric conversions are becoming very common today and as such, there is a great deal of information that can be found with a little digging. I suggest searching in the forums on rcuniverse.com or the like if this is the direction you would like to go. The equipment has become much less expensive in the last five years, so finding older, lightly used motors and batteries can help save you from spending a lot for your first plane.

We will do some simple unpowered glide tests after construction is complete to inspect the low speed flight handling and do some initial control trimming. This may be the best way to get the beginner pilot acclimated to the controls, as the speeds and altitudes are usually so low that damage is unlikely to occur in the event of an accident.

We will begin by building the fuselage, the body of the airplane. For this, we will transfer the profile of the fuselage onto the foam board and fold the board in half so that we can cut out both sides of the airframe in the same step. This will ensure that both sides are nearly identical and save us the time of cutting each out individually. Once cut out, we will remove the paper and reinforce the foam with balsa sheeting.

Make a photocopy of the fuselage's profile section directly from the plan. Since this plane has some length it will require a couple sheets of paper to accomplish. Glue or tape the profile to the foam board, trying to conserve as much space as possible. Plain white glue is sufficient for this step, but taping it down in sections is preferred so we can use the profile again later.

Cut a rectangular section about twice as wide as the profile.

We will make a cut down the length of the board's center, but not through the sheet. The idea here is that the blade penetrates about 1/2 to 3/4 of the thickness of the board to facilitate folding. Refer to photo 1 for clarification - notice the plan is attached to the board and the board is folded along its center.

Note: We will be using this technique throughout the project - from now on I will refer to it as a partial cut.

Apply a small amount of white glue to the inside of the board to bond
the two halves together. Once dry, carefully and cleanly cut the area where the firewall is illustrated on the profile. In the photo, a red line is drawn where you should make your cut. Try to keep the blade perpendicular to the surface of the board and use a straightedge or metal rule to guide you. We must remove this material so that the balsa ply firewall we build later can bond directly to the balsa sheeting we add to the outside of the foam sides.

Begin cutting the outline of the fuselage on the inside of any outer sheeting the plan illustrates. Look at photo 3; here I'm comparing the fuselage on the plan to the profile I used to create the foam sides. Notice that the top and bottom sheeting is not included on the profile; we must allow for clearance here so that when we add that sheeting it doesn't make the fuselage taller than specified. Once all the excess material has been removed, trace over the bulkhead lines with a ballpoint pen. This will leave indents in the foam for later reference. Use a square to draw lines across the top and bottom of the sheets so that the same procedure can be done on the other side. See photo 4.

To separate the two halves and remove the paper, run hot water over the sheets while gently bending them. The paper can then be peeled off without too much trouble. Try not to kink the foam in the process, but its not a detriment to the structure if you do.

Aside: Although not necessary, I chose to remove the paper to reduce weight at the cost of rigidity. The paper adds some stiffness to the foam board, which may be advantageous; some planes that use foam board exclusively may not mention removing the paper, just be aware that removing it will make the foam flimsy. The next step, however, will increase the stiffness of the sides significantly when we be bond balsa sheeting to strategic areas of the foam sides.

Tracing balsa profile.

The area on the top edge of the fuselage between F-5 and F-6 is the wing saddle. This area must be flat up to the forward dowel pin, illustrated on the plan. Position the profile section you used earlier on a piece of 3/32" sheet so that the wing saddle area is on the edge of the sheet. Since we want the sheeting to strengthen the forward section of the airframe without adding too much weight, we'll have it extend under the wing saddle on top and taper to a point midway across the wing chord on the bottom. In photo 5 I have the ruler spanning from the top of bulkhead F-6 to the midpoint on the bottom; transfer these measurements to the balsa when you trace the profile.

Cut the balsa along the diagonal line using a razor saw or sharp knife - use the rule to guide the blade so the cut is clean. Flip the excess material over so you can use the same angle for the other side, this way we can ensure the angles are the same. Apply a drop of CA glue to each end of the piece you traced the profile to and attach it to the remaining material, careful to align the edges. This is the same operation we did for the foam boards which will help us make exact duplicates we can separate later.

Cut out the profile

Carefully cut along the lines you traced. It is best to leave a bit of extra material so you can sand to the final shape with a sanding block. Once you're satisfied with the way the profile looks, slide a knife blade between the sheets and gently pry them apart. Since only a small amount of glue was used, there should be a minimal amount of damage to the wood. We now have to exact copies of the profiles. Use the profile plan to transfer measurements of the firewall location to the balsa - this will aid with aligning the foam board.

Balsa strips

We also need some balsa strips to extend to the tail end of the fuselage. Measure onto another sheet of 3/32" balsa two 1/2" wide strips, extending the entire length of the sheet and cut off. Use these strips to make the extensions to the tail as I show in the photos. The strips will have to be forced into alignment with the fuselage since it is curved, so make sure you hold them in position before making any cuts; it works best to lay the strips on the profile plan to make cuts marks. Once one set has been cut, use them to make a set for the other side of the fuselage.

Note: The horizontal stabilizer passes through the lower strip and must be trimmed. It is best to do it now, before the halves are complete. You'll have noticed that the strips will try to overlap at the tail section, but once clearance is made for the horizontal stabilizer, no further trimming will be necessary. Refer to photos 9 -12.

Nose section

Mix a batch of two-part epoxy adhesive per directions, enough to coat one side of each of the forward balsa supports. Mixing a small amount of isopropyl alcohol (90% or higher) to the epoxy will bring it to brushing consistency. Use a brush to apply a coat to one side of the balsa sheets, careful not to put any on the marks where the firewall will be attached. Position the sheets on the foam sides and place on a sheet of wax paper to prevent squeeze-out from adhering to the work surface. a stack of books is a great way to distribute an even load onto the sheets - but be careful the balsa doesn't shift while stacking.

Tail section

It is best to let the epoxy set for a couple hours before handling again. Once cured, the tail support strips can be prepared. Recall that the bottom strips must be forced into position, though; to do this you will need a surface that you can push pins into - something like a cork board for posting notes.

Position the foam sheet to take advantage of the edge of the board; if there is no frame on your board clamp a length of square stock balsa to butt the top edge of the turtle deck to (the turtle deck is the top of the fuselage behind the wings). Apply the epoxy to both the foam and the balsa and use T-pins to hold in position. Allow to fully cure before handling.

Tip: After cutting out the firewall and bulkheads, ensure they
are square by measuring diagonally from corner to corner. If they are square, the measurements should be the same. If they are different, check your measurements again and trim as necessary.

Firewall

The firewall will be constructed from two layers of 3/32" balsa and epoxy. Cut out two 2-1/8" squares from leftover materials. Apply epoxy to both faces you intend to join but orient the grain pattern perpendicular to each other to increase strength by limiting bending moments. Clamp with spring clamps or place under a hefty weight, be aware of squeeze-out. Once cured, choose a side as a base for trimming - use sand paper and sanding block with a square to make sure all sides are perpendicular to each other. The bottom edge of the firewall needs to be tapered to fit the floor of the fuselage. Mark a line on one edge at 2" from the opposite side, this is the "front". Cut along this line holding the saw blade at an angle so that the edge will taper to the edge on the other side. Some trimming will be necessary to achieve the proper angle - trim material from the "back" side since it is still a bit longer than necessary. Refer to the photos. Continue trimming until the angle matches the plan and sand smooth.

The width still needs to be refined to the proper size. To maintain the
proportions of the plan, trim 1/8" off one side, making the firewall a total 2" wide.

Attach 1/4" square stock around the bottom and sides on the front. Sand the bottom to match the angle.

For glow engine installation only

Mark holes for mounting the glow engine to the firewall. In the event that your engine has not mounting plate, construct a beam-style mount to be added later. Information on this mounting system can be found with a search - be aware that there are multiple versions and each will have its own advantages and drawbacks. The Cox Black Widow I'm using uses a mounting plate and integral tank, so the bolt pattern must be transferred to the firewall and holes drilled prior to installing the firewall (for convenience sake). The holes can be drilled at a later time, but it it easiest when the firewall can be laid flat.

Fuselage floor (Doubler)

The floor of the nose section is one length of 3/32" sheet extending from the firewall back to F-6, making it 10-1/16" long. The length must be accurate to account for the curvature of the bottom of the nose. The width is 2-3/16". Once cut out measure 1/4" in on each side down the length of the sheet; this will serve as a guide for attaching the sides. Double check that the ends are perfectly square using the diagonal measurements.

Bulkheads F-5 and F-6

The two bulkheads will be made from 3/32" sheet. These will be narrower than the firewall due to the foam board, at 1-5/8" wide. The height will be the same for each: 2-1/2". Use 1/4" square stock around the perimeter of both bulkheads, attached with CA glue.

Start by sanding the edges of the fuselage sides with a sanding block. Knock down any high spots where the balsa sides join. Try to focus on making the foam and balsa even and the edges square so they both make contact when joined to the doubler. Sand the balsa sheeting to remove excess dried epoxy and smooth the grain of the wood.

Bulkheads

Place the profile plan over the inside of the fuselage halves. Transfer marks for the bulkheads with a marker or pen. Apply epoxy to the firewall section of the right fuselage side and the right edge of the firewall and bulkheads F-5/6. Only one side will be joined first so that they can be checked for squareness. These three bulkheads are critical to maintaining the geometry of the fuselage, so be as accurate as possible. Use weights and T-pins or other objects to hold them in place while the epoxy dries. The use of a fast setting epoxy may be helpful here so you can hold the bulkhead in position. If necessary, each bulkhead can be joined one at a time to aid set up.

Once the epoxy cures, check the bulkheads for squareness again. Make any adjustments if necessary by pressing lightly to deform the foam board. Apply epoxy to the opposite edges and place the left side of the fuselage in position. Use T-pins to anchor the bulkheads in alignment with their corresponding marks. Turn the fuselage onto its side and place an evenly distributed load on the other side. A book works well here also. Once cured, measure diagonally from the corner of F-5 to the opposite corner of F-6 and repeat on the other corners (see photo). Ensure these measurements are the same to check squareness.

Fuselage floor

The front of the floor must be notched to fit between the sides of the at the firewall. Wet the outside with water to make the wood easier to manipulate to the shape of the fuselage bottom. Attach with epoxy (quick setting) applied to the bottom of F-5/6 and CA glue on the bottom of the firewall (to hold on position) and place some weights to hold the rear portion in contact - be sure to check alignment. Let the epoxy cure then carefully position the sides even with the floor and use CA glue to adhere in place.

Tail

Join the tail section by first trimming the contacting surfaces. Pinch the halves together and estimate the angle between them and trim to suit. The ends need to be even with each other to prevent skewing of the fuselage. Once trimmed, join with epoxy.

The floor of the tail section and turtle deck will be made from 1/16" sheeting. The top and bottom can be cut at this point, but only the bottom will be attached. Measure a 2-1/8" x 13-5/8" rectangle and draw a line down its center. Mark the bottom center of F-6 and align the center of the sheet that was just cut with this mark and the tail. Trace the outside edge of the fuselage onto the sheet and trim. Glue along the edges with CA. Sand edges and transition from 3/32" to 1/16" with 320 grit sand paper and sanding block.

Hold-down dowels

Cut two 1/4" dowels to 3-1/8". Mark their positions on both sides of the fuselage and bore a 1/4" hole at each location; I have positioned the rear dowel so that it can be glued to bulkhead F-6. This can be accomplished with a drill or by carefully cutting with a hobby knife. Install with epoxy.

Transfer measurements of both horizontal and vertical stabilizers onto a piece of 3/16" balsa; if necessary the rudder and elevator control surfaces can be separated from the stabilizers to fit on the wood sheet. Note that the plan only shows one half of the horizontal stab. Measure inside each edge 3/16" so that both are scaled smaller - we will be attaching 3/16" square stock around the perimeter for strength using CA glue.

Cut out the rudder and elevator and use the same procedure.

Once the glue dries, use a sanding block to knock the excess material off and round the edges.

The original wings were constructed using the built-up method, a timely and material-costly process. Instead we will use the Armin Wing method, detailed here: flitetest

Since flitetest has a tutorial on this method, I won't reiterate it here. The steps are very simple and use only hot glue and foam board. I have constructed the wing to the same dimensions in the plan, so you would want to make a 6" chord wing using the method in the linked video. Only a small adjustment in the dimensions is necessary and I did not include aileron control surfaces, since I wanted the model to be rudder/elevator only. Use the remainder of the 1/4" dowel that was used for the hold-down pins. I made the dowel extend half way into each wing, leaving the outer section without a spar. This will still be plenty strong and ensure the wing stays together in the center. I also did not use the packing tape as a covering method, since I am not sure of its fuel resistance. For electric flight, the packing tape is fine. Using the dowel, however, will eliminate any dihedral and reduce stability. To keep the dihedral, I warped the dowel with water and a bending jig, set to the proper 6 degrees. Pour boiling hot water over the dowel and hold in position with a gloved hand.

When joining the wing halves use a combination of epoxy and foaming polyurethane glue (Gorilla glue). Apply Gorilla glue to the spar before inserting into each wing. Place epoxy in the wing joint and elevate the opposite wing tip to the prescribed 6". After curing, wrap the joint section with packing tape to reinforce the area. Cap the wing tips with 3/32" balsa and epoxy.

For an even stronger wing joint, a layer of fiber glass cloth and epoxy can be used, I chose to forego this however, for weight savings. Although, the materials are lightweight on their own, the wing is nonetheless heavy for its size. A similarly sized wing made with traditional methods may be lighter but the durability, material cost, and time investments make up for the weight.

Attach the turtle deck that was cut out earlier with CA glue being careful to align the center mark with mark on top of bulkhead F-6 and tail. A line should extend down the center to show where to attach the vertical stabilizer.

Cut a slot on both sides of the fuselage where the horizontal stabilizer resides. Test fit the elevator and make marks to set alignment. Apply a small amount of CA to a corner to hold in place. Using a tape measure, check the distance from the nose sheeting to the outer corner of the trailing edge of the tail. Check the measurement on the other side and compare. These two should match before cementing in place. Once satisfied with the alignment, apply more CA to hold firmly. Cut a strip of 1/4" x 1/8" balsa to attach where the elevator and fuselage meet.

Add a spar to the trailing edge of the Rudder, made out of 3/16" x 1/4" balsa; make sure the bottom edge of the spar is tapered to allow elevator throw. Apply CA to the bottom edge of the rudder and carefully align on the fuselage. Be sure to check that the Vertical stabilizer is perpendicular to the horizontal stabilizer with a square or measuring from the top of the rudder to the outer corner of the elevator. Glue a 1/4" triangular piece of balsa to both sides of the rudder base and the bottom side of the elevator base.

The skid plate is made from 3/32" balsa sheet. Follow the contour illustrated on the plan for the skid plate formers (two on each side) and glue into position. Sheeting is 3/32" laid with the grain oriented transversely to the roll axis of the airframe. Wet the sheeting to help conform to the formers before gluing with CA.

The final sheeting in front of the wing saddle doubles as a hatch to access the engine mounting bolts and electrics. I created a frame of 3/32" balsa to glue on the top edge and then cut another sheet to fit over the forward area to be held down with screws or magnets.

The covering I chose for this plane is a fuel proof, heat shrinking plastic called Monokote (Top Flight brand). For an all electric aircraft, simple packing tape can be used to cover the entire airframe. I won't show the steps for this process, as it is lengthy and there is a great deal of information that already exists that will be far more helpful to a beginning builder. Follow this link for a tutorial on covering with tape. Follow this link for a tutorial on covering with Monokote plastic film covering.

To attach the control surfaces to the tail appendages, I used the old fashioned method of sewing them on. There is a great deal of info on this method as well with a simple search. Places like rcgroups, rcuniverse and others have loads of discussions on this as well as several other popular methods for attaching the surfaces. I'm leaving it up to the builder to decide which method best suits their skill level or ability. Just know that there are advantages and disadvantages to them all, so do your research.

Installing radio equipment and balancing the model need to be accomplished at the same time since the bulk of the weight in the aircraft are these components. First, mark the Center of Gravity (CG) on the bottom side of the wings, based on the plan. Set all the equipment inside the fuselage and attach the wings. Try to have everything sitting where you want it to be when it is flying. Slowly lift the model with fingers positioned at the CG marks. Note if the nose or tail rises. If one end of the plane moves more than about a 1/4" in either direction of the pitch axis, reposition the electrical components and test again. Although having the model hang perfectly level is good, a general rule is to have the nose hanging down somewhat, as this is a safer attitude for flight. Having the tail hanging low, on the other hand, is totally unacceptable and must be mended. The model should also be balanced along the roll axis, by lifting with one finger under the tail and one in the center of the nose. Repeat several times and note if there is a tendency to roll to one side or the other. Make sure the wing is centered on the fuselage. It may be necessary to add a small amount of weight to a wing tip; start with coins taped to the wing tip on the opposite side of the roll to see if there is a change. If a significant amount of weight is needed to balance the model, recheck the positioning of your fingers and the wing in the wing saddle - it should not require much.

Flight testing is necessary before any powered flights can occur. Be sure that the aircraft's battery and the transmitter battery are fully charged. It is best to conduct these tests on a flat surface with tall grass to soften the airplane's landing. It will take some practice to get some distance, but starting with the model held low, as if from a seated position, launch into a slight breeze to help increase lift. Having a partner to help is good as it gives time for the pilot to transition from launching to controlling the model. At this point, however, allowing the model to glide back to earth uncontrolled helps with studying any bad tendencies it may have in slow flight. The weight of this model (with the Armin wing) will severely limit its gliding abilities, so keeping that in mind, don't get overzealous and try to start launching with the intent to pull any altitude. Build up to a standing launch and get used to what it may try to do. Many tutorials exist for first time R/C pilots, but one of the best learning tools is to find a veteran pilot and have that person fly it under power for the first time. They will be able to comment on any poor flight characteristics better than anyone. Join an R/C flying club and get to know some members, most are very helpful and affordable for beginner pilots as part of incentive programs.

Good luck and happy flying.