Radio Control: Pylon Racing

Radio Control: Pylon Racing

Bill Hager 4 Holly Springs Dr., Conroe TX 77302

Introduction

Here we are in the dead of winter — or, for those up north, storm-and-blizzard season. It's building season, and I thought you might be interested in this article by Fog Tanner Jr., taken from the NMPRA newsletter. After several discussions on wing construction at the Nats and in our district, I decided to put it into writing. Paul Page asked me to write another article — you guys can blame him.

Given ten racers, you'll normally find five to ten different ways of doing the same thing. Most work some of the time, some work all the time, and some only work for the guy doing them (Phred's Law of Mechanics). It's one of those learning things that makes racing fun and challenging.

The wing-sheeting method we use at CRS Racing works for us and several others. It isn't the only way, but it works.

Wing construction overview

Everyone is always looking for ways to save wing weight and have a strong wing — often mutually exclusive goals. The speeds and long wings in use today have required changes from wing-building methods of ten years ago. Carbon fiber has been used in various stress areas for years, but it's not a cure-all: the strength of a wing is still only as good as the wood structure.

If you use very light wood you get a flexible (and sometimes poorly controlled) wing. If you use very stiff/heavy wood you get a wing that resists bending (can be like a shovel) and may over-roll. To get the stiffness without excessive weight we've started using Skyloft and a composite sheeting method.

Materials and tools

• Foam cores (in the blocks they are cut from)
• Building board, square / right triangle, straightedge, pen for alignment marks
• Radial arm saw (for slotting cores)
• Basswood or spruce spars (avoid 1/8" plywood for the spars)
• Glu-It (Pica) or equivalent aliphatic glue (sandable)
• Toothpicks and rubber bands (temporary hold)
• Waxed paper (roughly 24" x 20")
• Skyloft (unwoven nylon fabric similar to silkspan) — available from Dave Brown
• EZ-Lam or similar epoxy for lamination (we use 60-minute hardener)
• Cheap 3" touch-up roller (Shur-line style) and disposable sleeves
• Squeeze board (press) or flat surface and lots of weight (200–300 lb preferred)
• Sandpaper 180–220 grit

Preparing and aligning foam cores

1. Keep the foam cores in the blocks they were cut from and place them on your building board.
2. Use a square or right triangle and draw pen alignment lines on each end and in the middle (front and back). These lines will be used later to align core and sheeting.
3. Measure where the landing gear plate will be located on the cores. Transfer the back of the plate to the top of the wing — do not measure over the curved top surface (you will come up short).
4. Reassemble the cores into the blocks, set them on a flat surface, and use a square to transfer the landing-gear line to the top of the core.
5. On the top of the core use a straightedge and draw a line parallel to the leading edge, out 18 inches. If you intend to sweep the wing back, measure the sweep difference at 18" from the leading edge and draw the appropriate line.
6. The two cores together should have a straight line perpendicular to the centerline where the back of the landing-gear plate will be glued.

Cutting the spar slot

1. Place the core top-side up in supports (old blocks) and lay a straightedge on the line you've drawn.
2. Slide the tip of the radial arm saw blade against the straightedge so it contacts front and back of the blade; make sure you are cutting the back side of the line (toward the trailing edge).
3. Cut an 18" slot through each core. Repeat for the other core.

Making and gluing spars

• Make two spars from basswood or spruce. These woods bend slightly and avoid creating a stress riser like hard 1/8" plywood can.
• Spars tie the top and bottom skins together and provide real strength. Some builders use hard balsa or plywood spars, but these can cause the skin to push out on the bottom during turns.
• Use Glu-It (Pica) or other sandable aliphatic glue. Apply a light glue film — avoid globs and wipe off excess.
• Place two toothpicks about 1/2" from both sides of the spar and use a rubber band around the toothpicks to hold tension; this keeps the cores from separating while the spar glue sets.
• Place a 24" x 20" strip of waxed paper over the spars (top and bottom), assemble the back blocks, set on a flat surface, stack weight (the author uses ~60 lb) and let the glue set. This ensures the spars are straight and the cores won't separate when sheeted.

Preparing and joining sheeting

1. Weigh all the sheeting and assemble four stacks (right- and left-hand stacks) weighing the same.
2. Arrange boards so:

• Heaviest wood that can bend goes on the top leading edge.
• Heaviest wood that can't bend goes on the bottom leading edge.

1. Glue the four boards into sheets using Glu-It or another sandable glue. Avoid CyA for long glue seams — it can form a hard line that will show after sanding.
2. To glue seams:

• Run 3/4" masking tape along the seam.
• Flip the sheets, open two at a time to form an inverted V, run a thin bead of glue on the joint, lay flat, and wipe excess.
• Let dry, remove tape, and lightly block-sand seams with 180–220 grit.

Laying out Skyloft and sheeting panels

1. Lay the cores over the wing sheeting to mark leading and trailing edges. A convenient method: assemble a block, board, and core, make marks 3/4"–5/8" (depending on preference), and repeat for all boards. Mark each board with its location (e.g., top-left, bottom-right).
2. Lay out Skyloft over the boards and mark along the border of the foam cores. Cut the Skyloft to fit and identify which board each piece belongs to.

Epoxy lamination with Skyloft

Notes:

• For bonding the sheeting to foam cores with Skyloft in the composite, epoxy is the preferred adhesive. We use EZ-Lam with 60-minute hardener for ample working time.
• Use a cheap 3" touch-up roller with disposable sleeves to apply epoxy.

Procedure:

1. Lay out a piece of plastic at least 6" larger than the wing on all sides.
2. Mix epoxy in small batches. Some 60-minute epoxies can gel faster if mixed in large volumes — be conservative.

• The author mixes about 1/4 fl oz for the first half and about 1 fl oz for the second; only about 1/8–1.5 fl oz actually ends up on each panel.

1. Pour some epoxy on the roller and a little on the plastic. Saturate the roller as you would with paint.
2. Roll a thin, even film of epoxy over the board. You may get a thick start; roll it out to an even coat. Excess epoxy can be rolled off onto another board.
3. Lay the pre-cut Skyloft on the respective panel. The Skyloft may wrinkle slightly; smooth it with your hands. The goal is to make it damp with epoxy, not saturated.

• Warning: If you oversaturate the Skyloft you will add a lot of weight (and the panel will be overly stiff).

1. Roll the wrinkles out of the Skyloft without adding more epoxy — just get it damp.
2. Roll a tack coat of epoxy on the core side that you will bond first, assemble bottom board and core into the bottom block. Roll a tack coat on the top of the core and assemble the top sheet and top block.
3. Add some weight to keep everything from moving; Skyloft tends to allow slipping.

Repeat for the other wing panel.

Pressing, compression targets, and cure

• You can either set the composite on a flat surface and stack weights or use a squeeze board/press. We recommend a squeeze board that can put 200–300 lb on the composite. That amount of pressure forces the epoxy into the Skyloft from both sides; we have not experienced wing skin delamination with this method.
• If using a press:

1. Measure the core assembly at each end and in the middle when you draw your alignment lines and note those dimensions.
2. Add 3/16" to each measurement to allow for tip and bottom sheeting, then subtract 1/32" from that total — this is the target thickness to which you compress the composite.
3. Compress each measured station to the target number and tighten the press accordingly.

• Let the assembly set for 36–48 hours before handling.
• Epoxy may gel in ~8 hours, but can still move.
• At 24 hours some epoxies are not totally stable.
• Full cure can take around seven days (and longer in cold climates), but the assembly is usually safe to work with after 36–48 hours.

After the epoxy sets, remove the composite from the squeeze board or remove the weights and take off the wing panels. Don't flex the panels immediately — let the epoxy cure further without disturbance, then complete the wing.

Finishing notes

• This is one proven way of sheeting a wing — it produces straight wings at target weight without failures in our experience.
• Some builders complain it takes too much time, adds weight, or is unnecessary. Our wings are coming out straight and on weight, and we have not had structural wing failures.
• If you doubt the added strength, try breaking a standard panel and a composite panel — the composite is significantly tougher (David Layman found it strong enough to damage part of his garage-door frame).

J.P. Hanway’s Thompson Trophy Racer (brief)

I promised more on J.P. Hanway's Thompson Trophy Racer. J.P. was building an airplane to compete in the Thompson Trophy class at the 1995 Galveston model air races. The challenge was to pick an airplane that would be competitive and easy to build. The first criteria required an inline engine and no flying wires. The second criteria dictated a low-wing monoplane with a slab-sided fuselage and fairly straight lines.

After about a month of research, J.P. selected the Keith Rider R-4, a.k.a. Schoenfeldt "Firecracker." It wasn't the prettiest airplane, but it met the criteria better than other candidates. The R-4 was built by Keith Rider around 1935 and campaigned on the 1936 air-race circuit. Two years later Schoenfeldt purchased the aircraft, cleaned up the airframe, and fitted it with a Menasco Super Buccaneer 250 hp engine.

Transcribed from original scans by AI. Minor OCR errors may remain.