Author: Don DeLoach
Edition: Model Aviation - 2010/04
Page Numbers: 27,28,29,30,31,32,34
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Ellipsix

by Don DeLoach

Overview

The best thing about competition free flight (FF) is the wide variety of model types and sizes. There are glow-powered gas aircraft that span 10 feet and indoor rubber airplanes that are as little as paper clips. Somewhere between those extremes is the handheld Outdoor Catapult Glider (AMA event 142).

These models are compact, quick to build, inexpensive, and easy to fly. In the time-and-money-per-unit-of-fun department, no event delivers like Catapult Glider.

For the beginner, these aircraft are a perfect entry point for experiencing the joys of FF. Best of all, they can be made to fly great with little experience or expertise on the part of the builder.

Balsa for FF

Balsa varies widely in density, from as much as 15 to as little as 4 pounds per cubic foot. In FF, light balsa is crucial for top performance, because our models are generally smaller and less tolerant of high wing loadings than the average 10-pound RC aircraft.

In FF we seek the least-dense balsa available. This is known in the industry as "contest grade," typically 6 lb/ft³ and less — approximately half the density of average balsa found in many hobby shops or RC kits.

If you have a good local hobby shop, take time to sort through sheets for contest-grade wood. If not, mail-order from Mountain Models (Colorado Springs). Owner Brian Eberwein weighs and sorts every sheet of wood he receives; when you buy "Contest" balsa from him, you can be certain it is 6 lb/ft³ density or less.

C-grain wood is preferable for all flight surfaces because it is the most warp-resistant. It looks mottled, like mother-of-pearl, whereas A- and B-grain have much straighter, parallel grain patterns. C-grain is rare in stores but can be found if you sort through enough sheets.

Above all, do not use balsa that is even the slightest bit twisted. Catapult Gliders fly extremely fast in the climb and cannot tolerate warps.

Specifications

  • Type: AMA (event 142) FF handheld Catapult Glider
  • Skill level: Suitable for beginners
  • Wingspan (flat): 18"
  • Wingspan (projected): 16.3"
  • Wing area: 50 sq in (projected)
  • Length: 18"
  • Weight range: 0.7–0.9 oz (20–25 g prototypes weighed 22–25 g balanced)
  • Wing loading: 2.3 oz/sq ft
  • Power: 7– to 9-inch loop of 1/4" x .042" FAI Tan rubber, on 6" handle
  • Construction: Balsa, plywood, tubular carbon tailboom
  • Finish: Nitrate dope or Minwax Helmsman Spar Urethane
  • Cost to construct: $5–$10 in materials, $10–$15 DT timer
  • Time to construct: 4–6 hours (two evenings)
  • Flight duration: Two-minute maximums ("maxes") in competition

Flight Trim Notes

  1. Balance at 55% root chord; add nose weight as required.
  2. Very slight (1/8") stabilizer tilt for left glide; 1/32" left rudder.
  3. 1/16" x 1/4" x 3/4" wash-in wedge on left wing.
  4. Launch in steep (45° to 75°) right bank (45° recommended).
  5. Prototypes weighed 22–25 grams balanced.

Rear view: 1/8" stab tilt for left glide (1/4 scale).

Plans and Detail Notes

Fuselage Left Side View:

  • All material 4–6 lb/ft³ "contest" balsa unless otherwise noted.
  • Fasten well with CA and microballoons.
  • Sand fin to symmetrical airfoil shape.
  • Catapult: 6" dowel with 9" max. loop of 1/4" FAI rubber.

Left Wing (Flat View):

  • 1/16" offset (washout).
  • Balsa wash-in wedge under left wing from light 1/16".
  • Inset .026" wire line guide (left wing only).
  • Airfoil high point: sharp, sand flat. Leave 1/32" at TE square; 1/32" upsweep on lower surface.
  • Spanwise wing taper:
  • Main panels: 3/16"
  • Mid panels: 3/16" > 1/8"
  • Tip panels: 1/8" > 1/32"

Horizontal Stabilizer:

  • 1/16" sheet.
  • Taper—sand stab from 1/16" thick at centerline to 1/32" thick at tips.
  • Sand stab to flat-bottom airfoil.
  • 1/64" ply leading-edge protection.

Finish:

  • 2–3 coats of thinned dope OR 1–2 coats of Helmsman polyurethane (green can).
  • Sand with 400 between coats.

"Lend Your Weight" — NFFS www.freeflight.org

Construction

Materials for flight surfaces:

  • Wing: one piece of contest 3/16" balsa at least 3-1/2" x 18".
  • Fin and horizontal stabilizer: roughly 8" of 1/16" x 3" width balsa (contest density).
  • Use C-grain for warp resistance where possible.

Do not use any balsa that is even slightly twisted. Catapult Gliders are intolerant of warps.

Cutting and Leading-Edge Protection

  • Cut the wing, fin, and stabilizer from sheet balsa with a sharp #11 X-Acto blade or scalpel.
  • Lightly moisten a 1/64" plywood strip and attach it to the wing leading edge using medium CA. This protects against gouges from rough landings.

Stabilizer and Fin

  • Taper-sand the stabilizer from full thickness at the centerline to 1/32" at the tips, then sand to a flat-bottom airfoil. High point roughly 30% back from the LE. Avoid pressing too hard with sandpaper to prevent crushing fibers and causing warps.
  • The fin should be tapered to 1/32" thickness at its top edge and sanded to a symmetrical airfoil. Dab medium CA on the LE of both stabilizer and fin for dent protection. Final-sand with 400-grit paper.

Wing Shaping and Dihedral

  • Allow at least an hour to shape and final-sand the wing airfoil.
  • Start rough-shaping with a razor plane; if unavailable, use 60-grit sandpaper. Work with long, smooth strokes.
  • Mark the airfoil high point with pencil or a strip of flexible masking tape to preserve a sharp high point.
  • Sand the airfoil aft of the high point: this area (from the high point to the TE) should be a straight chordwise taper with no upper camber. Leave the back of the TE approx. 1/32" thick.
  • Reposition the tape and shape the forward portion of the airfoil, which has camber. Use 100- then 220-grit paper. Include the 1/32" quarter-round upsweep on the lower surface.
  • Polish-sand the wing with 400-grit paper. Cut the five dihedral breaks using a fine razor saw; use little pressure and keep the saw straight.
  • Sand dihedral bevels into the wing-panel glue joints using a flat, straight base (workbench edge or plate of glass). Butt the panel against the edge and sand, tilting the block slightly to approximate the bevel. Check progress against a ruler.
  • Glue each panel together with medium CA, matching dihedral angles on each wing half. When angles match, join the wing halves at the center dihedral break.

Fuselage

  • Ellipsix is designed to use the pop-up tailboom fuselage system developed by Stan Buddenbohm. Purchasing at least one of his prebuilt fuselages is recommended.
  • To fabricate at home: start with a piece of hard 1/8" x 1/2" balsa about 8" long for the fuselage pod where the wing is glued.
  • Tailboom: .120" OD tubular carbon with ~.020" wall thickness.
  • Hinge base for tailboom: extremely hard 3/32" balsa (or basswood) with 1/64" plywood sides. Fasten securely and align perfectly to the tailboom.
  • Tailboom hinges to the pod via a 1/8" diameter aluminum pop rivet friction-fit through 1/32" plywood "ears" on either side of the pod's rear. Ensure the fuselage pops up freely and easily with a single #8 rubber band — hang-ups equal flyaways.

Assembly

  • Glue on the flight surfaces, starting with the wing and taking care to align perfectly. Carelessness here makes trimming in high-speed launches difficult.
  • Glue the stabilizer onto the top of the fuselage with a slight tilt for left glide: left stab tip 1/8" higher than right when viewed from rear. Too little tilt is better than too much.
  • Apply finish to flight surfaces: normally two or three coats of 50/50 thinned nitrate dope, sanding lightly with 400-grit between coats. Options:
  • Mix a small amount of talcum powder with dope as a sanding-sealer.
  • Use a light coat of clear Minwax polyurethane, smeared off with a paper towel.
  • Color the model for visibility: Design Master spray paint (Michael's) is bright and lightweight. "Carnation Red" and "Holiday Red" are good for tops; "Flat Black" on undersides improves visibility in cloudy conditions.

Final Touches

  • Securely attach 1/32" plywood launching and rear hooks with thin CA.
  • Attach a 1/32" plywood doubler to one side of the nose for extra durability.
  • Test DT operation on the ground repeatedly to ensure reliability.

Timer and hold-down rigging:

  • Rig Spiderwire hold-down line to the timer to capture the front portion of the carbon tailboom and lock it into position under the midchord point beneath the wing.
  • Route the line back to the TE and up over the top surface of the wing. Attach a 2–3" loop of the elastic thread supplied with the timer and experiment with tension.
  • Target: timer should rotate approximately a half revolution in slightly more than two minutes for two-minute "maxes."
  • Better than elastic: a metal spring (available from Stan Buddenbohm) or one wound from .009" music wire on a .078" mandrel. Springs are consistent across temperatures and do not dry out like elastic.
  • Test the timer several times on the ground to ensure the tailboom locks solidly under tension and the wing pops up every release. If the tailboom binds, sand it narrower or increase rubber band tension. DTs must work every time.
  • Don't forget the 1/16" wash-in balsa wedge glued on the left main wing panel. This keeps the model from spinning left in a nosedive or turbulence. Ellipsix uses little incidence and a rearward CG, so careful trimming is essential.

Flying and Trimming

  • Balance the model at 55% per the plans. Do a couple of hand glides and observe tendencies.
  • If it is stallish, reduce incidence via the tailboom adjustment screw. If the glider dives, add incidence until the nose comes up near a stall. Aim for a floating glide on the verge of stall with a left-circle diameter roughly 50–75 ft.
  • For the maiden catapult launch:
  • Bend in a small amount of left rudder (approx. 1/32"–1/20").
  • Set DT to 5–10 seconds and hook up the Ellipsix to the rubber.
  • Pull back fully (full power), bank the model 45° to the right (hold launch stick in left hand), pitch nose 45° up, and release. The model should pitch nearly vertical while doing a wide left roll, transitioning to glide around 100 ft.
  • Rolling on launch burns some energy, but a consistent transition is more important than marginal altitude gains. Left rudder controls the left launch roll. As trimming progresses, you can reduce left rudder and incidence for a nearly vertical launch with little roll, but be prepared for spotty transitions and occasional death dives — those can yield higher altitude.
  • Anytime you change incidence, retrim the glide using CG shift. If transition problems persist, add clay to the left wingtip.
  • Trimming for top performance is individual: arm span and rubber loop length determine launch power. Long-armed fliers have an advantage. The AMA's 9" max loop is often too long for normal arm spans; 7" loops have worked well for many.
  • Make your launch stick from a large-diameter dowel (broomstick) for extra grip and extension. Use a 1/2" OD screw eyelet in the top of the dowel; ensure the stick does not extend past 6". In contests, tie several fresh loops — rubber typically wears out after four or five full-power launches.

— Don DeLoach [email protected]

Sources

  • Mountain Models

(719) 630-3186 www.mountainmodels.com

  • Stan Buddenbohm (catalog of FF Glider items, DT fuselages, timer springs; hosted on Krempetz FF Glider site)

Box 1177 Boulevard, CA 91905 www.schnable.net/hosted/amaglider/index.html

  • Michael's Craft Stores

(800) 642-4235 www.michaels.com

  • Cabela's (for 20-lb test Spiderwire fishing line)

(800) 237-4444 www.cabelas.com

  • FAI Model Supply (for Badge BL-7 DT timers, Master Airscrew razor planes, Zona razor saws, 1/4" rubber)

(570) 882-9873 www.faimodelsupply.com

  • Peck-Polymers (FF items, scalpels and blades, razor planes, carbon tubing, balsa)

(720) 833-9300 www.peck-polymers.com

Contact: Don DeLoach 831 E. Willamette Ave. Colorado Springs, CO 80903

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