Author: Louis Joyner
Edition: Model Aviation - 2008/02
Page Numbers: 152,153,154,155
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Free Flight Duration

Discus-launch Gliders: history, kits, and building tips

Louis Joyner [email protected]

DISCUS-LAUNCH gliders look like oversized hand-launched gliders (HLGs). A 3-foot wingspan is typical.

Instead of the usual overhand, near-vertical javelin throw, discus-launch gliders are gripped by the wingtip and thrown with a circular motion that is slightly above horizontal. The discus action is not only much easier on the arm, but it also yields a considerably higher launch altitude.

Early experiments (circa 2004) with discus-launch free flight gliders were based on RC discus-launch designs. In England Phil Ball and Andy Hewitt developed 5-foot-span models that used timer-operated auto rudder and stabilizer to control launch trajectory, transition, and glide. The aircraft were successful but complicated by the built-up wing construction and auto surfaces.

Soon another British modeler, Mark Benns, came up with a smaller model, spanning approximately 36 inches, that used a conventional solid-balsa wing and required no auto surfaces. Flight adjustments were built in. The wing was set at roughly 2° positive incidence relative to the stabilizer. (Typical javelin-launch gliders are set close to zero.)

Instead of the usual flat stabilizer, Mark used a V-shaped version that was slightly skewed on the tailboom (leading edge offset to the left). The bottom-mounted rudder was set for a slight right turn. The stabilizer and rudder offsets were carefully set to counterbalance the strong left turn of the discus launch and produce a right circle in the glide. Right-handed fliers grip the model’s left tip in the right hand and launch with a counterclockwise rotation.

Thanks to several free flight publications, the Internet, and a few enterprising kit manufacturers, discus launch had become mainstream by the time the 2007 Nats took place. Bruce Kimball was one of a number of top American glider fliers to make the transition from javelin to discus launch. He experimented with the larger auto-surface models but soon settled on the smaller glider. Nine of the top 10 places in HLG at the Nats went to discus-launch models.

“The whole idea was to make it a simple model; when you start adding gadgets a lot of people will complain,” said Bruce. “These models are simple enough that everybody can build and fly them.”

Compared to traditional javelin-launch aircraft, the new discus-launch gliders seem heavy. Bruce said:

“My first model was 62 grams, but I think that was too light. The glider I won with at the 2006 Nats was 72 grams; 75–90 grams is a good range, though models seem to tolerate being a heavier weight with no problem. Len Surtees has one model at 120 grams and he says that it flies well. I know that Tim Batiuk said that a couple of his gliders were over 100 grams.”

If you want to give discus launch, also called tip launch, a try, the best suggestion is to start with a kit. The two most popular are Stan Buddenbohm’s Dynamo Hum II and Mark Benns’s Spin-Up 36 (see sources for more info). Even if you decide to start from scratch, as I did, order plans. It will save you a great deal of time and aggravation.

I picked a short kit (plans and tailboom) from Len Surtees at the 2007 Nats. Figuring that my learning curve might include some broken models, I decided to set up fixtures and “mass produce” three identical models.

On Bruce Kimball’s recommendation I ordered some tapered carbon-fiber kite spars for booms. Everything else was the usual hobby-shop balsa and plywood. I made a few construction notes.

  • The typical way to shape a glider wing is to taper the balsa sheet from root to tip and then sand in the airfoil. Instead I cut the wing to its planform shape, added basswood edging at the leading and trailing edges, did some rough planing, and then sanded the airfoil from high point to the trailing edge using a fixture. Typical glider airfoils feature a straight-line upper surface from the high point back to the trailing edge.

The fixture is a piece of 3/4-inch birch plywood measuring roughly 7 x 20 inches with a strip of 3/8-inch pine glued approximately 5 inches from one long edge. The exact position of the 3/8 strip was chosen to give the desired taper angle from high point to the trailing edge at the wing root.

A sanding block bridges from the edge of the plywood board to the 3/8 strip. A stop on the underside of the sanding block keeps it from sliding forward.

The block is moved in a spanwise direction. By keeping the wing trailing edge approximately 1/4 inch from the edge, sanding automatically produces both a straight taper from high point to trailing edge and a taper in wing thickness from root to tip.

  • The V-shaped stabilizer turned out to be easier to make than I had anticipated.

After sanding the 3/32-inch balsa stabilizer to shape and giving it one coat of finish, I ran a strip of masking tape along the centerline on the flat underside. I made a straight cut along the top centerline with a razor saw. Then I used a small triangular file to open the cut to the correct angle. It took only a few seconds to file down to the masking tape. Then I added glue and propped up one side of the stabilizer to the correct angle.

  • I probably spent too much time agonizing over what to use for the hinge pin. Most discus-launch models use what’s called a broken-back DT. The wing and front fuselage are a separate assembly from the tailboom, stabilizer, and rudder. A removable hinge pin holds the two components together and allows the boom to pivot up for DT.

Some people use a nylon screw or even a pop rivet for the hinge, but I was worried about long-term wear from taking the model apart for transport or storage. I ended up using an aluminum screw post, also called a binder post. Although it is an office-supply item, it is easier to find at a local Ace Hardware store than at a big-box office-supply store.

I bored out a short length of thick-wall aluminum tubing (i.e., rear peg material from FAI Model Supply) with a 13/64 drill bit (another Ace Hardware item) and epoxied the tubing in the boom assembly to provide a durable pivot. The female portion of the screw post was trimmed to length so the screw could be tightened all the way without pinching in the fuselage sides and binding movement of the boom for DT.

I hope you will give discus launch a try. Your arm will thank you. To help, following are some additional places to find more information, kits, and materials.

  • Free Flight Digest, June–July 2007: kit review of Stan Buddenbohm’s Twirly Bird discus-launch glider, written by Larry Kruse.
  • Free Flight Digest, December 2006: kit review of the Mark Benns-designed Spin-Up 36 kitted by Sting Aero Products, written by Dohrman Crawford.
  • Aeromodeller, April 2007: kit review of Spin-Up 36 written by Bob Bailey (Aeromodeller is now published as the free flight section within Aviation Modeller International magazine).
  • Free Flight Forum 2004: “Discus-Launched Gliders” by Phil Ball and Andrew Hewitt — an extensive discussion of design, development, and construction of Phil and Andy’s large (approximately 60-inch-span) auto-surface discus-launch gliders.
  • Free Flight Forum 2005: “Further Experiments with Discus Launched Gliders and the Development of Big Bertha & Slippery Sam” by Andrew Hewitt. In this follow-on to the 2004 Forum piece, Andy details the development of smaller (39-inch-span) models with solid-balsa wings. They still incorporate timer-actuated auto surfaces.
  • Free Flight Forum 2005: “Update on Last Year’s Report on Free Flight Discus HLG—The 60-inch All Singing and Dancing Model” by Phil Ball. This features further exploration of the large auto-surface glider.
  • Free Flight Forum 2006: “Gadget-Free Discus-Launched Gliders” by Mark Benns. The Spin-Up 36’s designer details his design approach to discus launch without the use of auto surfaces. If you don’t read anything else, read this article. (Copies of the Free Flight Forum are available from Bob Stalick, NFFS Publications Service. Check the National Free Flight Society [NFFS] website for additional information.)
  • 36th Annual Report of the National Free Flight Society: “2005 Models of the Year—Discus Launched Gliders: ‘Stork’ & ‘Flingitucus’” by Andy Hewitt and Phil Ball. This includes three-views and information about the two large auto-surface-equipped models. (Check with NFFS Publications for availability.)
  • Discus Launched Gliders by Kurt Krempetz. Contains information and three-views of Kurt’s Round-A-Bout design. It’s available online at the AMA Glider website, which also contains photos and videos of a variety of discus-launch models, tips for converting traditional javelin-launch models to tip launch, and a variety of plans and construction tips for all types of HLGs and catapult gliders.
  • Hand Launch Glider Airfoils by Curt Stevens. Although written about conventional HLGs, Curt details the glider airfoil he developed almost a half century ago that is still the choice for many discus-launch designs.
  • Sting Aero Products sells Spin-Up 36 and Spin-Up 42 discus-launch kits and short kits, tailbooms, Smoothie and Cooltube viscous DTs, and other HLG and catapult glider kits. Sting Aero founder, noted Australian glider flier Len Surtees, recently sold the company to Tim Goldstein. All sales are now through Tim’s A2Z Corporation’s Peck-Polymers division.
  • Stan Buddenbohm’s discus-launch kits include the Twirly Bird, Meerkat, Dynamo Hum, and Dynamo Hum II. Five of the top 10 places in Open outdoor HLG at the 2007 Nats went to fliers using the Dynamo Hum II. Stan also sells fuselage kits, plans, and instructions for discus-launch models. You can find more information on the AMA Glider website.
  • FAI Model Supply carries Sting Aero Products kits and components, Ikara, Button, and Badge viscous DT timers, and many other free flight kits, supplies, and accessories.
  • Stevens AeroModel sells pop-up fuselage kits for discus-launch gliders, available in two sizes and with or without viscous timer. Kits feature laser-cut balsa-and-plywood front sections and carbon-fiber tailbooms.
  • Hang-em High Fabrics has tapered carbon-fiber kite spars for use as tailbooms. The G-Force Skinny (item WIND-SK32) is 32.5 inches long and weighs 7.5 grams.

Razor Planes

After 50 years my Little Giant razor plane was starting to show its age, and I used up my stash of Gillette Super Blue blades long ago. (The stainless-steel blades that replaced the Super Blues in the 1970s were worthless for planing balsa.) It was time to find a replacement.

The German-made Balsa-Hobel plane seemed like a reasonable replacement and, allowing for inflation, the $14 price tag was in line with what the Little Giant cost a half century ago. The big surprise was what razor blades cost now. The Solingen steel blades are $1.35 each. I figured 10 blades would get me through the three discus-launch wings if I were careful.

I placed an order with Tim Goldstein (A2Z Corporation), and a few days later the plane arrived. It even came with one blade. This tool is a joy to use. It makes nice, wide shavings and no chatter. Best of all, I turned three 4 x 36 balsa sheets into wings, and I'm still on the first blade.

Finding Balsa

Longtime Free Flight Digest editor Walt Rozelle, who died last October, once told me that he found the best sheet of balsa he had ever seen in the display rack at a craft store. The sheet of 4-pound-per-cubic-foot C-grain was mixed in with the usual rock-hard sheets that could pass for basswood.

In a typical Walt gesture, he gave the piece of balsa to an indoor modeler who could make better use of it.

Ever since I've been pawing through the balsa bins at chain craft stores and occasionally find a good sheet of light C-grain. But I hate to think how much time and gas I've wasted.

Now Peck-Polymers offers an alternative. Each sheet of its contest balsa is individually weighed and sorted. Most sizes are available in your choice of A/B- or C-grain. Density ranges are 4.00–4.99, 5.00–5.99, and 6.00–6.99 pounds per cubic foot.

"As you can imagine the supply is limited, particularly in the real light 4-pound stuff," said Peck-Polymers owner Tim Goldstein. "But many people are glad to pay the price to get the density sheets they really want instead of buying 20 sheets from others and hoping to get what they need."

For more information contact A2Z Corp.

2007 Nats Video

Alan Abriss's latest Nats video is now available as a two-hour DVD. Coverage of the weeklong outdoor free flight event includes Dawn Unlimited, ROW (rise-off-water), and the new electric categories.

The DVD is $20 plus $3 shipping and is available from Homegrown Television Productions.

Sources

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