Discus-launch gliders

Model Airplane News, Jan 2003 by Garwood, Dave

NEW OLYMPICS-STYLE TECHNIQUE FOR IMPRESSIVE RESULTS

DO YOU LOVE THE SIMPLICITY OF hand-launched gliders but hate having a strained shoulder after a day of throwing them? When flying hand-launch, do you wish you had longer flight times to find thermals before the next relaunch? Do you envy the impressive launch heights achieved by truly athletic pilots? Discus-launch gliders (DLGs) tackle all three problems.

Hand-launch gliders (HLGs) are the purest form of RC soaring; it's just you, your sailplane and the air. There's no high-start or winch to set up; you just heave the plane and go hunting for lift. Few things in RC flight are as satisfying as chucking a 60-inch glider, coring a thermal and circling up until the plane is a speck in the sky. But in the effort to get ever higher launches, sometimes, we damage a shoulder joint, strain finger muscles or other body parts and have to lay off hand-- launching for weeks or months. Now we have the planes and the technique to achieve launch heights that are two or three times as high, as well as longer initial flight times, with vastly reduced wear and tear on your body.

A moderately fit launcher can achieve a launch height of 40 to 50 feet in a light wind using the traditional overarm, or "javelin," launch technique. The new "discus launch," "tip launch, or "sidearm launch" method can easily double or triple the launch height-even for non-athletes. First, you need an airframe that can withstand the centrifugeal loads of the launch, the increased speed at launch and still fly well. When you have learned the launch technique, you'll be able to achieve higher launches that allow longer search times and increase the chance of finding lift on every launch and during every HLG flying session.

Model Airplane News has assembled a council of experts to help you learn the basics so you can try this technique. The advisors are: Bruce Davidson, a 2001 AMA Nationals HLG champion; Mark Drela, an aeronautics professor at MIT and avid HIG/DLG designer; and Denny Maize of Polecat Aeroworks, the manufacturer of the X-Terminator, the 2001 HLG national champion airframe.

They are enthusiastic about the practice and the potential of DLGs, and they generously shared their experiences with these model aircraft. Denny Maize not only supplied a ready-to-fly X-Terminator Pro, but also taught me enough technique by email for me to succeed at sidearm launching at my first DLG flying session. I have a desk job and can, unfortunately, answer "Yes" to all three questions in the opening paragraph, but after a single day at DLG, my interest in HLG has been renewed.

DLG ADVANTAGE

Model Airplane News: DLG is a significant advance in HLG. It enables higher launches with reduced strain on joints and muscles. It allows less physically capable individuals to participate in HLG-the "purest" form of RC model-sailplane flight.

Mark Drela: Yep. Seems to me that it has also leveled the field at HLG contests. Flying ability has become more important than physical prowess.

Bruce Davidson: I agree that the new sidearm launching technique makes it easier for people of all physical abilities to enjoy hand-launch sailplanes.

Denny Maize: Joe Wurts put it best when he said, "It allows the old, fat, gray-haired guy a chance to be competitive."

DLG DESIGN AND DEVELOPMENT

MAN: DLG is hard on airframes. Improved designs and stronger materials are needed.

Denny: Phew; there's an understatement. The winter of 2001/2002 was a biggie in design and development. Everyone in the business was going crazy strengthening airframes and playing with wing layups. The worst part of the problem involved stress loads that we didn't fully understand at the time. Boom flex, fuselage side loads and wing loads were different from what we had dealt with until DLG came along. Every time we thought we had a problem worked out, someone found a way to break it! There was one heck of a lot of R&D going on at a very fast pace.

Mark: The loads aren't much more severe than in a very strong javelin launch-- maybe twice as big. But they are in quite different places and are primarily sideways rather than up and down. The structure can be designed appropriately. Composite materials definitely make a more durable DLG; a built-up balsa/spruce DLG would be almost impossible to execute in a reasonable weight and durability.

Bruce: The stresses are in different places with the new launch technique, but with a good design, it doesn't have to be hard on the airframe for everyday flying. Competition is a different thing, though; anything used for competition has a shorter life.

MAN: Including the readily observable strengthening needed to handle the centrifugal launch loads and higher air speeds at launch, what are the challenges in designing for DLG?

Mark: First, the flexing of the tail boom screws up trajectory. Most of the kite booms are more flexible than is ideal. The large-diameter Allegro-Lite boom solved that problem. There are now comparable booms available. Second, slop of control linkages is challenging to prevent at light weights and low friction. Third is flaperon flutter. For discus launching, you want thick flaps, but aero performance wants thin flaps-tough tradeoff. Fourth is airfoils. Although these were a challenge, I think the latest crop is quite close to ideal and is probably within manufacturing tolerances.