RADIO CONTROL SOARING
Mike Garton 506 NE 6th St., Ankeny IA 50021 E-mail: mike@iastate.edu
This month I'll feature some of Carl (Mac) McBurnett's latest airplanes. Mac manufactures composite gliders in San Antonio, TX. His Two-Meter Dartar was released roughly a year ago; in the fall he worked with Pat Bowman to create a prototype airplane called the DS‑Demon, expressly for dynamic soaring (DS); and the Texas Twister discus‑throw hand‑launched glider (HLG) is Mac's latest addition.
The DS‑Demon
The DS‑Demon is the most exotic of the new airplanes. It was created for the relatively new sport of dynamic soaring, which is a way of flying models through wind gradients to extract energy from them. Imagine flying loops in a region where the air is moving faster up high. The top of the loop is flown in the same direction as the faster‑moving air. Like the water wheel on an old mill, energy can be extracted from the fluid.
A couple of years ago Joe Wurts demonstrated that this could be done on the downwind side of Parker Mountain (in southern Oregon) with an RC glider. When conditions are right, the airplane can build up to an excess of 150 mph with four or five circles. Pilots report a huge adrenaline rush — the speed is addictive. If the pilot keeps turning circles with the airplane, it eventually reaches a speed at which structural failure occurs.
To date, there has been no glider that could DS indefinitely on a windy day at Parker Mountain. F3B airplanes are the natural candidates, but even they blow up when pushed too hard. That is where the DS‑Demon comes in.
Pat Bowman designed the Demon and Mac built it. It has a 108‑inch wingspan with an RG15 airfoil. Flying weight is six‑and‑a‑half pounds, and most of the weight is carbon in the wings. The model was computer‑designed for 60 positive and 25 negative Gs. It has been flight‑tested at Parker on several occasions.
One day, with the Santa Ana winds howling, the stabilizer bellcrank broke. Forces on the tail get extreme doing loops at 150 mph. The wings and fuselage survived the mishap, and the Demon has come back for more. Now the airplane uses a 1/4‑inch carbon joiner rod just for the stabilizers.
A web link at the end of the column has pictures and MPEG video clips of the Demon in action. While viewing the video clips on your computer, make sure you turn on the sound to hear the airplane scream.
Right now the Demon is just a prototype. The model will go into production after Pat and Mac are satisfied with the flight‑test program.
There have been many technological advances in Formula 1 race cars that have trickled down to production cars. The same thing happens in RC soaring; F3B has driven development of fast molded airplanes. Now dynamic soaring is the harshest test environment available for soaring. Mac is applying some of the things he learned on the DS‑Demon project to his other production designs.
The Texas Twister
The Texas Twister is a 1.5‑meter hand‑launched glider designed for discus‑style launching. Right‑handed throwers grip the left wingtip with their palm facing down. After spinning around once, the airplane is released. Some pilots choose to throw without the body spin by just twisting their trunks.
The Twister uses a highly thinned SD6063 airfoil, which is one of the fastest glider airfoils available. I would wager that this airplane can beat 60‑inch‑span slope racers when ballasted to the same weight. The Twister has less than two‑thirds the frontal area of an RG15‑equipped sloper. With the ailerons, it probably turns cleaner too.
The planform is double‑taper with moderate aspect ratio. Mac precuts skin hinges, including ailerons, and was happy to see them on a production kit. Special high‑density blue foam and large carbon darts were required to stand up to the stresses of discus‑launching.
Many design‑test iterations went into this airplane. Each time the model broke, Mac would change the design a little, build a new airplane, and continue the test program. The models finally stopped breaking after five generations.
The fuselage is much like a long fiberglass tube reinforced with carbon. Most of the cross‑sections are circular. This fuselage is stiffer than the pod‑and‑boom types; it is the stiffest HLG fuselage I have seen. Strength is similar to that of the 1999‑vintage Maple Leaf airplanes (very strong). Carbon pushrods are standard for this kit.
The Twister’s V‑tail is vacuum‑bagged fiberglass over foam with large doubled carbon darts. It mounts into a saddle on the bottom of the fuselage. Mac tells me that the tail‑fuselage joint is critical on this airplane — the surface must be well scored. A two‑hour epoxy is recommended.
To better visualize the forces applied to the tail of a discus‑launch airplane, imagine sticking the tail of an HLG out the window of your car at 55 mph and then turning its tail so it is not aligned with the wind. You get the idea.
Advertised flying weight is 8–8.5 ounces with specific radio gear. My Twister came in at 8.6 ounces using four C510 servos, a Berg receiver, and a four‑cell 110 pack. The nose is long enough to balance with any battery. Mac usually puts his servos under the wing, but they can be moved way forward if necessary.
An up‑elevator preset is used for launch. This airplane does not need a rudder preset; it tracks pretty well without it. The Twister is zippy in flight and is intended for intermediate to expert pilots.
Most of the tricks John Roe taught me for a DJ Aerotech Wizard (see the March column) apply to this airplane. Do not move the center of gravity further back than four inches aft of the trailing edge. I originally tested mine with a two‑cell battery; I replaced it with a four‑cell pack to get better servo response.
There is a great deal of technique and finesse in discus‑throwing. Chris Oster has been reporting consistent dead‑air times of one minute, 20 seconds with his Texas Twister. Imagine how much sky this fast HLG covers in one minute, 20 seconds!
You cannot expect to reach your full potential the first few times out, but it is important to keep an open mind, study the better throwers, experiment systematically, and practice a lot. My first day discus‑throwing, the throws were only roughly 45 feet, resulting in 40‑second flights; the 26 inches of December snow and winter gear made spinning difficult.
Multiple sources tell me that once you have the technique down, discus throws will be approximately 50% higher than good javelin throws. If you are not very good at javelin throws, your discus throws may be 100% higher. They put less stress on the body, but more stress on the airplane.
The Twister is the first commercially viable discus‑launch airplane with flaperons; as of my January deadline, it is still the best one with flaperons. The flaperons will be especially helpful in turbulent conditions. Five degrees of camber increases the climb rate in a thermal — I was able to climb out quickly, even over the snow cover. Forty‑five degrees slows it to a crawl for landing. A couple of degrees of reflex can be used for fine trim.
Once you get the discus technique down, this airplane will be a serious contender in HLG competition. Just when I thought HLG technology had plateaued, someone has raised the bar a couple more notches.
The Dartar Two‑Meter
The Dartar Two‑Meter is a much tamer airplane than the Twister. Its low wing loading and SA7035 airfoil make it very friendly. Like the Twister, this Two‑Meter can be built in seven to ten hours. Wing control surfaces are pre‑cut with skin hinges. The servos are prewired, including a hole for wiring. The wing comes as halves; the builder joins the two parts to make a one‑piece bolt‑on wing. It is vacuum‑bagged carbon and fiberglass skins over foam.
This Two‑Meter has more carbon in the wing lay‑up than any Two‑Meter I have ever heard of. There are six large pieces of unidirectional carbon stacked on the top skin. The first full‑span piece starts at the leading edge and ends at the control‑surface cutouts. Each of the five additional darts is approximately an inch narrower.
The bottom of the wing has roughly half as much carbon as the top skins. Most manufacturers would make two or four airplanes with that much carbon. The wing structure has been proof‑tested with full peel‑to‑the‑metal launches using a ten‑ball bearing winch in 35‑mph winds.
The Two‑Meter Dartar's fuselage is fiberglass with carbon reinforcement. It has a shallow saddle in the top of the wing and a slip‑on nose cone. I replaced the two stock metal wing bolts with nylon ones so the wing should shear off in the event of a hard crash, minimizing damage to the fuselage and the wing.
Mac is known for using minimal epoxy resin to make a part. My model's fuselage weighed 3.2 ounces before I installed the radio gear. It is strong enough for a decent pilot. Those pilots who cartwheel often would be wise to request a beefier lay‑up from Mac.
The Two‑Meter Dartar can be ordered with a conventional stabilizer or a V‑tail. The tail saddle is flat to allow mounting the conventional stabilizer. I used triangular balsa to cradle the V‑tail on my airframe. The conventional and V‑tail are made from fiberglass vacuum‑bagged over thin foam. The tails are light and work fine.
The Two‑Meter Dartar and the Twister come complete with a big bag of hardware. They even include twisted servo wire and Deans connectors for the wing servos. Special pull‑pull lines are used to drive the tail. When Mac told me the pull‑pulls were working fine during the Two‑Meter's DS trials, it convinced me to use these. Mac also included carbon pushrods, but he warned that they are heavier than the pull‑pulls.
With a plain fuselage, a larger‑than‑stock battery, and extra wood in the tail, my model came out at 27 ounces. This is 11 ounces lighter than my best full‑house, zoom‑launching Two‑Meter. The Dartar Two‑Meter would have weighed 25 ounces if I had built it completely stock and used the recommended radio gear.
In flight the Dartar Two‑Meter is quite a floater. I can turn very tight and work the highest lift. Mac chose a fairly conservative triple‑taper performance with a touch of washout so the model does not tip‑stall, even during slow‑camber circles.
This airplane is a blast to hand‑launch. It takes less effort to catch a thermal from a hand‑launch than it does to set out the hi‑start. The slow speed makes landings easy.
The Dartar must be ballasted for windy days. Ballast is bolted into the fuselage above the tow hook.
If you have ever wished for a prefab airplane that can climb out easily from a hand‑launch and do rad zoom‑launches, here it is.
Sources
- DS‑Demon pictures and online video clips: www.bludart.com/demon/demon.html
- BluDartar Two‑Meter and Texas Twister: Carl McBurnett, 7506 Legend Point Dr., San Antonio TX 78244, (210) 662‑9503, www.bludart.com/
Transcribed from original scans by AI. Minor OCR errors may remain.
