File: 02joyner.lt1.doc
[Headline: Dan Berry’s modified Marval]
The design of Free Flight (FF) models is often an evolutionary process. Modifications over time improve performance, make a model stronger, and take advantage of changes in technology.
Marvin Mace’s Marval series of Power models offers a good example of this process. The original Marval, selected as a Model of the Year by the National Free Flight Society (NFFS) in 1993, was a “locked-up” model with no auto surfaces and conventional balsa construction. The Super Marval, a Model of the Year in 2007, utilizes a timer-actuated auto rudder and auto stabilizer, as well as a tubular carbon-fiber fuselage.
(Note: Marvin came up with the name to honor a fellow modeler Mark Valerius.)
Arkansan Dan Berry has been flying the Marval design for years and was spotted at last year’s Nats with what looked like a Super Marval 560. Not quite. “It’s the original Marval 560 with slightly different construction,” said Dan. “The Super Marval has bunt and auto surfaces.”
For his new model, Dan decided to adapt the five-panel wing and the fuselage construction from the Super Marval, but not use the auto surfaces. The most noticeable change was the tubular carbon-fiber fuselage instead of the original Marval’s balsa box.
“The tube is from the last batch that Seelig made,” noted Dan. (The late Hans Seelig was a Germany F1C flier who developed a popular multifunction mechanical timer.)
“The tube requires some adventures setting the firewall, pylon, and stabilizer mount,” Dan said. “The firewall is plywood set into the tube, pinned, then another layer of plywood added to match the backplate mount diameter. Three layers of 3/4-ounce fiberglass cloth were added all around, using West Systems epoxy.
“My pylon is 1/16-inch sheet balsa over ribs. The bottom rib is 1/2-inch thick, but most gets sanded away to fit the tube. The pylon is covered with Polyspan for strength. The pylon helps reinforce the tube where it is cut out for the timer.
“The stabilizer mount is 3/32-inch plywood with a balsa block to fit the boom,” Dan continued. “I used 3/4-ounce fiberglass cloth cut on a 45° bias. This is glued underneath the platform and wraps around the tube. I do that on all of my stabilizer mounts now.”
For the timer start switch to activate the timer at the moment of launch, Dan borrowed an idea he got years ago from the late Tom McLaughlan. “It’s actually pretty simple to make with hand tools … maybe 15 minutes,” said Dan. “Just make certain that it works before attaching the timer mount plate inside the fuselage. It is hard to fiddle with after it is in place.”
The switch is bent from 1/32-inch wire. It goes through a short length of 1/4-inch dowel mounted in the plywood timer mount plate. Inside the model, the wire bends down, then forward to engage the timer. A .015-inch wire spring wrapped twice around the dowel pulls the wire back from the timer, allowing it to run.
A rubber band is attached to the outside of the switch wire and hooked to the fuselage a few inches back. This keeps the outer part of the hook back, which holds the other end of the wire forward, stopping the timer. To launch, the model is gripped with the right hand over the rubber band and then the rubber band is unhooked at the rear using the left hand. Upon launch, the rubber band releases and the timer starts.
(Note: A detailed construction article about the Super Marval 560 appeared in the June 2009 issue of Model Aviation. Full-size plans [#1029] are available through the AMA Plans Service.)
Carbon-Fiber Fuselages
Although the Seelig fuselage that Dan used is no longer in production, similar booms are still made by Matt Gewain. Gail Gewain and her husband, Matt, started CST Sales roughly 25 years ago to provide carbon fiber and other composite materials for modelers and researchers.
Last summer, the couple sold the company and retired. Well, almost. Matt is still producing the tapered carbon-fiber fuselages once sold through CST. Now the booms are only available directly from Matt.
Three sizes with front-end diameters of 1.25, 1.5, and 1.625 inches are available. They weigh 52, 65, and 74 grams respectively. Their overall length is approximately 46 inches. Booms are $75 each and as many as six can be shipped in a single box for $20.
FF Team Selected
Mongolia will host the 2015 Free Flight World Championships this summer. Representing the US at this biennial contest will be F1A Towline Glider fliers Peter Barron, Jim Parker, and Shlomi Rozenzweig. F1B Wakefield Rubber fliers are Bill Booth, Blake Jensen, and Greg Simon. The F1C Power team members are Ron McBurnett, Mike Roberts, and Charlie Stiles.
Trimming a Folder
At the Nats last summer, I watched Charlie Stiles test-fly one of his F1C models: a folder. As the name implies, the wings fold to reduce drag during the five-second engine run then unfold for the glide. This model, by Ukrainian modeler and former World Champion Artem Babenko, was selected as a NFFS Model of the Year in 2011 and detailed in the Symposium Report that year.
Folded, the wingspan is approximately 4 feet; open, the wingspan doubles to 8 feet. When folded, the wing’s surface area is decreased and the airfoil is symmetrical, reducing drag and lift. Open, the wing section becomes a high-lift, undercambered airfoil to maximize glide time.
Trimming a power model of any type requires careful adjustment of the high-speed, powered climb and the transition to the slower glide phase. With a folder, this is complicated by the unfolding of the wing, which occurs a second or two after the motor stops and the model bunts from its vertical climb to horizontal.
Working with Charlie at this trim session was Steve Spence, multiple-time F1A Glider team member and son of the late Henry Spence, a four-time F1C team member. Steve had considerable experience working with his father flying folder models.
“With folders, a bad power pattern can mess up the unfolding,” Steve noted. Initial test flights are made with the wings held in the folded position by rubber bands. “Get the power pattern right, then start releasing the wings,” he said.[dingbat]
SOURCES:
NFFS
www.freeflight.org
Matt Gewain
[email protected]
AMA Plans Service
(800) 435-9262, ext. 507
www.modelaircraft.org/shop/plans
