Electric Flight - September 2013

Remembering the basics

checking flight current and bearing
temperatures can go a long way toward heading
off issues before they become serious problems,
but these tricks don’t take the place of regular
visual inspection. Worn control links, loose
screws, and worn-out head dampers won’t
show up in the data logs, and won’t get hot, but
they can still cost you degraded performance
and perhaps a painful and avoidable crash.
another benefit of flybarless heads is that
inspection and maintenance are so much
simpler, but you still have to inspect all linkages.
Finally, don’t forget the servos: modern digital
servos are mechanical marvels, but they don’t
last forever. check the servos periodically to
make sure they’re running smoothly, centering
repeatedly, and showing minimal gear lash.

Museum Scale H- 34

Darrell Sprayberry of Dalton, GA has earned a reputation as one of the finest scale helicopter modelers in the world, and at this year’s Toledo Weak Signals Expo he placed first in the Helicopter class with his fantastic scratch-built H- 34 Choctaw.

Also known as the Sikorsky S- 58, the H- 34 is an iconic design–perhaps the best piston-engine utility helicopter in the world before the arrival of turbine helis like the UH-1 Huey. Sadly, models of this outstanding helicopter are very rare, owing in part to the difficulty of getting it to balance with nitro or gasoline power. Fortunately, this is no problem with electric power, where the battery pack can be placed well forward.

Darrell’s - 34 is a masterpiece, with outstanding design engineering capped off by awesome scale details. He started the build in 2011, and the huge fuselage plug attracted lots of attention at that year’s IRCHA Jamboree. Since retiring last year he has focused more heavily on the project, and the result is really something to see. I spent some time talking with Darrell about this entirely original design and our wide-ranging discussion provided an interesting glimpse into the active mind of a world-class modeler.

Asked what scale the - 34 was built to, Darrell laughed and
said, “Seven feet long.” He went on to explain, “What I found with
these big helis is that seven feet is the magic number. Any bigger
than that, and they get really hard to transport. For me the main
requirement is that it has to fit in the back of my Jeep.”
Expanding on this common sense approach, Darrell remarked,
“I’m not big on modeling with a computer. I have a 4x8 white bench,
and I’ll lay the hardware out to get an idea of how things will fit
together. It helps to see things actual size.”

This year’s Toledo Expo featured some outstanding helicopters, but without question the class of the field was Darrell Sprayberry’s impressive scratch-built Sikorsky H- 34. This big machine was two years in the making and features a wealth of ingenious details.

Like any great scale model, the closer you look, the more you see. The H- 34 features a scale four-bladed rotor head and perfectly understated surface detailing.

The epoxy fiberglass fuselage was laid up in custom molds pulled off Darrell’s hand-carved plug. The body includes functional details like sliding doors and windows, and clamshell doors in the nose provide access to the battery packs.

The mechanics are also scratch-built and designed to fit inside the “doghouse” on top of the fuselage so that the scale cabin is completely open. The four-blade head has 880mm blades, for a total rotor span of 2.25 meters (89 inches). Hitec HS-7940TH high-voltage servos and a JR 11X radio provide the flight control.

Darrell flies electric helis exclusively, and he’s a big believer in the
outstanding KDE motors, so he chose a 700XF-495 for power. The
controller is a Castle Creations Ice-120HV, and the 12S battery pack
is made up of two 8000-6S packs inside the magnetic clamshell
doors in the nose. Even with the 8000-12S battery, the big machine
still requires four pounds of lead to balance, and all-up weight is “in
the high 30s.”
Head speed will be in the 1,000rpm range, but Darrell noted,
“I always let the helicopter choose the exact head speed. Any
given heli has a particular speed where it’s happiest.” Like many
full-scale helicopters, the H- 34 has a proportionally smaller tail
rotor diameter than is the norm for model helis. Because of this
and because he likes to run realistically low head speeds, Darrell
uses a higher tail ratio than is typical for pod and boom helis. The
H- 34’s torque tube-driven tail has a gear ratio of 6:1, so even at
a low 1,000rpm head speed, the 4-blade tail rotor is still turning
6,000rpm, providing plenty of holding power.

What’s next? Darrell has acquired the molds and tooling for IndyRC’s outstanding 1/7-scale AH- 64 Apache and his company, Unique Aircraft ( uniqueaircraft.com), will offer the Apache, Choctaw and other high-quality products to scale heli enthusiasts. Given Darrell’s fertile mind, you can count on some amazing things to come.