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Step One: Choosing the
Servo
Servo
choices are central to any thought about linkages. I was
told once that you can never have enough servo for the job.
Many factors play a role in servo selection.
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Troy
Newman w/Supreme |
Speed is
good, but more critical for us in pattern modeling are power
(torque) and centering or precision. Many of the servos on
the market today have plenty of power, but their centering
leaves something to be desired. Digital servos are best at
centering, as a general rule. However, some brands just
don’t center as well as others.
Linkage
Matters
We
sometimes spend lots of money to get the latest and greatest
digital servo and then throw a piece of music wire at the
linkage, with a Z-bend in it and a sloppy, oversized servo
wheel hole. When you buy the right servo for the task, you
need to consider the linkage that connects your fingers via
radio waves to the aileron, rudder, elevator, and throttle.
Linkages need to be stiff, and friction free. You need the
connections to be tight and free of any slop.
Next is a
look at the geometry involved in how the radio and servos
work. These principles are critical to good linkage setup.
Stiff lines
and slop-free connections are important. There are many
products offered on the market as pushrods. I'd recommend at
least a 4-40 pushrod for use in pattern models. This would
include Dubro's 4-40 metal pushrods. They're the cheapest
and they work. They're not the lightest but they'll move
the surface without breaking or failing under the loads we
apply.
On the
other hand, weight's a big factor. Dubro 4-40 pushrods
tend to get heavy. And since pattern flyers are always
looking to save weight, the carbon fiber pushrod systems
work the best. They're stiffer, lighter, and easier to deal
with than old metal rods.
The most
readily available carbon fiber rods are made by Central
Hobbies. These have titanium ends and are very light and
stiff. Remember: the pushrods are your connection between
fingers and control surface. So I choose Central Hobbies'
carbon fiber pushrods. They’re easy to use, and I've never
had one fail. In fact, I've never heard of one failing or
breaking, when it was correctly assembled.
To check
for slop or bend in a particular linkage, turn the radio on
with that servo connected and then apply a small amount of
force to the control surface. Wiggle the control back and
forth, watching both the servo and linkage. There'll almost
always be something in terms of gear slop or some give.
You'll want to minimize this as much as possible.
Nylon-geared servos are one solution for gear slop.
Likewise, a rigid servo tray or mounting rails are
important.
Next: Clevis Links & Ball Links
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