Like It Hot
Troubleshooting a hot tail servo
Text & photos by Jim Ryan
This topic came up recently in a conversation with a friend whom I’ve
been helping with his first “large” (600-class) scale project. He chose an
Align T-Rex 600 as the basis for his build, and he’s been logging time in
the stock pod and boom configuration while he builds his scale body. He
reported that the tail servo was getting extremely hot (144ºF, as measured
with a laser infrared thermometer), and he was justifiably concerned that a
tail-servo failure might be imminent.
Troubleshoo Ting Mechanics
For any mechanical problem, it’s important to follow a logical troubleshooting sequence, starting with the most obvious and easily corrected
item. In the classic Billy Bob Thornton film Sling Blade, the owner of the
Keven Schauz shows off Compass Model’s out
standing eXo 500 at the 2016 IRCHA Jamboree.
Flying 3D imposes heavy demands on the tail servo.
A hot tail servo can be a sign of impending failure,
but a systematic troubleshooting program can cure
the problem before disaster strikes.
Hot tail servos are a topic that comes up from time
to time, and I’d like to try and shed some light on the
causes and fixes for this condition. Let me start by
saying that servos should not get hot, and if they do,
there’s a problem that needs to be fixed. Aluminum
servo cases, sometimes with finned heat sinks, have
become popular, but these should only be letting the
servo run cooler. We’ve used plastic cases for decades,
and we weren’t melting servos.
lawnmower shop asks the lead character, Karl, to see why a balky rototiller
refuses to start. Karl glances in the tank and laconically remarks, “It ain’t got
no gas in it.” Always check the easy stuff first.
For any servo problem, the first and most obvious issue is friction or
binding. Disconnect the pushrod at the servo and see if it slides freely
throughout the range of travel. Even if it seems free enough, bear in mind
that friction can increase in flight under the resistance of main rotor torque
or from a vibrating pushrod binding in the tail-boom guides. Make sure the
pushrod guides are properly aligned. Likewise, make sure the pitch-control
arm pivots freely and that the pitch-control slider isn’t dragging. A tiny drop
of oil on the slider should be part of your regular maintenance program.
If you’re confident that the control linkages aren’t binding, the next thing
to check is that the flybarless unit or tail servo isn’t trying to run the pitch-control slider past its limit of travel. Depending on your flybarless setup or
During setup for most flybarless units and tail gyros, you can set the limits of tail
travel. Make sure the pitch control slider stops a couple of millimeters short of the
rotor hub or gear case. If the servo is trying to drive the pitch slider too far, it will
stall and heat up quickly. Always use the innermost hole on the tailservo arm that
will still allow a full range of travel.