Seems folks are overdure for a reality-check on the care and feeding of
their servos;
Specifications derived to advertise servo torque specifications utilize
a benchtop fixed voltage power supply which will supply current in
excess of the unit under test (UUT). Every test I have seen utilize such
a supply.
In real world servo applications the voltage varies, as seen by the
servo motor, as a direct result of the applied load.
The series-losses to achieve those great specifications put forth by the
OEM are limited to the length of the pigtail.
In the real world applications the factors that depress voltage as seen
by the servo motor include:
- series resistance from the lead type and gauge (expressed as awg)
- battery impedence (internal - NiMH has more resistance than NiCD)
- switch losses
- connector losses
With the above losses present as soon as you turn on your radio, The
voltage at the servo motor is already significantly lower than what is
present at the O/P of the battery pack, with no flight loads present.
Under maximum load, say during launch, whatever that may be for a given
airframe and type(s) of servos employed, the voltage seen at the hardest
working servo(s) will be the lowest. It could be 2/3 or less than what
the pack measures at it's O/P.
So now you have a servo attempting to hold a position under launch, on a
control surface whose surface area meets or already exceeds the torque
requirement as set out by the manufacturer's specifications.
It should be pretty clear that multiple servos under similar loads and
attempting to hold a commanded position will fail and move to some other
position.
Other things folks should start to realize about now is that if someone
with X airframe states that 10 degrees of camber will give the highest
launch, the next guy running the same airframe who compromized on his
set-up, or has a superior set-up, comparisons become futile.
This aircraft with an inferior electrical system will possibly require
12 degrees or more, in an attempt to achieve the same desireable launch
camber. I say "attempt", because at some point no matter what is
commanded, the servo may never achieve the desired positon due to
inadequate PWR at the load. The motor literally goes backwards against
current flow until the load seen be the control surface abates.
Remember...the OEM specifications are derived at a fixed voltage with
whatever the current the servo desires from the power supply to
determine maximum available torque (this commonly recognized measurement
is taken @ 1.0" from the output shaft to move a load a fixed distance).
No matter what, the servos in your aircraft will NEVER come close to this.
If your goal is to see the best flight performance from whatever
airframe you employ, don't scrimp on the electrical system.
Simon Van Leeuwen
RADIUS SYSTEMS
PnP SYSTEMS - The E-Harness of Choice
Cogito Ergo Zooom
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