Model Forum / Radio Controlled / Helicopters / August 2003

Hey guys,

I have had about four incidents over the last couple years where the
tail suddenly and violently let go.  Two of those resulted in crashes.
I first blamed my trusty Telebee and replaced it with a GY401.  But
about a year later it happened again.  And recently I had a couple
obvious tail swings that recovered but scared the crap outa me!  

After talking it over with a friend who traced his similar problems on
an ECO8 to ESD, I decided to see if I could measure any static charge
on my Raptor.  Well, I remembered that I have an antique electrostatic
voltmeter and this evening I tried some experiments.  I attached it to
the engine and to the mainshaft and twirled the tail rotor by hand.
WOW!  At about 60 RPM at the tail, the voltmeter read about 100V.  By
twirling the tail rotor a little faster I could easily pin the meter
at 120V.  The relative humidity in the basement is about 50% right
now.  By extrapolation, that probably means I can easily generate
thousands of volts in flight!  All that is needed is a spark gap.  And
the Raptor has several.  The most notable is the small gap between the
tail boom rear support screws and the boom itself.  In this case, I'm
wondering if it isn't a better idea to leave them long and let them
intentionally contact the boom.  

I tried another experiment.  My antenna runs from the skid to the tail
fin and parallel to the boom.  I connected the voltmeter to the engine
and the antenna and again, by twirling the tail rotor by hand I could
generate about 100V between the antenna and the engine.  A glitch (or
worse) waiting to happen if the receiver ever discharges to anything!
I got similar readings between the engine and the tail pushrod.  And
about double that between the mainshaft and the tail shaft although
there is probably not an easy discharge path there....

I am going to try simply grounding the boom to the engine and see if
that cuts the charge down by enough to remove the threat of a spark.
If not, I may end up grounding the mainshaft and tail shaft either
through the bearings or with little wire wipers.  

I know the issue of static discharge on helis with belt driven tails
is a well known problem, but I'm wondring how many people actually
deal with it.  

Paul