dragon_not_fly
Heliman
Location: US
| Tail Motor Problems Take 4And the saga continues... I have finally reached 70 flights on the same Colco
Thunderbird II tail motor with my setup which I have in detailed described above.
At flight 70, as the flight progressed, I needed FULL right yaw trim tab and even
1/4 right rudder stick deflection from center line to keep the nose of the helicopter
pointed straight, this as a side note, was never necessary to do when the tail motor was younger(had less flights on it), even when flight pack voltage ran way down when the heli was stuck deep within GE(Ground Effect). So, I unbolted the tail motor and desoldered it from TREC's motor lead wires and then took a small
jewelers flat headed screw driver and proceeded to take the motor cap off. I was
surprised to find what I did; what I found was no brush pads! The brush pads as
I call them, are the gun metal like blocks that rub agaisnt the commutator of the
motor to make electrical contact. They had completely worn down to nothing
and the only thing that was completing the electrical circuit was the brush pad
springs as I call them. In side the motor cap there was a fine graphite looking
dust or powder in which this powder was originally the brush pads! One of the
brush pad springs even had a small "bubble" on the outside of it do to heating.
I noticed at around flight number 57 a need for increasing amount of right yaw
trim tab was necessary as the flight progressed to keep the nose straight. At flight
number 60, I needed also some right rudder to help in keeping the nose pointed
straight. I am estimating that this tail motor only has may be 10 to 15 at most
flights left on it before even the brush pad springs "burn through" leaving ab-
solutely nothing to make electrical contact with the commutator anymore.
So, using the Colco Thunderbird II tail motor and my setup I am estimating
that I can only get up to 80 flights on one tail motor before it will completely
fail. I wondered why a tail motor; that is, a brushed D/C electric motor loses
power over time, in power I mean, RPM. Apparently, a brushed motor's per-
formance really boils down to just how well the brushes make contact with the commutator and how much of the brushes are left to make contact! As the motor
wears over time, more trim is necessary to compensate for decreased power
and hence less RPM of the motor. By increasing trim you increase the amount
of volts hitting those brushes which will help to increase RPM and power, but
will actually reduce the life of the brushes! It is a vicious cycle. There is a point
that I have found where not even FULL right yaw trim tab is enough to compen-
sate for decreased RPM such that even right rudder is needed to help keep the
nose pointing straight. When you reach this point, the REVO-MIX(revolutions
mix) is out far enough such that the rudder trim tab as I say is out of band; that is,
even FULL right yaw trim tab is not enough to fight agaisnt main rotor torque.
With TREC I have two options to bring the rudder trim tabs back into effectiveness; that is, the rudder trim tab is sufficient with adjustment to help
maintain correct/sufficient revo-mix, I can either increase the throttle curve or
increase the mix gain for the LiPO cell count in question. The problem with this
is that by increasing the throttle curve you increase voltage that is being applied
to the tail motor brushes which increases current while also increasing heating.
These negative side effects will decrease the life of the tiny motor brushes. I
decided not to change a single parameter within TREC since I knew this would
aggravate tail motor brush destruction, but would just learn to fly while holding
in rudder to compensate for left nose drift while even in a nose in hover! As a
side note, holding all other things constant, when tail motor RPM on a fixed
pitch tail rotor decreases, the amount of air that it pushes or blows if it is on the
right side of the helicopter if facing the rear of the heli with a main rotor that spins
clockwise if facing the top of the heli, the amount of air that it pushes decreases.
If the amount of air that it pushes decreases, the less force it will exert or push
against the tail of the helicopter where torque direction is a 180 degree opposite
arc to tail motor/rotor force while facing the rear of the helicopter. So, where to now? Well, I tried in vain in repairing the tail motor brushes in the helicopter's 70 flight tail motor with no success, so I placed a brand new one on and have started a new log of how long this one will last and at which flight the rudder trim tab goes out of band holding all other things constant. I always have a trick up my sleeve though, see in the next post.... |
IF,
