josok wrote:
> Why did the prop run into super fine pitch?
Thanks for the information, but it still doesn't explain why the prop
went into super fine pitch in the first place. Was it commanded by the
prop controller?
I don't have a Woodcomp propellor yet, but it appears to me that control
of the prop pitch motor is some sort of feedback loop. In normal
operation, I assume the prop controller won't fling the pitch back and
forth between the end limits, but make small changes, evaluate the
result, and not go to super fine if the RPM is already very high.
Apart from this, I may assume that this system has been working
correctly in the past, and that going to super fine pitch and high RPM
was not part of your normal flying procedure. A burnt out motor can not
be the cause of going to super fine pitch, as a burnt out motor does not
do anything anymore, so I think something first powered the motor to
super fine pitch before the motor burned out. Maybe the motor burned out
because of this untintended move to super fine pitch. If this is the
case, the burning out of the motor was not the cause, but a result of
something else going wrong.
> To resume: !st Endstops did not work, Woodcomp supplied component
> failure. 2nd: Motor burnt
The end stops, if they were there, would have been an *additional*
safety, but not for throwing the pitch against its limits over and over
again.
> The diode across the
> switches is of a 5 A type, where the pitch motor can draw + 15A.
Diodes over switches are not there to let the main current through, but
to extinguish the opposite voltage that appears when you suddenly take
the power away from a coil. Since the motor is a coil, it will create a
spark everytime when you disconnect it, and the diode is there to
extinguish it. There is no reason why the diode should have the same amp
rating as the current demand of the pitch motor. 5 Amps sound generously
to me for this task, I bet 1 amp would have been fine as well.
> The
> motor, for which i could not find any specifications, looks to be a 6
> V type.
The rating is something that the manufacturer comes up with, for
continuous use. There is no physical difference between a 12 Volt or 6
Volts motor. It is just a rating that describes the maximum voltage for
contineous usage. The same motor, with better cooling, or for shorter
durations, can handle a higher voltage.
It is fully acceptable that Woodcomp used a 6 Volt motor for the purpose
of being used in short bursts, in a very efficiently cooled area.
> As stated before: There is no mechanical endstop in the SR 3000
> (Thanks David for the addition) Not in any Woodcomp 3000, being
> "normal", reverse or feathering. Proper working of the fine endstop
> relies only on proper working of the electrical circuit. This
> circuit, contradictory what David states, is NOT double. it relies on
> a single, underrated diode!. There are 2 fine endstops microswitches,
> probably because their rating is insufficient. As any electrical
> engineer can tell you, doubling contacts does not double the
> switching capacity.
Are they wired in series or in parallel? You are suggesting that they
are connected in parallel, and if so, I agree that this is a very dumb
thing. However, I hate not to have a Woodcomp propellor here, but I'm
betting that these switches are wired in series. In this case, if one
fails, you still have the other one.
> There will be NO indication that either the microswitches or the
> diode has failed, until also the motor burns and you are in superfine
> pitch at 500 feet and at 50 knots :-(
Still, I wonder whether it is normal operation to rely on the
microswitches to stop the pitch motor, or that the propellor controller
is supposed to never try to command the propellor against its safety limits.
Even without mechanical end stops, diodes, and microswitches, the system
would have had to function normally. Until the moment that somewhere,
something catastrophically started to power up the pitch motor.
In the reverse circuit, there are no safety microswitches for superfine,
because the prop is intentionally moved through its super fine setting.
It is therefore likely that a failure occured in the reverse circuit.
Something tried to reverse the prop, the controller was fighting this,
with the result that the CB popped and the prop was stuck in super fine.
Since you made some modifications in the reverse circuit, it is not
really fair to blame Woodcomp for the catastrophic result.
> If you keep flying with a SR 3000: Please add a fine endstop check to
> you checklist for every take-off.
This sounds to me as a wise thing to do. I will for sure add it to my
check list.
Thanks!
--
Frans Veldman
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