josok wrote:
> [quote:f042f8651c]You first state that the lack of an end-stop in the
> Woodcomp propellor was the cause. Later on, you blame diodes and
> relays of inferior quality. [/quote:f042f8651c]
>
> Sorry, no. The fine pitch endstop is formed by two microswitches,
> parelled by one diode. Never mentioned a relay.
You are right. My slip up.
> Iv've tried to explain that, let's try again, real slow now, for the
> technically handicapped amongst us.
>
> Say you fly at 100 knots, and the controller has adjusted the rpm, as
> you have demanded, to 5000 rpm. Now, if you reduce throttle slightly
> the controller reduces the pitch to maintain the 5000 rpm that you
> asked for. Right?
So, let's assume for the moment I'm technically handicapped, and let me
ask just to be sure I don't misunderstand you:
If you throttle back from take-off power to cruise climb, the controller
tries to move the blades to the finest pitch possible, and it is the
task of the endswitches to prevent the blades from moving even further?
With other words, with every throttle move, the blades will always
travel to one of the endstops?
If this is the case, I can understand how a failed endstop immediately
caused the blades to travel to a non flyable position.
As I'm technically handicapped, I picture the working of the prop and
controller as follows:
Say you fly at 100 knots, and the controller has adjusted the rpm, as
you have demanded, to 5000 rpm. Now, if you reduce throttle slightly,
the controller reduces the pitch somewhat *without fully traveling to
the end stop*, measure the rpm again, evaluate if still below the target
rpm, reducing the pitch a little bit more, evaluate again, and stop when
the engine again is at 5000 rpm. The end switch has not been used in
this scenario, because the prop went never to its finest position in the
first place.
Even with the end switches shorted, you won't notice anything.
Until something else goes wrong...
For a failure, you need an additional factor, something that tries to
move the blades to an even finer pitch *while the rpm is already above
its target*.
I'm still looking for a scenario where the rpm can rise to 6200 rpm, the
blades are in super fine, and something is feeding power to the pitch
motor to move the blades even further. What is this "something". Once
again: even with all micro-switches shorted, the pitch motor won't run,
because power is fed only to these switches if something (the
controller, the reverse pitch relay) is sending power to the pitch motor
with the purpose of reducing the pitch. The end switches won't see any
power, shorted or not, unless something tries to move the pitch motor
even further.
> It will keep reducing the pitch at any further
> reducing of throttle, to keep up the RPM.
Past the desired RPM?
> [quote:f042f8651c]Some of us are intrigued by a message from you,
> written the 19th of may 2007, in which you wrote: "For reason of
> simplicity, exactly in a high stress situation (most of my landings
> will be :-) ) managed to do away with the change-mode switch. The
> 3-pole reverse-unreverse switch now operates a relay, that changes
> the mode. This latched and warning labeled switch is IMHO security
> enough against accidental operation. [/quote:f042f8651c]
>
> Some of us? Intrigued? I fail to see where this connects to this
> thread other then do distract from the facts.
Let me spell it out: In a normal situation, when powering back from
take-off power to cruise power, the end/stops/limit switches never come
into play, because the controller cuts the power to the pitch motor long
before the fully finest position is reached.
In order for the limit switches to show their failure, something has to
keep sending power to the prop circuit, now putting all bets on the
limit switches to prevent the prop from going past fully fine. Once
again: I think it is not common practice to cruise along with the prop
in fully fine, so the end switches should never have been exercised. But
something tried to move the blades to fully fine, and apparently even
past fully fine.
Now, as I studied the schematics, there are only two power sources for
the prop pitch motor: the controller, and the reverse circuitry. One of
these circuits had to be inadvertently sending power to the pitch motor,
while the rpm was already way up.
By definition, the reverse circuitry has to bypass the microswitches,
because its purpose is to intentionally move the blades past fully fine.
That makes the reverse circuitry suspect number one.
If I then read that you have modified the reverse circuit, it is not
difficult to start to wonder if this has had something to do with it.
> Safety bypass? Please! If "my" relay would fail, it would either blow
> the CB immediatly, leaving the pitch unchanged, or simply leave the
> pitch motor without power, again leaving the pitch unchanged.
If I recall correctly: on the Woodcomp there is a third slip ring, for
adding power to move the prop beyond its normal flying operating range.
This is used for reverse, or feathering. You need to put power on this
ring to feather or go to reverse.
This third ring is not connected to the controller, so the controller is
no longer suspect here.
Normally you put power on this third ring with a safety latched switch.
It appears from your message that you replaced the function of this
switch by a relay.
In this case, an inadvertently close of the relay contacts would
catastrophically power the pitch motor, bypass the end limit switches by
its very nature, keep power on the pitch motor until it finally gives up
the ghost. The controller, who sees the rpm going up, tries frantically
to move the pitch motor the other way via the other slip rings. This
will for sure pop the circuit braker. And will keep popping the circuit
breaker as long as rpm is too high, and the third slip ring remains
powered up.
A failure of the reverse switch/relay would exactly give you the
catastrophic results that you described. And the limit switches do not
even have to fail for this scenario to happen exactly as described. This
is the only "one-failure" scenario that I can think off, all other
scenario's require multiple failure points are not impossible, but less
likely.
> actual switching was done by a 4 pole 3 way switch. That switch could
> only do what it was supposed to do. I've shown the circuit to
> experts, and i have had the actual wiring checked by my inspector.
> His comment was "Better standard of wiring then seen sofar on
> experimentals"
I do not suspect the wiring failed, but that the relay failed. Relais do
fail sometimes.
I would love to see the schematic, to see what you have done. Then we
can see which components need to fail to cause exactly the same results
as you experienced.
I'm sure there will be multiple scenario's.
However, all these scenario's would have in common that something else
failed before the failed(?) end switches came into play.
> You are classifying a pretty fail-safe setup as a safety bypass,
> without even knowing the details.
I would love to know the details.
The whole problem is that you blame the end switches, and lack of
physical end stops, completely ignoring the fact that something must
first inadvertently power up the pitch motor to its fully finest pitch
before these end limits would come into the picture.
> I doubt very much if this is
> caused by confusion or else. You seem to be constructing things. It's
> a pity.
I'm just trying to solve the puzzle. My own safety depends on it.
If I would just walk away from any part that has been involved in an
accident somewhere, I could't build an airplane. It is necessary to
construct how things went wrong, before you can really assess whether
the part is safe or unsafe. Is a Rotax engine unsafe, because someone
crashed with it? Dunno, unless I get a clear description of what was the
cause. Same with the Woodcomp prop.
Just be assured that my intention is not to blame you in anyway. In
fact, I never got involved in the discussion about your accident. But
now you so openly blame Woodcomp and completely ignoring the fact that
some "non-woodcomp-part" had to be feeding power to the pitch motor for
the failure to occur, it is our (Europa community) mutual interest to
put things into perspective.
--
Frans Veldman
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