Jos,
I am still very sad and sorry about your loss of your plane. You have been a
great contributor to the Europa builder community and I do appreciate this
a lot.
I still try to figure out what exactly could have contributed to your
accident. So far the only creditable information what happened came from you
and it is consistent. What I believe you stated is:
1. You took of with a high power setting
2. A little later on climb out the engine started to rev up more but you
lost forward trust. This, you explained later, was caused by the prop going
to super fine mode.
This is defiantly something I can not understand what went wrong on the
prop side. You pointed towards the limit switches and the diodes. However, I
can't understand here the failure mode. I still assume that, at the initial
climb out, everything was ok and the blades where somewhere in the middle
( means permissible)operating travel range. This means that all the limit
switches had been depressed ( at least the ones from the outer and the
middle ring). Now I can go through all combinations of failure modes on the
diodes and switches and I can not understand how the DC motor got a command
to travel to fine. The worst thing what could have happened is that the
switch on the outer ring would have failed to open, which means the prop
would have not been able anymore to travel to coarse position but the prop
went into fine.
Why??
The constant speed controller should have seen a control deviation with the
increased motor speed and therefore command to go to course but not to fine.
I could see 3 single failure points causing this.
1st failure mode = "loss of feedback" (motor RPM). That could have set the
controller to command less pitch (means travel to fine).
The loss of feedback could in a somehow failsafe control scheme could have
been detected by a dual mean of feedback signal (I doubt that you had this)
or (but less reliable) with an control algorithm to detect an impossible
amount of change in RPM over a short interval of time. I don't know if any
countermeasures are implemented.Do you know?
2nd A short in the driving output transistor or FET. Typically, if a
transistor or FET fails, it will short circuit. This means the motor will
continuously commanded to go in a specific direction. If that would have
happened then only the fine stop switch would have stopped the motor. You
also could detect this in your controller and counteract (disable output)
with a couple of differently methods.
3rd controller failure. Frozen microprocessor (controller) program. Here
would only help redundancy again.
All those failure modes are not part of the actual prop but the controlling
circuit which did not came from Woodcomp.
All those problems could have still potential been taken care of (if you
would have had enough time) by a kill switch and transfer to an emergency
manual command switch. It is my understanding that you had a 3rd party
controller. Did your installation still provided this emergency mode?
I could see, only if you had two failures accumulated simultaneously by one
of the 3 above failure modes AND the micro switch on the outer ring failed
or the motor failed you would not have been able to recover.
Another(4th) remote possibility could have been that the DC motor might
came completely detached from the 3 worm gears so the prop had no counter
force and drifted (freewheeled) towards fine pitch setting.
So with all this I would like to ask the question:
Do you have a report by an independent person or agency stating what exactly
failed or do you know exactly what failed?
Best regards
Michael Grass
(Yes I bought also a Woodcomp and would just like to get a better
understanding what went wrong so I can plan accordingly)
----- Original Message -----
From: "josok" <josok-e@ukolo.fi>
Sent: 2009-03-18 16:36
Subject: Re: Europa-List: Woodcomp Propellers
>
> Just for the record: 1: The blades did not go in reverse but in superfine
> pitch. 2: This is the first time that a mechanical stop is ever mentioned.
> Woodcomp certainly never even pointed at the possibility. IMHO, if you
> power the motor directly, you can spin the blades all around on ANY
> Woodcomp. Until the motor burns out, which will be very quickly. As far as
> i know only MT manufactures electrical variable speed propellers with
> mechanical endstops. 3: The problems i encountered were because of non
> suitable microswitches, diode and motor. The same hardware can be found in
> any Woodcomp in flight adjustables, wether those are straight, reverseble
> or sail.
>
> It's everybodies (and in the UK the LAA's) choice of how much risk you
> want to take. I can imagine it's difficult to scrap something you layed
> out your money for. I realize it's bad news for many happy pilots. I also
> would not forgive myself if i would not take this stand. Now everybody may
> continue to throw stones at the bad news prophet :-)
>
> Regards,
>
> Jos Okhuijsen
>
>
> Visit - www.EuropaOwners.org
>
>
>
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