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Mark Burton
Joined: 10 Jan 2006
Posts: 74
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 Posted: Fri Mar 20, 2009 1:56 am Post subject: Electric VP Prop Reliablity |
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Folks,
What follows is a general comment and is not related to any specific failure of an electric VP prop system.
Jos is right to question the quality of the electrics (motor/switches/diodes/wiring, etc.) on the Woodcomp SR3000. However, in my opinion, the SR3000 is not substantially different in quality compared to the other low-cost electric VP props. In fact, I would rate another low-cost prop as having a far worse reliability record than the SR3000 (motor burnouts, intermittent operation, etc.) I also have experience of yet another manufacturer's products that have exhibited electrical failures. So although Woodcomp could almost certainly do better, they are not alone in this regard.
This potential for failure of the propeller system must be acknowledged by the pilot and they should be mentally prepared for that eventuality. So, whatever brand of VP prop you have (cheap or expensive), be prepared for it to stop functioning at any time.
To reduce the chance of that happening, inspect the prop/hub/wiring regularly. Ensure that the brushes are in good condition (slip rings clean) and that the pitch change mechanism operates freely (but not with excessive backlash) throughout the normal pitch range.
Most importantly, check that the range of pitch movement is such that the aircraft can still fly with the pitch stuck at any position in that range.
With regard to hard pitch stops that come into effect if a switch fails, it would be sensible to actually test that the fine pitch hard stop is in a position that will help save your hide. I suggest that having determined the correct position of the electrical switch, measure the static RPM with the pitch on the limit and then (carefully) override the switch and move the pitch to the hard limit (use a low voltage perhaps to move the pitch slowly to the stop?) You then run the engine again and compare the static RPM with the static RPM you achieved with the pitch on the electrical limit. You can decide on what is an acceptable rise in RPM. Probably, not much given that the electrical limit was likely to be set close to the finest position acceptable for safe operation.
Finally, make sure that the pilot (you) fully understands how the pitch is
controlled (controller modes, manual operation, etc.) and what the
appropriate controller settings are for the various flight scenarios.
As part of the pre-flight self-briefing before every flight, acknowledge the fact that the pitch may freeze just as you acknowledge the fact that the donkey may quit at any time.
Regards,
Mark
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Mark Burton
Joined: 10 Jan 2006
Posts: 74
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| Posted: Fri Mar 20, 2009 5:01 am Post subject: Re: Electric VP Prop Reliablity |
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In my previous message, I talked about checking that the fine pitch
hard stop was in a sensible position. I forgot to mention that if you want to do that check, be very careful not to damage the switches in the hub because with some hub designs (not all) if you overrun the limit switches, something will break (most likely the switch). So, before you attempt such a test, look at the switches and how they are activated and convince yourself that when the pitch moves to the hard limit, nothing will be damaged.
You may feel that this test is not required as the manufacturer will have set the hard stop to a suitable position. Personally, I no longer believe prop manufacturers when they say they have set the limit switches to the correct position. I am similarly sceptical about the position of hard limits.
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