mark.bitterlich(at)navy.m
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 Posted: Thu May 19, 2016 7:09 am Post subject: [Non-DoD Source] Questions for the Yak 55 operators. |
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The SK10 (not SD10) is a Permanent Magnet Generator (Alternator really since it outputs AC and not DC) that is totally unregulated, meaning that the faster that it spins the more voltage comes out. That voltage is fed to an external module that includes a rectifier and voltage regulator. The voltage regulator is of the type that uses a voltage reference and a NPN PASS transistor. This voltage can be adjusted within a small range uses a multi-turn potentiometer. It can also include an electrolytic filter capacitor and an over-voltage protect circuit that is stand alone.
Personally, I do not recommend this unit unless the owner is singularly concerned with weight and nothing else, and realizes the risks inherited with this design, that also get passed on to the NEXT owner.
Designs that use a pass transistor such as this one, often suffer from failure where that transistor SHORTS instead of OPENS. When this happens, (not "if", but "when"), unregulated voltage is then supplied directly to the main power rail in the aircraft. A sort of slow motion catastrophe then occurs.
Depending on the condition of the battery at the time, the over-voltage causes excessive charging current. Typically YAK and CJ owners use Gel Cell batteries. The over-charge causes them to vent gas, which over time causes the battery to lose capacity and no longer hold down the over-voltage being supplied, which then as the battery charge current decreases causes the aircraft rail voltage to go UP. Depending on how tolerant your avionics are, the next thing that can happen is failed radios, failed transponders, etc. This is not supposition, I have seen it happen on two aircraft equipped with this design.
The designer compensated and offered an optional over-voltage kit. It may now come standard, I am not sure. This kit was a "crow-bar" design, in that it had a voltage trigger set point, typically around 30-32 volts, where when the voltage from the system reached this point, a short circuit was presented to a circuit breaker, when then opened, removing power to a relay, and when the relay opened voltage from the whole system was removed from the aircraft. The PMG continued to generate power, it just was no longer applied to the aircraft.
Another slow motion event common to this device is simply the aircraft battery getting old and losing capacity over time. All batteries (more or less) do this on their own. But keep in mind that there are things in the aircraft that have current surges. If you have your transponder, your radio transmitting, and just happen to turn on Pitot Heat (even by accident) with a weak battery, you have just exceeded the 10 amp limitation of the SK-10 design. The result is typically that the PASS transistor will short, and you are off to the races with that event previously described.
In addition to the above, the SK-10 actually has moving parts that are actually unprotected and exposed in the engine compartment.
It is a unique and somewhat odd design for aircraft which has one basic and sole purpose. Weight savings. That is IT. For the few pounds added, the SK-35 and LS1A regulator is a MUCH better choice, extremely reliable and with 35 amps of power generation that will pretty much satisfy any need, and if that is not enough, B&C also makes an SK-45.
The SK-10 itself is well designed. The voltage regulator and over-voltage protect, and the need to add filter capacitor, etc., is not the best situation to deal with and if I had one in my aircraft, it would be the first thing I would replace.
Of course I told you all this already John, but here it is again.
Good Luck,
Mark
p.s. For multiple radios and transponders burnt up, contact Hubie Tolson and ask about his Super SU-26.
p.p.s. It now has an SK-35 and LS1A
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