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Old 01-22-2023, 01:21 PM
jrtens's Avatar
jrtens jrtens is offline
Join Date: Apr 2018
Location: Seattle
Posts: 293

See the Rotax 912iS Heavy MM, chapter 24-20-00 which you can download from
2019 12iS SLSA
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Old 01-22-2023, 01:48 PM
thiggins thiggins is offline
Join Date: Mar 2020
Location: Saluda,NC
Posts: 196
Donation 2023 paid
ASEL INST COMM Tail-wheeler
RV-12iS builder (Empennage complete, Wings complete!, Fuselage well under way !, finish kit ordered, Garmin avionics ordered, engine ordered, shop out of order )
Got a hangar, and moved some large pieces in!
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Old 01-22-2023, 06:22 PM
subpar_bucker subpar_bucker is offline
Join Date: Oct 2016
Location: Cranberry Twp, PA
Posts: 73

Thanks all!
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Old 01-23-2023, 06:47 AM
BobbyLucas's Avatar
BobbyLucas BobbyLucas is offline
Join Date: Oct 2019
Location: Belleville, MI
Posts: 401

I'm curious... Since generators can operate in 'reverse,' acting as motors and therefore as generator-starters, why does the engine have a conventional starter motor?
Bobby Steinmetz
Van Buren TWP, MI
RV-12iS #121228
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Old 01-23-2023, 07:27 AM
Piper J3's Avatar
Piper J3 Piper J3 is offline
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 2,856

Rotax has alternator built into field-windings on back of engine case. Not the same as Tesla. My Prius however, doesn't have starter motor. Main drive motor (dynamic brake) back-drives the Internal Combustion Engine (ICE) to start it. ICE starts / stops dozens of times when driving....
Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 830

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father - CFI) - USAAF 1st Lt. Captain B-17H
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Old 01-23-2023, 09:30 AM
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BobbyLucas BobbyLucas is offline
Join Date: Oct 2019
Location: Belleville, MI
Posts: 401

So Lockwood just told me that they are dynamos feeding AC into combined rectifier/voltage regulators. But when I google dynamo it says they produce DC, not AC.

The article below says magnetos (AC), dynamos (DC), and alternators (AC) are all types of generators. If our engines are producing AC, then the conclusion is that they must use either magnetos or alternators. Lockwood said there are no commutators, which would mean they must be magnetos... My suspicion is that they are dynamos feeding DC to the rectifier-regulators, and the Lockwood tech just got that detail wrong. If that's the case, then why use a rectifier at all? To protect against reverse polarity?

(Sorry if this is thread hijacking, but I hope it's pertinent)
Bobby Steinmetz
Van Buren TWP, MI
RV-12iS #121228
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Old 01-23-2023, 09:57 AM
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rcarsey rcarsey is offline
Join Date: Jul 2012
Location: North Brunswick, NJ
Posts: 406

Originally Posted by subpar_bucker View Post
I have a dumb question prompted by a question I was asked by a friend: the 912iS engine's electrical system is powered by 2 generators correct? It sounds like it is possible to install external alternators if desired based upon this video:
Yes, See the Rotax Installation Manual, Chapter 24-00-00, Page 17. Page 33 has a schematic of how you'd integrate it with the rest of your aircraft. Its belt-driven and completely separate from the Fuse Box.. so you can think of it as an alternator in a traditional aircraft engine.

I think people use the terms generator and alternator a little loosely. See here for an explanation. Someone smarter than me will have to say if the Rotax internal "generators" are indeed true "generators". All I know is that they output AC and the external rectifiers mounted on the Fuse Box convert it to DC.
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Rob Carsey, Winfield Park, NJ
RV-12iS (N713) / Completed 12/2020 / 350hrs and counting
APRS Track or ADSBExchange Track
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Old 01-23-2023, 11:12 AM
h&jeuropa h&jeuropa is offline
Join Date: Jun 2008
Location: Kalamazoo, MI
Posts: 352

As rcarsey states, the terms get used loosely. This article attempts to explain the terms,

Rotax 912, 914 and 912iS use similar power generating systems. They consist of multiple stationary coils mounted to the engine and magnets mounted on the crankshaft that rotate within the coils. This produces AC voltage. The AC voltage is delivered to an external rectifier/regulator where it is converted to DC.

The coil assemblies are limited in the amount of current (amps) they can create and the rectifier/regulator is also limited in the amount of current (amps) it can produce/control.

The output of the coils varies with the rpm of the magnets past the coils, which is why a Rotax at idle produces very little electrical energy. It starts to produce significant energy above about 2500 rpm.

Hope this helps,

Jim Butcher
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Old 01-23-2023, 12:12 PM
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BobbyLucas BobbyLucas is offline
Join Date: Oct 2019
Location: Belleville, MI
Posts: 401

So much reading today, but very interesting and enlightening.

Here's my current understanding:

Generator: Any device that converts mechanical (or chemical) energy into electrical energy.
Dynamo: A generator that produces pulsing direct current through the use of a commutator.

Alternator: A generator that produces alternating current.
Magneto: (permanent magnet synchronous generator) An alternator that uses rotating permanent magnets (ilo field coils).

So it seems to me that our engine uses internal magentos, which are a type of alternator, which in turn are a type of generator. So everyone is right! (Except the dynamo guy)

Yes, no? Certainly makes sense to me.

P.S. After Rob's post, I went and looked at the wires coming into the voltage regulator from the 'generator,' and there are three, so 3-phase AC, yeah?
Bobby Steinmetz
Van Buren TWP, MI
RV-12iS #121228
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Old 01-25-2023, 09:16 PM
kshunz kshunz is offline
Join Date: Jan 2023
Location: Seattle, WA
Posts: 3

Originally Posted by jrtens View Post
Welcome to VAF Scott. I think you will find that the 12iS is a great little airplane. Where is your flying club?
Thanks! It's the Boeing Employees Flying Association, in the Seattle area (Renton and Paine).
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