Rotax 912 External Alternator

[jnmeade]

Does anyone have any personal experience with adding the external alternator to the Rotax 912?

[drseti]

The internal Rotax 912 alternator puts out only 15 amps. If you are avionics-heavy, that's just not enough. There is an accessory drive available on the back of the prop gearbox, to which an auxiliary 20 amp generator can be bolted. The drive opening comes covered with a seal plate from the factory, but Rotax has a gear, shaft, and bushing kit available to mount there, as an accessory. In fact, the 912 that Eric Tucker brought for the Rotax Heavy Maintenance course had a prop governor mounted to this accessory drive. Looked pretty straightforward.

The generator assembly was factory installed on my SportStar, but doesn't look too difficult to add. Since it's a geared drive, no belts or pulleys are required.

[roger lee]

Rotax test question: Internal Rotax trigger coils produce? advertised 20 amps.
18 amps peak and 16 amps continuous.
How many trigger coils? 5
1 for the tach the other 4 for power.
The external generator puts out 40 amps and is run by belt drive. You shouldn't run both internal and external at the same time.

[drseti]

16 amps continuous.

Ok, I stand corrected (having said 15).

The external generator puts out 40 amps and is run by belt drive.

Well, yes, there is a belt-driven one available (i didn't realize it put out 40 amps). That's not the one Evektor uses. Their installation is definitely 20 amps, and gear driven, mounted to the accessory housing.

[Jim Stewart]

Rotax test question: Internal Rotax trigger coils produce? advertised 20 amps.
18 amps peak and 16 amps continuous.
How many trigger coils? 5
1 for the tach the other 4 for power.
The external generator puts out 40 amps and is run by belt drive. You shouldn't run both internal and external at the same time.

Of the five trigger coils, one is for tach and four are to time the spark, two for the upper plugs and two for the lower plugs. None of these coils produce significant amounts of power, just voltage pulses to indicate speed and position of the generator shaft.

There are also ten pole pieces with stator windings to generate power. One of the stator pole pieces powers the upper ignition system, one powers the lower ignition system, and the remaining eight supply power to the rectifier assembly that ultimately powers the aircraft electrical system.

This is how we end up with 2 independent ignition systems and one independent aircraft power source.

Why 2 pickup coils per ignition system? I suspect that they are positioned in such a way that the more advanced pickup is spaced further away from the rotor or is made less sensitive than the retarded pickup. At idle, insufficient voltage would be generated by the advanced pickup and only the retarded pickup would fire the spark plugs. Above idle, the increased speed would cause the advanced pickup to generate enough voltage to fire the plugs first. At least that's how a clever engineer would do it.

[roger lee]

The two yellow wires that come out of the rear flywheel / coil housing produce 18-22 VAC. These go to the reg/rec that converts to our DC.

[KSCessnaDriver]

Does anyone have any personal experience with adding the external alternator to the Rotax 912?

Never added it, but I've flown 912's with the 40 amp alternator installed. Didn't seem any different, other than that even with 2 engines, both running the 40 amp alternator, I could have enough stuff running to not have enough power.

Granted, my current ride has 4 200 amp alternators that I can easily not have enough power from.

[zaitcev]

There is an accessory drive available on the back of the prop gearbox, to which an auxiliary 20 amp generator can be bolted. (. . .)
The generator assembly was factory installed on my SportStar, but doesn't look too difficult to add.

Although our ultimate expert, Roger, does not seem to believe in this mystery alternator, I saw it on Tecnam Sierra, even took a picture. It is required for IFR certification in Europe (yes they fly LSAs IFR there, crazy huh). I cannot guarantee that both may be used together. The 20A add-on may be only a standby in case your main one quits while in IMC.

[jnmeade]

From Aircraft Spruce - note this is a spline drive and would probably need an adapter to fit the Rotax gearbox.

The SD-20 is a high-performance spline-driven alternator that mounts on a standard vacuum pump accessory pad. It may be used either as a primary or a stand-by alternator for a rated output 20 amps @ 2500 alternator RPM.

Constructed of all NEW materials (with absolutely NO "re-manufactured" content), the SD-20 is designed for durable service, with heavy-duty sealed ball-bearings, two cooling fans, a dynamically balanced rotor, and a special "shear section" designed into the drive coupling. The SD-20 is available for both 14 and 28 volt applications. and is externally-regulated. It may be appropriately matched with our LR3c, LS-1A, or SB1B Controllers (depending) on your application and electrical system configurations). Measuring 4.6" wide and 6" deep, and weighing only 5.75lbs., the SD-20 will clear the tachometer cable and oil filter on stock Lycoming engines.

SD20 ALTERNATOR
20 AMPS (HOMEBUILT)

[roger lee]

I didn't say I didn't believe in the external alternator, it's a must for some applications, but in the Tecnam Sierra that has less avionics and amp pull than a CT it doesn't need it. I just worked on a Tecnam today. The best way to know, whether you are building or an MFG, is to write down all your load draws and see what the total amp draw really is. It would be kind of superfluous if you only had a 12 amp draw system to put a 40 amp external alternator on.

[David]

I build my RV12 as an experimental so I could add things like the External Alternator. A few photos below during the build.





I did have to modify a few things to make it fit. As Roger said it is not recommended to run both, the internal generator and the external alternator at the same time.

I installed a Vertical Power (VPX) and isolated the Gen and the Alternator. The VPX starts up with the Alternator as the main and the Rotax Gen is off. Should I have a failure of the Alternator I will get a warning on my GRT via the VPX and switch the Alt off and the Gen on through the VPX. I would also shut down avionics so I don’t draw too much from the Rotax Gen ie.. Second GRT, ADSB, the Garmin 796 (to Battery) and others as needed

[drseti]

From Aircraft Spruce - note this is a spline drive and would probably need an adapter to fit the Rotax gearbox.

Per Eric Tucker of Rotax fame, the optional Rotax accessory drive kit does include the required spline adapter.

The SD-20 is a high-performance spline-driven alternator that mounts on a standard vacuum pump accessory pad. It may be used either as a primary or a stand-by alternator for a rated output 20 amps @ 2500 alternator RPM.

This is exactly the same unit that Evektor installs on the SportStar.

FWIW, the Evektor AOI indicates that in their installation, both alternators are to be run at the same time.

[jnmeade]

I did have to modify a few things to make it fit.

thanks very much for the very useful pictures.
Is that the oil filter under cylinder # 2? Did you have to modify how you mounted the oil filter?

[drseti]

Is that the oil filter under cylinder # 2?

I think what you're seeing there is the stainless steel muffler, Jim. The oil filter is the smaller, black object located in front of the cylinder and below the external alternator.

[jnmeade]

Is that the oil filter under cylinder # 2?

I think what you're seeing there is the stainless steel muffler, Jim. The oil filter is the smaller, black object located in front of the cylinder and below the external alternator.

You are right on the location. Thanks. I'm assuming no modification is needed.