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RV-12iS throttle position for start

fehdxl

Well Known Member
The RV-12iS POH provided by Van’s says the throttle should be at 50% +/-5%… why is this? Seems way too much power for start…and it prevents watching Gen A take over the load from Gen B. I’ve been opening it 1/16” to 1/8” and it starts just fine and at about 1800rpm. Looking to learn the why behind 50%. Thanks! -Jim
 

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The RV-12iS POH provided by Van’s says the throttle should be at 50% +/-5%… why is this? Seems way too much power for start…and it prevents watching Gen A take over the load from Gen B. I’ve been opening it 1/16” to 1/8” and it starts just fine and at about 1800rpm. Looking to learn the why behind 50%. Thanks! -Jim

Do you see where it says 2500 rpm for 5 seconds in the POH? Probably to excite the fields on the alternators /generators so that they start recharging the battery and generate positive amperage to handle the electrical load of the fuel pumps.

Did you know that Rotax wants you to maintain 2500 rpms until oil temp is at or above 120F for run up, to minimize wear to the clutch and gearbox dogs in the gearbox?

So it seems that 2500 rpms is not excessive, instead, it's necessary.
 
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I set 42%, gives about 2500RPM, then i can increase to 2600 RPM to see the switchover of Gen and observe the loading increase and Batt voltage.
 
That’s all true. But starting too high and you won’t see GEN B come on line, then when above 2500 for 5 seconds watch GEN A take over the load. It’s an electric airplane…need to ensure both GEN B and A are working. Starting at 50% doesn’t allow the check of both.

Do you see where it says 2500 rpm for 5 seconds in the POH? Probably to excite the fields on the alternators /generators so that they start recharging the battery and generate positive amperage to handle the electrical load of the fuel pumps.

Did you know that Rotax wants you to maintain 2500 rpms until oil temp is at or above 120F for run up, to minimize wear to the clutch and gearbox dogs in the gearbox?

So it seems that 2500 rpms is not excessive, instead, it's necessary.
 
Karl, What are you physically looking at or setting to make that happen? Maybe I need to just draw a line where 2200-2300 rpm is, and start it with the throttle at that line. Thx.

I set 42%, gives about 2500RPM, then i can increase to 2600 RPM to see the switchover of Gen and observe the loading increase and Batt voltage.
 
The percent throttle value is specifically that. A % value of the how much the throttle plate is opened.
It should not be confused with percent power. Throttle position of 50% is way less than 50% power.

Having said that, in many instances, using 50% throttle position for engine start is way too much in my opinion.
The RV-12iS POH lists that value because that is what Rotax specifies in there generic engine operation check list.
We have found that in warmer ambient temps, 35-40 percent works well to get a nice smooth start that doesn't jump to an excessively high RPM. In that condition, with an engine that has recently run, 35% seems to work very well.

During the winter, particularly in very cold conditions, it will generally need 50+%
 
Some addition good nuggets of information in the Rotax operating manual.

Fly safe!
 

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My RV-12iS has Dynon HDX and I am not able to determine how to if the generators switch over at 2,500 during engine start. I've seen it on the G3X.

Anyone figure this out?
 
If I understand your question correctly, when the ammeter goes from negative (discharging the battery) to positive, you’re on the generator. You should notice your HDX getting a bit brighter too.
 
We have found that in warmer ambient temps, 35-40 percent works well to get a nice smooth start that doesn't jump to an excessively high RPM.

After reading this thread I decided today to attempt a start at 40% throttle. Before today I always turned the key with the throttle at 50% immediately reducing RPM to 2300 - 2500 which leaves the GEN A/GEN B still needing performed. When starting today I still had to reduce RPM immediately but not by nearly as much.

These hot summer days and for hot starts I will turn the key with throttle at 35%. I always learn something reading vansairforce forums!
 
If I understand your question correctly, when the ammeter goes from negative (discharging the battery) to positive, you’re on the generator. You should notice your HDX getting a bit brighter too.

Thanks Bob. I had that question also. Maybe the POH should just say “if the amps are positive it’s switched over”
 
My RV-12iS has Dynon HDX and I am not able to determine how to if the generators switch over at 2,500 during engine start. I've seen it on the G3X.

Anyone figure this out?

Let your oil pressure get up around 55-60 psi or more at 2000 to 2200 rpm first, for 5-10 seconds, then proceed to 2500+ RPMS... It's a 2 step procedure.

The A and B lane switches lighted, once over 2500 rpm for 5 seconds, CHANGES from Lane B lit to Lane A lit, on your switch panel, not on the glass EFIS. That tells you you're charging at a positive rate and on the alternator.
 
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The A and B lane switches lighted, once over 2500 rpm for 5 seconds, CHANGES from Lane B lit to Lane A lit, on your switch panel, not on the glass EFIS. That tells you you're charging at a positive rate and on the alternator.

NO! Your lane lights remain unlit. (They are fault lights) The only indication is a significant jump in amps as your battery begins to charge. For example, with the Earth X battery, my amps usually go up to around 16 to 18 amps. So while you are advancing the throttle, you should be looking at the ammeter for this jump.
 
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NO! Your lane lights remain unlit. (They are fault lights) The only indication is a significant jump in amps as your battery begins to charge. For example, with the Earth X battery, my amps usually go up to around 16 to 18 amps. So while you are advancing the throttle, you should be looking at the ammeter for this jump.

This is correct. The Lane A and Lane B lights are *fault* lights only, and neither should be lit in normal operations, including during the startup sequence and while the generators kick in. The ammeter is the thing you need to keep an eye on.

If one or both lights are ever lit or flashing, refer to the RV-12iS POH for a list of what the combination of lights means.

From RV-12iS POH 7-12:

7.7.2 ROTAX 912 iS GENERATOR TRANSITION
After engine start, the Rotax 912iS generators will supply
power to the engine computers but not the main bus.
After the engine speed exceeds approximately 2,500
RPM for a few seconds the K1/K2 relay shown in Figure
7-7 will switch, Generator B will begin supplying power to
the main bus and Generator A will supply power to the
engine computers. This transition will be noted on the
Gen Amp Meter (Shown on the EFIS screen)

A table explaining possible Lane Fault light indications and related actions to take are in the POH, in Section 3-4.

Please don't operate the aircraft without being familiar with the contents of the POH.
 
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One additional detail…
Even if using 35% throttle position to start a 912iS, the RPM will always exceed 2000-2200. Sometimes it will even exceed the alternator switchover RPM of 2600. Rotax expects this to happen and specifically specifies it for a few seconds to activate the switchover. You are THEN supposed to retard the throttle to keep RPM below 2500 until oil temp reaches 120 F.
 
Just learned this about the Rotax 912iS display function last week.

With one or both Lanes on and the engine page displayed, pressing the menu button reveals a FADEC Information button. On that display, it shows the throttle position in percentage… which IMO is the factual answer on where the throttle is positioned for engine start.

Here’s a picture.

Fly safe!

-Jim
 

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Just learned this about the Rotax 912iS display function last week.

With one or both Lanes on and the engine page displayed, pressing the menu button reveals a FADEC Information button. On that display, it shows the throttle position in percentage… which IMO is the factual answer on where the throttle is positioned for engine start.

Here’s a picture.

Fly safe!

-Jim
Not needed for RV-12iS because that same infor is displayed on the EMS portion of the screen beside the tach indication.
 
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