RPM limiter for ROTAX 914

Since the discussion has shifted to this post instead of the duplicate, here’s what was previously stated:

Hi Kevin,

The propeller pitch and governors (constant speed) essentially function as a rev limiter. When set up and adjusted correctly, they will ensure you don't exceed the maximum 5800 RPM limit.

Both FAA and EASA regulations mandate redundant safety systems to ensure continuous operation in the event of a failure. As a result, engine overspeed protection must be reliable and avoid introducing unnecessary risks. This is why aircraft engines rely on governors rather than traditional rev limiters. Rev limiters, which cut off fuel or ignition, could introduce risks in aviation, especially during critical phases like takeoff or climb, where sudden power loss could compromise safety. Instead, aircraft engines use gradual, controlled systems to manage RPM.

Ken, to address your concerns about "flying at altitude possibly leading to engine over-revving":

When flying at altitude, it’s generally assumed that you’re either cruising or climbing. In both scenarios, if you have a fixed-pitch or ground-adjustable propeller, it's crucial to monitor both RPM and manifold pressure, adjusting the throttle accordingly. If the manifold pressure gets too high, the Turbo Control Unit (TCU) will regulate the boost by opening the wastegate, and it does this effectively.

While it’s uncommon for a 914 engine to overspeed, it can happen. In 99% of cases, this is due to operator error, improper propeller pitch setup, or incorrect TCU configuration. Installing a rev limiter is only a temporary fix and doesn't address the root cause. It could also compromise safety, which is why it's not permitted in certified aircraft.

And yes, using a constant-speed propeller with the 914 is the best solution.

Thanks RW.

Can you expound more on why, other than that a rev limiter is not allowed on certified aircraft, it is still a bad idea? Specifically, a little more color on the statement "running an underloaded prop and limiting its RPM with a rev limiter." If we are willing to ignore the legacy prohibition on certified aircraft, what are the specific downsides to adding a rev limiter?

There are certainly instances where one might take off at sea level and go mountain flying at 8000 feet.

Hi All,

I have installed several Ignition module pairs on 912. They are highly programmable electronic modules. You can set an RPM limiter, but as RW mentioned, it is not recommended. There is also an interesting feature like possibility to smoothly map ignition advance vs rpm.... that can also be used to protect an engine from over-speed , but it's rather used to have better start and super smooth idle.

Imagine approaching a short airfield obstacle while accelerating within the ground effect to reach an appropriate climb speed and the limitation system cuts the engine power......

My opinion is: it is better to save your life than the engine.

I also think that if you have to "artificially" protect an aircraft engine from the over-speed, it means your prop or its governor (CS controller) is not set properly.

Cheers!

Ken Ryan wrote:

Thanks RW.

Can you expound more on why, other than that a rev limiter is not allowed on certified aircraft, it is still a bad idea? Specifically, a little more color on the statement "running an underloaded prop and limiting its RPM with a rev limiter." If we are willing to ignore the legacy prohibition on certified aircraft, what are the specific downsides to adding a rev limiter?

There are certainly instances where one might take off at sea level and go mountain flying at 8000 feet.

Hi Ken

The idea of allowing a rev limiter to protect the engine in an aircraft is not wise.  The idea that at high RPM, I would assume you are also in high boost, we then shut off (interrupt)  the ignition frightens me.  The engine is running boost (the question was re the type 914) and then we stop the spark which then also interrupts the hot gas outflow....I see many problems.  Perhaps someone who has actually done this can comment because it is just not a good idea to me. 

If you have to use a rev limiter to control your RPM then you are not getting the full thrust of the engine in my book.  The fine pitch that would allow this might sound good for climb perhaps but at WOT level flight you are not getting good performance.  The prop needs to have enough load to bite the air and give you thrust.  

As for sea level to 8000 ft fixed pitch works fine without a turbo.  With a turbo you have to compromise to get full performance as the engine power is capable of pushing the prop beyond the limits if you do not pull back on the power or have an inflight adjustable prop.  Most with fixed pitch understand they have to throttle back, I do not think relying on the rev limiter is a good idea for the reasons I have stated.

I have a question for the persons who may have used this.  As the flywheel is permanent magnet then as long as it spins it is putting out power to charge the models.  In a normal shutoff the system grounds as it is a normally hot feed.  In the case of the aftermarket parts do they ground the ignition to stop the spark or ???.  If you fail to ground the system with the flywheel still spinning it will damage the SCR (the triggering gate inside the module).  Perhaps they have some slick way to work around that. 

my views anyway.

Cheers

Thanks RW. Great info, as always.