TESTING - 3 blade prop in RV-12 iS

[redbaron]

Be sure to check out the WHIRLWIND 3 Blade for the RV-12

[rvbuilder2002]

Originally Posted by rv9builder

Yes, Van’s considered a fixed-pitch prop during RV-12 development. If I remember correctly, they decided to go with an adjustable prop so builders can fine-tune the prop to their specifications. If your plane is based in high mountains, you might want your prop set up differently than a plane based at sea level.

Correct.

The recommended pitch setting is not meant as an absolute value, but as a good starting point that will work for most RV-12 pilots because most fly from relatively low altitude airports.

As for 3 blade performance #'s?
Still not ready to go public with anything yet.
I will say that based on performance #'s to date, if the decision is made to offer it as an option, that still may not include the ULS engine. The reason for that is the iS engine has it's peak torque better focused on the climb RPM zone than the ULS does. In simpler terms... even though the iS and ULS are both rated at 100 HP, the prop on the iS needs to be adjusted to a higher pitch angle to get the same climb RPM when compared to the pitch setting used on the ULS.
This helps make the compromise between setting for good climb and good cruise speed on the 3 blade a bit closer to what it is for the two blade, but still not quite equal.
This may not be the case for the ULS engine because its peak torque is at a higher RPM, but that is only a guess since at this point we haven't tested it on a ULS equipped RV-12.

I should also correct my previous post where I said there was no noticeable level of smoothness difference compared to the two blade. After getting the 3 blade really well balanced dynamically, there is a noticeable difference to the two blade (also well balanced).

[BigJohn]

My particular airplane has always had a noticeable vibration in the mid RPM range, even though it is smooth at idle and cruise power. I have been told by someone who works on multiple 12’s that this has been seen in on other airplanes as well. Yes, my blades are matched and have been dynamically balanced. The working theory is that this is a harmonic vibration endemic to a certain engine/gearbox/prop combination.

I would be very interested to hear Scott’s thoughts on this after having tested the three-blade.

[rvbuilder2002]

I have noticed some roughness on some RV-12's when operating in the mid range of the power band such as throttled back on a decent or low power in the pattern.
The Bing carb's on the ULS engine have three control stages (idle/low RPM, mid range, and high power/WOT). I think the mid range is a bit on the rich side on some engines and that this is what causes the roughness. Not propeller/gearbox/airframe interaction.
My reasoning for this is because the iS engine RV-12 does not exhibit any mid range roughness and not all ULS engines seem to, but I only have experience with one iS engine so far, and I have zero experience flying any other airframes with the ULS so I can't say this with absolute certainty.

[Jolly]

Have seen two Sensenich 68" 3 blade props on RV12's about 3 years ago and all is good and the performance was quite good.

The Bing carbs on the 912ULS in fact run leaner in a mid range rpm around 4500-4800 and richen out as you go above 5K to WOT. WOT is the richest to protect the engine. This is why some may get a mild vibration in the low 4K's when descending at reduced power settings. The reason the higher rpms run richer is set up that way to help stave off detonation and stay away from lean issues at WOT power like take off. You may even see slightly increased EGT's at times around 4500-4800 rpm. This can depend on the prop pitch setting and carb setup and sync. The 912iS engine regulates fuel much better at rpm and altitude so that occasional roughness in the mid range is not there. In many countries the 912iS uses a variable or constant speed prop and they may run at 5500 rpm during cruise where the US group tends be be around 5100-5400 in cruise and we use ground adjustable props. Neither engine was ever designed to run in the 4K's as its main cruise rpm setting.

[Piper J3]

The Bing 64 is a constant velocity carb where the manifold vacuum diaphragm controls the venturi opening and air/fuel mixture. This design feature also provides auto mixture compensation as altitude varies by comparing manifold pressure to atmospheric pressure at altitude – very nice. We synch our carbs at idle so the throttle plates in each carb behave very similar off idle. This is necessary to prevent rough running at slow speeds, which would be detrimental to the prop reduction gearbox. The cruise, or high end of the throttle, is automatically synched when large air flow brings the main jet into full function.

The midrange is where the two carburetors will vary and cause uneven power L-R. Carburetors are not linear and vary mostly in midrange. Not a whole lot can be done to synch midrange – the idle synch would have to be compromised which we don’t want to do for the reason mentioned above. Midrange we are only making a small percentage power and still operating above slow speed where the gearbox shouldn’t be operated, so all is good.

The 912iS is FADEC so all regimes are controlled with oxygen and airflow sensors for air/fuel flow in both L-R manifolds. This is what allows smooth operation in midrange.

[BigJohn]

I ran a plot showing RPM, and Left and Right EGT during a recent flight, and noted when the roughness occurs. Interestingly, my EGT's track almost perfectly between the two carbs, except in the mid to high 4000's, where the roughness occurs. During those periods the EGT's diverge by about 50 degrees. I think this supports the theories mentioned above, that the roughness stems from the carbs not generating equal power in the mid-ranges.

If one had the time and resources (I don't) it would be interesting to swap the carbs on a rough engine with those on a non-rough engine and see what happens.

I guess this is what sells fuel injected engines!

[RFSchaller]

John,

In the first 200 hours of operation my engine was smooth at all RPM. Then it developed a slight roughness when I pull back the power to idle for descent and landing. I’ve swapped out my spark plugs, rerouted my vent hoses, tried AVGAS and had my carbs overhauled all to to no avail. I found a Brit blog that suggests adding a half turn out to the idle jets screws. I’m in the middle of my annual, and I have made the change. I hope it solves the problem. I should know in a week or so when I put the plane back in the air.

Rich

[subpar_bucker]

I realize this may be an unfair question for Scott, but I'm getting closer to ordering my 12iS engine kit and I'm wondering if he can characterize the timeline for when this decision will be made. Is it weeks, months, or years?

[rvbuilder2002]

Originally Posted by subpar_bucker

I realize this may be an unfair question for Scott, but I'm getting closer to ordering my 12iS engine kit and I'm wondering if he can characterize the timeline for when this decision will be made. Is it weeks, months, or years?

Sorry, I don't have an answer to that question.