What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

TESTING - 3 blade prop in RV-12 iS

E. D. Eliot

Well Known Member
Was at Van's yesterday - looking around, returning parts that I over-ordered, picking up new parts. Walked out to the hanger to look at what's new just as Scott returned from an iS demo flight. Beautiful airplane! Noticed that they are 'testing a three blade prop. Asked if it was a CATO prop and if it was ok to list that info here. Was told that it was likely (not sure) a CATO, that they were testing it, and that it was ok to post here. A beautiful prop. Hope that it works out ok. Took a photo but there is no way that I am able to post it here - sorry.
 
Was at Van's yesterday - looking around, returning parts that I over-ordered, picking up new parts. Walked out to the hanger to look at what's new just as Scott returned from an iS demo flight. Beautiful airplane! Noticed that they are 'testing a three blade prop. Asked if it was a CATO prop and if it was ok to list that info here. Was told that it was likely (not sure) a CATO, that they were testing it, and that it was ok to post here. A beautiful prop. Hope that it works out ok. Took a photo but there is no way that I am able to post it here - sorry.

Very interesting!!! Love to hear what Van?s thinks.
 
Was at Van's yesterday - looking around, returning parts that I over-ordered, picking up new parts. Walked out to the hanger to look at what's new just as Scott returned from an iS demo flight. Beautiful airplane! Noticed that they are 'testing a three blade prop. Asked if it was a CATO prop and if it was ok to list that info here. Was told that it was likely (not sure) a CATO, that they were testing it, and that it was ok to post here. A beautiful prop. Hope that it works out ok. Took a photo but there is no way that I am able to post it here - sorry.

It was nice meeting you yesterday Ed.
Sorry I had to run right away.... we were late getting to a meeting after returning from a test flight evaluating the propeller performance.

One correction.... the propeller being test is a Sensenich 3 blade ground adjustable. It is an updated version from the original design we tested early in the RV-12 development program and decided not to offer in the kit.

The decision on whether to offer this newer design as an option is still under evaluation.
 
It was nice meeting you yesterday Ed.
Sorry I had to run right away.... we were late getting to a meeting after returning from a test flight evaluating the propeller performance.

One correction.... the propeller being test is a Sensenich 3 blade ground adjustable. It is an updated version from the original design we tested early in the RV-12 development program and decided not to offer in the kit.

The decision on whether to offer this newer design as an option is still under evaluation.

Scott..would love to hear what the test pilot thought relative in smoothness compared to 2 blade...whether Van?s decides to offer as option..
 
Scott..would love to hear what the test pilot thought relative in smoothness compared to 2 blade...whether Van?s decides to offer as option..

There is no noticeable difference in smoothness between the standard two blade and the three blade being tested as long as both have had the blade pitch set accurately and a dynamic balance has been done.

A few other comparisons...

The 3 blade is between 1-2 lbs lighter than the two blade.

It makes the lower cowl more difficult to remove and install (will require a longer nose gear leg slot if it is officially adopted)

It has zero indexing capability for the blade pitch so each blade must be 100% adjusted manually (not a big deal for those that want smooth running and do that anyway)

The prop can be removed from the engine with the blades still installed, so no repitching is necessary if removed for 500 hr gear box inspections, etc.

Comparisons in performance will have to wait.......
 
There is no noticeable difference in smoothness between the standard two blade and the three blade being tested as long as both have had the blade pitch set accurately and a dynamic balance has been done.

A few other comparisons...

The 3 blade is between 1-2 lbs lighter than the two blade.

It makes the lower cowl more difficult to remove and install (will require a longer nose gear leg slot if it is officially adopted)

It has zero indexing capability for the blade pitch so each blade must be 100% adjusted manually (not a big deal for those that want smooth running and do that anyway)

The prop can be removed from the engine with the blades still installed, so no repitching is necessary if removed for 500 hr gear box inspections, etc.

Comparisons in performance will have to wait.......

Found this posting from 2 months ago. Any additional info available on the testing process at Van's?
 
Prop for RV-12

Since Van’s likes the prop for the RV-12 to be at an exact pitch then why not commission a fixed pitch prop, either 2 or 3 blade, that will be pitched exactly as specified for an RV-12?
I wonder if Van’s has ever considered a fixed pitch prop for the RV-12? My guess is that sometime over the last 10 years they have. I would be interested in hearing the plus and minus for a fixed pitch prop for the RV-12.
Thanks.
 
Last edited:
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.
 
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).
 
Three-blade smoother in mid-RPM?s?

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.
 
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.
 
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.
 
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.
 
Just a little more information..........

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! ;)
 
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
 
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?
 
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.
 
Hi John,

The things that tend to cause the carbs to be rough at idle are heavy floats (they need to be weighed to rule these in or out even if you just did it 10 hours ago). The idle jet is clogged, but that should show up on a carb sync with a really high vacuum on the offending carb. An air leak around the rubber intake sockets for the carbs or at the air intake manifold where they screw down on top of the heads. Usually from someone taking these off and failing to make sure the "O" rings aren't pinched. This can and does cause detonation. Tweaking the mixture screw on the bottom by 1/4 turn should not cause a roughness. The 1.5 turns out is usually good for almost all people unless you live in an extreme condition. A choke being partly open and not closing all the way or a cable hanging up and not in sync with the other side.


Last week at CPS and the Heavy maint. Rotax class out of the way.:)
 
It has been 2 months now since the last post regarding the new Sensenich 3 blade test prop installation at Van's. I thought it might be worth resurrecting this old post, hoping for a performance update. Anyone have a progress report?

Tom
 
I have no insider information, but it seems to me if there is an update on this topic it will likely be announced at Oshkosh :)
 
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.

I beg to differ with you....mid range synch @ 3800 rpm (called off idle) is where you synch the needle valve circuit of your carb (done first before you synch idle setting. Done right...these carbs function well.

Following the Rotax 9 Series IRMT class that Jolly teaches....I have been called to help synch several different types of 912 uls installations as well as RV-12’s. What i’ve Found to be the root cause of guys having problems getting a good synch accomplished is incorrectly installed/adjusted carb cables (one or both with to much slack)...basic carb installations stuff..
 
Last edited:
I beg to differ with you....mid range synch @ 3800 rpm (called off idle) is where you synch the needle valve circuit of your carb (done first before you synch idle setting. Done right...these carbs function well.


I disagree with your method and the training given at the Rotax Service Specialty classes does as well.

The critical detail to having good gear box longevity is carbs that are WELL synced at the low end of the RPM range.

If they are after you do your adjustment in the middle of the range then great. If they aren't, the low end should never be compromised in an effort to get the middle exact.
I guarantee that doing so will cost you $$$$ and down time.
 
I disagree with your method and the training given at the Rotax Service Specialty classes does as well.

The critical detail to having good gear box longevity is carbs that are WELL synced at the low end of the RPM range.

If they are after you do your adjustment in the middle of the range then great. If they aren't, the low end should never be compromised in an effort to get the middle exact.
I guarantee that doing so will cost you $$$$ and down time.

No disagreement Scott...idle synch is done following “off idle adjustment” as per Van’s maint manual...it just wasn’t clear imho...until my Rotax class...the fact that there are 3: distinct fuel flow circuits in these carbs wasn’t clear until the class...idle and off idle (mid-range controlled by needle valve circuit) are the two synched. The cruse/take-off power circuit (main jet) is not synched.

The fact that you disagree with what was taught...does not make you right!
 
Last edited:
As Scott says? the critical point for synching the carbs is at idle and just-off-idle when the engine is making low RPM and low power. At low speeds power pulses are no dampened well by the prop (read flywheel) and the gearbox gets hammered. Synching the carbs is really matching power settings for this critical phase of operation. The Rotax gearbox cannot tolerate power imbalance at low speeds ? this is why a minimum of 1800 RPM (Green Arc) is required. Slower speeds than this and you?ll kill the gearbox in short order. Power imbalance at mid-range and highspeed is tolerated well because the crankshaft, gearbox, and prop are all running fast and have inertia and mass to dynamically dampen power pulses.

It?s imperative to understand the difference between a conventional horizontally opposed engine (Cont. & Lycoming) and a Rotax. Conventional engines have a single carburetor that feeds all cylinders relatively equal. All cylinders pretty much get the same fuel/air mixture and the engine hums along well at all speeds. No gearbox to worry about, so slow speeds aren?t a problem.

The 4-cylinder Rotax design is really two (2) separate power sources that share a common crankshaft and gearbox. What makes the engine behave this way is each cylinder bank is controlled by its own carburetor. Both left and right cylinder banks must produce similar power at slow speeds for the reasons stated above.
 
No disagreement Scott...idle synch is done following “off idle adjustment” as per Van’s maint manual...it just wasn’t clear imho...until my Rotax class...the fact that there are 3: distinct fuel flow circuits in these carbs wasn’t clear until the class...idle and off idle (mid-range controlled by needle valve circuit) are the two synched. The cruse/take-off power circuit (main jet) is not synched.

The fact that you disagree with what was taught...does not make you right!

For clarity,
When I am talking about sync at idle, I do not mean at the idle stops. That can be tweaked a bit with the stop screws, but only after the sync is good at 1700 - 1800 RPM
That should be a focus point since that is the minimum you should be idling at for anything other than very short periods of time.
If you first sync at mid range throttle, there is no adjustment you can make to sync at 1800 and have it not effect the mid range.
At 1800 RPM, the carb's are still in the idle circuit range.

And I don't disagree with what I was taught. I was taught to focus on the sync while the carbs were operating in idle circuit mode.
 
Last edited:
As Scott says? the critical point for synching the carbs is at idle and just-off-idle when the engine is making low RPM and low power. At low speeds power pulses are no dampened well by the prop (read flywheel) and the gearbox gets hammered. Synching the carbs is really matching power settings for this critical phase of operation. The Rotax gearbox cannot tolerate power imbalance at low speeds ? this is why a minimum of 1800 RPM (Green Arc) is required. Slower speeds than this and you?ll kill the gearbox in short order. Power imbalance at mid-range and highspeed is tolerated well because the crankshaft, gearbox, and prop are all running fast and have inertia and mass to dynamically dampen power pulses.

The 912 engine suffers from rather serious torsional vibration below about 1400 rpm and a propeller on a geared engine does not act as a damper like a flywheel inserted between the engine and the gearbox might. Quite the opposite in fact as larger a MMOI here almost always exacerbates TV issues in this rpm range on most 4 cylinder engines and a reason why Rotax specifies a maximum MMOI for propellers.

I wish I'd shot some video of our EFI 912 on a test stand transitioning through this range years ago. Truly scary how much the TV would flex the entire test stand and a clear picture of why Rotax warns against operation in the low rpm ranges. It's there even with perfectly synched carbs or a single TB in our case.

Balancing carbs is vitally important but not the cause of the TV issues at low rpm on this engine.
 
For clarity,
When I am talking about sync at idle, I do not mean at the idle stops. That can be tweaked a bit with the stop screws, but only after the sync is good at 1700 - 1800 RPM
That should be a focus point since that is the minimum you should be idling at for anything other than very short periods of time.
If you first sync at mid range throttle, there is no adjustment you can make to sync at 1800 and have it not effect the mid range.
At 1800 RPM, the carb's are still in the idle circuit range.

And I don't disagree with what I was taught. I was taught to focus on the sync while the carbs were operating in idle circuit mode.


Scott....again, could not agree more. I, when I first started flying my -12, I adjusted my idle to 1550 rpm (which you are correct stating from there to correct synch you would have to choose using “off-idle” compromising toward idle jet synch to effect smooth low rpm operation). As I gained experience flying my -12; I have found that setting idle to around 1775-1800 rpm {thanks to Roger aka Jolly} (synching using idle stop screws); then using off-idle for mid-range synch...you wind up with gauge needles staying dam near perfect....and a very smooth running engine.

The difference between 1550 and 1775 rpm, having very little difference in landing performance...once set up on stabilized approach (a slightly higher nose attitude)...just my two cents. I say again...pay attention to cable set up on carbs!

Edit: Sorry for the thread drift guys...just my passion for “Smooth Running Engines”...didn’t want to get your knickers in a twist Scott!
 
Last edited:
The difference between 1550 and 1775 rpm, having very little difference in landing performance...once set up on stabilized approach (a slightly higher nose attitude)...just my two cents. I say again...pay attention to cable set up on carbs!

Edit: Sorry for the thread drift guys...just my passion for “Smooth Running Engines”...didn’t want to get your knickers in a twist Scott!

Then I guess you and Roger have your own opinion that no one else has ever publicly agreed with, and if you look through the forums you will find that many have disagreed with.

Nothing twisted here....
Just trying to save people money from mistakenly following bad advice.
 
About 4 months ago I posted that I increased my idle screws to 2 turns open as suggested by a U.K. blog to try to eliminate roughness at flight idle. After about 70 Hours I can report it made no difference in the slight roughness at flight idle and it increased my fuel consumption by about 10%. I have returned the screws to the ROTAX recommended 1 1/2 turns. The slight roughness is unchanged, but the fuel consumption dropped back to what it was.
 
Getting back to the original point of this thread: does the lack of an announcement on this topic from Van's mean the three bladed prop is not going to be offered for the 12iS?

I've gone back and re-read all of Scott's posts on this thread. It sounds like there's little/no performance benefits to the 3 bladed prop. There is a benefit in terms of the smoothness being better after balancing. Maintenance may be easier because the prop can be removed and then reattached without requiring setting the pitch again assuming the cowling has been modified to clear the front gear leg.

I haven't yet ordered my engine. I'm trying to decide if I want to go forward with the three bladed prop myself which will require EAB, or if I want to stick with the factory specified two bladed prop. The smoother ride may be enough to convince me that EAB is the right path for me.
 
Getting back to the original point of this thread: does the lack of an announcement on this topic from Van's mean the three bladed prop is not going to be offered for the 12iS?

I've gone back and re-read all of Scott's posts on this thread. It sounds like there's little/no performance benefits to the 3 bladed prop. There is a benefit in terms of the smoothness being better after balancing. Maintenance may be easier because the prop can be removed and then reattached without requiring setting the pitch again assuming the cowling has been modified to clear the front gear leg.

I haven't yet ordered my engine. I'm trying to decide if I want to go forward with the three bladed prop myself which will require EAB, or if I want to stick with the factory specified two bladed prop. The smoother ride may be enough to convince me that EAB is the right path for me.

The chance of a 3 blade being offered by Van's is not dead, just moving slowly at the moment because of OSH and working with Sensenich on an opportunity to test a set of blades slightly refined based on the flight testing done so far.
I will try and remember to update this thread when we have any new info to share.
 
Since I'm all about cruise performance.. and live at 100' MSL, I doubt a 3-blade prop will be of much interest to me. Maybe mountain folk feel different about it..

That said, I see value in a product like the -12 that offers factory-supported/endorsed options. Almost all of the options offered (aoa, g3x, dual LL, etc) can be implemented by the builder after certification.. but I like knowing that someone more knowledgeable than me OK'd the idea and tested it. I'm too new to building to feel confident that I can put a 3-blade prop on and I won't fall out of the sky... or cut a huge hole in the wing to install a light..and feel confident that the wing wont rip open in the wind.

So I think options like 3-blade props, an option to install camera mounts in approved locations.. IFR GPS, VOR receiver/antennas, etc add flexibility and value to the -12 platform.
 
Since Van?s likes the prop for the RV-12 to be at an exact pitch then why not commission a fixed pitch prop, either 2 or 3 blade, that will be pitched exactly as specified for an RV-12?
I wonder if Van?s has ever considered a fixed pitch prop for the RV-12? My guess is that sometime over the last 10 years they have. I would be interested in hearing the plus and minus for a fixed pitch prop for the RV-12.
Thanks.

Check out CATTO, they have a three blade and soon a 2 blade fixed pitch for the-12.
 
There is an article about propellers in the January 2019 issue of Kitplanes. It says that for lower horsepower engines, a two blade propeller is more efficient. But a 3 blade propeller is quieter, has better climb performance at low airspeed, and has more ground clearance.
 
There is an article about propellers in the January 2019 issue of Kitplanes. It says that for lower horsepower engines, a two blade propeller is more efficient. But a 3 blade propeller is quieter, has better climb performance at low airspeed, and has more ground clearance.

Isn't that true for any application? RV, Bonanza, Cessna, Cirrus, whatever.
 
whirlwind

Yes Red Baron, I've read about these props in Kitplanes Magazine, and am seriously in doubt to build one on my 12. Numbers would surely help!
 
3 Blade announcement from the mother ship

Looks like Vans is making it official.

If you?re at #OSH19 next week be sure to stop by the Van?s booth! Among the things we?ll have to show you will be this three-blade sensenichprop that will be available for the RV-12 and -12iS later this year!
 
Hope we get some official performance numbers to go along with it!

I think the general feeling (earlier in this very thread?) was that 3 blades is good for climb and 2 blades was good for cruise.

I suppose we can look at the demo airplane's trip (on an adsb website) to oshkosh to see how the speeds differed from last year's trip :D
 
Looks like Vans is making it official.

If you?re at #OSH19 next week be sure to stop by the Van?s booth! Among the things we?ll have to show you will be this three-blade sensenichprop that will be available for the RV-12 and -12iS later this year!
I feel stupid, where'd you see the announcement? I can't find it. And we do need some real numbers in order to make an informed decision.

One deal killer for me with any 3-blade prop is that most of them need their own special shipping boxes and they can't be UPS'd, rather they must go by truck which is a hassle and $$$. I understand with prop though the blades easily detach making for a compact shipping box.
 
If anyone is contemplating a 3-blade prop upgrade... I might be interested in your used 2-blade set-up. Most interested in just buying a matched set of blades with no hub or hardware.
 
Back
Top