Setting Whirlwind GA-RW3B pitch on 912ULS

[3kdscf]

Am I going crazy or is the Whirlwind GA-RW3B propeller manual wrong? What pitch do you use with this combo? The manual clearly states in two places that for the 70" prop I have that the pitch should be set to 17 degrees. The problem is that when I use their setting (confirmed with two different digital levels), I cant even use full power on a 65mph climb without going over 5800 rpm.


After that I did experiments at 19.5 degrees and 21 degrees.


PERFORMANCE MEASURMENTS (with 22" tundra tires)
21 degrees: 109mph @5200rpm. ROC 880fpm @ 1200lbs & 4000'
19.5 degrees:104mph @5200rpm. ROC 950fpm @ 1200lbs & 4000'


CLIMB RPM @ 65mph
21 degrees: 5050rpm
19.5 degrees: 5500rpm


If it was just a performance tradeoff, I would choose the 21 degree pitch and take the faster cruise with only a small decrease in climb performance. Many people seem to say that its bad to "lug the engine" by having full power climb under 5300. Does anyone know if this is actually stated in Rotax documentation or know what is the reasoning for this recommendation?


Thanks!

Relevant pages from Whirlwind manual:
2685426855

[jrevens]

What is your static wide open throttle RPM? As stated, 17 deg. is just a starting point.

[Dave S]

Quote "If it was just a performance tradeoff, I would choose the 21 degree pitch and take the faster cruise with only a small decrease in climb performance. Many people seem to say that its bad to "lug the engine" by having full power climb under 5300. Does anyone know if this is actually stated in Rotax documentation or know what is the reasoning for this recommendation?"

Yes, but 5300 is wrong, at least for the ULS

Rotax states that at full throttle the engine must be operated at a minimum RPM of 5200. At least on the 912ULS - the other 912 versions may or may not be different. Reduced throttle settings not an issue.

"lugging" is relative - it's the specific engine spec that counts which takes into account a number of items like compression ratio, spark timing, max density altitude, octane rating, etc.

If a person operates full throttle below 5200 on the ULS they are tempting detonation.

Until recently I had not actually seen an engine destroyed by detonation - not a Rotax but a honda auto conversion and that happened within the first 5 hours on the ground. The engine was aluminum compost after that.

[3kdscf]

Rotax states that at full throttle the engine must be operated at a minimum RPM of 5200. At least on the 912ULS - the other 912 versions may or may not be different. Reduced throttle settings not an issue.

Thanks Dave! I missed that in the Rotax manuals. 19.5 degree pitch which gives 5500 on a 65mph climb is the way I will go. Just need to go back in my notes and see what the corresponding static RPM is to insure its in the 5100-5300 range specified.

[3kdscf]

For those wondering where Dave S got the "minimum 5200rpm WOT" from, its in SL-912-016 under section 3.1.2 "Performance recommendations". Took me a bit of googling to find it :)

[3kdscf]

What is your static wide open throttle RPM? As stated, 17 deg. is just a starting point.
Hey John,

Thanks! When I go to the hangar tomorrow I will check my notes and make sure the static RPM is in the 5100-5300rpm range specified with my current pitch setting of 19.5 degrees. I cant remember precisely as I did a lot of experiments.

[3kdscf]

IMO the Whirlwind recommendations do not make any sense and the Rotax requirement of keeping RPM above 5200rpm at wide open throttle upon climb is the important one to satisfy. Whirlwind says to set the WOT static rpm to 5100-5300 but if I set the static WOT to be anything less than 5200, I wont be able to satisfy the Rotax requirement. Here is what I found:


17.0 degrees: WOT static cant be tested without exceeding 5800rpm
19.4 degrees: 5450rpm WOT static, 5500rpm 65mph climb
20.2 degrees: 5330rpm WOT static, 5340rpm 65mph climb
21.0 degrees: 5050rpm WOT static, 5060rpm 65mph climb


Again, no idea why they recommend 17 degree pitch as the starting requirement because it does not meet their own recommendation of WOT static 5100-5300rpm. I'd rather keep at least 100rpm over the 5200rpm minimum specified by Rotax so I will be using the 20.2 degree pitch setting.

[jrevens]

I think I might have seen something from Kitfox which recommended a higher initial setting for the angle. Of course you set it at whatever it takes. Whirlwind may even be aware that the recommendation is a little too fine and just haven't gotten around to changing it in their installation documentation. Wish it wasn't so, but that is not unusual for many companies, in my experience. Just out of curiosity, have you checked the accuracy of your tachometer?

[3kdscf]

I think I might have seen something from Kitfox which recommended a higher initial setting for the angle. Of course you set it at whatever it takes. Whirlwind may even be aware that the recommendation is a little too fine and just haven't gotten around to changing it in their installation documentation. Wish it wasn't so, but that is not unusual for many companies, in my experience. Just out of curiosity, have you checked the accuracy of your tachometer?

Hi John,

I must have missed the Kitfox recommendation. As far as checking the accuracy of my tach, yes and no. I have both the Grand Rapids EIS2000 which gives the RPM in digital format as well as the analog AviaSport Rotax 912 tach and they both agree. Because they both trigger off the same signal provided by the engine, its not a true validation like as would be provided by a handheld laser RPM meter.

[desertdave]

Reviving this thread. I installed a Whirlwind prop, replacing my IVO and feel I lost quite a bit of airspeed. What are you guys finding as the sweet spot with your 100hp 912 ULS?

[efwd]

what are you using for pitch setting?
My 912is set between cruise and climb gets me about 115TAS in level flight. Im getting 5250rpm static WOT. I found that if I set the pitch any finer I would Redline the engine too easily once level.

[Geek]

what are you using for pitch setting?
My 912is set between cruise and climb gets me about 115TAS in level flight. Im getting 5250rpm static WOT. I found that if I set the pitch any finer I would Redline the engine too easily once level.

Mine is set to 17 degrees right now but no flight time on it. I got about 5300 static at WOT. That's with the 75" prop. Interested to know what you are running Eddie.

Geek

[efwd]

I referenced the rpm because I don't recall what the original setting I put in. By that I mean, when the airframe was level etc etc. Since that time, someone here had mentioned that it wasn't necessary to level the airframe each time pitch is changed. So, I just started at the baseline (that I don't recall) and started changing pitch (while in three point) until I found 5250rpm. You seem to be very close to my setting. I wouldn't change it. I think I originally set it for 5500 static WOT but if I pushed the nose over once I reached pattern altitude, I would easily reach 5800 rpm if I wasn't quick on the throttle. I also didn't enjoy the more turbulent days because each thermal would require I be hypervigilant as well. 5250rpm is suiting me fine.

[jiott]

That's what I use, 5250 rpm static, with my Whirlwind 70" GA-RW3B on a 912uls. A good compromise.

[Geek]

Thanks Eddie and Jim. I'll chime in later once I fly this thing. Still playing FAA but in the meantime - I got a hangar so that's progress!!!

G

[Geek]

For the definition of redundant - see redundant. Sorry about the double pump.

G

[desertdave]

Test flew the plane this morning and 5250 static is the sweet spot I was looking for. Climbs great at 65mph and cruses about 100mph indicated in the traffic pattern at 5200 RPM. Thanks all!

[jrevens]

I’m kinda’ late to the game here, but I just rechecked what I have mine set at… 21 deg. measured in level attitude. Same engine & prop as you. I’m at a higher field elevation than you Dave, but at 7,500 msl and 5350 rpm I cruise at a little over 120 mph. I’ve easily taken it up to 18,000’. Like Eddie, any finer pitch & it’s too easy to over-rev, & over-all performance is compromised. That’s just my experience, FWIW.

[efwd]

Okay, I'm back in the air. Fixing to fly to Phoenix today. I set up my pitch a bit more coarse than I had previously. I am currently 100 rpm less than what I had earlier (5250). So, at 5150 static WOT gives me about 10mph faster cruise than I had earlier. I took it out over the ocean where the air was very smooth and at 5500rpm I am now achieving 123mph TAS. If I run it at 5100 cruise I am now getting the speeds I once had while at 5500 rpm, about 110mph TAS.