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:
setting-pitch2.jpgsetting-pitch.jpg

[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.

[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

[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.

[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]

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