Steve6443 wrote:
No, generic statement from Rotax concerning the 912 series. They recommend flying between 5000 and 5500 rpm
Can you point me to where this is stated, please?
The Rotax operators manual (from the official Rotax site) has a table of recommended settings for cruise for each engine.
LeSving wrote:
The Rotax operators manual (from the official Rotax site) has a table of recommended settings for cruise for each engine.
Indeed, but it doesn’t say that you should cruise between 5000 and 5500 rpm. Neither does Rotax Service Letter SL-912-016.
There are some interesting measurements about optimal speeds for Rotax here – https://www.stolspeed.com/nid/46
Very interesting (and well done) testing of props. He didn’t test the Duc propeller though. They are considered to be a notch better than anything else, but haven’t seen it in any comparison like that.
From the newest Operators Manual
Airborne_Again wrote:
No, generic statement from Rotax concerning the 912 series. They recommend flying between 5000 and 5500 rpm
Can you point me to where this is stated, please?
On the Rotax Owner’s forum – even though the topic there concerns the 912ULS, it seems a general statement passed on during Rotax service classes. There are plenty of threads on that forum discussing this topic such as this.
LeSving wrote:
He didn’t test the Duc propeller though. They are considered to be a notch better than anything else, but haven’t seen it in any comparison like that.
We ordered our Sportstar with a Duc propeller because it did not require 500 hr overhauls like the standard prop did. I was quite surprised when the POH claimed substantially worse performance with that propeller – both cruise speed and climb rate. So far my experience with the aircraft suggests that the POH is overly pessimistic, but I’m still collecting data.
Steve6443 wrote:
On the Rotax Owner’s forum – even though the topic there concerns the 912ULS, it seems a general statement passed on during Rotax service classes. There are plenty of threads on that forum discussing this topic such as this.
I’d rather rely on the manufacturer’s recommendations and not on third-hand statements from a web forum. It is noteworthy that Rotax Service Letter SL-912-016 “Essential information regarding engine behavior, performance and manifold pressure data” which strongly cautions against low rpm/high MP operations with MOGAS 95 does not have any general advice against low cruise rpms. I suppose that Rotaxes in the US run on leaded AVGAS to a much greater extent than in Europe and that may have something to do with the recommendation of running at 5000+ rpm.
(An exception to relying on manufacturer’s recommendations is when extensive experience and independent testing show that those recommendations are overly cautious, e.g. concerning overhaul intervals and LOP operation of normally aspirated Lycoming engines.)
The problem with these techniques on the majority of basic ULs is that there is no way for the pilot to regulate manifold pressure separately from rpm. They only have one lever.
gallois wrote:
They only have one lever.
Several have manual variable pitch. We had it on the club’s Faeta (with an iS engine), but replaced it with a fixed pitch due to wear and tear of the rather skinny mechanism, and for simplicity for training. Almost every single one has at least on ground adjustable pitch. How the pitch is supposed to be adjusted somewhat accurately according to the table provided by Rotax, without a readout of the manifold pressure, is a bit mysterious to me. The exception is the iS engine. It has this (automatic) power/ECO mode which tends to mess it up a bit. The iS is best run with an in flight variable pitch IMO.
If everybody (including aircraft manufacturers) actually read all the documents Rotax hands out, they would be all running at 5000 rpm in cruise and fueled by lead free, ethanol free 98 octane 
