M.A.P.

Hi Sean,

Which Rotax manual did you get those numbers from? I can't find the numbers you've referenced in any of the Rotax manuals I've looked at.

The 912 Operators Manual Ed. 4, Rev. 1 (current version) has a table on page 5-6 which recommends operating the 912 S/ULS (when equipped with a CS prop) at
• 27 in.Hg at 5500 RPM,
• 26 in.Hg at 5000 RPM and
• 26 in.Hg at 4800 RPM.
The MAP figures you say you're typically using appear to be close to the recommended figures as shown in the OM.

As for the RPM, the Airmaster manual indicates that at normal Rotax RPM's, the regulator will attempt to keep the RPM's within approximately ±100 RPM of the target engine speed. If the regulator is failing to regulate within these limits, I'd speculate that the prop is sitting on the fine pitch stops and there's not enough engine power (MAP) to reach the desired RPM at that airspeed.

Is the "fine" pitch indicator on the regulator flashing green when this is happening? Per the manual, this would be an indication that the prop has reached the respective limit switch and the governor is attempting (and hopefully failing) to drive the prop further past that limit. If this is in fact the case, then until you have enough power and/or airspeed to reach the set RPM's (and the governor moves the prop off the fine pitch stop) you're sitting behind what is essentialy an expensive and complicated ground adjustable prop. Chapter 8 of the Airmaster manual covers the proper setting of the propeller pitch stops.

Hi Tim,

Thanks for your response & comments.

I am more than happy with the performance of the Airmaster/912ULS in the Sonex. This is not a concern about performance but rather my wish to "check in" with the Forums experts and get their take on my observations.

As for the origin of my Rotax RPM/MAP - I will have to go back to what I researched about 2.5 years ago, before the aircraft first flew. I dimly recall a graph that showed a difference between 95 RON & 98 RON - could this be the small difference in your figures compared with mine?

"...........essentialy an expensive and complicated ground adjustable prop. "

No way! my Victorian friend - I have experince with a ground adjust prop. The CS takes Take-Off, through to Cruise, to a whole new level. I doubt that it is a cost effective option but there is no denying it works extremely well.

FYI: at 5000 rpm my Sonex is loping along at 130 knots indicated (137 knots true) for about a 13L/hr fuel burn - not too shabby.

Hi Sean,

While I reckon I'm pretty good at interpereting technical documents, I wouldn't call myself an expert by any means. What I've learned about Rotax engines has mostly been in aid of keeping my Gazelle in an airworthy condition (as well as a healthy dose of curiosity). At any rate, could the graph you mentioned be the one on page 7 of SL-912-016R2 "Essential information regarding engine behavior, performance and manifold pressure data..."?

It can unfortunately sometimes be a bit tricky to work ot what people are trying to say when you've only got the text to work off - it sounded to me that your original post was saying that you were having trouble with the revs not reaching 5450 RPM..I apologise if my bit on prop pitch was unwarranted.

FYI: at 5000 rpm my Gazelle is plodding along at 75 KIAS (80 KTAS) for about a 14 l/hr fuel burn - I'm jealous.

Hi Tim,

No concerns about Max engine RPM. Interested to get comments on the MAP reading compared with engine rpm.

I assume that Airmaster have deliberately/conservatively set the pitch stops, to keep the engine rpm slightly below Max Continuous. This is fine by me.

I only use Max rpm for Take-Off, preferring economy over speed. My standard Cruise range  is 5000-5300 rpm, most often in the 5000-5100 rpm range.

SL-912-016R2 does seem to be the document/graph, I dimly recalled, showing diffrent MAP for 95 & 98 RON but doesn't really address my enquiry other than to suggest (if my reading is correct) that my engine is operating well within the graphs max for knock/detonation prevention.

Dont be jealous -  the Sonex is fast, which is great for touring. The downs side is, poor carrying  capacity in both weight & volume and has a higher stall ergo less chance of surviving a forced landing/crash.

Few aircraft manage to have the best of both ends of the performance envelope.If you can afford an ATEC Faeta or a Pipistrel Virus SW, you will get low stall, great carrying capacity and high speed cruise. If you are into homebuilding you can't go past a Sonerai/Rotax, as built by Robin Austen ; 

http://worldrecordplane.com/  

https://www.homebuiltairplanes.com/forums/threads/vh-srs-170-knot-sonerai-ii.34596/

I'm puzzled by what you say "With the exception of TO, all settings have an RPM limit at about 5450."

To me it sounds as though someone has messed with something! 

I have the same engine and prop in a Europa. The controller holds very well to the factory set rpm figures in auto mode - 5000 in cruise mode, 5400 in climb mode, 5700 in T/O mode (so that within the permitted 100rpm variation you won't exceed the Rotax 5800rpm limit). Note that the controller is only interested in varying pitch to maintain constant rpm at the value you have selected. It is not trying to optimise pitch/rpm combinations.

In any mode, WOT will give me about 28" MAP (I wouldn't use that in cruise, but it can do it, and with 98 0ctane Mogas or with Avgas it's within Rotax permitted limits on the revised performance curve in SL-912-016R2) By reducing throttle I can run at lower MAP with no change in rpm (not very much in T/O setting.) 

The pitch range is limited by microswitches not mechanical stops. These must be adjusted to suit the aircraft, they are not factory preset.

In cruise my fast setting is 26" giving me about 130kts at 5000rpm. 24" gives me about 110kts at 5000rpm. I can go down to about 20" before the rpm starts to drop, meaning the prop has gone to fully fine. At 15" the rpm is down to about 4000 and just about sustains level flight.

I hope that helps, not confuses. Just to labour the point, the cruise/climb/takeoff selector knob might just as well be labelled 5000/5400/5700. It is purely setting a reference rpm.