preferred cruise setting?

[207WF]

I think we are all over the map on this: some cruising 5000 rpm or below, and at the other extreme, Ed advocating 5500.  For most of the 7 years on my SW - with the prop set slightly to the coarse end of the approved Roger Lee setting (i.e., 5550 WOT at normal altitudes), I have used 5200 rpm for cruise.  This gives me 112 knots true and 4 2/3 gph, averaged over many long cross countries.  This year on the trip from SoCal to Osh, I experimented with 5300 rpm.  This gives me 115 knots true and 5.0 gph.  Also, the bird seems to run a little cooler on the oil temps and just a bit smoother at the higher rpms.  Getting 3 more knots for only 1/3 gph seems like a good deal, and with the cooler temps and smoother engine, I am going to convert to 5300.     WF 

[Ed Cesnalis]

Prior to agreeing ( we won't ) on a preferred cruise setting we would have to agree on a preferred prop pitch.

• Prop pitch - my first priority is to keep all available power accessible. My cruise altitudes are 8.5, 9.5, 10.5 & 11.5  so WOT @ ~10,000 allows me access to almost all of the power available.  I'm giving up some at low altitude's but to access max power down low you would be cruising in excess of 75% power so optimizing for a high altitude cruise makes sense even if you live at sea level.  WOT @ 7,500 = 5,500 is the coarsest I would go which is best performance at 75%.  Above 7,500' 75% is not available so WOT is needed to avoid low power settings.
• Cruise-setting.  If you are below 7,500' throttle back to a 75% setting otherwise WOT.

I have used this thinking on every NA engine that I have flown behind ( or in front of ) because I have always flown at high altitude.  It is not specific to Rotax.  I always flew my IO-360 WOT as well. 

[FlyingMonkey]

I use 5000rpm for just sightseeing, 5200rpm for cruise if I'm going somewhere, and 5400rpm if I'm trying to get somewhere fast.  This is usually at 3000-7500ft.  The engine seems to like all these settings equally well.

[paul m]

Looking for some thoughts on how my plane's prop is pitched.  Tonight tested WOT and got following results.  Typical flights are between 3,000 and 5,000 msl.

[Roger Lee]

A 912is engine. It is a tad over pitched. Flatten it by .5 to .75 degrees. Try to be around 5650 +/- (5600-5700)

[paul m]

thanks Roger

[FlyingMonkey]

WF, adding to what I mentioned in my post above, I flight plan the following numbers:

 

5000rpm  =  4.5gph  =  ~107kt

 

5200rpm  =  5gph  =  ~115kt

 

5400rpm  =  5.5gph  =  ~120kt

 

These burns are probably a little on the conservative side (meaning higher than actual), but they seem to work for me and have not left me short of fuel.  Clearly 5200rpm is the best compromise of speed and fuel burn, but if you are trying to get somewhere fast than the 5400rpm setting works well.  I suggest you give 5400rpm a try, my engine really likes that setting as a fast cruise and runs pretty cool there.  My prop is pitched flatter than yours though, so YMMV.

[Roger Lee]

His 912is engine will be in a good economy mode at 5200 or less, but above that he will burn more fuel than the CTSW. They have a target throttle position of under 92%. If you don't care about the extra fuel nothing wrong with higher rpms.

[Ed Cesnalis]

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His 912is engine will be in a good economy mode at 5200 or less, but above that he will burn more fuel than the CTSW. They have a target throttle position of under 92%. If you don't care about the extra fuel nothing wrong with higher rpms.

 

Roger,

 

Isn't the carbed version pretty similar?  When approaching WOT ~92%  the main jet gets control from the needle circuit and goes full rich.

[Roger Lee]

Hi Ed,

The fuel injected engine is different than the carbs because it has a throttle position sensor tied into the ECU that can actually meter fuel where the carb can't.

[Ed Cesnalis]

The technology is different but what is the same is that WOT has no economy associated with it except that you probably use WOT at altitude where you burn less.

 

In both cases you have to retard the throttle to get into better economy and avoid full rich.

[mocfly]

I keep it to the firewall (~ 5400)until I am about 5 miles out and never have burned more than 5.5--5.7 Gph.

[Roger Lee]

Hi Ed,

 

Higher altitude is where the 912is shines over the carbs. It can adjust far better than our carbs and can fly at lower rpms and keep the same speed. I was flying along side Duane one day and at low altitude and the carbs seem to be a little better.  At low altitudes I could run at 5100-5200 and he was 5300-5400+. At 9K he was down at 5100 and I was up at 5300 and using far more fuel at about 25%. The higher we went the worse the carbs performed and the better the fuel injection performed. He was able to run less rpm than me and save more fuel. 

 

Note:

Individual prop pitch can and will play a part in all our results.