Cruise rpm and prop pitch

".....5000 to 5500 is what the engine was intended to operate at..."

I see nothing anywhere in my Rotax engine manual that came with it saying anything like that.

On the contrary....I see a table listing several cruising power ratings of 100% , 75%, 65%, and 55% with corresponding RPMs of

5500, 5000, 4800, and 4300.....

To me that implies Rotax's blessings for optionally cruising as low as 4300 RPM.

It also has above that table the phrase "Engine operation is permitted without restriction between full power and power requirements of propeller. However, for economic reasons it is recommended  to run  the engine to the following data:..."

I don't see how it can be clearer than that Rotax says it's OK to cruise as low as 4300.

Good ideas. I guess I have to play with it. My prop has pins that you use to set the pitch and it only takes a few minutes. It was set at about pin#0 or 1 when I got the plane last year so before I flew to OSH I upped it to#4 I should have played with it more but it did increase my speed 12 to 15MPH but WOT was only around 5200 . Fuel flow jumped about 1 gph.

From Roger's advice I changed it to#3 which gave me about 5600 plus WOT this resulted it a speed increase of 8 to 10 MPH . For most runways it climbs fine but for short field I will change it to #1.

So my question is can I run with #4 for a long cross country with nice long runways?

Fred....

It sounds to me like you need a setting #3.5 for your prop. ?

one question: Did you mean that when you had it set to the high pitch #4 that fuel consumption increased by 1 GPH? My take on cruising higher pitch, just as with cruising flat land in too bear in a car, should result in lower fuel consumption. (It would be interesting fir you to calculate your fuel consumption at different pitches in miles per gallon instead of GPH.)

i'm out of my depth here, but, if fuel efficiency went down at #4 setting that to me hints that at that high drag in that high pitch prop the engine or carb is somehow not optimally matched. I vaguely remember Brian Carpenter saying something about pitching too high can damage the engine.

For me to would be a tough call to decide whether to do a long cross country you describe with the #4 or #3. But, Since the #4 offers barely any edge in speed I think I'd stick with the more standard #3 with WOT at 5600. You never know  when you might be very happy to have this extra climbing ability.

Yeah consumption got worse and I think Rodger mentions that in his old posts. Like to said I would need to calculate it to be sure and it didn't make sense to me either I would think lower rpm should use less fuel but I guess WOT dumps the same amount in no matter what RPM.

I didn't really get a chance to really test the rpm on 4 very much so I might try it again for a little longer ride.

The 3 did climb pretty slow from 8000 to 10500 I think I was down to about 100FPM.

Oh... BTW:

You wrote ... "My prop has pins that you use to set the pitch and it only takes a few minutes. "

That sounds almost as easy as the pitch setting on my IvoProp. Curious what model/brand that prop is? (Many ground-adjust props require a protractor, skill, peristance, LOTS of time, and even trial and error to adjust right.)