Cruise rpm and prop pitch

You wrote

".....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.

This is correct but does not mean that you can adjust your prop to 4300 rpm at WOT.

For you to have a well balanced setup for climbing and cruze and have the engine working as per the Rotax specs with a fixed or ground adjustable prop you should adjust to have aprox. 5100 rpm in climbing and 5500 to 5650(better) in level flight.

This is what Roger Lee reported. That way the engine will be running well in all the permitted range.

If you are needing to adjust to 4300 at WOT to have the needed performance may be you are under-powered and should consider to change the engine to a more powerful one.

Regards

Hi Gen..

Thanks for your thoughts.

you wrote....

"....If you are needing to adjust to 4300 at WOT...."

No one, and certainly not me, was or is suggesting anyone ever set pitch to run 4300 at WOT. I don't see how you got the idea I or anyone was suggesting that.

(FWIW, . Generally,personally, I suggest most do well setting pitch to run somewhere between 5600 and 5800 WOT in level flight.)

I don't think we're really disagreeing ... except somehow you thinking I was saying it's OK to set pitch for 4300 WOT.. Please consider  rereading.

I was pointing out that per Rotax documents they explicitly say it's OK to cruise...operate the engine  at between 4300 to 5500 sustained.  The other place we might differ is your earlier suggestion that it's bad to cruise with RPM below the 5000 range. As I said, neither my Rotax Manual or any official Rotax document I've ever seen saying anything like "it's best to stay in the 5000 to 5500 RPM range"....

If you like cruising in that 5000 to 5500 range that's fine. But per my post, my experience, and my Rotax manual there  are advantages to cruising at some lower RPMs.

regards,

Al

I agree with you I misunderstood the text  

Yeah I was asking if I could pitch the prop for a long cross country with a WOT of 5200 but I was also asking if it is OK to cruise around on a nice evening at lower RPM say 4500. Some of the planes I fly with travel in the 80 MPH range.

Thank you for all the input.

" I was also asking if it is OK to cruise around on a nice evening at lower RPM say 4500."

My personal answer (if it's not already obvious) is it's generally just fine to do that; that (assuming you haven't set prop pitch too high) flying low RPM saves wear and fuel, and reduces cockpit noise level.... I do it often when I'm not in a hurry to get anywhere. Sometimes I fly with fuel consumption as low as 2.5 gallons per hour. If someone's goal includes building hours rather than miles, flying slow is also an economical way to do that.

Only "but" is something like "....but if you detect that yours happens to seem to vibrate more or run less smooth at 4500, hunt around for another RPM either a bit above it or a bit below it, where sound and feel are sweeter." And of course the obvious, don't fly so slow that you don't have a decent margin between your cruise speed and your stall speed.

Alex