Inflight adjustable prop questions

Hi William,

Great question. I will be very intersted in the responses from our Rotax experts.

You probably already have the following but just in case, this is what I get from the Rotax Operators Manual:

4800 RPM, 65% /26.5Hg”/13L/h

5000 RPM, 75% / 27.2 Hg

5200 RPM 27.5 Hg”/15L/h

5500 RPM 28 Hg”/19L/h

Expressed in a slightly diffrent way:

Max Cruise 94 hp @ 5500 rpm/ 28 Hg /19Lh

Econ Cruise 65 hp @ 4800 rpm/26.5 Hg /13L/h

The suggestion is, that the above settings are within Rotax operating recommendations.

I am a bit confused about your settings.

WOT and 5500-5800 for less than 5 minutes in the TO and initial climb is within parameters and gets your best power for climb.

However WOT in any other part of the pattern is too much, downwind should be cruise or less.

Cruise is normally defined as 75% power so probably 4800-5200 rpm and MAP of 23-25 inches

again "cruise" should not be WOT but about 65-75% power 4800-5200 rpm and about 23-25 MAP.

You imply that WOT at 5600 rpm is a MAP of 25...this seems low unless you are operating at high density altitude. I often see 28+ MAP at WOT, so perhaps check your inlet for restrictions.

Aircraft piston engines are most reliable when NOT operated at Full power continuously set a reasonable RPM for cruise and adjust MAP. At altitude of course you can use WOT as it will not result in a high MAP, so at say 10,000 feet I usually only get 22-23 MAP at WOT and operating continuously there is OK, but I need to be cautious when descending.

There is a side note to the chart that says (if I remember correctly) that the limits shown only apply at WOT AND below 3500 ft (@ ISA?). Thus, to me, “normal flying”  should be no issue. 
Given a certain cruise speed keeping revs up in the range of 5000-5200 rather than at something around 4500 (with an extensive pitch of the prop) is said to be better because of lower side forces on the pistons.

I cannot double check but expert explanations on that matter sound plausible to me.