RPM

Yes it is bad for the engine to fly continuously at such a low RPM. The engineering is complex but basically it puts a lot of strain on the conrod and other components.

AS the others say around 5200 rpm is optimal.

If you want to know more ride a bicycle in top gear up a hill vs in the right gear and find out which feels better

Dear Gaston,

what is the best on a long trip by car? Full throttle (if you can)? On a long trip it's the best to drive economical. It's the same at your aircraft.Most of the discussion about the best RMP at ROTAX engine based on full throttle. Yes at full throttle your propeller needs pitch that enables 5200 to 5500 or a little bit higher RPM. Which kind of 912 you have? Which kind of propeller you have - fixed prop or constant or what? I have only enough experiance of fixed props: The pitch of my propeller enables 5400 RPM at full throttle in horizontal flight because at fixed prop you have to find a compromise. If you have constant prop maybe 5500 can be OK. But the RPM ist not all unless you only want to fly at full throttle (for fixed props). Economical flight starts at 65 to 75% - depends on your aircraft (aerodynamic). So if the prop is adjusted well (5300-5500 at full throttle in horizontal flight) maybe the economical setting can be at 4800 to 5000 RPM. Lower RPM dont hurt the engine if you are not in full throttle. Yes the ROTAX 912 (not injection) is desighned at a stationary engine, the apropriate setting of the ignition point starts at 4800 - better 5000 RPM.

If you have a constant prop (in flight training i had a constant prop) my FI recomends 5200 (manifold pressure 25) at the entire range of economical (crousing) flight (maybe this can be 65 to 75 %).

Dear HKK,

First - Car engines & operating conditions are not as for aircraft - there is much in common but direct comparisons are invalid due to the almost diametrically apposed demands on each.

Ground based vehicles:

Have multiple gears, to facilitate the engine operating within its optimum torque range, irrespective of vehicle speed.

They have variable ignitions, to change the timing of the combustion process, for good power/torque delivery from slow - fast engine speed/rpm.

In general the engines are optimised for acceleration and almost constantly varying torque demands to meet road/traffic conditions.

Are generally heavy due to engine construction type & materials, cooling systems that must operate effectively from a standstill to cruise speed, in ambient temperatures from -50C - +50C

Fuel econamy is tied to ground speed (mph/ kph) not trip speed. In fact the faster you go the more fuel will be consumed.

 Aircraft engines have:

At best, a limited gear selection, in a constant speed prop and no gears for a fixed prop

Ignition optimised for a narrow engine speed range. Operating outside this range will deliver sub optimal fuel/power consumption

Engines that are designed for a constant power delivery within a narrow rpm range 

Constructed to keep weight low (high power: weight ratio) with just enough cooling to prevent the engine from self destructing usually optimised for airborne/movement condition (aircraft don't do well standing still with the engine running).

Fuel econamy is tied to fuel consumption (how ever you wish to measure it) per hour and  to trip time. ie for a given fuel consumption per hour, the faster the trip is made, the lower the fuel consumption.

For the sake of academic discussion….  

I have a hard time with the idea that cruising at lower RPM is a problem as long as you are not demanding a high load on the engine at those lower RPM.  If you have a fixed pitch prop, set to get say 5600 WOT in level flight, then when you back off to a lower RPM (say 4800) you have unloaded the engine greatly. If you had your prop set to require full throttle (or nearly full) at 4800 RPM, then I agree this would be a problem because the engine would be heavily loaded, and essentially lugging.  Rotax supports this notion.  Service letter SL-912-016R1 does recommend against cruising below 5200 RPM at WOT, but has no restrictions against doing so at lower throttle settings.  With the carbureted engine I do understand the advantage of reaching an RPM that optimizes engine timing and efficiency, but there is nothing I see to suggest that not doing so could damage the motor. Some engine/prop combinations may not operate as smoothly at lower RPM, but that is not true for all.  

For constant speed propellers, Rotax has recommended settings to keep the manifold pressure lower at reduced RPM, for instance they show a 55% power setting at 4300 RPM and 24 In/Hg manifold pressure. There is nothing Rotax publishes that recommends against cruising at 55% at this reduced manifold pressure.

I fly behind a 912iS Sport with a ground adjustable prop set for 5600 WOT level at 5000’.  I typically cruise around 5200 which is kind of a sweet spot for the engine and it gets me 110 kts true.  But if I wanted to lope along at 80 kts to build hours, I would have no problem with cruising at 4600 RPM and would not worry about damaging the engine. The Rotax performance graph for the iS motor shows the cruise RPM between 4400 and 5000 (see attached graph).  Clearly Rotax does not think this is a problem.

Hi Jeff

I agree with almost all of what you have said, with the exception of setting your "adjustable prop set for 5600 WOT level at 5000"

In setting up your engine/prop to achieve this you have compromised the take- off (TO) performance/load on your engine, also reduced the pilots option for terrain/hazard avoidance (short field) and possibly negatively impacted on the aircrafts density altitude capabilities.

The Rotax recommendation for min RPM of 5200 is for all load/high power conditions (TO) - this is unlikely to be achieved by setting WOT the way you have BUT will be achieved if you follow Rotax recommendation for setting Static WOT at 5200 rpm.

Further there is a difference (in my mind) between Cruise and Loiter conditions - Cruise is configuring the aircraft as if you are actually going somewhere (min 5200rpm) Loitering is sort of drifting, admiring the scenery, checking out something on the ground  - maintaining altitude at a low power setting, depending on aircraft could be in the vicinity of say 4000rpm, no problemo!