Actual Power

Jeff

Yup, you got it.  it is important to remember that torque and RPM also are a factor of what is going on in an engine.  When we do hit the peak torque point the engine can still climb in RPM and this is partly due to the unloading of pressure in the cylinder head and cylinder head breathing.  The sweet spot is just between peak torque and maximum continuous power loading.  The engine will accelerate smoothly and quickly for the most part.  Overloading the engine (too much prop pitch for example) will cause a great deal of stress within the engine, particularly the crankshaft and crankcase, it normally leads to detonation in the engine.  

Geared engines are like having a torque multiplier is another way to look at it.  If we were to calculate the power at prop shaft speed you would need to multiply the torque by the gear ratio.  At least that was the way it worked for me. 

Cheers

It should be noted that the measured HP numbers have changed over time, generally in the direction of being more honest. Pre-1970 "gross HP" numbers were measured at the crankshaft, but with no exhaust, intake or attached accessories. Other cheats included a minimal amount of lighter weight oil. SAE Net HP is still a crankshaft measurement but under as-installed-in-the-vehicle conditions. Modern EU ratings are now similar (though required by law to use kW as the primary unit).

Rotax HP ratings are therefore relatively honest. HOWEVER, Lycoming/Continental numbers, having been done in ancient times are not. The O-200 100 hp rating makes considerably less than the 912ULS 100 hp.

RW, 

"The sweet spot is just between peak torque and maximum continuous power loading". 

Thats interesting. The max torque of the 912iS Sport is around 5000 RPM, and my engine just "feels" the best cruising at 5250 RPM. It seems to want to run there at the top of the ECO band, and that's exactly half the way between max torque and max continuous power.    

Agreed, I also use 5200 rpm as my cruise for the IS engine. Luckily I have a variable pitch prop so that makes it easy to do that.

Hi Jeff

The iS SPORT is a bit of an anomaly in that the big change, about a year or more after its introduction, was an increase power at mid range.  This was done by bigger and longer intake runners, larger air plenum and software changes.  The difference was enough that the overload clutch also had to be changed to prevent slipping.  (from 500 NM up to 600 NM) 

Given what we know about the HP calculation it stands to reason there was an increase at full power also.  This however was never reported as the acceptance to the EASA TC was already done and so it remained a 100 HP engine on paper.  The gain in mid range was actually about 5 to 6 HP, a massive increase considering.  It however made it much better to match fixed pitch propellers on that engine.  Even so officially there is no increase in peak power.

Cheers