gator2:
I’m stretching some brain cells here, but the 5 min limit is neither one nor the other but all of the above.
Most helicopters are derated for overall performance requirements. If you dig into Part 27 and Part 33 you will see lists of limiting factors used, to include drivetrains, fluid temps, system RPMs and component life.
While ersa made a valid point between fixed and rotor wing apps, this is an apples to oranges comparison by the sheer fact that airplanes generate lift through airspeed (via prop rpms) and helicopters directly through engine rpms. A helicopter engine operated at fixed wing limits would run out of steam at altitude. Not to mention require the drivetrains to be built much stronger.
The 5 min limit is one of many standard certification items to ensure an aircraft can perform throughout its flight envelope, type design,and mfg’r requirements. Ever wonder why a twin engine helicopter has an aircraft based 5 min limit with both engines on line, but when OEI the limit drops to 2 ½ minutes per engine? Me too.
All said and done, when Robbie certified the R44 I would bet all design items were considered on the 5 min limit and not just 2 of them.
And just to throw another wrench into it so to speak, if your 44 is operated Part 91 in the States there is no FAA requirement to overhaul the engine at 2200 hours. So that kind of “limits” the engine theory.
But as with any operational limits, abuse them at your peril.
W1