Why Uk pilots cruise in low 4000's?

lol it is crazy . Yeah I got a Kiev prop on mine . It's annoying seeing everyone out the U.K. cruising about 5k and for some reason we are low 4k when that not how Rotax say to fly them , I need to find out ??

Ok this myth that the Rotax 912 will somehow be damaged by driving around the sky below some certain rpm just drives me bonkers.  There is nothing, I say again, nothing in any Rotax manual, service instruction, service bulletin, notice, or any other document cautioning against cruising around the sky at or below any rpm or range aside from our maximum (5,800), our five minute time limit above maximum continuous (5,500), and oh, a minimum idle rpm is also to be set. However, Rotax did publish a document that cautions one’s propeller should be pitched to achieve a minimum static rpm of 5,200 whilst on the ground at Wide Open Throttle.  But, once airborne, fly at whatever rpm that keeps you aloft...and have no fear that cruising about in the 4,000 rpm or so range (I do as well) will harm your engine.  cheers!

It is not a myth, take a Rotax the heavy maintenance class with Eric Tuckerand who works for Rotax and you'll hear all about the resonance between the front and back cylinders in the crankshaft when operated between 3000 and 5000 rpm due to the different lenghts of the intake manifolds (I.E. it's the intake manifold tuning) the engine was designed and intended to operated above 5000rpm. The symptoms and effects can vary based on installation and propeller combinations.

Thanks Garrett. Did Eric Tucker give an example of what damage this resonance could cause over time and why Rotax hasn’t issued any cautions or advisories about operating in this range?

Jim, I took the Rotax classes 9 years so I do not remember exactly what he said so I found and copied this off the CT forum and it explains it better than I can.

So, what happens at low flight RPM? The firing impulses are spaced further apart, and the resulting pulsing resonates and excites the dogs and causes them to chatter. The dogs are the only thing transmitting the torsional motion from the prop gear to the prop shaft, the prop gear is freewheeling. Anyways, this chattering is amplified by the propeller, which is basically a giant flywheel on the front of the engine. These cause prop shaft harmonics which are further amplified by the large diameter of the prop shaft gear, and transmitted to the teeth of the gear, causing accelerated wear. Depending on the aircraft and prop combinations, some aircraft are more sensitive than others.

The 912 UL and 914 do not experience this problem as severely, because the dogs mesh more closely and they have a lower compression.

The resonant frequency is due to the pulsing between the front and rear cylinders, and reverberates through the engine crankshaft too. So you have all of these resonating parts between 3,500 to 4,300ish. This isn't in the manuals because the expectation is that you will make use of the engine horsepower, and not oversize. The european line of thinking is to size engines for the application and use to the full extent, instead of the american way which is shove as much horsepower as possible under the engine and use a tiny portion of it.