I'm not afraid to open pandora boxes; I agree with Rotax recommendations to
cruise
at 5000rpm or above and set the idle not below 2000rpm. Why? Because these
power plants have a TBO rating that allows them to perform their intended task
all the way up to the full allocated time at a price point that allows
replacement
at half life, if you on-sell the used unit, or replacement at zero TBO,
i.e run it at full power and then throw away the unit... it will last the entire
TBO without failure; and that is all it needs to do as far as the factory
is concerned.... if i was a manufacturer with a business plan thats how i would
do it as well; and would be thrilled to supply a brand new fresh unit every
1600 or 2000 hours of use.
Reasons why I believe the 80hp 912UL can be cruised under 5000rpm; it has a
different
valve timing profile and flatter torque curve than the ULS; reasons I believe
the 912 idle speed can be lowered to 1600rpm; new style composite foam
core props are extremely low mass and do not allow the spur gears in the gearbox
to chatter as would be the case with heavier solid blades; if you can hear
the gearbox chattering at idle then the idle speed is too low... why I believe
cruising at under 5000rpm has no affect on gearbox life; the propeller is under
constant load in cruise regardless of rpm; how are the spur gears going to
chatter and wear out prematurely if they are operating at constant load? Why
mechanical
parts fail; material failure due to physical impact, friction heat,
and sustained harmonic resonance leading to disruption of atomic attachment. The
last reason possibly being why rotax recommends the operational rpm they do...
however the physical proof of powerplants continuing to operate under condition
well past their TBO times causes me to doubt this.
Nail a VW beetle engine to a powerpole and run it at WOT see how long it runs
continuously...
nail another one to the same pole and run it at 75% power see how
long it runs for continuously... minus the lower time from the higher time
and divide the answer by the length and thickness of the said pole... perform
the same two tests repeatedly with a variety of different length and thickness
poles to ascertain which length and thickness pole offers the longest lasting
engine running time before failure of either the pole or the engine... if
dimensions
of pole dimensions fail to affect service life of either test unit then
it can be ascertained that the pole is not a mitigating factor and it is
inconclusively
perhaps the rom range chosen for each test performed.
I have an 80hp ULS in my 325kg mono with 310hrs on it since bulk strip
overhaul...
it cruises all day long unstressed @100kt IAS @ 4700rpm with a 3 blade 64"
Bolly prop set at 19deg pitch and 12lph fuel burn rate... with the idle set at
2000rpm over the fence I'm remaining within ground affect for the entire length
of the field at roundout; at 1600rpm I am on the ground where I intend to
be, and on the ramp my gearbox is not chattering... guaranteed when this
powerplant
reaches its TBO's it will still be well within rotax wear limitations for
continued service life on condition.
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