Graham,
Perhaps other air-cooled aero engine owners will back me up here, but I'm
not sure what you mean by "...that it is not possible to optimise the
performance of any air cooled engine, the heat transfer from thick metal to
air is not efficient enough". If you mean that the closer control of
cylinder head temps is better with fluid cooling I won't disagree, but
bearing in mind that most of the world's reciprocating aero engines are
direct drive, air cooled I think your statement needs clarification.
Also, the Europa is NOT an aircraft which needs to have a large propeller
turning less than 2000 rpm for optimum high speed operation. At (even) a
cruise speed of 160kts a 58" propeller turning 3000 rpm is working very
well. We sell Jabiru engines to airplanes much faster than the Europa (i.e.
with a wider speed range) with no adverse feedback on high-end performance.
Lastly, the VP propeller performance has more to do with the Rotax's
relatively peaky power and torque curves rather than the wider speed range
of the aircraft. While I do agree that a VP prop is "a nice thing to have"
for general flying and gives more flexibility in hot/high environments and
where shorter take-off is needed, my experience is that a good fixed-pitch
or ground adjustable wood or composite prop. is very hard to beat for the
money. I'm saying this as the owner of an Airmaster prop on our Jabiru
demonstrator - it's a 'nice thing to have" but not essential for the needs
of the aircraft or engine in most cases.
Andy Silvester
Suncoast Sportplanes, Inc.
www.suncoastjabiru.com
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