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Suddenly there is more room at the Seagull’s nest; the wings are
gone to the paint shop. They should be back tomorrow and from my own “weight
watch maniac’s” view, I can barely wait to see how much the paint has added.
Each wing weighed about 26 kg (57 lbs) including all attachments, flaperon
and wing tip before painting. The painted wings will also represent the first really ready
parts and a milestone in the whole process. Another milestone is that we now have an engine. Considering
possible alternatives of getting it from the Scandinavian Jabiru dealer in
Stavanger Norway, made us go for the quickest which was to jump into the car
and go pick it up. We made the 2800 km trip in three days with two overnight
stops with relatives in Oslo. The shortest route was closed for the winter
and the one available was scenic, narrow, much longer and had 53 tunnels
between Oslo and Stavanger (yes, we counted them on the way back). There was
also an unexpected number of toll plazas that on the return trip, when many
of them were closed, rapidly drained our bulk of Norwegian currency. The
aftermath of that is still yet to be seen. The Jabiru was immediately hung in place to verify the needs for
cowling modifications. The Rotax PSRU moves the prop shaft up related to the engine
cylinders and we were prepared to some interference with the upper engine
cowl. That assumption was right but only a slight lifting of the upper cowl
AFT part is needed. In all other aspects the Jabiru fits excellent and with a
three inch prop shaft extension, the unwanted nose-up CoG. moment caused by
the engine, decreases some 15 % compared to the Rotax installation. In Stavanger we also discussed suitable props a lot. The Jabiru
dealer Paul, recommended a two bladed light, fixed pitch wooden propeller.
Because of the prototype evaluation, we just thought that an adjustable would
give us more options in finding the best pitch. We have learned however, that
a fixed prop isn’t that expensive and the rpm’s we get with this at different
speeds, will still tell us a lot about how improvements should be made. It is maybe a little harder to predict the performance of a
pusher like the LN-3 than on a tractor design. Even if the theoretical
calculations give you a certain picture, the reality might give you something
different. Not necessarily worse, it can also be better. Experienced people used to be cautious about the fuselage shape
up-streams the propeller. If it’s too steep, a premature offset of the air
stream will adversely affect the prop efficiency. Since the LN-3 is a tandem
with a relatively narrow fuselage, we are on the right side to what available
praxis tells. A good pusher installation is actually more efficient than a
tractor since the high speed prop wash flows over much less of the airplane
surface. Thanks for
your interest Lage |